In complex sensory environments human observers can selectively process relevant events by directing their gaze and/or attention towards particular sources of stimulation. Neurophysiological and neuropsychological evidence orienting. The research described in this proposal has two general aims: 1) to further characterize the functional contributions of different anatomical components of the overt orienting system with an emphasis on the reflexive and voluntary aspects of oculomotor control; 2) to investigate the hypothesis that the visual pathways preferentially tuned to low spatial frequency information are most effective in controlling the direction of attention and gaze.
The first aim will be pursued in a series of studies that evaluates the effects of frontal and parietal damage on a) basic visual and auditory thresholds using a signal detection analysis; b) the ability of these patients to execute purely voluntary directed-manual and oculomotor responses in the absence of stimulus guidance; c) the ability of these patients to execute stimulus-guided manual and oculomotor responses. This work is intended to elucidate the role of cortical structures in overt orienting control by assessing the impact of different lesions on the sensory, attentional and motoric components of the orienting process. Our analysis will allow for more accurate specification of the component deficits underlying hemineglect and associated disturbances in oculomotor control that frequently result from unilateral parietal or frontal lesions. Reflexive aspects of orienting will be investigated in a series of experiments examining the production of extremely short latency saccades (i.e. express saccades). We will explore the hypothesis that express saccades are 1) primarily mediated by the superior colliculus, 2) released by lesions involving the frontal eye fields and 3) enabled by cortical processes which operate independently in the separated hemispheres of commissurotomy patients. Finally the second aim will be pursued in a series of experiments that tests the hypothesis that low spatial frequencies preferentially control overt and covert orienting, using both normal observers and patients with focal cortical damage. This issue will be evaluated by varying the spatial frequency content of stimuli used in a variety of detection, visual search and orienting tasks. We will examine whether 1) saccade latencies depend on the spatial frequency of the targets, 2) spatial precuing differentially influences detection latencies for high and low frequency targets, and 3) the magnitude of neglect and simultaneous extinction vary with spatial frequency. These experiments should help clarify the relative contributions of the magnocellular and parvocellular pathways in mediating oculomotor performance and the deployment of attention.
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