Neural adaptation

Neural adaptation or sensory adaptation is a change over time in the responsiveness of the sensory system to a constant stimulus. It is usually experienced as a change in the stimulus. For example, if one rests one's hand on a table, one immediately feels the table's surface on one's skin. Within a few seconds, however, one ceases to feel the table's surface. The sensory neurons stimulated by the table's surface respond immediately, but then respond less and less until they may not respond at all; this is an example of neural adaptation. Neural adaptation is also thought to happen at a more central level such as the cortex.^[1]

Weight training

Studies have shown that there is neural adaptation after as little as one weight training session. Strength gains are experienced by subjects without any increased muscle size. Muscle surface recordings using electromyographic (SEMG) techniques have found that early strength gains throughout training are associated with increased amplitude in SEMG activity. These findings along with various other theories explain increases in strength without increases in muscle mass. Other theories for increases in strength relating to neural adaptation include: agonist-antagonist muscle decreased co-activation, motor unit synchronization, and motor unit increased firing rates.^[2]

Neural adaptations contribute to changes in V-waves and Hoffmann's reflex. H-reflex can be used to assess the excitability of spinal α-motoneurons, whereas V-wave measures the magnitude of motor output from α-motoneurons. Studies showed that after a 14 week resistance training regime, subjects expressed V-wave amplitude increases of ~50% and H-reflex amplitude increases of ~20%.^[3] This showed that neural adaptation accounts for changes to functional properties of the spinal cord circuitry in humans without affecting organization of the motor cortex.^[4]

Visual

Adaptation is considered to be the cause of perceptual phenomena like afterimages and the motion aftereffect. In the absence of fixational eye movements, visual perception may fade out or disappear due to neural adaptation. (See Adaptation (eye)).^[5] When an observer's visual stream adapts to a single direction of real motion, imagined motion can be perceived at various speeds. If the imagined motion is in the same direction as that experienced during adaptation, imagined speed is slowed; when imagined motion is in the opposite direction, its speed is increased; when adaptation and imagined motions are orthogonal, imagined speed is unaffected.^[6] Studies using magnetoencephalography (MEG) have demonstrated that subjects exposed to a repeated visual stimulus at brief intervals become attenuated to the stimulus in comparison to the initial stimulus. The results revealed that visual responses to the repeated compared with novel stimulus showed a significant reduction in both activation strength and peak latency but not in the duration of neural processing.^[7]

Pain

While large mechanosensory neurons such as type I/group Aß display adaptation, smaller type IV/group C nociceptive neurons do not. As a result, pain does not usually subside rapidly but persists for long periods of time; in contrast, one quickly stops receiving touch or sensory information if surroundings remain constant.

Habituation vs adaptation

The terms neural adaptation and habituation are often confused for one another. Habituation is a behavioral phenomenon while neural adaptation is a physiological phenomenon, although the two are not entirely separate. During habituation, we have some conscious control over whether we notice something to which we have become habituated. However, when it comes to neural adaptation, we have no conscious control over it. For example, if we have adapted to something (like an odor or perfume), we cannot consciously force ourselves to smell that certain odor. Neural adaptation is tied very closely to stimulus intensity; as the intensity of a light increases, your senses will adapt more strongly to it.^[8] In comparison habituation can vary depending on the stimulus. With a weak stimulus habituation can occur also immediately but with a strong stimulus the animal may not habituate at all^[9] e.g. a cool breeze versus a fire alarm. Habituation also has a set of characteristics that must be met to be termed a habituation process.^[10]

Rhythmic behaviors

Short-term adaptations

Short term neural adaptations occur in the body during rhythmic activities. One of the most common activities when these neural adaptations are constantly happening is walking.^[11] As a person walks, the body constantly gathers information about the environment and the surroundings of the feet, and slightly adjusts the muscles in use according to the terrain. For example, walking uphill requires different muscles than walking on flat pavement. When the brain recognizes that the body is walking uphill, it makes neural adaptations that send more activity to muscles required for uphill walking. The rate of neural adaptation is affected by the area of the brain and by the similarity between sizes and shapes of previous stimuli.^[12] Adaptations in the inferior temporal gyrus are very dependent on previous stimuli being of similar size, and somewhat dependent on previous stimuli being of a similar shape. Adaptations in the Prefrontal Cortex are less dependent on previous stimuli being of similar size and shape.

Long-term adaptations

Some rhythmic movements, such as respiratory movements, are essential for survival. Because these movements must be used over the course of the entire lifetime, it is important for them to function optimally. Neural adaptation has been observed in these movements in response to training or altered external conditions.^[11] Animals have been shown to have reduced breathing rates in response to better fitness levels. Since breathing rates were not conscious changes made by the animal, it is presumed that neural adaptations occur for the body to maintain a slower breathing rate.

Transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is an important technique in modern cognitive neuropsychology that is used to investigate the perceptual and behavioral effects of temporary interference of neural processing. Studies have shown that when a subject’s visual cortex is disrupted by TMS, the subject views colorless flashes of light, or phosphenes.^[13] When a subjects’ vision was subjected to the constant stimulus of a single color, neural adaptations occurred that made the subjects used to the color. Once this adaptation had occurred, TMS was used to disrupt the subjects’ visual cortex again, and the flashes of light viewed by the subject were the same color as the constant stimulus before the disruption.

Drug induced

Neural adaptation can occur for other than natural means. Antidepressant drugs, such as those that cause down regulation of β- adrenergic receptors, can cause rapid neural adaptations in the brain.^[14] By creating a quick adaptation in the regulation of these receptors, it is possible for drugs to reduce the effects of stress on those taking the medication.

Post-injury

Studies in children with early childhood brain injuries have shown that neural adaptations slowly occur after the injury.^[15] Children with early injuries to the linguistics, spatial cognition and affective development areas of the brain showed deficits in those areas as compared to those without injury. Due to neural adaptations, however, by early school-age, considerable development to those areas was observed.

Current research

Many universities are examining the effects of Neural Adaptation and how it can be used to improve current medical and rehabilitation techniques.

See also

• Acclimatization (neurons), the process by which neural adaptation is usually believed to occur
• Adaptive system
• fMRIa
• Olfactory adaptation

References