Figure Caption: Sensory areas of the cortex. Represented here are somatosensory perception (touch, hot/cold, position), vision and hearing. The dark blue sections represents brain areas that mediate the association of these perceptions.

Syndromes of Perceptual Impairment

A number of distinct neuropsychology syndromes derive from defects in visual, somatosensory and auditory perception. They represent fundamental defects in perception rather than complex defects of perceptual elaboration, memory, reasoning or motor function. They usually involve only the posterior parts of the hemispheres and the presence of these syndromes has strong localizing significance.

Visual Field Cut

Lesion of the primary visual perception area of the right or left occipital lobe will produce a clear loss of visual perception from the contralateral visual field. Patients are usually aware of the deficit and do not neglect that side of the visual field. They behave as if the entire visual fields exist and they are blind for one side. This is in contrast to patient who can see objects in the affected field but neglects them. The neglect disorder is attributed to parietal lobe lesions. Since many brain illness may injure both areas, visual field defects may also exist in conjunction with visual neglect.

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Cortical Blindness

If both occipital lobes are injured then the patient is in a state of cortical blindness. The patient is unable to process visual information and behaves in a similar fashion to someone who suffers a peripheral blindness. However, some patients deny their blindness and attempt to behave as if they have vision. This state of denial of cortical blindness is called Anton's Syndrome. Many patients with Anton's syndrome have associated parietal lobe lesions and sensory neglect. They may deny sensory deficits in other modalities in addition to vision. Some patients have a general dementia, and others are recovering from coma and delirium when they manifest denial of cortical blindness.

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Central Achromatopsia

This refers to a loss of color perception as a result of lesions of the optic nerve or occipital lobe. The disorder can affect both visual fields or a single hemi-field. Patients are usually aware of the deficit and report the world as gray, or "dirty". Patients will correctly answer questions pertaining to color concepts or descriptions of objects that include color (e.g., "what is the color of blood?). Central achromatopsia may also affect one color more than another. Lesions of the fusiform gyrus of the medial occipital lobe is associated with this syndrome.

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Hemianesthesia

Lesion of primary somatosensory areas will produce a loss of tactile, proprioceptive and other sensations arising from the skin surface on the side contralateral to the lesion. Patients are usually aware of the loss of sensation and do not neglect the affected side. If the patient neglects the contralateral side and behaves as if the side does not exist then the lesion probably includes association areas in the parietal lobe that mediate complete body schema. Often hemianesthesia co-exists with hemi-inattention.

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Hemineglect

Patients with hemineglect syndrome can accurately perceive sensory information but behave as if it does not exist. Since a lesion usually affects only one hemisphere most neglect syndromes only involve sensory information on one side (i.e.., hemi). Their primary sensory systems (e.g., vision ) are intact, but they do not attend to or behave consistent with perception. They behave as if they only perceive information from the unaffected side. Patients with severe forms of hemi-neglect may even refuse to accept that the affected limbs even belong to them. They may even complain that someone else's leg is in bed with them.

Patients with hemineglect who are touched on the affected side may report being touched on the intact side. This error is called allesthesia. Allesthesia may be present in any sensory modality.

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Motor Impersistence

Patients with lesion of the right hemisphere may have difficulty sustaining motor actions or gestures. For example, they may be unable to keep their eyes closed for a sustained period. Although this is primarily a defect in motor performance, it may result from a deficit in sensory perception of the action sequence.

Hemispatial Neglect (Unilateral Spatial Neglect)

Patients with this type of neglect have difficulty orienting their bodies in space as well as solving problems that have a spatial or visual component. In general, the entire spatial array is compressed into the spatial portion represented by the intact side of sensory perception. For example, when asked to draw a symmetrical object, the side contralateral to the brain lesion is left blank. When asked to draw a clock, patients with hemispatial neglect will compress the lateral dimension of the circular clock face and draw all twelve numbers the one side ipsilateral to the brain lesion.

Patients with this form of neglect also fail to orient to objects in the space contralateral to the lesion. For example, they may only wash or dress one side of the body, ad may eat only the food on one side of the plate. They may also read only words on the intact side of space. Writing production will also only cover the intact side of space. Inability to dress because of neglect is called "Dressing Apraxia".

