SACCADE-RELATED ACTIVITY IN THE LATERAL INTRAPARIETAL AREA .1. TEMPORAL PROPERTIES - COMPARISON WITH AREA 7A

1. The cortex of the inferior parietal lobule (IPL) contains neurons whose activity is related to saccadic eye movements. The exact role of the IPL in relation to saccades remains, however, unclear. In this and the companion paper, we approach this problem by quantifying many of the spatial and temporal parameters of the saccade-related (S) activity. These parameters have hitherto been largely unstudied. 2. The activity of single neurons was recorded from Macaca mulatta monkeys while they were performing a delayed-saccade task. The analysis presented here is based on 161 neurons recorded from the lateral intraparietal area (LIP), a recently defined subdivision of the IPL; and 54 neurons recorded from the neighboring part of the IPL, area 7a. Overall, 409 IPL neurons were isolated in this study. 3. The typical activity of IPL neurons during the delayed-saccade task has three basic phases: light sensitive (LS), memory (M), and S. These basic phases are common to neurons of both areas LIP and 7a. In each phase (LS, M, and S), individual neurons may or may not be active. Most LIP neurons, however, are active in more than one phase. 4. To compare the activity levels of different neurons, the actual firing rate was weighted by each neuron's background level, yielding an "activity index" for each neuron, in each phase of the task. We calculated the activity index for the LS and M phases and for three phases related to the saccade: a presaccadic (Pre-S), a saccade-coincident (S-Co), and a postsaccadic (Post-S) phase. For area LIP neurons the median values of the activity index were high for the LS, M, Pre-S, and S-Co activities, and slightly lower in the Post-S period. In area 7a the median values were low for the LS phase and, in particular, for the M and Pre-S phases, somewhat higher coincident with the saccade, and high post-saccadically. 5. In area LIP, in each phase, 49-63% of the neurons had excitatory activity, and 10-17% had inhibitory responses. 6. In contrast, in area 7a excitatory responses were most frequent in the Post-S phase (56%). Excitation was particularly infrequent during M (28%) and Pre-S (22%). The incidence of inhibitory responses varied too (4-18%). The time course of inhibition was roughly opposite that of excitation; the highest frequency of inhibitory responses occurred during the saccade. 7. The latency of the S activity was defined as the time, relative to the beginning of the saccade, when the activity became significantly higher than background. In area LIP, latencies ranged from 200 ms before the saccade to 200 ms after the saccade. The mean latency in area LIP was 10.5 ms before the saccade. In area 7a the latencies ranged from 50 ms before the saccade to 380 ms after the saccade. The mean latency in area 7a was 140 ms after the saccade. 8. A neuron was considered presaccadic if its activity varied significantly from background by the time the saccade started. In other words, the latency of the neuron's S activity had to be less-than-or-equal-to 0 ms. In area LIP, 72% (61/85) of the neurons were presaccadic. In area 7a, only 18% (6/33) of the units were presaccadic. 9. The offset time of the saccadic activity was defined as the time the activity outlasted the saccade. The S activity usually lasted well beyond the saccade. The median offset time for LIP neurons was 120 ms. The median offset time for 7a neurons was 300 ms (but note that the S activity of most area 7a neurons only started postsaccadically). 10. The duration of the S activity was computed from its latency and offset and from the duration of the saccadic movement (typically 60 ms). For both areas LIP and 7a the distribution of durations was approximately Gaussian. The parameter values of the distributions were also similar for both areas (median 210 ms in both LIP and 7a). Thus the duration of the S activity is usually considerably longer than the duration of the saccade itself. 11. In area LIP the onset latencies of the LS activity ranged from 50 to 270 ms, with median 110 ms. In area 7a, although the range was similar to that of LIP, typically the LS latencies were longer (median 160 ms). 12. In summary, area LIP contains vigorous activity in anticipation of the saccade, during the LS, M, and Pre-S periods. Area 7a is active mainly during and after the saccade. These results suggest that area LIP participates in the planning of saccades. Area 7a, however, probably primarily subserves different functions.