Posterior parietal cortex contributions to cross-modal brain plasticity upon sensory loss

A nasal chemosensation-dependent critical window for somatosensory development

Nasal chemosensation is considered the evolutionarily oldest mammalian sense and, together with somatosensation, is crucial for neonatal well-being before auditory and visual pathways start engaging the brain. Using anatomical and functional approaches in mice, we reveal that odor-driven activity propagates to a large part of the cortex during the first postnatal week and enhances whisker-evoked activation of primary whisker somatosensory cortex (wS1). This effect disappears in adult animals, in line with the loss of excitatory connectivity from olfactory cortex to wS1. By performing neonatal odor deprivation, followed by electrophysiological and behavioral work in adult animals, we identify a key transient regulation of nasal chemosensory information necessary for the development of wS1 sensory-driven dynamics and somatosensation. Our work uncovers a cross-modal critical window for nasal chemosensation-dependent somatosensory functional maturation.

When Imagination Feels Like Reality: A Case Study of False Memories and Maladaptive Daydreaming in Visual Impairment

Background

When a person experiences maladaptive daydreaming (MD), they spend a prolonged period daydreaming with a strong sense of presence. The symptoms of MD are often excessive, interfere with functioning, and are linked to distress and comorbid mental disorders. In this paper, apparent false memory is described in the context of a woman with MD and visual impairment due to a progressive eye condition. Her vivid daydreams seemed indistinguishable from actual memories. Case Report. A 35-year-old woman with a lifelong MD reported three incidents of fabricating detailed false memories of events that her family confirmed never occurred: obtaining a new job, miscarrying twins, and hospitalization for COVID-19. She experienced anxiety and shame when the stories were disproven. The assessment confirmed MD, PTSD, OCD, and other disorders. Her verbal memory was below average, especially for longer narratives. Her misattributions of daydreams as real-life memories may relate to reliance on vivid mental images over deteriorating vision and source monitoring deficits.

Conclusion

This first reported case of confabulations in an individual with MD and visual disability suggests daydreams could potentially be mistaken for actual events in some MD cases. While sensitive, more research is needed on the prevalence of false memories among individuals with MD. The default mode network, prefrontal cortex, and their connectivity may be implicated in generating vivid daydreams and misattributing them to actual episodic events. Understanding the relationship between sensory impairments, dissociation, and susceptibility to memory distortions could inform interventions to improve reality testing for some MD patients.

Mixed Selectivity Coding of Content-Temporal Detail by Dorsomedial Posterior Parietal Neurons

The dorsomedial posterior parietal cortex (dmPPC) is part of a higher-cognition network implicated in elaborate processes underpinning memory formation, recollection, episode reconstruction, and temporal information processing. Neural coding for complex episodic processing is however under-documented. Here, we recorded extracellular neural activities from three male rhesus macaques (Macaca mulatta) and revealed a set of neural codes of "neuroethogram" in the primate parietal cortex. Analyzing neural responses in macaque dmPPC to naturalistic videos, we discovered several groups of neurons that are sensitive to different categories of ethogram items, low-level sensory features, and saccadic eye movement. We also discovered that the processing of category and feature information by these neurons is sustained by the accumulation of temporal information over a long timescale of up to 30 s, corroborating its reported long temporal receptive windows. We performed an additional behavioral experiment with additional two male rhesus macaques and found that saccade-related activities could not account for the mixed neuronal responses elicited by the video stimuli. We further observed monkeys' scan paths and gaze consistency are modulated by video content. Taken altogether, these neural findings explain how dmPPC weaves fabrics of ongoing experiences together in real time. The high dimensionality of neural representations should motivate us to shift the focus of attention from pure selectivity neurons to mixed selectivity neurons, especially in increasingly complex naturalistic task designs.

Implications of Neural Plasticity in Retinal Prosthesis

Retinal degenerative diseases such as retinitis pigmentosa cause a progressive loss of photoreceptors that eventually prevents the affected person from perceiving visual sensations. The absence of a visual input produces a neural rewiring cascade that propagates along the visual system. This remodeling occurs first within the retina. Then, subsequent neuroplastic changes take place at higher visual centers in the brain, produced by either the abnormal neural encoding of the visual inputs delivered by the diseased retina or as the result of an adaptation to visual deprivation. While retinal implants can activate the surviving retinal neurons by delivering electric current, the unselective activation patterns of the different neural populations that exist in the retinal layers differ substantially from those in physiologic vision. Therefore, artificially induced neural patterns are being delivered to a brain that has already undergone important neural reconnections. Whether or not the modulation of this neural rewiring can improve the performance for retinal prostheses remains a critical question whose answer may be the enabler of improved functional artificial vision and more personalized neurorehabilitation strategies.

