Biased Collision Judgments by People with Left but not Right Hemianopia

Abstract

Purpose: To characterize collision judgment behaviors in patients with homonymous hemianopia (HH). Specifically, to evaluate whether there were asymmetries in perceived safe passing distance thresholds for obstacles on the blind and seeing sides; in prior studies normal-sighted had no difference between their right and left side judgments on this task.

Methods: 27 HHs (11 RHH, 13 LHH, & 3 LHH with Left Hemispatial Neglect (LHSN)) seated facing a projector screen (94 × 79°) performed collision judgments for a life-sized human figure appearing at various distances from a simulated walking path. Scanning was allowed, limiting blindside detection failures to 9% (of which 83% were for figures appearing > 60cm from the walking path). Data were fitted with a cumulative Gaussian to calculate a collision threshold (50%) for right and left sides.

Results: LHH showed significant (p=0.005) asymmetry in judgments such that the left threshold was much smaller than the right (median left 37cm, right 50cm). Thus LHHs reported the figure as NOT posing a collision hazard when it was much closer to the path of simulated movement on the left. The 3 with LHSN had even greater asymmetry (left 27cm, right 49cm). Patients with RHH showed no difference (p=0.8) (median right 41cm, left 46cm). Line bisection error for RHHs & LHHs was near 0cm whereas LHSNs bisected 0.7cm to the right.

Conclusions: Asymmetry was measured in our sample of LHSNs, but also unexpectedly in LHHs without clinically measurable LHSN. Possible explanations are 1) blindside detection failures (but these were low and similar for RHH and LHH), 2) cognitive de-emphasis of left-side hazards, or 3) a shift in egocentric perception of straight-ahead. As the width of the collision envelope (right and left thresholds combined) was identical for LHH & RHH, a uniform shift of LHHs egocentric perception to the right seems the most plausible explanation. Results need to be verified in a larger population along with lesion analysis, and compared against specific measurements of egocentric localization (ie. visual open loop pointing) and against behaviors during actual mobility.

Keywords: 584 low vision • 611 neuro-ophthalmology: cortical function/rehabilitation • 759 visual impairment: neuro-ophthalmological disease