A comparison of the contrast effects in sound localization in the horizontal and vertical planes

The effect of a background sound on the auditory localization of a single sound source was examined. Nine loudspeakers were arranged crosswise in the horizontal and the median vertical plane. They ranged from -20 degrees to +20 degrees, with the center loudspeaker at 0 degree azimuth and elevation. Using vertical and horizontal centimeter scales, listeners verbally estimated the position of a 500-ms broadband noise stimulus being presented at the same time as a 2 s background sound, emitted by one of the four outer loudspeakers. When the background sound consisted of continuous broadband noise, listeners consistently shifted the apparent target positions away from the background sound locations. This auditory contrast effect, which is consistent with earlier findings, equally occurred in both planes. But when the background sound was changed to a pulse train of noise bursts, the contrast effect decreased in the horizontal plane and increased in the vertical plane. This discrepancy might be due to general differences in the processing of interaural and spectral localization information.     

Eye position and visual attention influence perceived auditory direction

The singular and joint effects of eccentric visual fixation and peripheral visual attention upon the perceived auditory median plane (PAMP) of the head were examined in four subjects. They positioned a hidden, horizontally-moving sound source to the perceived median plane of the head, while fixating a light 20 degrees or 45 degrees to the right or left of the actual median plane, and attending to another light in peripheral vision. Analysis by multiple linear regression yielded a model for each subject which indicated the nature and weighting of those factors contributing to variance reduction in PAMP judgments. Three subjects showed a strong linear attention effect, resulting in shifts of the PAMP away from the locus of attention. Similarly, a strong linear fixation effect was discovered in two subjects, a weak effect in another, and a nonlinear, inverse fixation effect in the last. Additional factors were noted for some subjects. Possible explanations of the observed effects were discussed. The study indicates that both visual attention and visual fixation are critical factors in experiments on auditory space perception although inter-individual differences do exist in the nature and strength of the effects.     

The influence of the acoustic context on vertical sound localization in the median plane

The influence of background sounds (frames) on vertical localization of single sound sources (targets) was examined in four experiments. Loudspeakers (five targets and four frames) were positioned in the median plane, ranging from +30 degrees to -30 degrees above and below the subject's ear level. The subjects determined the vertical position of the targets by either verbal judgments or manual pointing. Frame and target sounds were presented concurrently or successively with a 1-sec interval; both consisted of (1) 300-Hz square waves, (2) noise, or (3) targets of noise and frames of 300-Hz square waves. Particularly in the second condition, the subjects consistently shifted the apparent target positions away from the frame locations. This contrast effect persisted even 1 sec after the offset of the frames. No effect was found with different waveforms for the frame and the target. Results are related to recent findings indicating a similar effect in the azimuthal dimension. Possibly the effect is based on a mechanism in which the auditory system adapts to recently heard sound source positions.     

Development of multisensory spatial integration and perception in humans

Previous studies have shown that adults respond faster and more reliably to bimodal compared to unimodal localization cues. The current study investigated for the first time the development of audiovisual (A-V) integration in spatial localization behavior in infants between 1 and 10 months of age. We observed infants' head and eye movements in response to auditory, visual, or both kinds of stimuli presented either 25 degrees or 45 degrees to the right or left of midline. Infants under 8 months of age intermittently showed response latencies significantly faster toward audiovisual targets than toward either auditory or visual targets alone They did so, however, without exhibiting a reliable violation of the Race Model, suggesting that probability summation alone could explain the faster bimodal response. In contrast, infants between 8 and 10 months of age exhibited bimodal response latencies significantly faster than unimodal latencies for both eccentricity conditions and their latencies violated the Race Model at 25 degrees eccentricity. In addition to this main finding, we found age-dependent eccentricity and modality effects on response latencies. Together, these findings suggest that audiovisual integration emerges late in the first year of life and are consistent with neurophysiological findings from multisensory sites in the superior colliculus of infant monkeys showing that multisensory enhancement of responsiveness is not present at birth but emerges later in life.     

Infants' coordination of auditory and visual depth information.

