Redefining the tonotopic core of rat auditory cortex: physiological evidence for a posterior field

Previous physiological studies have identified a tonotopically organized primary auditory cortical field (AI) in the rat. Some of this prior research suggests that the rat, like other mammals, may have additional fields surrounding AI. We, therefore, recorded in the Sprague-Dawley rat extracellular responses of single neurons throughout AI, and continued posteriorly to verify the existence of a posterior field (P) and to compare the neuronal properties in the two regions. Acoustic stimuli, including tones, bandpass noise, broadband noise, and temporally modulated stimuli, were delivered dichotically via sealed systems. Consistent with previous findings, AI was characterized by an anterior-to-posterior tonotopic progression from high to low frequencies (ranging from >40 kHz to <1 kHz). A frequency reversal at the posterior border of AI marked entry into a second core tonotopic region, P, with progressively higher frequencies encountered further posteriorly, up to a point (approximately 8 kHz) where cells were no longer tone responsive. Nevertheless, bandpass noise was an effective stimulus in P, enabling characterization of cells up to 15 kHz. Compared with AI, the frequency tuning of response areas was relatively broader in P, the response latency was often longer and more variable, and the response magnitude was more commonly a nonmonotonic function of stimulus level. In both fields, most neurons were binaurally influenced. The presence of multiple auditory cortical fields in the rat is consistent with auditory cortical organization in other mammals. Moreover, the response properties of P relative to AI in the rat also resemble those found in other mammals. Finally, the physiological data suggest that core auditory cortex (temporal area TE1) is composed not only of AI as previously thought, but also of at least two other subdivisions, P and an anterior field (A). Furthermore, our physiological characterization of TE1 reveals that it is larger than suggested by previous anatomical characterizations.