On the use of the normalized correlation as an index of interaural envelope correlation

The purpose of the present study was to obtain new empirical observations that would help determine the form of interaural envelope correlation that accounts for listeners' sensitivity to binaural information conveyed by high-frequency stimuli. In a four-interval, two alternative task, listeners detected which interval contained a 4-kHz tone added antiphasically to diotic, 200-Hz-wide noise (NoS pi). The "nonsignal" intervals contained the tone added homophasically (NoSo). Discriminability (d') was measured as a function of S/N for values between -30 and +30 dB. For all S/Ns, overall level was 70 dB SPL. Using a similar procedure, listeners were also tested in "direct" discrimination tasks where changes in envelope correlations were produced in one of two manners. First, changes in the normalized interaural envelope correlation were produced by varying the degree to which the envelopes covaried. This was accomplished by "mixing" two independent Gaussian noises. Second, changes in the normalized interaural envelope correlation were produced by adding dc values to the envelopes of the stimuli. Overall, in both experiments, listeners' ability to discriminate changes in interaural envelope correlation (from a base correlation of 1.0) was well accounted for by the normalized correlation. It is also shown that the normalized correlation of the envelope accounts for classic data concerning discriminability of interaural time differences at high frequencies as a function of depth of modulation.