Visual motion analysis for pursuit eye movements in area MT of macaque monkeys

We asked whether the dynamics of target motion are represented in visual ar ea MT and how information about image Velocity and acceleration might be ex tracted from the population responses in area MT for use in motor control. The time course of MT neuron responses was recorded in anesthetized macaque monkeys during target motions that covered the range of dynamics normally seen during smooth pursuit eye movements. When the target motion provided s teps of target speed, MT neurons showed a continuum from purely tonic respo nses to those with large transient pulses of firing at the onset of motion. Cells with large transient responses for steps of target speed also had la rger responses for smooth accelerations than for decelerations through the same range of target speeds. Condition-test experiments with pairs of 64 ms ec pulses of target speed revealed response attenuation at short interpulse intervals in cells with large transient responses. For sinusoidal modulati on of target speed, MT neuron responses were strongly modulated for frequen cies up to, but not higher than. 8 Hz. The phase of the responses was consi stent with a 90 msec time delay between target velocity and firing rate. We created a model that reproduced the dynamic responses of MT cells using di visive gain control, used the model to visualize the population response in MT to individual stimuli, and devised weighted-averaging computations to r econstruct target speed and acceleration-from the population response. Targ et speed could be reconstructed if each neuron's output was weighted accord ing to its preferred speed. Target acceleration could be reconstructed if e ach neuron's output was weighted according to the product of preferred spee d and a measure of the size of its transient response.