Cross modal plasticity

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Cross modal plasticity can reorganize connections between the four main lobes as a response to sensory loss. Gray728.svg
Cross modal plasticity can reorganize connections between the four main lobes as a response to sensory loss.

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 cortices that no longer receive sensory input. [1]

Contents

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. [2] 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.

For example, blind individuals show enhanced perceptual and attentional sensitivity for identification of different auditory stimuli, including speech sounds. 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. [3]

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 cortices. [4] Furthermore, the somatosensory cortex acts as a hub region of nerve connections in the brain for the early blind but not for the sighted. [5] 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. [6] 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 cortices 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. [7] Although there were no cross modal connections at the start, the early blind were able to develop connections between the somatosensory and visual cortices while sighted controls were unable to. Early or congenitally blind individuals have stronger cross modal connections the earlier they began learning Braille. [8] 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. Perhaps due to these cross modal connections, sensory testing studies have shown that people who are born blind and read braille proficiently perceive through touch more rapidly than others. [9] Furthermore, tactile spatial acuity is enhanced in blindness [10] [11] and this enhancement is experience-dependent. [12] [13]

Plasticity in the deaf

Cross modal plasticity can also occur in pre-lingual deaf individuals. A functional magnetic resonance imaging (fMRI) study found that deaf participants use the primary auditory cortex as well as the visual cortex when they observe sign language. [14] Although the auditory cortex no longer receives input from the ears, the deaf can still use specific regions of the cortex to process visual stimuli. [15] Primary sensory abilities like brightness discrimination, visual contrast sensitivity, temporal discrimination thresholds, temporal resolution, and discrimination thresholds for motion directions do not appear to change in the loss of a modality like hearing. However, higher-level processing tasks may undergo compensating changes. In the case of auditory deprivation, some of these compensations appear to affect visual periphery processing and movement detection in peripheral vision. [16]

Deaf individuals lack auditory input, so the auditory cortex is instead used to assist with visual and language processing. Auditory activations also appear to be attention-dependent in the deaf. However, the process of visual attention in the deaf is not significantly different from that of hearing subjects. [17] Stronger activations of the auditory cortex during visual observation occur when deaf individuals pay attention to a visual cue, and the activations are weaker if the cue is not in the direct line of sight. [18] One study found that deaf participants process peripheral visual stimuli more quickly than hearing subjects. [19] Deafness appears to heighten spatial attention to the peripheral visual field, but not the central one. [20] The brain thus seems to compensate for the auditory loss within its visual system by enhancing peripheral field attention resources; however, central visual resources may suffer. [21]

Improvements tend to be limited to areas in the brain dedicated to both auditory and visual stimuli, not simply rewriting audio-dedicated areas into visual areas. The visual enhancements seem to be especially focused in areas of the brain that normally process convergence with auditory input. This is specifically seen in studies showing changes in the posterior parietal cortex of deaf individuals, which is both one of the main centers for visual attention but also an area known for integrating information from various senses. [22]

Recent research indicates that in attention-based tasks such as object tracking and enumeration, deaf subjects perform no better than hearing subjects. [23] Improvement in visual processing is still observed, even when a deaf subject is not paying attention to the direct stimulus. [24] A study published in 2011 found that congenitally deaf subjects had significantly larger neuroretinal rim areas than hearing subjects, which suggests that deaf subjects may have a greater concentration of retinal ganglion cells. [25]

Sign language

Deaf individuals often use sign language as their mode of communication. However, sign language alone does not appear to significantly change brain organization. In fact, neuroimaging and electrophysiology data studying functional changes in visual pathways, as well as animal studies of sensory deprivation, have shown that the enhancement in attention of peripheral visual processing found in deaf individuals is not found in hearing signers. [26]

The peripheral visual changes are seen in all forms of deaf individuals – signers, oral communicators, etc. [27] Comparative fMRIs of hearing speakers and hearing early signers, on the other hand, show comparable peripheral activation. The enhancement in attention of peripheral visual processing found in deaf individuals has not been found in hearing signers. It is therefore unlikely that signing causes the neurological differences in visual attention. [28]

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. [29] 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. [30] This activation is further evidence that cross modal plasticity is attention dependent.

