Role of stimulus ambiguity in conditional learning.

Three experiments using rats and the conditioned emotional response procedure examined the notion that when a conditioned stimulus (CS) is paired with a reinforcer (US), that CS must be ambiguous if the CS-US association is to become the target of conditional control. CS ambiguity was manipulated by varying whether the CS had been preexposed prior to conditioning. In Experiments 1 and 2, it was demonstrated that a cue that accompanied pairings of a CS and shock acquired conditional control over the CS-shock association when that CS had been preexposed, but not when it was novel. The measure of conditional control in Experiments 1 and 2 was the ability of the (conditional) cue to enhance responding to the target CS. Experiment 3 used a blocking procedure to show that this enhancement reflected an amplification of the target CS's effective associative strength. These findings extend existing knowledge of the conditions required for conditional cue formation.

Early experience and reinforcer quality in delayed flavour-food learning in the rat.

The ability of hungry rats to associate flavours with the consequences of ingesting glucose solutions was studied in three experiments. Experiment 1 used a procedure in which on some days one flavour, e.g. cinnamon, was presented and followed after 20 min by 20% glucose, while on other days a second flavour, e.g. wintergreen, was presented, but not followed by any event. During this training, subjects who received quinine-tainted glucose increased their consumption of the predictive flavour relative to groups given no quinine, but quinine tainting did not affect conditioned preference for the predictive flavour in choice tests. With the aim of discovering whether prior experience of a variety of foods improves ability to learn new flavour-calorie associations, Experiment 2 and used a similar procedure to compare subjects raised on a varied diet ("supermarket" rats) with controls previously given only chow. Contrary to expectation, the supermarket rats showed some impairment both on this delay task and, in Experiment 3, on one using a "mixtures" procedure. This involved presenting a mixture of one cue flavour with glucose-quinine on some days and a mixture of a second cue with an equally palatable saccharin solution on other days. Acquisition was particularly rapid in control subjects, reaching asymptote after only two flavour-glucose pairings. It was concluded that neither a decrease in palatability, as in Experiment 1, nor prior experience with a range of foods, as in Experiments 2 and 3, improve a rat's ability to associate a new flavour with calories.