Abstract-Persistent elevated neuronal activity has been identified as the neuronal correlate of working memory. It is generally assumed in the literature and in computational and theoretical models of working memory that memory-cell activity is stable and replicable; however, this assumption may be an artifact of the averaging of data collected across trials, and needs experimental verification. In this study, we introduce a classification scheme to characterize the firing frequency trends of cells recorded from the cortex of monkeys during performance of working memory tasks. We examine the frequency statistics and variability of firing during baseline and memory periods. We also study the behavior of cells on individual trials and across trials, and explore the stability of cellular firing during the memory period. We find that cells from different firing-trend classes possess markedly different statistics. We also find that individual cells show substantial variability in their firing behavior across trials, and that firing frequency also varies markedly over the course of a single trial. Finally, the average frequency distribution is wider, the magnitude of the frequency increases from baseline to memory smaller, and the magnitude of frequency decreases larger than is generally assumed. These results may serve as a guide in the evaluation of current theories of the cortical mechanisms of working memory. © 2007 IBRO. Published by Elsevier Ltd. All rights reserved.Key words: spike trains, monkeys, memory networks, parietal cortex, prefrontal cortex, computational models.The electrophysiological study of the neuronal basis of working memory in primates has traditionally focused on the changes in single-cell average frequency that may occur during the mnemonic retention of a stimulus cue in delayed-response tasks. Experiments dealing with this issue have led to the identification of cells, generally labeled "memory cells," that show a persistent increase in their average firing frequency (AF) during the memory period of a memory task (Fuster, 1997). Memory cells have been identified in multiple cortical regions, including prefrontal (Fuster and Alexander, 1971;Fuster, 1973;Niki, 1974;Niki and Watanabe, 1976;Funahashi et al., 1989;Miller et al., 1996;Rao et al., 1997;Romo et al., 1999), parietal (Gnadt and Andersen, 1988;Koch and Fuster, 1989;Andersen et al., 1990;Barash et al., 1991; Fuster, 1996, 1997), and inferotemporal Jervey, 1981, 1982;Miyashita and Chang, 1988;Fuster, 1990;Miller et al., 1993;Chelazzi et al., 1993Chelazzi et al., , 1998Colombo and Gross, 1994;Gibson and Maunsell, 1997) cortex. A variety of studies have shown that memory cells in all three of these associative regions are involved in the retention of a given sensory cue for a prospective motor response. It has also been shown that cells within a given region can retain associated items of more than one modality (Haenny et al., 1988;Maunsell et al., 1991;Colombo and Gross, 1994;Bodner et al., 1996;Gibson and Maunsell, 1997; Fuster, 1997,...