Dr Amir-Homayoun Javadi

Article

• Javadi, A., Patai, E., Marin-Garcia, E., Margois, A., Tan, H., Kumaran, D., Nardini, M., Penny, W., Duzel, E., Dayan, P., & Spiers, H. (2019). Backtracking During Navigation Is Correlated with Enhanced Anterior Cingulate Activity and Suppression of Alpha Oscillations and the ‘Default-Mode’ Network. Proceedings of the Royal Society B: Biological Sciences, 286. doi:10.1098/rspb.2019.1016
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  Successful navigation can require realizing the current path choice was a mistake and the best strategy is to retreat along the recent path: ‘back-track’. Despite the wealth of studies on the neural correlates of navigation little is known about backtracking. To explore the neural underpinnings of backtracking we tested humans during functional magnetic resonance imaging on their ability to navigate to a set of goal locations in a virtual desert island riven by lava which constrained the paths that could be taken. We found that on a subset of trials, participants spontaneously chose to backtrack and that the majority of these choices were optimal. During backtracking, activity increased in frontal regions and the dorsal anterior cingulate cortex, while activity was suppressed in regions associated with the core default-mode network. Using the same task, magnetoencephalography and a separate group of participants, we found that power in the alpha band was significantly decreased immediately prior to such backtracking events. These results highlight the importance for navigation of brain networks previously identified in processing internally-generated errors and that such error-detection responses may involve shifting the brain from default-mode states to aid successful spatial orientation.
• Crowley, R., Bendor, D., & Javadi, A. (2019). A review of neurobiological factors underlying the selective enhancement of memory at encoding, consolidation, and retrieval. Progress in Neurobiology. doi:10.1016/j.pneurobio.2019.04.004
  Abstract View in KAR
  How is the strength of a memory determined? This review discusses three main factors that contribute to memory enhancement - 1) emotion, 2) targeted memory reactivation, and 3) neural reinstatement. Whilst the mechanisms through which memories become enhanced vary, this review demonstrates that activation of the basolateral amygdala and hippocampal formation are crucial for facilitating encoding, consolidation, and retrieval. Here we suggest methodological factors to consider in future studies, and discuss several unanswered questions that should be pursued in order to clarify selective memory enhancement.
• Javadi, A., Patai, E., Marin-Garcia, E., Margolis, A., Tan, H., Kumaran, D., Nardini, M., Penny, W., Duzel, E., Dayan, P., & Spiers, H. (2019). Prefrontal Dynamics Associated with Efficient Detours and Shortcuts: A Combined Functional Magnetic Resonance Imaging and Magnetoencenphalography Study. Journal of Cognitive Neuroscience. doi:10.1162/jocn_a_01414
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  Central to the concept of the ‘cognitive map’ is that it confers behavioural flexibility, allowing animals to take efficient detours, exploit shortcuts and avoid alluring, but unhelpful, paths. The neural underpinnings of such naturalistic and flexible behaviour remain unclear. In two neuroimaging experiments we tested human subjects on their ability to navigate to a set of goal locations in a virtual desert island riven by lava, which occasionally spread to block selected paths (necessitating detours) or receded to open new paths (affording real
  shortcuts, or false shortcuts to be avoided). Detours activated a network of frontal regions compared to shortcuts. Activity in right dorsolateral prefrontal cortex specifically increased
  when participants encountered tempting false shortcuts that led along suboptimal paths that needed to be differentiated from real shortcuts. We also report modulation in event-related
  fields and theta power in these situations, providing insight to the temporal evolution of response to encountering detours and shortcuts. These results help inform current models
  as to how the brain supports navigation and planning in dynamic environments.
• Patai, E., Javadi, A., Ozubko, J., O’Callaghan, A., Ji, S., Robin, J., Grady, C., Winocur, G., Rosenbaum, S., Moscovitch, M., & Spiers, H. (2019). Hippocampal and retrosplenial goal distance coding after long-term consolidation of a real-world environment. Cerebral Cortex. doi:10.1093/cercor/bhz044
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  Recent research indicates the hippocampus may code the distance to the goal during navigation of newly learned environments. It is unclear however, whether this also pertains to highly familiar environments where extensive systems-level consolidation is thought to have transformed mnemonic representations. Here we recorded fMRI while University College London and imperial College London students navigated virtual simulations of their own familiar campus (> 2 years of exposure) and the other campus learned days before scanning. Posterior hippocampal activity tracked the distance to the goal in the newly learned campus, as well as in familiar environments when the future route contained many turns. By contrast retrosplenial cortex only tracked the distance to the goal in the familiar campus. All of these responses were abolished when participants were guided to their goal by external cues. These results open new avenues of research on navigation and consolidation of spatial information and underscore the notion that the hippocampus continues to play a role in navigation when detailed processing of the environment is needed for navigation.
• Brunec, I., Robin, J., Zita Patai, E., Ozubko, J., Javadi, A., Barense, M., Spiers, H., & Moscovitch, M. (2019). Cognitive mapping style relates to posterior-anterior hippocampal volume ratio. Hippocampus. doi:10.1002/hipo.23072
  Abstract View in KAR
  As London taxi drivers acquire ‘the knowledge’ and develop a detailed cognitive map of London, their posterior hippocampi (pHPC) gradually increase in volume, reflecting an increasing pHPC/aHPC volume ratio. In the mnemonic domain, greater pHPC/aHPC volume ratios in young adults have been found to relate to better recollection ability, indicating that the balance between pHPC and aHPC volumes might be reflective of cross-domain individual differences. Here, we examined participants’ self-reported use of cognitive map-based navigational strategies in relation to their pHPC/aHPC hippocampal volume ratio. We find that greater reported cognitive map use was related to significantly greater posterior, relative to anterior, hippocampal volume in two separate samples of young adults. Further, greater reported cognitive map usage correlated with better performance on a self-initiated navigation task. Together, these data help to advance our understanding of differences between aHPC and pHPC and the greater role of pHPC in spatial mapping.
• Stimpson, N., Davison, G., & Javadi, A. (2018). Joggin’ the Noggin: Towards a Physiological Understanding of Exercise-Induced Cognitive Benefits. Neuroscience & Biobehavioral Reviews, 88, 177-186. doi:10.1016/j.neubiorev.2018.03.018
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• Kortteenniemi, A., Javadi, A., Wikgren, J., & Lehto, S. (2017). Progression of adverse effects over consecutive sessions of transcranial direct current stimulation. Clinical Neurophysiology, 128, 2397-2399. doi:10.1016/j.clinph.2017.09.112
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• Nejati, V., Salehinejad, M., Nitsche, M., Najian, A., & Javadi, A. (2017). Transcranial Direct Current Stimulation Improves Executive Dysfunctions in ADHD: Implications for Inhibitory Control, Interference Control, Working Memory, and Cognitive Flexibility. Journal of Attention Disorders. doi:10.1177/1087054717730611
  Abstract View in KAR View full text
  Objective: This study examined effects of transcranial direct current stimulation (tDCS) over the DLPFC and OFC on major executive functions (EFs) including response inhibition, executive control, working memory, and cognitive flexibility/task switching in ADHD.
  Methods: ADHD children received (a) left anodal / right cathodal DLPFC tDCS and (b) sham stimulation in experiment one and (a) left anodal DLPFC / right cathodal OFC tDCS (b) left cathodal DLPFC / right anodal OFC tDCS and (c) sham stimulation in experiment two. The current intensity was 1 mA for 15 min with a 72-hr interval between sessions. Subjects underwent Go/No-Go task, Nback test, WCST and Stroop task after tDCS.
  Results: anodal lDLPFC tDCS most clearly affected executive control functions (e.g., WM, interference inhibition), while cathodal lDLPFC tDCS improved inhibitory control. Cognitive flexibility/task switching benefited from combined DLPFC-OFC, but not DLPFC stimulation alone.
  Conclusion: Task specific stimulation protocols can improve EFs in ADHD.
• Javadi, A., Glen, J., Halkiopoulos, S., Schulz, M., & Spiers, H. (2017). Oscillatory Reinstatement Enhances Declarative Memory. The Journal of Neuroscience, 265. doi:10.1523/jneurosci.0265-17.2017
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  Declarative memory recall is thought to involve the reinstatement of neural activity patterns that occurred previously during encoding. Consistent with this view, greater similarity between patterns of activity recorded during encoding and retrieval has been found to predict better memory performance in a number of studies. Recent models have argued that neural oscillations may be crucial to reinstatement for successful memory retrieval. However, to date, no causal evidence has been provided to support this theory, nor has the impact of oscillatory electrical brain stimulation during encoding and retrieval been assessed. To explore this we used transcranial alternating current stimulation (tACS) over the left dorsolateral prefrontal cortex (DLPFC) of human participants (n = 70, 45 females, age mean (SD) = 22.12 (2.16)) during a declarative memory task. Participants received either the same frequency during encoding and retrieval (60-60 Hz or 90-90 Hz) or different frequencies (60-90 Hz or 90-60 Hz). When frequencies matched there was a significant memory improvement (at both 60 and 90 Hz) relative to sham stimulation. No improvement occurred when frequencies mismatched. Our results provide support for the role of oscillatory reinstatement in memory retrieval.
• Bernardoni, F., Geisler, D., King, J., Javadi, A., Ritschel, F., Murr, J., Reiter, A., Rössner, V., Smolka, M., Kiebel, S., & Ehrlich, S. (2017). Altered medial frontal feedback learning signals in anorexia nervosa. Biological Psychiatry. doi:10.1016/j.biopsych.2017.07.024
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  Background
  
