Affiliations: [a] Department of Ophthalmology, The Johns Hopkins University, School of Medicine, Baltimore, Maryland, USA | [b] Department of Biomedical Engineering, The Johns Hopkins University, School of Medicine, Baltimore, Maryland, USA | [c] Department of Physiology I, Erasmus University, Rotterdam, The Netherlands
Note: [*] Reprint address: Dr. M. Shelhamer, 355 Woods/Wilmer, The Johns Hopkins Hospital, 600 N. Wolfe Street, Baltimore MD 21205.
Abstract: Previous experiments show that altered visual feedback can change VOR gain. Such changes also presumably occur when eyeglasses are donned and doffed, or when bifocals are worn. In these cases, a nonvisual cue accompanies the required gain adjustment (frames on/off for eyeglasses, looking up/down for bifocals). We set out to show that a subject can establish two VOR gains, and to determine if one of the associated nonvisual cues alone is sufficient to subsequently determine which gain to employ. Each of three subjects sat in a rotating chair inside an OKN drum during 2 hours of sinusoidal rotation at 0.2 Hz, 30∘/s peak. For 10 minutes the chair and drum counterrotated, driving VOR gain toward 1.7, while subjects looked up 20∘. Chair and drum were then coupled for 10 minutes, driving gain toward zero, during which subjects looked down 20∘. This sequence was repeated for 2 hours. Immediately thereafter, VOR gains were measured while subjects looked alternately up and down, using 20∘ to 40∘ step rotations. A fixation target, presented before and after each step, provided accurate gain determination by measuring the size of the re-fixation saccade. Results show a consistent reduced VOR gain looking downward (average 6%) and increased gain looking upward (average 8%). We conclude that humans carn adjust their VOR gain dependent on a situational context; we speculate that this context can take many forms.
