Spatial coordinates of human auditory working memory.

The accuracy of localizing remembered sound sources was investigated by employing a delayed-response task, where a small light spot, projected onto a screen by a laser diode attached to the head, had to be spatially aligned with either actual or remembered stimulus positions. Systematic errors indicated overestimation of the eccentricity of remembered targets compared to direct stimulus localization. This overestimation increased with prolonged response delay, suggesting that the coordinates of memorized space are distorted with respect to perceived actual sound location and that this distortion increases as a function of time.

Spatio-temporal constraints for auditory--visual integration.

The perceptual coherence of auditory and visual information is achieved by integrative brain processes. Specialized single neurons with spatial and temporal interactions of auditory and visual stimuli have been demonstrated by several neurophysiological studies. The present, psychophysical, study investigates possible perceptual correlates of these neuronal features. Subjects had to indicate the point of subjective spatial alignment (PSSA) for a horizontally moving visual stimulus that crossed the position of a stationary sound source. Auditory and visual stimuli consisted of periodic pulses that were systematically varied in their phase relationship or repetition rate. PSSAs obtained for continuous visual stimuli served as a reference. When sound and light pulses were coincident in phase at a repetition rate of 2 Hz, PSSAs were shifted by approximately 3 degrees in a direction opposite to the movement of the visual stimulus (with respect to the reference condition). This shift markedly decreased when the temporal disparity exceeded approximately 100 ms and disappeared near phase opposition (250 ms disparity). With 4 Hz repetition rate (temporal disparity < or =125 ms), there was no significant effect of phase relationship on PSSAs, but still an approximately constant shift with respect to the reference value. Variation of the repetition rate resulted in almost constant shifts in PSSA of approximately 3 degrees between 1 and 4 Hz and a linear decrease (slope 0.27 degrees /Hz) with higher repetition rates. These results suggest a spatio-temporal 'window' for auditory-visual integration, that extends over approximately 100 ms and approximately 3 degrees : when auditory and visual stimuli are within this window, they are always perceived as spatially coincident. These psychophysical findings may be related to properties of bimodal neurons such as have been demonstrated by neurophysiological recordings in midbrain and cortex.

Effect of gaze direction on sound localization in rear space.

The effect of eccentric eye position on the localization of sound in rear space was investigated, using a two-alternative forced-choice method in combination with a visual fixation task. The azimuthal position of the rear sound was perceived as shifted slightly (mean 1.2 degrees ) to the left of the subjects' median plane when fixation was 30 degrees to the right, or to the right when fixation was 30 degrees to the left. Combined with previous studies on localization in frontal space, this finding suggests that eye-position signals influence processing of binaural, but not monaural spectral, cues for directional hearing.

Dynamic vision based on motion-contrast: changes with age in adults.

Data are presented for a computerized test of dynamic vision in a sample of 1006 healthy subjects aged between 20 and 85 years. The test employed a form-from-motion stimulus: i.e., within a random-dot display, Landolt rings of the same average luminance as their surroundings become visible only when the dots within the ring are moved briefly, while those of the surround remain stationary. Thus, detection of gap location is based upon motion contrast (form-from-motion) rather than luminance contrast. With the size and exposure duration of the centrally presented ring held constant, motion contrast was manipulated by varying the percentage (between 20 and 100%) of moving dots within the ring. Subjects reported gap location (left, right, top, bottom). A gradual decline of dynamic vision with age was found for all motion-contrast levels. Beyond 70 years of age, chance-level performance occurred in almost half of the subjects. The data provide the basis for applications including diagnostic screening for glaucoma, visual disturbances in brain-damaged patients, as well as assessment of the dynamic vision of drivers of motor vehicles and athletes.

Primacy and frequency effects in absolute judgments of visual velocity.

