Vibrotactile reaction times in normal dextrals were measured for the two hands separately when either hand was located at each of seven possible positions: 90 degrees, 45 degrees, and 15 degrees to the left and right of the chest midline, and at the midline itself (0 degrees). Reaction times for the two hands did not differ and there was no Hand by Position interaction. At 90 degrees left, reaction times were significantly slower than at any other position except 45 degrees right. However, none of the other positions, including 45 degrees right, differed from each other. Performance in this task, therefore, was relatively uniform from 90 degrees right to 45 degrees left, but markedly slower at 90 degrees left. This far-left-side disadvantage may reflect a difficulty (for dextrals) in focussing covert attention in the far-left part of space for a block of trials. Since vibrotactile reaction times are sensitive to attentional factors in normal subjects, the paradigm should allow quantification of the clinical symptoms of the hemineglect syndrome; some preliminary observations of this syndrome with another vibrotactile design are reported.

Two vibrotactile RT experiments were performed with the stimulated and responding hand held on one or other side of the body. Subjects could or could not predict which hand would be .stimulated and have to respond, and overt gaze was directed to that or to the other (inoperative) hand. The effects were studied of hemispatial asymmetries (present only in the absence of position uncertainty), spatial eompatability effects (occurring only in its presence), and overt (directed gaze) and covert (expectancy based) directed attention. Sustained and transient aspects of holding and shifting attention were seen to be important determinants of hemispatial asymmetries.RKSIIMK Deux experiences dc T.R. vibrolactils ont cte conduiles, avec la main stimulee et executantc placee de Tun ou I'autre cote du corps. I.cs sujets pouvaient ou nc pouvaient pas prcdirc quelle main allait ctrc stimulee el devaicnt rcpondre, leur regard etant dirige vers I'une ou I'autre main inoperantc. Ont etc cludics les effcts des asyrnetries hcmispatiales (prcsentes sculcnicnt en I'absence d'inccrtitude dc position), les effcts de compatibilitc spatiale (survenant seulcmcnt en sa presence) et I'attcntion dirigee ouvcrtc (regard dirige) ct voilce (basee sur l'expcctation). Les aspects transitoires et soutcnus du mainlien et du dcplacemenl de I "attention sont vus comnie des determinants importants des asymetries hcmispatiales.Classical laterally effects (e.g., a right-ear or right-visual field superiority for verbal material) are usually ascribed to the prepotency of the crossed contralateral pathways which connect receptor surfaces on one side of the body with sensory and motor areas in the opposite (left for language) cerebral hemisphere (see Kimura, 1967). However, in a recent scries of auditory, visual, tactual, and kinesthetic experiments, we have found that performance may be determined by the exlracorporcal spatial location of the stimulus or response, rather than by the ear of entry, hand, or visual field (Bradshaw, Nettleton, Pierson, Wilson, & Nathan, 1987; see also e.g., Corballis, 1986; Morais & Bcrtelson, 1975;Williams, 1986). These findings, while not compatible with traditional anatomical-connectivity models, may be explained, however, by a hemispace account of latcrality effects. This account suggests that each hemisphere is responsible for mediating events which occur in the contralateral hemispatial field, with improved performance when the location of the stimulus or response event is contralateral to the specialized hemisphere.*Wc thank Bob Wood, John Dick, Frank Devlin, and GcolT Mead lor their continued assistance in the design, construction and maintenance olapparalus, and gratefully acknowledge the financial support we continue to receive from the Australian Research Grants Scheme. Address reprint requests to J.

Patients with left hemispatial neglect resulting from right hemisphere damage performed a vibrotactile reaction time (RT) task with the stimulated and responding (right) limb placed either ipsilaterally in right hemispace, or across the midline in contralateral (left) hemispace. In 8 patients who had undergone little or no rehabilitative training, responses made on the left were very much slower (by an average of 45 ms) than responses made on the right. Four out of 5 rehabilitated patients who had been taught compensatory strategies to overcome their difficulty in attending to events on the left, and who were tested with eyes open, were faster on the left, a pattern opposite to the small (9 ms) but significant right-side advantage shown by normal subjects. However, when 4 of the rehabilitated patients were tested with their eyes closed, their performance reverted to that shown before rehabilitation, that is, a substantial left-side disadvantage. Overt and covert attentional strategies and spatial asymmetries in normal subjects and patients with unilateral attention deficits are discussed. The vibrotactile RT task provides a promising method for quantifying disability during the various phases of the neglect syndrome.

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