Behavioral detectability of single-cell stimulation in the ventral posterior medial nucleus of the thalamus.

In mammals, most sensory information passes through the thalamus before reaching cortex. In the rat whisker system, each macrovibrissa is represented by approximately 250 neurons in the ventral posterior medial nucleus (VPM) of the thalamus and approximately 10,000 neurons in a cortical barrel column. Here we quantify the sensory impact of individual thalamic neurons in the rat VPM. We first trained animals to report microstimulation of VPM. All animals learned to report microstimulation currents of 2-5 microA. We then evoked action potentials (APs) in single thalamic neurons close to the microstimulation site using juxtacellular stimulation, adding on average 17.8 APs to 2.6 spontaneous APs during 200 ms current applications. A population analysis revealed that animals responded equally often in single-cell stimulation trials as in catch trials without stimulation, suggesting that APs of single thalamic cells in VPM lead to either no or only a very weak perceptual effect. These results are surprising given the relatively small number of VPM neurons and our previous observations that single neurons in other parts of the vibrissal system do have an impact on perception or motor output. Our findings therefore suggest that neural representations in whisker thalamus are more distributed than in other whisker-related structures.