Amorphosynthesis, also called a hemi-sensory deficit, is a neuropsychological condition in which a patient experiences unilateral inattention to sensory input.Unsworth, C. A. . Cognitive and Perceptual Dysfunction. Philadelphia: Davis Company. This phenomenon is frequently associated with damage to the right cerebral hemisphere resulting in severe sensory deficits that are observed on the contralesional (left) side of the body. A right-sided deficit is less commonly observed and the effects are reported to be temporary and minor.Weintraub, S., Ahern, G. L., Daffner, K.R. & Price, B.H. . Right-sided hemispatial neglect. Neurology, 42: 223. Evidence suggests that the right cerebral hemisphere has a dominant role in attention and awareness to somatic sensations through ipsilateral and contralateral stimulation.Mesulam, M-M. . A cortical network for directed attention and unilateral neglect. Ann Neurol, 10: 309-325.Mesulam, M-M. . Large-scale neurocognitive networks and distributed processing for attention, language and memory. Ann Neurol, 28: 597-613.Heilman, K. M. & Valenstein, E. . Neglect and related disorders. New York: Oxford University Press. In contrast, the left cerebral hemisphere is activated only by contralateral stimuli.Heilman, K. M. & Van Den Abell, T. . Right hemispheric dominance for attention: the mechanism underlying hemispheric asymmetries of inattention. Neurology, 30: 327-330.Gitelman, D. R., Alpert, N., Kosslyn, S., Daffner, K., Scinto, L., Thompson, W. & Mesulam, M-M. . Functional imaging of exploratory attentional movements. Neurology, 44: 328.Reivich, M., Gur, R. & Alavi, A. . Positron emission tomographic studies of sensory stimuli, cognitive processes and anxiety. Hum Neurobiol, 2: 25-33. Thus, the left and right cerebral hemispheres exhibit redundant processing to the right-side of the body and a lesion to the left cerebral hemisphere can be compensated by the ipsiversive processes of the right cerebral hemisphere.Iachini, T., Ruggiero, G., Conson, M. & Trojano, L. . Lateralization of egocentric and allocentric spatial processing after parietal brain lesions. Brain and Cognition, 69: 512-520. For this reason, right-sided amorphosynthesis is less often observed and is generally associated with bilateral lesions.Weintraub, S., Daffner, K. R, Ahern, G. L., Price, B. H., & Mesulam, M-M. . Right-sided hemispatial neglect and bilateral cerebral lesions. J Neurol Neurosurg Psychiatry, 60: 342-344.
Anatomy Brain areas in the parietal lobes play an integral role in processing and interpreting somatic sensations from the body or environment.11. Blakemore, S. & Frith, U. . The Learning Brain. Oxford: Blackwell Publishing. The right parietal lobe is associated with sensory integration and perception whereas the left parietal lobe is believed to function at a more conceptual level involving speech, reading and writing. The central sulcus divides the frontal lobe from the parietal lobe which is located superior to the occipital lobe and posterior to the frontal lobe. The primary somatosensory cortex- the main processing center for tactile sensations- is positioned posterior to the central sulcus, on the post-central gyrus. The somatosensory system is also associated with the perception of temperature, taste, vision, proprioception and kinesthesia.Penfield, W., & Rasmussen, T. . The cerebral cortex of a man: A clinical study of localization of function. New York: Macmillan. Sensory receptors that are spread throughout the body [skin, organs, muscles, etc.] send sensory input signals to the cortex via sensory afferent neurons.Saladin, K.S. . Anatomy and Physiology. New York: McGraw-Hill. The parietal lobes then act as a main determinant for the summation of stimuli and spatial awareness.Fogassi, L. & Luppino, G. . Motor functions of the parietal lobe. Current Opinion in Neurobiology, 15: 626-631. In research by Denny-Brown and Banker,Denny-Brown, D. & Banker, B. Q . Amorphosynthesis from left parietal lesion. AMA Arch Neurol Psychiatry, 71: 302-313. a disturbance in the physiological process of perceiving somatic sensations was termed amorphosynthesis. This concept was associated with lesions of the parietal lobe leading to the ineffectual processing of sensory stimulus on the opposite side of the lesion.
Causes Amorphosynthesis is most closely related to damage of the right parietal lobe but instances of left parietal and bilateral damage have also been reported.Kim, M., Na, D.L., Kim, G.M., Adair, J.C., Lee, K.H. & Heilman, K.M. . Ipsilateral neglect: behavioural and anatomical features. J Neurol Neurosurg Psychiatry, 67: 35-38. The inattention to or suppression of somatic sensations on the contralesional side of the body can manifest in the cerebral processing centers that produce the sensory modalities for touch, taste, vision, smell, and proprioception.Association for Research in Nervous and Mental Disease . The brain and human behaviour. Ulster Med J., 27: 173-174. This phenomenon is frequently associated with other unilateral conditions such as hemispatial neglect, metamorphognosia, hemiplegia, hemisomatognosia, kinesthetic hallucination, anosognosia, balint optic ataxia, anaesthoagnosia and apraxia.Hinterbuchnes, L. . Aphasia. N Y Acad Med., 50: 589-601. The causes of cerebral brain damage to either hemisphere can include traumatic brain injury, stroke, infection, surgery or a tumor.Karnath, H. . Spatial orientation and the representation of space with parietal lobe lesions. Philos Trans R Soc Lond B Biol Sci., 352: 1411-9.
