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

Neuroplasticity

Neuroplasticity is a non-specific neuroscience term referring to the ability of the brain and nervous system in all species to change structurally and functionally as a result of input from the environment. Plasticity occurs on a variety of levels, ranging from cellular changes involved in...

 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

Blindness

Blindness is the condition of lacking visual perception due to physiological or neurological factors.Various scales have been developed to describe the extent of vision loss and define blindness...

 or pre-lingual deafness. In these instances, cross modal plasticity can strengthen other sensory systems to compensate for the lack of vision

Visual perception

Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...

 or hearing

Hearing (sense)

Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...

. 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

Visual cortex

The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....

 is still in active use, although it deals with information different from visual input. Studies found that the volume of white matter

White matter

White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...

 (myelinated nerve connections) was reduced in the optic tract

Optic tract

The optic tract is a part of the visual system in the brain.It is a continuation of the optic nerve and runs from the optic chiasm to the lateral geniculate nucleus....

, 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

Atrophy

Atrophy is the partial or complete wasting away of a part of the body. Causes of atrophy include mutations , poor nourishment, poor circulation, loss of hormonal support, loss of nerve supply to the target organ, disuse or lack of exercise or disease intrinsic to the tissue itself...

 of grey matter

Grey matter

Grey matter is a major component of the central nervous system, consisting of neuronal cell bodies, neuropil , glial cells and capillaries. Grey matter contains neural cell bodies, in contrast to white matter, which does not and mostly contains myelinated axon tracts...

, 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

Auditory system

The auditory system is the sensory system for the sense of hearing.- Outer ear :The folds of cartilage surrounding the ear canal are called the pinna...

 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

Transcranial magnetic stimulation

Transcranial magnetic stimulation is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain...

. 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

Transverse temporal gyrus

The transverse temporal gyri are found in the area of primary auditory cortex in the superior temporal gyrus of the human brain, occupying Brodmann areas 41 and 42. It is the first cortical structure to process incoming auditory information...

 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

Two Streams hypothesis

The two-streams hypothesis is a widely accepted, but still controversial, account of visual processing. As visual information exits the occipital lobe, it follows two main channels, or "streams". The ventral stream travels to the temporal lobe and is involved with object identification...

. 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

Scientific control

Scientific control allows for comparisons of concepts. It is a part of the scientific method. Scientific control is often used in discussion of natural experiments. For instance, during drug testing, scientists will try to control two groups to keep them as identical and normal as possible, then...

 were unable to. Early or congenitally blind individuals have stronger cross modal connections the earlier they began learning Braille

Braille

The Braille system is a method that is widely used by blind people to read and write, and was the first digital form of writing.Braille was devised in 1825 by Louis Braille, a blind Frenchman. Each Braille character, or cell, is made up of six dot positions, arranged in a rectangle containing two...

. 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

Sign language

A sign language is a language which, instead of acoustically conveyed sound patterns, uses visually transmitted sign patterns to convey meaning—simultaneously combining hand shapes, orientation and movement of the hands, arms or body, and facial expressions to fluidly express a speaker's...

. Although the auditory cortex no longer receives input from the ears

EARS

EARS may refer to:* Electoral software* Electronic Arts, Redwood Shores campus.* Emirates Amateur Radio Society...

, 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

Prelingual deafness

A profoundly prelingually deaf individual is someone who was born with insufficient hearing to acquire speech normally, or who lost their hearing prior to the age at which speech is acquired....

, 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

Post-lingual deafness

Post-lingual deafness is a deafness which develops after the acquisition of speech and language, usually after the age of six.Post-lingual hearing impairments are far less common than prelingual deafness...

 their experience with visual cues like lip reading

Lip reading

Lip reading, also known as lipreading or speechreading, is a technique of understanding speech by visually interpreting the movements of the lips, face and tongue with information provided by the context, language, and any residual hearing....

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.