Alteration in the perception of food quantity by rats induced by manipulations of hunger and food sweetness

Rats were trained to differentially press two levers after presentation of one or four food pellets or tones, trials of all four types occurring within each session. Different levers were correct after one pellet or one tone and after four pellets or four tones. The bisection function from sessions including trials of intermediate quantities differed for the two cues: the bisection point for tone quantities was near the harmonic mean, but it was near the geometric mean for food pellets. Thus, the perception of food quantity appeared to depend upon processes different from the perception of tone quantity. Decreases in food sweetness or in food deprivation shifted the bisection function to the left, indicating that decreasing the "value" of food increases perceived quantity. Altering food sweetness also attenuated the discrimination performance to a degree, suggesting that taste is an important factor in the perception of food quantity. The bisection procedure for determining perceptions of food quantity appears to be a useful indirect method of assessing the hedonic value of food, without the confounding effects of the response-facilitating actions of rewards.