THE MEASUREMENT OF THE LATERALIZATION OF NARROW BANDS OF NOISE USING AN ACOUSTIC POINTING PARADIGM - THE EFFECT OF SOUND-PRESSURE LEVEL

The effects of varying interaural time delay (ITD) and interaural inte nsity difference (IID) were measured in normal-hearing subjects as a f unction of eleven frequencies and at sound-pressure levels (SPL) from 60 to 90 dB SPL and at 25-dB sensation level. Using an ''acoustic'' po inting paradigm, the IID of a 500-Hz narrow-band (100 Hz) noise (the ' 'pointer'') was varied by the subject to coincide with that of a ''tar get'' ITD stimulus. ITDs of 0, +/-200, and +/-400 mu s were obtained t hrough total waveform delays of narrow-band noise (NBN), including env elope and fine structure. The results of this experiment confirm the t raditional view of binaural hearing for like stimuli: There is little perceived displacement away from 0 IID at frequencies of 1250 Hz and a bove. In the low frequencies, subjects required IIDs greater than the expected 10 dB to perceive a fully lateralized image, and they varied in the maximum value of IID that they required, regardless of frequenc y. Our subjects did not always perceive the intracranial locations of ITD targets symmetrically: When the signal was delayed to one ear, the resultant matching IID was often different in magnitude than for the same ITD target delayed to the opposite ear for the identical frequenc y. The results of two subjects suggested that people with asymmetric n ormal hearing have adapted to their asymmetry for lateralization tasks : The subjects were found to lateralize toward the ear with the greate r SPL stimulus, regardless of the ear to which the signal was delayed, when signals of equal SL were presented, and toward the leading ear w hen signals of equal SPL were presented (unequal SL). Increasing the p resentation levels above 60 dB SPL had an effect on the perception of high-frequency ITD targets: As the intensity level increased, the slop es of the IID versus ITD functions increased indicating better discrim ination of ITD. This study is in agreement with other studies in provi ding strong evidence of individual differences in lateralization exper iments. These individual differences might be attributable to differen tial sensitivity to ambiguous time stimulus cues, differential task se nsitivity, age effects, threshold asymmetries, or criterion variabilit y.