Findings from the Garner-paradigm do not support the "how" versus "what" distinction in the visual brain

In this study we challenge the widely accepted suggestion that visual perception, but not visual control of action, processes object shape in a holistic manner (Ganel and Goodale, 2003, Nature). The finding that perceptual judgments but not visuomotor acts, such as grasping are affected by variations along an irrelevant object dimension (Garner-interference) is held up as strong evidence that vision for perception and vision for action are processed within different areas of the human brain. Here, we suggest that it is possible to explain the observed dissociation without recourse to functional and/or neurological dissociations between vision for perception and vision for action. In three experiments, we demonstrate that it is possible to either elicit or eliminate a Garner-interference effect in a perceptual task by varying the temporal profile of the perceptual response. Additionally, we show that it is also possible to induce a Garner-interference effect in a visuomotor task when the task is designed in a way which discourages participants from making online changes to their ongoing motor response. We suggest that the crucial factor for the occurrence of a Garner-interference is whether reaction time is measured such that it actually reflects the total duration of the response selection processes or not. According to our results, it seems therefore no longer appropriate to use the Garner-paradigm in support of the notion that the processing mechanisms in perception and action are dissociated.