Can we perceive aspects of the visual world without any conscious awareness we are doing so? In other words, is there such a thing as unconscious perception or subliminal perception (perception occurring even though the stimulus is below the threshold of conscious awareness?). Common sense suggests the answer is “No”. However, there is strong evidence that the correct answer is “Yes”. However, we need to be careful in the terms we use. A thermostat responds appropriately to temperature changes and so could be said to exhibit unconscious perception!
Some of the most important evidence suggesting visual perception does not require conscious awareness comes from research on blindsight patients with damage to early visual cortex (V1). Blindsight “refers to the rare ability of V1-damaged patients to perform visual tasks … even though these patients claim not to consciously see the relevant stimuli” (Ko & Lau, 2012, p. 1401).
KEY TERMS
Subliminal perception
Perceptual processing occurring below the level of conscious awareness that can nevertheless influence behaviour.
Findings
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 field. This was done by van der Stigchel et al. (2010). Two patients with blindsight were instructed to make an eye movement towards a target presented in their sighted field. The target was either presented on its own or at the same time as a distractor in the blind field. The direction of the eye movement was influenced by the distractor’s presence even though the patients were not consciously aware of it.
GY (discussed earlier) is a much-studied blindsight patient. He has extensive damage to the primary visual cortex in the left hemisphere and a smaller area of damage in the right parietal area caused by a car accident in childhood. In a study by Persaud and Cowey (2008), GY was presented with a stimulus in the upper or lower part of his visual field. On some trials (inclusion trials), he was instructed to report the part of the visual field to which the stimulus had been presented. On other trials (exclusion trials), GY was told to report the opposite of its actual location (e.g., “Up” when it was in the lower part).
What did Persaud and Cowey (2008) find? GY tended to respond with the real rather than the opposite location on exclusion and inclusion trials, suggesting he had access to location information but lacked any conscious awareness of it (see Figure 2.25). In contrast, healthy individuals showed a large difference in performance on inclusion and exclusion trials, indicating they had conscious access to location information. These findings suggest the involvement of conscious processes was much greater in healthy individuals than in GY.
Evidence that visual processing is very different in the intact and blind visual fields was reported by Persaud et al. (2011) in a study on GY. They manipulated the stimuli presented to his intact and “blind” visual field so his performance when judging the location of a vertical grating was comparable in both fields. In spite of this, GY indicated conscious awareness of far more stimuli in the intact field than the blind one (43% of trials vs. 3%, respectively). Of most importance, there was substantially more activation in the prefrontal cortex and parietal areas to targets presented to the intact field.
Figure 2.25
Estimated contributions of conscious and subconscious processing to GY’s performance in exclusion and inclusion conditions in his normal and blind fields.
Reprinted from Persaud and Cowey (2008). Reprinted with permission from Elsevier.
Overgaard et al. (2008) gave their blindsight patient, GR, a visual discrimination task. She decided whether a triangle, circle or square had been presented to her blind field. In one experiment, Overgaard et al. used a four-point scale of perceptual awareness: “clear image”; “almost clear image”;
“weak glimpse”; and “not seen”. In another experiment, GR indicated on a yes/no basis whether she had seen the stimulus.
Consider the findings when the four-point scale was used. There was a strong association between the level of perceptual awareness and the accuracy of her performance when stimuli were presented to her blind field. She was correct 100% of the time when she had a clear image, 72% of the time when her image was almost clear, 25% of the time when she had a weak glimpse and 0% when the stimulus was not seen. Thus, GR used the four-point scale appropriately. If the data for “clear image” and “almost clear image” are combined, GR claimed to have awareness of the stimulus on 54% of trials. On 83% of these trials she was correct.
When Overgaard et al. (2008) used the yes/no measure, GR seemed to have a much lower level of conscious awareness of blind-field stimuli. She indicated she had seen the stimulus on only 21% of the trials and she was right on 86% of those trials. Thus, the use of a sensitive method (i.e., four-point scale) to assess conscious awareness suggests degraded conscious vision sometimes underlies blindsight patients’ ability to perform at above-chance levels on visual tasks. Thus, the notion that blindsight patients lack all conscious visual experience in their blind field may sometimes be incorrect.