  In their relentless pursuit of thinness, individuals with anorexia nervosa (AN) engage in maladaptive behaviors (restrictive food choices, over-exercising) which may originate in altered decision-making and learning.
  
  Methods
  
  In this fMRI study we employed computational modelling to elucidate the neural correlates of feedback learning and value-based decision making in 36 female AN patients and 36 age-matched healthy volunteers (12-24 years). Participants performed a decision task which required adaptation to changing reward contingencies. Data were analyzed within a hierarchical Gaussian filter model, which captures inter-individual variability in learning under uncertainty.
  
  Results
  
  Behaviorally, patients displayed an increased learning rate specifically after punishments. At the neural level, hemodynamic correlates for learning rate, expected value and prediction error did not differ between the groups. However, activity in the posterior medial frontal cortex was elevated in AN following punishment.
  
  Conclusion
  
  Our findings suggest that the neural underpinning of feedback learning is selectively altered for punishment in AN.
• Brunec, I., Javadi, A., Zisch, F., & Spiers, H. (2017). Contracted time and expanded space: The impact of circumnavigation on judgements of space and time. Cognition, 166, 425-432. doi:10.1016/j.cognition.2017.06.004
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  The ability to estimate distance and time to spatial goals is fundamental for survival. In cases where a region of space must be navigated around to reach a location (circumnavigation), the distance along
  the path is greater than the straight-line Euclidean distance. To explore how such circumnavigation impacts on estimates of distance and time, we tested participants on their ability to estimate travel time
  and Euclidean distance to learned destinations in a virtual town. Estimates for approximately linear routes were compared with estimates for routes requiring circumnavigation. For all routes, travel times
  were significantly underestimated, and Euclidean distances overestimated. For routes requiring circumnavigation, travel time was further underestimated and the Euclidean distance further overestimated.
  Thus, circumnavigation appears to enhance existing biases in representations of travel time and distance.
• Mutz, J., & Javadi, A. (2017). Exploring the neural correlates of dream phenomenology and altered states of consciousness during sleep. Neuroscience of Consciousness, 3. doi:10.1093/nc/nix009
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  Do people routinely pre-activate the meaning and even the phonological form of upcoming words? The most acclaimed evidence for phonological prediction comes from a 2005 Nature Neuroscience publication by DeLong, Urbach and Kutas, who observed a graded modulation of electrical brain potentials (N400) to nouns and preceding articles by the probability that people use a word to continue the sentence fragment (‘cloze’). In our direct replication study spanning 9 laboratories (N=334), pre-registered replication-analyses and exploratory Bayes factor analyses successfully replicated the noun-results but, crucially, not the article-results. Pre-registered single-trial analyses also yielded a statistically significant effect for the nouns but not the articles. Exploratory Bayesian single-trial analyses showed that the article-effect may be non-zero but is likely far smaller than originally reported and too small to observe without very large sample sizes. Our results do not support the view that readers routinely pre-activate the phonological form of predictable words.
• Javadi, A., Emo, B., Howard, L., Zisch, F., Yu, Y., Knight, R., Pinelo Silva, J., & Spiers, H. (2017). Hippocampal and prefrontal processing of network topology to simulate the future. Nature Communications, 8, 14652. doi:10.1038/ncomms14652
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  Topological networks lie at the heart of our cities and social milieu. However, it remains unclear how and when the brain processes topological structures to guide future behaviour during everyday life. Using fMRI in humans and a simulation of London (UK), here we show that, specifically when new streets are entered during navigation of the city, right posterior hippocampal activity indexes the change in the number of local topological connections available for future travel and right anterior hippocampal activity reflects global properties of the street entered. When forced detours require re-planning of the route to the goal, bilateral inferior lateral prefrontal activity scales with the planning demands of a breadth-first search of future paths. These results help shape models of how hippocampal and prefrontal regions support navigation, planning and future simulation.
• Jünger, E., Javadi, A., Wiers, C., Sommer, C., Garbusow, M., Bernhardt, N., Kuitunen-Paul, S., Smolka, M., & Zimmermann, U. (2017). Acute alcohol effects on explicit and implicit motivation to drink alcohol in socially drinking adolescents. Journal of Psychopharmacology, 31, 893-905. doi:10.1177/0269881117691454
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  Alcohol-related cues can evoke explicit and implicit motivation to drink alcohol. Concerning the links between explicit and implicit motivation, there are mixed findings. Therefore, we investigated both concepts in 51 healthy 18- to 19-year-old males, who are less affected by neuropsychological deficits in decision-making that are attributed to previous alcohol exposure than older participants. In a randomized crossover design, adolescents were infused with either alcohol or placebo. Self-ratings of alcohol desire, thirst, well-being and alcohol effects comprised our explicit measures of motivation. To measure implicit motivation, we used money and drink stimuli in a Pavlovian conditioning (Pc) task and an Approach-Avoidance Task (AAT). Alcohol administration increased explicit motivation to drink alcohol, reduced Pc choices of alcoholic drink-conditioned stimuli, but had no effect on the AAT. This combination of results might be explained by differences between goal-directed and habitual behavior or a temporary reduction in rewarding outcome expectancies. Further, there was no association between our measures of motivation to drink alcohol, indicating that both self-reported motivation to drink and implicit approach tendencies may independently contribute to adolescents’ actual alcohol intake. Correlations between Alcohol Use Disorders Identification Test (AUDIT) scores and our measures of motivation to drink alcohol suggest that interventions should target high-risk adolescents after alcohol intake.
