Monocular and binocular distance cues: insights from visual form agnosia I(of III)

The human nervous system constructs a Euclidean representation of near (per sonal) space by combining multiple sources of information (cues). We invest igated the cues used for the representation of personal space in a patient with visual form agnosia (DF). Our results indicated that DF relies predomi nantly on binocular vergence information when determining the distance of a target despite the presence of other (retinal) cues. Notably, DF was able to construct an Euclidean representation of personal space from vergence al one. This finding supports previous assertions that vergence provides the n ervous system with veridical information for the construction of personal s pace. The results from the current study, together with those of others, su ggest that: (i) the ventral stream is responsible for extracting depth and distance information from "monocular" retinal cues (i.e. from shading, text ure, perspective) and (ii) the dorsal stream has access to binocular inform ation (from horizontal image disparities and vergence). These results also indicate that DF was not able to use size information to gauge target dista nce, suggesting that intact temporal cortex is necessary for "learned size" to influence distance processing. Our findings further suggest that in neu rologically intact humans, object information extracted in the ventral path way is combined with the products of dorsal stream processing for guiding p rehension. Finally, we studied the "size-distance paradox" in visual form a gnosia in order to explore the cognitive use of size information. The resul ts of this experiment were consistent with a previous suggestion that the p aradox is a cognitive phenomenon.