The influence of comparison between similar stimuli on the effectiveness of their unique features.

Four groups of rats received stimulus pre-exposure under conditions intended to produce different opportunities for stimulus comparison to occur. Groups AX/BX-L and AX/BX-S received alternating presentations of two compound flavors (AX and BX); the interval between these presentations was long (24 h) for group AX/BX-L, and short (5 min) for group AX/BX-S. Groups AX-L and AX-S matched groups AX/BX-L and AX/BX-S in their pre-exposure conditions except that they received presentations of water rather than presentations of BX. The effective salience of one of the unique stimulus features (A) was then assessed by using this flavor as a conditioned stimulus in a flavor-aversion procedure. It was found that aversion to A was learned about more readily after pre-exposure to AX and BX than after pre-exposure just to AX. However, there was no indication that the rate of conditioning to A was affected by the temporal interval between the presentations of AX and BX. These findings challenge the notion that stimulus comparison engages a process responsible for an increase in the salience of the unique stimulus features, but can be accommodated by the salience modulation mechanism proposed by Hall [Hall, G., 2003. Learned changes in the sensitivity of stimulus representations: associative and nonassociative mechanisms. Q. J. Exp. Psychol. 56, 43-55].