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I want to understand these two theories, how they are similar, and…
I want to understand these two theories, how they are similar, and how they are different:
According to the relative- waiting-time hypothesis (Jenkins, Barnes, & Barrera, 1981), the comparison is between how long one has to wait for the US during the CS versus how long one has to wait for the US during the intertrial interval (the interval from one US presentation to the next). When the US waiting time during the CS is much shorter than during the intertrial interval, the I/T ratio is high. Under these circumstances, the CS is highly informative about the next occurrence of the US, and high levels of responding occur. In contrast, with a low I/T ratio the US waiting time during the intertrial interval is similar to the US waiting time during the CS. In this case, the CS provides little new information about the next US, and not much conditioned responding develops.
The idea that conditioned responding depends on the information value of the CS has been developed in greater mathematical detail by Balsam and Gallistel (2009). Based on these calculations, Balsam and Gallistel concluded that “the CS is associable with the US … only to the extent that it reduces the expected time to the next US” (p. 77). Although this conclusion helps us understand the effects of the I/T ratio on conditioned responding, it is primarily applicable to situations that involve multiple conditioning trials (where there is a “next US”). These and related ideas (e.g., Gallistel & Gibbon, 2000) are difficult to apply to situations in which learning occurs in a single trial (e.g., taste-aversion learning or fear conditioning), and there is no “next US” (see also Domjan, 2003).
The Comparator Hypothesis
The relative-waiting-time hypothesis and related theories were developed to explain the effects of temporal factors in excitatory conditioning. One of their important contributions was to emphasize that conditioned responding depends not only on what happens during the CS but also on what happens in other aspects of the experimental situation.
The idea that both these factors influence learned performance is also central to the comparator hypothesis and its successors developed by Ralph Miller and his collaborators (Denniston, Savastano, & Miller, 2001; Miller & Matzel, 1988; Stout & Miller, 2007).
The comparator hypothesis was motivated by an interesting set of findings known as revaluation effects. Consider, for example, the blocking phenomenon (Figure 4.15). Participants first receive a phase of training in which CSA is paired with the US. CSA is then presented simultaneously with CSB, and this stimulus compound is paired with the US. Subsequent tests of CSB by itself show little response to CSB. As we discussed, the Rescorla-Wagner model interprets the blocking effect as a failure of learning to CSB. The presence of CSA blocks the conditioning of CSB.
The comparator hypothesis takes a different approach. It assumes that what is blocked is responding to CSB, not learning about CSB. If that is true, then responding to CSB should become evident if the block is removed somehow. How might that be accomplished? As it turns out, one way to remove the block to CSB is to eliminate responding to CSA after compound conditioning by presenting CSA repeatedly without the US. A number of studies have shown that such extinction of CSA following the blocking procedure unmasks conditioned responses to CSB (e.g., Blaisdell, Gunther, & Miller, 1999; Boddez et al., 2011). This is called a revaluation effect because it involves changing the conditioned value of a stimulus (CSA) that was present during the training of the target stimulus CSB. The unmasking of responding to CSB shows that blocking did not prevent the conditioning of CSB but disrupted the performance of the response to CSB.
Inspired by revaluation effects, the comparator hypothesis is a theory of performance rather than learning. It assumes that conditioned responding depends not only on associations between a target CS and the US but also on associations that may be learned between the US and other stimuli that were present when the target CS was being conditioned. These other stimuli are called the comparator cues and include the experimental context and other discrete CSs. In the blocking experiment, the target stimulus is CSB, and the primary comparator cue is the previously trained CSA that is present during the conditioning of CSB.
Another key assumption of the comparator hypothesis is that it only allows for the formation of excitatory associations with the US. Whether conditioned responding reflects excitation or inhibition is assumed to be determined by the relative strengths of excitation conditioned to the target CS as compared to the excitatory value of the comparator stimuli that were present with the target CS during training.
