When different images are presented to the two eyes, they compete for perceptual dominance, such that one image is visible while the other is suppressed. This binocular rivalry is thought to reflect competition between monocular neurons within the primary visual cortex. However, neurons whose activity correlates with perception during rivalry are found mainly in higher cortical areas, and respond to input from both eyes. Thus rivalry may involve competition between alternative perceptual interpretations at a higher level of analysis. To investigate this, we tested the effect of rapidly alternating the rival stimuli between the two eyes. Under these conditions, the perceptual alternations exhibit the same temporal dynamics as with static patterns, and a single phase of perceptual dominance can span multiple alternations of the stimuli. Thus neural representations of the two stimuli compete for visual awareness independently of the eye through which they reach the higher visual areas. This finding places binocular rivalry in the general category of multistable phenomena, such as ambiguous figures, and provides a new way to study the neural cause and resolution of perceptual ambiguities.

Visual object recognition is of fundamental importance to most animals. The diversity of tasks that any biological recognition system must solve suggests that object recognition is not a single, general purpose process. In this review, we consider evidence from the fields of psychology, neuropsychology, and neurophysiology, all of which supports the idea that there are multiple systems for recognition. Data from normal adults, infants, animals, and brain damaged patients reveal a major distinction between the classification of objects at a basic category level and the identification of individual objects from a homogeneous object class. An additional distinction between object representations used for visual perception and those used for visually guided movements provides further support for a multiplicity of visual recognition systems. Recent evidence from psychophysical and neurophysiological studies indicates that one system may represent objects by combinations of multiple views, or aspects, and another may represent objects by structural primitives and their spatial interrelationships.

& Subordinate-level object processing is regarded as a hallmark novel exemplars, and exemplars from novel species. Eventof perceptual expertise. However, the relative contribution of related potentials indicated that both basic-and subordinatesubordinate-and basic-level category experience in the acquisilevel training enhanced the early N170 component, but only tion of perceptual expertise has not been clearly delineated. In subordinate-level training amplified the later N250 component. this study, participants learned to classify wading birds and owlsThese results are consistent with models positing separate basic at either the basic (e.g., wading bird, owl) or the subordinate and subordinate learning mechanisms, and, contrary to perspec-(e.g., egret, snowy owl) level. After 6 days of training, behavioral tives attempting to explain visual expertise solely in terms of results showed that subordinate-level but not basic-level trainsubordinate-level processing, suggest that expertise enhances ing improved subordinate discrimination of trained exemplars, neural responses of both basic and subordinate processing. &

Two experiments (one using O and Q-like stimuli and the other using colored-oriented bare) investigated the oculomotor behavior accompanying parallel-serial visual search. Eye movements were recorded as participants searched for a target in 5-or 17-item displays. Results indicated the presence of parallel-serial search dichotomies and 2:1 ratios of negative to positive slopes in the number of saccades initiated during both search tasks. This saccade number measure also correlated highly with search times, accounting for up to 67% of the reaction time (RT) variability. Weak correlations between fixation durations and RTs suggest that this oculomotor measure may be related more to stimulus factors than to search processes. A third experiment compared free-eye and fixed-eye searches and found a small RT advantage when eye movements were prevented. Together these findings suggest that parallel-serial search dichotomies are reflected in oculomotor behavior.