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Timothy L Hodgson

Publications and source records attributed to Timothy L Hodgson.

7 recordsLinked to original sources
PubMed2007

The role of the lateral prefrontal cortex and anterior cingulate in stimulus-response association reversals.

Many complex tasks require us to flexibly switch between behavioral rules, associations, and strategies. The prefrontal cerebral cortex is thought to be critical to the performance of such behaviors, although the relative contribution of different components of this structure and associated subcortical regions are not fully understood. We used functional magnetic resonance imaging to measure brain activity during a simple task which required repeated reversals of a rule linking a colored cue and a left/right motor response. Each trial comprised three discrete events separated by variable delay periods. A colored cue instructed which response was to be executed, followed by a go signal which told the subject to execute the response and a feedback instruction which indicated whether to "hold" or "flip" the rule linking the colored cue and response. The design allowed us to determine which brain regions were recruited by the specific demands of preparing a rule contingent motor response, executing such a response, evaluating the significance of the feedback, and reconfiguring stimulus-response (SR) associations. The results indicate that an increase in neural activity occurs within the anterior cingulate gyrus under conditions in which SR associations are labile. In contrast, lateral frontal regions are activated by unlikely/unexpected perceptual events regardless of their significance for behavior. A network of subcortical structures, including the mediodorsal nucleus of the thalamus and striatum were the only regions showing activity that was exclusively correlated with the neurocognitive demands of reversing SR associations. We conclude that lateral frontal regions act to evaluate the behavioral significance of perceptual events, whereas medial frontal-thalamic circuits are involved in monitoring and reconfiguring SR associations when necessary.

Adolescent↗
PubMed2006

Role of the human supplementary eye field in the control of saccadic eye movements.

The precise function of the supplementary eye field (SEF) is poorly understood. Although electrophysiological and functional imaging studies are important for demonstrating when SEF neurones are active, lesion studies are critical to establish the functions for which the SEF is essential. Here we report a series of investigations performed on an extremely rare individual with a highly focal lesion of the medial frontal cortex. High-resolution structural imaging demonstrated that his lesion was confined to the region of the left paracentral sulcus, the anatomical locus of the SEF. Behavioural testing revealed that the patient was significantly impaired when required to switch between anti- and pro-saccades, when there were conflicting rules governing stimulus-response mappings for saccades. Similarly, the results of an arbitrary stimulus-response associative learning task demonstrated that he was impaired when required to select the appropriate saccade from conflicting eye movement responses, but not for limb movements on an analogous manual task. When making memory-guided saccadic sequences, the patient demonstrated hypometria, like patients with Parkinson's disease, but had no significant difficulties in reproducing the order of saccades correctly on a task that emphasized accuracy with a wide temporal segregation between responses. These findings are consistent with the hypothesis that the SEF plays a key role in implementing control when there is conflict between several, ongoing competing saccadic responses, but not when eye movements need to be made accurately in sequence.

Association Learning↗
PubMed2003

Memory-motor transformations are impaired in Parkinson's disease.

Parkinson's disease patients are known to suffer loss of dopaminergic input to the rostral caudate nucleus. Recent functional magnetic resonance imaging (fMRI) studies have implicated this structure in the transformation of spatial information in memory to guide action, suggesting that memory to motor transformations may be selectively impaired in Parkinson's disease. In order to investigate this possibility we tested a group of Parkinson's disease patients (PDs) using a memory-guided pointing task. Of interest was whether patients showed reduced accuracy in the task as a function of memory load. Twelve PD patients and 13 elderly controls were asked to recall single or four step target sequences with 2 time delays (500 and 3,500 ms). In all memory-guided conditions PD patients showed increased variability in memory-guided movement end-points. This effect was not affected by delay, number of items, or the sequence familiarity. The results are consistent with increased variability in memory-motor transformations in early PD, due to dopamine depletion within the rostral caudate nucleus.

Aged↗
PubMed2003

Differential cortical activation during voluntary and reflexive saccades in man.

A saccade involves both a step in eye position and an obligatory shift in spatial attention. The traditional division of saccades into two types, the "reflexive" saccade made in response to an exogenous stimulus change in the visual periphery and the "voluntary" saccade based on an endogenous judgement to move gaze, is supported by lines of evidence which include the longer onset latency of the latter and the differential effects of lesions in humans and primates on each. It has been supposed that differences between the two types of saccade derive from differences in how the spatial attention shifts involved in each are processed. However, while functional imaging studies have affirmed the close link between saccades and attentional shifts by showing they activate overlapping cortical networks, attempts to contrast exogenous with endogenous ("covert") attentional shifts directly have not revealed separate patterns of cortical activation. We took the "overt" approach, contrasting whole reflexive and voluntary saccades using event-related fMRI. This demonstrated that, relative to reflexive saccades, voluntary saccades produced greater activation within the frontal eye fields and the saccade-related area of the intraparietal sulci. The reverse contrast showed reflexive saccades to be associated with relative activation of the angular gyrus of the inferior parietal lobule, strongest in the right hemisphere. The frequent involvement of the right inferior parietal lobule in lesions causing hemispatial neglect has long implicated this parietal region in an important, though as yet uncertain, role in the awareness and exploration of space. This is the first study to demonstrate preferential activation of an area in its posterior part, the right angular gyrus, during production of exogenously triggered rather than endogenously generated saccades, a finding which we propose is consistent with an important role for the angular gyrus in exogenous saccadic orienting.

Adult↗
PubMed2002

The location marker effect. Saccadic latency increases with target eccentricity.

Previous studies have shown that the retinal eccentricity of target stimuli has surprisingly little effect on the latency of visually driven saccades. But up until now researchers have addressed this issue by presenting saccadic targets in an unstructured visual field. This contrasts with everyday vision in which eye movements are initiated to stimuli within a cluttered environment. The present experiment compared latencies for target onsets in an empty visual field with a condition in which continuously illuminated location markers "tagged" the possible target locations. Previous reports of no effect of eccentricity on latencies in an unstructured field were replicated. However, a significant effect of eccentricity was found when location markers were used. Interestingly this did not reflect a lengthening of latencies as would be predicted by a reduction in target discriminability. Instead, latencies were relatively facilitated to near-visual onsets in the location marker condition. It is concluded that under more natural viewing conditions the latency of saccades is likely to be modulated by the eccentricity of target stimuli. This effect can be explained by competitive attentional interactions in saccade target selection processes.

Adult↗
PubMed2002

Visual object memory and memory-guided saccades rely on shared mental representations.

We investigated the organisation of working memory processes by examining how a secondary memory task interferes with the accuracy of memory-guided saccades. A target was flashed at a random location, followed by a Kanji character. Subjects had to remember the location of the target and the Kanji character, and then they had to make a saccade towards the remembered target location. A second Kanji character was displayed and the subject had to decide if it was same or different. The performance of seven non-Kanji reader's were compared with six fluent Kanji readers in the task. Memorisation of Kanji characters was found to interfere with the accuracy of memory-guided saccades made by non-Kanji, but not by Kanji readers. These findings directly contradict accounts of working memory function which propose that spatial and visual object memory are functionally discrete.

Adult↗