Functional Convergence of Thalamic and Intrinsic Projections to Cortical Layers 4 and 6

Ascending sensory information is conveyed from the thalamus to layers 4 and 6 of the sensory cortical areas. Interestingly, receptive field properties of cortical layer-6 neurons differ from those in layer 4. Do such differences reflect distinct inheritance patterns from the thalamus, or are they derived instead from local cortical circuits? To distinguish between these possibilities, we utilized in vitro slice preparations containing the thalamo-cortical pathways of the auditory and somatosensory systems. Responses from neurons in layers 4 and 6 that resided in the same column were recorded using whole-cell patch clamp. Laser-scanning photostimulation via uncaging of glutamate in the thalamus and cortex was used to map the functional topography of thalamo-cortical and intracortical inputs to each layer. In addition, we assessed the functional divergence of thalamo-cortical inputs by optical imaging of flavoprotein autofluorescence. We found that the thalamo-cortical inputs to layers 4 and 6 originated from the same thalamic domain, but the intracortical projections to the same neurons differed dramatically. Our results suggest that the intracortical projections, rather than the thalamic inputs, to each layer contribute more to the differences in their receptive field properties.