Main  Work Departmental seminar
June 23, 2002
Department of Biomedical Engineering, Technion - IIT
Effects of eye movements and linear and nonlinear response properties in striate cortex (V1) of alert monkeys
Igor Kagan
Advisors: Prof. Moshe Gur, Prof. D. Max Snodderly
Abstract
The striate, or primary visual cortex (V1) in primates is the first cortical area where functional processing of visual information begins. The study of visual functions in alert animals is beneficial because experimental conditions are very close to "natural vision", but are complicated by eye movements. We recorded eye movements and extracellular responses from single cells and clusters of cells in V1 of alert macaques performing a fixation task, while presenting an extensive array of visual stimuli. We sought to correlate visual stimulation (input), modified by eye movements, with the resulting neuronal activity (output). We found that fixational eye movements strongly affect the firing of all V1 neurons. However, these effects can be consistently predicted from receptive field (RF) properties of a particular cell combined with the information about stimulus spatiotemporal transformations imposed by the eye movements. To achieve the precision required for detailed characterization of RFs, the effects of eye movements were minimized in the real-time using a feedback compensation for shifts in eye position ("image stabilization"), and also off-line during data processing. We found that most cells in alert monkey V1 have overlapping increment (responsive to bright) and decrement (responsive to dark) activating regions (ARs) and nonlinear response properties (complex cells), while the minority have nonoverlapping ARs and [quasi]linear response properties (simple cells). However, the behavior of our complex cells differ from the type of nonlinearity usually ascribed to complex cells in anaesthetized monkeys and cats, and can not be predicted from RFs' spatial maps. For example, the responses of complex cells to drifting sinusoidal luminance gratings exhibited diversity ranging from unmodulated firing (F0 harmonic) to frequency doubling (F2) to pseudolinear (F1) to "subF1" modulation, depending on stimulus parameters. Since existing models of cortical cells do not account for the behavior of complex cells, our results suggest an alternative complex cell model based on interactions between increment and decrement ARs and their surrounds, representing the basic functional unit of primate V1.
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