Adaptive interactions in the visual pathway

Nicholas A. Lesica and Garrett B. Stanley
Division of Engineering and Applied Sciences
Harvard University

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In a natural setting, the mean luminance and contrast of the visual world are constantly changing and multiple adaptive mechanisms are concurrently active. These adaptive mechanisms play an essential part in the neural encoding scheme by increasing the rate of information transmission, differential sensitivity, and signal to noise ratio. While visual adaptation has been studied for many years, the interactive effects of the different mechanisms have not been characterized. Extracellular recordings of responses to random stimuli were made from retinal ganglion cells and visual cortex in anesthetized rodents and primates. Initially, either mean luminance or contrast was held fixed, while the other feature was varied. The adaptive phenomena observed during these trials are in agreement with previous studies. An increase in contrast resulted in a decrease in the peak of the temporal receptive field to between 50% and 90% of its initial value over a time course of 10-15 seconds. Similar results were observed for changes in mean luminance. In order to investigate the interactive effects of mean luminance and contrast adaptation, responses to stimuli where both features were varied simultaneously were recorded. Preliminary results indicate that increasing mean luminance has a multiplicative effect on contrast adaptation in the visual cortex, i.e. increasing mean luminance increases the percent change in the peak of the temporal RF. Efforts are underway to use these data to create a model of adaptive interaction where the gain of the temporal receptive field is modulated by the mean luminance and contrast of the stimulus. Such a model can be used to predict the neuronal firing rate in response to more naturalistic stimuli with varying mean luminance and contrast.