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NeuroLogic - Centering on Upright Perception: Inner Ear Damage
Centering on Upright Perception: Inner Ear Damage
Date: June 13, 2014
What influences dizziness?
Dizziness so disabling that sufferers can’t tell up from down can be a symptom of damage to the inner ear or other senses. But in many cases, the problem instead appears to stem from a disruption of the processes in the brain that translate input coming from the inner ears about the pull of gravity and the eyes about our visual sensations into what is known as upright perception.
The human brain has an automatic capacity to know which way is up even when our bodies are askew, says Johns Hopkins neurologist Amir Kheradmand. But exactly where the processing network for upright perception is located within the brain was unknown—giving researchers no area to target for treatment.
In a new study, Kheradmand and Johns Hopkins colleagues David Zee and Adrian Lasker pinpointed a likely location for this processing site: an area in the human brain that seems to play an important role in the subconscious recognition of which way is up and which is down.
Recruiting eight healthy subjects for the study, the Johns Hopkins team placed each person in a dark room and showed them lines illuminated on a screen. The researchers instructed the subjects to report the orientation of the lines by rotating a dial to the right, left or straight.
The subjects then received transcranial magnetic stimulation (TMS) to an area in the right parietal cortex. Studies in stroke victims with balance problems suggested that damage to that part of the brain was involved in upright perception. For this part of the experiments, each subject had an electromagnetic coil placed against the scalp in a 2-centimeter wide location across the right parietal lobe, behind the ear. This spot was found by mapping a small cortical region of the parietal lobe in one subject.
At the identified location, the subjects received 600 electromagnetic pulses over the course of 40 seconds. After each session, the subjects were again asked to show researchers which way each illuminated line on the screen was oriented. The effects of TMS, which painlessly and noninvasively delivers electromagnetic currents to precise locations in the brain that can temporarily disrupt the function of the targeted area, wore off quickly and the subjects could again be tested on another day.
The researchers found that each subject reported that his or her sense of being upright was skewed in the same way after TMS in the same spot in the parietal cortex: the supramarginal gyrus.
Kheradmand says the study’s results raise the possibility that TMS could potentially be used to treat chronic dizziness. “If we can disrupt upright perception in healthy people using TMS, it might also be possible to fix dysfunction in the same location in people with dizziness and spatial disorientation,” he says. “We’re excited that this could someday be a key to helping people who have dizziness and spatial disorientation to feel better.”