Figure 4.

Relating trial-by-trial fluctuations in neural activity and behavior. a, b, Histogram (with bins logarithmically spaced) of R² between trial-to-trial fluctuations in behavior (reach speed, a; initial direction, b) and trial-to-trial responses of individual neurons, with drift removed from both. Control histogram for null hypothesis of no correlation (gray) was determined by correlating neural activity with behavioral data from different sessions with matched experimental conditions. Data are combined from both monkeys, but results hold for each. c, d, Analogous to a and b, but correlating behavior to fluctuations in PMd/M1 population response via optimal linear estimator (see Materials and Methods). e, f, Summary of population behavior R² analysis (c, d) but correlating behavior to neural firing rates observed in different trial epochs. Each data point represents distribution median, and error bars denote the smallest interval containing 68% of data points; colors are analogous to a–d. “pre-trial” is a 1 s window before the trial begins. “trial start” begins with the appearance of a central start target; “hold” begins when the monkey has moved to that target. “motor prep.” begins with the appearance of the visual reach target and ends with the go cue. We also repeated the analysis using the neural firing rates measured during each of the four quartiles of the preparatory period. (For more details on the trial timeline, see Materials and Methods.)