The Judd illusion is an illusion of perceived location: endpoint and unmarked midpoint positions are perceived to be shifted in a direction opposite to the direction of the arrow heads. We have previously shown (Dunn, 2003) that, contrary to expectations, both pointing and perceptual line-matching are affected by this illusion but that the pattern of effects are different across modality. One observed difference was an asymmetry in the magnitude of the pointing effect (not present in the matching data) depending upon target position relative to the participant’s midline. Here we explore two possible causes (hemispace Vs biomechanical) by comparing left and right handed pointing for left and right handed individuals, to Judd targets in various locations either side of the midline. Our results demonstrate a hemispace advantage for ipsilateral dominant hand-targets (only) located just left or right of the midline. This is consistent with evidence in the literature and probably reflects a combination of spatial processing factors, including hemishephere specialization for motor attention in the parietal lobe.

Dunn, A. K. (2003). A dual route account of vision for action and perception: The effects of the Judd illusion on pointing and line matching. Unpublished Ph.D. Thesis, York, UK.