Using the preferential-looking procedure infants 5, 7, and 9 months of age were presented two videoimages side by side on separate monitors accompanied by a soundtrack that matched one of the images. Each infant was presented: (1) a stationary drum-beating toy paired with the same toy approaching and receding in depth, to assess infants' recognition both that changing sound amplitude is a property of an object that is moving in space and that constancy in amplitude is a property of a stationary object; (2) a drum-beating toy moving horizontally paired with one approaching and receding in depth, to assess infants' recognition that systematic increases and decreases in amplitude accompany object movement in a particular dimension, namely depth; and (3) two identical toys alternately approaching and receding in depth but out of phase (i.e., one approaching while the other is receding), to assess infants' recognition that increases and decreases in amplitude accompany a particular type of object movement in depth. Measures of mean duration of looking time indicated that the 5-month-olds looked reliably to the correct videoimage only for the stationary toy paired with the constant amplitude sound. The 7-month-olds recognized that changes in amplitude accompany object movement in depth but did not coordinate auditory with visual depth information as well as older infants. The 9-month-olds looked reliably to the correct videoimages in all conditions. Possible contributing factors to these developmental trends in performance are discussed.     

Direct measurement of the curvature of visual space

We consider the horizontal plane at eye height, that is all objects seen at the horizon. Although this plane visually degenerates into a line in the visual field, the 'depth' dimension nevertheless gives it a two-dimensional structure. We address the problem of intrinsic curvature of this plane. The classical geometric method is based on Gauss's original definition: The angular excess in a triangle equals the integral curvature over the area of the triangle. Angles were directly measured by a novel method of exocentric pointing. Experiments were performed outside, in the natural environment, under natural viewing conditions. The observers were instructed not to move from a set location and to maintain eye height, but were otherwise free to perform eye, head, and body movements. We measured the angular excess for equilateral triangles with sides of 2-20 m, the vantage position at the barycenter. We found angular excesses and deficits of up to 30 degrees. From these data we constructed the metric. The curvature changes from elliptic in near space to hyperbolic in far space. At very large distances the plane becomes parabolic.     

Imaginal repositioning in everyday environments: effects of testing method and setting

Two experiments examined spatial knowledge access after imagined perspective switches in everyday environments. Blindfolded volunteers had to point to target objects in a well-known surrounding while imagining being repositioned into different spatial perspectives defined as self-rotations of the observer in the horizontal plane (0 degrees , 45 degrees , and 135 degrees ). Testing was either conducted in the space the target locations had been learned in (actual room testing), or while participants were away from this room, but should imagine being situated there (remote room testing). Experiment 1, in which perspective switches were tested on a trial-to-trial basis, revealed increases in pointing latency and error as a function of the amount of angular disparity between real and imagined perspective under actual and remote room conditions. Experiment 2, in which knowledge access was tested in blocks of multiple trials per perspective, showed increases in pointing latency with angular disparity for actual space testing, and to a much lesser degree for remote space testing. Implications of both findings for theoretical accounts of knowledge access after imaginal perspective switches are discussed, and recommendations for using different testing methods and settings are given.     

Perception of symmetry in infancy: the salience of vertical symmetry and the perception of pattern wholes

Four experiments were conducted to assess converging aspects of 4-month-old infants' perception of symmetry in visual patterns. Experiments 1 and 2 manipulated the structure and orientation of comparable patterns in order to evaluate the specialty of vertical symmetry. Infants showed no preference among vertically symmetrical, vertically repeated, and obliquely symmetrical patterns, but they processed vertically symmetrical patterns more efficiently than either vertically repeated patterns or obliquely symmetrical patterns. Experiment 3 manipulated the spatial separation of pattern components in order to determine the ability of young infants to integrate and coalesce information in visual patterns that is distributed in space. Infants processed vertically symmetrical patterns whose components were contiguous or nearly contiguous about the vertical axis (0 to 2.5 degrees separations) more efficiently than discontiguous patterns (5 and 10 degrees separations). Thus, extreme spatial separation about the vertical meridian caused infants to lose the advantage for vertical symmetry, and by inference their holistic perception of the visual pattern. Experiment 4 manipulated the organization of individual components of a vertical pattern in order to examine further infants' sensitivity to perceptual organization and synthesis of pattern form. Infants discriminated vertically symmetrical patterns from asymmetrical patterns with a vertical organization, thereby demonstrating sensitivity to the symmetrical organization of the pattern above their perception of components in the pattern. The results of these four experiments together corroborate and extend previous findings that vertical symmetry has a special status in early perceptual development and that infants can perceive pattern wholes.     

