I propose to study physiological mechanisms of neural selectivity that underlie the control of cortical gain to match the contrast range of the visual environment, and that underlie perception of motion, especially binocular mechanisms that contribute to motion in depth. We will make physiological measurements in areas 17 & 18 of the cat, an animal whose motion mechanisms parallel those in the primate/human, and compare them with models that mimic both functions. To study mechanisms we will use conventional stimuli generated on an oscilloscope, and also an array of 1 to 16 optimally oriented, independently and randomly modulated bars, i.e., spatiotemporal """"""""white noise"""""""", to provide cortical measurements that are simultaneously rich in space and time. An important feature of the stimulus is its complete representation of negative as well as positive response phases, because its high average power increases a neuron's average firing rate.
Our first Aim i s to understand the nonlinear basis for cortical cells to adapt their contrast gain to the contrast range in their receptive fields, an important mechanism for extending their linear range without sacrificing sensitivity. We will begin by checking that addition of white noise does not change a neuron's classification. Then, we will manipulate the neuron's gain by closely interleaving white-noise periods of two different contrast (power) levels. The magnitude and temporal properties of contrast gain are reflected directly in the amplitude and shape of responses (""""""""kernels"""""""") to impulsive l-bar white-noise components, and the time course of the adaptive mechanism is indicated by changes in these parameters following a change in contrast.
Our second Aim i s to follow-up on new psychophysical evidence for excellent selectivity for motion-in-depth. We will measure in cortical cells l-bar responses and 2-bar interactions in each eye and between the eyes by using independent but simultaneous white-noise bar arrays in each eye. These measures will demonstrate any differences in preferred velocity and in point-to-point mapping between the eyes, which I propose code 3- dimensional aspects of motion. Interactions will be convolved with stimuli moving along straight trajectories in depth to predict a set of responses that we would not have time to measure directly, and to examine in measured neurons and in models the dependence of selectivity on linear and nonlinear mechanisms.
