We have used a combination of recording techniques to investigate the functional organization of ferret auditory cortex. The location and tonotopic organization of primary auditory cortex (A1) is well characterized in the ferret, yet there has been little investigation of the location and response properties of non-primary fields. Our aim was to use optical imaging of intrinsic signals in combination with single-unit recording as tools for investigating auditory cortex in this species.

Optical image acquisition was performed using green (546 nm) light. Tonotopicity was mapped using pure tones presented as a sequence of increasing frequency, typically lasting 10-14 s and covering the range of 0.5-50 kHz. The sequence repeated itself continuously and the resulting optical signals were cross correlated with the stimulus. The time course of the optical response was of the order of several seconds and was superimposed on a pronounced vasomotion signal typically with a period of 8 s. However, using these relatively slow sequences, a clear stimulus-driven signal was observed. Best frequency maps were created by measuring the phase of the zero crossings of the modulated component of the optical signal. After imaging in each animal was completed, a mapping study was carried out using single unit recording. Experiments were performed under halothane anaesthesia with a neuromuscular blocker (pancuronium bromide). ECG and EEG were monitored and at the end of each experiment the animal was then overdosed with sodium pentobarbitone.

We observed at least three frequency reversals, which we attribute to A1, the anterior auditory field and a third non-primary area that may be homologous to the cat’s secondary auditory field. The best frequencies of units recorded matched the frequency responses that were revealed by measuring intrinsic optical signals.

We also investigated periodotopicity using these approaches. Pitch stimuli were amplitude-modulated (AM) tones with a high-frequency carrier (6 kHz), high-pass iterated ripple noise, and high-pass click trains. Schulze et al. (2002) have recently used AM tones to identify a periodotopic organization in low frequency A1 in the gerbil. We also found that optical signals in low frequency A1 were sensitive to the modulation of AM tones, but the same locations were not sensitive to the period of high-pass click trains or iterated ripple noise. Our results therefore do not support the existence of a pitch map in the low frequency areas of ferret auditory cortex.

This work was supported by The Wellcome Trust and MRC Centre for Cognitive Neuroscience.