Dreams and cross-modal plasticity in the blind
Updated: Nov 18, 2020
Can plasticity of the occipital cortex explain the phenotype of dreams in the blind?
What does the phenotype of dreams in the blind suggest about structural and functional changes in the occipital - “visual” - cortex of early blind (EB) individuals? In this essay I will describe some characteristics of blind people’s dreams, and then put forward cross-modal plasticity as a possible explanation for the differences between oneiric sensory elements in blind vs. sighted individuals.
While sighted people experience mainly visual impressions in their dreaming life, most studies reveal that in the dreams of the EB auditory imagery is the most frequently experienced sensory modality, followed - in decreasing order - by tactile, gustatory and olfactory sensory components. Late blind (LB) individuals, instead, retain some form of visual imagery, and tend to experience more oneiric tactile impressions than sighted controls (SC), but they do not always have enhanced auditory imagery in their dreams (3-5, 9).
The likely reason why LB individuals retain visual imagery is that, before they lost sight, their occipital cortex “got used” to processing visual information from the retina; later on, after blindness onset, this part of the LB brain can still produce visual impressions when stimulated, as in rapid-eye-movement (REM) sleep. In the EB, instead, the occipital cortex could never specialise in visual processing, and therefore it fails to produce any kind of visual imagery (2, 5, 9). However, the occipital cortex of the EB has some extraordinary abilities that distinguish it from that of most LB and SC individuals. Only to mention auditory processing, several studies report that the occipital cortex of EB participants was activated during tasks such as sound localisation and detection of changes in auditory stimulation (6-8, 10).
The unique features of the “blind visual cortex” could be underpinned by visual-auditory cross-modal plasticity. Simply put, in EB individuals the absence of visual stimuli may lead to the creation of new connections between the occipital cortex and areas of the brain involved in auditory processing, or to the unmasking of existing connections which are normally inhibited in the presence of vision (2). This cross-modal plasticity, which has been observed in blind moles (1), may also explain why auditory imagery is the most common sensory modality in EB people’s dreams. However, more research is needed to further elucidate the differences in dream phenotype between EB, LB and SC, and to understand the means by which the occipital cortex of the EB is reorganised to process non-visual sensory inputs.
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