Effects of eccentric viewing in orientation discrimination

Vestibular deficits are particularly evident in individuals with central visual field loss (CFL): two thirds of those with CFL complain of dizziness and instability, and have an increased rate of falls, injury, oscillopsia, and a fear of falling. Correct assessment of spatial orientation and movement is essential for fall-free walking and standing. Orientation information is normally provided through a weighted combination of visual, vestibular, proprioceptive and somatosensory signals. In CFL the problem is compounded by a largely deteriorated visual signal and a potentially mis-calibrated vestibular system. To assess these changes, we first set out to study the effects of eccentric viewing alone on orientation judgements in 8 healthy controls (6F) across the lifespan (22-74yo). Participants were seated in complete darkness with the head upright. A bar (length=10.5º vis. ang.) was flashed on a monitor either in the center or 7.5º to the right or left of fixation for 41 ms. The width of the bar was scaled with eccentricity (M-scale= 0.333), but the length of the bar was kept the same. Observers indicated if it was rotated clockwise or counterclockwise of vertical in a 2AFC design. Participants performed 504 trials (7 orientations, 2 directions, 3 positions, 12 repetitions), after a short set of practice trials. The bias (Fig1A) and the discrimination sensitivity (Fig1B) were estimated (probit fit) at each eccentricity, for each participant. There was no significant difference in bias or sensitivity for bars presented at the three eccentricities across our observers (p > 0.2, 1-way rm ANOVA, Tukey correction). Our pilot data suggest orientation discrimination is preserved when viewing peripherally, up to 7.5º visual angle. For large, high visibility stimuli any changes in subjective visual vertical observed in individuals with CFL are unlikely to be due to differences in viewing eccentricity due to the loss of the fovea. Changes in judgement bias are likely to be representative of a less reliable reference frame re gravity due to degradation of available vestibular information.