Aural localization of silent objects by active human biosonar: neural representations of virtual echo-acoustic space

Objective Evaluation of Obstacle Perception Using Spontaneous Body Movements of Blind People Evoked by Movements of Acoustic Virtual Wall

Obstacle perception using sound is the ability to detect silent objects, such as walls and poles. It is very important for blind people to recognize their environment using acoustic information through their auditory sense when walking or conducting various daily activities. In this paper, to develop an objective method for evaluating the degree of obstacle perception acquisition in the education and rehabilitation of the blind, the authors measured the spontaneous body movements evoked by the approach of an acoustic virtual wall. Ten blind persons who have experienced obstacle perception in their daily life, and seven sighted persons with no such experience participated in the experiment. The reciprocal (approach and receding) movements of the virtual wall were presented using simulated reflected sound, and the spontaneous body movements of the subjects were measured. As the results indicate, eight of the ten blind participants showed large maximum values for the correlation function between the wall and their body movements, whereas six of the seven sighted participants showed small maximum values. These results indicate that body movements can be used for an objective evaluation of obstacle perception. In particular, it was determined that the maximum value of the correlation function is the most appropriate for such an evaluation, because it does not depend on the subject’s physique.

Impact of blindness onset on the representation of sound categories in occipital and temporal cortices

The ventral occipito-temporal cortex (VOTC) reliably encodes auditory categories in people born blind using a representational structure partially similar to the one found in vision (Mattioni et al.,2020). Here, using a combination of uni- and multivoxel analyses applied to fMRI data, we extend our previous findings, comprehensively investigating how early and late acquired blindness impact on the cortical regions coding for the deprived and the remaining senses. First, we show enhanced univariate response to sounds in part of the occipital cortex of both blind groups that is concomitant to reduced auditory responses in temporal regions. We then reveal that the representation of the sound categories in the occipital and temporal regions is more similar in blind subjects compared to sighted subjects. What could drive this enhanced similarity? The multivoxel encoding of the ‘human voice’ category that we observed in the temporal cortex of all sighted and blind groups is enhanced in occipital regions in blind groups , suggesting that the representation of vocal information is more similar between the occipital and temporal regions in blind compared to sighted individuals. We additionally show that blindness does not affect the encoding of the acoustic properties of our sounds (e.g. pitch, harmonicity) in occipital and in temporal regions but instead selectively alter the categorical coding of the voice category itself. These results suggest a functionally congruent interplay between the reorganization of occipital and temporal regions following visual deprivation, across the lifespan.

Neuroplastic Reorganization Induced by Sensory Augmentation for Self-Localization During Locomotion

Sensory skills can be augmented through training and technological support. This process is underpinned by neural plasticity in the brain. We previously demonstrated that auditory-based sensory augmentation can be used to assist self-localization during locomotion. However, the neural mechanisms underlying this phenomenon remain unclear. Here, by using functional magnetic resonance imaging, we aimed to identify the neuroplastic reorganization induced by sensory augmentation training for self-localization during locomotion. We compared activation in response to auditory cues for self-localization before, the day after, and 1 month after 8 days of sensory augmentation training in a simulated driving environment. Self-localization accuracy improved after sensory augmentation training, compared with the control (normal driving) condition; importantly, sensory augmentation training resulted in auditory responses not only in temporal auditory areas but also in higher-order somatosensory areas extending to the supramarginal gyrus and the parietal operculum. This sensory reorganization had disappeared by 1 month after the end of the training. These results suggest that the use of auditory cues for self-localization during locomotion relies on multimodality in higher-order somatosensory areas, despite substantial evidence that information for self-localization during driving is estimated from visual cues on the proximal part of the road. Our findings imply that the involvement of higher-order somatosensory, rather than visual, areas is crucial for acquiring augmented sensory skills for self-localization during locomotion.

Early visual cortex response for sound in expert blind echolocators, but not in early blind non-echolocators

Echolocation is a perceptual and navigational skill that can be acquired by some individuals. Regarding blind people, this skill can help them "see" the environment around them via a new form of auditory information based on echoes. Expert human echolocators benefit from using this technique not only in controlled environments but also in their everyday lives. In the current study, we investigate the effect of echolocation on blind people's auditory spatial abilities at the cortical level. In an auditory spatial bisection task, we tested people who are early blinds and early blind expert echolocators, along with sighted people. Our results showed that there is similar early activation (50-90 ms) in the posterior area of the scalp for both early blind expert echolocators and sighted participants, but not in the early blind group. This activation was related to sound stimulation, and it is contralateral to the position of the sound in space. These findings indicate that echolocation is a good substitute for the visual modality that enables the development of auditory spatial representations when vision is not available.