What do blindsight patients experience?
It is harder than you might imagine to decide exactly what blindsight patients experience when presented with visual stimuli to their blind field. For example, the blindsight patient GY described his experiences as “similar to that of a normally sighted man who, with his eyes shut against sunlight, can perceive the direction of motion of a hand waved in front of him” (Beckers & Zeki, 1995, p. 56).
On another occasion GY was asked about his qualia (sensory experiences). He said, “That [experience of qualia] only happens on very easy trials, when the stimulus is very bright. Actually, I’m not sure I really have qualia then” (Persaud & Lau, 2008, p. 1048).
Blindsight patients vary in their residual visual abilities, and so it makes sense to assign them to different categories. Danckert and Rossetti (2005) identified three sub-types of blindsight:
1 Action-blindsight: These patients have some ability to grasp or point at objects in the blind field because they have some use of the dorsal (“where”) stream of processing (see Figure 2.3). Baseler et al. (1999) found that GY showed activation in the dorsal stream (but not the ventral or “what” stream) to visual stimuli presented in the blind field. This is the most studied sub-type.
2 Attention-blindsight: These patients can detect objects and motion and have a vague conscious feeling of objects in spite of reporting they cannot see them. They can make some use of the dorsal stream and motor areas.
3 Agnosopsia: 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.
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A demonstration of blindsight
Weiskrantz (e.g., 2004) distinguished between Type 1 blindsight and Type 2 blindsight. Type 1 (similar to agnosopsia) includes blindsight patients with no conscious awareness of visual stimuli presented to the blind field. In contrast, Type 2 (similar to attention-blindsight) includes patients with some awareness of such stimuli.
An example of Type 2 blindsight was found in patient EY, who “sensed a definite 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 tested many times may start to rely on indirect evidence. For example, patients’ partial 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.
Ko and Lau (2012) argued that blindsight patients may have more conscious visual experience than is usually believed. Their key assumption was as follows: “Blindsight patients may use an unusually conservative criterion for detection, which results in them saying ‘no’ nearly all the time to the question of ‘do you see something?’” (Ko & Lau, 2012, p. 1402). This excessive caution may occur because damage to the prefrontal cortex impairs the ability to set the criterion for visual detection appropriately. Their excessive conservatism or caution may explain why blindsight patients’ reported visual experience is so discrepant from their forced-choice perceptual performance.
Support for Ko and Lau’s (2012) position comes from Overgaard et al.’s (2008) finding (discussed earlier) that blindsight patients were excessively reluctant to admit having seen stimuli presented to their blind field. Ko and Lau cite further support for their position in research showing blindsight patients often have damage to the prefrontal cortex in addition to primary visual cortex.
Evaluation
There are various reasons for accepting blindsight as a genuine phenomenon. First, blindsight has been reported in studies in which potential problems with the use of subjective (and possibly distorted) verbal reports have apparently been overcome (e.g., Persaud & Cowey, 2008). Second, there are studies in which evidence for blindsight did not depend on subjective verbal reports (e.g., van der Stigchel et al., 2010). Third, there are functional neuroimaging studies showing many blindsight patients have activation predominantly or exclusively in the dorsal stream (Danckert & Rossetti, 2005). The relevance of this is that the dorsal stream is strongly associated with non-conscious processing (Milner, 2012). Fourth, there is evidence for subliminal processing of emotional information (known as affective blindsight; see Chapter 15).
What are the limitations of research in this area? First, there are considerable differences among blindsight patients with several apparently possessing some conscious visual awareness in their allegedly blind field.
Second, many blindsight patients probably have more conscious visual experience in their “blind” field than appears from yes/no judgements about the presence of a stimulus. This happens because they are excessively cautious about claiming to have seen a stimulus (Ko & Lau, 2012; Overgaard, 2012).
Third, one of the most studied blindsight patients, GY, has nerve fibre connections within the visual system not present in healthy individuals (Bridge et al., 2008). This suggests some visual processes in blindsight patients may be specific to them. This would limit our ability to generalise from such
patients to healthy individuals.
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