Adelson & Movshon’s 1982 Nature paper, “Phenomenal coherence of moving visual patterns,” cited to this day, is a good illustration of the difference between science and pseudoscience.
The brand of pseudoscience I will be discussing here has several basic features.
First, it draws general inferences from post hoc observations of special cases while studiously ignoring cases that contradict these inferences, as well as logical inconsistencies within among their theoretical assumptions. The chief characteristic of real science – the ability to make forward-looking predictions that put its assumptions to the test – is lacking. Pseudoscience is a riskless game. When stories built around a set of special cases are perpetuated and elaborated over generations, then we are dealing with a pseudoscientific tradition.
Before going on to show that Adelson & Movshon and their successors are part of such a pseudoscientific tradition, I want to offer an imaginary analogue of how they operate.
Imagine that Martians come to Earth and discover a cylindrical object floating in a pond filled with fish. It happens to be what we call a cork. They then proceed to construct an account of why this object floats. Their explanation hinges on certain features of the particular object – e.g. that it is cylindrical, that it has a little nick on one side, that it is tan-colored, that it has a particular texture, that it’s contains a molecule with a particular structure; and also on certain features of the pond, e.g. that it contains fish. They write up their report, successfully submit it to Martian Nature and advance in their careers.
Naturally, an investigation of a wider set of objects and conditions would expose the Martians’ mistake in treating incidental features of the cork and pond as causal factors underlying the phenomenon of interest. To protect their pet “theory,” the Martians make sure that future studies of flotation always revolve around corks and ponds, on which they make more and more post hoc observations referencing more and more incidental features and around which they construct ever more elaborate ad hoc stories.When a colleague asks, “What about that floating light bulb over there?” they pretend not to hear, or smugly describe their ad hoc theories as “partial explanations.”
It should be obvious that constructing robust, general explanations of why objects float would require much more time, effort, ingenuity and a wider field of view than concocting casual explanations based on easily observed or incidental features of particular cases while failing to examine this explanation critically or acknowledge contradictory evidence. It should also be evident that the stories constructed by these Martians will not figure additively in the construction of the true explanation.
Why the obsession with Gabor patches?
If you’ve ever wondered why vision science for decades seems almost exclusively interested in stimuli consisting of circular areas of light and dark stripes – the famous “Gabor patches” – it is for this reason; to provide perpetual, if thinly cosmetic, cover for the simplistic, fragile, irrational ad hoc stories built around these forms.
The story Adelson & Movshon are offering about motion perception in this paper is such a story.
They begin their abstract by stating that:
When a moving grating is viewed through an aperture, only motion orthogonal to its bars is visible…
The statement is false; it only applies to a narrow set of conditions. A more complete picture is described in a 1935 text cited by Adelson & Movshon:
There exists a tendency for line motion to be perpendicular to line orientation, and also a tendency to perceive motion along one of the cardinal directions of space. Above all, however, the perceived direction of motion runs as parallel as possible to the direction of the edge of the aperture that the line happens to intersect. If the two aperture edges that are intersected simultaneously have different orientations, then the line pursues an intermediate direction; being a subject of psychophysical `self-organization’…” (Wallach, 1935).
Wallach’s is a much more subtle and complex description with profound and difficult theoretical implications.
Adelson and Movshon prefer the more easily digestible, ad hoc version.
Their claim is descriptively correct when the “aperture” is circular. If we were to change the shape of the aperture – to make it rectangular, for example – then Wallach’s general statement would remain true, but A & M’s would fail. To put it another way: Wallach’s claim applies as a general principle over all known cases; Adelson & Movshon’s claim is a description of a special case. General hypotheses founded on it cannot be seriously entertained as explanatory. They are not robust. Scientists respect the phenomena, take on challenges, and actively look for weaknesses in their accounts; pseudoscientists avoid challenges and turn a blind eye to contradictions.
The use of circles and the false statements about the nature of aperture motion continues. We may find the same unqualified claim about orthogonality in a review on motion perception and the brain by Born & Bradley (2005), who state simply:
A moving edge seen through an aperture appears to move perpendicularly to itself…
A very recent example of a study treating employing circular apertures and treating orthogonal motion as the general case are Junxiang et al (2019)/Journal of Neuroscience.
