It is tempting to assume that visual perception is a conscious process. However, that is not always the case. For example, there are patients with severe damage to VI (primary visual cor- tex) who suffer from blindsight. Such patients can respond appropriately to visual stimuli in the absence of conscious visual experience. After we have considered blindsight patients, we will discuss evidence from healthy indi- viduals relating to unconscious perception or
subliminal perception (perception occurring
below the level of conscious awareness).
Blindsight
Numerous British soldiers in the First World War who had received head wounds were treated by an Army doctor called George Riddoch. He found something fascinating in many of those to respond to a given colour even though there
were large changes in the background colour. Thus, cells in V4 (but not earlier in visual processing) exhibit colour constancy.
Barbur and Spang (2008) studied instan- taneous colour constancy, in which there is high colour constancy following a sudden change in illuminant. Use of fMRI revealed, as expected, that the computations involved in instantaneous colour constancy involved V4. Less expectedly, V1 (primary visual cortex) was equally involved, and there was also signifi cant activation in V2 and V3. These fi ndings suggest that areas other than V4 play an important role in colour constancy.
There is a fi nal point. We should not regard colour processing as being entirely separate from other kinds of object processing. For example, colour can infl uence perceived shape. Imagine looking at a garden fairly late on a sunny day with strong shadows cast by the trees. It is easier to work out object boundaries (e.g., of the lawn) by using differences in colour or chromaticity than in luminance. Kingdom (2003) found that gratings that look almost fl at can be made to look corrugated in depth by the addition of appropriate colour.
Evaluation
Colour constancy is a complex achievement, and observers often fall well short of complete constancy. In view of its complexity, it is unsur- prising that the visual system adopts an “all hands on deck” approach in which many fac- tors make a contribution. The most important factors are those relating to the visual environ- ment, especially context (local contrast, global contrast). Of special importance are cone- excitation ratios that remain almost invariant across changes in illumination. In addition, top-down factors such as our knowledge and memory of the familiar colour of common objects also play a role. Our understanding of the brain mechanisms underlying colour con- stancy has been enhanced by the discovery of cells in V4 responding to colour constancy.
What are the limitations of research on colour constancy? First, we lack a comprehen-
blindsight: the ability to respond appropriately
to visual stimuli in the absence of conscious vision in patients with damage to the primary visual cortex.
unconscious perception: perceptual
processes occurring below the level of conscious awareness.
subliminal perception: processing that occurs
in the absence of conscious awareness.
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dorsal stream (but not the ventral stream) to visual stimuli presented in the blind fi eld. This is the most studied sub-type.
Attention-blindsight
(2) : these patients can
detect objects and motion and have a vague conscious feeling of objects in spite of reporting that they cannot see them. They can make some use of the dorsal stream and the motion area (MT). Danckert et al. (2003) found that an intact posterior parietal cortex in the dorsal stream was essential for showing action-blindsight.
Agnosopsia
(3) : these patients deny any conscious awareness of visual stimuli. However, they exhibit some ability to discriminate form and wavelength and to use the ventral stream.
The phenomenon of blindsight becomes somewhat less paradoxical if we consider how it is assessed in more detail. There are generally two measures. First, there are patients’ subjec- tive reports that they cannot see some stimulus presented to their blind region. Second, there is a forced-choice test in which patients guess (e.g., stimulus present or absent?) or point at the stimulus they cannot see. Blindsight is defi ned by an absence of self-reported visual perception accompanied by above-chance performance on the forced-choice test. Note that the two measures are very different from each other. Note also that we could try to account for blindsight by assuming that subjective reports provide a less sensitive measure of visual perception than does a forced-choice test. This is an issue to which we will return.
There is one fi nal point. As Cowey (2004, p. 588) pointed out, “The impression is some- times given, however unwittingly, that blind- sight . . . (is) like normal vision stripped of conscious visual experience. Nothing could be further from the truth, for blindsight is characterised by severely impoverished dis- crimination of visual stimuli.”
Evidence
The most thoroughly studied blindsight patient is DB. He underwent surgical removal of the with injuries to the primary visual cortex (BA
17) at the back of the occipital area of the brain (see Figure 1.3). This area is involved in the early stages of visual processing, so it was unsurprising that these patients had a loss of perception in parts of the visual fi eld. Much more surprising was that they responded to motion in those parts of the visual fi eld in which they claimed to be blind (Riddoch, 1917)! Such patients are said to suffer from blindsight, which neatly captures the apparently paradoxical nature of their condition.