Plasticity after olfactory deficit or whisker trimming

Cross-modal plasticity can be mutually induced between two sensory modalities. For instance, the deprivation of olfactory function upregulate whisker tactile sensation, and on the other hand, the trimming of whiskers upregulates olfactory function. In terms of cellular mechanisms, the coordinated plasticity between cortical excitatory and inhibitory neurons is associated with these upregulations of sensory behaviors. [31] [32] [33]

Related Research Articles

<span class="mw-page-title-main">Sensory nervous system</span> Part of the nervous system responsible for processing sensory information

The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons, neural pathways, and parts of the brain involved in sensory perception and interoception. Commonly recognized sensory systems are those for vision, hearing, touch, taste, smell, balance and visceral sensation. Sense organs are transducers that convert data from the outer physical world to the realm of the mind where people interpret the information, creating their perception of the world around them.

<span class="mw-page-title-main">Parietal lobe</span> Part of the brain responsible for sensory input and some language processing

The parietal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The parietal lobe is positioned above the temporal lobe and behind the frontal lobe and central sulcus.

<span class="mw-page-title-main">Claustrum</span> Structure in the brain

The claustrum is a thin sheet of neurons and supporting glial cells, that connects to the cerebral cortex and subcortical regions including the amygdala, hippocampus and thalamus of the brain. It is located between the insular cortex laterally and the putamen medially, encased by the extreme and external capsules respectively. Blood to the claustrum is supplied by the middle cerebral artery. It is considered to be the most densely connected structure in the brain, and thus hypothesized to allow for the integration of various cortical inputs such as vision, sound and touch, into one experience. Other hypotheses suggest that the claustrum plays a role in salience processing, to direct attention towards the most behaviorally relevant stimuli amongst the background noise. The claustrum is difficult to study given the limited number of individuals with claustral lesions and the poor resolution of neuroimaging.

Stimulus modality, also called sensory modality, is one aspect of a stimulus or what is perceived after a stimulus. For example, the temperature modality is registered after heat or cold stimulate a receptor. Some sensory modalities include: light, sound, temperature, taste, pressure, and smell. The type and location of the sensory receptor activated by the stimulus plays the primary role in coding the sensation. All sensory modalities work together to heighten stimuli sensation when necessary.

Multisensory integration, also known as multimodal integration, is the study of how information from the different sensory modalities may be integrated by the nervous system. A coherent representation of objects combining modalities enables animals to have meaningful perceptual experiences. Indeed, multisensory integration is central to adaptive behavior because it allows animals to perceive a world of coherent perceptual entities. Multisensory integration also deals with how different sensory modalities interact with one another and alter each other's processing.

Sensory substitution is a change of the characteristics of one sensory modality into stimuli of another sensory modality.

In developmental psychology and developmental biology, a critical period is a maturational stage in the lifespan of an organism during which the nervous system is especially sensitive to certain environmental stimuli. If, for some reason, the organism does not receive the appropriate stimulus during this "critical period" to learn a given skill or trait, it may be difficult, ultimately less successful, or even impossible, to develop certain associated functions later in life. Functions that are indispensable to an organism's survival, such as vision, are particularly likely to develop during critical periods. "Critical period" also relates to the ability to acquire one's first language. Researchers found that people who passed the "critical period" would not acquire their first language fluently.

Neuroplasticity, also known as neural plasticity, or brain plasticity, is the ability of neural networks in the brain to change through growth and reorganization. It is when the brain is rewired to function in some way that differs from how it previously functioned. These changes range from individual neuron pathways making new connections, to systematic adjustments like cortical remapping or neural oscillation. Other forms of neuroplasticity include homologous area adaptation, cross modal reassignment, map expansion, and compensatory masquerade. Examples of neuroplasticity include circuit and network changes that result from learning a new ability, information acquisition, environmental influences, practice, and psychological stress.

<span class="mw-page-title-main">Thalamocortical radiations</span> Neural pathways between the thalamus and cerebral cortex

In neuroanatomy, thalamocortical radiations also known as thalamocortical fibres, are the efferent fibres that project from the thalamus to distinct areas of the cerebral cortex. They form fibre bundles that emerge from the lateral surface of the thalamus.

Sensory processing is the process that organizes and distinguishes sensation from one's own body and the environment, thus making it possible to use the body effectively within the environment. Specifically, it deals with how the brain processes multiple sensory modality inputs, such as proprioception, vision, auditory system, tactile, olfactory, vestibular system, interoception, and taste into usable functional outputs.

Body schema is an organism's internal model of its own body, including the position of its limbs. The neurologist Sir Henry Head originally defined it as a postural model of the body that actively organizes and modifies 'the impressions produced by incoming sensory impulses in such a way that the final sensation of body position, or of locality, rises into consciousness charged with a relation to something that has happened before'. As a postural model that keeps track of limb position, it plays an important role in control of action.

