Single Cortical Column in Area 17: The Distribution of Intrinsic Connections

Abstract

Intrinsic connections are one category of neuronal cortical connections, referring to axons that remain within the same cortical area and do not travel in the white matter. Our knowledge about these connections is based on extrapolations of results on the labelling of cells in different cortical layers, but there is no direct information about the complete connectivity of a single cortical column. To study how the population of cells sending axons to the same cortical column is distributed over the cortex we microiontophoretically injected horseradish peroxidase in single cortical columns of area 17 in cat. 3-D reconstruction of the region of labelled cells was performed by using serial frontal brain sections. It was shown that afferents from layer IV cells are short range (up to 0.5 mm) and from the supragranular (II, III) and the infragranular (V, VI) layer cells are long range (up to 5 mm). The regions with labelled cells in the supragranular and infragranular layers sending axons to the very same cortical column lie in register. For columns representing the central visual field, the distribution of labelling (in a tangential plane) is elongated in the mediolateral direction, and for peripheral columns there was a tendency towards elongation in the mediolateral and rostrocaudal directions. Moreover, the majority of labelled cells were located in regions representing peripheral parts of the visual field relative to the injection site. The described connections may represent the substrate for global linking tasks and underlie several psychophysical phenomena, such as meridional and peripheral effects.