Correlative single-unit mapping and neuroanatomical tracing was used to test a hypothesis that the input to a place- and modality-specific column of the monkey somatosensory cortex consists of a bundle of axons arising from an elongated, rodlike aggregation of relay cells extending through much of the anteroposterior length of the thalamic ventrobasal complex and receiving their inputs from a similarly elongated, place- and modality-specific aggregation of medial lemniscal axons. Microelectrodes introduced into the monkey VPLc or VPM nucleus horizontally from behind commonly encounter sequences of single units that have virtually identical receptive-field characteristics (in terms of place and modality) over distances of 500-800 μm. Microelectrodes introduced vertically from above encounter units with a systematically changing sequence of receptive fields as they descend. But vertical penetrations made at different anteroposterior coordinates often encounter units with the same receptive fields at the same depth. Angled penetrations traversing the nuclei from anterolateral to posteromedial successively encounter units whose receptive fields change rapidly, often over distances as little as 100 μm. Punctate injections of horseradish peroxidase made at electrophysiologically defined sites in the somatosensory cortex retrogradely label narrow, curving rods of cells that extend anteroposteriorly through most of the anteroposterior dimension of VPLc or VPM. Similarly oriented rodlike configurations of medial lemniscal terminations can be demonstrated in VPLc following punctate injection of anterograde tracers at physiologically defined sites in the dorsal column nuclei. The focal nature of the thalamic projection on the somatosensory cortex was further demonstrated by injection of minute amounts of anterograde tracer at defined sites in VPLc or VPM. The foci of thalamocortical terminations form parts of small strips that are particularly evident in sections cut tangential to the previously flattened postcentral gyrus. These results provide an anatomical and physiological confirmation of the hypothesis outlined in paragraph 1.
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