Using the Principles of Multisensory Integration to Reverse Hemianopia

The brain can develop conflicting multisensory principles to guide behavior

Midbrain multisensory neurons undergo a significant postnatal transition in how they process cross-modal (e.g. visual-auditory) signals. In early stages, signals derived from common events are processed competitively; however, at later stages they are processed cooperatively such that their salience is enhanced. This transition reflects adaptation to cross-modal configurations that are consistently experienced and become informative about which correspond to common events. Tested here was the assumption that overt behaviors follow a similar maturation. Cats were reared in omnidirectional sound thereby compromising the experience needed for this developmental process. Animals were then repeatedly exposed to different configurations of visual and auditory stimuli (e.g. spatiotemporally congruent or spatially disparate) that varied on each side of space and their behavior was assessed using a detection/localization task. Animals showed enhanced performance to stimuli consistent with the experience provided: congruent stimuli elicited enhanced behaviors where spatially congruent cross-modal experience was provided, and spatially disparate stimuli elicited enhanced behaviors where spatially disparate cross-modal experience was provided. Cross-modal configurations not consistent with experience did not enhance responses. The presumptive benefit of such flexibility in the multisensory developmental process is to sensitize neural circuits (and the behaviors they control) to the features of the environment in which they will function. These experiments reveal that these processes have a high degree of flexibility, such that two (conflicting) multisensory principles can be implemented by cross-modal experience on opposite sides of space even within the same animal.

Ameliorating Hemianopia with Multisensory Training

Hemianopia (unilateral blindness), a common consequence of stroke and trauma to visual cortex, is a debilitating disorder for which there are few treatments. Research in an animal model has suggested that visual-auditory stimulation therapy, which exploits the multisensory architecture of the brain, may be effective in restoring visual sensitivity in hemianopia. It was tested in two male human patients who were hemianopic for at least 8 months following a stroke. The patients were repeatedly exposed to congruent visual-auditory stimuli within their blinded hemifield during 2 h sessions over several weeks. The results were dramatic. Both recovered the ability to detect and describe visual stimuli throughout their formerly blind field within a few weeks. They could also localize these stimuli, identify some of their features, and perceive multiple visual stimuli simultaneously in both fields. These results indicate that the multisensory therapy is a rapid and effective method for restoring visual function in hemianopia.SIGNIFICANCE STATEMENT Hemianopia (blindness on one side of space) is widely considered to be a permanent disorder. Here, we show that a simple multisensory training paradigm can ameliorate this disorder in human patients.

Audio-visual stimulation for visual compensatory functions in stroke survivors with visual field defect: a systematic review

Background

Hemianopia is a complete or partial blindness in the visual fields of both eyes, commonly caused by cerebral infarction. It has been hypothesized that systematic audio-visual (AV) stimulation of the blind hemifield can improve accuracy and search times, probably due to the stimulation of bimodal representations in the superior colliculus (SC), an important multisensory structure involved in both the initiation and execution of saccades.

Methods

A narrative synthesis of the findings is presented to highlight how AV rehabilitation impacts on patients with hemianopia including visual oculomotor function, functional ability in activities of daily living, hemianopic dyslexia, visual scanning and searching tasks, maintaining of functional ability post training and the effect on brain multisensory integration by using neuroimaging.

Results

Sixteen studies were included (fourteen articles (188 participants) and two literature reviews). Results were grouped into AV training of hemianopia in adults and in children and then further grouped according to the AV task type: tasks measuring the training effects by comparing visual stimulation training to audio-visual training, localization abilities in homonymous hemianopia (HH) and AV integration in patients with HH.

Conclusion

Systematic AV training may improve the processing of visual information by recruiting subcortical pathways, and because most of the patients with visual cortex damage have an intact SC, it might be useful to use the bimodal AV training to activate retinotectal functions. Nevertheless, the underlying mechanisms supporting the reported positive effects are not currently understood. Systematic functional and/or structural imaging studies may help in understanding the underlying mechanism and inform the design of optimal training paradigms.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10072-022-05926-y.

Mechanisms of Plasticity in Subcortical Visual Areas

Visual plasticity is classically considered to occur essentially in the primary and secondary cortical areas. Subcortical visual areas such as the dorsal lateral geniculate nucleus (dLGN) or the superior colliculus (SC) have long been held as basic structures responsible for a stable and defined function. In this model, the dLGN was considered as a relay of visual information travelling from the retina to cortical areas and the SC as a sensory integrator orienting body movements towards visual targets. However, recent findings suggest that both dLGN and SC neurons express functional plasticity, adding unexplored layers of complexity to their previously attributed functions. The existence of neuronal plasticity at the level of visual subcortical areas redefines our approach of the visual system. The aim of this paper is therefore to review the cellular and molecular mechanisms for activity-dependent plasticity of both synaptic transmission and cellular properties in subcortical visual areas.

