Laminar differences in sizes, shapes, and response profiles of cutaneous receptive fields in the rat SI cortex

Quantitative techniques were used to demonstrate cortical layer differences in cutaneous receptive fields (RF's) in the rat SI cortex. Two- and three-dimensional (2-D and 3-D) RF maps were constructed showing the responsiveness of single neurons to standardized punctate stimulation of each of a matrix of points on the skin or the mystacial vibrissa pad. These allowed a visualization not only of the overall sizes of such RF's, but also their shape and "response profile". Initial experiments showed that the sizes and response profiles of such RF's were similar whether they were mapped by sinusoidal mechanical vibration of skin, punctate touch, or direct intracutaneous electrical stimulation. This method was used to quantitatively determine distoproximal lengths of RF's of single units recorded at different depths in the forepaw area of the SI cortex. Plots of these RF lengths as a function of cortical depth showed that the smallest RF's were found in the granular layers (IV and deep III). RF's up to double that size were found in supragranular layers, and up to triple that size in infragranular layers. 3-D maps of RF's in the granular layers showed sharp central response peaks surrounded by very steep dropoffs to the RF boundaries. In the whisker areas, granular layer RF's were typically circular in shape and contained from 1-4 whiskers. By contrast, in supragranular layers they were often elongated in shape, and were oriented along rows or columns of whiskers. RF's in layer V resembled large, high plateaus, often supporting clearly separated peaks. RF's mapped in the fore- and hindpaw areas were similar, but, even in the granular layers, were often slightly elongated along the limb axis. In all regions of the SI, both the locations and shapes of the granular layer RF's appeared to be conserved as subsets of other more topographically heterogeneous RF's encountered elsewhere in the column. These findings may correlate with patterns of axonal connectivity in the rat SI.