Toward a functional categorization of slow waves.

This study is concerned with slowly varying, long-duration brain event-related potential (ERP) components, referred to as Slow Wave activity. Slow Wave activity can be observed in the epoch following P3b, suggesting that it reflects further processing invoked by increased task demands, beyond the processing that underlies P3b. The present experiment was designed to distinguish Slow Wave activity related to specific types of task demands which arise during difficult perceptual (pattern recognition) and conceptual (arithmetic) mental operations. Three late ERP components that respond differentially in amplitude to manipulation of perceptual and conceptual difficulty were identified: 1) A P3b, with a topography focused about Pz, evidently related to the subjective categorization of easy and difficult conceptual operations, that increased when the subjective low-prohability operation was performed; 2) A longer latency, centroparietal positive Slow Wave that increased directly with perceptual difficulty but was not affected by conceptual difficulty; 3) A very long latency negative Slow Wave, broadly distributed over centroposterior scalp, that increased directly with conceptual difficulty while its onset was delayed when perceptual difficulty increased. DESCRIPTORS: Event-related potentials. Slow Wave, P3b, Mental arithmetic. Pattern recognition. Information processing. Several reports have described slowly varying, view). These "Slow Waves" can be observed in the long-duration brain event-related potential (ERP) epoch following P3b, suggesting that they reflect components whose amplitudes relate directly to task further processing invoked by increased task dedemands (see Ruchkin & Sutton, 1983, for a remands, beyond the processing that underlies P3b. Slow Wave activity has been found in a variety This research was supported in part by a U.S.P.H.S. of tasks. Comparisons of these data suggest that grant from the National Institute of Neurological and there are systematic differences in Slow Waves such Communicative Disorders and Stroke, NS 11199. We are that they appear to be reducible to two broad catindebted to Dr. Richard Coppola for providing laboratory egories based on their onset latencies, those reflectcomputer software and Mr. Howard Canoune for editorial jng gĵ jjer (1) perceptual operations or (2) concepthe remaining authors plttude tncreases when the sttmulus parameters are .Address requests for reprints to: D.S. Ruchkin, Demanipulated such that perception is made difficult partment of Physiology, School of Medicine, University as in signal detection, recognition, and localization of Maryland, Baltimore, Maryland 21201, USA. paradigms (Ruchkin, Sutton, Kietzman, & Silver, 340 Ruchkin, Johnson, MahafFey, and Sutton Vol. 25, No. 3 1980: Kok & de Jong, 1980; Parasuraman, Richer, & Beatty, 1982; Sutton, Ruchkin, Munson, Kietzman, & Hammer, 1982; McCallum, Curry, Cooper, Pocock, & Papakostopoulos, 1983). Slow Waves are also elicited when diflicult conceptual operations are required as in matching of linguistic stimuli (Sanquist, Rohrbaugh, Syndulko, & Lindsley, 1980), semantic judgment/incidental memorization (Neville, Kutas, Chesney, & Schmidt. 1986), complex mnemonic memorization activity (Karis, Fabiani, & Donchin, 1984), memory search (Okita, Wijers, Mulder, & Mulder, 1985; Kramer, Schneider. Fisk, & Donchin, 1986), retrieval from memory of abstract information (Rosier, Clausen, & Sojka, 1986), leaming when transitions occur in stimulus sequence-generating rules (Stuss & Picton, 1978; Stuss, Toga, Hutchison, & Picton, 1980; Johnson & Donchin, 1982), processing low-probability stimuli in a Bernoulli sequence' (N. Squires. K. Squires, & Hillyard, 1975), and mental rotation (Stuss, Saraziti, Leech. & Picton 1983; Peronnet & Farah, 1987; Johnson. Cox, & Fedio, 1987). The data from these studies indicate that there are at least three parameters of Slow Wave activity that vary as a function of task; 1) onset latency. 