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COPYRIGHT 2001 Massachusetts Institute of Technology
J.J. Marotta [1]
M.A. Goodale [2]
Abstract
The present study examined whether the learned pictorial depth cue of "familiar size" could be used to plan a reaching and grasping movement in the absence of binocular vision. Sixteen right-handed subjects were presented with two different arrays, under monocular and binocular viewing conditions, in which a range of different "grasp-sized" spheres that were lit from within could be presented in an otherwise darkened environment. In the "familiar-size" presentation array, only one "standard" sized sphere was presented, which gave subjects an opportunity to learn the relationship between the standard sphere's retinal image size and its distance. In the "multiple" spheres presentation array, subjects could not learn such a relationship because on any one trial, one of four different sphere sizes could be present. In a second experiment, the effects of this paradigm on six subjects' perceptual reports of distance were examined by having subjects slide their index fingers apart along a horizontal rod to indicate the estimated distance of the spheres. When familiar size could not be used as a cue to distance, subjects produced more on-line corrections in their reaching and grasping movements to the standard-sized spheres--but only under monocular viewing conditions. It appears that subjects are able to exploit the learned relationship between an object's distance and its projected retinal image size to help program and control reaching and grasping movements when binocular vision is not available. Although the influence of familiar size on subjects' perceptual estimates is less clear, it is clear that subjects' perceptual estimates show poor absolute scaling for distance. This result further supports the notion that under normal viewing conditions the visuomotor system uses binocular information to program and control manual prehension, but is able to use pictorial information when binocular vision is denied.
INTRODUCTION
Previous studies have demonstrated that binocular vision is a critical source of distance information used by the visuomotor system to control object-directed grasping. Nevertheless, even in the absence of binocular vision, people are still able to reach out and pick up objects reasonably accurately using only monocular cues (Jackson, Jones, Newport, & Pritchard, 1997; Dijkerman, Milner, & Carey, 1996; Marotta, Perrot, Nicolle, & Goodale, 1995; Marotta, Perrot, Nicolle, Servos, & Goodale, 1995; Servos & Goodale, 1994; Servos, Goodale, & Jakobson, 1992). One source of monocular information that subjects can use to help calibrate reaching and grasping movements, is pictorial information from the goal object itself, or the scene in which it is embedded (Marotta & Goodale, 1998; Marotta, Behrmann, & Goodale, 1997). Pictorial (or static monocular) cues have been used by artists for years to give an impression of three-dimensional structure on a two-dimensional canvas. Local perspective and shape cues not only enab le us to construct the spatial relations between objects in a scene, but with experience, can be used to calculate the actual distance and size of objects we wish to pick up.
Marotta et al. (1997) examined the use of pictorial information in individuals with visual form agnosia. These individuals have damage to the ventral cortical stream of projections involved in visual perception that run from primary visual cortex (V1) to inferotemporal cortex but have an intact dorsal cortical stream that runs from V1 to posterior parietal cortex, which is believed to be involved in the visuomotor control of action (Milner & Goodale, 1995). Although these individuals had no problem calibrating their grasp when binocular vision was available, they failed to calibrate their grasp when one of their eyes was covered. Indeed, they seemed unable to make use of the pictorial cues that healthy subjects use when binocular information is unavailable (Marotta et al., 1997). Taken together, these findings suggest that pictorial cues are normally processed by the perceptual mechanisms in the ventral stream and then passed on to the visuomotor mechanisms in the dorsal stream.
In an investigation into the role that individual pictorial cues play in the programming and control of manual prehension, subjects made fewer on-line adjustments in the trajectory of their limb and the aperture of their fingers when the elevation of a target object in the visual scene could be used as a reliable source of distance information. This learned relationship between elevation and distance was used only under monocular viewing conditions. When binocular vision was available, the presence or absence of this learned pictorial cue did not influence the execution of the grasping movement.
Another potentially useful pictorial cue for the programming and control of reaching and grasping under monocular viewing conditions is the depth cue of "familiar size." The use of familiar size as a cue to distance is quite straightforward. The visual angle subtended by an object is inversely proportional to its distance. If an organism knows the retinal image size of an object at one distance, then it can use image size to calculate the distance of that object at any distance. This very simple method requires either that the observer is genetically endowed with knowledge of the size of the object or that the observer has an opportunity to learn its size (Collett & Harkness 1982; Hochberg, 1972; Schiffman, 1966). In other words, it is the knowledge of the relationship between an object's size and its retinal image that forms the basis for the pictorial cue of familiar size. For example, if we see an adult elephant in the zoo that projects a small retinal image size, then we assume that the elephant is far a way from us. Of course, this conclusion would be untrue if it was the case that someone had replaced the real elephant with a small model of an elephant.
The role of pictorial cues in visual perception is part of a larger issue that turns on the traditional historical contrast between theories of "direct" and "indirect" perception. Indirect theories maintain that current sensory input cannot provide all that is required for perception and that the perceptual world is constructed instead through cognitive processes involving memory and inference that mediate or enrich that input (e.g., Rock, 1977; Helmholtz, 1890/1962). In contrast, theories of direct...
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