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Biomedical subjects

Todd C Handy

Publications and source records attributed to Todd C Handy.

9 recordsLinked to original sources
PubMed2006

Motor experience with graspable objects reduces their implicit analysis in visual- and motor-related cortex.

Motor-related regions of parietal and prefrontal cortices have been shown to selectively activate when observers passively view objects that afford manual grasping. Yet, it remains unknown whether these cortical responses depend on prior motor-related experience with the object being observed. To address this question, we asked participants to undergo fMRI scanning while viewing exemplars of two different categories of graspable objects: one associated with extensive motor experience (door knobs) and one associated with no self-reported motor experience (artificial rock climbing holds). Despite participants' lack of experience grasping climbing holds, these objects were found to generate a systematic response in several visuomotor-related regions of cortex-including left PMv and left AIP. Interestingly, however, the response to door knobs did not include activity in any motor-related regions, being limited instead to a comparatively small bilateral area of lateral occipital cortex, relative to the more spatially extensive response in occipital and temporal cortex that was observed for climbing holds. This result suggested that object-specific responses in both visual- and motor-related cortex may in fact negatively correlate with object-specific motor experience. To test this possibility, we repeated the experiment using participants having extensive self-reported experience grasping climbing holds (i.e., veteran indoor rock climbers). Consistent with our hypothesis, both climbing holds and door knobs generated activity limited to lateral occipital cortex. Taken together, these data support the proposal that repeated real-world motor experience with an object category may lead to reduced implicit analysis in both motor- and visual-related regions of cortex.

Adolescent↗
PubMed2005

Placing a tool in the spotlight: spatial attention modulates visuomotor responses in cortex.

Visual spatial attention has long been associated with facilitatory effects on visual perception. Here, we report that spatial attention can also modulate implicit visuomotor processing in dorsal regions of human cortex. Participants underwent fMRI scanning while performing a voluntary attentional orienting task that varied the category of a task-irrelevant object in the attended location (tool vs. non-tool). Data were then analyzed as a function of the attended location (left vs. right visual field) and the object category in that location. We found that the fMRI BOLD response in two visuomotor-related regions--the supplementary motor area (SMA) and the left inferior parietal lobule (IPL)--showed an interaction between the location of attention and the location of the tool in the bilateral display. Further, these responses were statistically distinct from those regions in dorsal cortex showing activity modulated only by the tool location or only by the attended location. While the effects of attending non-foveally within the visual field have been well documented in relation to visual perception, our findings support the proposal that voluntary visuospatial attention may also have consequences for the implicit planning of object-directed actions.

Adult↗
PubMed2005

Attention and sensory gain control: a peripheral visual process?

Attention-related sensory gain control in human extrastriate cortex is believed to improve the acuity of visual perception. Yet given wide variance in the spatial resolution of vision across the retina, it remains unclear whether sensory gain operates homogenously between foveal and nonfoveal retinotopic locations. To address this issue, we used event-related potentials (ERPs) in a variant of the canonical spatial attention task. Participants were cued to expect targets at either fixation (foveal targets) or at a location several degrees above fixation (parafoveal targets). At both target locations, manual reaction times were shorter for cued relative to uncued targets, indicating that attention was consistently oriented to the cued location. Nevertheless, attention-related increases in sensory-evoked cortical activity were only observed at the parafoveal target location, as measured by the amplitude of the lateral occipital P1 ERP component. A second experiment replicated this data pattern using targets with lower stimulus contrast, indicating that the absence of a P1 effect for foveal targets could not be attributed to a saturated P1 response under higher-contrast stimulus conditions. When considered in light of retinogeniculate projections to cortex showing systematic changes in their physiological organization beginning within a degree of visual angle of the fovea, our findings support the proposal that the strategic functions of visual attention may vary with the retinotopic location involved.

Adolescent↗
PubMed2004

From facial cue to dinner for two: the neural substrates of personal choice.

