Walter H. Ehrenstein

Sound localization with eccentric head position

This study investigates the influence of head-to-trunk position on auditory localization in humans. Various methods of head pointing, of two-alternative forced choice, and hand pointing were employed. Head-pointing toward actual sound sources in darkness, by using only the subjective median plane of the head as a reference, resulted in systematic underestimations of target eccentricity. The deviations of the terminal head position from the target shifted with a mean slope of approximately 0.1 degrees per degree change in head position. A corresponding shift in the localization of virtual sound sources (presented via headphones during eccentric head positions) was demonstrated by requiring forced-choice (left or right) responses with respect to the subjective median plane of the head. Head pointing toward remembered auditory targets in darkness resulted in undershoots similar to those found with actual targets. However, when a visual marker of the actual median plane of the head was additionally presented to the subject during these tasks (by a laser attached to the head that projected a spot onto a screen), sound localization was fairly accurate. Localization of eccentric auditory targets by using a swivel hand pointer also showed systematic errors similar to those found with head pointing in darkness when the head was simultaneously oriented toward the sound. When the head remained in alignment with the trunk, hand pointing resulted in overshooting responses. These results may be related to neural processes, presumably in the posterior parietal cortex, that transform auditory and visual spatial coordinates into a common, trunk-centered, frame of reference.

Auditory-visual spatial integration: A new psychophysical approach using laser pointing to acoustic targets

The alignment of auditory and visual spatial perception was investigated in four experiments, employing a method of laser pointing toward acoustic targets in combination with various tasks of visual fixation in six subjects. Subjects had to fixate either a target LED or a laser spot projected on a screen in a dark, anechoic room and, while doing so, direct the laser beam toward the perceived azimuthal position of the sound stimulus (bandpass-filtered noise; bandwidth 1-3 kHz; 70 dB sound pressure level, duration 10 s). The sound was produced by one of nine loudspeakers, located behind the acoustically transparent screen between 22 degrees to the left and 22 degrees to the right of straight ahead. Systematic divergences between sound azimuth and laser adjustment were found, depending on the instructions given to the subjects. The eccentricity of acoustic targets was generally overestimated by up to 10.4 degrees with an only slight influence of gaze direction on this effect. When the sound source was straight ahead, gaze direction had a substantial influence in that the laser adjustments deviated by up to 5.6 degrees from sound azimuth, toward the side to which the gaze was directed. This effect of eye position decreased with increasing eccentricity of the sound. These results can be explained by the interactive effects of four distinct factors: the lateral overestimation of the auditory eccentricity, the effect of eye position on sound localization, the effect of the retinal eccentricity on visual localization, and the extraretinal effect of eye position on visual localization.

Visual and proprioceptive shifts in perceived egocentric direction induced by eye-position.

The relation of three main effects of eye-position on perceived direction was investigated using a method of hand pointing in the horizontal plane: (1) Retinal eccentricity is overestimated with respect to the fovea by a constant factor of 2.6 degrees; (2) an extraretinal signal induces a shift in perceived visual direction (slope 0.12) that is opposite to the direction of eccentric gaze; and (3) the perceived position of the median plane of the head shifts toward the direction of eccentric eye-position (slope 0.23) while perceived trunk position remains unchanged.

Selective directional sensitivity in visual motion perception

We present two experiments demonstrating that: (i) the latency of perception of the position of a small visual target moving towards the fovea is shorter than that of the same target moving away from the fovea; (ii) the reaction time (RT) to onset of motion of the same type of target is also shorter when it moves towards the fovea; and (iii) the RT to onset of motion away from the fovea may be shorter when larger, textured stimuli are employed. The relation of the findings to the existence of two systems for visual motion information processing and to recent neurophysiological findings is discussed.

