Convergence of magno- and parvocellular pathways in layer 4B of macaque primary visual cortex

EARLY visual processing is characterized by two independent parallel pathways: the magnocellular stream, which carries information useful for motion analysis, and the parvocellular stream, which carries information useful for analyses of shape and colour(1-3). Although increasing anatomical and physiological evidence indicates some degree of convergence of the two streams, the pathway through layer 4B of primary visual cortex (V1) and on to higher cortical areas is usually considered to carry only magnocellular input(1-3). This is inferred from anatomical descriptions of local circuitry in V1, and functional studies of area MT, which receives input from layer 4B(4-7). We have directly measured the sources of local functional input to individual layer 4B neurons by combining intracellular recording and biocytin labelling with laser-scanning photostimulations(8-10). We found that most layer 4B neurons receive strong input from both magnocellular-stream-recipient layer 4C alpha neurons and parvocellular-stream-recipient layer 4C beta neurons. Thus higher cortical areas that receive input either directly or indirectly from layer 4B are likely to be more strongly influenced by the parvocellular pathway than previously believed.