Figure 5.

Center surround organization and lateral inhibition. (A) Spatial pattern of GrC responses to a short mf burst (two spikes at 500 Hz on eight contiguous mfs) at the time of the first spike (E) and of the second spike (E₂). E_2−bi indicates the response at the time of the second spike when inhibition is blocked. Since inhibition arises after excitation, it is does not affect generation of the first spike but markedly reduces generation of the second and following spikes. Thus, the influence of inhibition on the GrC response was obtained as (I = E−E_2−bi). The difference between excitation and inhibition (E–I) reveals that inhibition is especially effective in reducing excitation around the core, generating a Mexican hat profile. The plots were the average of 10 simulations using different random seeds for synaptic connectivity. (B) Spikegrams for all active GrCs ordered from center to periphery [data from one of the simulation used to make (A)]. The first spikes are indicated by red dots, the second and following spikes by black dots. In the center the first spike occurs about 5 ms after the stimulus, whereas in the surround it tends to occur later. The second spike, which is quite rare in control, becomes well evident when inhibition is blocked. (C) The spatial profile of inhibition (I) was subtracted from the profile of excitation (E) to obtain the E-I balance (E-I) along a section passing through the core of the corresponding plot in (A) (average data obtained from 10 simulations with different random seeds for synaptic connectivity). The larger extension of inhibition compared to excitation and the Mexican-hat profile of the E-I balance are evident.