Peak shift, prototypicality and aesthetic preference

[opening paragraph]: Ramachandran and Hirstein offer a number of interesting ideas about aesthetic preference. In this commentary I shall focus mainly on their ideas concerning peak shift and prototypicality. The authors give the example of a rat rewarded for responding to a rectangle (the S+) and not rewarded for responding to a longer triangle (the S-). They argue that the rat will respond even more to a more elongated rectangle. In fact, two phenomena are involved here. Peak shift refers to the fact that the rat responds more to a supernormal stimulus (the S++ or more elongated rectangular). Behavioural contrast refers to the fact that the rat responds more to the S++ than it did to the S+ before discrimination training took place. It does not detract from the authors’ main point, but we would be unlikely to find peak shift in the hypothetical experiment described. Peak shift only occurs when the S+ and the S- are very similar to each other (Hanson, 1959). Let us say that we train a rat to respond to a 1000 Hz tone and not to respond to a 500 Hz tone. No peak shift will be found. We will find peak shift if the S+ is 1000 Hz and the S- is 950 Hz. The conventional explanation for this is that the animal shows generalization. A rat trained simply to respond to a 1000 Hz tone will also respond a bit less to one of 990 Hz, even less to one of 980 Hz, and so on. In discrimination training, our rat would have a strong tendency to respond to a 1000 Hz tone but, because of generalization from the 950 Hz tone, it would also have some tendency not to respond to it. If we subtract the inhibitory gradient around 950 Hz from the excitatory gradient around 1000 Hz, we will end up with maximal responding around, say, 1010 Hz (the S++). Intuitively, we would expect the rat to respond more to the S++ but also less vigorously than it had to the S+ before discrimination training was begun. This is because the inhibitory gradient has been subtracted from the excitatory gradient. Surprisingly, we find behavioural contrast; the rat responds more vigorously to the S++ than it had to the S+. Grossberg (1975) argues that this is because there is a tendency to keep amount of activation on a layer of cortex constant.