We describe a scalp-recorded measure of tonotopic selectivity, the “cortical onset response” (COR) and compare the results between humans and cats. The COR results, in turn, were compared with psychophysical masked-detection thresholds obtained using similar stimuli and obtained from both species. The COR consisted of averaged responses elicited by 50-ms tone-burst probes presented at 1-s intervals against a continuous noise masker. The noise masker had a bandwidth of 1 or 1/8th octave, geometrically centred on 4000 Hz for humans and on 8000 Hz for cats. The probe frequency was either − 0.5, − 0.25, 0, 0.25 or 0.5 octaves re the masker centre frequency. The COR was larger for probe frequencies more distant from the centre frequency of the masker, and this effect was greater for the 1/8th-octave than for the 1-octave masker. This pattern broadly reflected the masked excitation patterns obtained psychophysically with similar stimuli in both species. However, the positive signal-to-noise ratio used to obtain reliable COR measures meant that some aspects of the data differed from those obtained psychophysically, in a way that could be partly explained by the upward spread of the probe’s excitation pattern. Our psychophysical measurements also showed that the auditory filter width obtained at 8000 Hz using notched-noise maskers was slightly wider in cat than previous measures from humans. We argue that although conclusions from COR measures differ in some ways from conclusions based on psychophysics, the COR measures provide an objective, noninvasive, valid measure of tonotopic selectivity that does not require training and that may be applied to acoustic and cochlear-implant experiments in humans and laboratory animals.

Most accounts of single- and multi-unit responses in auditory cortex under anesthetized conditions have emphasized V-shaped frequency tuning curves and low-pass sensitivity to rates of repeated sounds. In contrast, single-unit recordings in awake marmosets also show I-shaped and O-shaped response areas having restricted tuning to frequency and (for O units) sound level. That preparation also demonstrates synchrony to moderate click rates and representation of higher click rates by spike rates of non-synchronized tonic responses, neither of which are commonly seen in anesthetized conditions. The spectral and temporal representation observed in the marmoset might reflect special adaptations of that species, might be due to single- rather than multi-unit recording, or might indicate characteristics of awake-versus-anesthetized recording conditions. We studied spectral and temporal representation in the primary auditory cortex of alert cats. We observed V-, I-, and O-shaped response areas like those demonstrated in awake marmosets. Neurons could synchronize to click trains at rates about an octave higher than is usually seen with anesthesia. Representations of click rates by rates of non-synchronized tonic responses exhibited dynamic ranges that covered the entire range of tested click rates. The observation of these spectral and temporal representations in cats demonstrates that they are not unique to primates and, indeed, might be widespread among mammalian species. Moreover, we observed no significant difference in stimulus representation between single- and multi-unit recordings. It appears that the principal factor that has hindered observations of high spectral and temporal acuity in the auditory cortex has been the use of general anesthesia.

Sound spectra are represented by patterns of activity along the tonotopic axis ofthe cochlea. Cochlear implants can transmit spectra by stimulating tonotopicallyappropriate electrodes, but fidelity is limited by intracochlear spread of excitation. We aim to better evaluate present-day experimental stimulation procedures and, potentially, to improve transmission of spectra with novel stimulation modalities. As a first step, we are developing non-invasive measures of tonotopic spread of excitation that can be compared between normal-hearing cats and humans. These measures include psychophysics in the present study and scalp-recorded electrophysiology in a companion study (Guérit et al., 2021). Cats and humans detected pure-tone probes presented in continuous 1/8- and 1-oct noise-band maskers. Masker bandwidths were readily discernable in both species by the dependence of masked thresholds on probe frequencies. Thresholds were largely constant across the bandwidth of the 1-oct masker, whereas thresholds dropped markedly at frequencies away from the center of the 1/8-oct masker. Cats and humans differed in that the feline auditory filter centered on 8 kHz, which we measured using a notched-noise procedure, was 22% wider than published values for humans at the same center frequency. Also, thresholds for the cats in the 1-octmasker condition consistently were 1.0 to 3.2 dB higher than expected based on the estimated masker power in the feline auditory filter. The present psychophysical results parallel those in our companion electrophysiological study, thereby providing perceptual validation for that study. These psychophysical and electrophysiological methods will be valuable for future investigations of novel approaches for auditory prosthesis.