Loss of vision, whether full or partial, can have a substantial effect on auditory spatial abilities. Research has shown that for some tasks, individuals with vision loss exhibit enhanced auditory performance. For example, blind individuals performing azimuthal localization, echolocation, or distance discrimination tasks often show superior abilities compared to sighted individuals. However, other auditory tasks, such as bisection or elevation judgments, may result in significantly degraded performance for the blind. These findings have critical clinical implications, as individuals with vision loss rely heavily on auditory cues for various activities, including path planning, orientation, and obtaining spatial information about objects in the environment.
The perceptual restructuring hypothesis provides a framework for understanding why certain auditory abilities are enhanced while others are degraded following vision loss. This hypothesis comprises nine principles derived from a broad range of evidence within the literature. For instance, the principle of “Discrimination” posits that the ability to discern small changes in sounds is improved by blindness, based on evidence showing enhanced auditory discrimination abilities following complete vision loss. Another principle, “Age at Onset,” suggests that changes in auditory ability are more pronounced the earlier in life vision is lost. This principle highlights the critical role of early sensory experiences in shaping auditory abilities.
Despite the extensive research on fully blind individuals, there is limited evidence on whether the same principles apply to people with partial vision loss, and no data exist on absolute distance judgment tasks for this group. This study aims to investigate whether the age at onset of partial visual impairment (VI) affects performance in absolute distance judgment tasks. Some studies of fully blind participants are described briefly for various tasks before studies of participants with VI are presented.
Research on Auditory Abilities in Blind Individuals
Voss et al. tested fully blind participants with either early-onset visual loss (before 11 years of age), late-onset loss (after 16 years of age), and sighted controls for a minimum-audible-angle discrimination task. Sounds were presented either in front of the interaural plane or behind it. The performance (percentage of correct responses) for the front of the interaural plane was significantly higher for the early-onset group than for the late-onset and sighted groups, both of whom performed similarly. For sounds presented behind the interaural plane, the early- and late-onset groups performed similarly, and this was significantly better than sighted controls.
Wanet and Veraart examined spatial localization for judging the distance of tones in near space in early- and late-onset blind groups and sighted controls. Performance was impaired for the early-blind group only. Gougoux et al. investigated frequency-change discrimination for early-onset, late-onset, and normally sighted participants. Early-blind participants showed significantly better performance than the other two groups. Wan et al. also reported enhanced performance for early-onset blind participants compared with late-onset blind participants for frequency discrimination and pitch-timbre categorization tasks. These studies collectively indicate differences in auditory spatial abilities and frequency discrimination between individuals with early- and late-onset complete visual loss. Although late-onset loss is sometimes associated with changes in auditory spatial abilities, more often, similar performance is reported when late-onset blind and sighted controls are compared.
Auditory Abilities in Individuals with Partial Vision Loss
The term “visual impairment” (VI) encompasses a range of conditions involving partial visual loss, in which some vision is preserved. This is distinct from full visual loss, which involves complete blindness or light perception only. Studies examining people with VI have reported mixed results regarding their auditory abilities. For example, Lessard et al. reported poorer azimuth judgments by individuals with VI, with a shift in perceived sound location toward the center of an array of loudspeakers compared to sighted controls. Data from laboratory studies have shown that greater severity of VI is associated with increased judged sound source distances, and participants with VI disrupted the relationship between judged room size and sound source distance. On the other hand, other studies have reported enhanced auditory abilities in individuals with VI, including auditory azimuth judgments, dynamic sound localization, and self-reported enhanced abilities to locate the position of the speaker during a conversation and to follow as it switched from person to person. Other studies have shown no difference between VI and sighted controls for auditory abilities such as distance discrimination and static sound localization.
Investigating Absolute Distance Judgment in Partial Vision Loss
Currently, no studies have examined absolute distance judgments in early- and late-onset partial visual impairment. This study aimed to investigate whether the age at onset (before 10 years and after) affected absolute auditory distance judgments in people with VI. Previous studies have reported that fully blind individuals with late-onset visual loss made spatial judgments similar to those of sighted controls for various tasks. Therefore, it was hypothesized that early-onset VI participants would perform better in judged distance tasks compared to late-onset VI participants.
Participants and Methodology
A total of 52 participants were recruited from Sankara Nethralaya Eye Hospital in Chennai, India. To avoid confounding effects of age, participants were chosen to be aged 33 years or less. They were categorized into three groups: normally sighted controls (18 participants; 13 female; mean age, 21 years; range, 20 to 25 years), early-onset VI (24 participants; 10 female; mean age, 23 years; range, 18 to 31 years), and late-onset VI (10 participants; 2 female; mean age, 23 years; range, 18 to 33 years). The criteria for early- and late-onset VI were vision loss before (early) or after (late) 10 years of age, similar to criteria used in the literature for fully blind individuals. Group characteristics were analyzed using one-way analysis of variance (ANOVA) and independent-samples t-tests, showing no significant differences in age or visual acuity across the groups.
