INFLUENCE OF HEAD-TO-TRUNK POSITION ON SOUND LATERALIZATION

The effect of horizontal head position on the Introduction lateralizat ion of dichotic sound stimuli was investigated in four experiments. In experiment 1, subjects adjusted the interaural level difference (ILD) of a stimulus (band-pass noise) to the subjective auditory median pla ne (SAMP) while simultaneously directing the beam of a laser attached to the head to visual targets in various directions. The adjustments w ere significantly correlated with head position, shifting in a directi on toward the side to which the head was turned. This result was repli cated in experiment 2, which employed a two-alternative forced-choice method, in which stimuli of different no were presented and left/right judgments were made. In both experiments, the average magnitude of th e shift of the SAMP was about 1 dB over the range of head positions fr om straight ahead to 60 degrees to the side. The shift of the SAMP ind icates that any shift in head position induces a change in sound later alization in the opposite direction, i.e., the intracranial sound imag e is shifted slightly to the left when the head is directed to the rig ht and to the right when the head is to the left. In experiments 3 and 4, the effect of head position was compared with that of eye position by using the same methods as in experiment 2. Both shifts in SAMP, in duced by either head- or eye-position changes, are in the same directi on and, on average, of about the same magnitude (experiment 3), and he ad-and eye-position effects compensate approximately for each other du ring variations of head position when the gaze remains fixed to a visu al target in space (experiment 4).