Scanning hypothesis: visual perception synchronized with alpha activity

28

Abstracts /International Journal of Psychophysiology 25 (I 997) 17-84

based on the ideas that (1) the mental activity emerge as a result of synergic activity of the cortical selective neuronal groups, responsible for particular data processing; (2) these groups have the features of neuronal oscillators characterized with their own frequency properties; (3) the EEG and ERP spectral components reflect the activity of the main cortical oscillators; (4) the exact coincidence in frequency properties of two or more oscillators promotes their functional interaction. The brain image in IIM is realized in two versions. In interpolation map the cortical areas differ according to the normalized number of the spectral peaks coincidence between this and all other electrode sites, emphasizing the location of the connections centers, whereas ‘arrows’ version presents the actual pattern of cortical binds. Applied to the study of cortical connections in rather simple tasks such as left and right hand tingers movements, IIM reveals some advantages in comparison with the coherence analysis. In fact IIM is invariable to the oscillations phase shifts during the analysis epoch, this shift being quite probable in neuronal groups communications at mental functions performance. The IIM was used for the study of the brain connectivity in the thinking process. The specific patterns were found for particular mental operations, such as imaginative, spatial and abstract-verbal thinking. It is of importance that connectivity patterns could be obtained only while analyzing the primary effects of nervous excitation but not the secondary metabolic effects used in 3-D brain imaging methods. REACTIVITY OF EEG BURST SUPPRESSION DURING ANAESTHESIA K. Hartikainen’*, M. Rorarius’ and V. Jantti3. ‘Department of Physiology, Tampeer Brain Research Center, Tampere University, Medical School ‘Department of Anaesthesiology, Tampere University Hospital ‘Department of Clinical Neurophysiology, Oulu University Hospital We have studied the reactivity of the brain at deep levels of anaesthesia, during which electroencephalography (EEG) shows a burst suppression pattern. High amplitude bursts of cortical electrical activity are interrupted by periods of relative quiescence. Behind this phenomenon there is a non-linear mechanism, which forces the brain to switch between two stages, i.e., periods of high and low amplitude activity. Burst suppression has many dynamical features characteristic of non-linear systems. Burst-suppression pattern shows irregularity of the suppression periods and unpredictability of occurrence of bursts. Burst suppression induced by anaesthetics, such as isoflurane, is very reactive to external stimuli. Low amplitude suppression changes into a high amplitude burst with a latency of 300 to 500 ms from auditory, somatosensory or visual stimulus. However, all stimuli do not produce bursts. Regularly occurring stimuli do not produce bursts due to habituation, while novel stimuli of very low physical intensity can produce a burst. Bursts exhibit stimulus specificity; dif-

ferent modes of stimulation produce different patterns of bursts, while an identical stimulus produces repeatedly bursts with constant latency and waveform. This may reflect specific sensory processing during deep anaesthesia. Deep anaesthesia with burst suppression pattern provides a simple model of brain activity, when most of the noise that is present in the awake brain is filtered. Studies on sensory evoked bursts, with different stimulus parameters, provides tools for addressing questions concerning reactivity, arousal, habituation and possible short term memory of the preceding stimuli and the effects of different drugs on these brain functions. ANALYSIS OF SINGLE FOLLOWING PAINFUL LASER STIMULATION

TRIAL BRAIN RESPONSES AND NON-PAINFUL CO,

A.D. Towell* and S.G. Boyd The Hospital for Sick Children, Great Ormond Street and University of Westminster, London We have previously recorded CO, laser evoked averaged potentials with vertex positive (VP) components at 300 ms (VP3001 and 800 ms (VPSOO) corresponding to painful and non-painful stimulation which we have proposed to be mediated by A6 and C fibres respectively. However, analysis of averaged data and subjective ratings to heat and sharpness did not discriminate between the VP300 and VP800. We therefore recorded single trials in 12 adult subjects who received 20 painful stimuli and 20 non-painful stimuli delivered to the dorsum of the left hand, the order of intensity being counterbalanced between subjects. Following each stimulus subjects rated pain, sharpness and heat on a 10 point scale. Using single trial analysis the amplitude of the VP300 was greater than the VP800 and multiple regression statistics revealed that pain ratings were a significant predictor of amplitude for the VP300 whilst heat ratings were the best predictor for the VP800 (p < 0.06). No such relationships were seen with analysis of the signal averaged data. These results suggest single trial analysis is the optimal method for interpretation of laser evoked potential data and further supports the psychophysiological distinction between the VP300 and VP800. SCANNING HYPOTHESIS: VISUAL PERCEPTION CHRONIZED WITH ALPHA ACTIVITY

SYN-

LA. Shevelev* and V.M. Kamenkovich Dept. Sensory Systems, Institute of Higher Nervous Activity & Neurophsiology, S-a Butlerova Street, Moscow 117865, Russia The scanning hypothesis (Pitts and McCulloch, 1947, Bull. Math. Biophys., 9: 127-147), which postulates spreading of an excitatory wave over the visual cortex with frequency of the alpha rhythm, was tested in experiments with 27 human subjects. We compared the recognition of figures of different form and size or recognition of direction of apparent light motion when they were asynchronous or synchronized to

