EVOKED RESPONSE CORRELATES OF LETTER RECOGNITION

Transcription

1 ACTA NEUROBIOL. EXIP. 1980, 40 : Short communication EVOKED RESPONSE CORRELATES OF LETTER RECOGNITION I. RADILOVA, A. GRABOWSKA, T. RADIL-WEISS, L. MARAS 'and W. BUDOHOSKA Institute of Physiology, Czechoslovak Academy of Sciences Prague, Czechoslovakia Abstract. The positive late wave of the average evoked responses induced by tachistoscopic presentation of single letters had a higher amplitude when the letters were correctly recognized than when they were not. The results suggest that the late positive component of VER is a correlate of complex cognitive processes engaged in object's identification. Letter recognition in humans is a highly complex phenomenon which is usually studied by means of psychophysiological methods (1,.2, 4, 6, 14). Repeated tachistoscopic (i.e., short-lasting) presentation of stimuli and definite time relationship between the instant of stimulus and the course of the mental process studied (7, 8), as well as the latency period of recognition (12, 13) provide an opportunity to investigate the underlying neural processes by the method of evoked potential averaging. The aim of this experiment was to show whether the occipital average evoked responses show any differences when the stimulus letters are, or, respectively, are not recognized correctly. As the stimulus material is the same in both cases, any differences in the average evoked response shapes might be raleted to brain processes responsible for the recognition of semantic signals. An attempt was also made to find the electrophysiological evoked response correlates of the facilitatory influences between two letters observed in some previous experiments (1, 6, 7). In those studies letters presented in pairs were re-

2 cognized better than single letters. A preliminary report of these results was published (9). Altogether 44 experimental sessions were performed in 23 young adult (16 females and 7 males) subjects with normal vision. The subjects sat in comfortable chairs in a dark, electrically and acoustically shielded chamber and watched a screen placed 90 cm in front of the eyes and subtending a 3' X 2' visual angle. The stimulus material consisted of ten upper case, printed letters (A, B, C, E, F, L, 0, P, T, Z) white against an almost dark background; each letter subtended 56' by 38' visual angle. single letter pair of letters Fig. 1. Examples of stimuli used. The characters were presented for 20 ms either separately or in pairs (Fig. I), by means of a Kodak Carousel projector. The stimulus luminance was set for each subject individually before the experiment at a slightly suprathreshold level, so as to provide about 50 /a of correct recognitions. The recording electrode was placed upon the occipital scalp, 3 cm above the inion, whereas the indifferent one was attached to the left earlobe. The EEG signal was amplified by means of a solid state amplifier, monitored constantly on an osciloscope and fed into a DEC LAB 8lE computer. The system adopted in this experiment (15) consisted of a modular electronic control system (SUP), the above-mentioned computer and the projection devices. Each visual stimulus was presented 1,2 s after a warning beep (Fig. 2) calling the subject to be attentive and to fixate the middle of the screen. The Kodak Carousel projector and the shutter were operated by the system automatically. The computer sampled the EEG activity during a period which began 0.5 s before the shutter's opening and terminated 1 s after it, and than stored the sample in its memory. The subjects were pretrained and properly instructed to report their visual sensations after the presentation of each stimulus; their reports reached the experimenter via an acoustical intercom device. The experimenter then classified their answer as correct or incorrect by pressing one of the two buttons. The evoked potential, which was preliminarily stored i n the computer memory, was then addressed to one of its two averaging systems. The above setup solved the problem resulting from longer latencies of the subjective evaluations of the stimuli in comparison with the latencies of the evoked potentials. The system allowed us to classify single evoked responses into two groups according to the criterion of correct or incor-

