Magnification functions and receptive field sequences for submodality-specific bands in SI cortex of cats.

Electrophysiological data were collected from the forelimb region of somatosensory cortex in barbiturate-anesthetized cats using low-impedance microelectrodes in long slanting trajectories. Subsequently, the brains were fixed and stained with thionin to locate the electrode trajectories and to correlate cytoarchitecture with neural activity. Confirming earlier experiments, regions of cortex, preferentially responsive to one submodality of afferent input, were observed to stretch in mediolateral bands across SI. Separate magnification functions were calculated for each band. In the forelimb region the magnification functions for the deep and cutaneous RA bands could be approximated by linear functions while the data for the cutaneous SA band was best described by a second-order equation. At least in the forelimb region this magnification function applied only along the anteroposterior dimension of the band because the body representation is anisotropic; along the anteroposterior dimension of the band, receptive field loci may change only millimeters on the skin when the electrode moves through 1 mm of cortex, whereas when an electrode moves in a mediolateral direction they change centimeters on the forearm for an equivalent cortical distance. Each representation is separated from the others by a transition zone having a minimal width less than 200 micron. Within the transition zones, neurons often have receptive field loci which are a compromise between loci found in the adjacent representations. In these zones responses are often difficult to elicit. Electrode penetrations encountering these transition zones contain data which fulfill the receptive field and modality criteria for the boundary of a cortical column. However, within the mediolateral length of each submodality band we have found no unit with a dimension greater than 200 micron which has a boundary or transition zone. Evidence for units of smaller size cannot be obtained because of limitations in the data collection techniques. Thus, within each submodality-specific band, large portions of the somatotopic map appear to be a mediolateral continuum while relatively abrupt changes occur in receptive field locus and in submodality when the electrode passes through a transition zone between adjacent bands.