Effects of the Ebbinghaus illusion on children’s perception and grasping

We investigated the development of the Ebbinghaus illusion in children’s perception and grasping. A previous study (Hanisch et al. 2001) had reported negative illusion effects on 5- to 12-year-olds’ grasping as compared to their perception. We attempted to replicate this finding and to test different hypotheses based on a direct influence of the context elements on the trajectories of the fingers which could explain this reversal of the illusion effects. For 5- to 7- and 9- to 11-year-olds we observed the classical illusion effects in perception. Illusion effects were perfectly similar for perception and grasping in 9- to 11-year-olds, while there was a non-significant trend toward smaller illusion effects in grasping for the 5- to 7-year-olds. This could be due to a slightly different effect of the illusion on younger children’s grasping. However, it seems clear that there are no qualitative changes, as a reversal of the illusion effects in grasping of younger children. Finally, we show that our grasping data conform well to the motor literature for children’s grasping, thereby strengthening our conclusions.     

Are perception and action affected differently by the Titchener circles illusion?

 In the present study, we investigated the effects of the Titchener circles illusion in perception and action. In this illusion, two identical discs can be perceived as being different in size when one is surrounded by an annulus of smaller circles and the other is surrounded by an annulus of larger circles. This classic size-contrast illusion, known as Ebbinghaus or Titchener Circles Illusion, has a strong perceptual effect. By contrast, it has recently been demonstrated that when subjects are required to pick up one of the discs, their grip aperture during reaching is largely appropriate to the size of the target. This result has been considered as evidence of a clear dissociation between visual perception and visuomotor behaviour in the intact human brain. In this study, we suggest and investigate an alternative explanation for these results. We argue that, in a previous study, while perception was subjected to the simultaneous influence of the large and small circles displays, in the grasping task only the annulus of circles surrounding the target object was influential. We tested this hypothesis by requiring 18 subjects to perceptually estimate and grasp a disc centred in a single annulus of Titchener circles. The results showed that both the perceptual estimation and the hand shaping while grasping the disc were similarly influenced by the illusion. Moreover, the stronger the perceptual illusion, the greater the effect on the grip scaling. We discuss the results as evidence of an interaction between the functional pathways for perception and action in the intact human brain. Received: 9 September 1998 / Accepted: 25 January 1999     

Grasp effects of the Ebbinghaus illusion: obstacle avoidance is not the explanation

The perception-versus-action hypothesis states that visual information is processed in two different streams, one for visual awareness (or perception) and one for motor performance. Previous reports that the Ebbinghaus illusion deceives perception but not grasping seemed to indicate that this dichotomy between perception and action was fundamental enough to be reflected in the overt behavior of non-neurological, healthy humans. Contrary to this view we show that the Ebbinghaus illusion affects grasping to the same extent as perception. We also show that the grasp effects cannot be accounted for by non-perceptual obstacle avoidance mechanisms as has recently been suggested. Instead, even subtle variations of the Ebbinghaus illusion affect grasping in the same way as they affect perception. Our results suggest that the same signals are responsible for the perceptual effects and for the motor effects of the Ebbinghaus illusion. This casts doubt on one line of evidence, which used to strongly favor the perception-versus-action hypothesis. Electronic Publication     

Grasping visual illusions: Consistent data and no dissociation

The finding that the Ebbinghaus/Titchener illusion deceives perception but not grasping is usually seen as strong evidence for Goodale and Milner's (1992) notion of two parallel visual systems, one being conscious and deceived by the illusion (vision-for-perception) and the other being unconscious and not deceived (vision-for-action). However, this finding is controversial and led to studies with seemingly contradictory results. We argue that these results are not as contradictory as it might seem. Instead, studies consistently show similar effects of the illusion on grasping. The perceptual effects are strongly dependent on the specific perceptual measure employed. If, however, some methodological precautions are used, then these diverse perceptual results can be reconciled and point to a single internal size estimate that is used for perception and for grasping. This suggests that the Ebbinghaus illusion deceives a common representation of object size that is used by perception and action.     

