Selective responsiveness of medial prefrontal cortex neurons to the meaningful stimulus with a low probability of occurrence in rats.

Multi-unit neuronal activity was recorded in the medial prefrontal cortex (mPFC) of 13 chronically prepared male rats while they performed a two-tone discrimination task. Tones at 1000 and 2000 Hz were sequentially presented at intervals of 3-6 s. The duration of each tone was 0.8 s. Rats were trained to press a bar within 1.2 s after the cessation of the 1000 Hz tone (target), and not to press the bar when the other tone (non-target) was presented. Intracranial electrical stimulation (ICS) of the medial forebrain bundle was given as a reward immediately after the rats had correctly responded to the target tone. Probability of the target occurrence was either 30% or 70% in different sessions. When the target tone was presented on only 30% of the trials, the mPFC neurons in the majority of rats tested (10/13) exhibited phasic excitation about 100 ms after the onset of the target tone. However, when the target tone occurred on 70% of the trials, mPFC neurons in most of rats (11/13) did not show excitatory responses, and in some of them (5/13) were inhibited. No mPFC neurons exhibited significant responses to the non-target tone, regardless of its probability. These results suggest that the mPFC neurons selectively respond to meaningful events with a low probability of occurrence.