Tactile discrimination is the ability to differentiate information through the sense of touch. The somatosensory system is the nervous system pathway that is responsible for this essential survival ability used in adaptation. There are various types of tactile discrimination. One of the most well known and most researched is two-point discrimination, the ability to differentiate between two different tactile stimuli which are relatively close together. Other types of discrimination like graphesthesia and spatial discrimination also exist but are not as extensively researched. Tactile discrimination is something that can be stronger or weaker in different people and two major conditions, chronic pain and blindness, can affect it greatly. Blindness increases tactile discrimination abilities which is extremely helpful for tasks like reading braille. In contrast, chronic pain conditions, like arthritis, decrease a person's tactile discrimination. One other major application of tactile discrimination is in new prosthetics and robotics which attempt to mimic the abilities of the human hand. In this case tactile sensors function similarly to mechanoreceptors in a human hand to differentiate tactile stimuli.

<span class="mw-page-title-main">Somatosensory system</span> Nerve system for sensing touch, temperature, body position, and pain

In physiology, the somatosensory system is the network of neural structures in the brain and body that produce the perception of touch, as well as temperature (thermoception), body position (proprioception), and pain. It is a subset of the sensory nervous system, which also represents visual, auditory, olfactory, gustatory and vestibular stimuli.

Extinction is a neurological disorder that impairs the ability to perceive multiple stimuli of the same type simultaneously. Extinction is usually caused by damage resulting in lesions on one side of the brain. Those who are affected by extinction have a lack of awareness in the contralesional side of space and a loss of exploratory search and other actions normally directed toward that side.

Sensory maps are areas of the brain which respond to sensory stimulation, and are spatially organized according to some feature of the sensory stimulation. In some cases the sensory map is simply a topographic representation of a sensory surface such as the skin, cochlea, or retina. In other cases it represents other stimulus properties resulting from neuronal computation and is generally ordered in a manner that reflects the periphery. An example is the somatosensory map which is a projection of the skin's surface in the brain that arranges the processing of tactile sensation. This type of somatotopic map is the most common, possibly because it allows for physically neighboring areas of the brain to react to physically similar stimuli in the periphery or because it allows for greater motor control.

Many types of sense loss occur due to a dysfunctional sensation process, whether it be ineffective receptors, nerve damage, or cerebral impairment. Unlike agnosia, these impairments are due to damages prior to the perception process.

Haptic memory is the form of sensory memory specific to touch stimuli. Haptic memory is used regularly when assessing the necessary forces for gripping and interacting with familiar objects. It may also influence one's interactions with novel objects of an apparently similar size and density. Similar to visual iconic memory, traces of haptically acquired information are short lived and prone to decay after approximately two seconds. Haptic memory is best for stimuli applied to areas of the skin that are more sensitive to touch. Haptics involves at least two subsystems; cutaneous, or everything skin related, and kinesthetic, or joint angle and the relative location of body. Haptics generally involves active, manual examination and is quite capable of processing physical traits of objects and surfaces.

Crossmodal attention refers to the distribution of attention to different senses. Attention is the cognitive process of selectively emphasizing and ignoring sensory stimuli. According to the crossmodal attention perspective, attention often occurs simultaneously through multiple sensory modalities. These modalities process information from the different sensory fields, such as: visual, auditory, spatial, and tactile. While each of these is designed to process a specific type of sensory information, there is considerable overlap between them which has led researchers to question whether attention is modality-specific or the result of shared "cross-modal" resources. Cross-modal attention is considered to be the overlap between modalities that can both enhance and limit attentional processing. The most common example given of crossmodal attention is the Cocktail Party Effect, which is when a person is able to focus and attend to one important stimulus instead of other less important stimuli. This phenomenon allows deeper levels of processing to occur for one stimulus while others are then ignored.

<span class="mw-page-title-main">Daphne Bavelier</span> French professor and cognitive neuroscientist

Daphné Bavelier is a French cognitive neuroscientist specialized in brain plasticity and learning. She is full Professor at the University of Geneva in the Faculty of Psychology and Educational Sciences. She heads the Brain and Learning lab at Campus Biotech in Geneva, Switzerland.

Hey-Kyoung Lee is a neuroscience professor at Johns Hopkins University. She studies cross-modal plasticity between visual and auditory systems.

References

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