3 rect recognition of the physically identical groups of stimuli and to average each group separately. Fig. 2. The timing of consecutive steps controlled by the programming system (SUP) during each trial of stimulus presentation. For each trial, the experimenter classifies the evoked response according to the correctness of recognition. After a warning beep (acoustical stimulation) the EEG activity is sampled (synchronous pulse). Then the shutter of the Kodak projector is opened (shutter control). During the 6 s delay time the subject reports on his perception and the experimenter classifies (sorte code) the single evoked potential preliminarily stored in the memory. The last step is the automatic change of a stimulus slide in the Caroussel projector (Kodak control). Then the whole cycle starts again. The average evoked responses (compi1.ed of about 60 single evoked potentials) were characterized, under the above-mentioned experimental conditions (small stimuli of low luminance), mostly by a long latency and slow (peak latency about ms) positive wave (Fig. 3). According to the aims of this study, two types of average evoked response comparisons were performed: (1) Comparisons of the average responses evoked by letters which were correctly vs. incorrectly recognized, separately for single letters and for pairs of letters (in the latter case the evoked responses were classified according to correct or incorrect recognition of both letters. (2) Comparison of the average responses evoked by correctly recognized single letters and correctly recognized pairs of letters. The examples of the first-type comparisons for single letters are given in Fig. 3. Amplitude differences between average responses evoked by correctly and incorrectly recognized stimuli exceeding about 20 /a of total amplitudes were considered significant. The

4 incidence of cases when the late positive wave was higher for correct than for incorrect recognition, when the two waves were equal and when the wave was higher for incorrect recognition, is expressed Fig. 3. Average evoked responses after single letter presentations elicited in four different subjects (A, B, C, D) by the same stimuli uder two conditions: when they are correctly (continuous line) or incorrectly (dotted line) recognized. In subject A, the same experiment was repeated twice (A, and Az). Notice that ms after the stimulus presentation, the two curves begin to differ, the "correct recognition" curve having a higher amplitude. in Fig. 4. From the total amount of 16 experimental sessions with single letters the amplitude of the late positive wave was higher for correct recognition 11 times, no difference was found 4 times and the opposite result was recorded once. It may be concluded that the higher amplitude of this wave in the case of correct recognition of single letters in comparison with incorrect recognition of the same stimuli may be considered a regular finding. No systematic differences were found when evoked responses for correct and incorrect recognized pairs of letters were compared (Fig. 4 PL). In order to find electrophysiological correlates of the facilitatory interactions between two simultaneously presented letters, a comparison of the average responses evoked

5 by single letters and by pairs of letters was made. This comparison also failed to show any systematic differences. Fig. 4. Amplitude prevalence of the slow positive evoked potential wave in two types of experiments: SL, single letters; PL, pairs of letters. 1 C > I, amplitude is higher iln 8 case of correct recognition : C < I, higher for incorrect recognition; C = I, amplitudes are equal under both conditions. The ordinate shows the 2 number of experimental sessions. 'a c >I Ce 1 C =I Our finding of the differences in the late positive wave in cases of correct vs. incorrect single letter recognition is in accordance with observations (3, 5, 16, 17, 19-21) of the socalled P300 late positive deflection in the evoked responses which has a higher amplitude when the stimulus is correctly detected, when the ambiguity of the signal is reduced, as well as when the subject is certain of his judgment. Many authors (e.g. 10, 11) show that the P3O0 wave is elicited by attentive processes directed either to an anticipated target signal or to a new unpredictable stimulus. Another possibility was suggested by Ritter and Voughan (18) who postulated that P3O0 wave is a correlate of central cognitive processes. In most studies very simple stimuli, e.g. flashes, were most frequently used. It seems that the task which required subjects just to detect stimuli also did not engage more complex cognitive processes. In the present study we used letters as a material. It can be supposed that perception of these semantic sig~als involved processes of detailed estimation and identification of stimuli. The procedure used in this experiment (warning beep before each stimulus presentation, the use

