User:Jlue00/Extinction (Neurology) proposal

Introduction
Extinction is a neurological disorder that causes unilateral spatial neglect. 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 (towards the left side space following a right lesion) and a loss of exploratory search and other actions normally directed toward that side.

Physiology/Characteristics
Unilateral spatial neglect is a relatively common deficit that most frequently arises after right brain damage. The physiology of the symptom shows a lack of awareness for sensory events located in the contralesional side of space (towards the left side space following a right lesion) and a loss of exploratory search and other actions normally directed toward that side.

Grouping Effects
Considerable processing can still take place prior to the level at which extinction arises. If extinction is a pathological exaggeration of the normal difficulty of multiple concurrent targets, then it should be reduced if the two competing events are grouped together. Recent findings from right-parietal patients with left extinction confirm the prediction suggesting that grouping mechanisms may still operate despite the spatial bias of the patient and influence whether a particular stimulus will reach the patient's awareness.

Task Effects on Extinction
This is the idea that enumerating a few visual elements may exploit special `subitizing' mechanisms allowing individual elements to be processed together as a single numerable group rather than each being attended in a serial manner, as for the counting of larger sets. The change in task-set produced a reduction in extinction. Nevertheless, the general principle may be similar to that of grouping described above; extinction is reduced when the concurrent events can be attended as a single perceptual unit, becoming allies rather than competitors in the bid to attract attention.

Extinction and Multiple Targets
Extinction patients can detect a single left-sided event in isolation, missing this only when presented in combination with another event further to the right. Extinction can be observed within vision, taste, olfaction, hearing or touch, and even between two events in separate sensory modalities. This suggests that a patients’ spatial bias is most detrimental when multiple events compete for attention, which is common to daily life.

Tactile
Patients with tactile extinction are aware of being touched on a contralesional limb, but seem unaware of similar contralesional touch if touched simultaneously on their ipsilesional limb. In the tactile, extinction occurs in the domains of at the level of the hands, the face-neck, the arms-legs, both in case of symmetrical and asymmetrical stimulations, or between the two sides of a single body-part.[3] Extinguished tactile stimulus does not access consciousness but it may interfere with perception of the ipsilesional one. Considerable processing can still take place prior to the level at which loss of awareness arises. The extinction can also rise in bilateral conditions.[16] In a patient study, bilateral trials with extinction still revealed residual early components over the right hemisphere in response to the extinguished left touches. When somatosensory neural activity in the right hemisphere was reduced in amplitude when compared to the one by right hand stimulation on the left hemisphere.[17] So it can be concluded that tactile extinction is defined in conditions of bilateral stimulation and perhaps unilateral stimulation as well. Extinction arises at a high level of tactile input processing.[3]

Visual Extinction
Visual/spatial extinction, also known as pseudohemianophopia, is the inability to perceive two simultaneously stimuli in each visual field. Those who show spatial extinction can detect a single item in both the left and right visual fields but, under certain conditions of bilateral double simultaneous stimulation (DSS), fails to detect the item in one field[11]. It is thus believed that extinction is caused by sensory neglect, and that extinction reflects an attentional deficit rather than a contralesional deficit in primary perceptual processing.[1] In visual extinction this attentional deficit in perception applies mainly to attention in the relevant dimension. Visual extinction is greatest when objects either have the same color or the same shape.

Studies suggest that brain damage to the parietal lobe causes sensory neglect and that in turn causes extinction. Spatial neglect specifically leads to visual extinction. Neglect often follows right inferior parietal damage, and is characterized by impaired attention and lack of awareness for stimuli on the contralesional (left) side of space.[21] Any kind of brain damage can lead to neglect, things like stroke, brain tissue death, or tumors, and cause the unilateral damage to one side of the parietal lobe. Overall a person with parietal brain damage still has intact visual fields.

One way to reduce the effects of extinction is to use grouping of items. Brightness and edge based grouping reduces visual extinction and they act in an additive way.[11] Grouping with similar shapes also reduces the effects of extinction. This suggests that the attentional deficit in extinction can be compensated, at least in part, by the brain’s object recognition systems.

While the parietal lobe deals with sensation and perception, the amygdala controls the perception of fear and emotion. This means that by utilizing the perception abilities of the amygdala that emotional properties of contralesional stimuli can be extracted despite pathological inattention and unawareness.[21] This is because the ability of the amygdala to perceive fear is autonomous and without conscious effort and attention. Unfortunately studies have shown that perception of fear can become habituated so it can be unreliable to reduce extinction by use of the amygdala.

Auditory Extinction
Auditory extinction is the failure to hear simultaneous stimuli on the left and right sides. This extinction is also caused by brain damage on one side of the brain where awareness is lost on the contralesional side. Affected people report the presence of side specific phonemes, albeit extinguishing them at the same time. This points to the fact that auditory extinction, like other forms of extinction, is more about acknowledging a stimulus in the contralesional side than about the actual sensing of the stimulus.

Just like other forms of extinction, auditory extinction is caused by brain damage resulting in lesions to one side of the parietal lobe. Auditory extinction appears to be a rather common phenomenon in the acute state of a vascular disease.[14] The acute state of the vascular disease usually leads to neglect which then in turn leads into neglect. Neglect then leads to the auditory extinction. The number of lesions causes an additive effect when occurring in combination with a recent damage.[5]

When it comes to treating and recognizing the occurrence of auditory extinction most sound can still be perceived with the other ear. For the nature of sound, which possess directionality but still fills space, makes it more amenable to misattribution of source location.[6] This is said to be the ‘prior entry’ effect. This is when a stimuli occurring at an attended location receive privileged access to awareness is enhanced at attended relative to unattended locations.[12]

Chemical Extinction
Little is known on the side of occurrence of unilateral extinction or neglect for sensory modalities, which are traditionally thought to project to the brain in a predominantly uncrossed fashion, such as olfaction and taste (8). To date, only a limited number of investigations concerning the suppression of (or competition among) spatial information processed through the so-called chemical senses have been reported. A number of various different reasons may account for this lack of research. First, the distinction between pure chemical versus somatosensory information is often problematic. Second, it is widely assumed that olfaction and taste are senses that are not specialized for conveying spatial information (3).

