Demand Characteristics on Visually Guided Grasping.

A model of vision separating visual processing into motor and perceptual subsystems has been a subject of considerable interest within the domain of cognitive neuroscience. Goodale and Milner (1992) proposed the two streams hypothesis, theorizing that the ventral and dorsal visual pathways arising from early visual cortex mediate perceptual and visuomotor functions, respectively. One of the more controversial lines of support for the two streams hypothesis comes from the finding that grasping resists the effect of visual illusions (e.g., Aglioti et al. 1995). Although many studies have replicated this finding, others have not (e.g., Franz et al., 2001). The present study aims to better understand these discrepancies by examining the influence of demand characteristics, an untested factor in this field referring to the participants' awareness of the research focus. This is done by manipulating the background information (herein referred to as cover story) provided to the participants before testing commences. By investigating whether cover stories affect grasping behaviour and perceptual judgements within a Ponzo illusion paradigm, this research attempts to clarify the mixed outcomes previously reported in the literature. The results confirmed prior findings that dissociated the effect of a depth cue illusion on grasping and perceptual estimates of target size. The use of cover stories did not influence either the participants' perceptual reports or their motor responses. The findings reaffirm a classic perception-action dissociation while prompting a re-evaluation of the conflicting findings in the field. [ABSTRACT FROM AUTHOR]