The receptive field properties of layer IV simple cells in cat visual cortex are largely determined by specific connections from the lateral geniculate nucleus of the thalamus. Thalamic inputs, however, make up only a relatively small percentage of the synapses onto simple cells. This raises the question: how can a small number of synapses play a dominant role in the responses of cortical neurons? We have found that neighboring thalamic cells with similar and overlapping receptive fields often fire precisely synchronized spikes, within one millisecond of each other. This may provide a mechanisms to strengthen the relatively spare thalamic input to cortex.
To assess the importance of this synchronized firing, we have recorded from a cortical neuron simultaneously with two and sometimes three presynaptic neurons in the thalamus. In doing so, we were able to compare the efficacy of the thalamic inputs when they occurred in isolation and when they occurred in pairs. The efficacy of the nearly simultaneous spikes was on average of greater than the sum of the efficacies of each thalamic neuron in isolation. The greater efficacy of the simultaneous spikes suggests that the thalamic inputs are not independent, but have a synergistic effect on the cortical target. This synergy could be produced by nonlinear biophysical interactions between two simultaneous EPSPs, by the cooperative effect of a larger presynaptic network, or by a high threshold for firing the postsynaptic neuron.