THE CS AS A “ SIGNAL”
In describing discrimination, it seems natural to speak about the CS+ and CS– as though they were signals for the animal, providing information about things to come. And in fact this way of thinking about the CS helps us understand several aspects of conditioning, including why the rate at which conditioning develops depends on how the CS and US are related to each other in time.
Conditioning happens most efficiently when the CS precedes the US by some opti-mum interval—usually a half-second or so, or perhaps a few seconds at most. If the interval between the CS and US is increased beyond this optimum, the effectiveness of the pairing drops sharply. But we also don’t want the interval between these stimuli to be too short. In fact, presenting the CS and US simultaneously is usually ineffective in establishing an association—and the backward procedure, presenting the US before the CS, is even worse (Rescorla, 1988; Figure 7.10).
These facts make perfect sense if we think of the CS as a signal warning the organ-ism that it should prepare itself for the upcoming US. To see why, imagine a mountain road that has a dangerous hairpin turn. How should drivers be warned about this turn? The best warning would be a “Caution” sign just before the turn (analogous to forward pairing with a short CS-US interval). This sign would be informative, and—crucially— would allow the driver enough time to prepare for the upcoming maneuver. But it’s important not to place the sign too far ahead of the turn. Suppose a Caution sign is posted 100 miles before the turn (forward pairing with an extremely long CS-US interval). In that case the driver might not connect the sign with what it signifies—or just as bad, he might have forgotten about the sign by the time he reaches the curve. Things would be worse still, though, if the sign were prominently displayed right in the middle of the hairpin turn (simultaneous pairing), because now the warning comes too late to be of any use. Worst of all, the driver might suspect a degree of malevolence if he discovered the sign placed on the road just beyond the turn (backward pairing), although he’d probably be grateful that he didn’t find the sign at the bottom of the ravine.