P300 and Stimulus Categorization: Two Plus One is not so ... - CiteSeerX

PSVCHOPHYHOLOGY © 1980 by Hie Society for Psychophysiological Research, Inc.

Vol. 17. N o . 2 Printed in U.S.A.

P300 and Stimulus Categorization: Two Plus One is not so Different from One Plus One RAY JOHNSON, JR. AND EMANUEL DONCHIN Cognitive Psychophysiology Laboratory, Department of Psychology, University of Illinois, Champaign

ABSTRACT Event related brain potentials (ERPs) were recorded from subjects who were instructed to count one of three, equally probable tones presented in a random sequence. In another condition, the subjects had to count one of two stimuli, one of which was presented with a probability of .33. The data support the view that the pattern of variation of P300 amplitude with the sequential structure of the series depends on the category to which events are assigned, rather than on the individual stimuli eliciting the P300. Furthermore, the data support the idea that the amplitude of P300 elicited by task-relevant stimuli is determined by the subjective probability associated with the eliciting event. DESCRIPTIONS: ERP, P300, Subjective probability. Stimulus categorization. The assessment of subjective probabilities is of considerable importance in the study of the P300 component ofthe human event-related brain potential (ERP). When subjects are presented with a series of Bernoulli events, the magnitude of the P300 elicited by each stimulus is inversely related to its prior probability (Duncan-Johnson & Donchin, 1977; Roth, Ford, Lewis, & Kopell, 1976; Tueting, Sutton, & Zubin, 1970). In these studies, the variable controlling the magnitude of P300 amplitude was assumed to be the prior probabilities of the stimuli. Yet, the prior probability of a stimulus accounts for only part of the variance of P300. In a series of reports (Duncan-Johnson, 1978; DuncanJohnson & Donchin, 1977; Johnson & Donchin, This research was supported by the Advanced Research Projects Agency's Cybernetic Technology Office, under ONR Contract #N-000-i4-76-C-0002 to E. Donchin. The authors wish to thank Christopher D. Wickens, Connie C. Duncan-Johnson, Gregory L. Chesney and Jack Isreal for their helpful comments on the manuscript. A preliminary report of this study was presented at the Seventeenth Annual Meeting of the toteTi977'' ^'y*^^°P'^y'^°^*'S^'=^ ^"'^^*^' Philadelphia, OcT^- ' ,, .,. o u c r j,T inepresent address ofthe first author is: Stanford University

1979; K. Squires, Petuchowski, Wickens, & Donchin, 1977; K. Squires, Wickens, N. Squires, & Donchin, 1976), it has been demonstrated that the P3(X) elicited by a sequence of Bernoulli events varies from trial to trial despite constant prior probability. These studies further showed that this variability can be attributed to the specific sequence of stimuli presented on the trials immediately preceding each stimulus. To account for this variability in P300 amplitude, K. Squires et al. (1976) proposed a model which assumes that the subjective probability (or "expectancy'') associated with the outeome of each event varies from trial to trial. According to this model, the effect of prior probability on P300 is modulated by an exponentially decaying memory trace for past presentations of each Stimulus. Thus, a Stimulus (A) induces an expectation, which decays over SUCCessive trials, that it will be repeated (AA). The subject's expectancy is further modulated, although tO a lesser extent, by the occurrence of sequences of stimulus alternations (i.e., ABABA). In this formulation, these two factors, which are related tO the preceding Sequence of stimuli, combine with the prior probabilities o f each stimulus to determine ,

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School of Medicine, Department of Psychiatry and Behavioral ^ ^ subjective probablhty assigned to each outcome Sciences, Stanfoixl, CA 94305. of a trial. There appears to be an inverse monotonic Address requests for reprints to: Dr. Emanuel Donchin, Cog- relationship between the subjective probability of a nitive Psychophysiology Laboratory, Department of Psychol- Stimulus and the amplitude of the P300 which it ogy. University of Illinois, Champaign, Illinois 61820. elicits. 167 0048-5772/80/010167-12$l .20/0 © 1980 The Society for Psychophysiological Research, Inc.

