An electrophysiological measure of tonotopic selectivity in normal-hearing humans and cats

We describe a non-invasive electrophysiological (EEG) measure of tonotopic selectivity and compare the results between humans and cats. Sequences of 50-ms tone-burst probes were presented at 1-second intervals against a continuous noise masker, and the averaged cortical onset response (COR) to the probe was measured using EEG electrodes placed on the scalp. The noise masker had a bandwidth of 1 or 1/8th octave, geometrically centred on 4000 Hz for humans and 8000 Hz for cats. Probe frequency was either -0.5, -0.25, 0, 0.25 or 0.5 octaves re 4000/8000 Hz. The COR was larger for probe frequencies more distant from the noise geometrical centre, 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 a companion paper. 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. We argue that although COR measures are affected by some factors that differ from those that influence psychophysical masked detection thresholds, they can reveal differences in the width of excitation patterns produced by different stimuli. We also argue that the paradigm may be effectively applied to cochlear-implant experiments in humans and animals.