Hemi-spatial neglect is more frequent following right parietal lobe lesions. It has been theorized that the right hemisphere maintains the complete body schema and visual spatial array for both sides of space. However, the left hemisphere only maintains the right side of space. When the right hemisphere is injured, there is a severe unilateral neglect of the left side of space. However, left hemisphere lesions result in far less neglect of the right side of space because the right hemisphere is able to mediate orientation and cognition involving both sides.

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Balint Syndrome

This syndrome results from the injury of both parietal lobes. It consists of three cardinal symptoms: 1) paralysis of gaze, in which the patient cannot look into the peripheral field; 2) optic ataxia, in which the patient cannot use visual information to accurately coordinate actions; 3) central fixation of dynamic visual attention, in which the patient has difficulty attending to the peripheral fields when actively attending to the environment. Performance may be worse in one hemifield, usually the right.

Patients with Balint's syndrome appear to neglect the peripheral parts of the visual fields and have great difficulty integrating the information from all parts of the visual fields into a whole perception. For example, when shown a picture of common scene, such as a sporting event, they may only describe individual details, such as the clothing worn by one person. Although they might describe numerous such details, they may never realize that the picture depicts a sporting event. This ability to describe and recognize details but inability to recognize the whole visual array is called simultanagnosia.

The optic ataxia associated with Balint's syndrome is manifested as difficulty estimating distances in visual space and coordinating actions consistent with the proper spatial arrangement. For example, if patients are asked to touch an object with the finger, they often point and miss. When asked to pour water from a pitcher to a glass, they invariably miss the glass.

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Sensory Extinction

Some patients with a lateralized lesion may respond to a stimulus presented only to the contralateral side. However, when two stimuli are presented, one to each side, the patient will only report perceiving the stimulus presented on the intact ipsilateral side. These perceptual errors made to simultaneous presentations are called sensory extinctions. These extinctions may occur in the visual, auditory and tactile modalities, singly or in combination. Extinctions are a manifestation of hemi-neglect and many patients with sensory extinctions have recently recovered from complete hemi-neglect.

Hemiakinesia

Patients with hemi-neglect may not spontaneously use the limbs contralateral to the site of brain lesion. However, when requested to use the limb or when attention is focused on the limb, the patient may actually show good muscle strength and ability to coordinate the limb. Such patients will also have difficulty moving the head and eyes to look toward the side of space they are neglecting. These disordered motor actions are generally called hemi-akinesia.

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Assessment

Examination of sensory disorders begins with a systematic testing of each sensory system. This is accomplished by presenting unilateral and bilateral stimuli to each modality. For example, visual perception is examined in the clinic by first holding both hands in front of the patient, one in each of the left and right visual fields. The examiner then moves a fingers of one hand and then the other hand, in random fashion, and asks the patient to indicate which side has a moving finger. This represents unilateral stimulation. Then the examiner moves the fingers of both hands and asks the patient to report whether a finger on the left or right, or both fingers are moving. This is bilateral stimulation. If the patient cannot see that a finger on the left or right is moving under a condition of unilateral stimulation then the patient has a visual field cut or neglect. If the patient is able to correctly respond in the condition of unilateral stimulation but says that one finger is moving when both fingers are moving (bilateralstimulation), then the patient is neglecting the side that was not reported. Auditory and Tactile perception and neglect are examined in the same fashion. For example, the patient hears auditory tones presented in one ear and then the other (unilateral), as well as tones presented simultaneously to both ears.

Simultanagnosia is assessed by having the patient count a number of elements, such as dots on a page. Notice whether the patient tends to focus on only the central part of the array of objects. Patients are also asked to describe a common scene. If the patient only describes specific details and does not explore the entire visual area then simultagnosia may be present.

Visual neglect is also examined by having the patient complete tasks in which the must cross out all of the letters printed randomly on a sheet of paper. If neglect is present, the patient will cross out only the letters on the intact side. Patients are also requested to bisect a horizontal line. Patients with nelect will place the bisection closer to their intact visual field and not in the middle of the line.