A condition in which individuals with eye disease form vivid and detailed visual hallucinations sometimes mistaken for visual perception.
Patients with Charles Bonnet syndrome have increased activity in brain areas specialised for visual processing when hallucinating (ffytche et al., 1998).
In addition, hallucinations in colour were associated with increased activity in areas specialised for colour processing.
In visual perception, bottom-up processes inhibit activation in parts of the visual cortex (e.g., BA37). The impoverished bottom-up processes in Charles Bonnet syndrome permit spontaneous activation in areas associated with the production of hallucinations (Kazui et al., 2009).
Anyone (other than those with eye disease) suffering from visual hallucinations is unlikely to remain at liberty for long. How do we avoid confusing images and perceptions? One reason is that we generally know we are deliberately constructing images, which is not the case with perception. Another reason is that images contain much less detail than perception. Harvey (1986) found people rated their visual images of faces as similar to photographs from which the sharpness of the edges and borders had been removed.
Figure 3.17
The approximate locations of the visual buffer in BA17 and BA18 of long-term memories of shapes in the inferior temporal lobe, and of spatial representations in posterior parietal cortex, according to Kosslyn and Thompson’s (2003) anticipation theory.
Interactive exercise:
Kosslyn – mental imagery
Research activity:
Kosslyn
Imagery resembles perception
If visual perception and visual imagery involve similar processes, they should influence each other. There should be facilitative effects if the contents of perception and imagery are the same but interference effects if they differ.
Pearson et al. (2008) found evidence of facilitation. They studied binocular rivalry – when two different stimuli are presented one to each eye, only one is consciously perceived at any given moment. If one of the two stimuli is presented shortly before the other, that increases the chances it will be perceived in the binocular rivalry situation. Pearson et al. found this when observers initially perceived a green vertical grating or a red horizontal grating.
This facilitation effect was greatest when the orientation of the grating under binocular rivalry conditions was the same as the initial orientation and least when there was a large difference in orientation.
KEY TERM
Binocular rivalry
When two different visual stimuli are presented one to each eye, only one stimulus is seen; the seen stimulus alternates over time.
Pearson et al. (2008) also considered what happened when the initial single grating was imagined rather than perceived. The pattern of facilitation in binocular rivalry was remarkably similar to that observed when the initial single grating was perceived.
So far as interference is concerned, we will consider a study by Baddeley and Andrade (2000). Participants rated the vividness of visual or auditory images under control conditions (no additional task) or while performing a second task. This second task involved visual/spatial processes or verbal processes (counting aloud repeatedly from 1 to 10). The visual/spatial task reduced the vividness of visual imagery more than that of auditory imagery because the same mechanisms were involved on the visual/spatial task and visual imagery tasks.
According to Kosslyn (1994, 2005), much processing associated with visual imagery occurs in early visual cortex (BA17 and BA18), although several other areas are also involved. Kosslyn and Thompson (2003) considered numerous neuroimaging studies. Tasks involving visual imagery were associated with activation of early visual cortex in half of them. The findings were most often significant when the task involved inspecting the fine details of images, when the task focused on an object’s shape rather than an object in motion, and when sensitive brain-imaging techniques were used.
Ganis et al. (2004) compared patterns of brain activation in visual perception and imagery. There were two main findings. First, there was extensive overlap in the brain areas associated with perception and imagery. This was especially so in the frontal and parietal areas, perhaps because perception and imagery involve similar control processes.
Second, visual imagery was associated with activation in only some brain areas involved in visual perception. Kosslyn (2005) estimated that visual imagery tasks are associated with activation in about two-thirds of the brain areas activated during perception.
Figure 3.18
Slezak (1991, 1995) asked participants to memorise one of the above images. They then imagined rotating the image 90 degrees clockwise and reported what they saw. None of them reported seeing the figures that can be seen clearly if you rotate the page by 90 degrees clockwise.
Left image from Slezak (1995), centre image from Slezak (1991), right image reprinted from Pylyshyn (2003), with permission from Elsevier and the author.
Figure 3.19
The extent to which perceived (left side of figure) or imagined (right side of figure) objects could be classified accurately on the basis of brain activity in early visual cortex and object-selective cortex. (ES = extrastriate retinotopic cortex; LO = lateral occipital cortex; pFs = posterior fusiform sulcus.)
From S.H. Lee et al. (2012). Reproduced with permission from Elsevier.
Imagery does not resemble perception
Have a look at Figure 3.18, which consists of the outlines of three objects. Start with the object on the left and form a clear image. Then close your eyes, mentally rotate the image by 90 degrees clockwise and decide what you see. Then repeat the exercise with the other objects. Finally, rotate the book through 90 degrees. You found it easy to identify the objects when perceiving them even though you probably could not when you only imagined rotating them.
Slezak (1991, 1995) carried out research using stimuli very similar to those shown in Figure 3.18. No observers reported seeing the objects. This was not a deficiency in memory – participants who sketched the image from memory and then rotated it saw the new object. Thus, the information contained in images cannot be used as flexibly as visual information.
S.-H. Lee et al. (2012) found evidence for important differences between imagery and perception. Participants viewed or imagined various common objects (e.g., car, umbrella) and activity in early visual cortex and areas associated with later visual processing (object-selective regions) was assessed.
Attempts were made to work out which objects were being imagined or perceived on the basis of activation in these areas.
What did Lee et al. (2012) find? First, activation in all brain areas assessed was considerably greater when participants perceived rather than imagined objects. Second, objects being perceived or imagined could be identified with above-chance accuracy on the basis of patterns of brain activation except for imagined objects in primary visual cortex (V1; see Figure 3.19).
Third, the success rate in identifying perceived objects was greater based on brain activation in areas associated with early visual processing than those associated with later processing. However, the opposite was the case with respect to identifying imagined objects (see Figure 3.19). These findings point to an important difference between imagery and perception: processing in early visual cortex is very limited during imagery for objects but is extremely important during perception. Imagery for objects depends mostly on top-down processes based on object knowledge rather than on processing in early visual cortex.