Humans and animals confuse lateral mirror images, such as the letters "b" and "d," more often than vertical mirror images, such as the letters "b" and "p." Experiments were performed to find a neural correlate of this phenomenon. Visually responsive pattern-selective neurons in the inferotemporal cortex of macaque monkeys responded more similarly to members of a lateral mirror-image pair than to members of a vertical mirror-image pair. The phenomenon developed within 20 milliseconds of the onset of the visual response and persisted to its end. It occurred during presentation of stimuli both at the fovea and in the periphery.

Rollenhagen, Julianne E. and Carl R. Olson. Low-frequency oscillations arising from competitive interactions between visual stimuli in macaque inferotemporal cortex. J Neurophysiol 94: 3368 -3387, 2005. First published May 31, 2005 doi:10.1152/jn.00158.2005. Some neurons in the inferotemporal cortex (IT) of the macaque monkey respond to visual stimuli by firing action potentials in a series of sharply defined bursts at a frequency of about 5 Hz. The aim of the present study was to test the hypothesis that the oscillatory responses of these neurons depend on competitive interactions with other neurons selective for different stimuli. To test this hypothesis, we monitored responses to probe images displayed in the presence of other already visible backdrop images. Two stimuli were used in testing each neuron: a foveal image that, when displayed alone, elicited an excitatory response (the "object") and a peripheral image that, when displayed alone, elicited little or no activity (the "flanker"). We assessed the results of presenting these images separately and together in monkeys trained to maintain central fixation. Two novel phenomena emerged. First, displaying the object in the presence of the flanker enhanced the strength of the oscillatory component of the response to the object. This effect varied in strength across task contexts and may have depended on the monkey's allocating attention to the flanker. Second, displaying the flanker in the presence of the object gave rise to sometimes strong oscillations in which the initial phase was negative. This was all the more striking because the flanker by itself elicited little or no response. This effect was robust and invariant across task contexts. These results can be accounted for by competition between two neuronal populations, one selective for the object and the other for the flanker, if it is assumed that the visual responses of each population are subject to fatigue.

Neurons in the visual cortex of the macaque monkey exhibit a variety of competitive behaviors, including normalization and oscillation, when presented with multiple visual stimuli. Here we argue that a biophysically plausible cortical circuit with opponent inhibition, spike-frequency adaptation, and synaptic depression can account for the full range of behaviors. The governing parameter is the strength of inhibition between competing neuronal pools. As the strength of inhibition is increased, the pattern of network behavior shifts from normalization mode to oscillatory mode, with oscillations occurring at progressively lower frequency until, at the extreme, winner-take-all behavior appears.