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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]

Perceiving and grasping an object present an animal with different sets of computational problems. The solution in primates entails the specialization of separate neural networks for visual processing with different object representations. This explains why the Ebbinghaus illusion minimally affects the grasping hand's in-flight aperture, which normally scales with target size, even though the size of the target disc remains misperceived. An attractive alternative account, however, posits that grasps are refractory to the illusion because participants fixate on the target and fail to attend to the surrounding context. To test this account, we tracked both limb and gaze while participants made forced-choice judgments of relative disc size in the Ebbinghaus illusion or did so in combination with grasping or manually estimating the size of one of the discs. We replicated the classic dissociation: grasp aperture was refractory to the measured illusory effect on perceived size, while judgments and manual estimates of disc size were not. Importantly, the number of display-wide saccades per second and the percentage of total fixation time or fixations directed at the selected disc failed to explain the dissociation. Our findings support the contention that object perception and goal-directed action rely on distinct visual representations. This article is part of a discussion meeting issue 'New approaches to 3D vision'. [ABSTRACT FROM AUTHOR]