Chromatic properties of neurons in macaque MT

Abstract

We have studied the responses of MT neurons to moving gratings, spatially modulated in luminance and chromaticity. Most MT neurons responded briskly and with high contrast sensitivity to targets whose luminance was modulated, with or without added chromatic contrast. When luminance modulation was removed and only chromatic stimulation was used, the responses of all MT neurons were attenuated. Most were completely unresponsive to stimulation with targets whose modulation fell within a "null" plane in color space; these null planes varied from neuron to neuron, but all lay close to the plane of constant photometric luminance. For about a third of the neurons, there was no color direction in which responses were completely abolished; almost all of these neurons had a definite minimum response for chromatic modulation near the isoluminant plane. MT neurons that responded to isoluminant targets did so inconsistently and with poor contrast sensitivity, so that only intensely modulated targets were effective. Whereas the best thresholds of MT neurons for luminance targets are close to behavioral contrast threshold, the thresholds for isoluminant targets lie considerably above behavioral contrast threshold. Therefore, although some MT neurons do give responses to isoluminant targets, they are unlikely to be the source of the chromatic motion signals revealed behaviorally.

Processing of first- and second-order motion signals by neurons in area MT of the macaque monkey

“…On the one hand, we feel secure in asserting that MT does not appear to function as a homogeneous "cue-invariant" motion analyzing system, which processes information about all kinds of motion. Not only do second-order stimuli fail to elicit reliable direction-selective responses from most MT cells, but we have previously shown that chromatically modulated targets similarly fail to elicit a vigorous selective response in these cells (Gegenfurtner et al, 1994). But to establish what MT, globally, does not do, does not answer the question of where and how second-order motion signals are processed.…”

Uneven integration for perception and action cues in children's working memory

“…Our "dorsal" action cue may therefore activate both dorsal-stream representations guiding a retrieval action, and ventral-stream representations that perceptually recognise a distinctive feature such as the hook. Likewise, although our "ventral" colour cue is likely to be irrelevant for dorsal-stream action planning, there is evidence for some processing of colour in the dorsal stream (Gegenfurtner et al, 1994). Therefore we note that our cues (or indeed any cues) are unlikely to elicit purely ventral or dorsal stream representations.…”

Colour in action: evidence for a redundancy signal effect when driving motor responses by combined colour and spatial cues

“…They found that only targets that changed spatial location elicited automated limb modifications, and therefore only cues from the dorsal pathway seem to contribute. However, their results may be explained by the recent physiological and psychophysical findings which show that neurons in the dorsal pathway (area MT) respond to high chromatic contrast (Gegenfurtner et al, 1994;Schiller & Colby, 1983, see also . Therefore, it is possible for the motor system to take advantage of a colour signal in the dorsal motor pathway.…”

“…However, these studies do not distinguish between the detection of a chromatic contrast and the perception of colour. Whereas the former is possible based solely on dorsal signals (Gegenfurtner et al 1994;, the latter is presumed to be a ventral domain. We distinguish between the two by having the subjects make decisions based on the perceived colour of a stimulus, rather than the mere presence of a chromatic contrast.…”