Responses of neurons in macaque MT to unikinetic plaids
Authors: Pascal Wallisch
J Anthony Movshon
Journal: Journal of Neurophysiology (2019)
Discipline: NEUROSCIENCE & BEHAVIOR
Keywords: MT; direction selectivity; electrophysiology; motion integration; motion perception.
Affiliations: Center for Neural Science, New York University, New York, New York.

Abstract:  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 &amp; 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. 

Retracted: No

PubPeer comments:
  The specificity of neuronal responses is limited by what is known as the u201caperture problemu201d u2013 if only a single oriented contour of a moving object is in the receptive field (RF), the neuron can only signal motion orthogonal to the orientation of the contour.  An individual grating is perceived as moving orthogonal to its orientation. The above statements are at best misleading and at worst false. As I have noted in several PubPeer comments and a related [blog post] , it is false to imply that the visual system solves the  aperture problem by producing a percept of  contours moving orthogonal to their orientation. The claim is, for example, contradicted by the well-known barberpole illusion. The claim does apply when the aperture is a circle, and from the figures provided, this is the case in question. But - just as in the case of Adelson Movshon (1982), the authors seem to be treating this special case as the general case for the purpose of drawing general conclusions about neural function (as illustrated in the first quote above). Oddly enough, the authors do bring up the barber-pole illusion, saying that the perceptual effects of their  unikinetic plaids  are conceptually similar to barberpole stimuli. In fact, the barberpole illusion, with its simple stripes, is *conceptually* identical to the  individual grating stimuli, not to the  unikinetic stimuli. As such, the perceived motion in the barberpole violates prediction; the stripes do not appear to move in a direction orthogonal to their orientation. **What if the authors were to use, e.g. rectangular apertures to present their  unikinetic stimuli?** Would the outcomes be *as different* as they are when we change the aperture shape from round to rectangular in the case of  individual gratings ? Would the stimuli remain  conceptually similar to the barberpole illusion? Would the neural speculations (at least to the authors satisfaction) still fit? The authors unqualified references to a neuron s  receptive field are also theoretically problematic. The term currently has no theoretical description, as the older conception has long proved invalid and is referred to as  the classical receptive field. De facto definitions may be found in the methods sections of contemporary papers, each set of authors ascertaining its characteristics via multifarious ad hoc procedures. I ve discussed the problems with the concept in other PubPeer posts, including one on the highly-cited Olshausen and Field (1996), and comments linked therein. Wallisch and Movshon cite the Simoncelli and Heeger s (1998)  A model of neuronal responses in visual area MT as a kind of ideal case:  ...few neurons exhibit the idealized pattern of responses that could be expected from the 425 Simoncelli-Heeger model. Simoncelli and Heeger, in their turn, appear to base their model on the same false claim discussed at the beginning of this comment. They cite Adelson and Movshon in the following way:  The V1 neurons described thus far are not selective for stimulus velocity. Rather, they are selective only for the component of velocity orthogonal to their preferred spatial orientation (Adelson Movshon, 1982). Adelson and Movshon (1982) did not come into contact with any neurons; their experiments were psychophysical. Thus Simoncelli-Heeger based their model on a false fact and a mischaracterization of Adelson and Movshon, and, twenty years later, Wallisch and Movshon treat this model as a baseline. ,