Figure 4.

Response of different network structures to center-surround stimulus configuration. (A) The stimulus activated the entire patch as shown. The hypercolumn at recording site “1” is part of the center and the hypercolumn at site “2” is part of the surrounding texture. δ refers to the difference in the firing rates at “1” and “2”. The results obtained were averages from 10 trials. (B) Cartoon depicting connection schemes employed to demonstrate the effect of center-surround stimulus configuration on different network structures. The arrow denotes long-range inhibition mediated by BC or MC when they assume extreme values of orientation specificity. (C) We used stimuli of three different strengths (g_1, g_2, g₃) for this experiment and hence the three panels. The red bar denotes the period of stimulus presentation and plotted are the difference in the average activity of PC at “1” and “2” when the stimulus was presented to different network structures. When the inhibition was mediated by orientation unspecific inhibitory neurons, regardless of their presynaptic dynamics, that is depressing (BC) or facilitating (MC), the global inhibition to the minicolumn at the center was less than the surround (see text for more description). Hence, this resulted in negative δ for Type 2, Type 3, Type 4 and Type 5 network structure. On the contrary, δ was positive only for the network structure with long-range inhibition inhibition mediated via orientation specific MC (Type 1). (D) For this experiment, we used stimuli of two different strenghts (g₄, g₅) and hence the two panels. Here again δ refers to the difference in the firing rates at “1” and “2”. Lowering τ_facil of PC-MC_LO and PC-MC_LR connections lowers the duration of postive δ. When long-range inhibition was mediated by orientation specific BC (Type 6), the duration of positive δ furthur deteriorated. The initial positive δ was followed by a negative δ due to the depressing synapses between PC and BC (see text).