The ventral intraparietal area (VIP) is located at the end
of the dorsal stream. Its neurons are known to have receptive-field
characteristics similar to those of MT and MST neurons, but
little is known about the temporal characteristics of VIP
cells' responses. How fast are directionally selective
responses evoked in the ventral intraparietal area after viewing
optic flow patterns, and what are the temporal properties of
these neuronal responses? To examine these questions, we recorded
the activity of 37 directionally selective ventral intraparietal
area (VIP) neurons in two awake macaque monkeys in response
to optic flow stimuli with presentation times ranging from 17
ms to 2000 ms. We found a minimum response latency of 45 ms,
and a median latency of 152 ms. Of all neurons, 10% showed early
response components only (response latency < 150 ms and no
activity in 500–2000 ms interval after stimulus onset),
55% only late response components (response latency >150
ms and sustained activity in 500–2000 ms interval), and
35% both early and late response components. Early responses
appeared to very brief stimulus presentations (33-ms duration),
while the late responses required longer stimulus durations.
The directional selectivity was independent of optic flow duration
in all cells. These results suggest that only a subset of neurons
in area VIP may contribute to the fast processing of optic flow,
while showing that the temporal properties of VIP responses
clearly differ from the temporal characteristics of neurons
in areas MT and MST.