Causes of Amorphosynthesis are: Cerebrovascular accidents that have affected cerebral hemispheres, such as occlusion of the right middle cerebral artery.Adam and Victor's Principles of Neurology Diffused brain lesion and lesions on other parts of the central nervous system (CNS).Cherington, Michael, and Philip Yarnell. "Amorphosynthesis on the Chess Board." Scandinavian Journal of Rehabilitation Medecine 7, no. 4 (February 1975): 176-78. .
Types The amorphosynthesis of sensory stimuli is associated with different perceptual and conceptual effects relative to the severity of damage to the parietal lobe. The degree of sensory suppression has been explored with bilateral and ipsilateral double stimulation methods in patients with either extensive or superficial parietal lesions.Fazlullah, M. . Tactile perceptual rivalry and tactile-amorphosynthesis in the localization of cerebral lesions. Postgrad Medical Journal, 32: 338-346.Brozzoli, C., Dematte, M. L., Pavani, F., Frassinetti, F. & Farne, A. . Neglect and extinction: within and between sensory modalities. Restor Neurol Neurosci, 24: 217–232.Haan, B., Karnath, H. O. & Driver, J. . Mechanisms and anatomy of unilateral extinction after brain injury. Neuropsychologia, 50: 1045–53.Kim, M., Na, D. L., Kim, G. M., Adair, J. C., Lee, K. H. & Heilman, K. M. . Ipsilesional neglect: behavioural and anatomical features. Journal of Neurology, Neurosurgery & Psychiatry, 67: 35–38.Vaishnavi, S., Calhoun, J., Southwood, M. H. & Chatterjee, A. . Sensory and response interference by ipsilesional stimuli in tactile extinction. Cortex, 36: 81–92. Complete extinction is commonly observed in which patients with extensive right parietal damage show complete and constant inattention to tactile stimuli on the contralesional side of the body. Incomplete extinction is frequently associated with lesions that are less extensive or superficial in nature. This phenomenon is supported by studies showing that if two stimuli are simultaneously applied to both sides of the body, the patient [with their eyes closed] will ignore the stimulus that is applied to the affected side and report a tactile sensation from the unaffected side alone. If each side of the body is separately stimulated, then each stimulus is correctly reported without delay. Incomplete sensory suppression has also been observed using ipsilateral double stimulation to one side of the body. Results indicate that stimulation to a proximal and distal segment [for example, the face and hand] on one side of the body will result in a distal [hand] stimulus suppression, to which the patient will report feeling only the proximal [face] stimulation. Further evidence suggests that the parietal lobe gives rise to the processing of attention and awareness that is necessary for sensory perception. In studies of double stimulation in which the patient has their eyes open, incomplete extinction is eliminated when attention is directed to the application of stimulus on the affected side. This phenomenon is not observed in patients with complete extinction in which there is extensive damage to the parietal lobe, suggesting that the subsequent sensory suppression is not affected by expectant attention
Subtypes of amorphosynthesis, depending on the type of deficit, have been referred to as tactile amorphosynthesis, visual amorphosynthesis, and amorphosynthetic apraxia of speech or writing
Treatment Treatment of amorphosynthesis is often carried out by a variety of clinicians, neuropsychologists, physical therapists, occupational therapists, caretakers, speech-language pathologists and optometrists, depending on the severity and type of sensory suppression.Pierce S. R. & Buxbaum L. J. . Treatments of unilateral neglect: A review. Archives of Physical Medicine and Rehabilitation, 83: 256–268. Rehabilitation consists of developing an individualized treatment plan that is designed to help the person address the deficits that are affecting them. Trained professions can help to improve communication and are primarily advised to direct attention to the contralesional [affected] side of the body. Although not all deficits have seen improvements after therapy, evidence suggests that many patients are able to live independently following treatment implementationHellweg, S. & Johannes, S. . Physiotherapy after traumatic brain injury: A systematic review of the literature. Brain Injury, 22: 365–373. 27.Watson, M. . Do patients with severe traumatic brain injury benefit from physiotherapy? A review of the evidence. Physical therapy Reviews, 6: 233-249.
Signs and symptoms
S. Fazlullah, in his article Tactile Perceptual and Tactile-Amorphosynthesis in the Localization of Cerebral Lesions (1956), provides a detailed explanation of the specific signs and symptoms in amorphosynthesis caused by left and right parietal lobe lesions.
Left Parietal Lobe Lesion
Gerstmann syndrome: A patient is unable to recognize his/her finger. A patient is unable to differentiate right from left. A patient makes mistakes while writing. A patient is unable to name things. Parietal apraxia: A patient is unable to understand or execute actions. Constructional apraxia: A patient has trouble drawing.
Right Parietal Lobe Lesion Anosognosia A patient cannot perceive a defective function such as hemiplegia, or paralysis of one side of the body. Hemiasomatognosia A patient cannot focus attention on the left side of the body and believes that this side of the body feels “strange.” Metamorphognosia A patient perceives part of the body as too heavy or thick. Corporeal agnosia A patient loses sensation in the left side of the body and mistakenly believes that an extremity has been lost. Phantom Sensations A patient believes that a part of the body has doubled. Transposition of Parts of the Body A patient neglects the left side of the body, always performing actions with her right and searching for her left extremities in places other than the hospital bed, such as a locker. Constructional apraxia When asked to arrange, draw, or copy a simple model of one- to three-dimensional figures, a patient consistently neglects important details on the model's left side. For example, when asked to draw a figure of a few matchsticks, the patient would only draw the matchsticks on the figure's right side. Disorientation of space: A patient is unable recognize depth of space. Agnosia of left portion of space: