If suppression in IT results from fatigue as a result of responses to briefly presented images, the reduction in response to a second stimulus should be proportional to the magnitude of the response to the initial stimulus. An overall reduction in the firing rate to the second stimulus might arise through any number of intracellular processes that simply result from recent excitation of the cell-a form of fatigue of the mechanisms in the cell that give rise to its spiking output ( Carandini 2000 Clifford et al. First, simple adaptation, narrowly defined as an adjustment in sensitivity in a neuron as a result of its recent response, could underlie repetition-induced suppression. There are a variety of potential influences of stimulus history that could be associated with repetition-induced suppression in IT. 1994 McMahon and Olson 2007 Miller et al. For example, in the inferotemporal cortex (IT) of the monkey-an area important for processing information about object shape-presenting two identical stimuli in sequence results in a substantially (≈50%) reduced overall response to the second stimulus. Concomitantly, neural responses in the visual system are strongly modulated by recent stimulus history ( Clifford et al. We repetitively observe the same object across multiple time scales (seconds, minutes, hours, days, and weeks). Under natural viewing conditions, all objects are observed within the context of a stimulus history. The data suggest that suppression in IT is a property of the input and recurrent circuitry in IT and is not directly related to the degree of response in the recorded neuron itself. The selectivity of the sustained response was preserved in spite of substantial suppression of the early part of the response. The dynamics of the response suggests that different parts of the input and recurrent circuitry that gives rise to neural responses in IT are differentially modulated by repetition suppression. Neural selectivity to a continuum of related images was enhanced if the first stimulus produced a weak response in the cell. Repetition suppression was also limited in duration, peaking at ≈ 200 ms after the onset of the second (test) image and disappearing before the end of the response. The data show that the degree of suppression does not depend directly on the response evoked by the first stimulus in the recorded neuron. Here, we examined the time course of suppression in IT by varying both the duration and stimulus content of two stimuli presented in sequence. The characteristics of repetition suppression in IT are poorly understood, and the details, including the interaction between the content of the first and second stimulus and the time course of suppression, are not clear. In addition, repetition suppression may have important perceptual consequences. Understanding the mechanisms underlying repetition suppression may provide important insights into the circuitry that generates responses in IT. In the inferotemporal cortex (IT)-an area important for processing information about object shape-there is a substantially reduced response to the second presentation of an image. Neural responses throughout the sensory system are affected by stimulus history.
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