Retinal location and its effect on the spatial distribution of visual attention

A series of studies tested for distractor compatibility effects across wide target/distractor distances (0.6 degree to 20 degrees of visual angle). The effects of precue condition, constant/varied target location, horizontal/vertical distractor distance, and foveal/peripheral presentation were studied. Results show strong compatibility effects across wide distances when distractors are at peripheral retinal locations. When both stimuli were presented at the same peripheral location in opposite hemifields, compatibility effects were evident within an area of at least 2.5 degrees of visual angle. In contrast, when foveally placed distractors were used, compatibility effects were found primarily with target letters positioned near. The findings suggest that distance effects are not homogeneous across retinal location.     

Stabilization and destabilization of perception--action patterns influence the self-organized recruitment of degrees of freedom

The recruitment of an additional biomechanical degree of freedom in a unimanual rhythmic task was explored. Subjects were asked to synchronize adduction or abduction of their right index finger with a metronome, the frequency of which was increased systematically. In addition, haptic contact on or off the metronome beat was provided. Results showed that the pattern exhibiting the highest intrinsic stability recruited the vertical plane more than did the less stable one. Moreover, presence and location of haptic contact modulated coordination stability and therefore induced changes in the recruitment of the vertical plane. Thus, a trade-off was shown between coordination stability on the horizontal plane and recruitment of the vertical one. These findings suggest that recruitment of degrees of freedom is governed by general principles of coordination dynamics.     

Perceptual space in the dark affected by the intrinsic bias of the visual system

Correct judgment of egocentric/absolute distance in the intermediate distance range requires both the angular declination below the horizon and ground-surface information being represented accurately. This requirement can be met in the light environment but not in the dark, where the ground surface is invisible and hence cannot be represented accurately. We previously showed that a target in the dark is judged at the intersection of the projection line from the eye to the target that defines the angular declination below the horizon and an implicit surface. The implicit surface can be approximated as a slant surface with its far end slanted toward the frontoparallel plane. We hypothesize that the implicit slant surface reflects the intrinsic bias of the visual system and helps to define the perceptual space. Accordingly, we conducted two experiments in the dark to further elucidate the characteristics of the implicit slant surface. In the first experiment we measured the egocentric location of a dimly lit target on, or above, the ground, using the blind-walking-gesturing paradigm. Our results reveal that the judged target locations could be fitted by a line (surface), which indicates an intrinsic bias with a geographical slant of about 12.4 degrees. In the second experiment, with an exocentric/relative-distance task, we measured the judged ratio of aspect ratio of a fluorescent L-shaped target. Using trigonometric analysis, we found that the judged ratio of aspect ratio can be accounted for by assuming that the L-shaped target was perceived on an implicit slant surface with an average geographical slant of 14.4 degrees. That the data from the two experiments with different tasks can be fitted by implicit slant surfaces suggests that the intrinsic bias has a role in determining perceived space in the dark. The possible contribution of the intrinsic bias to representing the ground surface and its impact on space perception in the light environment are also discussed.     

The development of infant discrimination of affect in multimodal and unimodal stimulation: The role of intersensory redundancy

This research examined the developmental course of infants' ability to perceive affect in bimodal (audiovisual) and unimodal (auditory and visual) displays of a woman speaking. According to the intersensory redundancy hypothesis (L. E. Bahrick, R. Lickliter, & R. Flom, 2004), detection of amodal properties is facilitated in multimodal stimulation and attenuated in unimodal stimulation. Later in development, however, attention becomes more flexible, and amodal properties can be perceived in both multimodal and unimodal stimulation. The authors tested these predictions by assessing 3-, 4-, 5-, and 7-month-olds' discrimination of affect. Results demonstrated that in bimodal stimulation, discrimination of affect emerged by 4 months and remained stable across age. However, in unimodal stimulation, detection of affect emerged gradually, with sensitivity to auditory stimulation emerging at 5 months and visual stimulation at 7 months. Further temporal synchrony between faces and voices was necessary for younger infants' discrimination of affect. Across development, infants first perceive affect in multimodal stimulation through detecting amodal properties, and later their perception of affect is extended to unimodal auditory and visual stimulation. Implications for social development, including joint attention and social referencing, are considered.     

