Cognition, the process of abstract thinking, requires acquisition of accurate data about persons and objects in the environment through perception and the capacity to encode and retrieve memories of prior perceptions and thoughts. Perceptual recognition and identification of persons and objects rests upon the 5 senses (sight, hearing, smell, taste and touch). Vision, hearing, smell and taste arise from brain processing of information fed by the cranial nerves of the head which are wired directly to the brain. Touch is mediated by the peripheral nerves and spinal cord. A TBI can disrupt one, some or all of the 5 senses.
Anosmia, the inability to smell, frequently accompanies frontal lobe injury, because the olfactory nerve bulbs which transmit smell data from the nose to the brain run directly under the medial frontal lobes. When the frontal lobes are subjected to traumatic forces which jerk them back and forth across a bony segment of skull called the cribiform plate, the olfactory bulbs get crushed or shredded. Anosmia produces inability to taste food, since taste is not merely a function of taste buds on the tongue but is largely a function of smells associated with foods. Studies of the brain show our sense of smell is structurally and functionally integrated with emotion and memory.
Smells can trigger profound feelings and rekindle old memories in an instant, whether they be the smells of sex, of a morning walk in a pine forest or the smells of an old leather baseball glove. This is because the olfactory bulbs are wired to the piriform lobe of the cortex in the antero-medial temporal lobe, where axonal projections synapse with the amygdala (the part of the brain which appraises the emotional tone of situations) and entorhinal portion of the hippocampus (which plays the major role in preparation of episodic memories for long term storage, and retrieval of such memories from long term storage).
The piriform lobe does not replicate individual smells in a point by point grid (as the visual cortex does for images), but “associates” with other parts of the brain to create a gestalt perception. J. Neuroscience 20(18): 6974-6982. Hence, destruction of the olfactory bulbs (with loss of input to the piriform) robs the victim, not just of the ability to identify select smells, but to remember and enjoy past experiences hooked up in time and space with those smells.
Blurry vision often follows trauma to the occipital lobes where the primary visual cortex fuses the separate visual streams transmitted through the optic nerves from the retinas of the left and right eyes. Problems with touch (such as numbness or hypersensitivity) accompany damage to the somato-sensory strip in the parietal lobes.
Astereognosia, the inability to explore and discern the tactile properties of objects through exploration with the thumb and forefinger, follows damage to the superior parietal lobe. Problems with hearing accompany damage to the temporal lobe (where sounds are processed), to the auditory nerve which feeds sound data to the temporal lobe or to the hair cells in the inner ear. Balance is a systematic integration of sight, hearing and posture. Damage to any of these can produce dizziness or dysequilibrium.
Perceptual disorders are very consequential. They tend to cut the person off, to varying extents, from other people and the world around them. This partial black out of sensory information is isolating and leads to uncertainty and anxiety. Fortunately the perceptual distortions which accompany a TBI often improve with time as the brain or cranial nerve heals and as the person learns to compensate for the distortions. However, some perceptual deficits are permanent.
Following a TBI, it is important to identify which of your 5 senses is “off,” in what ways and by how much. Next, you should visit the appropriate specialist for testing. The starting place is the neurologist, ENT doctor or physiatrist. They in turn will refer patients for audiograms (hearing tests), neuro-ophthalmologic or neuro-optometric testing, smell testing and the like.
What can be done to increase function depends on the site and extent of the damage. For example, where head trauma has shaken and damaged hair cells in the inner ear, a cochlear implant can help by sending sound data directly to the auditory nerve bypassing the hair cells. Where cranio-facial trauma damages the retinas, implanted electrodes can stimulate the visual cortex at the back of the brain. However, if the deafness or blindness is “cortical,” i.e. a result of damage to the areas of the brain which process and integrate sound or visual data, the deficits are harder to overcome, and require the patient to engage in systematic exercises to stimulate regrowth of cells.
The leading agency of the federal government for research into perceptual disorders is the National Institute on Deafness and Other Communication Disorders or NIDCD, which has a website at http://www.nidcd.nih.gov.
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