Association Cortex Is Essential to Reverse Hemianopia by Multisensory Training

Hemianopia induced by unilateral visual cortex lesions can be resolved by repeatedly exposing the blinded hemifield to auditory-visual stimuli. This rehabilitative "training" paradigm depends on mechanisms of multisensory plasticity that restore the lost visual responsiveness of multisensory neurons in the ipsilesional superior colliculus (SC) so that they can once again support vision in the blinded hemifield. These changes are thought to operate via the convergent visual and auditory signals relayed to the SC from association cortex (the anterior ectosylvian sulcus [AES], in cat). The present study tested this assumption by cryogenically deactivating ipsilesional AES in hemianopic, anesthetized cats during weekly multisensory training sessions. No signs of visual recovery were evident in this condition, even after providing animals with up to twice the number of training sessions required for effective rehabilitation. Subsequent training under the same conditions, but with AES active, reversed the hemianopia within the normal timeframe. These results indicate that the corticotectal circuit that is normally engaged in SC multisensory plasticity has to be operational for the brain to use visual-auditory experience to resolve hemianopia.

An Audiovisual 3D-Immersive Stimulation Program in Hemianopia Using a Connected Device

BACKGROUND Homonymous hemianopia is a loss of conscious vision in one hemifield, strongly affecting everyday life. Audiovisual stimulation programs improve visual perception in the blind hemifield; however, they use large equipment operated in clinical settings. Such treatments require frequent visits at the clinic, hampering the patient's adherence and compliance. In one hemianopia patient, we tested a 4-week dynamic audiovisual rehabilitation program in the stand-alone, remotely controlled, virtual-reality, head-mounted display Oculus Go and measured the effect on visual perception. CASE REPORT A 15-year-old Caucasian male was diagnosed with a right homonymous hemianopia with splitting of central fixation after a traumatic occipital contusion at age 7 months. Visual assessment showed impaired binocular contrast sensitivity and retinal sensitivity. Fixation stability and visual fields were strongly affected. After a 4-week audiovisual rehabilitation program, including 3 hours 20 minutes of stimulation, the contrast sensitivity, fixation stability, and paracentral visual perception were significantly enhanced, improving quality of life. CONCLUSIONS This pioneering work reports the use of virtual-reality in a head-mounted display to provide an audiovisual stimulation protocol for low-vision rehabilitation in a hemianopia patient. Real-time data recording and remote control of the stimulation program demonstrate that such rehabilitation treatment can be performed by the patient at home without interruption of care, decreasing the burden of disease. Beneficial effects on visual function were measured according to clinical guidelines of low-vision assessment. Improvement in visual function and quality of life challenge the prevailing belief that post-acute vision loss is both permanent and unchangeable.

A Multidimensional, Multisensory and Comprehensive Rehabilitation Intervention to Improve Spatial Functioning in the Visually Impaired Child: A Community Case Study

Congenital visual impairment may have a negative impact on spatial abilities and result in severe delays in perceptual, social, motor, and cognitive skills across life span. Despite several evidences have highlighted the need for an early introduction of re-habilitation interventions, such interventions are rarely adapted to children’s visual capabilities and very few studies have been conducted to assess their long-term efficacy. In this work, we present a case study of a visually impaired child enrolled in a newly developed re-habilitation intervention aimed at improving the overall development through the diversification of re-habilitation activities based on visual potential and developmental profile, with a focus on spatial functioning. We argue that intervention for visually impaired children should be (a) adapted to their visual capabilities, in order to increase re-habilitation outcomes, (b) multi-interdisciplinary and multidimensional, to improve adaptive abilities across development, (c) multisensory, to promote the integration of different perceptual information coming from the environment.

Reversing Hemianopia by Multisensory Training Under Anesthesia

Hemianopia is characterized by blindness in one half of the visual field and is a common consequence of stroke and unilateral injury to the visual cortex. There are few effective rehabilitative strategies that can relieve it. Using the cat as an animal model of hemianopia, we found that blindness induced by lesions targeting all contiguous areas of the visual cortex could be rapidly reversed by a non-invasive, multisensory (auditory-visual) exposure procedure even while animals were anesthetized. Surprisingly few trials were required to reinstate vision in the previously blind hemisphere. That rehabilitation was possible under anesthesia indicates that the visuomotor behaviors commonly believed to be essential are not required for this recovery, nor are factors such as attention, motivation, reward, or the various other cognitive features that are generally thought to facilitate neuro-rehabilitative therapies.

Interhemispheric visual competition after multisensory reversal of hemianopia

Unilateral lesions of visual cortex have the secondary consequence of suppressing visual circuits in the midbrain superior colliculus (SC), collectively producing blindness in contralesional space (“hemianopia”). Recent studies have demonstrated that SC visual responses and contralesional vision can be reinstated by a non‐invasive multisensory training procedure in which spatiotemporally concordant visual‐auditory pairs are repeatedly presented within the blind hemifield. Despite this recovery of visual responsiveness, the loss of visual cortex was expected to result in permanent deficits in that hemifield, especially when visual events in both hemifields compete for attention and access to the brain's visuomotor circuitry. This was evaluated in the present study in a visual choice paradigm in which the two visual hemifields of recovered cats were simultaneously stimulated with equally valent visual targets. Surprisingly, the expected disparity was not found, and some animals even preferred stimuli presented in the previously blind hemifield. This preference persisted across multiple stimulus intensity levels and there was no indication that animals were less aware of cues in the previously blind hemifield than in its spared counterpart. Furthermore, when auditory cues were combined with visual cues, the enhanced performance they produced on a visual task was no greater in the normal than in the previously blind hemifield. These observations suggest that the multisensory rehabilitation paradigm revealed greater inherent visual information processing potential in the previously blind hemifield than was believed possible given the loss of visual cortex.