• Dormal, V., Javadi, A., Pesenti, M., Walsh, V., & Cappelletti, M. (2016). Enhancing duration processing with parietal brain stimulation. Neuropsychologia, 85, 272-277. doi:10.1016/j.neuropsychologia.2016.03.033
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  Numerosity and duration are thought to share common magnitude-based mechanisms in brain regions including the right parietal and frontal cortices like the supplementary motor area, SMA. Numerosity and duration are, however, also different in several intrinsic features. For instance, in a quantification context, numerosity is known for being more automatically accessed than temporal events, and durations are by definition sequential whereas numerosity can be both sequential and simultaneous. Moreover, numerosity and duration processing diverge in terms of their neuronal correlates. Whether these observed neuronal specificities can be accounted for by differences in automaticity or presentation-mode is however not clear. To address this issue, we used brain stimulation (transcranial random noise stimulation, tRNS) to the right parietal cortex or the SMA combined with experimental stimuli differing in their level of automaticity (numerosity and duration) and presentation mode (sequential or simultaneous). Compared to a non-stimulation group, performance changed in duration but not in numerosity categorisation following right parietal but not SMA stimulation. These results indicate that the right parietal cortex is critical for duration processing, and suggest that tRNS has a stronger effect on less automatic processes such as duration.
• Javadi, A., Beyko, A., Walsh, V., & Kanai, R. (2015). Transcranial Direct Current Stimulation of the Motor Cortex Biases Action Choice in a Perceptual Decision Task. Journal of Cognitive Neuroscience, 27, 2174-2185. doi:10.1162/jocn_a_00848
  Abstract View in KAR
  One of the multiple interacting systems involved in the selection and execution of voluntary actions is the primary motor cortex (PMC). We aimed to investigate whether the transcranial direct current stimulation (tDCS) of this area can modulate hand choice. A perceptual decision-making task was administered. Participants were asked to classify rectangles with different height-to-width ratios into horizontal and vertical rectangles using their right and left index fingers while their PMC was stimulated either bilaterally or unilaterally. Two experiments were conducted with different stimulation conditions: the first experiment (n = 12) had only one stimulation condition (bilateral stimulation), and the second experiment (n = 45) had three stimulation conditions (bilateral, anodal unilateral, and cathodal unilateral stimulations). The second experiment was designed to confirm the results of the first experiment and to further investigate the effects of anodal and cathodal stimulations alone in the observed effects. Each participant took part in two sessions. The laterality of stimulation was reversed over the two sessions. Our results showed that anodal stimulation of the PMC biases participants' responses toward using the contralateral hand whereas cathodal stimulation biases responses toward the ipsilateral hand. Brain stimulation also modulated the RT of the left hand in all stimulation conditions: Responses were faster when the response bias was in favor of the left hand and slower when the response bias was against it. We propose two possible explanations for these findings: the perceptual bias account (bottom-up effects of stimulation on perception) and the motor-choice bias account (top-down modulation of the decision-making system by facilitation of response in one hand over the other). We conclude that motor responses and the choice of hand can be modulated using tDCS.
• Javadi, A., Tolat, A., & Spiers, H. (2015). Sleep enhances a spatially mediated generalization of learned values. Learning & Memory (Cold Spring Harbor, N.Y.), 22, 532-536. doi:10.1101/lm.038828.115
  Abstract View in KAR
  Sleep is thought to play an important role in memory consolidation. Here we tested whether sleep alters the subjective value associated with objects located in spatial clusters that were navigated to in a large-scale virtual town. We found that sleep enhances a generalization of the value of high-value objects to the value of locally clustered objects, resulting in an impaired memory for the value of high-valued objects. Our results are consistent with (a) spatial context helping to bind items together in long-term memory and serve as a basis for generalizing across memories and (b) sleep mediating memory effects on salient/reward-related items.
• Javadi, A. (2015). Modulation of the pre-supplementary motor area reduces the sense of agency (Commentary on Cavazzana et al.). The European Journal of Neuroscience, 42, 1887-1888. doi:10.1111/ejn.12954
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• Schaal, N., Javadi, A., Halpern, A., Pollok, B., & Banissy, M. (2015). Right parietal cortex mediates recognition memory for melodies. The European Journal of Neuroscience, 42, 1660-1666. doi:10.1111/ejn.12943
  Abstract View in KAR
  Functional brain imaging studies have highlighted the significance of right-lateralized temporal, frontal and parietal brain areas for memory for melodies. The present study investigated the involvement of bilateral posterior parietal cortices (PPCs) for the recognition memory of melodies using transcranial direct current stimulation (tDCS). Participants performed a recognition task before and after tDCS. The task included an encoding phase (12 melodies), a retention period, as well as a recognition phase (24 melodies). Experiment 1 revealed that anodal tDCS over the right PPC led to a deterioration of overall memory performance compared with sham. Experiment 2 confirmed the results of Experiment 1 and further showed that anodal tDCS over the left PPC did not show a modulatory effect on memory task performance, indicating a right lateralization for musical memory. Furthermore, both experiments revealed that the decline in memory for melodies can be traced back to an interference of anodal stimulation on the recollection process (remember judgements) rather than to familiarity judgements. Taken together, this study revealed a causal involvement of the right PPC for memory for melodies and demonstrated a key role for this brain region in the recollection process of the memory task.
• Javadi, A., & Spiers, H. (2015). Neuroscience: teleporting mind into body and space. Current Biology, 25, R448-R450. doi:10.1016/j.cub.2015.04.010
  Abstract View in KAR
  Brain imaging and a novel 'body-swop' illusion reveals distinct parietal-premotor and parietal-hippocampal networks involved in constructing a sense of body-ownership and self-location, with the posterior cingulate mediating between them.