6 of many different stimuli with equal probability of their presentation) helped to maintain active attention during the whole experiment. It is possible that.small fluctuations in attention did occur, but if we consider that conditions of the experiment were such as to provide about 50 /o of correct recognitions, it can be postulated that the incorrect recognitions did not result from the fluctuations of attention but rather from the lack of information caused by short lasting, low intensity stimulation. Thus it can be speculated that the late positive component of VER is a correlate of complex cognitive processes engaged in the object's identification. The authors are indebted to Dr. L. Szymafiskl and Mr. M. Indra, electrical engineers, for their valuable assistance in preparing the experimental setup and in the computer data processing. BUDOHOSKA, W., GRABOWSKA, A. and JABEONOWSKA, K Interaction between two letters in visual perception. Acta Neurobiol. Exp. 35: BUDOHOSKA, W., GRABOWSKA, A. and JABEONOWSKA, K The effect of interaction between elements of familiar and unfamiliar patterns. Pol. Psychol. Bull. 10: CAMPBELL, F. W. and KULIKOWSKI, J. J An electrophysiological measure of the psychophysical contrast threshold. J. Physiol. 217: 5455P. CARR, T. H., LEHMKUHLE, S. W., KOTTAS, B., ASTOR-STETSON, E. C. and ARNOLD, D Target position and practice in the identification of letters in varying contexts. A word superiority effect. Percept. Psychophys. 19: COOPER, R., MCCALLUM, W. C., NEWTON, P., PAPAKOSTOPOULOS, D., POCOCK, P. V. and WARREN, W. J Cortical potentials associated with the detection of visual events. Science 196: ESTES, U. K The locus of inferential and perceptual processes in letter identification. J. Exp. Psychol. Gen. 104: GRABOWSKA, A. and BUDOHOSKA, W Interaction between two spatially separated letters presented in succession. Acta Neurobiol. Exp. 39: GRABOWSKA, A., FERSTEN, E. and BUDOHOSKA, W Facilitation of letter recognition as a result of activity of central nervous system. Pol. Psychol. Bull. 10: GRABOWSKA, A., RADILOVA, I., RADIL, T. and MARAS, L Letter recognition and evoked potentials. Act. Nerv. Super. 21, 1: HAIDER, M., SPONG, P. and LINDSLEY, D. B Attention, vigilance and cortical evoked potentials in humans. Science 145: HILLYARD, S. A,, HINK, R. F., SCHWENT, V. L. and PICTON, T. W Electrical signs of selective attention in the human brain. Science 182: JABEONOWSKA, K. and BUDOHOSKA, W Hemispheric differences in the visual analysis of the verbal and nonverbal material in children. Acta Neurobiol. Exp. 36:

7 13. JABEONOWSKA, K. and BUDOHOSKA, W Specific visual disability in relation to ontogeny of brain asymetry. Acta Neurobiol. Exp. 38: JUOLA, Y. F., LEAVITT, D. D. and CHOE, C. S Letter identification in word, nonword and single-letter display. Bull. Psychon. Soc. 4: RADIL, T., RADILOVA, I., BOHDANECKP, Z., INDRA, M., LANSKP, P. and MARAS, L A system for psychophysiological correlation between subjective interpretation and bioelectric manifestation of visual stimuli. Act. Nerv. Super. 21: RADILOVA, I., RADIL, T. and MARAS, L Detection of the number of stimuli and evoked potentials. Act. Nerv. Super. 21: RADILOVA, I. and RADIL, T Electrophysiological correlates of perception and cognition (in Russian). Stud. Psychol. 21: RITTER, W. and VAUGHAN, H. G. Jr Averaged evoked responses in vigilance and discrimination. Science 164: SUTTON, S., BRAREN, M., ZUBIN, J. and JOHN, E. R Evoked potential correlates of stimulus uncertainty. Science 150: SUTTON, S., TUETING, P., ZUBIN, J. and JOHN, E. R Information delivery and the sensory evoked potential. Science 155: Accepted 20 June 1980 I. RADILOVA, T. RADIL-WEISS and L. MARAS, Institute of Physiology Czechoslovak Academy of Sciences, Bude~ovickB. 1083, Prague 4-KRC, Czechoslovakia. A. GRABOWSKA and W. BUDOHOSKR, Nencki Institute of Experimental Biology, Pasteura 3, Warsaw, Poland,