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Parallel and Serial Neural Mechanisms for Visual Search in Macaque Area V4
Narcisse P. Bichot,1*Andrew F. Rossi,2,3Robert Desimone1,4
To find a target object in a crowded scene, a face in a crowdfor example, the visual system might turn the neural representationof each object on and off in a serial fashion, testing eachrepresentation against a template of the target item. Alternatively,it might allow the processing of all objects in parallel butbias activity in favor of those neurons that represent criticalfeatures of the target, until the target emerges from the background.To test these possibilities, we recorded neurons in area V4of monkeys freely scanning a complex array to find a targetdefined by color, shape, or both. Throughout the period of searching,neurons gave enhanced responses and synchronized their activityin the gamma range whenever a preferred stimulus in their receptivefield matched a feature of the target, as predicted by parallelmodels. Neurons also gave enhanced responses to candidate targetsthat were selected for saccades, or foveation, reflecting aserial component of visual search. Thus, serial and parallelmechanisms of response enhancement and neural synchrony worktogether to identify objects in a scene. To find a target objectin a crowded scene, a face in a crowd for example, the visualsystem might turn the neural representation of each object onand off in a serial fashion, testing each representation againsta template of the target item. Alternatively, it might allowthe processing of all objects in parallel but bias activityin favor of those neurons that represent critical features ofthe target, until the target emerges from the background. Totest these possibilities, we recorded neurons in area V4 ofmonkeys freely scanning a complex array to find a target definedby color, shape, or both. Throughout the period of searching,neurons gave enhanced responses and synchronized their activityin the gamma range whenever a preferred stimulus in their receptivefield matched a feature of the target, as predicted by parallelmodels. Neurons also gave enhanced responses to candidate targetsthat were selected for saccades, or foveation, reflecting aserial component of visual search. Thus, serial and parallelmechanisms of response enhancement and neural synchrony worktogether to identify objects in a scene.
1 Laboratory of Neuropsychology, National Institute of Mental Health (NIMH), National Institutes of Health, Bethesda, MD 20892, USA. 2 Laboratory of Brain and Cognition, National Institute of Mental Health (NIMH), National Institutes of Health, Bethesda, MD 20892, USA. 3 Department of Psychology, Vanderbilt University, Nashville, TN 37203, USA. 4 McGovern Institute for Brain Research at the Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
* To whom correspondence should be addressed. E-mail: bichotn{at}mail.nih.gov
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