The role of directionally selective neurons in the perception of global motion

Abstract

Dynamic random dot targets consisting of many localized motion vectors have been used to study the pooling of local motion signals into a global motion percept (Williams and Sekuler, 1984). In such displays, the dots are displaced with a constant step size and the direction of motion for each dot is chosen at random from a specified distribution. When the distribution extends over 360 deg, the display consists only of local random motion of individual dots and no coherent motion is reported. However, when the distribution is less than 360 deg (biased), the stimulus appears to flow in a single direction. We examined the effects of reducing the number of directionally selective (DS) cortical neurons on this integration process. Normal cats and cats with severely reduced proportions of DS neurons were trained on 2 direction discrimination tasks. The discrimination of opposite directions was examined while varying either the range of directions of local motion, or the proportion of dots moving with biased distribution. When all dots in the display were directionally biased, cats with reduced numbers of DS neurons performed the task as well as normal cats and humans (threshold range: 280-320 deg). However, when the proportion of biased dots decreased, these animals had severe deficits. Thus, in the absence of noise, even a very small number of DS neurons can perform spatial pooling of local directional signals, and support normal discrimination of opposite directions. However, a full complement of directional detectors appears necessary when the motion signal is masked by noise. The discrimination of small differences in direction revealed far more severe deficits, even when all the dots in the display were directionally biased (no noise). Cats with few DS neurons could not discriminate differences <40 deg, whereas normal cats discriminated differences of 9-12 degs. This deficit was even more pronounced when the targets were noisy or had a large range of directions. Thus, small differences in direction cannot be discriminated by a reduced complement of DS neurons, even in the absence of noise. This loss of accuracy is most likely due to the reduced sensitivity of directional mechanisms in cats that show a loss of DS in a majority of cortical neurons.