Several theorists (e.g. Mishkin & Ungerleider, 1982) have argued that vision is used for two crucial functions (refer back to Figure 2.13). First, there is object perception (what is it?). Second, there is spatial perception (where is it?). There is good evidence (at least in macaque monkeys) that rather different brain systems underlie each of these functions:
1. There is a ventral pathway running from the primary visual area in the cortex to the inferior temporal cortex; this pathway is specialised for object perception (i.e., what is it?).
2. There is a dorsal pathway running from the primary visual area in the cortex to the posterior parietal cortex; this pathway is specialised for spatial perception (i.e., where is it?).
Some of the original research in this area was reported by Mishkin and Ungerleider (1982). They used a situation in which there were two food wells, each of which was covered by a lid. There was food in one of the wells, and monkeys were allowed to lift one lid in order to find it. Food was either associated with a specific lid pattern (object information) or with whichever food well was closer to a small model tower (spatial information). Monkeys whose inferior temporal lobes were removed had problems in using object information but not spatial information. In contrast, monkeys whose parietal lobes were removed experienced difficulty in using spatial information but not object information.
Neuroimaging evidence was reported by Haxby et al. (1994). They used two tasks with normal participants. There was an object-recognition task that involved deciding which of two faces matched a target face. There was also a spatial task that involved deciding which of two figures consisting of a dot and two lines was a rotated version of the target figure. PET data indicated that the occipital region of the cortex was activated as participants performed both tasks. However, the pattern of activation differed elsewhere in the cortex. The objectrecognition task produced heightened activation in the inferior and medial temporal cortex, whereas the spatial task led to increased activation in the parietal cortex. These patterns of activation are as predicted by the theory.
Milner and Goodale (1995, 1998) have developed and extended these theoretical ideas in several ways. They drew a distinction between vision for perception and vision for action (see earlier in the chapter). Both these systems use object and spatial information. However, they do so in different ways, with different representations being used for recognition and for visually guided action. According to Milner and Goodale, the dorsal pathway may be of greatest value in providing an answer to the question "How do I interact with that object?". That contrasts with Mishkin and Ungerleider (1982), who claimed that the dorsal pathway provides information to answer the question "Where is that object?".
Some of the most convincing evidence for the notion of separate visual systems for perception and for action has come from the study of brain-damaged patients. It was predicted that there would be a double dissociation: some patients would have reasonably intact vision for perception but severely impaired vision for action, and others would show the opposite pattern.
Half of the double dissociation consists of patients with optic ataxia. According to Georgopoulos (1997, p. 142), such patients "do not usually have impaired vision or impaired hand or arm movements, but show a severe impairment in visually guided reaching in the absence of perceptual disturbance in estimating distance." For example, consider a study by Perenin and Vighetto (1988). Patients with optic ataxia experienced great difficulty in rotating their hands appropriately when given the task of reaching towards and into a large oriented slot in front of them.
Which parts of the brain are damaged in optic ataxia? The answer varies from patient to patient. However, "The brain damage in cases of optic ataxia has been localised in the parietal cortex..., its underlying white matter and/or the posterior part of the corpus callosum" (Georgopoulos, 1997, p. 142).
The other half of the double dissociation consists of some patients with visual agnosia (see Chapter 4). This is a condition in which there are severe problems with object recognition. DF is the most studied patient having visual agnosia coupled with fairly good spatial perception. In spite of having reasonable visual acuity, DF was unable to identify any of a series of a selection of drawings of common objects. However, as was pointed out by Milner, Carey, and Harvey (1991), DF "had little difficulty in everyday activity such as opening doors, shaking hands, walking around furniture, and eating meals. she could accurately reach out and grasp a pencil orientated at different angles."
In a study by Goodale and Milner (1992), DF held a card in her hand, and looked at a circular block into which a slot had been cut. When she was asked to orient the card so that it would fit into the slot, she was unable to do so, suggesting that she has very poor perceptual skills. However, DF performed well when she was asked to move her hand forward and inset the card into the slot.
Carey, Harvey, and Milner (1996) obtained additional evidence of DF's ability to use visual information to guide her actions. She was given the task of picking up rectangular shapes differing in width and orientation. She was able to do this as well as normal individuals. However, DF did not show normal grasping behaviour when trying to pick up more complex objects (e.g., crosses) in which two different orientations are present together.
Which areas of the visual cortex are intact and damaged in DF? MRI indicated that most of the primary visual cortex is still intact. According to Milner and Goodale (1998, p. 8), it is reasonable to assume that, "the ventral stream is severely damaged and/or disconnected in DF (an assumption that is quite consistent with her pattern of brain damage)."
There are three exciting theoretical implications of research in this area. First, as Milner and Goodale (1998, p. 2) pointed out, "Standard accounts of vision implicitly assume that the purpose of the visual system is to construct some sort of internal model of the world outside." Thus, it is common to focus on vision for perception and to de-emphasise vision for action.
Second, Milner and Goodale (1998) argued that many visual illusions (e.g., geometric illusions) occur because of the processing of the visual input by the ventral system. According to Milner and Goodale (1998, p. 10), "the dorsal system, by and large, is not deceived by such optical illusions." Thus, it is predicted that the dorsal pathway or "where" system allows us to make accurate eye and hand movements with respect to illusory figures that we misperceive. For example, Wong and Mack (1981) re-presented a target stimulus after a 500-millisecond interval in the same location as before. The surrounding frame had been moved, so that the participants had the illusion that the target's position had changed. However, their eye movements were directed accurately to the actual position of the target. Similar findings have been obtained with other visual illusions. Gentilucci et al. (1996) obtained similar findings with the Muller-Lyer illusion (see Figure 3.3). The participants were asked to point to various parts of the illusion. There were small effects of the illusion on hand movements, but these effects were much smaller than those obtained in perceptual judgements of the Muller-Lyer illusion (see earlier in the chapter).
Third, as Milner and Goodale (1995, 1998) implied, it is likely that vision for action makes use of rather different information than does vision for perception. Vision for action (based on the dorsal pathway) uses short-lasting, viewpoint-dependent representations, that is, the representations are influenced by the angle of viewing. In contrast, vision for perception (based on the ventral pathway) may use long-lasting, viewpoint-
independent representations, that is, the representations rely on stored knowledge and are not influenced by the angle of viewing (see Chapter 4). According to Milner and Goodale (1998, p. 12), the dorsal system is designed to guide actions purely in the here and now, and its products are consequently useless for later reference.it is only through knowledge gained via the ventral stream that we can exercise insight, hindsight and foresight about the visual world.
What about future research? The theoretical approach so far has focused on the differences and separateness of the dorsal and ventral streams. Accordingly, "One of the important questions that remains to be answered is how the two streams interact both with each other and with other brain regions in the production of purposive behaviour" (Milner & Goodale, 1998, p. 12).
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