Attending to Contrast

stimulus that drives the neuron to fire and can be a meaningful way to characterize a neuron's responses. However, by studying the full distribution of stimuli preceding a spike rather than just the average of this distribution , the authors show that quickly varying dynamic stimuli have a more subtle effect on H1, so that there Attending to Contrast are essentially two relevant functions of the stimulus (roughly, the velocity and acceleration of the motion signal) that affect the firing of the neuron. In this ap-Studies of the neural mechanisms underlying attention proach, the number of parameters, as well as their rela-build on the foundation of classical studies of the pri-tion to the stimulus, are determined by the structure of mate visual system that have focused on understanding the data. Having found this simplified parameterization, how visual images are transduced, encoded, and ana-it is possible to fully characterize the nonlinear input/ lyzed by the brain. In most of these classical studies, output relation of the neuron. anesthetized animals were presented with visual stimuli Next, Brenner et al. show that H1 adapts to the vari-while physiologists recorded the activity of neurons car-ance of the velocity distribution of the motion signal, as rying information from the retina to the lateral geniculate it must do when the fly switches from straight flight to nucleus of the thalamus and then on to the hierarchically chasing behavior, for example. It is important to remem-organized visual cortices. As Hubel and Wiesel (1959) ber that this is an adaptation to the statistics of an were among the first to demonstrate, the sensitivity of ensemble of stimuli rather than to a particular stimulus each neuron in the visual cortex to patterns of light and or stimulus feature. They emphasize that all measurable dark can be described by a receptive field, which is the higher order statistics of the spike train adapt, not just pattern of light and dark that maximally excites the cell. the firing rate. Yet, they find that the adaptation can In a typical cell in cortical area V1, for example, the be fully described in terms of a single parameter, the receptive field might be described as two vertically ori-" stretch factor, " which determines how the neuron ented dark regions flanking a vertically oriented light matches its limited dynamic range of spike rates to the region: a vertical band of light flanked by darkness …