Abstracts /International Journal of Psychophysiology25 (1997) 17-84 different phases of the EEG alpha wave recorded over the occipital cortex. Reliable dependence of visual recognition on alpha wave phase was found: 1) Better recognition of small figures at relatively later phases of alpha wave was found, while of big figures (up to 13*) - at earlier phases; 2) If the motion acclerated away from the fixation point and was synchronized with the alpha wave it was more likely to be perceived, whereas if it was towards the fixation point it was less likely to be seen. However, perception of constant-velocity motion was not changed by linking it to the alpha wave phase; 3) Under flicker stimulation through closed eyes with EEG alpha rhythm frequency, 11 out of 12 subjects perceived clear illusory visual objects (in descending probability: rings, circles, spirals or spiral springs, funnels and curved crossed bars) and the frequency of optimal stimulation was closely correlated with individual dominant alpha frequency (r = 0.86, p < 0.01). These results indicate that the EEG alpha wave reflects the periodical excitatory process that evenly scans the visual cortex from its near periphery to its central area and that this process plays a substantial role in visual processing and recognition. TEMPORAL NISM

LIMITS OF AN INTEGRATION

MECHA-

E. Sxelag*(i), K Rymarczykt’), M. Deutschman(‘), E. Poppel(*) and N .v . Steinbiichel@). “‘Dept. of Neurophysiology, Nencki Institute of Experimental Biology, 02-093 Warsaw, POLAND “‘Institute for Medical Psychology, 80-336 Munich, GER-

Temporal integration - one of mechanisms at the highest level of a taxonomy of time perception allows to integrate information over time and to bind, up to 3 s, successive events into one unit. Here we report experimental evidence on potential hemispheric differences in upper (Exp. 1, 2) and lower (Exp. 3) limits of this binding process and on factors affecting these limits. In Exp. 1 and 2, subjects grouped together dichotically presented beats of a metronome, to create mentally a subjective rhythm. The subjects either reported verbally how many beats they were able to integrate (Exp. 11, or pressed a button at the beginning and at the end of the integration sequence (Exp. 2). The integration interval lengths were analyzed for different metronome frequencies presented to the right ear (predominantly addressing the left hemisphere) or to the left ear (right hemisphere). The results showed that successive beats were integrated up to app. 3 s. The length of integration periods depended on the frequency of presented beats (the higher frequency the shorter integration interval). There was no hemispheric asymmetry in the length of the integration periods. The results suggest that though the upper limit of intergration depends on the mental content of the information processed, it constitutes, however, a general intra-hemispheric process of a major complexity. In Exp. 3, subjects reacted as fast as possible or with a defined time delay to a tone by pressing a button with the

29

right hand (left hemisphere) or with the left hand (right hemisphere). Requested delays varied between 200 and 750 ms in steps of 50 ms. The surprising result was that for short delays (requested response times 350 ms) variability was much larger than for longer delays (above 350 ms), especially for responses with the right hand. Thus, precise temporal control on consciously mediated actions sets only in after a rather long delay. Neuronal mechanisms underlying this lower limit of integration appear to be different in the two hemispheres, the left hemisphere showing surprisingly higher temporal variability. The results are discussed with respect to speech perception. TRANSIENT SIDERED

NEGLECT:

WHY IT SHOULD BE CON-

M. Jehkonen*, J-P. Ahonen, P. Dastidar and G. Molnar Tampere University Hospital, P.O. Box 2000, FIN-33521 Tampere, Finland The aim of this study was to evaluate the spontaneous recovery of visual neglect in a sample of 24 neglecters suffering from right hemisphere infarct. The patients were examined twice, three and ten days after the onset, by a neuropsychologist and a neurologist. A MRI or CT study was carried out once to verity the localization and the size of the lesion. The main method in assessing visual neglect was the Behavioral Inattention Test. Out of the 24 neglect patients 38% had recovered up to ten days after the onset, thus showing transient neglect (TN). The star cancellation test of the Behavioral Inattention Test was the most sensitive measure to detect reversible neglect (89% of the cases). The TN-group had less visual field defects (22% vs. 53%) and motor defects (33% vs. 47%) than patients having persistent neglect. The MRI/CT-findings of the TNgroup did not differ from the neglecters in five cases. In this study visual neglect remitted in 38% of the cases studied in acute phase. To select patients for intensive treatment of visual neglect, we should be able to differentiate the TN-patients from those having persistent neglect, already in the acute phase. EVENT-RELATED POTENTIALS DURING LEXICAL DECISION TASK. DIFFERENCES IN N400 BETWEEN CONCRETE AND ABSTRACT WORDS Junk0 Ito*, Sachiko Koyama’ and Yasuhiro Nageishi’ Central Clinical Laboratory, Division of Neurophysiology, Kyoto University School of Medicine, Kyoto ‘Department of Integrative Physiology, National Institute for Physiology, Okazaki *Teaching profession center, Asahi University, Gifu, Japan Patients with mild aphasia often show impairment of reading in addition to speech and listening comprehension. Recalling concrete words is easier than recalling abstract words for