Motion-onset visual-evoked potentials as a function of retinal eccentricity in man

Visual-evoked potentials were elicited by the motion-onset of a black-and-white square-wave grating of 2.4 cycles/deg that drifted from right to left at a velocity of 3 deg/s. The center of the 2 x 2 deg stimulus field was binocularly viewed either foveally or at eccentricities of 6, 12, or 20 deg in the lower visual field along the vertical meridian. Peak-to-peak amplitudes P1-N2 and N2-P2 were found to decrease non-linearly as a function of eccentricity. The VEP-amplitudes were standardized by setting each foveal value to 100%, and a relative measure was derived for peripheral values given by the ratio of the peripheral to the foveal values. The decrease of the relative VEP-values with eccentricity was significantly smaller than that of the relative cortical magnification factor of striate cortex in man, whereas it agreed fairly well with that of the relative point-image size of the area MT in Macaque monkey. In this respect, the motion-onset VEP is distinct from the pattern-reversal VEP, the amplitude of which decreases much more rapidly with retinal eccentricity; hence, it may involve different generating structures of the brain.

Influence of head-to-trunk position on sound lateralization

Abstract The effect of horizontal head position on the lateralization 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 plane (SAMP) while simultaneously directing the beam of a laser attached to the head to visual targets in various directions. The adjustments were significantly correlated with head position, shifting in a direction toward the side to which the head was turned. This result was replicated in experiment 2, which employed a two-alternative forced-choice method, in which stimuli of different ILD were presented and left/right judgments were made. In both experiments, the average magnitude of the shift of the SAMP was about 1 dB over the range of head positions from straight ahead to 60° to the side. The shift of the SAMP indicates that any shift in head position induces a change in sound lateralization in the opposite direction, i.e., the intracranial sound image is shifted slightly to the left when the head is directed to the right 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, induced by either head- or eye-position changes, are in the same direction and, on average, of about the same magnitude (experiment 3), and head- and eye-position effects compensate approximately for each other during variations of head position when the gaze remains fixed to a visual target in space (experiment 4).

Neck-proprioceptive influence on auditory lateralization.

Abstract The effect of transcutaneous vibration of the posterior neck muscles on the lateralization of dichotic sound was investigated in human subjects. Two-alternative forced-choice (left/right) judgements were made on acoustic stimuli presented with different interaural level differences via headphones during neck-muscle vibration. A shift of the subjective auditory median plane toward the side contralateral of vibration was found, indicating that the sound was perceived as shifted toward the side of vibration. The mean magnitude of the vibration-induced intracranial shift was 1.5 dB. The results demonstrate a neck-proprioceptive influence on sound lateralization and suggest that this proprioceptive input is used for a central-nervous transformation of auditory spatial coordinates onto a body-centered frame of reference.

Varying the Strength of the Munker—White Effect by Stereoscopic Viewing

In the Munker—White effect grey target bars appear lighter when they are flanked by white bars, and darker when they are flanked by black bars. It is shown that the effect is enhanced if the patterns are presented stereoscopically so that the grey bars appear either behind the grating, in which case they are seen as a rectangle that is occluded by the white bars of the grating, or in front of the grating, so that they form a transparent rectangle. These results are explained in terms of object perception: contrast enhances differences between an object and its surroundings, whereas assimilation reduces differences within an object.

Development of dynamic vision based on motion contrast

Abstract The development of dynamic vision was investigated in 400 healthy subjects (200 females and 200 males) aged between 4 and 24 years. The test consisted of a computer-generated random-dot kinematogram in which a Landolt ring was briefly presented as a form-from-motion stimulus. Motion contrast between the ring and background was varied in terms of the percentage of dots moving coherently within the ring in four levels (100%, 50%, 30%, and 20%). The subject’s task was to indicate the position of a gap in the ring (left, right, top, bottom). Results show a clear increase in performance with age for all motion contrast levels, with the greatest changes for the lowest levels. Adult performance was reached at the age of 15 years. Luminance-based static acuity measured with the Landolt test was poorly correlated with acuity for its form-from-motion analogue.

Auditory-visual shift in localization depending on gaze direction

The effect of eye position on the spatial congruence of the perceived direction of auditory and visual cues was investigated, using a two-alternative forced choice method in combination with a visual fixation task. The azimuth of the sound was perceived as slightly shifted to the left of a visual reference when the gaze was directed to the left, and to the right when the gaze was to the right. The maximum magnitude of this relative auditory-visual shift was 4.7 degrees over a range of fixation angles from 45 degrees to the left to 45 degrees to the right. The observed auditory-visual shift may reflect an incomplete transformation of spatial coordinates within auditory and visual neural representations, as suggested by neurophysiological recordings in the primate midbrain.