Grasping visual illusions: Consistent data and no dissociation

The finding that the Ebbinghaus/Titchener illusion deceives perception but not grasping is usually seen as strong evidence for Goodale and Milner's (1992) notion of two parallel visual systems, one being conscious and deceived by the illusion (vision-for-perception) and the other being unconscious and not deceived (vision-for-action). However, this finding is controversial and led to studies with seemingly contradictory results. We argue that these results are not as contradictory as it might seem. Instead, studies consistently show similar effects of the illusion on grasping. The perceptual effects are strongly dependent on the specific perceptual measure employed. If, however, some methodological precautions are used, then these diverse perceptual results can be reconciled and point to a single internal size estimate that is used for perception and for grasping. This suggests that the Ebbinghaus illusion deceives a common representation of object size that is used by perception and action.     

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.     

Pointing movements are affected by size-contrast illusions

 The influence that the perceived size of visual targets has on the characteristics of pointing movements was investigated in the present study. A size-contrast illusion, known as the Ebbinghaus or Tichener circles, was employed. In this illusion, a target circle surrounded by several smaller circles is perceived to be larger than a target circle of the same physical size surrounded by several larger circles. Movement times of open-loop pointing responses directed to the perceptually smaller target circle were significantly longer than the movement times of pointing responses directed to the perceptually larger target circle. The extent of this difference was similar to that observed when pointing responses were directed at physically different-sized target circles that were not surrounded by other circles. In addition, when the perceptually smaller circle was enlarged so that it appeared to be the same size as the perceptually larger circle, the movement times became equivalent. This evidence supports the contention that the relative rather than the absolute size of the target has a major impact on the control and execution of pointing movements. Such a conclusion contradicts those made previously concerning grasping movements made under similar conditions and implies that pointing responses are more directly influenced by visual perceptual processing than grasping responses. Received: 3 September 1998 / Accepted: 15 December 1998     

Manual size estimation: a neuropsychological measure of perception?

Manual size estimation (participants indicate the size of an object with index finger and thumb) is often interpreted as a measure of perceptual size information in the visual system, in contrast to size information used by the motor system in visually guided grasping. Because manual estimation is a relatively new measure, I compared it to a more traditional perceptual measure (method of adjustment). Manual estimation showed larger effects of the Ebbinghaus (or Titchener) illusion than the traditional perceptual measure. This inconsistency can be resolved by taking into account that manual estimation is also unusually responsive to a physical variation of size. If we correct for the effect of physical size, manual estimation and the traditional perceptual measure show similar illusion effects. Most interestingly, the corrected illusion effects are also similar to the illusion effects found in grasping. This suggests that the same neuronal signals which generate the illusion in the traditional perceptual measure are also responsible for the effects of the illusion on manual estimation and on grasping.     

The effect of the Ebbinghaus illusion on grasping behaviour of children

Within the context of the Ebbinghaus illusion, adults regularly misjudge the physical size of a centre disc, yet scale their hand aperture according to its actual size. Separate visual pathways for perception and action are assumed to account for this finding. The dorsal visual stream is said to elaborate on egocentric (visuomotor), while the ventral stream is involved in allocentric transformations (object recognition). This study examines the ontogenetic development of this dissociation between perception and action in 35 children between the ages of 5 and 12 years. We report four major results. First, when children judged object size without grasping the disc, their judgements were deceived by the illusion to the same extent as adults. However, when asked to estimate size and then to grasp the disc, young children's (5-7 years) perceptual judgements became unreliable, while adults were still reliably deceived by the illusion in 80% of their trials. Second, the younger the children, the more their aperture was affected by the illusional surround. Discs of the same size were grasped with a smaller aperture when surrounded by a small annulus, although they were perceived as being larger. Third, young children used the largest safety margin during grasping. Fourth, the reliance on visual feedback decreased with increasing age, which was documented by shorter movement times and earlier maximum hand opening during grasping in the older children (feedforward control). Our results indicate that grasping behaviour in children is subject to an interaction between ventral and dorsal processes. Both pathways seem not to be functionally segregated in early and middle childhood. The data are inconclusive about whether young children predominantly use a specific visual stream for either a perceptual or motor task. However, our data demonstrate that children were relying on both visual processing streams during perceptual as well as visuomotor tasks. We found that children used egocentric cues to make perceptual judgements, while their grasping gestures were not exclusively shaped by viewer-centred but also by object-centred information.     

