Nonlinear brain dynamics: Working memory load and attention

40

Abshacts

/International

Journal of Psychophysiology

recorded. The signals can be flashing lights, auditory clicks, tactile pulses, electrocutaneous shocks, or combinations thereof in intermodal comparisons. The capacity of the brain to process information on the level of signal processing can be assessed with this new neuropsychological method. Preliminary results obtained with the method are reported and the determinants of the speed of signal processing in the brain are reviewed. NONLINEAR BRAIN DYNAMICS: WORKING MEMORY LOAD AND ATTENTION G. Sammer*, Institute of Psychology, University of Hamburg, Von-MellePark 11, D-20146, Germany In the present study effects of both working memory load and attentional processes without memory scanning on the dimensional complexity of the EEG were investigated. A 12 x 12 grid was displayed on a computer screen. Starting at one of the units, a trail passed about 18 units (duration t: 21 set; 3, 6 or 9 changes in direction), followed either by an identical or a deviant probe-trail. Ten subjects were instructed to continuously check the actual probe-trail for the first deviation from the previously memorized trail and to immediately indicate it’s detection by pressing a button. At the end of each probe-trail a simple reaction time measurement was implemented to maintain the subject’s attention even after the detection of a deviation. Dimensional complexity and spectral power were computed for EEG-periods (8 sec.) before and after the deviation (n = 18) respectively the mid-unit (identical probes; n = 18). Both trials without correct responses and trials with early or late deviations were excluded from further processing. The EEG was recorded from 17 standard positions (lo-20 system) against linked mastoids (bandpass 2.56-23 Hz). Main results were a higher dimensional complexity and reduced anterior theta after correctly detected deviations (deviant probes) and a decreased dimensional complexity in relation to the prolonged search demand with identical probes. These effects reached statistical significance already on the basis of single case analysis (ANOVA). The scalp distribution of the dimensional complexity was different for attentional and memory scanning processes. The results are in correspondence with recent findings which relate a decrease of EEG-dimensional complexity to increased mental activity over the involved brain regions. E. Lorincz* and M. Fabre-Thorpe (1). School of Psychology, University of St Andrews, Fife, Scotland, KY 16 9 JU, U.K. (1) CERCO, EHESS, UMR 5549, 133 route de Narbonne, 31062 Toulouse Cedex, France. The parietal cortex is now well established as being one of the primary structures of the dorsal pathway, involved in the processing of sensori-motor information necessary for goal

25 (I 997) 17-84

directed movements. Modulated by different structures involved in attentional, mnesic and motivational processes, lesions of the parietal cortex produce complex deficits in humans as well as in monkeys. However, the main impairments involve space perception and exploration, being thought to be due to problems in perceiving the relative position of objects, or the inability to shift attention. In this study, we investigated if similar deficits can be reproduced in cats by lesionning the areas widely thought to be the homologue of the posterior parietal lobe of humans. In order to examine space perception and exploration, cats were trained to perform a two-choice visuo-motor task in which they were required to point to and depress one of two levers (targets) in response to a visual cue. Two types of spatial associations were tested: compatible and incompatible. For the compatible associations the visual cue was presented at the same location as the correct target. For the incompatible associations, the visual cue and the target were not only spatially dissociated but also located in different hemifields. Moreover, in this associations the visual cues were either far or near from the targets. The bilateral lesions of the areas 5 and 7 impaired mildly and transiently both the accuracy of guided movements and only the performance for the compatible associations. In contrast, cats consistently failed to select the correct target when performing incompatible associations although they did not resort to the compatible behavior pattern. Moreover, no difference was observed depending on the eccentricity of the visual cue. These results strongly suggest that the parietal cortex plays a major role in complex cognitive processes in cats. VISUAL-CORTEX ACTIVATION BY SOUNDS IN HUMANS WITH EARLY- AND LATE-ONSET BLINDNESS T. Kujala*, K. Alho’, M. Huotilainen’, R.J. Ilmoniemiz, A. Lehtokoski’, A. Leinonen’, T. Rinne’, 0. Salonen3, J. Sinkkonen’, C.-G. Standertskjold-Nordenstam3 and R. Naatiinen’ ‘Cognitive Brain Research Unit, Dept. Psychology, P.O. Box 13, FIN-00014, University of Helsinki *BioMag laboratory, 3Dept. Radiology Helsinki University Central Hospital, FIN-00290, Helsinki, Finland It is commonly held that cross-modal reorganization may occur in an immature but not in a mature brain. This belief was put in a test in the present study by recording auditotyevoked responses in early-blinded (before the age of 2) and late-blinded (12-28 years after birth) adults and in sighted control subjects. The subjects were presented with sound sequences containing 600 Hz standard tones (p = 0.9) replaced occasionally by 660 Hz deviant tones (p = 0.1). During the recordings, the subjects were either counting deviant stimuli or ignoring the stimulation. There were no differences in the front-back distributions of the mismatch negativity, elicited by deviant tones even when the stimuli were ignored. The N2 and P3, elicited during active detection of deviant tones, were posterior in distribution in the blind compared with that in the sighted. In order to confirm that the activity elicited by deviant