Effects of CS concentration on long-delay taste aversion learning in preweanling and adult rats

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The effect of sucrose solution (CS) concentration on toxiphobia conditioning under both immediate and long-delay CS—US intervals was investigated in preweanlings (18-day-old) and adult (60- to 80-day-old) rats. The main effect of delay interacted with age. With the immediate CS-US interval aversions by preweanlings and adults were equivalent but at the 1-hr delay performance was significantly poorer for the preweanlings. The effect of CS concentration among the immediate and 1-hr CS—US delay groups also depended upon the animal's age. Among adults, the more concentrated the CS the poorer the taste aversion. Floor effect at the short delays, however, may have produced this effect. When a 3-hr CS—US delay was employed, the more concentrated the CS, the better the aversion. Among infants, a U-shaped function best fit the data with the intermediate concentration producing the weakest taste aversion. These findings are discussed in terms of associative and nonassociative processes involved in long-delay learning.