The visual system is supposed to extract distance information from the environment in order to scale the size and distance of objects in the visual scene. The purpose of this article is to challenge this account in three stages: First, I identify three shortcomings of the literature on vergence as our primary cue to near distances. Second, I present the results from two experiments that control for these shortcomings, but at the cost of eradicating vergence and accommodation as effective distance cues (average gain of y = 0.161x + 38.64). Third, I argue that if all our cues to distance are either (a) ineffective (vergence; accommodation; motion parallax), (b) merely relative (angular size; diplopia), or (c) merely cognitive (familiar size; vertical disparity), then the visual system does not appear to extract absolute distance information, and we should be open to the possibility that vision functions without scale.Keywords: vergence, accommodation, visual distance, visual scale, visual cognition
IntroductionScale has two components: size and distance. Specifically, it is concerned with differentiating a small object up close from a large object far away, even though both may have the same visual angle. Ptolemy (c.160 AD) first articulated visual scale in these terms, arguing that size wasn't just a function of visual angle (vs. Euclid, c.300 BC), but visual angle appropriately scaled by distance information (see Hatfield, 2002). But the problem is where does the distance information come from? Ptolemy could rely on 'extramission': the length of the rays emitted, and then returning, to the eye. As an 'intromission' theorist, al-Haytham (c.1021) had no such luxury and relied on familiar size instead. But Descartes (1637) regarded familiar size as a merely cognitive cue and so, along with Kepler (1604), outlined three optical / physiological cues that could plausibly replace Ptolemy's 'extramission' thesis: 1. vergence (the angles of the eyes), 2. accommodation (the curvature of the intraocular lens), and 3. motion parallax (the change in the visual scene from the motion of the observer). (which was not peer-reviewed) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity.The copyright holder for this preprint . http://dx.doi.org/10.1101/371948 doi: bioRxiv preprint first posted online 2
VergenceThe visual system's ability to extract distance information from vergence was one of visual psychophysics' earliest concerns, and was answered affirmatively by Hueck (1838) (2001) presented subjects with a small 0.57° disc in darkness for 5s, then after 5s in complete darkness asked subjects to match a visible reference to the disc's distance.They found subjects were close to veridical between 20cm and 40cm, but distances were increasingly underestimated beyond that: 60cm was judged to be 50cm, and 80cm judged to be 56cm. Nonetheless, they concluded that their results were consistent with , at least in reaching space: '…the results of our experiment indicate that vergence can be u...