• Javadi, A., Hakimi, Z., Barati, M., Walsh, V., & Tcheang, L. (2015). SET: a pupil detection method using sinusoidal approximation. Frontiers in Neuroengineering, 8, 4. doi:10.3389/fneng.2015.00004
  Abstract View in KAR
  Mobile eye-tracking in external environments remains challenging, despite recent advances in eye-tracking software and hardware engineering. Many current methods fail to deal with the vast range of outdoor lighting conditions and the speed at which these can change. This confines experiments to artificial environments where conditions must be tightly controlled. Additionally, the emergence of low-cost eye tracking devices calls for the development of analysis tools that enable non-technical researchers to process the output of their images. We have developed a fast and accurate method (known as "SET") that is suitable even for natural environments with uncontrolled, dynamic and even extreme lighting conditions. We compared the performance of SET with that of two open-source alternatives by processing two collections of eye images: images of natural outdoor scenes with extreme lighting variations ("Natural"); and images of less challenging indoor scenes ("CASIA-Iris-Thousand"). We show that SET excelled in outdoor conditions and was faster, without significant loss of accuracy, indoors. SET offers a low cost eye-tracking solution, delivering high performance even in challenging outdoor environments. It is offered through an open-source MATLAB toolkit as well as a dynamic-link library ("DLL"), which can be imported into many programming languages including C# and Visual Basic in Windows OS (www.eyegoeyetracker.co.uk).
• Brunec, I., Chadwick, M., Javadi, A., Guo, L., Malcolm, C., & Spiers, H. (2015). Chronologically organised structure in autobiographical memory search. Frontiers in Psychology, 6, 1-9. doi:10.3389/fpsyg.2015.00338
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• Javadi, A., Schmidt, D., & Smolka, M. (2014). Adolescents adapt more slowly than adults to varying reward contingencies. Journal of Cognitive Neuroscience, 26, 2670-81. Retrieved from http://www.mitpressjournals.org/doi/abs/10.1162/jocn{\_}a{\_}00677 http://www.ncbi.nlm.nih.gov/pubmed/24960048
  Abstract View in KAR
  It has been suggested that adolescents process rewards differently from adults, both cognitively and affectively. In an fMRI study we recorded brain BOLD activity of adolescents (age range = 14-15 years) and adults (age range = 20-39 years) to investigate the developmental changes in reward processing and decision-making. In a probabilistic reversal learning task, adolescents and adults adapted to changes in reward contingencies. We used a reinforcement learning model with an adaptive learning rate for each trial to model the adolescents' and adults' behavior. Results showed that adolescents possessed a shallower slope in the sigmoid curve governing the relation between expected value (the value of the expected feedback, +1 and -1 representing rewarding and punishing feedback, respectively) and probability of stay (selecting the same option as in the previous trial). Trial-by-trial change in expected values after being correct or wrong was significantly different between adolescents and adults. These values were closer to certainty for adults. Additionally, absolute value of model-derived prediction error for adolescents was significantly higher after a correct response but a punishing feedback. At the neural level, BOLD correlates of learning rate, expected value, and prediction error did not significantly differ between adolescents and adults. Nor did we see group differences in the prediction error-related BOLD signal for different trial types. Our results indicate that adults seem to behaviorally integrate punishing feedback better than adolescents in their estimation of the current state of the contingencies. On the basis of these results, we argue that adolescents made decisions with less certainty when compared with adults and speculate that adolescents acquired a less accurate knowledge of their current state, that is, of being correct or wrong.
• Schad, D., Jünger, E., Sebold, M., Garbusow, M., Bernhardt, N., Javadi, A., Zimmermann, U., Smolka, M., Heinz, A., Rapp, M., & Huys, Q. (2014). Processing speed enhances model-based over model-free reinforcement learning in the presence of high working memory functioning. Frontiers in Psychology, 5, 1-10. doi:10.3389/fpsyg.2014.01450
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• Javadi, A., Brunec, I., Walsh, V., Penny, W., & Spiers, H. (2014). Transcranial electrical brain stimulation modulates neuronal tuning curves in perception of numerosity and duration. Neuroimage, 102, 451-457. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/25130301
  Abstract View in KAR
  Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation method with many putative applications and reported to effectively modulate behaviour. However, its effects have yet to be considered at a computational level. To address this we modelled the tuning curves underlying the behavioural effects of stimulation in a perceptual task. Participants judged which of the two serially presented images contained more items (numerosity judgement task) or was presented longer (duration judgement task). During presentation of the second image their posterior parietal cortices (PPCs) were stimulated bilaterally with opposite polarities for 1.6s. We also examined the impact of three stimulation conditions on behaviour: anodal right-PPC and cathodal left-PPC (rA-lC), reverse order (lA-rC) and no-stimulation condition. Behavioural results showed that participants were more accurate in numerosity and duration judgement tasks when they were stimulated with lA-rC and rA-lC stimulation conditions respectively. Simultaneously, a decrease in performance on numerosity and duration judgement tasks was observed when the stimulation condition favoured the other task. Thus, our results revealed a double-dissociation of laterality and task. Importantly, we were able to model the effects of stimulation on behaviour. Our computational modelling showed that participants' superior performance was attributable to a narrower tuning curve - smaller standard deviation of detection noise. We believe that this approach may prove useful in understanding the impact of brain stimulation on other cognitive domains.
• Howard, L., Javadi, A., Yu, Y., Mill, R., Morrison, L., Knight, R., Loftus, M., Staskute, L., & Spiers, H. (2014). The hippocampus and entorhinal cortex encode the path and Euclidean distances to goals during navigation. Current Biology : CB, 24, 1331-1340. doi:10.1016/j.cub.2014.05.001
  Abstract View in KAR
  BACKGROUND
  
