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The dual visual systems picture itself is supported by a large and impressively varied body of evidence. Such evidence includes the original neuropsychological studies of patient DF (Milner et al. ; Milner and Goodale ; Goodale and Milner ), a swathe of neuroimaging experiments (e.g., Le et al. ; James et al. ), a wide variety of single cell recordings and other experimental interventions in monkeys (Taira et al. ; Sakata ), and psychophysical experiments involving normal human subjects (Bridgeman et al. ; Goodale et al. ; Aglioti et al. ; Fecteau et al. ). Of these, the Aglioti et al.results are both the best known and the most controversial. They involved normal subjects engaging in reaching and grasping movements whose target was subject to a perceptual size illusion, and seemed to show that grip aperture was sensitive to the realsize of the target, whereas judgments based on conscious visual experience were inﬂuenced by the illusion. The explanation, according to (Milner and Goodale ), was that grip aperture was determined by the dorsal ‘vision-for-action’ system operating independently of the ventral ‘vision-for-perception’ stream whose codings, on this interpretation, informed both conscious experience and verbal response. More recently, a similar effect has been shown using the so-called ‘hollow face illusion’. In this illusion a concave model of a human face appears convex, because of the inﬂuence of top-down knowledge concerning normal human faces. This suggests it is a purely ventral stream-based illusion. Kroliczak et al. ()showed that in a task in which subjects were asked to ﬂick small targets off the actually hollow (though visually convex) face, the ﬂicking movements found the real (non-illusory) locations of the targets. According to Milner and Goodale: This demonstrates that the visuomotor system can use bottom-up sensory inputs [. . .] to guide movements to the veridical locations of targets in the real world, even when the perceived positions of the targets are inﬂuenced, or even reversed, by top-down processing. (Milner and Goodale , p. 245). Such demonstrations, and the more general issue of perception-action dissociations and visual illusions, have spawned a large and complex literature, some of which we shall touch upon later in this treatment (for some balanced reviews of the topic, see Carey ; Jacob and Jeannerod , Chapter 4). There are, ﬁnally, compelling computational and information-processing considerationsthatspeak in favour of a dual visualsystemarchitecture (though just how this in turn lines up with the issues concerning conscious and nonconscious vision is open to question, as we shall see later). The online control of motor action requires the extraction and use of radically different kinds of information (from the incoming visual signal) than do the tasks of recognition, recall, and reasoning. The former requires a constantly updated, (multiply) egocentrically speciﬁed, and exquisitely distance and orientation-sensitive encoding of the visual array. The latter requires the computation of objectconstancy (objects do not change their identity every time they move in space) and the recognition of items by category and signiﬁcance irrespective of the ﬁne detail of location, viewpoint, and retinal image size.