Functional near-infrared spectroscopy during optic flow with and without fixation

Abstract

Background and purpose: Individuals with visual vertigo describe symptoms of dizziness, disorientation, and/or impaired balance in environments with conflicting visual and vestibular information or complex visual stimuli. Physical therapists often prescribe habituation exercises using optic flow to treat these symptoms, but there are no evidence-based guidelines for delivering optic flow and it is unclear how the brain processes such stimuli. The purposes of this study were to use functional near-infrared spectroscopy (fNIRS) to explore cerebral activation during optic flow, and determine if visual fixation had a modulating effect on brain activity.

Methods: Fifteen healthy participants (7 males and 8 females; mean age 41 years old) stood in a virtual reality environment and viewed optic flow moving unidirectionally in the yaw plane with and without fixation. Changes in cerebral activation were recorded from the bilateral fronto-temporo-parietal and occipital lobes using fNIRS.

Results: Cerebral activation was greater with visual motion than while viewing a stationary scene. Greater cerebral activation in the bilateral fronto-temporo-parietal lobes was observed when optic flow was viewed with fixation.

Discussion and conclusions: Optic flow activates the bilateral fronto-temporo-parietal regions of the cerebral cortex. This activation is greater while viewing optic flow and a fixation target, providing preliminary evidence supporting the use of a fixation target during habituation exercises.

Fig 1. Virtual reality environment.

The participant was exposed to optic flow in a three-screen wide field-of-view virtual reality environment. Participants faced the front screen that was 1.5 meters away, and were tethered to the ceiling to prevent falls.

Fig 2. fNIRS head cap.

The participant wore a fNIRS head cap that consisted of 11 sources (capital letters A-K) and 20 detectors (numbers 1–20) on the scalp, distributed between the left fronto-temporo-parietal (left), occipital (center), and right fronto-temporo-parietal (right) regions.

Fig 3. Estimated spatial maps.

Estimated spatial maps (T-test) of the oxyhemoglobin data collected using fNIRS for the change in cerebral activation between fixation and no fixation conditions using all source-detector combinations. The color bar represents the results of the T-statistic (T-score). Areas in red indicate greater cerebral activation (increased oxyhemoglobin) and areas in blue indicate lesser cerebral activation (decreased oxyhemoglobin) during the comparison. Thick lines indicate areas with significant activation (p < 0.05).