There is evidence from neuropsychology that visuospatial processing is not a unitaiy phenomenon and can be further subdivided into at least two parts, visual and spatial. Using PET scans, Jonides, Smith, Koeppe, Awh, Minoshima and Mintun (1993) reported data which suggest that the storage of images occurs in the occipital lobe while the record of its position in space takes place in the parietal lobe. This is anatomical evidence for separate visual and spatial stores in the visual sub-systems in working memoiy. Farah, Hammond, Levine and Calvanio (1988) also argued that "there exists evidence that in animals and in humans the representation of the visual appearance of stimuli and the spatial location of stimuli are subserved by distinct, independent systems" (p445). Furthermore, they suggest that this visual and spatial distinction is also maintained in mental imagery and that visual and spatial imageiy consists of separate and independent representations. In this sense, it is argued that it is not a question of whether imagery is visual or spatial but whether a particular task depends more on one kind of representation than another. They describe a patient, L.H., who shows a clear dissociation in performance on visual and spatial imageiy tasks. L.H.'s performance on visual imagery tasks (involving colour, size comparisons and classification of shapes and animal tails) was very poor compared with controls. His performance on spatial imagery tasks (involving various image transformations and memory for spatial locations), however was within the normal range. L.H. had damage to the right temporal lobe, as well as damage to temporo-occipital regions in both hemispheres.
In the neuropsychology literature, the case of L.H. is often contrasted with that of E.L.D who is argued to show intact visual imagery but to have a deficit in spatial processing (Hanley, Young and Pearson, 1991). E.L.D. performed well on the same visual tasks used by Farah et al (1988) but had a poor performance on the Corsi blocks test and the Brooks matrix task Hanley et al (1991), however, interpreted E.L.D's pattern of performance as suggesting that she has a deficit in the part of the visuospatial system that processes incoming visuospatial information but that the part of the visuospatial system involved in the retrieval of visuospatial information from long-term memory is untouched. They argue that this is consistent with Logie (1989) who proposed that short-term memoiy comprises of an active rehearsal process and a functionally separate store. By this reasoning, E.L.D.'s passive store would be intact and able to receive information directly from long-term memory, but the active part of the store which manipulates and maintains information could be damaged.'The contribution of long-term storage means that the case of E.L.D. is not a perfect double dissociation with the case of L.H. Moreover, although E.L.D. is shown to have damage to the right hemisphere, the damage is widespread and extends into the right frontal lobe.
Piggott and Milner (1993) argue that loss of spatial memory is often associated with hippocampal damage (though this does not seem to be the case with L.H ), while visual memoiy appears to rely on the right temporal lobe. They report a group of patients with right anterior temporal damage who show impairments in the recognition and recall of faces, complex visual scenes and geometric designs. Therefore, although there is some disagreement in the literature over neuroanatomy.
there is clear evidence from neuropsychology that there are functionally distinct visual and spatial stores in the VSSP.
Baddeley and Lieberman (1980) investigated whether the system involved in the control processes of imagery employs visual or spatial coding. They used Brooks' matrix task (Brooks, 1967). The secondary suppression tasks involved subjects either having to discriminate between light and dark squares (a visual task) or having to keep a torch beam on a photo-sensitive cell on a pendulum while blindfolded (a spatial task). Baddeley and Lieberman found that remembering the nonsense sentences was dismpted more by the visual task than the pendulum while the Brooks spatial task was disrupted more by the task with the pendulum. Both a peg-word and a method of loci mnemonic were disrupted by concurrent tracking, whereas a rote rehearsal strategy was unaffected. On the assumption that the concurrent tracking task was predominantly a spatial task that was relatively free from visual input, Baddeley and Lieberman (1980) concluded that the VSSP in working memory relies on spatial rather than visual coding. They proposed a simple analogy in which the VSSP was a screen on which spatial information may be represented. Although the paper focuses more on spatial processing than visual processing, however, they do not rule out the existence of visual processing within working memory. Although they argued that the VSSP relies more on spatial processing they maintained that this "does not ... preclude the occurrence of a parallel system or component concerned with pictorial or nonspatial visual representation" (p537). Indeed, they suggested that the demonstration of "blindsight" by Weiskrantz, Warrington, Sanders and Marshall (1974) would support the notion of two separate visual systems, one dealing with where an object was seen, the other dealing with identification of what was seen.