2) topography, and 3) polarity. Comparisons across reports in which Slow Wave onset latency can be estimated suggest that this component may be initiated at the time when the additional processing effort is required. Thus, onset latencies are relatively short (150-450 ms) for difficult perceptual operations (e.g., detection and identification of an external stimulus). Although Slow Wave onset overlaps P3b for perceptual processing, its duration is markedly longer than P3b. In contrast, for mental processes that follow perception (conceptual operations), the range of Slow Wave onset latencies increases to 300-800 ms. Slow Wave topography also varies across tasks. Slow Waves associated with perceptual difficulty are generally positive over posterior and central scalp. In some studies it was also positive, but lower in amplitude, over frontal scalp (Johnson & Donchin, 1978; Kok & de Jong, 1980), while in other studies the frontal aspect of Slow Wave was negative (McCallum et al., 1983; Ruchkin, Sutton, Kietzman. & Silver. 1980; Ruchkin, Sutton, & Stega, 1980; Sutton et al., 1982). When a feedback task 'The Slow Wave activity in Oddball tasks may be viewed as a special case of the tasks involving detection of changes in the sequence-generating rule, since their amplitudes are inversely related to event probability. Their relatively short duration may reflect the relative ease of categorizing events as having either a high or a low probwas combined with difficult signal detection. Slow Wave was larger centrally for the combined task than for signal detection alone (Johnson & Donchin, 1978; Ruchkin, Sutton, & Stega, 1980). There is an even greater variety of Slow Wave topographies associated with various conceptual operations. For example, positive Slow Waves, maximal over centro-posterior scalp, have been obtained during reactivation of stored abstract itiformation (Rosier et al., 1986), semantic matching (Sanquist et al., 1980), and rule leaming (Stuss & Picton, 1978; Stuss et al., 1980; Johnson & Donchin, 1982). A Slow Wave that was positive over posterior scalp, negligible at central scalp, and negative over frontal scalp was elicited by low-probability stimuli in a Bemoulli sequence (N. Squires et al., 1975). Karis et al. (1984) obtained a relatively large, frontal-maximal positive Slow Wave in an were employed during overt memorization, while Neville et al. (1986) obtained a Slow Wave that was large and positive over both parietal and frontal scalp in a semantic judgment/incidental memory paradigm. However, the Neville et al. experiment also involved perceptual difficulty, so their Slow Wave data may have also reflected perceptual operations. In memory scanning experiments, Okita et al. (1985) found short duration (400-700 ms) negative Slow Waves that were maximal over centro-posterior scalp, while Kramer et al. (1986) reported longer-duration (500-1200 ms) negative Slow Waves that were maximal over frontal scalp. Stuss et al. (1983) and Peronnet and Farah (1987) reported centro-parietal negative Slow Waves during mental rotation of visual images, and Stuss et al. showed that the late negative component was not present for other tasks involving complex figures. Peronnet and Farah demonstrated that the amplitude of their negativity was directly related to the extent to which the figure had to be rotated. Finally, Johnson et al. (1987) obtained a negative Slow Wave with a frontal topography in a mental rotation experiment, although it is unclear to what extent this activity reflects true rotational processing since the stimuli could be verbally encoded. All of the above studies have the common characteristic that Slow Wave amplitude increased as a function of task demand and/or with behavioral signs of improved processing efficacy (e.g., better recall in memorization paradigms). Latency differences in these studies suggest that Slow Wave reflects when the processing occurs, while topographic differences as a function of task suggest that the Slow Waves may reflect the nature of the additional processing, in the sense that different tasks may in.\fay. 1988 Functional Categorization of Slow Waves 341 volve different tieural generators. With some exceptiotis, there appears to be a pattem in these results such that negative Slow Waves are associated with scanning and mental imagery while positive Slow Waves are associated with memory storage, rule learning, and perceptual operations. These differences suggest that by suitable manipulation of task type and task difficulty, it may be possible to delineate ERP reflections of specific types of sustained mental operations. The experiment reported here was designed to study the similarities and differences in Slow Wave components associated with perceptual (pattern recognition) and conceptual (mental arithmetic) operations. Our approach was to manipulate independently two levels of pattem recognition and three levels of arithmetic difficulty randomly from trial to trial.