Responses of neurons in macaque MT to unikinetic plaids

Response properties of MT neurons are often studied with “bikinetic” plaid stimuli, which consist of two superimposed sine wave gratings moving in different directions. Oculomotor studies using “unikinetic plaids” in which only one of the two superimposed gratings moves suggest that the eyes first move reflexively in the direction of the moving grating and only later converge on the perceived direction of the moving pattern. MT has been implicated as the source of visual signals that drives these responses. We wanted to know whether stationary gratings, which have little effect on MT cells when presented alone, would influence MT responses when paired with a moving grating. We recorded extracellularly from neurons in area MT and measured responses to stationary and moving gratings, and to their sums: bikinetic and unikinetic plaids. As expected, stationary gratings presented alone had a very modest influence on the activity of MT neurons. Responses to moving gratings and bikinetic plaids were similar to those previously reported and revealed cells selective for the motion of plaid patterns and of their components (pattern and component cells). When these neurons were probed with unikinetic plaids, pattern cells shifted their direction preferences in a way that revealed the influence of the static grating. Component cell preferences shifted little or not at all. These results support the notion that pattern-selective neurons in area MT integrate component motions that differ widely in speed, and that they do so in a way that is consistent with an intersection-of-constraints model. NEW & NOTEWORTHY Human perceptual and eye movement responses to moving gratings are influenced by adding a second, static grating to create a “unikinetic” plaid. Cells in MT do not respond to static gratings, but those gratings still influence the direction selectivity of some MT cells. The cells influenced by static gratings are those tuned for the motion of global patterns, but not those tuned only for the individual components of moving targets.

Feature Contribution to Motion Signal Predicted by Magnitude of Intersection-of-Constraints Projection

Yo and Wilson (1992 Vision Research32 135 – 147) reported that Type II plaids move in the vector sum (VS) direction at short durations. Bowns ( Vision Research in press) showed that the result did not generalise to all Type II plaids. Computational analysis of the stimuli revealed an alternative explanation of the result in terms of features that were shown to move in a similar direction to the VS. When features (or VS) moved in a different direction to the intersection of constraints (IOC) the plaids were often perceived to move in the direction of the feature. It is hypothesised that a feature will contribute only when there is sufficient projection of the IOC in the feature direction. The direction of the features and the IOC for the stimuli used by Bowns was computed for a set of plaids that shifted from being perceived in the feature direction to the IOC. If projection is critical, then it should decrease as the stimuli shift direction. This was confirmed. A different set of plaids comprising a feature that moved in the opposite direction to the IOC and varied in terms of the magnitude of projection was presented to subjects in a forced-choice task. The plaids moved as predicted from the hypothesis. These results show a correlation between the amount of projection of the IOC in the direction of a feature and the incidence of perceived movement in the direction of the feature. The hypothesis that perceived movement of a plaid is influenced by the degree of projection of the IOC onto another available motion signal such as a displaced feature is supported.