|Budget Amount *help
¥13,200,000 (Direct Cost : ¥13,200,000)
Fiscal Year 2000 : ¥2,600,000 (Direct Cost : ¥2,600,000)
Fiscal Year 1999 : ¥4,200,000 (Direct Cost : ¥4,200,000)
Fiscal Year 1998 : ¥6,400,000 (Direct Cost : ¥6,400,000)
1. We investigated neuronal mechanisms of how expectancy is acquired and how it is modified. We trained monkeys on either spatial delayed response task or delayed reaction time task using several different kinds of reward.
2. In the delayed response task, the animal could obtain the reward for the correct task performance. The same kind of reward was used continuously for about 50 trials. When the reward was changed without warming, the animal acquired the expectancy for the newly presented reward within 2 to 5 trials. This learning process was observed both in animal's behavioral latency and in neuronal activity of the prefrontal cortex. It was not possible to determine which changes occurred earlier between behavioral latency and neuronal activity.
3. In the delayed reaction time task, the animal was required to perform a simple key press response after a go signal presentation. Four consecutive trials constituted a block, and 4 different kinds of reward were given always in the same order within a block : i.e., sweat potato - raisin - cabbage - no reward (food reward) or orange juice - water - grape juice - no reward (liquid reward). Thus, it was possible for the animal to know what reward would be given on each trial, and the animal developed the expectancy for the outcome in each trial. In the primate orbitofrontal cortex, there were three kinds of expectancy related neurons : i.e., (1) neurons that monitored which (1^<st>, 2^<nd>, 3^<rd>, or last) trial the animal was faced with, (2) neurons which were involved in judging what specific reward would be given in the current trial, (3) neurons which showed activity changes in relation to the expectancy of the specific reward. The activity of reward expectancy related orbitofrontal neurons depended on the motivational significance of the reward, that is, the more the animal was hungry or thirsty, and the more preferable the reward was, the larger (or the smaller depending on a neuron) the activity changes were.