Figure 9.

Synaptic plasticity adjusts the readout scheme according to task demands, illustrated by simulations of a fine motion discrimination (Purushothaman and Bradley, 2005). Sample moves in the fixed reference direction. Test stimuli are inclined by Δθ = {±0.5°,±1° ... ±3°} relative to the reference direction. The task is to judge whether a test stimulus is inclined clockwise ( Δθ > 0) or counterclockwise ( Δθ < 0) relative to the reference. After learning, the choice-selective populations in the decision circuit encode clockwise/counterclockwise (instead of match/nonmatch) decisions, and hence are labeled as CW and CCW. (A) Spatio-temporal dynamics of the synaptic strengths. Differences of the synaptic strengths Δc = c^CW − c^CCW are color coded for comparison neurons with all preferred directions. x-axis: trial number, y-axis presynaptic neurons labeled by their preferred directions. Through learning a connectivity profile emerges, such that neurons tuned clockwise and counterclockwise relative to the reference preferentially target the CW- and CCW-selective populations, respectively. (B) Psychometric function for the fine motion discrimination. Psychometric threshold is ~ 1°-2°. (C,D) Strengths of synaptic connections to the CW-selective (red, $c_{ME}^{CW}$ and $c_{MS}^{CW}$ ) and CCW-selective (blue, $c_{ME}^{CCW}$ and $c_{MS}^{CCW}$ ) populations after learning. Activity of each neuron is gradually weighted in the decision process, whereby higher weights are assigned to the most sensitive neurons tuned 40°- 70° away from the reference direction.