Binaural summation of the acoustic reflex

The ways in which summation of energy or of information occurs have long been used to explore sensory mechanisms. In the case of the acoustic reflex, some binaural summation is known to occur, but both data and specification of method have been sketchy. Accurate estimates of mean and standard deviation were therefore sought to compare binaural acoustic reflex summation (BARS) with binaural loudness summation (BLS) to determine whether these have a common basis.

A specified method was developed for determining reliable acoustic reflex values from admittance/intensity functions. Subjects were 34 university students; the ages ranged from 18 through 25 yr. For each, the contralateral component of the binaural stimulus was presented at a level above the ipsilateral component corresponding to the difference in monaural reflex thresholds ("physiologically equivalent").

BARS had a mean of 4.4 dB (SD = 3.5 dB), which was significantly different from the BLS (mean = 7.6 dB, SD = 1.2 dB). The contralateral-ipsilateral difference had a mean of 11.0 dB (SD = 5.1 dB). The BARS estimate was not appreciably larger than that found in studies lacking correction for physiologic equivalence.

Although the presence of a contralateral component elicits a summation or facilitation effect, the particular intensity value contributes little to the magnitude of that effect.