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Cross modal plasticity

Cross modal plasticity is the adaptive reorganization of neurons to integrate the function of two or more sensory systems. Cross modal plasticity is a type of neuroplasticity and often occurs after sensory deprivation due to disease or brain damage. The reorganization of the neural network is greatest following long-term sensory deprivation, such as congenital blindness or pre-lingual deafness. In these instances, cross modal plasticity can strengthen other sensory systems to compensate for the lack of vision or hearing. This strengthening is due to new connections that are formed to brain cortexes that no longer receive sensory input.

Plasticity in the blind
Even though the blind are no longer able to see, the visual cortex is still in active use, although it deals with information different from visual input. Studies found that the volume of white matter (myelinated nerve connections) was reduced in the optic tract, but not in the primary visual cortex itself. However, grey matter volume was reduced by up to 25 % in the primary visual cortex. The atrophy of grey matter, the neuron bodies, is likely due to its association with the optic tract. Because the eyes no longer receive visual information, the disuse of the connected optic tract causes a loss of grey matter volume in the primary visual cortex. White matter is thought to atrophy in the same way, although the primary visual cortex is less affected.

Through cross modal plasticity, the auditory and visual cortexes are much more interconnected in the early blind than in people who can see. This connectivity enhances the abilities of the auditory system in the blind, making them more able in auditory tasks like syllable detection. However, this increased connectivity makes the auditory system dependant on the visual cortex for detecting sound. The spatial detection of sound can be interrupted in the early blind by inducing a virtual lesion in the visual cortex using transcranial magnetic stimulation. By using the visual cortex to strengthen the auditory system, cross modal plasticity ties the two systems together. Cross modal plasticity also spreads out the processing of auditory information between these two systems. This spreading causes less activity in the auditory cortex, as it was found that the blind use their Heschl’s gyri less when performing an auditory detection task. Because the visual system is able to take on some of the processing duties of the auditory system, areas like the Heschl's gyri aren't as important in listening tasks for the deaf.

The somatosensory cortex is also able to recruit the visual cortex to assist with tactile sensation. Cross modal plasticity reworks the network structure of the brain, leading to increased connections between the somatosensory and visual cortexes. Furthermore, the somatosensory cortex acts as a hub region of nerve connections in the brain for the early blind but not for the sighted. With this cross-modal networking the early blind are able to react to tactile stimuli with greater speed and accuracy, as they have more neural pathways to work with. One element of the visual system that the somatosensory cortex is able to recruit is the dorsal-visual stream. The dorsal stream is used by the sighted to identify spatial information visually, but the early blind use it during tactile sensation of 3D objects. However, both sighted and blind participants used the dorsal stream to process spatial information, suggesting that cross modal plasticity in the blind re-routed the dorsal visual stream to work with the sense of touch rather than changing the overall function of the stream.

Experience dependence
There is evidence that the degree of cross modal plasticity between the somatosensory and visual cortexes is experience-dependent. In a study using tactile tongue devices to transmit spatial information, early blind individuals were able to show visual cortex activations after 1 week of training with the device. Although there were no cross modal connections at the start, the early blind were able to develop connections between the somatosensory and visual cortexes while sighted controls were unable to. Early or congenitally blind individuals have stronger cross modal connections the earlier they began learning Braille. An earlier start allows for stronger connections to form as early blind children have to grow up using their sense of touch to read instead of using their sight.

Plasticity in the deaf
Cross modal plasticity has also been reported in the brains of the deaf. A functional magnetic resonance imaging (fMRI) study found that the primary auditory cortex was used by deaf subjects alongside the visual cortex when they observed sign language. Although the auditory cortex no longer receives input from the ears, it is still used by the deaf to process sign language. There is no hearing component to sign language, so the auditory cortex is instead used to assist with visual and language processing. Auditory activations also appear to be attention-dependant in the deaf. Stronger activations of the auditory cortex during visual observation occur when a deaf subject pays attention to a visual cue, and the activations are weaker if the cue is not in the direct line of sight.

Cochlear implants
Another way to see cross modal plasticity in the deaf is when looking at the effects of installing cochlear implants. For those who became deaf pre-lingually, cross modal plasticity interfered with their ability to process language using a cochlear implant. For the pre-lingual deaf, the auditory cortex has been reshaped to deal with visual information, so it cannot deal as well with the new sensory input that the implant provides. However, for post-lingual deaf their experience with visual cues like lip reading can help them understand speech better along with the assistance of a cochlear implant. The post-lingual deaf do not have as much recruitment of the auditory cortex as the early deaf, so they perform better with cochlear implants. It was also found that the visual cortex was activated only when the sounds that were received had potential meaning. For instance, the visual cortex activated for words but not for vowels. This activation is further evidence that cross modal plasticity is attention dependent.