Zwei interessante Unterschiede zu Unterschiede beim Sehen zwischen Männern und Frauen.
1. Das räumlich-zeitliche Denken
Die erste Studie behandelt das räumlich-zeitliches Denken:
Cerebral cortex has a very large number of testosterone receptors, which could be a basis for sex differences in sensory functions. For example, audition has clear sex differences, which are related to serum testosterone levels. Of all major sensory systems only vision has not been examined for sex differences, which is surprising because occipital lobe (primary visual projection area) may have the highest density of testosterone receptors in the cortex. We have examined a basic visual function: spatial and temporal pattern resolution and acuity.
We tested large groups of young adults with normal vision. They were screened with a battery of standard tests that examined acuity, color vision, and stereopsis. We sampled the visual system’s contrast-sensitivity function (CSF) across the entire spatio-temporal space: 6 spatial frequencies at each of 5 temporal rates. Stimuli were gratings with sinusoidal luminance profiles generated on a special-purpose computer screen; their contrast was also sinusoidally modulated in time. We measured threshold contrasts using a criterion-free (forced-choice), adaptive psychophysical method (QUEST algorithm). Also, each individual’s acuity limit was estimated by fitting his or her data with a model and extrapolating to find the spatial frequency corresponding to 100% contrast.
At a very low temporal rate, the spatial CSF was the canonical inverted-U; but for higher temporal rates, the maxima of the spatial CSFs shifted: Observers lost sensitivity at high spatial frequencies and gained sensitivity at low frequencies; also, all the maxima of the CSFs shifted by about the same amount in spatial frequency. Main effect: there was a significant (ANOVA) sex difference. Across the entire spatio-temporal domain, males were more sensitive, especially at higher spatial frequencies; similarly males had significantly better acuity at all temporal rates.
As with other sensory systems, there are marked sex differences in vision. The CSFs we measure are largely determined by inputs from specific sets of thalamic neurons to individual neurons in primary visual cortex. This convergence from thalamus to cortex is guided by cortex during embryogenesis. We suggest that testosterone plays a major role, leading to different connectivities in males and in females. But, for whatever reasons, we find that males have significantly greater sensitivity for fine detail and for rapidly moving stimuli. One interpretation is that this is consistent with sex roles in hunter-gatherer societies.
Viele Testosteronunterschiede sind ein Zeichen dafür, dass in einem bestimmten Bereich Geschlechtsunterschiede auftreten. Hier wurde also gerade Bewegung deutlicher von Männern wahrgenommen. Aus der Studie:
Figure2 shows that females and males have similar sensitivities at low spatial frequencies; but with increasing spatial frequency, males’ higher sensitivity becomes more and more apparent. (An earlier study that used a much more restricted range of spatial and temporal frequencies, found a similar pattern of male–female differences ) The higher sensitivity of males at middle and high spatial frequencies may be common to mammals: the same pattern of sensitivities has been found, using behavioral techniques, in hooded rats
Auch hier bestehen also keine Unterschiede in allen Bereichen, sondern diese zeigen sich mit ansteigenden Anforderungen.
Und die evolutionäre Herleitung:
A plausible reason for sex-differences in spatio-temporal resolution stems from the period when hominids “descended from the trees” and ventured onto the savannas of Africa. Dwellers in forested regions have limited distances over which they must detect and identify objects, be they predators, enemies, or food. On open plains, however, the views are much longer. This places a premium on acuity, for early detection; we reiterate that we find the largest effect sizes at the high spatial frequencies and that our extrapolated acuity data show that males have a 10% advantage in acuity.
The sex-differences in vision might relate to different roles of males and females of early hunter-gatherers; males, being generally larger and more powerful, would have to detect possible predators or prey from afar and also identify and categorize these objects more easily. It is noteworthy that sensitivities to low spatial frequencies is enhanced by temporal modulation; in the real world, retinal images are rarely stationary – objects move and the observer moves.
Evidence for the “hunter-gatherer hypothesis” can be found in studies of visuo-spatial abilities of existing hunter-gatherers: a large meta-analysis of such studies showed that in general males performed better than females . Furthermore, there are significant sex differences in “near-vision” and “far-vision”: males are generally better for accurately perceiving and estimating sizes of targets in far-space . In monkeys there are different populations of cortical neurons dealing with eye-hand coordination and perception of objects in near- or far-space . And this perceptual dichotomy may be related to different neuronal populations associated with the “ventral and dorsal pathways” from primary visual cortex to the higher cortical areas that process visual information. The ventral pathway is from primary visual cortex to infero-temporal cortex and deals with vision-for-identification; the dorsal pathway to the parietal lobe deals more with spatial localization, or vision-for-action [82,83]. Parenthetically, however, we should note that this dorsal/ventral dichotomy is not quite as clear as originally posited (e.g. [84,85]).
