INHIBITION SYNCHRONIZES SPARSELY CONNECTED CORTICAL-NEURONS WITHIN AND BETWEEN COLUMNS IN REALISTIC NETWORK MODELS

Networks of compartmental model neurons were used to investigate the b iophysical basis of the synchronization observed between sparsely-conn ected neurons in neocortex. A model of a single column in layer 5 cons isted of 100 model neurons: 80 pyramidal and 20 inhibitory. The pyrami dal cells had conductances that caused intrinsic repetitive bursting a t different frequencies when driven with the same input. When connecte d randomly with a connection density of 10%, a single model column dis played synchronous oscillatory action potentials in response to statio nary, uncorrelated Poisson spike-train inputs. Synchrony required a hi gh ratio of inhibitory to excitatory synaptic strength; the optimal ra tio was 4 : 1, within the range observed in cortex. The synchrony was insensitive to variation in amplitudes of postsynaptic potentials and synaptic delay times, even when the mean synaptic delay times were var ied over the range 1 to 7 ms. Synchrony was found to be sensitive to t he strength of reciprocal inhibition between the inhibitory neurons in one column: Too weak or too strong reciprocal inhibition degraded int ra-columnar synchrony. The only parameter that affected the oscillatio n frequency of the network was the strength of the external driving in put which could shift the frequency between 35 to 60 Hz. The same resu lts were obtained using a model column of 1000 neurons with a connecti on density of 5%, except that the oscillation became more regular. Syn chronization between cortical columns was studied in a model consistin g of two columns with 100 model neurons each. When connections were ma de with a density of 3% between the pyramidal cells of each column the re was no inter-columnar synchrony and in some cases the columns oscil lated 180 degrees out of phase with each other. Only when connections from the pyramidal cells in each column to the inhibitory cells in the other column were added was synchrony between the columns observed. T his synchrony was established within one or two cycles of the oscillat ion and there was on average less than 1 ms phase difference between t he two columns. Unlike the intra-columnar synchronization, the inter-c olumnar synchronization was found to be sensitive to the synaptic dela y: A mean delay of greater than 5 ms virtually abolished synchronizati on between columns.