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The following two sections describe impairments of attention and voluntary movements, two skills which can be thought of as relying on established cognitive routines. Although there is no clear cut distinction from knowledge stores, attention and movement are primarily actions rather than answers. Perhaps because they comprise action routines, these two areas have not been illuminated by graded tests, in the way that cognitive functions mediated by specific knowledge stores have been.

(i) THE NEGLECT DISORDERS

There is considerable evidence in the literature that a specific class of spatial disorders can result from brain lesions, which render the patient inattentive to events or certain

categories of stimuli in the hemispace contralateral to the lesion. It is thought to occur as a result of disruption of the processes governing attention. These unilateral neglect

disorders, as they are known, are most commonly the result of damage to the right cerebral hemisphere. This observed asymmetry has led to speculation that the right hemisphere has a bilateral role in attentional processes, whereas the left hemisphere only monitors right space.

The neglect disorders are of considerable theoretical interest because they illustrate fractionations of various attentional mechanisms ( in the reading of lines of text, individual words, reproduction of pictures, drawing, descriptions of remembered scenes and in putting on clothes, for example). The range of difficulties experienced by this group of patients has been extensively studied.

De Renzi (1982, p85) quotes an early and clinically useful test for detecting neglect that was described by Axenfeld in 1894. It simply requires the patient to bisect horizontal lines. Colombo, De Renzi, and Faglioni (1976), found that 45 of their 103 patients with

the side ipsilateral to their lesion, the remaining three displaced toward the contralateral side. Information about this phenomenon was extended by Heilman and Valenstein (1979), who demonstrated that the amount of the displacement was sensitive to the spatial position of the line. There was no advantage from indicating the whole line to the patient before the bisection.

Hecaen and Marcie (1974) reported a patient who crowded all his written words into one half of a sheet of paper. Albert (1973) asked his patients to cross through diagonal lines which were drawn all over a sheet of paper. Clear omissions were made by the patients with neglect. Similarly De Renzi and Faglioni (1967), asked their patients to copy small numbers of crosses drawn on one sheet of paper onto a blank sheet. Again, there were omissions by the patients with neglect.

There is even some evidence that, not only are stimuli on the contralateral side ignored, but stimuli on the ipsilateral side may draw attention to them. De Renzi et al (1989) compared 18 patients with right hemisphere unilateral lesions with 10 control subjects on two tests of neglect. The first was to read aloud a sentence. The second was to point to circles drawn over a page. On this basis, 8 of the right brain-damaged group were judged to neglect. The main experiment was the presentation of sets of four letters in the right half of the screen. The position of the letters in the horizontal row was not found to affect the reaction times of either the control or non-neglecting right brain-damaged group.

In contrast, the patients with visual neglect showed a marked progressive decrease in reaction time with the target letters’ increasing distance from the centre. When the target letter coincided with the fixation point, in the centre of the screen, the patients with visual neglect took three times as long as the patients without visual neglect. The authors speculate that this might be the result of an uncontrollable gaze deviation towards the rightmost extremity of structured space.

(ü) DISORDERS OF VOLUNTARY MOVEMENT

A wide range of voluntary actions has been demonstrated to be sensitive to acquired brain damage. These include single repetitive movements (e.g. Wyke, 1967), unfamiliar actions and action sequences (e.g. De Renzi et al, 1980), familiar gestures (e.g. Hecaen,

1978), object use (e.g. Poeck and Lehmkuhl, 1980) and constructional tasks. This latter class is perhaps the most likely to degrade performance on conventional tests of

intelligence and these will be discussed in more detail.

Constructional apraxia is characterised by failures in the organisation of complex actions in space. Tests of copying and drawing are widely used to detect this disability. Benton (1962) developed a set of geometric drawings which patients copy. An error score is obtained, which is compared to the distribution of scores recorded from a normal sample. Other investigators have studied patients’ difficulties when constructing block designs (e.g. Arrigoni and De Renzi, 1964).

There have been suggestions that the types of apraxic difficulties following right and left hemisphere lesions are qualitatively different. For example, Hecaen and Assal (1970) tested 32 patients with unilateral cerebral lesions on tasks involving drawing, copying and arranging stick patterns. Differences were noted in the performance of the right and left hemisphere groups. If landmarks were given, the left hemisphere group was helped, which the authors suggest means that the left hemisphere patients were failing to plan their constructions effectively during the original trial.

Another difference demonstrated was that if the scoring was on the basis of absence of correct lines, then the right hemisphere group was best; however if the scoring was on the basis of extra lines wrongly placed, then the left hemisphere group was best. Interestingly, apraxics who also presented with language disturbance obtained more benefit from

furnished landmarks than those apraxics without aphasia. This thoughtful study by Hecaen and Assal (1970) serves to highlight the complicated interdependence of the mental skills

which underlie even simple constructional tasks.