Figure 5. Exogenous, movement-unrelated SC spikes had the greatest impact on movement metrics when they occurred peri-saccadically.

(A) Individual trial spike rasters across all neurons ≤ 4.5 deg in eccentricity and all movements towards RF locations from experiment 1. The spike rasters are sorted based on the time of the visual burst (peak firing rate after stimulus onset) relative to saccade onset (bottom left: trials with visual bursts earlier than microsaccades; top right: trials with visual bursts later than microsaccades). The spike rasters are plotted in gray except during the interval 30–100 ms after stimulus onset (our visual burst interval; Figure 2) to highlight the relative timing of the visual burst to movement onset. Spikes in the visual burst interval are color-coded according to the observed movement amplitude on a given trial (legend on the left). As can be seen, microsaccades were enlarged when extra-foveal SC spiking (stimulus-driven visual bursts) occurred right before and during the microsaccades (see marginal plot of movement amplitudes in D). (B) Same as A, and with the same trial sorting, but with burst timing now aligned to movement peak velocity. (C) Same as A, B, and with the same trial sorting, but with burst timing now aligned to movement end. The biggest amplitude effects occurred when the exogenous ‘visual’ spikes occurred pre- and intra-saccadically, but not post-saccadically. (D) Microsaccade amplitudes (20-trial moving average) on all sorted trials in A–C. Blue horizontal lines denote the range of trials for which there was a significant increase in movement amplitudes (Materials and methods). Note that the numbers of trials are evident in figure. Figure 5—figure supplement 1 shows similar results from experiment 2, and Figure 5—figure supplement 2 shows similar results from the far neurons of experiment 1.

Figure 5—figure supplement 1. Analyses similar to those in Figure 5 but from experiment 2.

This figure is formatted similarly to Figure 5, but now using data from experiment 2 (the spatial frequency task; neurons ≤ 4.5 deg in preferred eccentricity). Very similar results can be seen.

Figure 5—figure supplement 2. Analyses similar to Figure 5 but for the far neurons of experiment 1.

The same temporal relationship between the peripheral visual bursts and the microsaccade onset times existed for the movements that had increased amplitudes. The only difference in this case was that the overall behavioral impact on the movement amplitudes (D) was smaller than with the near visual bursts (consistent with Figure 3—figure supplement 1). Note that this could reflect a lower likelihood of proper temporal alignment of far peripheral visual spikes with the population motor bursts for microsaccades, according to the novel hypothesis of Jagadisan and Gandhi, 2019. Indeed, in Figure 6 and Figure 6—figure supplement 2, we found that with proper temporal alignment, the impacts of individual movement-unrelated spiking on microsaccade amplitudes were quantitatively similar for both near and far neurons.