Spatial variations of visual-auditory fusion areas

The tolerance to spatial disparity between two synchronous visual and auditory components of a bimodal stimulus has been investigated in order to assess their respective contributions to perceptual fusion. The visual and auditory systems each have specific information-processing mechanisms, and provide different cues for scene perception, with the respective dominance of space for vision and of time for hearing. A broadband noise burst and a spot of light, 500 ms in duration, have been simultaneously presented to participants who had to judge whether these cues referred to a single spatial event. We examined the influence of (i) the range and the direction of spatial disparity between the visual and auditory components of a stimulation and (ii) the eccentricity of the bimodal stimulus in the observer's perceptual field. Size and shape properties of visual-auditory fusion areas have been determined in two dimensions. The greater the eccentricity within the perceptual field, the greater the dimension of these areas; however, this increase in size also depends on whether the direction of the disparity is vertical or horizontal. Furthermore, the relative location of visual and auditory signals significantly modifies the perception of unity in the vertical plane. The shape of the fusion areas, their variation in the field, and the perceptual result associated with the relative location of the visual and auditory components of the stimulus, concur towards a strong contribution of audition to visual-auditory fusion. The spatial ambiguity of the localisation capabilities of the auditory system may play a more essential role than accurate visual resolution in determining fusion.     

Auditory psychomotor coordination and visual search performance

In Experiments 1 and 2, the time to locate and identify a visual target (visual search performance in a two-alternative forced-choice paradigm) was measured as a function of the location of the target relative to the subject's initial line of gaze. In Experiment 1, tests were conducted within a 260 degree region on the horizontal plane at a fixed elevation (eye level). In Experiment 2, the position of the target was varied in both the horizontal (260 degrees) and the vertical (+/- 46 degrees from the initial line of gaze) planes. In both experiments, and for all locations tested, the time required to conduct a visual search was reduced substantially (175-1,200 msec) when a 10-Hz click train was presented from the same location as that occupied by the visual target. Significant differences in latencies were still evident when the visual target was located within 10 degrees of the initial line of gaze (central visual field). In Experiment 3, we examined head and eye movements that occur as subjects attempt to locate a sound source. Concurrent movements of the head and eyes are commonly encountered during auditorily directed search behavior. In over half of the trials, eyelid closures were apparent as the subjects attempted to orient themselves toward the sound source. The results from these experiments support the hypothesis that the auditory spatial channel has a significant role in regulating visual gaze.     

Developmental changes in associations between auditory-visual events

The goal of the present study was twofold: to examine the influence of two amodal properties, co-location and temporal synchrony, on infants' associating a sight with a sound, and to determine if the relative influence of these properties on crossmodal learning changes with age. During familiarization 2-, 4-, 6- and 8-month-olds were presented two toys and a sound, with sights and sounds varying with respect to co-location and temporal synchrony. Following each familiarization phase infants were given a paired preference test to assess their learning of sight-sound associations. Measures of preferential looking revealed age-related changes in the influence of co-location and temporal synchrony on infants' learning sight-sound associations. At all ages, infants could use temporal synchrony and co-location as a basis for associating an auditory with a visual event and, in the absence of temporal synchrony, co-location was sufficient to support crossmodal learning. However, when these cues conflicted there were developmental changes in the influence of these cues on infants' learning auditory-visual associations. At 2 and 4 months infants associated the sounds with the toy that moved in synchrony with the sound's rhythm despite extreme violation of co-location of this sight and sound. In contrast, 6- and 8-month-olds did not associate a specific toy with the sound when co-location and synchrony information conflicted. The findings highlight the unique and interactive effects of distinct amodal properties on infants' learning arbitrary crossmodal relations. Possible explanations for the age shift in performance are discussed.     

