Understanding the sensory neural code involves not only characterizing the neural representation of the sensory input at various stages of processing in the pathway, but also how these representations are transformed between the various brain regions. The thalamocortical circuit in the rat vibrissa system has been utilized as a model system by a number of laboratories to address this question. In the rat vibrissa pathway, cells in the layer IV of the primary somatosensory cortex receive convergent input from multiple cells within the ventral posterior medial (VPM) nucleus of the thalamus, and it is widely believed that interactions between these thalamic cells contribute to the cortical representation. Although the time course of homosynaptic and heterosynaptic interactions in the cat visual pathway has been previously studied, how the spikes from different VPM cells interact in vivo to drive cortical cells is not as well understood in the vibrissa pathway, nor is the full functional consequence of this phenomenon in the context of coding sensory stimuli.

We investigated how neural activity of multiple presynaptic neurons interacted to affect the firing of postsynaptic neurons in the rat vibrissa system. By simultaneously recording single unit activity of multiple cells in the VPM nucleus and barrel cortex of anesthetized Sprague-Dawley rats in response to weak whisker stimulation, we examined interactions between presynaptic spikes of VPM cells which project to the same barrel cell target. For homosynaptic pairs, we asked how multiple closely-timed spikes in a thalamic event interacted in driving a cortical event. For heterosynaptic triplets, we were interested in how spikes from a presynaptic neuron interacted with closely-timed thalamic events from another presynaptic neuron in driving a common cortical target. In our preliminary analyses, we found that an event from a single thalamic neuron with 2 spikes had higher efficacy in driving a cortical response than that with 1 spike, and a thalamic event with 3 spikes had higher efficacy than that with2 spikes. However, the efficacy appears to saturate with increasing numbers of spikes within a thalamic event. Furthermore, preliminary data also suggested that a thalamic event with one or two spikes was statistically more effective in driving the cortical cell when coincident with a spike from another VPM cell also projecting to the same cortical cell target.