![]() ![]() The sudden presentation of near stimuli necessarily introduces three confounding cues:ĭiplopia: When the stimuli are initially presented, they are seen as double. Subjects are typically sat in complete darkness with their vergence in a resting state, and then stimuli are suddenly presented at distances as close as 20cm. My concern with these two papers, and indeed with the literature as a whole, is the way the stimuli are presented. The purpose of this paper is to challenge this conclusion. This is particularly effective in the near visual space corresponding to arm’s length.’ Nonetheless, they concluded that their results were consistent with Mon-Williams & Tresilian (1999), at least in reaching space: ‘…the results of our experiment indicate that vergence can be used to reliably evaluate target 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. (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 a strong linear relationship between the vergence specified distance and the perceived distance of the point of light ( y = 0.86 x + 6.5 for distances between 20cm and 60cm). The point of light was aligned with the visual axis of the right eye, whilst the vergence demand of the left eye was varied with base-in and base-out prisms. Mon-Williams & Tresilian (1999) asked subjects to point with a hidden hand to the distance of a point of light viewed in darkness. ![]() Distance estimates plotted against the vergence demand of the stimulus in Mon-Williams & Tresilian (1999) (left) and Viguier, Clément, & Trotter (2001) (right). The contemporary literature is comprehensively reviewed in Appendix A, but suffice to say the effectiveness of vergence as a cue to near distances (up to 2m) is considered settled, and the only real question is whether vergence should be regarded as a veridical distance cue or not? Two papers, Mon-Williams & Tresilian (1999) and Viguier, Clément, & Trotter (2001), are generally cited as authoritative, and their results are summarised in Fig.1.įig.1. The visual system’s ability to extract distance information from vergence was one of visual psychophysics’ earliest concerns, and was answered affirmatively by Hueck (1838) Meyer (1842) Wheatstone (1852) and Wundt (1862). motion parallax (the change in the visual scene from the motion of the observer). ![]() accommodation (the curvature of the intraocular lens), and 3. 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. As an ‘intromission’ theorist, al-Haytham (c.1021) had no such luxury and relied on familiar size instead. 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. Euclid, c.300 BC), but visual angle appropriately scaled by distance information (see Hatfield, 2002). Ptolemy (c.160 AD) first articulated visual scale in these terms, arguing that size wasn’t just a function of visual angle (vs. 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. Scale has two components: size and distance. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |