Neural Coding Of Visual Stimuli

National Institute Of Mental Health

Investigators

Linked publications & trials

Abstract

Interpreting the information encoded in single neuronal responses requires knowing which response features carry information. Despite a great deal of study, however, it is still not completely train which response features are important. To describe a neuronal response completely we must specify the arrival time of each spike. However, we are interested less in the spike train itself than in its role in transmitting information. Therefore, only those aspects of the response that carry unique information need be included. Previously we showed that all of the information carried by neuronal spike trains requires specifying only the spike count distribution (which is approximately truncated Gaussian), the variation in firing rate with a bandwidth of less than 30 Hz, the equivalent of measuring spike counts in 30 ms wide bins), and the interval histogram. If these features completely describe single neuronal responses, they contain all of the information available from those responses, no matter what representation of the response is chosen. The reason the spike count distribution (that is, knowing how many times each spike count occurs) is so important is that the temporal coding depends almost completely on the spike count. Intuitively this seems clear when we realize that the more spikes that are present, the richer the potential temporal code. Thus, the influence of variation in the number of spikes that occurs with successive presentations of a stimlus must be properly taken into account when estimates of neuronal coding are made. In the past the statistical models of neuronal responses were used because they work, but there has been no clear theoretical understanding. If the responses arise from a random process with a certain overall pattern in time, the responses must follow certain well-known statistical rules. Over the past year we have shown that the order of spikes in responses exactly follow these rules, given by the statistical properties codified by order statistics. One of the advantages using the order statistic representation is that it allows exact knowledge of the amount of information carried by neuronal responses if the spike count distribution and the average variation of firing rate can be measured. This is a principled way to make this measurement. Using a simple reformulation of the basic formula of order statistics, we have derived a decoder that will decode neuronal responses millisecond-by-millisecond as the response evolves for any neuron that has spikes with times that appear to be stochastically determined. This algorithm can form the basis of an instant-by-instant neuronal controller. Finally, given these extremely accurate statistical models of neural responses, we studied what happens when they are applied to small populations (pairs) of neurons. Our measurements in two brain regions, primary motor cortex and inferior temporal cortex, show that all of the patterns of spikes including simultaneous spikes are related to the same measurements, i.e. the rate variation and the spike count distribution, plus the correlations of the spike counts taken over 100's of milliseconds. Thus, it appears that the patterns of spikes seen across neurons are directly related to the slow variations in firing rate. Thus, it appears that the same measurements that are decoding single neuronal responses are also adequate decoding the information carried in the population activity.