Cognitive neuroscienceVariability in neuronal activity in primate cortex during working memory tasks
Section snippets
Single unit recording
The database for this study consists of 961 cells. In that sample, there are 521 parietal cells recorded from areas 2, 3, 5, and 7 of one monkey during performance of a haptic delayed-matching task (Zhou and Fuster, 1996); 291 prefrontal cells recorded from areas 6, 8, 9 and 46 of another monkey during performance of a CM audiovisual task (Bodner et al 1996, Fuster et al 2000); and 149 prefrontal cells recorded primarily from area 9 and the dorsal portion of area 46 of a third monkey during
Statistics by memoranda
Unless otherwise noted, the analyses presented in the Results section were conducted in a memorandum-specific manner (see Experimental Procedures). The cell-specific responses to each memorandum were pooled together (see Experimental Procedures). The results of the analysis on the pooled data are presented below. A summary of the major statistical results for the various analysis described in detail below is shown in Table 2. Detailed statistical summaries are presented in the appendix.
Parietal
In
Systematic fluctuations over trials
To address the question of whether there are systematic fluctuations over trial number, two additional analyses were performed. First, we assessed the correlation of the delay period AF on consecutive trials in cells showing significant delay-activated memorandum-specific responses (see Experimental Procedures). The distribution of consecutive-trial correlation coefficients for all three datasets is presented in Fig. 9a. In parietal cortex, substantial numbers of cells showed high
Inter-trial variability and population averaging
To address the question of how the variability in the response across trials is related to the size of the population, we generated population firing rate data for populations ranging in size from 1 cell to 109 cells, 10,000 times for each population size (see Experimental Procedures). The mean population firing rate for all population sizes was between 21.7 and 21.8 Hz. The results for the SD of the population firing rate are presented in Fig. 10. The CV (SD/mean) of the population firing rate
Discussion
In the present study we have shown that the cellular average frequency distributions are broad during both the baseline and delay periods of a working memory task. During the delay period of working memory tasks, cells with significant inhibitory or excitatory changes in average frequency (relative to baseline) are common, at least in parietal cortex. Although these delay-period changes can be large (>10 Hz), most are generally small (<5 Hz), especially in prefrontal cortex. Typically, large
Conclusions
In this study, we have characterized the frequency behavior of cortical cells in working memory tasks. The statistics and variability of cortical firing suggest that the mnemonic retention of stimuli is achieved by widely distributed, multifunctional cortical networks that vary substantially in both their composition and behavior across multiple trials; no reliable, distinct, memory-related bistable states could be clearly identified. These conclusions are based on the identification of cells
Acknowledgments
We thank William Bergerson and Bradford Lubell for technical support. We also acknowledge David Rex for his help in the use of the LONI Pipeline Processing Environment (NIH/NCRR 2 P41 RR13642) used in carrying out the frequency analysis. This work was supported by NIMH grants MH-25082 and MH-51697, a grant from the Whitehall Foundation, and by the MIND Institute. C. Chow was supported by NIMH grant MH-01508, and a grant from the Sloan Foundation.
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