Reward modulates neuronal activity in the hippocampus of the rat.

In the hippocampus of freely moving rats, neurons have been recorded that fire predominantly when the animal travels through a particular area while exploring the environment (so-called 'place cells'). This study investigates if the neuronal firing characteristics of such cells are modulated by attention, expectation of reward or memory load. A total of 16 electrodes were implanted in the CA1 region of the hippocampus of 3-month-old Long-Evans rats. Using a tetrode recording system, single neurons were recorded while a rat explored an 8-arm maze and retrieved pellets at the end of each arm. It was found that 31 out of 67 neurons showed place cell characteristics, while the other cells either fired in more than one place or fired along whole arms of the maze. Interestingly, 11 of the 31 neurons showed enhanced firing activity when the animal entered a baited arm but did not fire when the arm was visited again after the bait had been retrieved. In a second experiment, only four out of eight arms were baited. Firing rates of 46 neurons were analysed, and all cells (spatial or non-spatial) fired more in baited arms than in non-baited ones (P<0.001). In a reversal task in which the previously unbaited four arms were subsequently baited, neuronal activity was increased in the newly baited arms (42 cells analysed, P<0.001). Since no alterations to the maze or cues have been made, we interpret the increased firing probability of neurons in baited arms compared to unbaited arms as a correlate for 'attention' or 'expectation'.