No sense organ is dedicated to time perception; therefore, the question of specific temporal processing mechanisms has been a recurrent matter of debate in the literature. Since Hoagland's (1933) original contribution suggesting that judgment of time was mediated by the reaction of some chemical pacemaker in the central nervous system, attention has been directed to the idea of a relationship between electroen-cephalographic (EEG) activity and the experience of time. A popular hypothesis was that the alpha rhythm of the electroencephalogram might provide the time base of an internal clock. The data, however, do not answer the question of whether there is a causal relation between time judgment and alpha frequency either in Werboff's

(1962) or in Treisman's (1984) studies. The latter found much less variation in the alpha frequency than in the concurrent time productions. As pointed out by Surwillo (1966, p. 392) "factors other than the alpha rhythm are required to account for variations in the experienced time." More recently some studies have focused on the event-related desynchronization (ERD) of the EEG rhythm (Mohl and Pfurtscheller, 1991), which has the potential to become a promising line of research. Nevertheless, the most familiar electrophysiological correlate of time estimation is a slow negative wave, called the contingent negative variation (CNV). Therefore, the first section of this chapter will focus on data providing evidence that the CNV subtends processing of temporal information, and we will carefully consider how the amplitude and the time course of this slow wave could "reflect the cognitive activity leading up to the formation of a temporal judgment," as pointed out by Ruchkin et al. (1977, p. 454). The second part of this chapter will examine whether findings on CNV topography and sources corroborate data from lesion and neuropsychological studies aimed at determining the neural bases of a specific mechanism for temporal processing.

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