Networks with lateral connectivity. II. Development of neuronal grouping and corresponding receptive field changes.

1. Using a three-layered network model defined in the previous paper, we studied the basic features of neurons in the cortical layer while the synaptic strengths of lateral excitatory connections were made modifiable by a Hebbian learning rule and a normalization process. 2. We found that neurons in the cortical layer formed groups through their lateral excitatory connections after the network was trained with sequential random dot stimulations. Neurons within a group connected tightly; neurons in different groups connected weakly. 3. The effects of model parameters and input parameters on the formation of neuronal groups were investigated. Results showed that the average size and rough shapes of groups were mainly determined by the spatial distribution of lateral connections within the cortical layer, irrespective of input parameters and training methods. Thus groups are structure dependent. 4. Lateral inhibition in the network is the only key factor that affects the grouping of neurons. Without an appropriate amount of distant inhibition, group formation does not occur. Group formation is very robust to all other parameters we tested. On the other hand, group locations are very easily disturbed by inputs or changes of parameters, suggesting that such neuronal groups are dynamically maintained. 5. With the development of neuronal groups, neurons can be divided into two response types. TN-1 neurons respond weakly to inputs and have small receptive fields or do not respond at all (silent); TN-II neurons, approximately 30-40% of all, respond strongly to inputs and have large receptive fields. The two types of neurons also differ with respect to response threshold and temporal firing patterns. After groups formed, receptive fields of TN-II neurons within the same group clustered spatially with high overlap, whereas receptive fields of TN-I neurons with detectable responses shifted systematically with the neuron's spatial location. 6. The two types of neurons are homogeneously distributed across the cortical layer. The population of each type of neuron produces a full representation of the input layer with weak or strong responses, respectively. 7. We concluded that neurons in the cortical network naturally assembled into functional groups. Such groups are dynamic and amenable to change by input stimuli. A fraction of neurons (30-40%) within the same group shares a similar receptive field and strongly respond together to stimuli, so that the network has more robust response to inputs. On the other hand, the responses of a large portion (60-70%) of neurons become weak or silent: these neurons are available for other (unknown) functional purposes.