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Object Perception: Psychophysics and Physiology in Monkeys


Logothetis,  NK
Department Physiology of Cognitive Processes, Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Logothetis, N. (1998). Object Perception: Psychophysics and Physiology in Monkeys. Poster presented at Environmental Structure, Statistical Learning and Visual Perception: 1998 CVS Symposium, Rochester, NY, USA.

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The notion of the temporal lobe being involved in object recognition—as it emerged from clinical and lesion studies—has received strong support from previous electrophysiological experiments in monkeys. The results of these experiments showed that neurons in the inferior temporal cortex (IT) respond to a variety of complex two-dimensional patterns, including figures of animate objects, such as faces, hands, and body parts. To better understand the role of this area in object recognition, we set out to determine whether the configurational selectivity found for IT neurons is specific for faces or body parts, or whether it can be generated for any novel object as a result of extensive training. Monkeys were trained to become "experts" at identifying exemplars of novel, computer-generated object classes. Critically, these objects had never been experienced by the monkeys, nor did they possess any inherent biological relevance. Nonetheless, after training, the animals learned to discriminate individual objects from a set of highly similar distractors, a task not unlike that of identifying a specific face or a particular bird species. Because all of the objects used in testing were composed of the same basic parts, good performance in this task had most likely to rely upon using holistic configurational information, and upon the detection of subtle shape differences. Physiological recordings from individual neurons in IT revealed a subpopulation of cells that were activated selectively by views of these previously unfamiliar objects. Many neurons fired selectively for a small set of views of spheroidal or wire objects that the monkey had learned to recognize from all viewpoints. The cells were most active when the target was presented from one particular view, and their activity declined as the object was rotated in depth. Remarkably, these cells could not be consistently activated by any other tested object, including numerous visually similar distractors and views of the target more than about 45 deg away from the preferred view. Attempts to simplify the objects lead, in most cases, to a significant reduction of the neurons' responses, a finding suggesting that some neurons in IT show the type of specificity to complex configurations, which was previously described for faces or other animate objects. Testing IT neurons with stimuli that can be perceived more than one way showed that these cells discharge for their effective stimulus only when this stimulus is perceived. Phenomenal suppression of the preferred pattern caused almost invariably a profound suppression of the cells' activity. IT thus seems to be very closely related not only to shape representation but also to the conscious perception of a visual object.