The predominate working hypothesis in the psychophysical literature has been a centralized internal clock model (Creelman 1962, Treisman 1963; for a review see Allan 1979), in which an oscillator beating at a fixed frequency generates tics that are detected by a counter. These models often assume that timing is centralized, that is, the brain uses the same circuitry to determine the duration of an auditory tone and for the duration of a visual flash.

The basic computational unit of clock theories involves an oscillator and a counter (Creelman 1962, Treisman 1962). Conceptually, the oscillator beats at some constant frequency, and each beat would then be counted by some sort of neural integrator. These ideas have not yet been expressed concretely in terms of the synaptic organization of a specific brain region.

However, as proposed by Meck and colleagues, clock-like mechanisms could be involved in timing on the scale of seconds and minutes (Meck 1996, Matell & Meck 2000).

..... the dominant model of temporal processing is the internal clock model. A prototypical clock model includes an oscillator (pacemaker) that emits pulses that are counted by an accumulator (Creelman, 1962; Treisman, 1963; Church, 1984; Gibbon et al., 1997). Within this framework, the pulse count provides a linear metric of time, and temporal judgments rely on comparing the current pulse count to that of a reference time. This model has proven effective in providing a framework for much of the psychophysical data relating to temporal processing (Church, 1984; Meck, 1996; Rammsayer and Ulrich, 2001). However, electrophysiological and anatomical support for the putative accumulator remains elusive, and mounting evidence indicates that clock models are not entirely consistent with the experimental data (for reviews see Mauk and Buonomano, 2004; Buhusi and Meck, 2005).

The standard model of temporal processing postulates a single centralized internal clock, which relies on an oscillator and an accumulator (counter) (Creelman, 1962; Treisman, 1963; Church, 1984; Grondin, 2001). The clock concept is generally taken to imply that the passage of time is counted in units that can be combined or compared linearly.

Traditionally, the manner in which durations in the seconds-to-minutes range are perceived, represented and estimated has been explained using a pacemaker–accumulator model. This model is relatively straightforward, and provides powerful explanations of both behavioural and physiological data. However, recent advances that challenge the traditional pacemaker–accumulator model have come from studies that use various modern techniques, which range from drug microinjection and ensemble recording in genetically modified and wild- type rodents to functional MRI (fMRI) and positron emission tomography (PET) in neurologically impaired and control humans. These data indicate that time might be represented in a distributed manner in the brain, and that telling the time is a matter of detecting the coincidental activation of different neural populations.

Human behavior based on the perception and timing in the range of seconds-to- minutes has traditionally been explained bythe pacemaker–accumulator model. The pacemaker- accumulator model (PAM), which is based on scalar expectancy or timing theory (Church, 1984; Gibbon et al., 1984; Meck, 1983), “is relatively straightforward, and provides powerful explanations of both behavioural and physiological data” (Buhusi & Meck, 2005; p. 755).

Briefly, the PAM model implicates the processing of temporal information via three synchronized modular information processing systems (see Buhusi & Mech, 2005.) The “clock” system consists of a dopaminergic pacemaker that regularly generates or emits neural ticks or pulses that are transferred (via a “gaiting” switch) to the accumulator, which accumulates ticks/pulses (neural counting) that correspond to a specific time interval. The raw representation of the stimulus duration in the accumulator is then transferred to working memory, a component of the PAM “memory” system. The contents of working memory are then compared against a “reference standard” in the long-term (reference) memory, the second component of the PAM memory system. Finally, the “decision” level of the PAM is conceptualized to consist of a comparator that determines an appropriate response based on a decision rule which involves a comparison between the interval duration value present in working memory and the corresponding duration value in reference memory. In other words, a comparison is made between the contents of reference memory (the standard) and working memory (viz., are they “close?”).

Various models of time perception have been suggested, the most popular being the internal- clock model (Gibbon, 1977). Here a series of pulses are produced by an internal pacemaker; these pulses are collated, counted and then compared to stored representations in order to allow the brain to judge durations and produce time estimations. Such models have been extensively studied using behavioural paradigms (Thomas &Weaver, 1975; Block, 1990).

Internal clock models based on neural counting provide a useful heuristic for explaining human performance on the temporal discrimination of brief intervals. It is not surprising, therefore, that the notion of a pacemaker-counter system represents a fundamental feature of most psychophysical models of temporal discrimination introduced over the last 4 decades (e.g., Allan, Kristofferson, & Wiens, 1971; Creelman, 1962; Gibbon, 1977; Killeen & Fetterman, 1988; Penton-Voak, Edwards, Percival, & Wearden, 1996; Treisman, 1963; Treisman, Faulkner, Naish, & Brogan, 1990).