In absolute judgment tasks, identical stimuli are rated higher (or lower) when presented in a series of more frequent small (or large) stimuli. Using visual stimuli differing in velocity, we show that this conventional frequency effect is largely modulated by the primacy effect--that is, by the stimuli occurring on the early trials of a run. In Experiment 1, a frequency-like primacy effect was obtained with equal-frequent velocities. Identical velocities were rated faster when mainly slow rather than fast ones occurred on initial trials. In Experiment 2, we contrasted the frequency effect and the primacy effect: In runs with frequent slow velocities, mainly fast ones occurred earlier, whereas in runs with infrequent slow velocities, mainly slow ones did so. Lack of differences of ratings in the two conditions suggests that the two effects canceled each other. In Experiment 3, when mainly frequent velocities occurred earlier, the conventional frequency effect was obtained. We conclude that the conventional frequency effect represents a combination of the primacy effect and the pure frequency effect.

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.

The scintillating grid illusion during smooth pursuit, stimulus motion, and brief exposure in humans.

The Scintillating Grid Illusion occurs when small white disks are superimposed onto the intersections of a grey-on-black Hermann grid. As a result illusory dark spots are seen at numerous crossings, flashing with each flick of the eye and changing their location and distribution with each saccade. The illusion is absent with steady fixation. The present study shows that saccadic eye movements are not necessary to produce the illusion. Rather, the illusion was also found to occur (i) during smooth pursuit movements when the grid was stationary, (ii) during smooth displacement of the grid with the gaze kept steady, and (iii) during brief exposures of the stationary grid. It is concluded that, while transient stimulation is essential for generating the illusion, reduction in effective luminance contrast resulting from brief exposure and high stimulus speed are responsible for reductions in its strength.

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.

Neck-proprioceptive influence on auditory lateralization.

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.

Development of dynamic vision based on motion contrast.

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 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.

Influence of head-to-trunk position on sound lateralization.

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 degrees 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).

A computer-assisted test for the electrophysiological and psychophysical measurement of dynamic visual function based on motion contrast.

A new test is described that allows for electrophysiological and psychophysical measurement of visual function based on motion contrast. In a computer-generated random-dot display, completely camouflaged Landolt rings become visible only when dots within the target area are moved briefly while those of the background remain stationary. Thus, detection of contours and the location of the gap in the ring rely on motion contrast (form-from-motion) instead of luminance contrast. A standard version of this test has been used to assess visual performance in relation to age, in screening professional groups (truck drivers) and in clinical groups (glaucoma patients). Aside from this standard version, the computer program easily allows for various modifications. These include the option of a synchronizing trigger signal to allow for recording of time-locked motion-onset visual-evoked responses, the reversal of target and background motion, and the displacement of random-dot targets across stationary backgrounds. In all instances, task difficulty is manipulated by changing the percentage of moving dots within the target (or background). The present test offers a short, convenient method to probe dynamic visual functions relying on surprathreshold motion-contrast stimuli and complements other routine tests of form, contrast, depth, and color vision.

Early demonstrations of subjective contours, amodal completion, and depth from half-occlusions: "stereoscopic experiments with silhouettes" by Adolf von Szily (1921).

If all-black figures are used, certain monocular appendages to binocular shapes are seen in depth, either nearer (when in a medial position) or further (when in a lateral position) than the binocular shape itself. These appendages also link to form subjective contours in front of the binocular shape or amodal completions behind it. These and other discoveries by von Szily, made before 1921, anticipate a number of modern findings.

Motion extrapolation and velocity transposition.

A study of the effect of the size of a moving target and the extent of its visible motion on motion extrapolation is reported. Targets (a horizontal pair of dots separated by either 0.2 or 0.8 deg) moved across a 10 deg rectilinear path and were then occluded. Observers pressed a key when they thought the leading dot of a hidden target had reached a randomly specified position (0-12 deg from the point of occlusion). In experiment 1, in agreement with velocity-transposition predictions, at moderate (5 deg s-1) and rapid (10 deg s-1) velocities extrapolation times were longer for large targets than for small ones. At slow velocity (2.5 deg s-1) this effect was reversed. In experiment 2 the effect of target size at moderate velocity was found for a short (2.5 deg) visible path. However, the extrapolation time increased with shorter (2.5 deg versus 10 deg) paths. A proposed account of these effects suggests that the visual system performs a spatiotemporal scaling, according to the velocity-transposition principle, not only of visible motion but also of extrapolated motion.

