LEARNING-INDUCED CHANGES IN RAT PIRIFORM CORTEX ACTIVITY MAPPED USINGMULTISITE RECORDING WITH VOLTAGE-SENSITIVE DYE

The piriform cortex (PCx) has a potential role in storage and recall o f olfactory information. This study is a first extensive investigation of the spatiotemporal distribution of activity in the PCx induced by learned sensory inputs following conditioning. In a conditioned group, rats chronically implanted with four electrodes in the olfactory bulb were trained to associate the electrical stimulation of a given bulba r electrode with a positive reinforcement, while stimulation of a diff erent electrode predicted a negative reinforcement. In a familiarized group, rats received the same protocol of daily electrical stimulation with no associated reinforcement. At the end of the conditioning or f amiliarization episode, activity evoked in the PCx was optically mappe d using a 144 photodiode array. In the anaesthetized rats, PCx maps we re recorded in response to stimulation of each of the four bulbar elec trodes using either high (0.5-1 mA) or low (0.1 mA) test current inten sities. Low intensity stimulation revealed that conditioning selective ly enhanced the probability of occurrence of a signal composed of a si ngle late (56-73 ms) component which occurred almost simultaneously on a large PCx area. In the conditioned group, high intensity stimulatio n through either of the four electrodes revealed a potentiation of the early (17-30 ms) disynaptic component of the PCx response in the most posterior part of the PCx as well as a homogeneous increase of the la te (39-52 ms) component spread over the PCx areas. These data suggest that learning induces synaptic changes at different nodes of the PCx c ircuitry.