Synaptic Plasticity Can Produce and Enhance Direction Selectivity
Figure 6
Reproducing the Properties of a Population of Midbrain Neurons
(A) Experimental data from a population of midbrain neurons in Eigenmannia (replotted from [17]). PSP depression plotted against direction selectivity observed in two conditions for each neuron. Asterisks with solid line: response of each neuron to the moving object. Circles with dashed line: response of the same neurons to the moving object but with concomitantly presented global synchronous gamma-band oscillations. Curves are the best fit second degree polynomials.
(B) Hypothetical distributions of synaptic properties in model populations of neurons. The background gradient is a region of Figure 4C where the addition of global synchronous oscillations enhances direction selectivity. Lines represent one-dimensional restrictions of the parameter space—these lines are four hypothetical distributions of parameter values across a population of neurons, lines labeled C through F.
(C–F) Model data plotted in the same manner as the experimental data shown in (A). The data shown in each of the plots in (C–F) correspond to the labeled lines in (B). In (C–F), the lower curve shows direction selectivity to the moving object and the upper curve shows direction selectivity to the moving object with concomitant oscillations. For this plot, the parameters determining the relative contributions of the depressing and the nondepressing synapses have been expressed as a count of more numerous, but individually weaker, synapses. We arbitrarily model the numbers of depressing and nondepressing synapses, as well as their contributions to the post-synaptic potentials as roughly equal. To achieve these approximate equalities, we multiplied the number of the depressing synapses by a weight of 0.2 and the nondepressing synapses by a weight of 0.01. The hypothetical distribution are: (C) constant total number of synapses (80) with variable ratio of numbers of depressing to nondepressing; (D) constant number of depressing synapses (80) with variable number of nondepressing synapses; (E) constant ratio of numbers of depressing to nondepressing synapses (5/3) with variable total numbers of synapses; (F) constant number of nondepressing synapses (12) with variable number of depressing synapses. (E) and (F) are most similar to the experimental data shown in (A).