High-field fMRI reveals tonotopically-organized and core auditory cortex in the cat

As frequency is one of the most basic elements of sound, it is not surprising that the earliest stages of auditory cortical processing are tonotopically organized. In cats, there are four known tonotopically organized cortical areas: the anterior (AAF), posterior (PAF), and ventral posterior (VPAF) auditory fields and primary auditory cortex (A1). Electrophysiological and anatomical evidence have suggested that AAF and A1 form core auditory cortex. The purpose of this investigation was to determine if high-field functional magnetic resonance imaging (fMRI) could be used to define the borders of all four tonotopically organized areas, identify core auditory cortex, and demonstrate tonotopy similar to that found using more invasive techniques. Five adult cats were examined. Eight different pure tones or one broad-band noise (BBN) stimuli were presented in a block paradigm during continuous fMRI scanning. Analysis was performed on each animal individually using conservative familywise error thresholds. Group analysis was performed by extracting data from fMRI analysis software and performing a battery of statistical tests. In auditory cortex, a reversal of the tonotopic gradient is known to occur at the borders between tonotopically organized areas. Therefore, high and low tones were used to delineate these borders. Activations in response to BBN as opposed to tonal stimulation demonstrated that core auditory cortex consists of both A1 and AAF. Finally, tonotopy was identified in each of the four known tonotopically organized areas. Therefore, we conclude that fMRI is effective at defining all four tonotopically organized cortical areas and delineating core auditory cortex.