Repetition Suppression in High and Low Order Areas of Macaque Visual Cortex

 In addition to recognizing objects, the visual system of our brains must adapt to different  conditions, such as periods of rapid change, like a busy intersection in a large city, and to periods  of stability, such as observing a painting in a museum. A key difference between these  conditions is the recency with which visual features are observed. In the case of the painting, one  might look at a particular feature for a long time and then return to it a short time later, whereas  in the case of the busy intersection things are constantly moving and changing and the same  thing is unlikely to appear at the same place again. One of the most robust ways that neurons in  the visual system respond to stimulus recency is through repetition suppression. Repetition  suppression is the phenomenon whereby a visual sensory neuron will reliably fire fewer spikes in  response to the second presentation of an identical stimulus than to its initial presentation. Moreover, repetition suppression is associated with behavioral improvements. Judgments about  object properties are faster for repeated versus non-repeated objects. Despite its prevalence and  possible impact on behavior, we know little about how repetition suppression arises within the  visual system. To shed light on the mechanisms and possible functional significance of repetition  suppression we have performed several experiments in awake rhesus macaques recording from  single neurons in multiple brain areas at different levels of the visual hierarchy while monkeys  viewed sequential displays in which we controlled the repetition of different aspects of the  stimulus. We found that surprisingly the degree of suppression depended not only on the  properties of the images but also on the preferences of the neuron. We found that repetition  suppression probably does not serve as a behaviorally relevant recency signal due its  homogeneous nature. Furthermore, we found no evidence to support that repetition suppression  is driven by neuronal fatigue as a mechanism. We also found that surprisingly repetition  suppression arises seemingly independently at multiple levels of the visual hierarchy without a  clear bottom-up or top-down origin. We have also found evidence that content outside a neuron’s  classical receptive field can have an impact on repetition suppression in a context dependent  manner, suggesting a possible role for lateral connections in the generation of repetition  suppression.