The response properties of neurons in different fields of the auditory cortex in the rat.

The auditory cortex (AC) of the rat has been the subject of many studies, yet the details of its functional organization are still not well understood. We describe here the functional organization of the AC in young rats (strain Long Evans, aged 30-35 days, anesthetized with ketamine/xylazine) on the basis of the neuronal responses to acoustic stimuli. Based on the neuronal responses to broad band noise (BBN) and pure tone bursts, the AC may be divided into the primary auditory cortex (AI) and three other core fields: anterior (AAF), suprarhinal (SRAF) and posterior (PAF) as well as an unspecific region (UR) inserted between the AI and AAF. The core fields are surrounded by a belt area. Neurons in the AI, AAF, SRAF and PAF showed well defined characteristic frequencies (CF) in response to pure tone stimulation; in contrast, UR neurons responded only at high intensities without a clear CF. Neurons responding only to BBN stimulation were found mostly in the belt area. The putative borders between the core fields were determined by changes in their tonotopic gradient; however, no tonotopic organization was found in the PAF. Neurons with the shortest response latencies to BBN stimulation were found in layer 4 (L4) and layer 6 (L6) in the AI, while those with the longest latencies in the superficial layers (L1/2) of the belt area. Similar principles of responsiveness were observed when the spike rate in response to BBN stimulation was evaluated, with the highest rate present in L4 of the AI and the lowest in L1/2 of the belt area. According to the shape of the peristimulus time histograms, the responses of neurons in the AC of the rat may be classified as pure onset, sustained, onset-sustained, double peak or late onset. The most dominant in all fields, as well as in all layers, was the pure onset response. Our findings offer further cues for understanding the functional organization of the AC in the rat.