The brains of people who lost their hearing early in life reorganize the visual map: the primary visual cortex and thalamus devote more neural resources to the peripheral visual field than to its center. This is the conclusion reached by researchers from the Universities of York and Sheffield. The study was published in the journal Proceedings of the National Academy of Sciences.
The reason is compensatory plasticity. For a hearing person, sound provides cues about where to turn their head and gaze, particularly at the edge of the visual field, where visual acuity is naturally lower. Without these auditory cues, deaf people use their vision as an “early warning system.” Numerous experiments have shown that they outperform hearing people in detecting sudden and moving targets in the periphery, at distances of 30 to 85 degrees from the center of gaze.
The team, led by Heidi Baseler, looked for evidence of this reorganization using functional magnetic resonance imaging. The researchers mapped the visual fields of 32 volunteers — 16 people who had lost their hearing early in life (hearing loss of more than 70 decibels before the age of four) and 16 hearing participants of the same age. The participants were shown expanding rings and rotating sectors with a high-contrast checkerboard pattern covering up to 72 degrees of visual space. Previous studies had not examined beyond 37.5 degrees and therefore missed the most informative region.
The researchers found that the far periphery — beyond 39 degrees from the center — was more strongly represented in deaf participants both in the primary visual cortex, where visual information is initially processed, and in the lateral geniculate nucleus of the thalamus, a relay station carrying signals from the retina to the brain's cortex. The greater representation of the visual periphery came at the expense of the area devoted to central vision. The authors believe this redistribution underlies the behavioral advantage of deaf people in peripheral vision.
The largest peripheral representations were found in participants who had used British Sign Language since childhood, followed by those who learned it later in life, while the smallest were observed in participants who did not use sign language. The sample sizes were too small for firm statistical conclusions, but the trend suggests that visual-language experience may strengthen the effect.
The changes begin as early as the retina: in previous research, the authors found that deaf people have a thicker layer of nerve fibers that collect signals from the edges of the retina. The researchers conclude that the developing brain can adapt — and potentially optimize — the visual system to suit a person's way of life.
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