The Hippocampus Is Needed for Complex Eye-Blink Conditioning

The hippocampus is not required for animals to acquire the simple form of eye-blink conditioning—specifically, delay conditioning—when little or no time passes between the end of the CS and the US, that we’ve described so far. But when the time intervals are longer (in what is called trace conditioning), animals without a hippocampus are unable to form conditioned eye-blink responses (Moyer et al., 1990). Removal of the hippocampus also prevents the formation of discrimination reversal conditioning—that is, conditioning in which the stimulus that previously served as CS now signals that no US is coming (Berger and Orr, 1983). So the neural plasticity underlying these more complicated forms of conditioning is still being actively investigated. For example, it appears that the hippocampus must make new neurons in order for animals to learn trace conditioning.