The Cross-Modal Effects of Sensory Deprivation on Spatial and Temporal Processes in Vision and Audition: A Systematic Review on Behavioral and Neuroimaging Research since 2000

One of the most significant effects of neural plasticity manifests in the case of sensory deprivation when cortical areas that were originally specialized for the functions of the deprived sense take over the processing of another modality. Vision and audition represent two important senses needed to navigate through space and time. Therefore, the current systematic review discusses the cross-modal behavioral and neural consequences of deafness and blindness by focusing on spatial and temporal processing abilities, respectively. In addition, movement processing is evaluated as compiling both spatial and temporal information. We examine whether the sense that is not primarily affected changes in its own properties or in the properties of the deprived modality (i.e., temporal processing as the main specialization of audition and spatial processing as the main specialization of vision). References to the metamodal organization, supramodal functioning, and the revised neural recycling theory are made to address global brain organization and plasticity principles. Generally, according to the reviewed studies, behavioral performance is enhanced in those aspects for which both the deprived and the overtaking senses provide adequate processing resources. Furthermore, the behavioral enhancements observed in the overtaking sense (i.e., vision in the case of deafness and audition in the case of blindness) are clearly limited by the processing resources of the overtaking modality. Thus, the brain regions that were previously recruited during the behavioral performance of the deprived sense now support a similar behavioral performance for the overtaking sense. This finding suggests a more input-unspecific and processing principle-based organization of the brain. Finally, we highlight the importance of controlling for and stating factors that might impact neural plasticity and the need for further research into visual temporal processing in deaf subjects.

Visual sensory stimulation interferes with people's ability to echolocate object size

Echolocation is the ability to use sound-echoes to infer spatial information about the environment. People can echolocate for example by making mouth clicks. Previous research suggests that echolocation in blind people activates brain areas that process light in sighted people. Research has also shown that echolocation in blind people may replace vision for calibration of external space. In the current study we investigated if echolocation may also draw on ‘visual’ resources in the sighted brain. To this end, we paired a sensory interference paradigm with an echolocation task. We found that exposure to an uninformative visual stimulus (i.e. white light) while simultaneously echolocating significantly reduced participants’ ability to accurately judge object size. In contrast, a tactile stimulus (i.e. vibration on the skin) did not lead to a significant change in performance (neither in sighted, nor blind echo expert participants). Furthermore, we found that the same visual stimulus did not affect performance in auditory control tasks that required detection of changes in sound intensity, sound frequency or sound location. The results suggest that processing of visual and echo-acoustic information draw on common neural resources.

Human Exploration of Enclosed Spaces through Echolocation

Some blind humans have developed echolocation, as a method of navigation in space. Echolocation is a truly active sense because subjects analyze echoes of dedicated, self-generated sounds to assess space around them. Using a special virtual space technique, we assess how humans perceive enclosed spaces through echolocation, thereby revealing the interplay between sensory and vocal-motor neural activity while humans perform this task. Sighted subjects were trained to detect small changes in virtual-room size analyzing real-time generated echoes of their vocalizations. Individual differences in performance were related to the type and number of vocalizations produced. We then asked subjects to estimate virtual-room size with either active or passive sounds while measuring their brain activity with fMRI. Subjects were better at estimating room size when actively vocalizing. This was reflected in the hemodynamic activity of vocal-motor cortices, even after individual motor and sensory components were removed. Activity in these areas also varied with perceived room size, although the vocal-motor output was unchanged. In addition, thalamic and auditory-midbrain activity was correlated with perceived room size; a likely result of top-down auditory pathways for human echolocation, comparable with those described in echolocating bats. Our data provide evidence that human echolocation is supported by active sensing, both behaviorally and in terms of brain activity. The neural sensory-motor coupling complements the fundamental acoustic motor-sensory coupling via the environment in echolocation.SIGNIFICANCE STATEMENT Passive listening is the predominant method for examining brain activity during echolocation, the auditory analysis of self-generated sounds. We show that sighted humans perform better when they actively vocalize than during passive listening. Correspondingly, vocal motor and cerebellar activity is greater during active echolocation than vocalization alone. Motor and subcortical auditory brain activity covaries with the auditory percept, although motor output is unchanged. Our results reveal behaviorally relevant neural sensory-motor coupling during echolocation.

Echolocation in humans: an overview

Bats and dolphins are known for their ability to use echolocation. They emit bursts of sounds and listen to the echoes that bounce back to detect the objects in their environment. What is not as well-known is that some blind people have learned to do the same thing, making mouth clicks, for example, and using the returning echoes from those clicks to sense obstacles and objects of interest in their surroundings. The current review explores some of the research that has examined human echolocation and the changes that have been observed in the brains of echolocation experts. We also discuss potential applications and assistive technology based on echolocation. Blind echolocation experts can sense small differences in the location of objects, differentiate between objects of various sizes and shapes, and even between objects made of different materials, just by listening to the reflected echoes from mouth clicks. It is clear that echolocation may enable some blind people to do things that are otherwise thought to be impossible without vision, potentially providing them with a high degree of independence in their daily lives and demonstrating that echolocation can serve as an effective mobility strategy in the blind. Neuroimaging has shown that the processing of echoes activates brain regions in blind echolocators that would normally support vision in the sighted brain, and that the patterns of these activations are modulated by the information carried by the echoes. This work is shedding new light on just how plastic the human brain is. WIREs Cogn Sci 2016, 7:382-393. doi: 10.1002/wcs.1408 For further resources related to this article, please visit the WIREs website.