2.4.1 Basic Timing Phenomena Fixed-Interval Timing

The fixed-interval (FI) schedule is perhaps the simplest demonstration of animal timing and was first described in Skinner (1938). The subject is rewarded for the first response that occurs after a fixed period of time has elapsed. Responses before the interval has elapsed have no effect, and once a reinforcement has been given, a new interval begins. This procedure typically produces what is known as the FI scallop, a pattern of responding characterized by an initial slow rate of response that gradually increases over the interval to reach its maximum rate at approximately the end of the interval. This pattern is essentially universal, being found in humans, rats, pigeons, fish (Talton et al., 1999), and many other species. This pattern of response is even seen at very long intervals and has been demonstrated in intervals up to 36 h long (Eckerman, 1999). Over extensive training, the scallop can become increasingly angular, with subjects responding at a very low rate during the first part of the interval and switching to a steady, high rate of response in the latter part of the interval (Schneider, 1969). The performance of the model under an FI schedule is

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