  Despite decades of research on spatial memory, we know surprisingly little about how the brain guides navigation to goals. While some models argue that vectors are represented for navigational guidance, other models postulate that the future path is computed. Although the hippocampal formation has been implicated in processing spatial goal information, it remains unclear whether this region processes path- or vector-related information.
  
  RESULTS
  
  We report neuroimaging data collected from subjects navigating London's Soho district; these data reveal that both the path distance and the Euclidean distance to the goal are encoded by the medial temporal lobe during navigation. While activity in the posterior hippocampus was sensitive to the distance along the path, activity in the entorhinal cortex was correlated with the Euclidean distance component of a vector to the goal. During travel periods, posterior hippocampal activity increased as the path to the goal became longer, but at decision points, activity in this region increased as the path to the goal became closer and more direct. Importantly, sensitivity to the distance was abolished in these brain areas when travel was guided by external cues.
  
  CONCLUSIONS
  
  The results indicate that the hippocampal formation contains representations of both the Euclidean distance and the path distance to goals during navigation. These findings argue that the hippocampal formation houses a flexible guidance system that changes how it represents distance to the goal depending on the fluctuating demands of navigation.
• Javadi, A., Schmidt, D., & Smolka, M. (2014). Differential representation of feedback and decision in adolescents and adults. Neuropsychologia, 56, 280-8. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/24513024 http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=3991323{\&}tool=pmcentrez{\&}rendertype=abstract
  Abstract View in KAR
  It is widely accepted that brain maturation from adolescence to adulthood contributes to substantial behavioural changes. Despite this, however, knowledge of the precise mechanisms is still sparse. We used fMRI to investigate developmental differences between healthy adolescents (age range 14-15) and adults (age range 20-39) in feedback-related decision making using a probabilistic reversal learning task. Conventionally groups are compared based on continuous values of blood oxygen level dependent (BOLD) percentage signal change. In contrast, we transformed these values into discrete states and used the pattern of these states to compare groups. We focused our analysis on anterior cingulate cortex (ACC), ventral striatum (VS) and ventromedial prefrontal cortex (vmPFC) as their functions have been shown to be critical in feedback related decision making. Discretisation of continuous BOLD values revealed differential patterns of activity as compared to conventional statistical methods. Results showed differential representation of feedback and decision in ACC and vmPFC between adolescents and adults but no difference in VS. We argue that the pattern of activity of ACC, vmPFC and VS in adolescents resulted in several drawbacks in decision making such as redundant and imprecise representation of decision and subsequently poorer performance in terms of the number of system changes (change of contingencies). This method can be effectively used to infer group differences from within-group analysis rather than studying the differences by direct between-group comparisons.
• Javadi, A., Winkler, I., & Maiche, A. (2014). Half and Half is Less than One-eighth and Seven-eighths: Asymmetry Affects Judgment of Numerosity. Procedia - Social and Behavioral Sciences, 126, 275-277. Retrieved from http://linkinghub.elsevier.com/retrieve/pii/S1877042814019569
  Abstract View in KAR
  Our visual perception is not perfect. It is prone to interference of configuration of elements and irrelevant dimensions. In this study, we aimed to investigate whether combinations of white and black dots can interfere with the perception of numerosity. Participants were asked to judge numerosity of two serially presented sets of dots, while the ratio of black and white dots in the second set changed. Our results showed that the interference follows a 4th order effect. Further studies needs to be done to investigate underlying mechanisms.
• Wiers, C., Javadi, A., & Bermpohl, F. (2013). Automatic approach bias towards smoking cues is present in smokers but not in ex-smokers. Psychopharmacology, 229, 187-97. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/23604335
  Abstract View in KAR
  RATIONALE: Drug-addicted individuals show automatic approach tendencies towards drug-related cues, i.e., an approach bias (ApB). Nevertheless, little is known about ApB in tobacco smokers and about the presence of ApB after smoking abstinence. OBJECTIVES: We investigated ApB to smoking cues in heavy tobacco smokers versus never-smokers and studied its relation to smoking characteristics and craving. Second, we compared ApBs of heavy smokers with biases of abstinent heavy smokers. METHOD: A group of current heavy smokers (n = 24), ex-smokers who were abstinent for at least 5 years (n = 20), and never-smokers (n = 20) took part in the experiment. An indirect smoking approach avoidance task was performed, in which participants were required to respond to pictures of smoking and neutral cues by pulling (approach) or pushing (avoid) on a joystick, according to the content-irrelevant format of the picture (landscape or portrait). Craving scores were examined using the Questionnaire of Smoking Urges. RESULTS: Heavy smokers showed an ApB for smoking cues compared to ex-smokers and never-smokers, which correlated positively to craving scores. There were no group differences in ApB scores for ex-smokers and never-smokers. CONCLUSION: These results suggest that ApBs for smoking cues are present in heavy smokers and decrease after long-term successful smoking cessation.
• Javadi, A., & Cheng, P. (2013). Transcranial direct current stimulation (tDCS) enhances reconsolidation of long-term memory. Brain Stimulation, 6, 668-74. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/23137702
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  A new and weak memory trace undergoes consolidation to gain resistance against interfering stimuli. When an encoded memory is recalled, it becomes labile and another round of consolidation, or reconsolidation, is required to restore its stability. Transcranial direct current stimulation (tDCS) is a non-invasive method of altering cortical excitability. The aim of this study was to examine the effects of tDCS on the reconsolidation of long-term verbal memory. Participants (n = 15) memorized words in the encoding session, then reactivated the memory of the words 3 h later using an old-new recognition task under anodal, cathodal and sham stimulation to the left dorsolateral prefrontal cortex (DLPFC). Finally, after another 5 h, they performed another round of the old-new recognition task and rated their confidence. Anodal tDCS during the second session resulted in significantly more words recognized in the third session as compared to cathodal and sham stimulation. Cathodal tDCS did not affect the recognition performance compared to sham stimulation. These results cannot be attributed to differences in response times and confidence ratings, as they were comparable in all conditions. In order to study whether the activation of a memory was crucial for the enhancing effects of anodal tDCS, a group of controls (n = 15) did not perform the recognition task in the second session but still underwent stimulation. Contrary to the main group, anodal stimulation did not enhance the memory performance for the control group. This result suggests that anodal tDCS over the left DLPFC can enhance the reconsolidation of long-term memory only when the memory has been reactivated.
