Figure 6.
Application of FA to data from V1 and PMd. See methods and Supplementary Figure 6 for a description of datasets. a. FA was applied to covariance matrices (#neurons × #neurons) of spike counts, taken in an analysis window that either ended at stimulus onset (pre-stimulus) or began just after stimulus onset (stimulus). The measured covariance matrix was approximated as the sum of a ‘network’ covariance matrix and a diagonal matrix of private noise. To produce the plots in b–g, network variances were averaged across the subset of neurons/conditions (48% and 30% for V1, 74% and 79% for PMd) whose distribution of mean rates was matched before and after stimulus onset (similar to Fig. 4). b. Estimated variances for one V1 dataset. Network variability declined more than private variability in both absolute (p<10−7) and relative (percent of initial value, p<10−7) terms (paired t-tests across conditions). c. similar plot for a V1 dataset from a second monkey: p<0.002 (absolute) and p<0.002 (relative). d. Summary comparison for V1. Changes in variability (stimulus minus pre-stimulus) were expressed in percentage terms. Data to the left of zero indicate that network variability underwent the larger decline. The distribution includes all conditions and both datasets. The mean and standard error is given by the black symbol at top (p<10−7 compared to zero, paired t-test). Gray symbols give individual means for each dataset. e. Same as b,c but for one PMd dataset (G20040123). Network variability declined more in absolute (p<0.005) and relative (p<0.001) terms. f. Similar plot for a second PMd dataset (G20040122): p<0.05 (absolute) and p<0.02 (relative). g. Summary comparison for PMd (distribution mean <0; p<10−4). h. Relationship between mean firing rate and network-level (shared firing-rate) variance. Data (same dataset as panel b) were binned by mean rate, and the average network variance (±SE) was computed for each bin. This was done both pre-stimulus (gray) and after stimulus onset (black). The average was taken across neurons and conditions (each datum being averaged was, for one condition, one element of the blue diagonal in panel a). Distributions of mean rates are shown at bottom. The analysis in b was based on the overlapping (mean-matched) portion of these distributions. i. Similar plot for PMd (same dataset as panel e).