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Even Blind People Can’t See the Darkness: A Neurologist’s Manuscript

by | May 1, 2025 | Manuscripts | 0 comments

Mahesh Mudi, Fellow in Paediatric Optometry, Binocular Vision and Cerebral Vision Impairment

Consultant Optometrist, Narayana Nethralaya, Bengaluru, India

 

Dynamics of visual function

Congenitally blind individuals, having never seen physical objects, cannot perceive images visually. The term “blindness” is often ambiguously used, with only one dictionary definition addressing the medical condition. Symbolic uses associate blindness with ignorance or lack, as seen in phrases like “blind fear” or “blind prejudice.” This metaphorical “darkness” underscores the divide between blind and sighted individuals. Blind writers’ express frustration over societal misconceptions, including the notion that blindness enhances other senses as a compensatory.

Experiment—I Experiment—II
  • Participants in a study by Cooper and Shepard (1973) were shown a letter, such as “ᴚ,” and timed as they determined if it was in its normal orientation or a mirror image. To do this, they retrieved a canonical “R” from memory and compared it to the presented stimulus. Visual or spatial representations can be derived from verbal descriptions, haptic input, or retrieval from semantic memory.(3)
  • Research indicates that congenitally blind individuals can perform mental rotation tasks using sensory inputs other than visual imagery. In a haptic version of the Shepard and Metzler task, they rotated shapes more slowly than those who became blind later in life, and both groups were slower than sighted, blindfolded participants. Longer encoding and response selection durations further highlight their reliance on alternative sensory processing.(4)

Table 1: This table summarises two distinct experiments involving blind individuals.

Neurologists Aspect 

The idea that blind individuals “live inside their bodies” is complex, as they experience the world through different sensory modalities. Neuroimaging studies show overlaps between cortical regions for visual perception and imagery, though they do not completely align. fMRI studies reveal that the primary visual cortex, crucial for seeing, does not directly mediate visual imagery in sighted individuals; instead, a network of spatial subsystems and higher-order visual regions does. Non-visual senses, like touch, can activate visual cortical regions. After sudden vision loss, the brain undergoes significant changes, with visual areas repurposed for other senses. (7,8)

Recent studies using MRI brain imaging techniques like voxel-based morphometry (VBM), diffusion tensor imaging (DTI), and diffusion tensor tractography (DTT) have looked at changes in Gray Matter (GM) and white matter (WM) in the brains of blind people. They indicate that there is noticeable loss of brain tissue in all parts of the visual pathway. This includes certain brain areas like the lateral geniculate and posterior pulvinar nuclei, as well as the striate and extra-striate visual areas, and the inferior temporal gyrus and lateral orbital cortex, which help with recognising objects. (9,10) EEG data reveal that the numerical distance effect has comparable parietal activity in both sighted and blind individuals. Research on blind people’s mental images involves simple imagery paradigms and spatial knowledge development, showing they better recall concrete, visualisable phrases than abstract ones. (11)

Interestingly, blind individuals may exhibit colour representation, making colour names appropriate for both sighted and blind people. Early vision deprivation may improve blind people’s hearing processing along the “where” pathway, but not the “what” route. Blind individuals are faster at detecting and locating auditory signals but slower at discriminating their frequency.(12) In early blind people, listening to sounds can activate areas in the brain related to vision, including parts in the back of the brain. This also involves areas in the cerebellum and parts of the brain that process touch. The inferior colliculus (IC) and dorsal lateral geniculate nucleus (LGNd) can transmit auditory information to the visual thalamus.(13)

PET and fMRI studies reveal that blind individuals utilise supra-modal brain regions, including the main visual cortex and extra-striate areas. Congenitally blind individuals show reduced grey matter but increased cuneus thickness due to less cell removal during early development. Post-mortem studies indicate early monocular blindness reduces ocular dominance in the affected eye’s visual cortex, with corresponding enlargement in the working eye’s columns.(7,8,14)

Conclusion 

Congenitally blind individuals rely on touch and hearing, which enable sequential data processing, unlike vision, which allows simultaneous processing. Consequently, their cognitive functions appear “sequential.”

 

References:

  1. Kleege, G. (2006). Blind rage: An open letter to Helen Keller. Gallaudet University Press.
  2. National Federation of the Blind. (n.d.). Blindness statistics. Retrieved from https://nfb.org/resources/blindness-statistics
  3. Cooper, L. A., & Shepard, R. N. (1973). The time required to prepare for a rotated stimulus. Memory & Cognition, 1(3), 246-250. https://doi.org/10.3758/BF03198104
  4. Shepard, R. N., & Metzler, J. (1971). Mental rotation of three-dimensional objects. Science, 171(3972), 701-703. https://doi.org/10.1126/science.171.3972.701
  5. Arcos, K., Harhen, N., Loiotile, R., & Bedny, M. (2022). Superior verbal but not nonverbal memory in congenital blindness. Experimental Brain Research, 240(3), 897-908. https://doi.org/10.1007/s00221-021-06304-4
  6. Gougoux, F., Zatorre, R. J., Lassonde, M., Voss, P., & Lepore, F. (2005). A functional neuroimaging study of sound localization: Visual cortex activity predicts performance in early-blind individuals. PLoS Biology, 3, e27. https://doi.org/10.1371/journal.pbio.0030027
  7. Dietrich, S., Hertrich, I., & Ackermann, H. (2013). Ultra-fast speech comprehension in blind subjects engages primary visual cortex, fusiform gyrus, and pulvinar – A functional magnetic resonance imaging (fMRI) study. BMC Neuroscience, 14, Article 74. https://doi.org/10.1186/1471-2202-14-74
  8. Burton, H. (2003). Visual cortex activity in early and late blind people. Journal of Neuroscience, 23, 4005-4011. https://doi.org/10.1523/JNEUROSCI.23-10-04005.2003
  9. Bauer, C. M., Merabet, L. B., & Merabet, K. (2017). Brain rewires itself to enhance other senses in blind people. PLOS ONE. Retrieved from ScienceDaily.
  10. Blakemore, E. (2017). Blind people’s brains rewire themselves to enhance other senses. Smithsonian Magazine. Retrieved from Smithsonian Magazine.
  11. Szucs, D., & Csépe, V. (2005). The parietal distance effect appears in both the congenitally blind and matched sighted controls in an acoustic number comparison task. Neuroscience Letters, 384(1-2), 11-16. https://doi.org/10.1016/j.neulet.2005.04.050
  12. Guerreiro, M. J. S., Putzar, L., & Röder, B. (2016). The effect of early visual deprivation on the neural bases of auditory processing. Journal of Neuroscience, 36(5), 1620-1630. https://doi.org/10.1523/JNEUROSCI.2559-15.2016
  13. Park, W. J., & Fine, I. (2022). The perception of auditory motion in sighted and early blind individuals. bioRxiv. https://doi.org/10.1101/2022.09.11.507447
  14. Leporé, N., Voss, P., Lepore, F., Chou, Y. Y., Fortin, M., Gougoux, F., Lee, A. D., Brun, C., Madsen, S. K., Toga, A. W., & Thompson, P. M. (2009). Brain structure changes visualized in early- and late-onset blind subjects. NeuroImage, 10, 1-12. https://doi.org/10.1016/j.neuroimage.2009.07.028

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