brain", the right hemisphere, appears to be responsible for spatial relations, so-called "gestalt" perception (exemplified by the ability to rapidly estimate the number of dots on a card after an extremely brief exposure, without actually counting each dot), and "part-to-whole" judgments (for example, matching arcs to circles). At least some aspects of musical perception may also be done by the right hemisphere (Milner, 1962).

The eminent neurosurgeon, Joseph Bogen, in a fascinating series of papers (Bogen, 1969a, 1969b; Bogen and Bogen, 1969), has speculated that the two sides of the brain utilize two different cognitive modes, one "propositional" (analytic, digital) and one "appositional" (analogic, synthetic). Only the intactness of the corpus callosum, the fibres connecting the two hemispheres, allows us the illusion that we have just one mind.

A variety of techniques have been used by researchers to ascertain "where things are" in the brain. At one time, researchers had to depend on "natural experiments", the unfortunate consequences of unilateral brain damage caused by tumor, strokes and man-made accidents (e.g. gunshot wounds). Correlations were made between the locus of a lesion and the type of impairment the patient suffered: for example, we know that in adults, aphasia is nearly always the result of injury to the left hemisphere (Russell, and Espir, 1961, report that 97 per cent of their 205 aphasics had left hemisphere damage).

While researchers continue to rely on data from brain damage to a large extent, more recently laterality in normal subjects has been investigated using several harmless techniques. In dichotic listening, subjects are presented with competing, simultaneous auditory stimuli (e.g. the right ear hears "ba" while the left ear hears "ga"). A right-ear advantage in response accuracy is thought to reflect left hemisphere processing for the stimulus presented. In adults, dichotic presentation of verbal stimuli typically results in a right-ear advantage (Kimura, 1961), while certain non-verbal stimuli (environmental sounds, musical chords; see Curry, 1967; Gordon, 1970) yield a left-ear advantage. The right-ear advantage for verbal stimuli typically seen in normal subjects is generally quite small, but it is reliable, and, when groups of subjects are used, it is usually statistically significant.

Other techniques include the use of EEG and AER (average evoked

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