Division of labour within the visual system: fact or fiction? Which kind of evidence is appropriate to clarify this debate?

The perception versus action hypothesis of Goodale and Milner (Trends Neurosci 15:20–25, 1992) and Milner and Goodale (The visual brain in action. Oxford University Press, Oxford, 1995) postulated two different pathways within the visual system—one for action and one for perception. With the help of pictorial illusions, evidence for this dissociation was found in various studies. There is an ongoing debate, however, as to whether or not this evidence is biased by methodological issues. Indeed, relevant and decisive data can come only from those studies that (1) match conditions appropriately with respect to task demands, (2) use illusions that do not provide any potential obstacles for the hand, (3) do not risk that grasping is either memory driven (when the target is not visible) or online corrected (due to a direct comparison of the grip aperture with the size of the target object), (4) do not confound differences between perception and action conditions with differences in visual feedback, and (5) correct for differences in response functions between grasping and perception. In following all these points outlined above we found support for the perception versus action hypothesis: grip aperture follows actual size independent of illusory effects, while perceived length as indicated by finger–thumb span clearly was subject to the illusion.     

Anticipated action consequences as a nexus between action and perception: evidence from event-related potentials

We used high-density event-related potentials (ERP) in a modified flanker paradigm to study the role of anticipated action consequences in action planning and the role of anticipation in the perception of action consequences. Prior to the experiment, participants were trained to classify target letters in a four-alternative forced-choice task; another letter was presented as an effect following each response. After participants had thus acquired the response-effect contingencies, in the experiment effect letters were presented as flankers to target letters. Effect-compatible flankers were letters that were learned as effects of the correct response to the target; effect-incompatible ones were learned as effects of other responses; neutral flankers were never presented as action effects. To help distinguish early and late effects of flankers on target processing, flankers were presented either simultaneously with the target or after a delay. We found that effect-incompatible flankers resulted in longer, than other flankers, time between the onset of the response-locked lateralized readiness potential and the response, indicating extended motor processing. ERP evoked by the effect-incompatible flankers differed from those evoked by other flankers in early perceptual component P1 and in later frontal component P2 reflecting stimulus evaluation and conflict detection. These results show that anticipating action consequences involves brain systems ranging from perceptual to executive; anticipated action effects constitute a link between perception and action.     

A temporal analysis of grasping in the Ebbinghaus illusion: planning versus online control

Recent work has shown that pictorial illusions have a greater effect on perceptual judgements than they do on the visual control of actions, such as object-directed grasping. This dissociation between vision for perception and vision for action is thought to reflect the operation of two separate streams of visual processing in the brain. Glover and Dixon claim, however, that perceptual illusions can influence the control of grasping but that these effects are evident only at early stages of the movement. By the time the action nears its completion any effect of illusions disappears. Glover and Dixon suggest that these results are consistent with what they call a 'planning and control' model of action, in which actions are planned using a context-dependent visual representation but are monitored and corrected online using a context-independent representation. We reanalysed data from an earlier experiment on grasping in the Ebbinghaus illusion in which we showed that maximum grip aperture was unaffected by this size-contrast illusion. When we looked at these data more closely, we found no evidence for an effect of the illusion even at the earliest stages of the movement. These findings support the suggestion that the initial planning of a simple object-directed grasping movement in this illusory context is indeed refractory to the effects of the illusion. This is not to suggest that more deliberate and/or complex movements could not be influenced by contextual information. Electronic Publication     

Temporal recalibration during asynchronous audiovisual speech perception

When a participant moves a hand-held target in complete darkness after an afterimage of that target has been obtained, an illusory increase (with movements away from the participant) or decrease (with movements towards the participant) in the apparent size of the afterimage is reported (the Taylor illusion, reported first in Taylor, J Exp Psychol 29: 1941). Unlike typical Emmert’s Law demonstrations, the Taylor illusion shows that a motor-related signal can be used to specify distance for the computation of real size. A study by Carey and Allan (Exp Brain Res 110: 1996) found that the Taylor illusion did not occur in a condition where an afterimage of one hand was obtained while the other hand performed a movement away from the participant from directly behind the first. It was proposed that, for the illusion to manifest itself, proprioceptive and visual information must be in strict “register” when the afterimage is obtained. To evaluate this hypothesis, 14 participants performed “towards” and “away” movements after obtaining afterimages of hand-held cards. Participants wore either plain lenses or prism lenses during the trials, the latter of which displaced visual stimuli 10° to the left. No significant difference was found between the two lens conditions in terms of the effect on the perception of the Taylor illusion. It was concluded that the illusory size distortions may depend on register of visual and proprioceptive position in terms of depth, rather than in the picture plane. Several suggestions for future studies of the Taylor illusion are proposed, and limitations of size judgements of afterimages are outlined.     