Dynamic complexity of visuo-motor coordination: an extension of Bernstein's conception of the degrees-of-freedom problem.

Extending Bernstein's spatial conception of the degrees-of-freedom problem in the human motor system, we introduce a method developed from the theory of non-linear dynamics that allows one to quantify the spatio-temporal, i.e. dynamic, complexity of visuo-motor coordination. The correlation dimension D is used to measure the effective number of dynamic degrees of freedom in the coordination that a subject uses when performing a visuo-motor tracking task. The validity of the estimator employed is demonstrated. Visuo-motor coordination had a low-dimensional (mean D-SD=6.07 -0.82) dynamic structure, which was consistent with deterministic chaos rather than with pure stochastic noise. D correlated with tracking performance, P. Both D and P were closely related to the degree of visuo-motor compatibility that the task presented to the subject. However, for short periods of training P increased, but D did not. As these seemingly contradictory results suggest, our dynamic conception of the degrees-of-freedom problem may reveal far more intricate visuo-motor interactions than Bernstein could identify on the basis of his spatial analyses of bodily movement patterns and by the methods of evaluation that were available to him at the time.

The growing-louder effect in short diotic stimuli.

Previous evidence from short monotic stimuli shows that a steady stimulus is perceived as growing louder; to be perceived as steady, the intensity of the stimulus must decrease. In the present study, 10 subjects heard a sequence of diotic tonal stimuli. Each stimulus lasted 1.5 sec. and increased, decreased, or remained steady in intensity; initial intensity was 40 dB SPL and carrier frequency was 1 kHz. Subjects made forced binary responses of "growing louder" or "growing softer" to each stimulus. Confirming the evidence from monotic stimuli, the mean value of changing intensity eliciting equal numbers of both responses was negative. Possible explanations for this growing-louder effect reside in (a) the percussive nature of many natural sounds and (b) selective responding to approaching sound-sources.

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.

The effect of eye position on auditory lateralization.

The present study examines whether the direction of gaze can influence sound lateralization. For this purpose, dichotic stimuli with variable interaural level difference (ILD) were presented under different conditions of visual fixation. In experiment 1, subjects with their head fixed directed their gaze to a given target, simultaneously adjusting the ILD of continuous pure tone or noise stimuli so that their location was perceived in the median plane of the head. The auditory adjustments were significantly correlated with gaze direction. During eccentric fixation, the psychophysical adjustments to the median plane shifted slightly toward the direction of gaze. The magnitude of the shift was about 1-3 dB, over a range of fixation angles of 45 degrees to either side. The eye position effect, measured as a function of pure-tone frequency, was most pronounced at 2 kHz and showed a tendency to decrease at lower and higher frequencies. The effect still occurred, although weaker, even when the eyes were directed to eccentric positions in darkness and without a fixation target. In experiment 2, the adjustment method was replaced by a two-alternative forced-choice method. Subjects judged whether sound bursts, presented with variable ILDs, were perceived on the left or right of the median plane during fixation of targets in various directions. Corresponding to experiment 1, the psychometric functions shifted significantly with gaze direction. However, the shift was only about half as large as that found in experiment 1. The shift of the subjective auditory median plane in the direction of eccentric gaze, observed in both experiments, indicates that dichotic sound is localized slightly to the opposite side, i.e., to the left when the gaze is directed to the right and vice versa. The effect may be related to auditory neurons which exhibit spatially selective receptive fields that shift with eye position.

A cross-modal aftereffect: auditory displacement following adaptation to visual motion.

It has been shown earlier that the perceived location of static sound-sources can be displaced (a) during visual motion and (b) following auditory motion. Here we combine these phenomena. The subject adapted to the horizontal visual motion of a surrounding drum, then (with the lights off) localized static sound-sources by setting the direction of a pointer. Adapting motion was clockwise or counterclockwise: the difference between each subject's settings following the opposite directions of adaptation showed small but consistent auditory displacements opposite to the adapting directions. This visual-auditory aftereffect, which is consistent with sensorineural data, challenges a general, if implicit, belief that aftereffects do not cross modalities.