In an extensive review of research by themselves and others—including studies from a wide range of paradigms—Milner and Goodale (2006) provided compelling evidence for their claim that humans and other primates possess a two-part visual system: one part being a ventral pathway whose activities lead to conscious visual experience (the “vision-for-perception” system), and the second being an older, dorsal pathway whose activities guide motor behavior in the absence of awareness (the “vision-for-action” system). To take just one of their examples, Loomis, da Silva, Fujita, and Fukusima (1992) found that subjects made systematic errors in their reports of the relative distances of two points on the ground, but that they could walk with great accuracy to any such point when asked to do so. If we assume that those verbal reports closely tracked perceptual experience, then clearly the sense of distance that guided their motor responses was independent of the distortions that occurred in their conscious experiences of distance.

While this and other observations recruited by Milner and Goodale (2006) involved a contrast between some aspect of experience and motor performance, a second theoretical proposal involved contrasting events within perception itself. That is, it has been suggested that several perplexing phenomena occur because humans possess two, sometimes conflicting, senses of distance—one of which we are aware of, and one of which is not, itself, directly available (although it influences our experience of size; Parks, 2001). For example, in one experiment, subjects who projected an afterimage onto a vertical surface all reported that that projected image increased in size when they backed away from that surface. Of course, this result can be explained by size scaling on the basis of increased distance. Paradoxically, however, most of those same subjects also reported that the image seemed to be moving toward them—which is to say, perceived distance apparently tended to decrease (Day & Parks, 1989)—leading, in part, to the later suggestion that two contradictory estimates of distance were simultaneously registered, one experienced only indirectly, through its effect on size scaling.

Obviously, this hypothesized duality is highly reminiscent of Milner and Goodale’s (2006) proposed coexistence of conscious and unconscious visual streams. Specifically, it may be that the sense of distance of which we are aware is produced within Milner and Goodale’s vision-for-perception stream, while the sense of distance involved in size scaling occurs in their vision-for-action stream.

And indeed, in an entirely independent development, Servos (2006) has since found that “D.F.,” a victim of damage to her vision-for-perception stream, responded quite erratically to a test of consciously registered distance, while retaining the ability to respond systematically and appropriately (albeit with some attenuation) to changes in distance in an afterimage size-scaling task. To this may be added an instance of the opposite pattern, resulting from damage to the other—vision-for-action—stream. That is, “Case 5” described in Ettlinger, Warrington, and Zangwill (1957), who suffered damage to that stream, apparently had normal conscious experiences of distance, but complained of difficulty in judging “the space to allow . . . as in parking a car.” That is, his lesions had deprived him of accurate and reliable size scaling. All of this clearly supports Servos’s conclusion that the distance registration that is necessary to successful size scaling occurs in a separate physiological stream. Furthermore, the two distances that arise in attempting to explain certain illusory events are thus matched by the different distances registered in Milner and Goodale’s (2006) two visual streams.

The important new point here is that, since the two senses of distance occur in separate pathways, their being sometimes mutually contradictory—such as, to take another example, in the size-scaling explanation of the moon illusion—engenders no perplexing paradox after all.Footnote 1

Equally gratifying is the additional light that this theory casts on the symbiotic relationship between the traditional realm of research in visual perception and the more recently developed field of visual neuroscience. On the one hand, it is widely accepted that findings and theory in the former may tell neurophysiologists “what to look for.” Here, on the other hand, we have a rarer instance of the latter informing efforts within the former, specifically by providing a physiological basis for theoretical assumptions that otherwise would be, at best, discomforting.