SEEING WITHOUT KNOWING

Destruction of small parts of the cortex, for example after stroke, tumour, surgery or gunshot wounds, can result in bizarre and unexpected symptoms. Colour and motion awareness and the strange phenomenon of blindsight In the syndrome known as cerebral achromatopsia, for example, patients lose all colour sensations, so the world appears to be in shades of grey (see Sacks, 1995, for a good example, and Zeki, 1993, for historical details). If the damage is restricted to a small portion of the lower surface of the occipital lobes, the loss of colour vision can occur without any other detectable anomaly: visual acuity is normal, as are depth perception, shape understanding, and so on. Recently, another syndrome has been associated with damage to a lateral part of the occipital lobe: akinetopsia. Someone with akinetopsia loses motion awareness, so that visual stimuli all look stationary even when they are moving. These patients notice if there is a change of stimulus location, but there is no sense of pure motion ccurring between the two successive locations (Zihl et al., 1983). Syndromes like this support the theory that humans possess many specialized processing areas, as do monkeys and other primates. These specialisms contrast with the general loss of subjective vision that follows lesions of the primary visual cortex, area V1. This has been strikingly demonstrated by rare cases of damage to V1 in one hemisphere of the brain. Vision is then affected in one half of visual space, so if your right visual cortex is damaged and you look straight ahead, everything to the left of you is in some way visually absent or missing. Interestingly, though, there are some visual stimuli that can still evoke behavioural responses in the ‘blind’ half of the visual field. For example, if you hold a stick in the blind field and ask the person, ‘Am I holding this stick vertically or horizontally?’ they will say, ‘What stick? I can’t see anything over there at all.’ So you say, ‘Well, I am holding a stick, so please guess what the answ r is.’ Amazingly, these patients will answer correctly most of the time, and much more often than they would by chance guessing. Their behavioural responses to large visual stimuli, including the location, motion and orientation, presented in the blind half of the visual field will be correct more than nine times out of ten. They cannot respond to the fine details of the scene, and they cannot initiate movements towards stimuli they have not been told are there, but something remains of their previous visual capacities within the blind half of the field. This phenomenon has been termed ‘blindsight’ (Weiskrantz et al., 1974). It has been of great interest in recent studies on how subjective awareness of the visual world arises (e.g. Zeki, 1993; Weiskrantz, 1997). The ventral and dorsal streams Leading away from area V1, a distinction is generally made between two broad streams of parallel visual processing (see figures 8.10 and 8.12 above). These were initially known as the ‘what’ and the ‘where’ stream, but t ere has been some dispute over the exact role of the latter, since some researchers believe it is also involved in the visual control of movements (the ‘how’ stream), not simply in locating objects. Partly for this reason, the streams have since become known as the ‘ventral’ and ‘dorsal’ treams, emphasizing their (uncontroversial) anatomical locations, not their more controversial functional roles. The ventral stream takes mainly parvo retinal input from V1 and flows towards the inferotemporal cortex, where cells respond to the sight of whole, complex three-dimensional objects (or at least to the constellations of features that characterize these objects). Damage to this stream impairs object recognition and knowing what objects are for (Milner & Goodale, 1995; Newcombe et al., 1987). This stream includes a specialized area that deals selectively with face recognition and which is damaged in the syndrome called prosopagnosia (as in the example of the man who mistook his wife for a hat: Sack , 1985). The dorsal stream, in contrast, takes mango input and runs into the parietal lobe. It deals with locating objects and with sensorimotor coordination, mostly occurring subconsciously. Damage to the parietal lobe can hamper the ability to grasp something with the hand or post a letter through the slot in a mailbox (Milner & Goodale, 1995). With right parietal lesions particularly, it becomes difficult to recognize objects from unusual points of view (such as a bucket from above), rotate an object mentally, read a map, draw, use building blocks, and pay attention to spatial locations especially on the left side of space (a phenomenon known as spatial ‘neglect’; Robertson & Halligan, 1999). In summary, these different lines of evidence strongly support the idea of parallel processing. However, they do not explain why our behaviour is not a bundle of reflex reactions to sensory stimuli. In the next sections, we consider the role of different types of cognitive knowledge in perception.