Figure 1.

Associative discrimination task. Rats were trained to respond to auditory stimuli of a high-frequency (4.5 kHz, Tone 1) or low-frequency (0.9 kHz, Tone 2) tone by licking to receive sucrose (S in A and B) after Tone 1 or not licking to avoid quinine (Q in A) or no sucrose (NS in B) after Tone 2. For data analysis, each behavioral trial was defined as a 14 s period that included a 4 s period before tone delivery, a 2 s period during tone, and an 8 s period after tone delivery. An interval of 10 ± 2 s occurred between these behavioral trials. To foster the rat's motivation and response, a bigger drop of sucrose was delivered if the animal responded within the initial 0–2 s window after the tone than if it responded within the subsequent 2–6 s window, after which no solution was delivered in response to licking. A, With the S/Q procedure, one group of animals was trained with partial reinforcement such that only 75% of sucrose-associated or quinine-associated trials were reinforced. B, With the S/NS procedure, another group of animals was trained to receive full reinforcement (100%) for sucrose-associated trials and no reinforcement (0%) for no sucrose-associated trials. The unit discharge was recorded together with EEG and EMG activity and licking. After recording and characterization over several trials, one neuron (per side per animal) was submitted to juxtacellular labeling with Nb for its immunohistochemical identification.