So, a false, simplistic assumption about motion perception has been imported into the neuroscience age and serves as the basis for explanations about brain function; and since neuroscience as a whole has adopted the post hoc/ad hoc approach to theorizing – as a result suffering from a replication crisis that shows no signs of abating, – and has adopted Orwellian language to hide its barrenness, this false assumption remains safe in its pseudoscientific cocoon, and the pseudoscientific tradition it underpins remains strong and has even colonized new lands.
More on Gabors
The stimuli Adelson and Movshon employ (and which are employed generally under the label “Gabor patches”) are not “gratings” consisting of simple lines or bars, with solid black areas alternating with solid white areas. Their colors gradually change from light to dark.
The use of these patterns is linked to another pseudoscientific notion, this one quite bizarre, irrational and, of course, unsupported. It is the idea, to quote Adelson and Movshon, that, “visual analyzing mechanisms selective for…spatial frequency exist at a relatively early stage in the visual pathway.”
The idea that mechanisms analyzing “spatial frequency” of patterns exist at any stage of the visual process is patently absurd. I’ve addressed the reasons why in a separate blog post; among them is the fact that there is no possible utility to such a function, nor is there any conceivable mechanism by which it could be achieved, given that this “analysis” first requires synthesis of the point stimulation on the retina.
The assumption was protected by, first and foremost, being deaf to reason and restricting experimental activity to manipulations of the features of this narrow category of patterns, describing effects post hoc, and and drawing ad hoc inferences presented as general principles.
This is exactly what Adelson and Movshon do here. They are drawing various technical-sounding inferences by acting like the Martians messing around with corks and ponds and pretending to make general discoveries about the nature of floating objects. If you pointed to a floating feather, or a floating light bulb, or a floating corpse, they would be at an utter loss for words. Similarly, if you pointed Adelson and Movshon, even today, to known facts of motion perception – you can read about them in an online translation of a chapter in an 82-year-old text (Metzger, 1937, translation made available by Brandeis University) – these practitioners of pseudoscience would be at a total loss. They would do what they’ve always done, simply look away. It’s worked well so far.
Update #1: Pseudoscientists never lose
It occurred to me after finishing this post that there is a very well-known effect that contradicts the “orthogonal motion” claim, and this is the barberpole illusion. Again, it’s the nature of pseudoscience that gross contradictions go unnoticed or politely ignored.
Update #2: Pseudoscientists never lose redux
Two recent articles illustrate the way that the dogma of “spatial frequency tuning” is protected by its followers. In “Mechanisms of Spatialtemporal Selectivity in Cortical Area MT” (2019/Neuron), Pawar et al describe how this selectivity is contingent on a variety of stimulus “features:”
“…even interactions among basic stimulus dimensions of luminance contrast, spatial frequency, and temporal frequency strongly influence neuronal selectivity. This dynamic nature of neuronal selectivity is inconsistent with the notion of stimulus preference as a stable characteristic of cortical neurons.”
Even when results are all over the place, the spatial tuning concept remains in place – it is now merely described as “unstable.” The idea that neurons are “signalling” spatial frequency via their firing rate but that that firing rate is contingent on a bunch of other factors is even more senseless than the simpler notion.
In “Single-Neuron Perturbations Reveal Feature-Specific Competition in V1,” (2019/Nature), Chettih & Harvey also find instability in their desired correlations:
“Precise levels of decoding accuracy were variable from experiment to experiment, depending on the number and tuning of imaged cells as well as overall signal quality. …This is of note because the tuning bias also causes different grating orientations to be more or less likely to be matched to the tuning preferences of photostimulated neurons.”
Again, the fundamental tenet that neurons are tuned to “spatial frequency,” as irrational as it is, is never questioned, despite needing to be qualified beyond recognition.
Naturally, the data in both papers is correlation-fished and fitted via assumptions – such as linear models, Gaussian “priors” – chosen because they make the math easy, not because of any rationale. None of the authors seems to have considered how their mathematical acrobatics and probability functions can illuminate how we see that elephant over there.