Denise D. J. de Grave

Illusions in action: consequences of inconsistent processing of spatial attributes

Abstract. Many authors have performed experiments in which subjects grasp objects in illusory surroundings. The vast majority of these studies report that illusions affect the maximum grip aperture less than they affect the perceived size. This observation has frequently been regarded as experimental evidence for separate visual systems for perception and action. In order to make this conclusion, one assumes that the grip aperture is based on a visual estimate of the objects size. We believe that it is not, and that this is why size illusions fail to influence grip aperture. Illusions generally do not affect all aspects of space perception in a consistent way, but mainly affect the perception of specific spatial attributes. This applies not only to object size, but also to other spatial attributes such as position, orientation, displacement, speed, and direction of motion. Whether an illusion influences the execution of a task will therefore depend on which spatial attributes are used rather than on whether the task is perceptual or motor. To evaluate whether illusions affect actions when they influence the relevant spatial attributes we review experimental results on various tasks with inconsistent spatial processing in mind. Doing so shows that many actions are susceptible to visual illusions. We argue that the frequently reported differential effect of illusions on perceptual judgements and goal-directed action is caused by failures to ensure that the same spatial attributes are used in the two tasks. Illusions only affect those aspects of a task that are based on the spatial attributes that are affected by the illusion.

Illusions as a tool to study the coding of pointing movements

Pictorial illusions bias our judgments about certain visual attributes. Such illusions are therefore only expected to influence a task if these attributes are used to perform the task. When pointing to a position, different visual attributes could be used to guide the hand: direction and distance (or length) of the required displacement (vector coding) or the final position (position coding). In this study we used the Brentano illusion (an illusion of length) to determine which attributes are used in pointing. Several conditions were tested in which the visibility of the hand and the stimulus were varied. The illusion influenced movements between two points along the shaft of the figure, but not movements perpendicular to the shaft. When the hand and/or target were invisible during the movement, the influence of the illusion increased. Pointing movements under different visual conditions were based on different relative contributions of position and vector coding. The contribution of vector coding was always rather modest.

Effects of the Ebbinghaus figure on grasping are not only due to misjudged size

It is not evident how the small effects of the flankers of the Ebbinghaus figure on peak grip aperture (PGA) should be interpreted. One interpretation is that the flankers influence the estimated size, which in turn influences the grasp. If this interpretation is correct, then only the size-dependent aspects of the grasping movement should depend on the spatial positions of the flankers. An alternative interpretation is that the effect on grip aperture is caused by a change in judgement of the required precision, in which case various aspects of the grasping movement could be influenced by the size and position of the flankers. We presented subjects with a display consisting of a central disk surrounded by four large or small flankers. The array of circular flankers could be rotated by 45°. There were two tasks: to reproduce the perceived size of the central disk, and to grasp the central disk. As in other studies, the reproduced size and the PGA were both influenced by the size of the flankers. The effect on reproduced size settings was independent of the flankers’ spatial position. Nevertheless, the flankers’ position did influence the final grip aperture and the grip orientation at PGA and at movement offset. Because the flankers changed more than only the PGA, we conclude that the effect of the flankers on prehension cannot only be because of misjudgement of the size of the central disk.

A metanalysis of the effect of the Müller-Lyer illusion on saccadic eye movements: No general support for a dissociation of perception and oculomotor action

Milner and Goodales (1995) proposal of a functional division of labor between vision-for-perception and vision-for-action is supported by neuropsychological, brain-imaging, and psychophysical evidence. However, there remains considerable debate as to whether, as their proposal would predict, the effect of contextual illusions on vision-for-action can be dissociated from that on vision-for-perception. Meta-analytical efforts examining the effect of the Müller-Lyer (ML) illusion on pointing (Bruno, Bernardis, & Gentilucci, 2008) or grasping (Bruno & Franz, 2009) have been conducted to resolve the controversy. To complement this work, here we re-analyzed 17 papers detailing 21 independent studies investigating primary saccades to target locations that were perceptually biased by the ML illusion. Using a corrected percent illusion effect measure to compare across different studies and across experimental conditions within studies, we find that saccadic eye movements are always strongly biased by the illusion although the size of this effect can be reduced by factors such as display duration and between-trials variability in display length and orientation, possibly due to a process of saccadic adaptation. In contrast to some reports, we find no general support for differences between voluntary and reflexive saccades or between saccades performed in conjunction with a pointing movement and saccades performed without pointing. We conclude that studies on the effect of the Müller-Lyer illusion do not provide evidence for a functional dissociation between primary saccades and perception.

