Few agree on whether temporal processing is centralized or distributed and which structures are involved. Indeed, if all neural circuits can intrinsically process temporal information, then virtually any circuit could be involved, and the location of temporal processing would depend on the nature and modality of the task at hand.

The holy grail of timing research is to understand the ‘time-dependent process’: a mechanism equivalent to a piezoelectric crystal in a man-made clock or the movement of a shadow on a sundial. This has proven an elusive goal, to the extent that ideas about how this mechanism might work remain near the level of conjecture. Researchers have had great difficulty in pinning timing- related activity in the brain to any specific type of function. This is largely because mosttime measurement tasks draw upon more than one process, making it difficult to tease the various components apart.

Circadian, interval and millisecond timing involve different neural mechanisms. In mammals, the circadian clock that drives metabolic and behavioural rhythms is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. This master clock coordinates tissue-specific rhythms according to light input and other cues — such as social information — that it receives from the outside world. The circadian timer relies on a molecular network of transcriptional feedback loops. On the other hand, interval timing depends on the intact striatum, but not on the intact SCN or cerebellum. In the interval- timing range, the striatum and the cerebellum might both be activated, possibly contributing to different aspects of performance as a function of the sequential stages of motor memory consolidation.

Brain responses in cognitive neuroscience reveal that the integration of successive auditory items already takes place at a very early processing stage, that is, in the first 200 ms after stimulus- onset. The process of integration is reflected by an early negative component of the event-related potential (ERP), the so-called mismatch negativity (MMN; e.g., Sussman,Winkler, Ritter, Alho, & Näätänen, 1999). In the auditory domain, the MMN is evoked whenever a change within a flow of repetitive stimuli can be identified. If, for example, a stream of constant auditory events (e.g., tones or short words) is given, the brain detects this regularity and forms a so- called standard. If this standard trace is violated by a deviant sound, an MMN is elicited (for a review, see, e.g., Näätänen, Tervaniemi, Sussman, Paavilainen, & Winkler, 2001). If stimuli are presented very rapidly, they are even more strongly integrated and form an auditory object that is perceived as a whole (Sussman, Ritter, & Vaughan, 1998).