A rapid and precise on-response in posterior parietal cortex

Abstract

The activity of neurons in the lateral intraparietal area (LIP) of the monkey predicts the monkey's allocation of spatial attention. We show here that despite being relatively high within the visual hierarchy, neurons in LIP have extremely short and precise visual latencies. Mean latency was 45.2 msec; the timing precision of the onset response was usually better than 4 msec. The majority of neurons had a pause in response after an initial burst, followed by more sustained visual activity. Previous attention allocation had no effect on either the latency or magnitude of the initial burst, but produced clear effects on the magnitude of the later sustained activity. Together, these data indicate that the initial burst in LIP visual response reflects an uncontaminated sensory signal. Information about stimulus onset is transmitted rapidly through the visual system to LIP; the on-response has a higher speed and temporal precision than realized previously. This information could be used to orient attention to novel objects in the visual environment rapidly and reliably.

Figure 1.

Responses of two neurons to the target and a ring. Histograms and rasters are aligned to stimulus onset. Raster plots show data from a subset of 20 consecutive correct trials. The inset shows the stimulus configuration with the target or ring in the RF (dotted line). Arrows indicate latency. Histogram bin width, 2 msec.

Figure 2.

Latencies after the target and ring. A, Cumulative distribution of latencies after the presentation of the target and ring. B, Plotting target latency against ring latency shows a consistent relationship between the two. The linear best fit line is in black. C, D, Distributions of the 5th (white columns) and 95th (gray columns) percentiles of latency after the target (C) and ring (D).

Figure 3.

Distribution of last prelatency (left plots) and first postlatency (right plots) spike probabilities for the target (A) and ring (B). Each row represents a single neuron. Brightness represents the probability of a spike occurring in a given bin; darker blocks show higher probabilities. Dots show the median time of the last (left plots) and first (right plots) spikes. Neurons were sorted by the median time of the first spike.

Figure 4.

Postlatency activity. A, C, Distribution of activity that was (black) or was not (white) significantly higher than the prestimulus activity for all 41 cells after the target (A) or ring (C). Each row shows data from a single cell. Cells were sorted by whether they had a period of inactivity (above the dashed lines) or not, and then by the number of white blocks. The vertical white lines are at 10 and 50 msec (see text). B, D, Distractor responses during the initial burst (B) or 50-70 msec postlatency (D). Data from trials in which the target preceded the distractor in the RF plotted against data from trials in which the target did not appear in the RF. p values from paired t tests.