The effects of parvocellular lateral geniculate lesions on the acuity and contrast sensitivity of macaque monkeys

The effects of ablating the visual pathway that passes through the parvocellular (dorsal) LGN were tested in 2 macaque monkeys by measuring acuity and both luminance and chromatic contrast sensitivity. Thresholds were tested monocularly before and after ibotenic acid was used to lesion parvocellular layers 4 and 6 of the contralateral geniculate. The injections were centered at the representation of 6 degrees in the temporal field on the horizontal meridian, and vision was tested with localized stimuli at this location. In addition, in one of the monkeys, a lesion was made in magnocellular layer 1 of the opposite geniculate, and the same thresholds were tested. Physiological and anatomical reconstructions demonstrated complete destruction of the target layers in 1 parvocellular lesions and in the magnocellular lesion, and sparing of the nontarget layers in the tested region. Parvocellular lesions caused a 3-4-fold reduction in visual acuity within the affected part of the visual field, while the magnocellular lesion did not affect acuity. Both luminance and chromatic contrast sensitivity, tested with stationary gratings of 2 c/degree, were severely reduced by parvocellular lesions, but not affected by the magnocellular lesion. However, when luminance contrast sensitivity was tested with 1 c/degree gratings, counterphase modulated at 10 Hz, it was reduced by both parvocellular and magnocellular lesions. This study demonstrates that the parvocellular pathway dominates chromatic vision, acuity, and contrast detection at low temporal and high spatial frequencies, while the magnocellular pathway may mediate contrast detection at higher temporal and lower spatial frequencies.