JOURNALOF NEUROPHYSIOLOGY Vol. 57, No. 1, January 1987. Printed in U.S.A. Temporal Encoding of Two-Dimensional Patterns by Single Units in Primate Inferior Temporal Cortex. II. Quantification of Response Waveform BARRY J. RICHMOND AND LANCE M. OPTICAN Laboratory of Neuropsychology, National Institute of Mental Health, and Laboratory of Sensorimotor Research, National Eye Institute, Bethesda, Maryland 20892 SUMMARY AND CONCLUSIONS 1. The purpose of this study was to describe how the responses of neurons in inferior tem- poral (IT) cortex represent visual stimuli. In the preceding paper we described the responses of IT neurons to a large set of two-dimensional black and white patterns. The responses to dif- ferent stimuli showed temporal modulation of the spike trains. This paper develops a method for quantifying temporal modulation and showsthat the stimulus determines the distri- bution over time, as well as the number, of spikesin a response. 2. The responses were quantified using an orthogonal set of temporal waveforms called principal components. The principal com- ponents related to each neuron were extracted from all the responses of that neuron to all of the stimuli, regardless of which stimulus elic- ited which response. Each responsewas then projected onto the set of principal components to obtain a set of coefficients that quantified its temporal modulation. This decomposition produces coefficients that are uncorrelated with each other. Thus each coefficient could be tested individually, with univariate statis- tics, to determine whether its relation to the stimulus was nonrandom. 3. The waveforms of the principal com- ponents are unconstrained and depend only on the responses from which they are derived; hence, they can assume any shape. Nonethe- less, the 2 1 neurons we analyzed all had prin- cipal components that belonged to only one of two sets. The two sets could be characterized by their first principal component, which was either phasic or tonic. This suggests that these neurons may useasfew astwo different mech- anisms in generating responses. 4. The first principal component washighly correlated with spike count, and both were driven by the stimulus. Higher principal com- ponents were uncorrelated with spike count, yet someofthcm wcrc alsodriven by the stim- ulus. Thus the principal components form a richer description of the stimulus-dependent aspects of a neuronal response than does spike count. 5. Bootstrap tests showedthat several prin- cipal components (usually 3 or 4) were deter- mined by the stimulus. Since higher principal components were not correlated with the spike count, the stimulus must have determined the distribution of spikes in the responseas well as their number. However, it is possible that the number and distribution of spikes are both determined by the same characteristics of the stimulus. In this case, the temporal modula- tion would be redundant, and a simple uni- variate measure would be sufficient to char- acterize the stimulus-response relationship. The next paper in this series shows that the distribution of spikes is independent of their number and conveys addition about the stimulus (1 2) . .a1 inform .ation INTRODUCTION The goal of this study is to develop a new methodology for studying the way neuronal spike trains represent visual patterns. Such an approach must be capable of analyzing data from experiments in which more than one 147 wnloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (143.088.066.066) on August 25, 20 Copyright © 1987 American Physiological Society. All rights reserved.