The Neuropsychology Of Visual Imagery

Farah (1984) carried out a review of imagery deficits following brain injuries using Kosslyn's theory to understand these deficits. She abstracted the general component processes and structures of the theory and analysed various test tasks in terms of them. She then showed that different deficiencies in brain-damaged patients could be traced to problems with particular components. For instance, Kosslyn's theory posits a process that generates images from long-term memory representations, so if this process is damaged then the patient should not be able to describe the appearance of objects from memory or draw objects from memory. However, the same patient should be able to recognise and draw visually presented objects because these involve component processes other than those used in image generation. Several studies have reported patients with this pattern of behaviour (e.g., Lyman, Kwan, & Chao, 1938; Nielsen, 1946).

Traditionally, imagery has been viewed as being a right hemisphere function (see Ehrlichman & Barrett, 1983, for a review). Farah challenged this view in arguing that at least one component— the imagery generation component—appears to be a left hemisphere function (see Farah, 1984; Farah, Peronnet, Gonon, & Giard, 1988; Kosslyn, 1987; Kosslyn, Holtzmann, Farah, & Gazzaniga, 1985). In a study involving split-brain patients, Farah, Gazzaniga, Holtzman, and Kosslyn (1985) have shown that the disconnected left hemisphere could perform a task requiring image generation when the right hemisphere could not; and that the right hemisphere could be shown to have all the components of the imagery task except for image generation (see also Kosslyn et al., 1985). This work has also dealt with the link between the imagery system and visual system (see Farah, 1988; Farah, Weisberg, Monheit, & Peronnet, 1990).

There has been considerable debate about the lateralisation of imagery processes (see Corballis, 1989; Farah, 1988; Goldberg, 1989; Kosslyn, 1987; Sargent, 1990). Some of this work has questioned the original evidence used by Farah (see Sargent, 1990), whereas other theorists have tried to argue that there are distinct types of imagery information involved in image generation that may arise in both hemispheres (see Kosslyn, 1987). The emerging consensus in this debate appears to be that the left hemisphere has a direct role in the generation of visual images, although the left may not be its sole preserve. Mechanisms in the right hemisphere do seem to play a role in image ROTATION (see Richardson, 1999). Both hemispheres are likely to contribute to image generation but in different ways (see D'Esposito et al., 1997; Farah, 1995; Kosslyn, Thompson, & Alpert, 1997; Tippett, 1992).

Kosslyn et al. (1993) have used PET techniques to investigate the localisation of imagery processing in the brain. They found that when subjects were instructed to close their eyes and evaluate visual mental images of uppercase letters that were either small or large, the small mental images engendered more activation in the posterior portion of the visual cortex whereas the large mental images engendered more activation in anterior portions of the visual cortex (see also Kosslyn, 1994, 1999). Continuing work in this vein has further supported these findings (D'Esposito et al., 1997; Kosslyn et al., 1997).

All of this research represents an important step from psychology into neuropsychology (see Kosslyn, 1999, for more of an overview). Apart from showing how psychological theories can include neuropsychological evidence, it also has important implications for the imagery-propositional debate. Farah (1984) has pointed out that in propositionalist terms there should be no difference between the recall and manipulation of information about the appearances of objects and information about other memory contents (e.g., historical facts or philosophical arguments). Hence, the occurrence of selective impairments to these types of information should be as likely as a selective impairment of imagery. However, specific impairments of historical ability do not occur but selective impairments of imagery do; moreover we can identify separate brain areas dedicated to this imagery ability.

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