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Whereas the expectancy model formulated by K. Squires et al. (1976) assumes that the ^ p l i t u d e of P300 vanes with the subjective probablhty of HernouUi outcomes, it might be argued that a smaller P300 is elicited by the last in a series of identical stimuli because neural responses to repeating stimuli are often diminished (Harris, 1943; Sharpless & Jasper, 1956). Explanations invoking noncognitive processes can be refuted by demonstrating that physically identical sequences of stimuU will ,

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have diflFerent effects on P300 amplittide if the instructions to the subjects are changed, or if the information available to the subject is vaned. This issue can be further clarified by considering an experiment in which one of three, rather than one of two, equiprobable stimuli can occur on any trial. when the subject is told to count only one stimulus, If the variance in P300 amplitude depends primarily on the interaction between successive physical stimuli, then such a series should be treated as a ^^,

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VoL 17, No. 2

Procedun g^^jects counted the occurrences of 1000-Hz tones ^hich, in the two-stimulus session, were presented in series with 1400-Hz tones. During the three-stimulus session, the uncounted tones were 1400 and 1800 Hz. All tones were 50 m^c in duration (10 msec rise/fall time), and were presented at an intensity of 80 dB SPL (re .0002 dynes/cm^). The stimuli were mixed with a continuous background of wide-band white noise at 55 dB SPL and S^if??^ MnauraUy through Telephonies earphones (TDH-39). Subjects were seated in a reclining lounge

^^^ ^ a dimly lighted room. A cross-hairfixationpoint ^ ^ continuously illuminated in one of the fourfieldsof an Iconix tachistoscope. The stimuli were presented in a random order at a fixed, 1350-msec interstimulus interval in blocks of 205 trials, Subjectsreceived11 blocks of trials, each lasting apprt)ximately 5 min during both conditions. The sequence of stimuli constituting each trial block was identical for all subjects. Furthermore, the sequence of counted, relative *? uncounted, stimuli was the same in both conditions. Subiects Were given a bonus of 20 cents Der block of tiials

sequence of tiiree equiprobable outcomes, and P300 .^ ^^^^ they reported the correct number of 1000-Hz amplitudes should be the same for all three stimuli. ^^^^^. ^^ JQ ^^^^ ^^^^ ji^^i^ ^^^^^ ^^ ^i^^i^ ^^^ ^f Altemately, P300 amplitude may depend on the ^^e actual number. Four subjects received the two-tone sequence of Bernoulli events with two possible cotidition during the first session, and 4 received it duritig outcomes, a counted or an uncounted event, with the second. The two experimental conditions were prethe latter outcome comprising two distinct stimuli, sented in separate, 3-hrrecordingsessions. In this case, the prior probabilities are .33 and .67 A * _ , , , , Apparatus for the counted and uncounted outcomes, resj^ctively. A larger P300 should be elicited by the Burden Neurological Institute Ag-AgCl eiecti-odes counted stimulus and a smaller P300 should be ^^""^ ^^®^ ^ ^^^ subject's scalp with collodion at F^, elicited by both uncounted stimuli. ^ - ^"^ \}:^^t "'^'^'** electrodes were used as a r™. . J • J . ...1. reference. The subject was grounded with a forearm Thepresentexpenmentwasdesignedtotestthese ^ , ^ ^ j ^ ^ ^ j ^ ^ electro-oculogjam (EOG) was recorded two hypotheses by presenting subjects with a bet^^en supra- and sub-orbital electrodes. Beckman stimulus series consisting of three, equiprobable biopotential electrodes were used for reference, ground, tones, only one of which was to be counted. In this and EOG electrodes. The signals were amplified by Grass way, it was possible to determine which categoriza- 7P122 amplifiers, set to a time constant of 2.5 sec (cf. tion of the stimuli is the more potent determinant of Duncan-Johnson & Donchin, 1979), and upper halfP300 amplitude—the categorization established by amplitude cutoff of 35 Hz (3 dB/octave rolloff). the subject's responses (count vs no count) or the Experimental control and data acquisition were mancategorization established by the physical prop- aged with a PDP 11/40 computer (see Donchin & Heffley, ^r ^u *• 1 • K^*u u A * 1975). The Signals from each of the four channels were erties of the stimulus senes. Although data pre- ^. ^ ^ ^ ^ ^^^^ ^^ 200 samples/sec for a 1150-msec sented by a number of investigators (Courchesne, ^^^ ^^^^ ^egan 150 msec prior to stimulus onset. The Hillyard, «& Courchesne, 1977; Friedman, Sim- EEG, EOG, and ERP waveforms were monitored son, Ritter, & Rapin, 1975; Kutas & Donchin, throughout the session. 1979) suggest that it is response-defined categoriza^ . ^ . tions which determine die relationship between P300 amplitude and prior probability, the relationFirst-order ERPs were computed by using data from all ship between sequential expectancies and P300 ^^^ t"^*^ «" ^^^^^ ^ particular stimulus was presented. »^«i;^,^