• Herbst, S., Javadi, A., van der Meer, E., & Busch, N. (2013). How long depends on how fast--perceived flicker dilates subjective duration. PLoS ONE, 8, e76074. doi:10.1371/journal.pone.0076074
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  How do humans perceive the passage of time and the duration of events without a dedicated sensory system for timing? Previous studies have demonstrated that when a stimulus changes over time, its duration is subjectively dilated, indicating that duration judgments are based on the number of changes within an interval. In this study, we tested predictions derived from three different accounts describing the relation between a changing stimulus and its subjective duration as either based on (1) the objective rate of changes of the stimulus, (2) the perceived saliency of the changes, or (3) the neural energy expended in processing the stimulus. We used visual stimuli flickering at different frequencies (4-166 Hz) to study how the number of changes affects subjective duration. To this end, we assessed the subjective duration of these stimuli and measured participants' behavioral flicker fusion threshold (the highest frequency perceived as flicker), as well as their threshold for a frequency-specific neural response to the flicker using EEG. We found that only consciously perceived flicker dilated perceived duration, such that a 2 s long stimulus flickering at 4 Hz was perceived as lasting as long as a 2.7 s steady stimulus. This effect was most pronounced at the slowest flicker frequencies, at which participants reported the most consistent flicker perception. Flicker frequencies higher than the flicker fusion threshold did not affect perceived duration at all, even if they evoked a significant frequency-specific neural response. In sum, our findings indicate that time perception in the peri-second range is driven by the subjective saliency of the stimulus' temporal features rather than the objective rate of stimulus changes or the neural response to the changes.
• Javadi, A., & Aichelburg, C. (2013). Training enhances the interference of numerosity on duration judgement. PLoS ONE, 8, e54098. doi:10.1371/journal.pone.0054098
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  The interference of magnitudes in different dimensions has been demonstrated previously, but the effect of training in one dimension on judgment of another has yet to be examined. The present study aimed to investigate the effect of training in numerosity judgment on judgment of duration. 32 participants took part in two sessions, 12 days apart, and had to judge which of two successive sets of items was presented longer. Half of the participants (training group) were additionally trained in 11 sessions to judge which one of the two successive sets of items was more numerous. It was found that the participants in the training group became more prone to the interference of numerosity on judging duration after training, when compared to the control group. Thus, being trained to more easily perceive the difference in number of items in the two sets affected the perception of duration. On the 3-month follow up session, no effect was found with 20 participants (n = 10 for each group). These findings indicate that the interference of magnitudes in different dimensions can be modulated by training. We discuss that this modulatory effect might be due to neural changes in shared brain regions between interfering magnitudes and/or is mediated by higher levels of perception.
• Ditye, T., Javadi, A., Carbon, C., & Walsh, V. (2013). Sleep facilitates long-term face adaptation Sleep facilitates long-term face adaptation. Proceedings of the Royal Society B: Biological Sciences, 230, 20131698. doi:10.1098/rspb.2013.1698
  Abstract View in KAR
  Adaptation is an automatic neural mechanism supporting the optimization of visual processing on the basis of previous experiences. While the short-term effects of adaptation on behaviour and physiology have been studied exten- sively, perceptual long-term changes associated with adaptation are still poorlyunderstood.Here,we showthat the integrationofadaptation-dependent long-term shifts in neural function is facilitated by sleep. Perceptual shifts induced by adaptation to a distorted image of a famous person were larger in a group of participants who had slept (experiment 1) or merely napped for 90 min (experiment 2) during the interval between adaptation and test com- pared with controls who stayed awake. Participants’ individual rapid eye movement sleep duration predicted the size of post-sleep behavioural adap- tation effects. Our data suggest that sleep prevented decay of adaptation in a way that is qualitatively different from the effects of reduced visual interfer- ence known as ‘storage’. In the light of the well-established link between sleep andmemory consolidation, our findings link the perceptual mechanisms of sensory adaptation—which are usually not considered to play a relevant role in mnemonic processes—with learning and memory, and at the same time reveal a new function of sleep in cognition.
• Hakimi, Z., Barati, M., & Javadi, A. (2013). A Distributed Intrusion Detection System Using Cooperative Agents. Life Science Journal, 10, 253-257. doi:10.1109/ICEECS.2014.7045229
  Abstract View in KAR
  Due to the extension of attacks to computer networks in the current decade, it is crucial to secure the networks in an appropriate and effective way. One solution is to have a host that supervises the situation for all the computers in the network and makes decision regarding possible attacks. This method is not effective in recently developed networks due to their large extension and high speed. We propose a new distributed intrusion detection system that is based on cooperative agents (‘DIDCA’). Agents are grouped into subsets of agents that are in connection with a leader on top. Leaders of different subsets communicate together to share all the information throughout the network. We simulated our method using DARPA 1999 database and compared it with Snort. Our results showed that our method achieved a better performance.
• Barati, M., Hakimi, Z., & Javadi, A. (2013). A Flow Based Horizontal Scan Detection Using Genetic Algorithm Approach. Life Science Journal, 10, 331-335.
  Abstract View in KAR
  An attacker has to 'scan' susceptible points of a network before attacking. There are several methods of detection of such behavior which are mostly based on thresholding. As the performance of these methods is highly dependent on the value of threshold, it is crucial to adjust this value appropriately. This adjustment is not always trivial. In this study we proposed a new method to optimize the parameters of the system using genetic algorithms (GA) based on network flows. Subsequently we compared our method with Snort. The results showed a superior performance as measured by the sensitivity index of d'.

