Picower Cent Learning/Memory, MIT, Cambridge MA 02139
D C Lyon, J Schummers, J Sharma, M Sur

Studies of contrast adaptation in V1 of anaesthetized animals have focused primarily on the effects on a cell’s firing rate to a single contrast presented several times in rapid succession or successive presentations of gradually increasing/decreasing contrasts. This approach has been useful for defining the time course of contrast or pattern adaptation. Yet, in awake monkeys, different levels of awareness or expectation may modify these adaptation effects. For example, sudden changes in luminance can invoke strong bottom-up or stimulus-driven processes that capture attention, while a sequence of stimuli can set up an expectation that is altered with an unexpected transition. To test the interaction of abrupt changes in luminance with adaptation we presented sequences of white or gray squares on a slightly darker gray background. The squares were presented for 1sec every 3sec to the fixating monkey, on the receptive fields of V1 neurons. In general cells showed moderate to weak adaptation when the same square was presented several times in a row, in that firing rate gradually decreased over repeated presentation of the same square. At points in the sequence where contrast increased “unexpectedly”, neurons fired as if the adaptation effect was extinguished. To further explore effects of expectation the monkey viewed sequences of gray – white squares, still spaced 2sec apart. Periodically, two white or two gray squares would appear in succession. In most of these cells general adaptation effects were apparent. Interestingly, contrary to what would be expected of adaptation, in some cells the average firing rate was higher following the second successive (oddball) presentation of a white or gray square. We conclude that the monkey was not only adapting to the luminance of the stimulus, but also to the repetitive sequence. The occurrence of an unexpected change in the stimulus sequence served to eliminate the neuron’s bottom-up adaptation.
Supported by: EY07023 and EY014521