The development of a human auditory localization response: a U-shaped function

Research during the past 10 years on the neonatal head-turn response to off-centred rattle sounds is reviewed, and various procedural and stimulus conditions that influence the probability of eliciting a correct response are identified. Also, the existence of a U-shaped developmental function is confirmed in a cross-sectional study of 104 infants between 3 days and 7 months of age. Neonates responded reliably, but slowly; the response decreased in frequency and magnitude between 1-3 months of age and increased again by 4-5 months of age. Speculation that this U-shaped function reflects a maturational shift in locus of control from subcortical to cortical structures was supported by the infants' responses to the presence effect (PE), which is thought to be cortically mediated. The PE was produced by playing the rattle sound through two loudspeakers with the output of one delayed by 5 ms, relative to the other; adults perceive only one sound at the leading loudspeaker. As predicted, neonates failed to respond to the PE, and the onset of correct PE responses corresponded closely to the upswing in the U-shaped function for SS responses. Other explanations for the temporary decline in orientation responses to sound are also discussed.     

Cognitive Constraints on the Development of Hierarchical Spatial Organization Skills

This study examines the mechanism behind an observed shift in young children's hierarchical organization skills. By 9 years of age, children, like adults, code the location of an object in a homogeneous space at two levels (fine-grained and categorical) and combine this information when reporting location. For points within a circle, multi-level location coding occurs for both radius and angle. Before age 9 years, children employ hierarchical coding only radially. It was hypothesized that younger children may either be unable to encode and/or process two dimensions simultaneously, or to impose the frame of reference required for angular subdivision. Tasks were designed to test these two hypotheses: one reduced the relevant dimensions required for coding, the other provided the frame of reference. Results indicate that 7-year-olds are able to form angular categories when the number of dimensions for coding is reduced, and that an explicit frame of reference does not facilitate mature angular category division. The acquisition of adult forms of spatial representation appears to be limited by encoding and/or processing capacities, rather than by spatial abilities.     

The effect of orientation on tactual braille recognition: optimal touching positions.

Subjects in five experiments matched tangible braille against a visible matching code. In Experiment 1, braille recognition suffered when entire lines of braille characters were tilted in varying amounts from the upright. Experiment 2 showed that tilt lowered performance for tangible, large embossed letters, as well as for braille. However, recognition was better for print letters than it was for braille. In Experiment 3, subjects attempted to match the upright array against embossed braille that was left/right reversed, inverted up/down, or rotated +180 degrees. Performance was close to that for normal braille in the left/right reversal condition, and very low for the +180 degrees rotation group. These results on braille tilt in the "picture plane" may reflect difficulty in manipulating the tangible "image." Braille recognition performance was not lowered when the visible matching array was tilted -45 degrees or -90 degrees from the upright but the tangible stimuli were upright. In Experiment 4, recognition of left/right reversed braille that was physically horizontal (on the bottom of a shelf) was compared with that of braille left/right reversed due to its location on the back of a panel, in the vertical plane. Braille recognition accuracy was higher with braille located vertically. An additional experiment showed the beneficial effect of locating braille in the vertical, frontoparallel plane, obtained with +90 degree rotated braille. It is proposed that optimal tactual performance with tangible arrays might depend on touching position, and on the physical position of stimuli in space. Just as there are good and poor viewing positions, there may be optimal touching positions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Monocular unmasking of noise-embedded patterns.

Binocular disparity cues may help an observer "unmask" a target in a background, thereby enhancing its detectability (e.g., Moraglia & Schneider, 1992). Here, we sought to determine whether similar effects could be produced by monocular displacement cues resulting from a two-frame sequential presentation of a Gabor pattern (a sinusoidal modulation of luminance combined with a Gaussian modulation of local contrast) embedded in an unvarying field of two-dimensional Gaussian noise. The Gabor in the second frame was spatially displaced relative to its location in the first frame; the horizontal displacement corresponded to a phase shift of the peak spatial frequency of the Gabor of 0 degree, 90 degrees, 180 degrees, 360 degrees, or 540 degrees. Monocular detection thresholds for the Gabor were appreciably lower for the 90 degrees, 180 degrees, and 540 degrees shift, than for the 0 degree and 360 degrees values. We explain these findings in terms of a model that constitutes the monocular analog of our summation model of binocular unmasking.