Adaptation properties of macaque MT neurons

We have investigated the adaptation properties of single neurons in area MT of anesthetized macaque monkeys using drifting coherent dot patterns. We compared the direction tuning properties of neurons before and after an adapting stimulus (40s) whose direction was either in the preferred, null, or near-preferred direction (30 degrees from optimal). Adaptation in the preferred and near-preferred direction strongly reduced the response of MT neurons to subsequent stimuli without altering their direction preference. Null motion adaptation caused a slight increase in the cells' response to subsequent stimuli, without altering their direction preference. Similar results were obtained with shorter duration (3s) adapting stimuli although the effect was substantially smaller. The data suggest that the primary effect of prolonged sensory drive is to reduce the gain of MT neurons rather than to affect their selectivity. To test whether the adaptation properties of MT neurons could underlie the motion aftereffect, cells were adapted to gratings drifting in the null direction for 40 seconds and tested with counterphase gratings. The response to counterphase gratings nearly doubled after null adaptation. Similarly, the response of MT cells to zero coherence random dot motion was moderately stronger after adaptation to coherent dot motion in the null direction. The substantial increase in the response of MT neurons to motion-balanced stimuli following null adaptation suggests a motion opponent computation in area MT and that adaptation effects in MT may form the neural substrate for the MAE.