Two illusions of perceived orientation: one fools all of the people some of the time; the other fools all of the people all of the time

Abstract

In a series of three separate experiments, we studied two different orientation illusions, in both of which vertical lines appear tilted as a result of being set against a tilted background pattern. The 'simultaneous tilt illusion' (STI), in which a target grating is viewed within an abutting tilted grating surround, is thought to originate early in the cortical processing of visual contours. In contrast, the 'rod-and-frame' illusion (RFI), which is induced by a distant tilted frame, is thought to originate much later in the perceptual processing system. In the first two experiments we found that orientation-guided action was virtually impervious to the RFI, whereas both perceptual experience and action were equally influenced by the STI. In the third experiment, in which the two illusions were pitted one against the other, an opposite pattern of effects emerged, such that visuomotor performance was now deceived by the illusion much more than was perceptual experience. This pattern of association and dissociation in the effects of visual illusions on perception versus action can best be explained within a two-visual-systems model of cortical processing.

2-D Tilt and 3-D Slant Illusions in Perception and Action Tasks

“…The effect of the illusions was clearly manifest in both the perception and action conditions. For the 2-D illusion, the magnitude of the effect was similar for perception and action, consistent with the results reported by Dyde and Milner (2002); and for the 3-D illusion, the magnitude of the effect on the action response was significantly greater than its effect on the perception response.…”

“…As discussed in section 1, Dyde and Milner (2002) have argued that the failure to find an illusory effect on action does not contradict the idea of separate processing streams, as such an effect might arise early in processing. Our prime motivation in the use of the 3-D illusion was that binocular cues are often considered to be particularly involved in the control of action tasks as, in principle, they provide accurate metric 3-D information about the world.…”

“…The influence of the 2-D induced-tilt illusion has been reported previously (Dyde and Milner 2002). However, owing to the equivocal nature of many experiments in this area, it is important to review such results, particularly as we employ a technique which allows a direct quantitative, in addition to a qualitative, comparison between the two domains.…”

“…A related complication, raised by Dyde and Milner (2002), is that finding no difference between the effects of an illusion on perception and action may not necessarily contradict the idea of separate processing streams. This is because a particular illusion may simply occur early enough in visual processing so that it affects both streams.…”

Evidence for a Proprioception-Based Rapid On-Line Error Correction Mechanism for Hand Orientation during Reaching Movements in Blind Subjects

“…Reach offset was defined as the first time the transport velocity toward the target panel decreased to Ͻ3% of peak velocity. For the letter-posting tasks, final hand orientation was determined by calculating the mean orientation during the 50 ms immediately preceding the detected reach offset (Dyde and Milner, 2002). This time period was used to avoid confounding the measured hand orientation with any passive mechanical rotation of the handle caused by contact with the task panel (Gosselin-Kessiby et al, 2008).…”