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Christopher Kennard
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Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2015) 27 (2): 377–386.
Published: 01 February 2015
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When briefly presented with pairs of words, skilled readers can sometimes report words with migrated letters (e.g., they report hunt when presented with the words hint and hurt ). This and other letter migration phenomena have been often used to investigate factors that influence reading such as letter position coding. However, the neural basis of letter migration is poorly understood. Previous evidence has implicated the right posterior parietal cortex (PPC) in processing visuospatial attributes and lexical properties during word reading. The aim of this study was to assess this putative role by combining an inhibitory TMS protocol with a letter migration paradigm, which was designed to examine the contributions of visuospatial attributes and lexical factors. Temporary interference with the right PPC led to three specific effects on letter migration. First, the number of letter migrations was significantly increased only in the group with active stimulation (vs. a sham stimulation group or a control group without stimulation), and there was no significant effect on other error types. Second, this effect occurred only when letter migration could result in a meaningful word (migration vs. control context). Third, the effect of active stimulation on the number of letter migrations was lateralized to target words presented on the left. Our study thus demonstrates that the right PPC plays a specific and causal role in the phenomenon of letter migration. The nature of this role cannot be explained solely in terms of visuospatial attention, rather it involves an interplay between visuospatial attentional and word reading-specific factors.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2005) 17 (2): 340–354.
Published: 01 February 2005
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Right-hemisphere patients with left neglect often demonstrate abnormal visual search, re-examining stimuli to the right while ignoring those to the left. But re-fixations alone do not reveal if patients misjudge whether they have searched a location before. Here, we not only tracked the eye movements of 16 neglect patients during search, but also asked them to click a response button only when they judged they were fixating a target for the very first time. “Re-clicking” on previously found targets would indicate that patients erroneously respond to these as new discoveries. Lesions were mapped with high-resolution MRI. Neglect patients with damage involving the right intraparietal sulcus or right inferior frontal lobe “re-clicked” on previously found targets on the right at a pathological rate, whereas those with medial occipito-temporal lesions did not. For the intraparietal sulcus patients, the probability of erroneous re-clicks on an old target increased with time since first discovering it; whereas for frontal patients it was independent of search time, suggesting different underlying mechanisms in these two types of patient. Re-click deficits correlated with degree of leftward neglect, mainly due to both being severe in intraparietal cases. These results demonstrate that misjudging previously searched locations for new ones can contribute to pathological search in neglect, with potentially different mechanisms being involved in intraparietal versus inferior frontal patients. When combined with a spatial bias to the right, such deficits might explain why many neglect patients often re-examine rightward locations, at the expense of items to their left.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2004) 16 (2): 318–330.
Published: 01 March 2004
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Active vision is a dynamic process involving the flexible coordination of different gaze strategies to achieve behavioral goals. Although many complex behaviors rely on an ability to efficiently switch between gaze-control strategies, few studies to date have examined mechanisms of task level oculomotor control in detail. Here, we report five experiments in which subjects alternated between conflicting stimulus-saccade mappings within a block of trials. The first experiment showed that there is no performance cost associated with switching between pro and anti saccades. However, follow-up experiments demonstrate that whenever subjects alternate between arbitrary stimulus-saccade mappings, latency costs are apparent on the first trial after a task change. More detailed analysis of switch costs showed that latencies were particularly elevated for saccades directed toward the same location that had been the target for a saccade on the preceeding trial. This saccade “inhibition of return” effect was most marked when unexpected error feedbacks cued task switches, suggesting that saccade selection processes are modulated by reward. We conclude that there are two systems for saccade control that differ in their characteristics following a task switch. The “reflexive” control system can be enabled/disabled in advance of saccade execution without incurring any performance cost. Switch costs are only observed when two or more arbitrary stimulus-saccade mappings have to be coordinated by a “symbolic” control system.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (2000) 12 (5): 894–907.
Published: 01 September 2000
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In this paper, we describe a novel approach to the study of problem solving involving the detailed analysis of natural scanning eye movements during the “one-touch” Tower-of-London (TOL) task. We showed subjects a series of pictures depicting two arrangements of colored balls in pockets within the uper and lower halves of a computer display. The task was to plan (but not to execute)the shortest movement sequence required to rearrange the balls in one half of the display (the workspace)to match the arrangement in the opposite half (the Goalspace)and indicate the minimum number of moves required for problem solution. We report that subjects are more likely to look towards the Goalspace in the initial period after picture presentation, but bias gaze towards the Workspace during the middle of trials. Towards the end of a trial, subjects are once again more likely to fixate the Goalspace. This pattern is found regardless of whether the subjects solve problems by rearranging the balls in the lower or uper visual fields, demonstrating that this strategy correlates with discreate phases in problem solving. A second experiment showed that efficient planners direct their gaze selectively towards the problem critical balls in the workspace. In contrast, Individuals who make errors spend more time looking at irrelevant items and are strongly influenced by the movement strategy needed to solve the preceding problem. We conclude that efficient solution of the TOL requires the capacity to generate and flexibly shift between control sets, including those underlying ocular scanning. The role of working memory and the prefrontal cerebral cortex in the task are discussed.