Cochlear nerve acoustic envelope response detection is improved by the addition of random-phased tonal stimuli

We test Lowenstein's de bias hypothesis as an alternative mechanism for the phenomenon sometimes called 'stochastic resonance'. Probe stimuli consisti ng of paired phase-locked tones at frequencies f(1) and f(2) (where f(2)-f( 1) = 800 Hz, f(1) > 4.5 kHz) and at equal intensity were used to generate s ynchronous 800 Hz cochlear nerve activity (envelope responses). When a back ground tone of the same intensity, with a frequency halfway between f(1) an d f(2), is presented simultaneously with the probe stimulus, the envelope r esponse amplitude typically decreases. Consistent with Lowenstein's hypothe sis, however, when the intensities of the probe and background tone are nea r the detection threshold of the envelope response (approximately 0-20 dB s ound pressure level), the simultaneous presence of the background tone ofte n increases the amplitude of the envelope response. At these same intensity levels, when the background tone precedes the probe stimulus, it decreases the amplitude of the response to the probe stimulus. The effects of simult aneous presentation of the probe and the background tone are frequency-depe ndent. becoming less pronounced or reversing as the frequency of the backgr ound tone departs from those of the probe stimuli. (C) 2001 Elsevier Scienc e B.V. All rights reserved.