Generalized Learning

The role of the hippocampus is well established in associative learning assays of the conditioned stimulus-unconditioned stimulus (CS-US) type (Ono et al., 1995; Thompson et al., 1982). Typically in CS-US trials an animal is trained to associate a neutral CS (e.g., an odor) with a previously meaningful US (e.g., pain or reward). The archetypical example is that of Pavlov and his bell-stimulated salivating dog. The paradigm takes advantage of the fact that the animal has some unconditioned response (UR) to the US, so that the experimenter can verify association of the CS by omitting the US while still observing the UR.

As one might expect, there is a clear relationship between the timing of various components of the CS-US and the ability of the animal to learn the association. For example, there is an optimal time of CS length that maximizes learning rate (Cooper, 1991). This is reminiscent of the credit-assignment problem and suggests that the duration of environmental signals may affect attention for those signals when they are subsequently paired with relevant stimuli.

At the level of the synapse, long-term potentiation (LTP) in the hippocampus is a form of learning based on the modulation of synaptic gain between the presynaptic and postsynaptic cell (Churchland and Sejnowski, 1994). N-methyl-D-aspartate (NMDA)-type glutamate receptors are required for LTP, and their voltage-gated properties necessitate that the postsynaptic cell fire for a certain temporal duration while the presynaptic cell fires in order for the NMDA receptors to be activated. NMDA activation operates on gene transcription (Bading et al., 1993), and this may be a mechanism for the modulation of long-term synaptic gain. Several phases exist for hippocampal LTP, and these have been compared to the various phases exhibited in vertebrate memory (DeZazzo and Tully, 1995). A recent finding suggests that this mechanism may also operate in a retrograde fashion by weakening synapses when the presynaptic cell fires after the postsynaptic cell action potential (Markram et al., 1997). This mechanism is not an event timer in the usual sense; it is more akin to a coincidence detector. But it does exhibit the flavor of event timers, and it possibly explains the contiguity of the CS-US sequence (see Matell and Meck, 2000; Matell et al., this volume).

The hippocampus appears to be required in order to get beyond coincidence-limited association. In trace conditioning assays, in which a puff of air follows a brief tone, the hippocampus is required for association of nonoverlapping stimuli (Thompson et al., 1982). Hippocampal lesions only allow learning when the CS and US are overlapping in time.

Hippocampal-type event timers offer substantial associative powers in the development of tool use and causal recollection. A Japanese macaque (Macaca fuscata) washing a potato must somehow learn to associate the improved potato with the action that preceded it. A more common example of this kind of temporally gapped causal association is the learned food avoidance response exhibited in a large number of vertebrates. If an animal becomes sick from eating a toxic food, it may learn to avoid that food in the future. There is a temporal relationship here because delay of adverse consequences inevitably limits the association of the food with its effects (Stephens and Krebs, 1986).

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