FM Forward Masking: Implications for FM Processing

Abstract

It has been argued and has been demonstrated in several species that there is specialized neural circuitry for processing frequency changes (FM). We have examined this possibility psychophysically in humans by examining the aftereffect of a brief sinusoidal FM masker on subsequent detection of FM. Using this FM forward masking paradigm, we have shown that: (1) there can be substantial masking; (2) this is not due to FM to AM conversion; (3) there is broad tuning for the modulation frequency of the FM; (4) the recovery period is approximately 300 ms; (5) similar results were seen at different carrier frequencies (4 kHz, 500 Hz). These results are qualitatively similar to those shown for AM forward masking (J Acoust Soc Am 118:3198-3210, 2005) and, as suggested in that paper for AM, are consistent with an adapting modulation filterbank for FM. More specifically, the notion is that there are hardwired neural processes selectively tuned to FM and that these channels adapt and recover over a relatively brief time scale (300 ms). An alternative model involves higher level, plastic processes such as noisy template matching and short-term memory. In this paper, we will highlight the basic psychophysical data and discuss the theoretical implications for models of FM processing.