Blindsight patients typically have extensive damage to V1. However, their loss of visual awareness in the blind fi eld is probably not due directly to the V1 damage. Damage to V1 has knock-on effects throughout the visual system, leading to greatly reduced activation of sub- sequent visual processing areas (Silvanto, 2008). There are at least ten pathways from the eye to the brain, many of which can be used by blindsight patients (Cowey, 2004). It appears that cortical mechanisms are not essential. Köhler and Moscovitch (1997) found that blindsight patients who had had an entire corti- cal hemisphere removed nevertheless showed evidence of blindsight for stimulus detection, stimulus localisation, form discrimination, and motion detection for stimuli presented to their removed hemisphere. However, those having a cortical visual system (apart from primary visual cortex) can perform more perceptual tasks than those lacking a cerebral hemisphere (Stoerig & Cowey, 1997). There is evidence that blindsight patients can often make use of a tract linking the lateral geniculate nucleus to the ipsilateral (same side of the body) human visual motion area V5/MT that bypasses V1.
Blindsight patients vary in their residual visual abilities. Danckert and Rossetti (2005) identifi ed three sub-types:
Action-blindsight
(1) : these patients have
some ability to grasp or point at objects in the blind fi eld because they can make some use of the dorsal stream of process- ing. Baseler, Morland, and Wandell (1999) found that GY showed activation in the
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at chance level when trying to detect a light presented to the blind area of the visual fi eld. However, the time they took to direct their eyes at a light presented to the intact part of the visual fi eld increased when a light was presented to the blind area at the same time. Thus, blindsight patients processed the light in the blind area even though they showed no evidence of detecting it when deciding whether it was present or absent.
One of the central issues is whether blind- sight patients genuinely lack conscious visual perception. Some blindsight patients may have residual vision, claiming that they are aware that something is happening even though they cannot see anything. Weiskrantz (e.g., 2004) used the term blindsight Type 1 (similar to Danckert and Rossetti’s, 2005, agnosopsia) to describe patients with no conscious awareness. He used the term blindsight Type 2 (similar to attention-blindsight) to describe those with awareness that something was happening. An example of Type 2 blindsight was found in patient EY, who “sensed a defi nite pinpoint of light”, although “it does not actually look like a light. It looks like nothing at all” (Weiskrantz, 1980). Type 2 blindsight sounds suspiciously like residual conscious vision. However, patients who have been tested many times may start to rely on indirect evidence (Cowey, 2004). For example, the performance of patients with some ability to guess whether a stimulus is moving to the left or the right may depend on some vague awareness of their own eye movements.
Evidence that blindsight can be very unlike normal conscious vision was reported by Persaud and Cowey (2008). The blindsight patient GY was presented with a stimulus in the upper or lower part of his visual fi eld. On some trials (inclusion trials), he was instructed to report the part of the visual fi eld to which the stimulus had been presented. On other right occipital cortex including most of the
primary visual cortex. He showed some per- ceptual skills, including an ability to detect whether a visual stimulus had been presented to the blind area and to identify its location. However, he reported no conscious experience in his blind fi eld. According to Weiskrantz, Warrington, Sanders, and Marshall (1974, p. 721), “When he was shown a video fi lm of his reaching and judging orientation of lines (by presenting it to his intact visual fi eld), he was openly astonished.”
Suppose you fi xate on a red square for several seconds, after which you look away at a white surface. The surface will appear to have the complementary colour (i.e., green). This is a negative after-effect (discussed earlier in the chapter). Weiskrantz (2002) found to his considerable surprise that DB showed this negative after-effect. This is surprising, because there was conscious perception of the after- image but not of the stimulus responsible for producing the afterimage! DB showed other afterimages found in healthy individuals. For example, he reported an apparent increase in the size of visual afterimages when viewed against a nearby surface and then against a surface further away (Emmert’s law). Thus, DB’s perceptual processing is more varied and thorough than previously believed.
Impressive fi ndings were reported by de Gelder, Vroeman, and Pourtois (2001), who discovered GY could discriminate whether an unseen face had a happy or a fearful expres- sion. He was probably responding to some distinctive facial feature (e.g., fearful faces have wide-open eyes), since it is improbable that he processed the subtleties of facial expression. The ability of blindsight patients to distinguish among emotional expressions in the absence of visual awareness is known as affective blind- sight (see Chapter 15).
It would be useful to study the perceptual abilities of blindsight patients without relying on their subjective (and possibly inaccurate) reports of what they can see in the blind fi eld. This was done by Rafal, Smith, Krantz, Cohen, and Brennan (1990). Blindsight patients performed
Emmert’s law: the size of an afterimage
appears larger when viewed against a far surface than when viewed against a near one.
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image was almost clear, 25% of the time when she had a weak glimpse, and 0% when the stimulus was not seen. Thus, the use of a sensi- tive method to assess conscious awareness sug- gests that degraded conscious vision sometimes underlies blindsight patients’ ability to perform at above-chance levels on visual tasks.
