Comment on "Single-trial spike trains in parietal cortex reveal discrete steps during decision-making"

Abstract

Latimeret al (Reports, 10 July 2015, p. 184) claim that during perceptual decision formation, parietal neurons undergo one-time, discrete steps in firing rate instead of gradual changes that represent the accumulation of evidence. However, that conclusion rests on unsubstantiated assumptions about the time window of evidence accumulation, and their stepping model cannot explain existing data as effectively as evidence-accumulation models.

Fig. 1

Experimental evidence in support of a gradual accumulation of evidence in LIP. (A) LIP neurons ramp, on average, during an RT task. Averages are sorted by RT quantile (color), using trials in which the monkey chose the direction associated with the choice target in the neuron’s response field. (Modified from [2], showing response from ~200ms after stimulus onset; see also [11] Figs. 2b,d). (B) LIP neurons undergo multiple incremental changes in firing rate on single trials. On this example trial, the monkey decided in favor of the green target in the neuron’s response field, consistent with the accumulated evidence from the sequence of shapes. (From movie 3 of [4]). (see [Yang_Movies, Kira_Movie1, Kira_Movie2] for more single-trial examples and [4, 5] for population analyses).

Fig. 2

Behavioral integration times are difficult to determine from the analyzed dataset but are certainly shorter than the full 500–1000 ms viewing durations. Data are from [12] (Figs 7A,D; with missing coherences kindly supplied by Latimer et al.) Open circles correspond to behavioral data obtained after the collection of the neural recordings ([1], Fig 7D). Smooth curves show fits of a bounded diffusion model, from which we estimate the median decision time ~250 ms across all motion strengths (methods explained in [13, 14]). The data actually analyzed in [1](Fig 7A) exhibit little dependence of accuracy on the range of viewing times tested and overall worse performance (filled stars).