Collinear facilitation is independent of receptive-field expansion at low contrast

Modulation of single-cell responses by compound stimuli (target plus flankers) extending outside the cell’s receptive field (RF) may represent an early neural mechanism for encoding objects in visual space, enhancing their perceptual saliency. The spatial extent of contextual modulation is wide. The size of the RF is known to be dynamically variable. It has been suggested that RF expansion when target contrast decreases is the real cause of effects attributed to modulation by flankers. This is not the case. We directly compared, in the same cells, the extent of RF size changes when stimulus contrast decreased with that revealed by systematically changing the target-and-collinear-flankers separation. We found that RF expansion at low contrast was not universal, and that the spatial extent of RF expansion, when it existed, was smaller than that of collinear flanker modulation. We conclude that the two processes in striate cortex work independently from each other.     

Consistency in exchange for inappropriately matched visual feedback? A comment on Franz and Gegenfurtner (2008) “Grasping visual illusions: Consistent data and no dissociation”

Franz and Gegenfurtner (2008) argue that the evidence for a division of labour within the visual system for action and perception is flawed because perception is often measured by manual estimation, which responds in general with a larger slope to a change of physical size than does adjusting. Therefore results obtained under manual estimation have to be corrected for this difference in slope: In a reanalysis of six studies grasping and perception were equally influenced by the illusion after this correction. However, closer inspection of methods reveals that visual feedback was confounded with conditions (suppressed vision while grasping vs. full vision while adjusting). We argue that studies can produce relevant and decisive data only when they (a) do not confound conditions with visual feedback, (b) do not allow online corrections of the action due to a direct comparison of the hand with the target, and (c) do not provide any risk of grasping being memory driven when the target is removed.     

Non-target flanker effects on movement in a virtual action centred reference frame

Visual selective attention is thought to underly inhibitory control during pointing movements. Accounts of inhibitory control during pointing movements make differential predictions about movement deviations towards or away from highly salient non-target flankers based on their potential cortical activation and subsequent inhibition: (1) Tipper et al. (Vis Cogn 4:1–38, 1997) “response vector model” predicts movements away from highly salient flankers; (2) Welsh and Elliott’s (Q J Exp Psychol 57:1031–1057, 2004a and J Mot Behav 36:200–211, 2004b) “response activation model” predicts movements towards highly salient flankers early in the response, that is resolved by a race for inhibition. To eliminate the confounds of physical properties, such as obstacle avoidance and information cues of non-target objects, pointing was conducted in a virtual environment (graphical user interface). Participants were 14 skilled computer users who moved a computer cursor with a mouse to virtual targets. Analysis revealed non-target flankers significantly interfered with movement consistent with action centred selective attention, and reflecting a proximity-to-hand effect. Spatial analysis revealed evidence of highly salient flankers attracting movement, and less salient flankers repelling movement, supporting Welsh and Elliott’s response activation model. These effects were achieved in a virtual 2D environment where interference caused by the physical properties of objects was less cogent.     

Opposite effects on perception and action induced by the Ponzo illusion

This study investigated the effects of the Ponzo illusion on three tasks: manual estimation of target width and peak grip aperture during pantomimed and natural prehension. The targets were three discs of 25 mm height and 20, 40 and 60 mm diameter. Illusory effects on perception were larger and less variable than effects on peak grip aperture during pantomimed reaching, and also were larger and less variable for targets at least 40 mm in diameter. Although a large, statistically significant perceptual illusion in the expected direction was induced for the 60-mm-diameter target, peak grip apertures during reaches to acquire the targets did not significantly differ due to high intersubject variability. However, in two of the six reaching conditions (two reaching tasks x three target sizes), individual differences in illusory effects on perception (perceived width of target placed over converging vs diverging lines) were strongly negatively correlated with individual differences in illusory effects on peak grip aperture during prehension (i.e., if object was perceived to be larger, peak grip aperture during reach was smaller). This unexpected correlation indicates that individuals with larger illusions of increased target size reached with reduced grip apertures. The strong relationship between effects on perception and action in two conditions may indicate shared visual processing by the perceptual and motor systems, with background visual information having opposite effects on the two systems.     

