Speed and accuracy of taste identification and palatability: impact of learning, reward expectancy, and consummatory licking

Despite decades of study, it remains a matter of controversy as to whether in rats taste identification is a rapid process that occurs in about 250-600 ms (one to three licks) or a slow process that evolves over seconds. To address this issue, we trained rats to perform a taste-cued two-response discrimination task (2-RDT). It was found that, after learning, regardless of intensity, the delivery of 10 mu l of a tastant (e.g., NaCl or monopotassium glutamate, MPG) was sufficient to identify its taste with maximal accuracy within 400 ms. However, despite overtraining, rats rarely stopped licking in one lick. Thus, a one-drop lick reaction task was developed in which subjects had to rapidly stop licking after release of a stop signal (tastants including water) to obtain rewards. The faster they stopped licking, the greater the reward. Rats did not stop licking after receiving either hedonically positive or negative stop signals, and thus failed to maximize rewards even when reinforced with even larger rewards. In fact, the higher the sucrose concentration given as a stop signal, the greater the number of consummatory licks elicited. However, with a stop signal of 2 mM quinine HCl, they stopped licking in similar to 370 ms, a time faster than that for sucrose or water, thus showing that in this rapid period, quinine HCl evoked an unpalatable response. Indeed, only when rats licked an empty sipper tube would they usually elicit a single lick to obtain a reward (operant licking). In summary, these data indicate that within 400 ms, taste identification and palatability, must either occur simultaneously or with marked overlap.