Synaptic Plasticity Can Produce and Enhance Direction Selectivity
Figure 3
Response, V∞(t), of the Two State Model to a Pulse Stimulus
(A,C) Initialized with a temporally and spatially uniform 50% grey stimulus.
(B,D) Initialized with global synchronous gamma-band oscillations. Solid lines represent responses to movement in the preferred direction, dashed lines to the non-preferred direction. The responses in each plot are aligned to the peak of the PSP from the depressing synapse. This facilitates the comparison between the preferred and non-preferred directions of movement, but as a result, the onsets of the moving stimuli are not aligned. In the preferred direction, the object passes first through the nondepressing part of the receptive field and then enters the depressing part. Parameters of the model for this and all remaining figures are the same as listed in Figure 2 except for the synaptic factors, which vary. For A and B (magenta): gn = 0.2, gd = 8. For C and D (green): gn = 0.6, gd = 4. The stimulus parameters used throughout for the pulse stimulus are fg = 20 Hz, Ag = 0.7, or Ag = 0 (respectively, with or without initializing global synchronous oscillations), t0 = −1,000 msec, ts = 0 msec, tf = 5,500 msec, φ = 135°; σ, p0, and p1 differed depending on direction of movement: σ = 1, p0 = −16p, p1 = −19p, (preferred direction); σ = −1, p0 = −15p, p1 = −19p (non-preferred direction).