The scalar property. Three types of behavioural procedure have traditionally been used to investigate interval timing in humans and other animals: estimation, production and reproduction. In humans, the first two protocols tend to rely on verbal instructions or responses, requiring the participant to translate between performance and a verbal representation of duration, which can lead to confounds. A more reliable approach, which can be used equally well with a wide variety of animal species, is to use a reproduction procedure, in which the subject is presented with a given criterion duration and then required to reproduce this duration (FIG. 2). Typically, the participant’s responses follow a normal distribution around the criterion duration, and the width of this response distribution is proportional to the criterion duration. The way in which the mean and standard deviation of the response distribution covary is usually referred to as the scalar property, and resembles WEBER’S LAW, which is obeyed by most sensory dimensions. The scalar property applies not only to behavioural responses, but also to neural activation as measured by ensemble recording, or by the haemodynamic response to timed events measured with fMRI

The theory of scalar timing, originally proposed by Gibbon (1977) and developed later with other colleagues (Gibbon, Church, & Meck, 1984) must surely rank as one of the most successful recent imports from the animal to the human psychological laboratories.

Scalar timing theory (Gibbon, 1977; Gibbon et al., 1984) proposes that subjective time produced by the underlying timing mechanism exhibits two properties. The first of these is mean accuracy, the requirement that mean estimates of some real time, t, should equal t. The second requirement is the scalar property itself, the requirement that standard deviations of time estimates grow as a constant fraction of the mean. This second property is

Scalar timing proposes that timed behaviour is regulated by a complex underlying mechanism, discussed in more detail later, involving an internal clock consisting of a pacemaker and accumulator with a switch connecting them, as well as memory and decision mechanisms, sometimes tested by constructing a coeffcient of variation statistic (standard deviation/ mean); thus the scalar property requires that this coefficient of variation remain constant as the duration timed changes.