Conference or workshop item

• Javadi, A., Ifram, F., Champion, I., & Boccara, L. (2016). Neural correlate of memory enhancement during physical exercise. In International Conference on Memory (ICOM2016). Budapest, Hungry.
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  It has been shown that physical exercise is beneficial for cognition, however, the neuronal mechanism underpinning this process is not yet clear. Therefore, we investigated the correlation of brain activity during physical exercise with memory performance. Eighteen participants were asked to memorise a set of stimuli. Subsequently they either cycled on an exercise bike for 30 minutes while their EEG was recorded, or sat on the exercise bike and watched a documentary for 30 minutes. After a 1.5 hour retention interval, they were asked to perform an old/new recognition task. In addition to improvement in memory performance (exercise 71.11% vs. rest 66.67%, p=0.02), the theta band activity of the DLPFC was correlated with the enhanced performance (r=-0.48, p=0.04).
• Javadi, A. (2016). Withdrawal symptoms of electrical brain stimulation in a probabilistic decision making task. In 6th International Conference on Transcranial Brain Stimulation 2016. Göttingen, Germany.
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  Question – It has been shown that transcranial electrical brain stimulation (TES) can improve many aspects of cognition, including decision making and learning. However, it has not been studied whether brain is capable of adapting itself to perform at least equally well without TES, after initially learning the task under influence of TES. We used a probabilistic learning task to investigate this question.
  Methods – Participants (n = 10) took part in two groups of active (n = 5) and sham (n = 5) transcranial direct current stimulation (tDCS). Each participant attended two experimental sessions. In both sessions participants were asked to perform a probabilistic decision making task. In this task participants adapted to changes in reward contingencies. Participants were presented with two options of which one of them was designated as the better one, leading to higher possibility of rewarding than punishing feedback. Participants were asked to maximise their gain by choosing the better option. The contingencies changed over the course of the trials. Consequently participants had to adjust to the changes in the environment. In the first session, participants in the active and sham tDCS groups, received 15 minutes and 16 seconds of anodal tDCS over their left dorsolateral prefrontal cortex, respectively. For all participants sham stimulation was administered in the second session. Number of rewards in both sessions was recorded and their difference was considered for analysis.
  Results – Comparison of difference of acquired rewards between the two sessions showed that participants in the active group tend to perform worse than those in the sham group.
  Conclusions – This result shows that learning under the influence of TES leads to adaptation which induces changes that might not be efficient without TES in a later phase. In more general terms, this result indicates that learning a task under the influence of TES leads to creation of a model which might not be valid anymore without TES.
• Javadi, A., & Ifram, F. (2016). Physical exercise improves long-term memory no less than transcranial direct current stimulation. In 6th International Conference on Transcranial Brain Stimulation 2016. Göttingen, Germany.
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  Question – It has been shown that electrical brain stimulation, in particular transcranial direct current stimulation (tDCS), can improve memory performance. Physical exercise has also been shown to be able to improve different aspects of cognition. The aim of this study was to investigate which of these methods is more effective in improvement of long-term memory. Those tDCS and physical exercise protocols were chosen that have been shown to be effective in improvement of long-term memory: tDCS during memorisation (targeting encoding phase) and physical exercise after memorisation (targeting consolidation phase). We expected to see improvement following application of both methods. No prediction was made on which method is more effective.
  Methods – Participants (n = 24) took part in three experimental sessions. They were asked to memorise a set of images (‘encoding’) for a later old/new recognition task (‘recognition’). In one of the sessions participants were asked to cycle for 30 minutes on an exercise bike following encoding. In the other two sessions they received either 15 minutes (‘active’ stimulation) or 16 seconds (‘sham’ stimulation) of 1.5 mA anodal tDCS applied over the left dorsolateral prefrontal cortex (left-DLPFC). Performance of the participants in the recognition phase was recorded for analysis.
  Results – Both physical exercise and active stimulation led to significant improvement of long-term memory performance compared to sham stimulation (paired sample t-test p < 0.01 and p < 0.05, respectively). Physical exercise, however, led to stronger improvement (cohen’s d effect size physical exercise > active tDCS).
  Conclusions – These results show that only 30 minutes of physical exercise can significantly improve long-term memory performance. Furthermore, this duration of physical exercise during consolidation was more effective than 15 minutes of tDCS during encoding. Considering that tentatively physical exercise lead to less adverse side effects as compared to electrical brain stimulation, physical exercise can be considered potentially a more effective method of cognitive enhancement, in particular in healthy participants.
• Javadi, A., & Assassi, Z. (2016). Investigating the role of parietal and prefrontal cortices in spatial working memory using tDCS. In 6th International Conference on Transcranial Brain Stimulation 2016. Göttingen, Germany.
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  Question – Working memory involves the temporary storage, processing, and manipulation of information. Previous studies have suggested that the posterior parietal cortex (PPC), and dorsolateral prefrontal cortex (DLPFC) are involved in spatial working memory, and spatial and object working memory, respectively. The aim of the present pilot study was to investigate whether modulation of the activity of the left PPC and the left DLPFC changes spatial, and object working memory performance. Transcranial direct current stimulation (tDCS) was used for this purpose. Based on past literature we expected to see improvement of both spatial and object working memory after tDCS of the left DLPFC as well as improvement of spatial working memory after tDCS of the left PPC.
