Disordered Perceptions In The Auditory System

Disordered perceptions in the auditory system Tinnitus  Initiated usually by loud noise, but maintained by persistent activity within the auditory cortex or subcortical structures. Auditory hallucinations  As in schizophrenia.  Related to selective subtle changes in brain morphology.  Deficits distributed over the left superior temporal, left temporoparietal and right prefrontal regions. Disorders perceptions in the visual system Associated visual agnosia  Unable to recognise objects through sight.  Lesions in the fusiform gyrus (FG), inferior temporal gyrus (ITG) and middle temporal gyrus (MTG). Prosopagnosia  Specific inability to recognise faces.  Face-selective neurons are located in the occipitootemporal cortex.  Post-mortem cases reveal damage to the occipitotemporal cortex, including the fusiform gyrus.  Face-selective neuron in monkey (at superior temporal lobe) responded to monkey and human faces, and to other cartoon faces; did not respond to scrambled faces or to other visual objects. Blindsight  Damage to peripheral visual cortex (V1) results in blindness.  Some subjects with damage to V1 can point to something moving across their visual field, even though there is no conscious awareness of the object.  Believed that this residual capacity is due to a parallel projection from the thalamus to the visual motion area (V5). Motion blindness  Damage to the visual motor area (V5) results in motion blindness.  Patients with V5 damage see only series of scenes, not smooth transitions (e.g. as in flowing of water). Synaesthesia  Perceptual combination of one or more senses.  Majority hear sounds as colours, or see numbers as colours. o fMRI studies have revealed activation of visual areas when listening to sounds.  One patient has been able to perceive touch when looking at other people being touched. o One possible region that mediates the conscious perception of touch on oneself during the observation of touch is the anterior insula. o fMRI studies have revealed activation of the anterior insula when subjects imagined themselves performing actions relative to someone else performing the same action, looked at pictures of their own face and identified their own memories. o Give its role in attribution to self, it is possible that the anterior insula activity, along with overactivation of the touch mirror system, accounts for why she perceives herself as direct target of the observed touch. Conclusions  Disordered sensations can be generated anywhere along the sensory pathways, from the sensory receptor to the sensory cortex.  Many cases of disordered perceptions are due to pathology (e.g. loss of sensitivity to faces, prosopagnosia), is associated with damage to the fusiform gyrus, which contains face-sensitive neurones.  Not all disordered perceptions are due to pathology: synaesthesia may be the result of functional overlap of sensory processing in adjacent sensory cortices.  Chronic phantom limb pain is believed to be maintained by reorganisation of the primary somatosensory cortex, in which the representation of normally innervated areas (e.g. face) expands into the denervated area (e.g. hand).  Mirror therapy can be used to alleviate phantom limb pain and other abberant sensations in an amputated limb: by reflecting an image (in a vertical mirror) of the intact hand to where the missing hand is placed, the brain can attribute the virtual hand to the missing hand – in the same way that a rubber hand can be “owned” by the brain.