The current experiment examined the neural substrates of response selection, comparing conditions that required participants to make criterion-free selections from sets of same-sex faces (i.e., inconsequential decision) to choosing a dinner date from opposite-sex faces (i.e., consequential decision). In each of these tasks, either a single face (i.e., no choice) or two or three faces (i.e., free choice) appeared for selection. The results revealed that regions of dorsal premotor cortex (PMd) and parietal cortex bilaterally, as well as an area along the medial surface of the superior frontal gyrus, were activated by both consequential and inconsequential decisions, thereby providing evidence for a common selection network. Consequential decisions were further indexed by activation of the insula/ventrolateral prefrontal cortex (BA 47) and the paracingulate gyrus (BA 32). The implications of these findings for current accounts of response selection and social-cognitive functioning are considered.

Adult↗
PubMed2003

Cortical and subcortical contributions to the representation of temporal information.

Converging evidence suggests that temporal representations of brief durations are derived subcortically. We tested split-brain patient JW in order to investigate whether these representations project bilaterally or unilaterally to cortex. Using visual stimuli to signal time intervals, JW was asked to compare the duration of a pair of standard stimuli that were presented bilaterally with a comparison stimulus that was presented to either the left or right visual field. Assuming the hand of response is controlled by the contralateral cerebral hemisphere, a hand by visual field interaction was predicted if the representation of stimulus duration was restricted to the cerebral hemisphere receiving the lateralized stimulus. However, we failed to observe this interaction for two different ranges of stimulus durations, both in the hundred (Experiment 2) to hundreds (Experiment 1) of milliseconds range. Instead, there was a consistent right hemisphere advantage in task performance. When the task then required a discrimination based on the physical size of the stimuli rather than their duration, an interaction between response hand and visual field was obtained (Experiment 3). Taken together, these results suggest that (1) even though the comparison stimulus was presented unilaterally, the representation of its duration was available to both cerebral hemispheres, and (2) a right hemisphere advantage in psychophysical tasks requiring the comparison of successive stimuli is observed for temporal and non-temporal judgments.

Cerebral Cortex↗
PubMed2003

Graspable objects grab attention when the potential for action is recognized.

Visually guided grasping movements require a rapid transformation of visual representations into object-specific motor programs. Here we report that graspable objects may facilitate these visuomotor transformations by automatically grabbing visual spatial attention. Human subjects viewed two task-irrelevant objects--one was a 'tool', the other a 'non-tool'--while waiting for a target to be presented in one of the two object locations. Using event-related potentials (ERPs), we found that spatial attention was systematically drawn to tools in the right and lower visual fields, the hemifields that are dominant for visuomotor processing. Using event-related fMRI, we confirmed that tools grabbed spatial attention only when they also activated dorsal regions of premotor and prefrontal cortices, regions associated with visually guided actions and their planning. Although it is widely accepted that visual sensory gain aids perception, our results suggest that it may also have consequences for object-directed actions.

Attention↗
PubMed2002

Extensive individual differences in brain activations associated with episodic retrieval are reliable over time.

The localization of brain functions using neuroimaging techniques is commonly dependent on statistical analyses of groups of subjects in order to identify sites of activation, particularly in studies of episodic memory. Exclusive reliance on group analysis may be to the detriment of understanding the true underlying cognitive nature of brain activations. In the present study, we found that the patterns of brain activity associated with episodic retrieval are very distinct for individual subjects from the patterns of brain activity at the group level. These differences go beyond the relatively small variations due to cyctoarchitectonic differences or spatial normalization. We quantify this individual variability by cross-correlating volumes of brain images. We demonstrate that individual patterns of brain activity are reliable over time despite their extensive variability. We suggest that varied but reliable individual patterns of significant brain activity may be indicative of different cognitive strategies used to produce a recognition response. We believe that individual analysis in conjunction with group analysis may be critical to fully understanding the relationship between retrieval processes and underlying brain regions.

Adult↗