Journal ofl-Apm mental Psychology I earning. Memory, and Cognition 1 W , Vol. I.S. No'. 4. X 10-822 Copyright 1992 by the American Psychological Association Inc 0278-7393/92/^3.00 Differential Effects of Voluntary Expectancies on Reaction Times and Event-Related Potentials: Evidence for Automatic and Controlled Expectancies Juliana Matt, Hartmut Leuthold, and Werner Sommer University of Konstanz, Konstanz, Federal Republic of Germany Expectancy has been used to explain the effects of stimulus sequences both on reaction times (RTs) and on the P300 component of the human event-related potential. However, there are conflicting views about the control obtainable over these underlying expectancies. We compared the effects of voluntary expectancies for stimulus changes or repetitions in random tone series on RTs and the P300. Ss responded according to either stimulus identity (Experiment 1) or stimulus sequence (Experiment 2). In both experiments RTs were strongly affected by event expectedness. P300 amplitude, on the other hand, was affected (as a trend) only in Experiment 2. The results suggest that there are at least 2 types of "expectancy," one that is largely automatic and inflexible, reflected in P300 amplitude, and a second, controlled process that is reflected mainly in RT. The latter type of expectancy appears to affect processing stages beyond stimulus evaluation and classification. Expectancy is a key concept in the explanation of the effects of the stimulus sequence both on choice reaction time (RT) and on the amplitude of the P300 component of the event- related potential (ERP). In a previous study, we (Sommer, Matt, & Leuthold, 1990) showed that the parallels between these sequential effects may be mostly superficial, and differ- ent theoretical concepts may be required to explain them. The present study extends these findings to the consequences of voluntary expectancies, which we manipulated to study subjects' control over the sequential effects. When subjects performed choice responses to random Ber- noulli series of two alternative stimuli, basically two kinds of sequential effects have been reported in RTs when the re- sponse-stimulus intervals (RSIs) are longer than 500 ms (for reviews see Kirby. 1980, and Kornblum, 1973). First, the RT to a given stimulus is shorter when it is preceded by a different as compared with the same stimulus. This so-called first-order alternation effect is explained by assuming that in random series subjects expect too many alternations. This expectancy bias facilitates processing of alternations and inhibits process- ing of repetitions, leading to relatively short and long RTs, respectively, for these events. For RSIs below 500 ms first- order repetition effects were commonly reported. This is explained by the automatic facilitation of the processing of any subsequent events by preceding stimulus repetitions. At sufficiently short RSIs the facilitating effects of successive This research was supported by the Deutsche Forschungsgemein- schaft. and portions of it were presented at the meeting of the Deutsche Gesellschaft fiir Psychophysiologie und ihre Anwendungen, Gie/3en. June 1990, and at the 30th Annual Meeting of the Society for Psychophysiological Research, Boston, October 1990. We thank Eric Soetens and three anonymous reviewers for their helpful com- ments on earlier drafts of the article. Correspondence concerning this article should be addressed to Juliana Matt. Fachgruppe Psychologie, Universitat Konstanz, Post- fach 5560. 7750 Konstanz, Federal Republic of Germany. repetitions may accumulate and produce higher order repeti- tion effects. A second kind of sequential effect is observed after contin- ued runs of stimulus repetitions or stimulus alternations, in which RT decreases with the length of that run (Remington, 1969; Soetens, Boer, & Hueting, 1985). Conversely, RT in- creases with run length, if the run is discontinued by the current stimulus. These so-called higher order sequential ef- fects are attributed to expectancies for the continuation of runs. Again, a stimulus that matches the expectancy is con- sidered to be processed faster than a stimulus that does not. Effects of the stimulus sequence, similar to those on RTs, have been reported also for ERPs and, in particular, for the P300 component of the ERP, an electrically positive deflec- tion at 300 ms or more after task-relevant stimuli. Generally, runs of stimulus repetitions and alternations produce small P300 amplitudes, if continued, and large amplitudes, if dis- continued by the present stimulus (Squires, Petuchowski, Wickens, & Donchin, 1977; Squires, Wickens, Squires, & Donchin, 1976). Accordingly, one might say that those higher order sequences that produce short RTs lead to small P300 amplitudes and those that produce long RTs elicit large P300s. Similar to the sequential effects in RTs at long RSIs, the sequential effects in P300 have also been explained by ex- pectancy. Unexpected events are thought to elicit large P300s and expected events, small ones. Thus, the higher order effects in both RTs and P300 amplitude are explained by a tendency of the subjects to expect the continuation of repetition or alternation runs. This seemingly straightforward parallelism does run, how- ever, into several problems when one takes a closer look. On an empirical level, the first-order alternation effect commonly reported in RTs at long RSIs is opposed to an equally com- mon first-order repetition effect in P300 amplitude for the 1- s or more interstimulus intervals (ISIs) at which it has been investigated (Chesney & Donchin, 1979; Duncan-Johnson & Donchin, 1977, 1982; Duncan-Johnson, Roth, & Kopell, 810