To show the timing of remapping in LIP, we present data from a free-viewing visual foraging task (
Mirpour et al., 2009). In this task, subjects are presented with an array of stimuli that remains on the screen for the duration of the trial. These typically include five identical distractors that never give a reward and five identical potential targets, one of which will be rewarded if it is fixated for 500 ms. This leads to a form of visual search in which the subjects visually forage among the stimuli, typically looking from target to target, waiting at each to see if they get the reward and then moving on.
Figures 4A and
4B show the mean population response of 52 LIP neurons recorded in the visual foraging task (
Mirpour & Bisley, 2016), aligned by fixation onset (i.e., the end of the saccade) and sorted by the identity of the stimulus (target or distractor) in the neuron's presaccadic response field (
Figure 4A) or postsaccadic response field (
Figure 4B). The blue trace in the
Figure 4C shows the mean difference in response between the two traces in
Figure 4B: This represents the strength of the signal of the postsaccadic response, and we will refer to it as the postsaccadic signal. Prior to fixation onset, the postsaccadic signal is close to zero, but shortly after the saccade ends, the mean difference ramps up to a plateau across a 25-ms period. It then stays at that approximate level for another 70 to 80 ms before ramping up again. The two arrows in the figure show the mean visual latency (VL) for these neurons (i.e., the time it takes from when a stimulus is presented in the response field to when the LIP neuron starts responding) and the population discrimination latency (DL), which is the time at which the activity from this population of neurons starts discriminating between the response to a target and the response to a distractor following array onset. The red trace in this figure shows the difference between the two traces in
Figure 4A (i.e., the presaccadic signal), sorted based on what was in the response field before the saccade. Until fixation onset, the presaccadic signal is relatively strong, and this drops to around zero within about 25 ms.
Figure 4D shows the same data set, but instead of showing the difference in response, it shows the percentage of neurons in each time bin that had a significant main effect of stimulus identity in the presaccadic response field (red), a main effect of stimulus identity in the postsaccadic response field (blue), or a significant interaction between the two (all
p < 0.01, analysis of variance).