ASYMMETRY IN REGARD TO THE EFFECTS OF MAGNOCELLULAR AND PARVOCELLULAR LESIONS

The placing of lesions in the magno- and parvocellular layers of the Lateral Geniculate Nucleus (LGN) of the visual stream has been used in attempts to assess the contributions of the two systems to various visual tasks. However, because there are about ten times as many parvocellular cells as magnocellular cells a lesion blocking the parvocellular input would be expected to have a larger deleterious impact than one blocking the magnocellular input. Thus, a visual task that depends upon all inputs, i.e. which is not linked specifically to either the magno- or parvocellular systems, would be expected to be more severely affected by a lesion in the parvocellular system than by one in the magnocellular system simply on the basis of the number of cells involved. A larger impact of a parvocellular lesion can, therefore, not be taken to mean that the task in question is specifically, or predominantly, linked to this system. Effects following magnocellular lesions (and not observed following parvocellular lesions), on the other hand, cannot be accounted for on the basis of cell number. There is, therefore, an asymmetry, in regard to the significance of the effects of lesions placed in the magno- and parvocellular layers of the LGN.