Psychophysical Tuning Curve and Critical Band as Determined by Masking in the Presence of FM sounds

A upward-sweeping FM sound and a downward-sweeping sound were used to estimate psychophysical tuning curves (PTCs) in simultaneous masking. In each experiment, masked PTCs for signal level of 15, 20, 30 and 35dB SL caused by the addition of a 800ms masker (FM sound) were determined as a function of masker frequency. The results were as follows: 1) When a upward-sweeping FM sound was used to estimate masked PTCs, the PTC for the higher signal level had the shallower low-frequency slope and steeper high frequency slope, and 10dB bandwidth for PTC coincided in restricted PTC. 2) When a downward-sweeping FM sound was used to estimate masked PTCs, the PTC for the higher signal level had the steeper low-frequency slope and shallower high-frequency slope, and 10dB bandwidth for PTC coincided in unrestricted PTC. 3) The PTCs as determined by masking in the presense of FM sounds apparently restricted subjects to listen off-frequency, and it was shown that the bandwidth of PTC properly refects the critical band. 4) The properties of these PTCs can be easily explained by a neural network model of lateral inhibition, therefore, we hypothesized that hierarchical processing is incorporated within inferior colliculus for parallel signal processing. 5) We concluded that frequency resolution and critical band filtering are therefore realized in a psychophysically relevant way in the autitory midbrain level.