When does action resist visual illusion? The effect of Müller–Lyer stimuli on reflexive and voluntary saccades

The primate visual cortex exhibits two anatomically distinct pathways (dorsal and ventral). According to the “two visual systems hypothesis” (TVSH) of Milner and Goodale (The visual brain in action. Oxford University Press, Oxford, 1995), this anatomical distinction corresponds to a functional division of labor between vision-for-action (dorsal) and vision-for-perception (ventral). This proposal is supported by evidence that, in healthy volunteers, perceptual responses are affected by visual illusions, whereas motor responses to the same illusion-inducing stimuli are not. However, previously we have shown that the amplitude of saccadic eye movements is modified by the Müller–Lyer illusion in a similar manner as perceptual responses. Here we extend this finding to reflexive and voluntary (memory-guided) saccades. We show that both types of saccade can be strongly affected by the illusion. In our studies, the effect on reflexive saccades was comparable to that usually observed with verbal reports (an effect size of 22 ± 8%), whereas the effect on voluntary saccades was smaller (11 ± 11%). In addition, both types of saccade provide evidence for the scaling bias usually observed in perceptual responses. We suggest that previous studies may have employed methods that generally reduced the effect of the illusion. Interpretations of dissociations between reflexive and voluntary saccades in terms of the TVSH appear to be premature.     

Is simple reaction time affected by visual illusions?

A number of studies have shown that while perceptual judgment is deceived by pictorial illusions, grasping and other kinds of motor behaviour are not. This is in keeping with the existence of two different cortical systems: a ventral stream subserving vision-for-perception and a dorsal stream subserving vision-for-action. The former is sensitive to illusions, the latter is not. Given this dissociation of functions, one wonders whether simple visuomotor reaction time (RT) follows the ventral or the dorsal rule in perceiving illusory figures. Answering this question might contribute to a better understanding of the different functions of the two systems. We carried out two experiments, one with the Ponzo and the other with the Ebbinghaus–Titchener illusion and found that RT is sensitive to both illusions with faster responses to stimuli appearing illusorily bigger than the others. These results show that motor action is subserved by the ventral system when that action directly reports the presence or onset of a target rather than when that action requires a spatial adjustment that reflects the physical features of the target.     

A reexamination of the size–weight illusion induced by visual size cues

The size–weight illusion induced by visually perceived sizes was reexamined to investigate whether this illusion is a sensory based or cognitive-based phenomenon. A computer-augmented environment was utilized to manipulate visual size information of target objects independently of their haptic information. Two physical cubes of equal mass (30.0 g) and size (3.0 × 3.0 × 3.0 cm) were suspended in parallel by wires attached to small graspable rings, in order to keep haptically obtained information constant between lifts. Instead of directly seeing each physical cube, subjects viewed 3D graphics of a cube with a wire and a ring that were precisely superimposed onto each physical cube. Seventeen subjects vertically lifted these augmented cubes, one after the other, by grasping the attached rings, and then reported their perception of cube heaviness. The graphical size of a comparison cube pseudo randomly varied for every comparison from 1.0 × 1.0 × 1.0 to 9.0 × 9.0 × 9.0 cm, while that of a standard cube remained constant (5.0 × 5.0 × 5.0 cm). Results indicated that the size–weight illusion frequently and systematically occurred for all the subjects such that when the comparison cube was relatively smaller than the standard cube, it was perceived to be heavier, and vice versa. As the size difference increased between the standard cube and the comparison cube, more subjects experienced the illusion, and vice versa. Follow-up tests showed occurrence of the size–weight illusion was significantly correlated with subject’s sensitivity to discriminate weight, but not with sensitivity to discriminate visual size. Results suggest that the size–weight illusion induced by only visual size cues in an augmented environment is sensory based, and depends on an individual’s integrated perception based on multimodal sensory information.