Perceptual learning without feedback and the stability of stereoscopic slant estimation

Subjects were examined for practice effects in a stereoscopic slant-estimation task involving surfaces that comprised a large portion of the visual field. In most subjects slant estimation was significantly affected by practice, but only when an isolated surface (an absolute disparity gradient) was present in the visual field. When a second, unslanted, surface was visible (providing a second disparity gradient and thereby also a relative disparity gradient) none of the subjects exhibited practice effects. Apparently, stereoscopic slant estimation is more robust or stable over time in the presence of a second surface than in its absence. In order to relate the practice effects, which occurred without feedback, to perceptual learning, results are interpreted within a cue-interaction framework. In this paradigm the contribution of a cue depends on its reliability. It is suggested that normally absolute disparity gradients contribute relatively little to perceived slant and that subjects learn to increase this contribution by utilizing proprioceptive information. It is argued thatögiven the limited computational power of the brainöa relatively small contribution of absolute disparity gradients in perceived slant enhances the stability of stereoscopic slant perception. DOI:10.1068/p3163 ôAddress to which all correspondence and requests for reprints should be sent. (1) Generally, knowledge about perceptual learning helps to develop hypotheses about the underlying mechanisms of percept construction in vision (Poggio et al 1992); learning demonstrates plasticity in the processing of the signals (Fiorentini and Berardi 1980). Bedford (1993, page 5) defined perceptual learning as follows: ``The general function of perceptual learning is to improve sensory systems, which is particularly important if there is malfunction. The processes responsible for perceptual learning do not represent new information from the environment external to the organism. ... Perceptual learning can be observed if, as a result of experience, the same proximal stimulus leads to a new percept.'' In the case of vision, perceptual learning will manifest itself by changes in what is actually seen as a result of practice. Note that, according to the definition, finding an effect of practice does not necessarily mean that perceptual learning has occurred. to the improvements in perceiving relative distance (eg Fendick and Westheimer 1983; Kumar and Glaser 1993; Fahle et al 1995) or in recognizing shape (eg Ramachandran 1976; Bradshaw et al 1995). These studies involved non-metrical aspects of stereoscopic vision requiring only depth ordering. They cannot, therefore, be used to estimate changes in metrical aspects of perceived depth as a result of perceptual learning. The estimation of stereoscopically perceived surface orientation is a metrical task. In this study this task is examined when subjects have not been provided with any feedback about their performance. Before we proceed, it is helpful to review a number of issues that play a relevant role in stereoscopically perceived surface orientation. 1.2 Stereoscopic surface slant and disparity gradient The orientation of a surface is specified by the term slant. Slant is the angle between the surface and a reference. A surface that is slanted about the vertical axis creates a horizontal disparity gradient: the horizontal visual angle subtended by the left eye's view of the surface is different from the visual angle subtended by the right eye's view. Observers' performance can be quite impressive when the disparity gradient is the only available signal for slant: practiced observers are able to detect a change in perceived slant with a standard deviation in disparity gradient of about 7 s of arc per degree of visual angle (Ogle 1938; Backus and Banks 1999; Backus et al 1999). Generally, an isolated disparity gradient (in otherwise completely dark surroundings) is an ambiguous signal for slant relative to the head (Helmholtz 1867/1962). For example, the gradient created by a frontoparallel planar stimulus that is presented straight ahead is identical to the horizontal disparity gradient of the same plane presented eccentrically with a different slant (Ebenholtz and Paap 1973; Gillam and Lawergren 1983; Backus et al 1999). This is illustrated in figure 1. In order to interpret an isolated disparity gradient for slant relative to the head, one must compensate for the position of the surface patch relative to the head. Signals about eye posture and several disparity types make this compensation possible (eg Mayhew and Longuet-Higgins 1982; Porrill et al 1990; Rogers and Bradshaw 1995; Banks and Backus 1998; Erkelens and van Ee 1998; Backus et al 1999). 1.3 Stereoscopic vision in the laboratory If an observer knows the location of a slanted surface, then, theoretically, the disparity gradient contains all the information that she or he needs for correct slant estimation. However, even when the viewing conditions are well specified, slant estimates measured in the laboratory are often not in accordance with geometrical prediction. Artificially induced disparity gradients involve conflicting stereoscopic and monocular slant cues (Banks and Backus 1998). In each image of a stereogram, monocular cues indicate that (a) (b) a a b Figure 1. The horizontal disparity gradients are identical in panels (a) and (b). (a) Because the plane is slanted with the right side away from the observer, the retinal angle subtended in the right eye is larger than in the left eye. (b) In order to interpret a disparity gradient one needs to compensate for the location of the surface relative to the head. 96 R van Ee