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Science 22 April 2005:
Vol. 308. no. 5721, pp. 529 - 534
DOI: 10.1126/science.1109676
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Research Articles

Parallel and Serial Neural Mechanisms for Visual Search in Macaque Area V4

Narcisse P. Bichot,1* Andrew F. Rossi,2,3 Robert Desimone1,4

To find a target object in a crowded scene, a face in a crowd for example, the visual system might turn the neural representation of each object on and off in a serial fashion, testing each representation against a template of the target item. Alternatively, it might allow the processing of all objects in parallel but bias activity in favor of those neurons that represent critical features of the target, until the target emerges from the background. To test these possibilities, we recorded neurons in area V4 of monkeys freely scanning a complex array to find a target defined by color, shape, or both. Throughout the period of searching, neurons gave enhanced responses and synchronized their activity in the gamma range whenever a preferred stimulus in their receptive field matched a feature of the target, as predicted by parallel models. Neurons also gave enhanced responses to candidate targets that were selected for saccades, or foveation, reflecting a serial component of visual search. Thus, serial and parallel mechanisms of response enhancement and neural synchrony work together to identify objects in a scene. To find a target object in a crowded scene, a face in a crowd for example, the visual system might turn the neural representation of each object on and off in a serial fashion, testing each representation against a template of the target item. Alternatively, it might allow the processing of all objects in parallel but bias activity in favor of those neurons that represent critical features of the target, until the target emerges from the background. To test these possibilities, we recorded neurons in area V4 of monkeys freely scanning a complex array to find a target defined by color, shape, or both. Throughout the period of searching, neurons gave enhanced responses and synchronized their activity in the gamma range whenever a preferred stimulus in their receptive field matched a feature of the target, as predicted by parallel models. Neurons also gave enhanced responses to candidate targets that were selected for saccades, or foveation, reflecting a serial component of visual search. Thus, serial and parallel mechanisms of response enhancement and neural synchrony work together 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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Science. ISSN 0036-8075 (print), 1095-9203 (online)