Relative contributions of two neural population responses to intensity coding

Abstract

Two neural population responses have been proposed to encode the 120-dB intensity range: the spread of excitation along the entire basilar membrane and neurons of different thresholds within a narrow region of the basilar membrane. This study tests relative contributions of the two population codes by measuring forward-masked intensity discrimination of a broadband noise (14 kHz) and pure tones from 125 to 8000 Hz in octave steps. The hypothesis is that the broadband noise and low-frequency tones rely more on the spread of the excitation code, whereas high-frequency tones rely more on the high-threshold neuron code. To obtain appropriate forward masker levels, three normal-hearing listeners balanced loudness of the broadband noise and tones to a 100-ms, 90-dB SPL, 1000-Hz standard. Intensity discrimination was measured for a 25-ms probe which had the same spectral characteristic as the masker and occurred after a 100-ms delay from the masker’s offset. The discrimination data showed that the midlevel hump systematically increases from the broadband noise and the low-frequency tone to the high-frequency tone conditions, indicating a greater contribution of high-threshold neurons to intensity coding [Zeng et al., Hear. Res.55 , 223–230 (1991)]. Backward-masked intensity discrimination was also measured and will be discussed.