Functional Organization of Ferret Auditory Cortex

Abstract

We characterized the functional organization of different fields within the auditory cortex of anaesthetized ferrets. As previously reported, the primary auditory cortex, A1, and the anterior auditory field, AAF, are located on the middle ectosylvian gyrus. These areas exhibited a similar tonotopic organization, with high frequencies represented at the dorsal tip of the gyrus and low frequencies more ventrally, but differed in that AAF neurons had shorter response latencies than those in A1. On the basis of differences in frequency selectivity, temporal response properties and thresholds, we identified four more, previously undescribed fields. Two of these are located on the posterior ectosylvian gyrus and were tonotopically organized. Neurons in these areas responded robustly to tones, but had longer latencies, more sustained responses and a higher incidence of non-monotonic rate-level functions than those in the primary fields. Two further auditory fields, which were not tonotopically organized, were found on the anterior ectosylvian gyrus. Neurons in the more dorsal anterior area gave short-latency, transient responses to tones and were generally broadly tuned with a preference for high (>8 kHz) frequencies. Neurons in the other anterior area were frequently unresponsive to tones, but often responded vigorously to broadband noise. The presence of both tonotopic and non-tonotopic auditory cortical fields indicates that the organization of ferret auditory cortex is comparable to that seen in other mammals.

Cortico‐cortical connectivity within ferret auditory cortex

“…Tracer injections were, where possible (17/23 injection sites; see Table 1), made in physiologically identified cortical regions. When physiological verification was not possible, the locations of the tracer injections were targeted and assigned to a particular cortical field based on our previous descriptions of ferret auditory cortex (Bizley et al, 2005) and were subsequently confirmed cytoarchitectonically (see below).…”

“…Signals were amplified and digitized for off‐line analysis. Acoustic stimuli were noise bursts and pure tones of varying frequency (150 Hz to 20 kHz) and intensity, as in Bizley et al (2005). …”

“…Our goal was to place targeted injections into the six areas characterized electrophysiologically so far, two in each region of the EG and labeled in bold in Figure 1B. In the MEG we targeted the A1 and the anterior auditory field (AAF), which are the tonotopically organized primary or core areas (Kowalski et al, 1995; Wallace et al, 1997; Nelken et al, 2004; Bizley et al, 2005), In the PEG, injections were placed in the two further tonotopic fields identified in ferret auditory cortex: the posterior pseudosylvian and the posterior suprasylvian fields (PPF and PSF). Finally, in the AEG, tracers were placed in the anterior dorsal field (ADF) and the anterior ventral field (AVF), which are not tonotopically organized.…”

“…The physiologically identified fields within each of these areas are not cytoarchitectonically distinguishable. Therefore subdivisions within the MEG, PEG, and AEG were made according to their known physiological organization (Bizley et al, 2005). On this basis, the caudal two‐thirds of the MEG was classified as the A1, with the AAF occupying the rostral one‐third.…”

“…The presence of multiple auditory cortical areas on the ectosylvian gyrus (EG) of this species was first demonstrated by using 2‐deoxyglucose autoradiography (Wallace et al, 1997) and subsequently confirmed by using optical imaging of intrinsic signals (Nelken et al, 2004) and single‐unit recording (Kelly et al, 1986; Kelly and Judge, 1994; Kowalski et al, 1995; Bizley et al, 2005). Although most electrophysiological recording studies have focused on the primary auditory cortex (A1) (Phillips et al, 1988; Kowalski et al, 1996; Schnupp et al, 2001; Fritz et al, 2003; Rabinowitz et al, 2011; Keating et al, 2013), the nonprimary auditory fields in this species are now receiving increasing attention (Nelken et al, 2008; Bizley et al, 2009, 2010, 2013; Walker et al, 2011; Atiani et al, 2014).…”