Making N Responses

In another paradigm animals have been trained to make a certain number of responses to obtain a reward. Platt and Johnson (1971) required rats to signal when they had completed N lever presses by poking their nose into a hole equipped with a photoelectric sensor. As shown in Figure 6.2a, the number of responses the rats made before head poking was roughly normally distributed around the required number. Scalar variance was also obtained; the standard deviation of the distribution of the obtained number of responses increased linearly with the required number of responses (see also Fetterman and MacEwen, 1989; Laties, 1972; Mechner, 1958; Rilling, 1967; Wilkie et al., 1979). Whalen et al. (1999) tested adult humans with an adapted version of the Platt and Johnson task. Adults were required to rapidly press a button a certain number of times without verbally counting. Results were indistinguishable from those of the rats. As shown in Figure 6.2b and c, scalar variance was found for both species, as indicated by the linear increase in the standard deviation in the response distribution with the mean number of responses required and the constant coefficient of variation obtained for both species (see also Cordes et al., 2001).

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