The Impact of Hearing Loss and Hearing Aid Usage on the Visuospatial Abilities of Older Adults in a Cohort of Combined Hearing and Cognitive Impairment

Introduction

It has been proposed that hearing loss may result in improved visuospatial abilities. The evidence for this assertion is inconsistent, and limited to studies in congenitally deaf children, despite older adults with age-related hearing loss constituting the vast majority of the hearing impaired population. We assessed visuospatial (visuoconstruction and visuospatial memory) ability in older adult hearing aid users with and without clinically significant cognitive impairment. The primary aim of the study was to determine the effect of hearing loss on visuospatial abilities.

Method

Seventy-five adult hearing aid users (HA) aged over 65 were recruited, out of whom 30 had normal cognition (NC-HA), 30 had mild cognitive impairment (MCI-HA), and 15 had dementia (D-HA). The Rey Osterrieth Complex figure test (ROCFT) copy, 3 min recall and 30 min recall tests were performed to evaluate the visuoconstructional and visuospatial memory abilities of the participants.

Results

There were significant differences between the ROCFT copy, 3 min recall, and 30 min recall among the three cohorts (p < 0.005). Compared with previously published normative data, the NC-HA performed significantly better in the ROCFT copy (p < 0.001), immediate recall (p < 0.001), and delay recall (p = 0.001), while the MCI-HA performed similarly to the expected norms derived from population (p = 0.426, p = 0.611, p = 0.697, respectively), and the D-HA performed below this norm.

Conclusion

Though visuospatial abilities tend to decline when the global cognitive functioning declines, we found suggestive evidence for positive effects of age-related hearing loss on visuospatial cognitive ability. Participants with mild cognitive impairment and hearing loss, who would have been expected to perform worse than normative data, were in fact performing as well as cognitively healthy subjects without hearing loss. Visuospatial ability could be targeted when providing rehabilitation for the older adults with hearing loss.

Multisensory deficits in dyslexia may result from a locus coeruleus attentional network dysfunction

A fundamental educational requirement of beginning reading is to learn, access, and rapidly process associations between novel visuospatial symbols and their phonological representations in speech. Children with difficulties in such cross-modal integration are often divided into dyslexia subtypes, based on whether their primary problem is with the written or spoken component of decoding. The present review suggests that starting in infancy, perceptions of audiovisual speech are integrated by mutual oscillatory phase-resetting between sensory cortices, and throughout development visual and auditory experiences are coupled into unified perceptions. Entirely separate subtypes are incompatible with this view. Visual or auditory deficits will invariably affect processing to some degree in both domains. It is suggested that poor auditory/visual integration may be diagnostic for both forms of dyslexia, stemming from an encoding weakness in the early cross-sensory binding of audiovisual speech. The review presents a model of dyslexia as a dysfunction of the large-scale ventral and dorsal attention networks controlling such binding. Excessive glutamatergic neuronal excitability of the attention networks by the Locus coeruleus-norepinephrine system may interfere with multisensory integration, with deleterious effects on the acquisition of reading by degrading graphene/phoneme conversion.

Cortical and Subcortical Circuits for Cross-Modal Plasticity Induced by Loss of Vision

Cortical areas are highly interconnected both via cortical and subcortical pathways, and primary sensory cortices are not isolated from this general structure. In primary sensory cortical areas, these pre-existing functional connections serve to provide contextual information for sensory processing and can mediate adaptation when a sensory modality is lost. Cross-modal plasticity in broad terms refers to widespread plasticity across the brain in response to losing a sensory modality, and largely involves two distinct changes: cross-modal recruitment and compensatory plasticity. The former involves recruitment of the deprived sensory area, which includes the deprived primary sensory cortex, for processing the remaining senses. Compensatory plasticity refers to plasticity in the remaining sensory areas, including the spared primary sensory cortices, to enhance the processing of its own sensory inputs. Here, we will summarize potential cellular plasticity mechanisms involved in cross-modal recruitment and compensatory plasticity, and review cortical and subcortical circuits to the primary sensory cortices which can mediate cross-modal plasticity upon loss of vision.