In response to my post on Leonard Sax's account of the "emerging science" behind the claim that "Girls and boys ... see the world differently", David Hilbert sent some additional information. David argues that Sax's account of sex differences in vision is even more misleading, and also more internally incoherent, than I had understood. Below, I've reproduced David's note, with his permission.
We're getting pretty far away from speech and language, it's true, but this started because David Brooks picked up on Sax's misleading description of some shaky findings on sex differences in perception of emotional faces, and concluded that boys and girls need to be given different books to read. I've also just discussed Sax's claim that "Girls and boys... hear differently", and therefore need different teaching styles and different classroom environments.
[Guest post by David Hilbert]
In reading your discussion of Sax's book I was struck by his purported explanation for the claimed difference in retinal thickness: "That's because the male retina has mostly the larger, thicker M cells while the female retina has predominantly the smaller, thinner P ganglion cells." Although I'm a philosopher, not a visual scientist, my research does require basic knowledge of retinal anatomy and physiology and I know that there are 7-9 times as many P-cells as M-cells in the primate retina. (My source here is a useful textbook, Wandell, B. A. (1995). Foundations of Vision. Sunderland, MA, Sinauer Associates, pp 121-122.) This statistic is hard to reconcile with the claimed predominance of M-cells in males. I have been unable to find any authoritative source for the claim that there are sex-differences in the ratio of M-cells to P-cells although I have found it repeated elsewhere. The only evidence I have found offered is to work backwards from differences in performance between males and females to claims about their inferred physiological basis. These kinds of inferences are notoriously tricky and I have not been impressed with the reasoning I have encountered. I went to Borders to look at Sax's book, I don't want to buy it, and the only source in the vicinity of the claim is the second of the rat papers. The only relevant material in that paper is the claim that, "cytoarchitectonic studies are warranted to determine the number, type and density of cells as well as the thickness of each of the cellular layers before such hypotheses can be validated." (p. 135) In other words they don't know the reason for the differences in retinal thickness that they measured.
I was able to find a paper that had data on the area of the magnocellular and parvocellular layers of the lateral geniculate nucleus of the thalamus in humans broken down by sex. (Andrews, T. J., S. D. Halpern, et al. (1997). Correlated Size Variations in Human Visual Cortex, Lateral Geniculate Nucleus, and Optic Tract. J. Neurosci. 17(8): 2859-68.) These layers in the LGN are the recipients of the projection from the M-cells and P-cells in the retina. Although the paper doesn't discuss sex differences I did compute the average ratio of the areas of the M- and P-layers for all subjects, female subjects and male subjects. The overall ratio was .324; for females .329 and for males .318. This very small sample (10 female hemispheres, 9 male hemispheres), contradicts Sax's claim although the difference is probably not significant. I am not an expert on this topic and there may be better data that I have not been able to find.
The citation to rat data for this type of claim is more irresponsible than you indicate. It would be hard to pick a sighted mammal with a visual system more different from the human one than the rat. Rats are nocturnal animals with rod dominated retinae. The human retina is cone-dominated and also is characterized by a number of specializations found only in primates. Among these are the P-cells which are not found in mammals other than primates (although there are analogues in the cat which partly explains why cats provide a much more useful model for human vision). The most bizarre claim in Sax's discussion of vision is found in the table on the page following the one you quote from. There he asserts that the M-cells receive input from the rods while the P-cells receive cone inputs. Notice that given his early claim about sex differences this would mean that males have primarily rod-driven vision. We now have an explanation for the insistence of men on wearing their sunglasses indoors.
[Guest post by David Hilbert]
[Here's the table that David is referring to, from p. 22 of Leonard Sax's book Why Gender Matters:
|P CELLS||M CELLS|
|Are wired predominantly to . . .||Cones||Rods|
|Are located mostly in . . .||The center of the retina (center of the field of vision)||All throughout the retina (entire field of vision, peripheral and central)|
|Are best adapted to detect . . .||Color and texture||Location, direction, and speed|
|Answer the question:||"What is it?"||"Where is it now? Where is it going? How fast is it moving?"|
|Ultimately project to:||Inferior temporal cortex||Posterior parietal cortex|
(more P cells than M cells)
(more M cells than P cells)
(Warning to unwary readers: some of the "facts" in this table are clearly false, and none should be relied on without checking in more reliable sources.)
I'd like to point out that Sax's presentation of this material is getting very wide distribution, and is having a significant impact on the way that serious people think about educational policy today. ]
[Update 5/20/2008 -- see Dr. Leonard Sax's response here, and some further discussion by me at
]Posted by Mark Liberman at August 22, 2006 10:33 AM