Abstract
Abstract: :
Purpose: To compare the validity and reliability of ball–tossing, a nonverbal index of perceived distance, to verbal reports of distance; to study the influence of vergence corollary discharge on these two indices of distance; and to study the influence of response speed on these indices. Methods: Nineteen normally binocular observers made 128 verbal and 128 ball–toss estimates of distance for targets shown at sixteen randomized distances between 30cm and 3.3M. Vergence corollary discharge – the sense of nearness derived from convergence – was induced by wearing "glasses" containing 9 prism diopters base–out prism and –1.50D spherical lenses in combination with the observer's refractive correction. For "fast" responses subjects completed their response within 1.5 seconds of target onset. For "slow" responses subjects were allowed 5 seconds to complete their response. The distance estimates were block randomized into four viewing conditions based on glasses and speed: (1) fast/no glasses, (2) slow/no glasses, (3) fast/glasses, (4) slow/glasses. Informed consent was obtained prior to experimentation. Results: The slopes of the regressions of response distance on target distance quantified responses. The overall slopes of verbal and ball–toss responses were similar at 0.91±0.0096 and 0.83±0.0097 respectively. The effect of the glasses was assessed by paired comparisons of glasses slope with no–glasses slope. For verbal reports the glasses did not alter slope for either slow responses (difference= –.013, t=–0.48, p=0.54) or fast responses (difference=.002, t=0.086, p=0.933). For ball tossing the glasses significantly reduced slope for fast responses (difference=–.104, t=–5.40, p<0.0001) but did not alter slope for slow responses (difference= –.049, t=–1.27, p=0.22). Conclusions: Ball–tossing and verbal responses performed similarly under normal viewing but only ball–tossing was influenced by the glasses, and then only when responses were made quickly. Ball–tossing is a valid index of perceived distance, is as reliable as verbal estimates, and can reveal corollary discharge effects on distance perception if it is done quickly. The time–dependent effect of vergence corollary discharge on distance perception suggests that vergence corollary discharge is an element of the "vision for action" component of space perception.
Keywords: space and scene perception • vergence • vision and action