Auditory Testing Procedures
All participants were screened to ensure normal or near-normal hearing, following procedures described by the British Society of Audiology. Pure-tone average better-ear hearing thresholds across 0.5, 1, 2, 4, 6, and 8 kHz were less than or equal to 25 dB hearing loss. The experimental procedure and possible consequences were described to all participants, who provided informed consent. The tenets of the Declaration of Helsinki were followed throughout testing, with ethical approval from the Anglia Ruskin University Ethics Panel and the Sankara Nethralaya Ethical Board.
Results and Interpretation
The results indicated that normally sighted controls generally provided the most accurate judgments for sounds presented at close distances, but systematically underestimated distances as the virtual source distance increased. This finding aligns with previous reports on sound source distance judgments in real environments and suggests that the virtualization methods used were adequate in simulating real-world environments.
The experiment’s conditions, particularly the simulated anechoic environment, provided limited cues to support distance judgments. The only available cue was the sound level at the listener’s ears. Although there was a significant effect of environment, distance judgments were not markedly more accurate for the reverberant room compared to the anechoic condition, even with the additional cue of the direct-to-reverberant ratio. This suggests that participants largely based their judgments on the level of the sounds reaching their ears, with differences across groups being greater for closer distances than farther ones.
Role of Visual Experience in Auditory Space Calibration
These findings can be interpreted in terms of the possible role of visual experience in calibrating auditory space. Absolute judgments, particularly with limited acoustic cues, tend to be less accurate than body-related judgments. Previous research has shown that participants born blind perform poorly in localizing brief auditory stimuli with respect to external acoustic landmarks or reproducing the spatial distance between two sounds. Such judgments require the calibration of auditory space using an external reference frame, which is more accurate when visual information is available. In contrast, blind participants perform similarly to sighted controls when localizing sounds with respect to their own hand or judging the distances of sounds from their finger, suggesting calibration using tactile/haptic information.
In this experiment, an external reference frame was likely required. None of the groups would have had much experience in judging the distance of sources 5 meters or more away, resulting in poor calibration for far space across all groups. However, the sighted group would have used visual information to calibrate auditory space for small to intermediate distances, accounting for their more accurate judgments at these distances compared to the VI groups, especially the early VI group.
Early-Onset Versus Late-Onset Vision Loss
Previous research has demonstrated that early-onset blindness leads to significant changes in auditory abilities, while late-onset blindness does not. This study’s results show that partial VI affects auditory distance judgments, consistent with the perceptual restructuring hypothesis. Principle P6 of this hypothesis states that visual cues are crucial for calibration, and Principle P9 highlights the importance of age at onset. These principles are based on evidence around the necessity of visual calibration for spatial tasks and the impact of early visual experiences on sensory abilities.
The current results also support other theoretical models, such as the cross-modal calibration hypothesis, which proposes that cross-sensory calibration occurs with the loss of a primary sense, and the perceptual deficiency hypothesis, which suggests that visual signals calibrate auditory spatial information. While these hypotheses have traditionally explained the effects of full visual loss, the current findings suggest they also apply to partial vision loss.
Need for Further Research
Further research is needed to determine whether early-onset vision loss or the overall duration of vision loss is the dominant factor affecting auditory abilities. Previous studies on fully blind individuals have shown that early onset, rather than the duration of vision loss, enhances auditory abilities. This indicates that a critical period in early development may play a crucial role in cross-modal calibration. Additionally, the effects of sex-related differences and other demographic factors, such as education, on auditory distance judgments in VI individuals warrant further investigation.
The criteria for defining early- and late-onset VI also require further exploration. This study used a cutoff age of 10 years, similar to previous research on fully blind individuals, but other studies have used different cutoff ages. For instance, research on congenitally blind and early-blind individuals (age at onset between 1.4 and 13 years) has shown that congenital blindness leads to greater changes in auditory abilities compared to early-onset blindness. Future studies should clarify how changes in auditory abilities are affected by age at onset in partially sighted individuals.
Conclusion
The study’s findings highlight that distance judgments are significantly different between early-onset VI participants and sighted controls for close distances in most conditions, with no significant differences observed between sighted controls and the late-onset VI group. These results suggest that early partial visual loss, but not late-onset visual loss, leads to significant changes in auditory distance judgments. These findings have important clinical implications for individuals with vision loss, emphasizing the need for tailored auditory training and rehabilitation programs to enhance their spatial awareness and orientation abilities. Further research is needed to fully understand the mechanisms behind these changes and to develop effective interventions for individuals with vision loss.
REFERENCE : https://journals.lww.com/optvissci/fulltext/2024/06000/effect_of_early_versus_late_onset_of_partial.15.aspx
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