Fixation locations when grasping partly occluded objects

When grasping an object, subjects tend to look at the contact positions of the digits (A. M. Brouwer, V. H. Franz, D. Kerzel, & K. R. Gegenfurtner, 2005; R. S. Johansson, G. Westling, A. Bäckström, & J. R. Flanagan, 2001). However, these contact positions are not always visible due to occlusion. Subjects might look at occluded parts to determine the location of the contact positions based on extrapolated information. On the other hand, subjects might avoid looking at occluded parts since no object information can be gathered there. To find out where subjects fixate when grasping occluded objects, we let them grasp flat shapes with the index finger and thumb at predefined contact positions. Either the contact position of the thumb or the finger or both was occluded. In a control condition, a part of the object that does not involve the contact positions was occluded. The results showed that subjects did look at occluded object parts, suggesting that they used extrapolated object information for grasping. Additionally, they preferred to look in the direction of the index finger. When the contact position of the index finger was occluded, this tendency was inhibited. Thus, an occluder does not prevent fixations on occluded object parts, but it does affect fixation locations especially in conditions where the preferred fixation location is occluded.

Why are saccades influenced by the Brentano illusion

In the Brentano version of the Müller-Lyer illusion one part looks longer and the other looks shorter than it really is. We asked participants to make saccadic eye movements along these parts of the figure and between positions on the figure and a position outside the illusion. By showing that saccades from outside the figure are not influenced by the illusion, we demonstrate that the reason that saccades along the figure are influenced is that the incorrectly judged length is used to plan the amplitude of the saccade. This finding contradicts several current views on the use of visual information for action. We conclude that actions are influenced by visual illusions, but that such influences are only apparent if the action is guided by the attribute that is fooled by the illusion.

Planning movements well in advance

It has been suggested that the metrics of grasping movements directed to visible objects are controlled in real time and are therefore unaffected by previous experience. We tested whether the properties of a visually presented distractor object influence the kinematics of a subsequent grasping movement performed under full vision. After viewing an elliptical distractor object in one of two different orientations participants grasped a target object, which was either the same object with the same orientation or a circular object without obvious orientation. When grasping the circular target, grip orientation was influenced by the orientation of the distractor. Moreover, as in classical visuomotor priming, grasping movements were initiated faster when distractor and target were identical. Results provide evidence that planning of visually guided grasping movements is influenced by prior perceptual experience, challenging the notion that metric aspects of grasping are controlled exclusively on the basis of real-time information.

The influence of the Brentano illusion on eye and hand movements.

When making an eye movement and a hand movement toward a visual target, the movements could be guided by visual judgments of direction and distance (or length) of the required displacement (vector coding), estimates of the final position (position coding), or both. Using the same information for the eyes and the hand is efficient; however, if this information contains an error, this causes both the eye and the hand to be incorrect. In this study, we tried to find out whether saccades and pointing movements use the same source of information when eye and hand movements are performed either concurrently or separately. Four experiments have been performed using the Brentano illusion, which primarily influences judgments of length but not those of position. This illusion only influences movements if the illusory length is relevant for the task, demonstrating that vector coding is involved. Subjects made saccades, pointing movements, or both between vertices of the Brentano illusion. The illusion influenced saccades and pointing movements when these movements were performed concurrently and separately, showing that the eye and the hand use vector coding. However, depending on the task, eye and hand movements were influenced to a different extent. This favors the interpretation that the eyes and the hand use a common motor command but each with a different relative contribution of vector coding.

Are the original Roelofs effect and the induced Roelofs effect caused by the same shift in straight ahead

We investigated whether the original Roelofs effect and the induced Roelofs effect are caused by the same shift in perceived straight ahead. Subjects were presented with a target within a frame in complete darkness. Target and frame could both be shifted to the left or right of objective straight ahead. On separate trials, subjects gave verbal estimates about the position of either the target or the frame. The eccentricity of the frame was underestimated (the original Roelofs effect). However, the perceived position of the target did not follow this misjudgement of the eccentricity of the frame (the induced Roelofs effect was not present). Thus, it is unlikely that both effects have a common origin in misjudging egocentric straight ahead.

Using a stick does not necessarily alter judged distances or reachability.

Background It has been reported that participants judge an object to be closer after a stick has been used to touch it than after touching it with the hand. In this study we try to find out why this is so. Methodology We showed six participants a cylindrical object on a table. On separate trials (randomly intermixed) participants either estimated verbally how far the object is from their body or they touched a remembered location. Touching was done either with the hand or with a stick (in separate blocks). In three different sessions, participants touched either the object location or the location halfway to the object location. Verbal judgments were given either in centimeters or in terms of whether the object would be reachable with the hand. No differences in verbal distance judgments or touching responses were found between the blocks in which the stick or the hand was used. Conclusion Instead of finding out why the judged distance changes when using a tool, we found that using a stick does not necessarily alter judged distances or judgments about the reachability of objects.