Much of the research on sex-effects among hunter-gatherers implicitly assumes that the differences are congenital. However, it has been reported that there is a significant sex-by-age interaction in infants, indicating important maturational factors : females have higher sensitivity at the peak of the CSF at six months, but not at four and eight months – it seems that the sex-related differences we find (overall male superiority) may not be true from birth.
The hunter-gatherer hypothesis correctly predicts that adult males will perform better for targets in far-space – the hunter must perceive and correctly aim at more distant targets – while females will be better for near-space – arguing that they are the gatherers and foragers for nearby foods . Our findings seem to fit this model: males indeed are much more sensitive to high spatial frequencies. However, the sensitivity difference at low spatial frequencies is relatively small and indeed females may do better for static or slowly moving targets (see Figure2), which would accord with attending to nearby, stationary objects.
Das könnte auch erklären, warum Männer gerade Ballsportarten so gerne mögen.
Eine weitere Studie behandelt Farbunterschiede:
Because cerebral cortex has a very large number of testosterone receptors, we examined the possible sex differences in color appearance of monochromatic lights across the visible spectrum. There is a history of men and women perceiving color differently. However, all of these studies deal with higher cognitive functions which may be culture-biased. We study basic visual functions, such as color appearance, without reference to any objects. We present here a detailed analysis of sex differences in primary chromatic sensations.
We tested large groups of young adults with normal vision, including spatial and temporal resolution, and stereopsis. Based on standard color-screening and anomaloscope data, we excluded all color-deficient observers. Stimuli were equi-luminant monochromatic lights across the spectrum. They were foveally-viewed flashes presented against a dark background. The elicited sensations were measured using magnitude estimation of hue and saturation. When the only permitted hue terms are red (R) yellow (Y), green (G), blue (B), alone or in combination, such hue descriptions are language-independent and the hue and saturation values can be used to derive a wide range of color-discrimination functions.
There were relatively small but clear and significant, differences between males and females in the hue sensations elicited by almost the entire spectrum. Generally, males required a slightly longer wavelength to experience the same hue as did females. The spectral loci of the unique hues are not correlated with anomaloscope matches; these matches are directly determined by the spectral sensitivities of L- and M-cones (genes for these cones are on the X-chromosomes). Nor are there correlations between loci of pairs of unique hues (R, Y, G, B). Wavelength-discrimination functions derived from the scaling data show that males have a broader range of poorer discrimination in the middle of the spectrum. The precise values for all the data depend on whether Newtonian or Maxwellian optics were used, but the sex differences were the same for both optical systems.
As with our associated paper on spatio-temporal vision, there are marked sex differences in color vision. The color-appearances we measured are determined by inputs from thalamic neurons (LGN) to individual neurons in primary visual cortex. This convergence from LGN to cortex is guided by the cortex during embryogenesis. We hypothesize that testosterone plays a major role, somehow leading to different connectivities for males and females: color appearance requires a re-combination and re-weighting of neuronal inputs from the LGN to the cortex, which, as we show, depends on the sex of the participant.
Aus einer Besprechung der Studie:
But when the researchers tested color vision in one of two ways—by projecting colors onto frosted glass or beaming them into their subjects’ eyes— women proved slightly better at discriminating among subtle gradations in the middle of the color spectrum, where yellow and green reside. They detected tiny differences between yellows that looked the same to men. The researchers also found that men require a slightly longer wavelength to see the same hue as women; an object that women experience as orange will look slightly more yellowish to men, while green will look more blue-green to men. This last part doesn’t confer an advantage on either sex, but it does demonstrate, Abramov says, that “the nervous system that deals with color cannot be wired in the exact same way in males as in females.” He believes the answer lies in testosterone and other androgens. Evidence from animal studies suggests that male sex hormones can alter development in the visual cortex.
While Abramov has an explanation for how the sexes see differently, he’s less certain about why. One possibility—which he cautions is highly speculative—is that it’s an evolutionary adaptation that benefited hunter-gatherer societies: Males needed to see distant, moving objects, like bison, while females had to be better judges of color when scouring for edible plants.