Robert S. Wall

Spatial hearing in children with visual disabilities.

A study is reported of the effect of early visual experience on the development of auditory space perception. The spatial hearing of thirty-five children with visual disabilities (twenty-two with congenital total blindness) was compared with that of eighteen sighted children and seventeen sighted adults. The tests provided a comprehensive assessment of spatial-hearing ability, including psychophysical estimates of spatial resolution in the horizontal, vertical, and distance dimensions, as well as measures of reaching and walking to the locations of sound sources. The spatial hearing of the children with visual disabilities was comparable to or some-what better than that of the sighted children and adults. This pattern held even when the group with visual disabilities was restricted to those children with congenital total blindness; in fact, some of those children had exceptionally good spatial hearing. These findings imply that the developmental calibration of human spatial hearing is not dependent on a history of visual experience. It seems likely that this calibration arises from the experience of changes in sound-localization cues arising from self-motion, such as turning the head or walking. As a practical matter, orientation and mobility instructors may reasonably assume that individuals with visual disabilities can use their hearing effectively in day-to-day travel situations.

Cortical activation during Braille reading is influenced by early visual experience in subjects with severe visual disability: A correlational fMRI study

Functional magnetic resonance imaging was performed on blind adults resting and reading Braille. The strongest activation was found in primary somatic sensory/motor cortex on both cortical hemispheres. Additional foci of activation were situated in the parietal, temporal, and occipital lobes where visual information is processed in sighted persons. The regions were differentiated most in the correlation of their time courses of activation with resting and reading. Differences in magnitude and expanse of activation were substantially less significant. Among the traditionally visual areas, the strength of correlation was greatest in posterior parietal cortex and moderate in occipitotemporal, lateral occipital, and primary visual cortex. It was low in secondary visual cortex as well as in dorsal and ventral inferior temporal cortex and posterior middle temporal cortex. Visual experience increased the strength of correlation in all regions except dorsal inferior temporal and posterior parietal cortex. The greatest statistically significant increase, i.e., ∼30%, was in ventral inferior temporal and posterior middle temporal cortex. In these regions, words are analyzed semantically, which may be facilitated by visual experience. In contrast, visual experience resulted in a slight, insignificant diminution of the strength of correlation in dorsal inferior temporal cortex where language is analyzed phonetically. These findings affirm that posterior temporal regions are engaged in the processing of written language. Moreover, they suggest that this function is modified by early visual experience. Furthermore, visual experience significantly strengthened the correlation of activation and Braille reading in occipital regions traditionally involved in the processing of visual features and object recognition suggesting a role for visual imagery. Hum. Brain Mapping 14:186–195, 2001.

Impact of optical devices on reading rates and expectations for visual functioning of school-age children and youth with low vision

Optical devices were prescribed and training provided for children with low vision in the state of Tennessee. Reading rates and comprehension rates were taken before introduction of the optical devices and after approximately six months of using the aids. Scores were also recorded of student, teacher, and parent expectations of the students’ visual functioning. Data show that students did not make significant changes in their silent or oral reading rates. However, it is important to note that they did make significant gains in comparison to the reading norms of their sighted peers. Data also show that expectation of students’ visual functioning increased in both students and their teachers. In contrast, parents’ expectations of students’ visual functioning decreased.

Low Frequency Sound as a Navigational Tool for People with Visual Impairments

For decades, a mistaken assumption in the habilitation of travel for people with visual impairments has been that the use of “echolocation” is a dominant feature of acoustic information for the guidance of locomotion. This paper outlines a theoretical model of acoustic information available in many travel situations. The acoustic information consists of a build up of low frequency sound along large structures such as walls. Useful frequencies are low enough that the experience of this build up is often one of feeling rather than hearing. Experimental validation of this model was accomplished by having participants with visual impairments walk along walls with varying types of auditory occlusion and through laboratory based manipulations of acoustic information. Results indicate that the model is appropriate. Useful acoustic information appears to extend up to 1 m from a wall. There is strong evidence that participants make use of binaural information, gauging the similarity of acoustic information reaching each ear to maintain orientation to a wall. Motion through the sound field near the wall makes the information more useful but it is unclear whether motion in certain directions or conditions is more useful than others.

The effect of stimulus bandwidth and subject position on horizontal‐plane localization with virtual source images

In an anechoic chamber normal‐hearing subjects performed a localization task in the frontal horizontal plane. The stimulus was a 200‐ms burst of filtered noise. Within a block of trials, half of the presentations (randomly determined) were ‘‘real’’—presented from single loudspeakers—and the other half were ‘‘phantoms’’—produced by the simultaneous activation of two loudspeakers at ±30° using a virtual source imaging technique [Takeuchi et al., J. Acoust. Soc. Am. 109, 958–971 (2001)]. Both phantom and real sources spanned the azimuthal range ±80°. When the stimulus was a 4 kHz low‐pass filtered noise, rms error was only slighly higher for phantom (D=7.1°) than for real (D=5.5°) sources. For 8 kHz low‐pass filtered noise, performance remained about the same for real sources, but increased for phantom sources (D=11.5°). Data will also be reported for conditions in which the subject’s position is systematically varied outside the ‘‘sweet spot.’’ Results will be discussed in terms of robustness of the virt...

Precision of sound localization measured by a reaching task

This study validates a reaching measure of sound localization for subsequent application to children. Seven adults reached for broadband sound sources while hand position was measured to within 2 mm. Sounds came from 18 regions in frontal reaching space, with the loudspeaker moved away just after stimulus offset. In the ‘‘visual’’ condition subjects watched until the sound ended, then closed their eyes and reached. In the ‘‘auditory’’ condition subjects were blindfolded. Precision of sound localization was estimated by comparing variability in the visual and auditory conditions: s=√s2A−s2V. Estimates were computed for horizontal angle, vertical angle, and distance for each target location. Results agreed reasonably well with conventional measures. For targets straight ahead at ear level, horizontal s=2.6°, vertical s=5.0°, and distance s=9.5%. Systematic variations occurred across target locations. This reaching task is a rapid, naturalistic way of measuring three‐dimensional sound localization. [Work sup...