The removal of binocular cues disrupts the calibration of grasping in patients with visual form agnosia

Abstract

The present study tested the idea that the visuomotor systems mediating prehension do not have independent access to pictorial cues processed by perceptual mechanisms. Individuals with visual form agnosia, whose perceptual systems are compromised but who have intact visuomotor control, were examined to determine whether they could use pictorial scene cues to calibrate manual prehension when binocular information was removed. The removal of binocular cues produced considerable disruptions in size-constancy of grip aperture, which, combined with earlier observations in normal subjects, suggests that binocular cues are of primary importance in calibration of grasping. In the absence of binocular vision, normal subjects can use pictorial information, information that is severely compromised in individuals with visual form agnosia, to compute the distance (and thus the size) of the goal object. Thus, individuals with visual form agnosia must rely on a retinal image that remains uncalibrated, leading to inaccurate calibrations of grip aperture. The fact that these individuals scaled their grasp much less accurately under the monocular viewing condition, despite showing normal binocular grasping, suggests that pictorial cues to depth, which are presumably processed by mechanisms mediating our perception of objects and events in the world, can be accessed by visuomotor mechanisms only indirectly. These results, together with others, suggest that the visuomotor system 'prefers' to use binocular information and uses pictorial cues only as a last resort.

2D But Not 3D: Pictorial-Depth Deficits in a Case of Visual Agnosia

“…Cortex, (2004) Haffenden and Goodale, 1999;Mon-Williams and Dijkerman, 1999;Marotta et al, 1997;Mon-Williams et al, 2001;Servos et al, 1992;Servos and Goodale, 1998). The main thrust of this literature has been in general agreement with De Renzi's (1982) proposal that binocular depth information may be useful in the control of spatial action (see also Dijkerman et al, 1996;Gallese et al 1995, Otto-de Haart et al, 1999, by the dorsal stream.…”

Perceptuo-Motor Interactions during Prehension Movements

“…Processing depth information along the dorsal stream is particularly dependent on binocular inputs, like stereopsis Shikata et al, 1996) and vergence (Mon-Williams et al, 2001;Quinlan and Culham, 2007). For instance, in visual agnosic patients, visuomotor performance is disturbed during monocular viewing of a target (Dijkerman et al, 1996(Dijkerman et al, , 2004Marotta et al, 1997). This suggests that, when binocular vision is unavailable, the extraction of depth information (that is crucial for visuomotor control) relies on pictorial cues like texture, illumination gradients, and perspective.…”

Perceptual constancy in judgments of egocentric distance: prevailing binocular information

“…Following the invention of stereoscope by Wheatstone that created a whole new area of study in visual perception, the development of random-dot stereograms (2) , the discoveries on neural wiring of binocular disparity encoding (3,4) , and the arising of computational theory in visual perception investigation, several findings and models on binocular depth perception increased knowledge until present status (1,(5)(6)(7)(8)(9)(10)(11) . The nature of binocular information may be a trade off between stereopsis and binocular suppression in order to providing stability and singleness of our visual world (8) .…”

“…Marotta and his colleagues (9,10) found that pictorial cues are useful for calibration of visuomotor tasks in the absence of binocular cues. In their experiments, observers reached by prehension or grip movements a visually available target, so specifying egocentric distances was vital for accurate accomplishment of those tasks.…”