Peak Shift in Remembering

If remembering is discriminative behaviour along the dimension of time and if, as Sargisson and White (2001) argued, generalisation around a peak can occur in this behaviour, then the peak shift which has been shown in discrimination along so many other stimulus dimensions, might also occur in remembering. To examine this hypothesis, 6 hens were trained in a delayed matching-to-sample procedure at delays of 2 and 4 s. The probability of reinforcement for correct responses was initially 0.9 at both delays until performance stabilised. A generalisation probe was then carried out by inserting unreinforced trials at delays of 0, 1, 1.5, 2.5, 3, 3.5, 4.5, 5 and 6 s in a session amongst normal training delay trials. The generalisation functions had a slight peak around 2 s. After further training, a second generalisation probe showed a slightly declining function. The probability of reinforcement at the 2 s delay was then dropped to 0.1, so that in the terms of the classical generalisation/peak shift paradigm, 2-s delay trials became S- and 4-s delay trials became S+. A third generalisation probe then was conducted. This resulted in a flat function from 0 s to 3 s, and a large, clear peak in discriminative performance at 4.5 s over all hens. After more of the same differential reinforcement training, a fourth generalisation probe showed a broad curve peaking at 3 s, with minima at 1 s and 6 s and a global maximum at 0 s. Another training condition was then run, with the probability of reinforcement at the 2-s delay dropped to 0, to see if increasing the "aversiveness" of S- would again result in a peak shift. A fifth generalisation probe was then conducted. This showed a sharp decline in discriminability at shorter delays, a dip around 2 s, and a very small area shift beyond 4 s, but no clear peak shift. This was interpreted as being due to overlearning, with the consequences of remembering at S- no longer significantly affecting performance at S+. A final training condition was then run, with S- moved from 2 s to 3 s with zero probability of reinforcement, and for only a short period, to prevent overlearning. It was predicted that this would cause peak shift to re-occur. A sixth generalisation probe was then conducted. This found a further decline in discriminability at shorter delays, a shift in the dip from 2 s to 3 s, and a large, clear peak at 4.5 s. This demonstration of peak shift in a remembering process would not have been predicted by any traditional theory of memory, but strongly supports the conception of remembering as discriminative behaviour along the stimulus dimension of time