Olfactory Extinction
Multiple case studies and investigations have been conducted on unilateral neglect within the visual, auditory, and tactile sensory modalities, but only three case studies have been reported on neglect within the olfactory sensory modality (4). It is still unclear whether humans can localize at all the source of the olfactory stimulation by distinguishing between odors that are processed through the right versus the left nostril. This is particularly true when the stimulus is a pure odorant rather than trigeminal, that is when the odor does not cause any somatosensory stimulation that is known to be encoded by the trigeminal system. It was discovered that when pure odorants such as hydrogen sulfide or vanillin were used as stimulants localization was random. On the other hand stimulation with carbon dioxide or menthol yielded identification rates of more than 96%. These results established the fact that directional orientation, considering single momentary odorous sensations, can only be assumed, when the olfactory stimulants simultaneously excite the trigeminal somatosensory system. Thus it is possible to distinguish between right and left side when the substances additionally or mainly excited the trigeminal nerve (9).

RBD patients with left tactile and visual neglect were reported to exhibit neglect and extinction of olfactory stimuli to the left nostril, in spite of the anatomically constrained projection of the olfactory input from that nostril to the intact left hemisphere. This ﬁnding was taken to suggest an impaired processing of all inputs from the contralesional side of space, regardless of whether such inputs were primarily directed to the damaged right hemisphere or the intact left hemisphere. Yet this interpretation is questionable because normal subjects appear unable to localize to a nostril a lateralized olfactory stimulus without the aid of an associated stimulation of the crossed trigeminal input from that same nostril. Further, and in keeping with the above notion, on a number of unilateral and bilateral olfactory stimulations those patients identiﬁed the left nostril input correctly, but misplaced it to the right nostril, possibly because of a rightward response bias related to left-sided neglect (8).

RBD patients who were affected by left tactile extinction were asked to identify and localize a series of bilaterally presented olfactory stimuli. The presence of an extinction-like phenomenon in the patients’ performance was reported. Speciﬁcally, when two different stimuli were delivered to each nostril, RBD patients consistently failed to report the stimulus delivered to the left nostril. The olfactory system predominantly projects its ﬁbers ipsilaterally thus these results are evidence supporting the representational theory of neglect. Also patients affected by olfactory extinction showed a large number of displacements in that the correctly-identiﬁed stimuli presented to the left nostril were described as being in the right nostril (3).

Nevertheless, it is not completely possible to determine the exact inﬂuence exerted by the nasal somatosensation in the olfactory extinction reported, since one of the odours considered as being pure odorants was later found to be processed probably also by the trigeminal. It appears that the human olfactory system is able to localize the source of the olfactory stimulation only when the odour elicits also a trigeminal response. This contradicts the idea that trained participants can localize both trigeminal stimuli and pure odorants between the two nostrils. Moreover recently it was shown that naive participants were able to reliably localize pure odorants between the two nostrils. Clearly, if the ability of the olfactory system to extract spatial information from non-trigeminal stimuli turns out to be true, new light could be shed on the extinction phenomena described for odours (3).

The olfactory sense also provides a unique mechanism to test the sensory and representational theories of unilateral neglect. Olfactory information projects predominantly to the ipsilateral hemisphere. Patients with a right hemisphere lesion show left sided neglect in other modalities and fail to respond to the left contralateral nostril, thus the representational theory  is supported. It was suggested that since the olfactory sensory pathways to the cerebral hemispheres were not crossed, a neglect should have occurred on the right side if a sensory loss were the cause of neglect. Neglect in olfactory sense is compared with its occurrence in the trigeminal sense, a sense stimulated in the same manner as olfaction (chemically through  the nasal passages)  but contralaterally innervated. Studies supporting the representational theory of unilateral neglect show that right hemisphere lesion patients with left unilateral neglect failed to respond to their left contralateral nostril on olfactory double simultaneous stimulation in spite of adequate olfactory  sensitivity. This demonstrated that the occurrence of unilateral neglect is not a function of sensory attenuation, in fact, olfactory sensitivity did not correlate with number of extinctions (4).

Extinction of Taste
The existence of extinction in taste is less explored than in olfaction. In the first reported case a patient with wide a parietal-occipital tumor and tactile extinction also showed extinction of taste on the left part of the tongue when two tastes were presented simultaneously on each hemi-tongue. The results showed the patient had unimodal taste extinction and displaced taste sensations under crossmodal taste-tactile stimulation which means that when the patient was touched on the right hemi-tongue and taste was applied to the left hemi-tongue, the patient reported bilateral taste stimulation suppressing the right touch and misplacing the left stimulus. However, to date there is still no clear evidence of the existence of pure taste extinction.

Multisensory
Evidence has shown that extinction in particular can emerge even when concurrent stimuli are presented in different sensory modalities. For example, tactile extinction can be modulated by visual events simultaneously presented in the space region near the tactile stimulation, reducing tactile perception. Another case would be, visual and tactile information can be integrated in other peripersonal space regions, such as around the face.