Synaptic Plasticity Can Produce and Enhance Direction Selectivity
Figure 2
Response of Model to a Moving Sine Grating Following 500 msec Initialization with 50% Grey
(A) The one-state model with the fast process alone, s = 1.
(B) The two-state model with both the fast and slow processes, s = 0.99. Blue and red (middle trace of each subpanel) shows membrane potential, V∞(t). Cyan and magenta shows firing rate, F(t) (beneath V∞(t), and timing of action potentials determined from F(t) by a Poisson process (above). Blue and cyan (top subpanel in both A and B): preferred direction (σ = 1); red and magenta (bottom subpanel): non-preferred direction (σ = −1). Small numbers are the expected action potentials in each cycle. Black dotted line indicates threshold for action potentials. In the one-state model, the direction index (defined in Model) is constant across all cycles at 0.48. In the two-state model, the direction index increases from 0.49 to 1—the two-state model becomes more direction selective with time. Other parameters of model are Rb = 5 Hz, Rc = 172 Hz × log(67 × 0.2), xn = −45°, xd = 45°, tD = 150 msec, tS = 3,000 msec, d = 0.4, gd = 15, gn = 0.7,V0 = 0.7, V0 = 70 mV, VE = 0 mV, Vt = −64 mV, Vr = −65 mV, tm = 30 msec, tR = 10 msec. Other parameters of the stimulus are t0 = −500 msec, ts = 0 msec, tf = 3,000 msec, f = 2 Hz, p0 = ∞, p1 = −∞.