V E S T I B U L A R STIMULATION, LEFT SOMATOSEN- SORY DEFICITS A N D SPATIAL H E M I N E G L E C T

G. Vallar Istituto di Clinica Neurologica, Universith di Milano, Milano, Italy

Somatosensory deficits produced by right-hemisphere lesions may be temporarily ameliorated by vestibular stimulation, which may also positively affect a number of manifestations of the neglect syndrome. Furthermore, neglect patients may dis- play conductance skin responses and normal somatosensory- evoked potentials to stimuli of which they are not aware. Taken together, these observations suggest that hemianaes- tbesia in neglect patients is due, at least in part, to the defective access to conscious experience of information which has undergone some early unconscious processing. This ab- sence of perceptual awareness, in turn, reflects a pathological lack of correspondence between somatotopic and perceptual egocentric representat ions of the body, which is partly re- stored by vestibular stimulation.

E V E N T - D E P E N D E N T DIF F E R E NC E S OF P3 AMPLI- TUDES BETWEEN SCHIZOPHRENICS AND H E A L T H Y CONTROLS

R. Verleger ~, M. Bode ", V. Arolt b and D. K6mpf d Depar tment of Neurology, University of Lribeck, Lribeck,

Germany and h Depar tment of Psychiatry, Liibeck, Germany

In a guessing paradigm, Levit et al. (1973) found that the event to be predicted evoked larger P3s in the uncertain than in the certain condition and that this effect was larger in normals than in schizophrenics. From informal observations made in our replication of this finding, we assumed that P3s were smaller in schizophrenics because the patients did not enjoy guessing. They preferred the well-structured certain condition. Here, therefore, we measured potentials evoked by all events (information, central event, account), predicting that schizophrenics' P3s would not be smaller with the infor- mation given in the certain condition and with the display of the money earned. In each trial, a light or a sound was presented. Subjects (15 schizophrenics and 15 control sub- jects) had to confirm this 'central event' by a delayed re- sponse, and then the balance of account was presented. In the certain condition, the central event was preceded by the appropriate information on the screen ('light' or ' sound') whereas in the uncertain condition, the word 'guess ' ap- peared. The control subjects had larger P3s than the schizophrenics with the central event, as usual, but as pre- dicted both groups had equally large P3s at the information and at the state of account. These results are consistent with the hypothesis that P3 is smaller in schizophrenics due to motivational factors.

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PRIMING OF THE W R O N G RESPONSE BY INVOLUN- TARY READING, AS M E A S U R E D BY THE LATERAL- IZED READINESS POTENTIAL

R. Verleger ~, T. Schulz h, A.-M. Striven ~' and E. Wascher ~' a Depar tment of Neurology, University of Lribeck, Lribeck, Germany and b Institute of Psychology, Ruhr-Universitiit Bochum, Bochum, Germany

In the Stroop task, the color of color words has to be named. If word and color are not congruent, overt responses get delayed by this conflict but usual event-related potentials (e.g., P3, N400) are not affected. The conflict might, however, be visible as wrong lateralisation of the readiness potential. To measure the lateralized readiness potential in the Stroop task, we required left- or right-hand responses instead of verbal responses. In order to still guarantee verbal mediation of the response, response mapping changed on each trial and was displayed on the screen before each Stroop stimulus (e.g., ' red ' and "yellow' on the left, 'b lue ' and 'green' on the right). We expected that, e.g., with the Stroop stimulus ' red ' in blue (correct response: blue, therefore right; wrong response: red, therefore left) the readiness potential would first be lateral- ized on the wrong side. In the first experiment, only some subjects' responses were delayed by incongruence. These sub- jects indeed exhibited an early lateralization on the wrong side. In the second, more difficult, experiment, incongruence delayed all subjects' response times (from 910 ms with congru- ent stimuli to 1090 ms with incongruent ones) and indeed an early lateralization was found for incongruent stimuli in the whole group at 350 ms. In conclusion, the dominant word activated its response very quickly. This finding suggests that inhibition of this wrong response tendency plays an important role for the response delay with incongruency.

A U T O N O M I C NERVOUS SYSTEM ACTIVITY AND M E N T A L W O R K L O A D

E. Vernet-Maury ~, C. Deschaumes-Molinaro ", G. Delhom- me h and A. Dittmar b ~' Laboratoire de Physiologie Neuro-Sensorielle, CNRS, Villeurbanne, France and b Laboratoire de Thermor6gula- tion, CNRS, Lyon, France

An original auditory generator apparatus was used to quantify mental workload. Special rules must be used in the dual task technique, and this apparatus is well adaptated to them. The primary task consisted of processing data and the secondary task consisted of response (pressing right or left pedal) at auditory stimuli requiring a binary choice. The subject re- sponses were quantified using error level and reaction time. Successively, the subject works under two strategies: correct or rapid.

Using this methodology, calculated values of the mental workload mark (C) were reliable and precise (C = 0.81 _+ 0.05).