Extrageniculostriate vision in the monkey: Discrimination of luminous flux-equated figures

Six monkeys (Macaca mulatta) were tested before and after histologically confirmed total removal of the striate area and complete degeneration of the lateral geniculate nuclei. Using Klüver's pulling-in technique with nonmovable transillu-minated targets, two discrimination problems were given at both a high and a low photometric level. In the first test, the targets were of equal area and differed in luminance and total luminous flux (B-F problem). In the second test, figures differed in luminance and area in an inverse proportion ( B A problem) which resulted in their being closely matched for total luminous flux. Postoperatively, the animals showed significant deficits on all tests. Every monkey, however, achieved criterion level of performance not only on the B-F but on the B A problem as well. Performance on the latter test did not change after dilatation of the pupils, or when minimal flux differences were eliminated as cues by making the rewarded target to have the lesser flux in half of the trials and the greater flux in the other half. These findings indicate that monkeys can be trained to discriminate between stimuli equated for total luminous flux after exclusion of the geniculostriate system. Although it is possible that other stimulus parameters are discriminated by such animals, our results do not necessarily negate Klüver's original concept. Indeed, destriated monkeys may still utilize a luminous flux cue to discriminate flux-equated figures through the detection of differences in the rate of change of this parameter during eyes, head and/or body movements.