cross-modal plasticity

cross-modal plasticity, the ability of the brain to reorganize and make functional changes to compensate for a sensory deficit. Cross-modal plasticity is an adaptive phenomenon, in which portions of a damaged sensory region of the brain are taken over by unaffected regions.

Well-established examples of cross-modal plasticity include sensory adaptations in persons affected by hearing or vision loss. Hearing loss often leads to heightened peripheral vision in the deaf, and the blind experience increased sensitivity to sound and touch. In deaf persons, auditory areas are at work during the processing of visual and somatosensory data, while in blind persons, the visual areas of the brain are active during the processing of somatosensory information, which relates to touch. The extent of the reorganization has an impact on the outcome of treatments, such as retinal or cochlear implants, which are ineffective if the visual or auditory cortex has been commandeered by other senses.

Characteristics

The effects of cross-model plasticity vary from person to person. The types of modifications depend on age, sensory experience, and the specific brain systems involved. For instance, chemosensory loss, which is the loss of the ability to detect the odour of chemicals, can lead to a decrease in sensitivity in other senses. Other sensory systems, including those used in language acquisition, form during distinct developmental periods. Therefore, the timing of the sensory deprivation is critical to the ability of the damaged region to reorganize or restore function and has profound implications for the education of deaf and blind children and the rehabilitation of patients with brain injuries.

Experience also affects the transformation of the brain. A blind person who reads Braille often will have an acute sense of touch, and a deaf person who communicates through sign language often will have sharp eyesight. In each case, the areas of the brain that process those functions likely have expanded into damaged regions.

Historical context

The sensory experience was once thought of as a process performed by a specific part of the brain hardwired to carry out that function. In the latter part of the 20th century, however, that perspective began to change, with the sensory experience increasingly coming to be viewed as an integration of input from multiple brain regions with numerous neural connections. Much of what is known about cross-modal plasticity stems from the pioneering research of American neuroscientist Paul Bach-y-Rita, whose groundbreaking experiments with blind patients in the 1960s advanced the idea of neuroplasticity. Bach-y-Rita’s father, Pedro, had made a remarkable recovery from a stroke owing to a strict rehabilitation program designed by Bach-y-Rita’s brother. Pedro, who had been left partially paralyzed after the stroke, eventually regained the ability to walk and talk. Bach-y-Rita was convinced that the plasticity of the human brain, in enabling it to reorganize after injury, was instrumental to his father’s recovery.

Bach-y-Rita’s first experiment involving sensory substitution was conducted by using a tactile vision substitution system (TVSS) that partially restored sight in congenitally blind persons. The system consisted of a chair embedded with 400 vibrating plates connected to a hand-cranked camera. With minimal practice, participants learned to recognize faces and objects, such as a telephone and a vase, through patterns of vibrations on their backs. The experiment demonstrated that somatosensory signals could be rerouted to the visual cortex, allowing the brain to substitute sight with touch. Subsequent technological advances led to increasingly sophisticated sensory substitution apparatuses; an example was a mouthpiece filled with electrodes that enabled deaf persons to “hear” with their tongue.