We next describe SAT adjustments in movement selleck chemicals neurons identified with the stochastic accumulation process (Hanes and Schall, 1996; Boucher et al., 2007; Ratcliff et al.,
2007; Woodman et al., 2008). Recent modeling specifies how visual neurons can provide the evidence that is accumulated by movement neurons (Purcell et al., 2010, 2012). Unlike visual neurons, movement neurons in FEF and SC project to omnipause neurons of the brainstem that are responsible for saccade initiation (Huerta et al., 1986; Langer and Kaneko, 1990; Segraves, 1992). Thus, they are uniquely poised to trigger saccades based on accumulating evidence. Movement neurons with no visual response are encountered less commonly than neurons with visual responses (Bruce and Goldberg, 1985; Schall, 1991). Here they comprised ∼10% of task-related neurons (n = 14). Many more neurons had both visual buy DAPT responses and presaccadic movement activity (n = 70); we will present data from these separately. We found four major adjustments in movement activity. First, the baseline shift reported earlier was significant in 29% of movement neurons (Figure S2A). Second, the rate of evidence accumulation varied with SAT condition (Figures 3A and 3B). For each movement neuron separately, we fit a regression line to the accumulating discharge rate in the 100 ms preceding the saccade on trials when the target was correctly located
in the RF. On average, the slope was lowest in the Accurate condition, intermediate in the Neutral, and largest in the Fast condition. We observed identical effects for visuomovement neurons (Figures S3A and S3B). Third, the magnitude of movement neuron activity at saccade initiation was lowest in the Accurate condition, intermediate in the Neutral, and highest in the Fast condition (Figure 3B; visuomovement neuron activity in secondly Figure S3B). Like baseline neural activity and mean
RT, this effect emerged immediately after a change in SAT cue (Figure S2C). Thus, SAT during visual search is accomplished in part through adjustment of the magnitude of neural activity producing responses. However, this result is puzzling because the direction of the change is opposite that of accumulator models that explain SAT through decreases in threshold with increasing speed stress. We will address this in detail below. Fourth, within each SAT condition, movement neuron activity accumulated to an invariant level at saccade initiation across RT quantiles (Figures 3C–3E; visuomovement activity in Figures S3C–S3E). This replicates previous studies from multiple laboratories and tasks: when SAT is not manipulated, or when task conditions cannot be predicted or remain constant, activity at saccade does not vary with RT (Hanes and Schall, 1996; Paré and Hanes, 2003; Ratcliff et al., 2007; Woodman et al., 2008; Ding and Gold, 2012).