  Methods – Participants (n = 12) attended three experimental sessions with different stimulation conditions. Fifteen minutes of 1.5 mA anodal tDCS was applied over either the left PPC, left DLPFC or occipital lobe as control condition, beginning five minutes before the task. Participants were required to perform a 2-back spatial and object working memory task; i.e., they were required to pay attention to both objects and their location. Abstract objects were presented on either of an eight locations placed on a virtual circle around the centre of the screen. In addition to a 2-back object working memory task, occasionally participants were asked to indicate the location of the object they saw two screens previously. Performance in three different conditions was measured for analysis: (1) 2-back working memory task for the cases in which both cue and target objects were presented in the same location (OWM-same), (2) in different location (OWM-different) and (3) spatial working memory (SWM).
  Results – Preliminary results showed that performance in the OWM-different condition was lower than OWM-same. There, however, was no significant different between different stimulation conditions.
  Conclusions – These results suggest that anodal tDCS to the left PPC and left DLPFC might not be able to modulate spatial, and object working memory performance. Further studies are needed to investigate the differential role(s) of the DLPFC and PPC in spatial and working memory.
• Javadi, A., Ifram, F., & Boccara, L. (2016). Theta band activity during physical exercise correlates with memory improvement. In Experimental Psychology Society Meeting. Durham, UK.
  Abstract View in KAR
  It has been shown that physical exercise is beneficial for cognition, however, the neuronal mechanism underpinning this process is not yet clear. In a study examining the immediate effects of physical exercise on long-term memory, we aimed to investigate the correlation of brain activity during physical exercise with memory performance. Healthy adult participants (n = 18) were asked to memorise a set of 80 stimuli. Subsequently, they were asked either to cycle on an exercise bike for ~30 minutes (2 minutes of warm-up + 6 blocks of 4-minute cycling 1-minute rest) while their EEG was recorded, or to sit on the exercise bike comfortably and watch a documentary for 30 minutes. After a 1.5 hour retention interval period, they were shown 160 stimuli (80 old and 80 new) and were asked to perform an old/new recognition task. The behavioural data showed an improvement in memory performance (exercise 71.11% vs. rest 66.67%, p=0.02). More importantly, the theta band oscillatory activity of an electrode over the dorsolateral prefrontal cortex (DLPFC) was negatively correlated with the enhanced performance (r=-0.48, p=0.04). Results highlight the modulatory effect of physical exercise on neuronal activity and a functional role of theta activity in memory enhancement during consolidation.
• Javadi, A., Beyko, A., & Walsh, V. (2015). Event-related electrical stimulation of motor cortices enhances performance in a hand-tapping task. In the 1st International Brain Stimulation Conference (Vol. 8, p. 389). Singapore: Elsevier Inc. doi:10.1016/j.brs.2015.01.244
  Abstract View in KAR
  Introduction: Transcranial electrical stimulation (TES) for about 20 min, has resulted in the majority of observed effects in related TES research. While this duration of stimulation has been recognised as safe for human use, some researchers incorporated an alternative method of administration incorpo- rating less than 3 minutes of stimulation, to investigate whether it still produces significant results. It has been shown that short duration stimulation significantly improved verbal memory performance on a word recognition task, in which less than 2 s of anodal stimulation administered per trial was sufficient. Methods: This study sought to investigate whether stimulation locked to Events (‘Event-related’ stimulation) was effective for the improvement of other types of memory, such as procedural motor memory. To test this, response times (RTs) for an implicit sequence- learning hand-tapping task using anodal tDCS over the primary motor cortex (PMC) were compared across Event-related, Constant, and Sham conditions (n ¼ 15 per condition). Participants were asked to tap the buttons on a response box with six buttons as soon as a cue light lit up in the button. Results: Differences in the slopes of RTs over blocks of trials were fitted with an exponential decay function, which revealed a sig- nificant difference in the exponential decay variable, comparing Event-related and Sham conditions (p ¼ 0.042). Discussions: These results show that Event-related stimulation was effective for the improvement of procedural motor memory as compared to Sham. The results of this is another evidence of effectiveness of short duration of stimulation and highlight the importance of synchronisation of stimulation with the ongoing task.
• Javadi, A., Emo, B., Howard, L., & Spiers, H. (2015). The human hippocampus represents the topological structure of the environment during navigation. In the British Neuroscience Association (Vol. 23, p. 374). Edinburgh.
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• Javadi, A., Glen, J., Halkiopoulos, S., Schulz, M., & Spiers, H. (2015). Transcranial alternating current stimulation can improve declarative memory. In the British Neuroscience Association (Vol. 23, p. 702). Edinburgh.
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• Schaal, N., Javadi, A., Halpern, A., Pollok, B., & Banissy, M. (2014). Right parietal cortex mediates memory for melodies: A tDCS study. In the Annual Meeting of the Psychonomic Society. California, USA.
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• Emo, B., Pinelo Silva, J., Javadi, A., Howard, L., & Spiers, H. (2014). How spatial properties of a city street network influence brain activity during navigation. In the Spatial Cognition 2014. Bremen, Germany.
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• Mavros, P., Javadi, A., Howard, L., & Spiers, H. (2014). A mobile application to record synchronised behavioural and EEG data during real-world wayfinding. In the Spatial Cognition 2014. Bremen, Germany.
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• Tolat, A., Javadi, A., & Spiers, H. (2014). Sleep suppresses the memory consolidation of lower-valued items in competition with higher-valued items. In the 9th FENS forum of neuroscience 2014. Milan, Italy.
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• Javadi, A., Winkler, I., & Maiche, A. (2014). Half and half is less than one-eighth and seven-eighth: asymmetry affects judgement of numerosity. In International Conference on Timing and Time Perception (Vol. 126, pp. 275-277). Corfu, Greece: Elsevier B.V. doi:10.1016/j.sbspro.2014.02.408
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• Stewart, L., Brewin, C., Muggleton, N., Javadi, A., & Tcheang, L. (2013). Hypervigilance without Threat: Eye-Movements Reveal Vigilant Behaviours. In The International Society for Traumatic Stress Studies 29th Annual Meeting. Philadelphia, USA. Retrieved from https://www.istss.org/meetings-events/meeting-archives/2013-annual-meeting-archives.aspx
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• Dormal, V., Javadi, A., Cappelletti, M., Pesenti, M., & Walsh, V. (2013). Enhancing time discrimination with right parietal transcranial Random Noise Stimulation (tRNS). In the 1st tDCS forum. Louvain-la-Neuve, Belgium.
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