Evaluation
There are various reasons for accepting blind- sight as a genuine phenomenon. First, there are studies indicating blindsight in which potential problems with the use of subjec tive (and possibly distorted) verbal reports have apparently been overcome (e.g., Persaud & Cowey, 2008). Second, there are studies in which evidence for blind- sight did not depend on subjective verbal reports (e.g., Rafal et al., 1990). Third, there are func- tional neuroimaging studies showing that many blindsight patients have activation predom- inantly or exclusively in the dorsal stream (see Danckert & Rossetti, 2005, for a review). This is important evidence because conscious visual perception is primarily associated with activa- tion in the ventral stream (Norman, 2002).
What are the problems with research on blindsight? First, there are considerable dif- ferences among blindsight patients, which led Danckert and Rossetti (2005) to identify three subtypes. As a result, it is hard to draw any general conclusions.
Second, there is evidence (e.g., Danckert & Rossetti, 2005; Overgaard, Fehl, Mouridsen, Bergholt, & Cleermans, 2008; Weiskrantz, 2004) that a few blindsight patients possess some conscious visual awareness in their allegedly trials (exclusion trials), GY was told to report
the opposite of its actual location (e.g., “Up” when it was in the lower part). GY tended to respond with the real rather than the opposite location on exclusion trials as well as inclusion trials when the stimulus was presented to his blind fi eld. This suggests that he had access to location information but lacked any conscious awareness of that information. In contrast, GY showed a large difference in performance on inclusion and exclusion trials when the stimu- lus was presented to his normal or intact fi eld, indicating he had conscious access to location information. Persaud and Cowey used the fi ndings from inclusion and exclusion trials to conclude that conscious processes were involved when stimuli were presented to GY’s normal fi eld but not to his blind fi eld (see Figure 2.17).
Overgaard et al. (2008) pointed out that researchers often ask blindsight patients to indicate on a yes/no basis whether they have seen a given stimulus. That opens up the pos- sibility that blindsight patients have some con- scious vision but simply set a high threshold for reporting awareness. Overgaard et al. used a four-point scale of perceptual awareness: “clear image”, “almost clear image”, “weak glimpse”, and “not seen”. Their blindsight patient, GR, was given a visual discrimination task (deciding whether a triangle, circle, or square had been presented). There was a strong association between the level of perceptual awareness and the accuracy of her performance when stimuli were presented to her blind fi eld. She was correct 100% of the time when she had a clear image, 72% of the time when her
Figure 2.17 Estimated contributions of conscious and subconscious processing to GY’s performance in exclusion and inclusion conditions in his normal and blind fi elds. Reprinted from Persaud and Cowey (2008), Copyright © 2008, with permission from Elsevier.
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This caused an increase of 18% in the cinema sales of Coca-Cola and a 58% increase in popcorn sales. Alas, Vicary admitted in 1962 that the study was a fabrication. In addition, Trappery (1996) reported in a meta-analysis that stimuli presented below the conscious threshold had practically no effect on consumer behaviour.
In spite of early negative fi ndings, many researchers have carried out studies to demon- strate the existence of unconscious perception. There are three main ways in which they pres- ent visual stimuli below the level of conscious awareness. First, the stimuli can be very weak or faint. Second, the stimuli can be presented very briefl y. Third, the target stimulus can be immediately followed by a masking stimulus (one that serves to inhibit processing of the target stimulus).
How can we decide whether an observer has consciously perceived certain visual stimuli? According to Merikle, Smilek, and Eastwood (2001), there are two main thresholds or criteria:
Subjective threshold
(1) : this is defi ned by an individual’s failure to report conscious awareness of a stimulus.
Objective threshold
(2) : this is defi ned by an individual’s inability to make accurate forced-choice decisions about a stimulus (e.g., guess at above-chance level whether it is a word or not).
Two issues arise with these threshold measures. First, as Reingold (2004, p. 882) pointed out, “A valid measure must index all of the perceptual information available for consciousness . . . and blind field. It is doubtful whe ther such
patients fulfi l all the criteria for blindsight. Third, consider one of the most-studied blindsight patients, GY, whose left V1 was destroyed. He has a tract connecting the undamaged right lateral geniculate nucleus to the contralateral (opposite side of the body) visual motion area V5/MT (Bridge, Thomas, Jbabdi, & Cowey, 2008) (see Figure 2.18). This tract is not present in healthy individuals. The implication is that some visual processes in blindsight patients may be specifi c to them and so we cannot generalise from such patients to healthy individuals.
Fourth, Campion, Latto, and Smith (1983) argued that stray light may fall into the intact visual fi eld of blindsight patients. As a result, their ability to show above-chance performance on various detection tasks could refl ect pro- cessing within the intact visual fi eld. However, blindsight is still observed when attempts are made to prevent stray light affecting performance (see Cowey, 2004). If blindsight patients are actually processing within the intact visual fi eld, it is unclear why they lack conscious awareness of such processing.