A Language
Since the middle of the last century, disturbances of language function have been reported to occur in the context of damage to the left cerebral hemisphere. There have been many studies which have sought a more precise localization for various specific aphasie deficits. Coughlan and Warrington (1978) demonstrated that patients with left temporal lobe
lesions were more impaired on naming tasks than patients who had suffered lesions to other regions of the left hemisphere. The finding was later replicated in a study of unilateral lesions (McKenna and Warrington, 1980).
Sentence comprehension can be impaired as a result of left hemisphere damage. The most widely used clinical test utilises abstract coloured shapes, which must be moved to
command (De Renzi and Vignolo, 1962). Swisher and Samo (1969) validated the Token Test for English subjects, using two shapes, two sizes and five colours of plastic tokens. This latter study compared aphasie patients with left hemisphere lesions, with a non- aphasic right hemisphere group. The control group consisted of hospitalized patients withouut cerebral damage. The left hemisphere aphasie group did less well on the Token Test overall than either the right hemisphere patients or the control group. In addition, the right hemisphere, non-aphasic group did less well than the controls on the last two
sections, which invoved more complicated commands.
De Renzi and Vignolo (1962) had noted that even patients who were apparently able to comprehend normal speech were shown to fail on the Token Test. This example illustrates a point that is very pertinent to cognitive assessment. Even when patients are fortunate enough to be able to continue with their routine daily tasks, the integrity of their intellectual fimctioning is far from proven. The everyday situation is predictable, rarely tests the limits of their knowledge and will often contain many extra cues and a great deal of redundant and repeated information.
For instance, small details of context may be essential to maintain appropriate fimction. McCarthy and Warrington (1987) report two patients who could follow instructions concerning two plastic tokens, as long as both tokens could be moved. However the patients were unable to follow the same instructions if one token was fixed; the
reprocessing necessary to work out how the other token must be moved to achieve the target arrangement defeated them.
B Arithmetic
Jackson and Warrington (1986) constructed a graded difficulty Arithmetic test consisting of 12 additions and 12 subtractions. There was a normal control group of 100 volunteers and two experimental groups with unilateral left and right hemisphere lesions. The left hemisphere lesion group performed significantly below the other two groups, between which there was no significant difference. The finding is in line with other studies, for example Dagenbach and McCloskey (1992), which have concluded that there is a store of arithmetic rules and number knowledge which can be disrupted by left hemisphere damage.
Specific knowledge about numbers can be impaired in one modality. Warrington (1982) described a patient, STH, who could hardly comprehend spoken numbers and was unable to make simple size comparisons between single digits presented aurally. However her aural comprehension of other words was demonstrated to be above average. There have been attempts to identify other cognitive functions, aside fi'om calculation rules, that are necessary for arithmetic problem solving. Some of these studies have implicated the right hemisphere and spatial skills in aspects of arithmetical problem solving, for example Hecaen (1962).
C Verbal Memory
Deficits in verbal short term memory can be demonstrated by asking the patient to repeat lists of digits. For example, Warrington and Shallice (1969) described a patient who could reliably repeat one, but not two, digits. The relation of this constraint to aural
comprehension is not straightforward (McCarthy and Warrington, 1990b), but it almost certainly compromises the encoding and manipulation of verbal material.
Other long term stores may be disrupted by damage to the left cerebral hemisphere. Of particular interest here is the verbal semantic system. For example, it is the semantic store which furnishes the knowlege that a canary is a yellow bird which is often kept as a pet and
sings. Patients have been reported with specific semantic impairments (e.g. Warrington, 1975). In addition categories of knowledge have been demonstrated to be differentially affected (McCarthy and Warrington, 1988).
(ii)THE RIGHT HEMISPHERE A Spatial Ability
From the disabling disorder of visual disorientation, described vividly by Holmes (1918), to problems with integrated spatial analysis, such as counting the number of blocks in a two dimensional line drawing (Warrington and Rabin, 1970), spatial impairments resulting fi'om right hemisphere damage have been extensively documented. Typically, the patient is no longer able to process spatial arrangements and relations efhciently. A test of spatial skills which has been shown to be differentially sensitive to right hemisphere damage is Elithom’s Mazes (1955). It consists of a triangular lattice with large black dots scattered across a number of the intersections. The patient must find a route which passes through a specified number of dots. The maze task was found to correlate highly with non-verbal and verbal intelligence tests, suggesting that there is a significant reasoning component to performance on this task.
Another spatial reasoning task that has been studied in the context of brain damage is the mental rotation of figures. Ratcliff (1979) used fi-ont and back view drawings of a human form, one hand of which was enclosed in a black circle. The patients, who had all suffered penetrating missile wounds during the Second World War, were required to say whether the circled hand was the right or the left one. For upright figures, lesion site was not a significant variable. However the right posterior brain-damaged group were significantly impaired when the human figures were inverted.
B Visual Perception
Cognitive impairments of visual perception have been documented at both a simple and a complex level of processing. The component discriminant functions of shape, colour,
acuity and localization have been shown to dissociate (e.g. Warrington, 1986). Object perception can also be affected and this is particularly striking if the patient is asked to identify pictures of objects from an angle that distorts the prototypical view (Warrington and James, 1988).
Empirical demonstrations of this particular pattern of impairment have used sets of
incomplete line drawings, which could not be identified by many patients with right parietal lesions, and photographs of everyday objects, taken from an atypical angle, which caused many right parietal patients to ignore the foreshortened long axis and thus fail to identify the objects (Warrington and Taylor, 1973).
Patients with a classical apperceptive agnosia would have no difficulty in describing the physical attributes of an object, but they would be unable to integrate their fragmented visual percepts into recognition of the object as a whole. For example, a pencil might be described as a long thin blue thing, which is flat at one end and pointed at the other. The patient would have no idea of the object’s name or use, because that would require them to perceive the whole item. They are no longer able to do this, because their store of object percepts cannot be used effectively.
c Visual Memory
There are visual memory systems which are similar in type to the verbal memory systems. Tasks used to document short term visual memory deficits include tachistoscopic exposure to groups of abstract symbols (e.g. Alajouanine, 1960) and a version of digit span which requires the patient to tap out a series of positions on a board (Milner, 1971). Visual memory functions also mediate the recognition of maps, scenes and routes (see Whiteley and Warrington, 1978). Spatial and perceptual learning has been demonstrated to rely on right hemisphere function (Kimura, 1963; Warrington, 1984).