Perception-action Dissociations Research Articles

Perception-Action dissociations depend on factors that affect multisensory processing.

Behavioral perception-action dissociations are widely used to test models of high-level vision, but debates concerning their interpretation have underestimated the role of multisensory mechanisms in such tests. Sensorimotor tasks engage multisensory processing in fundamentally different ways than perceptual tasks, and these differences can modulate dissociations in different ways based on task features. To test this idea, we compared perception and action using a well-understood size-contrast effect, the Uznadze illusion, and manipulated both unimodal and crossmodal stimulation as well as conditions that are known to favor or hinder multisensory integration. Results demonstrated that similar or dissociable effects on perception and action can be observed depending on factors that are known to affect multisensory processing. Specifically, such factors can cause a visual task to be affected by the illusion or remain fully unaffected, whereas a visuomotor task can be affected by the illusion, remain immune from the illusion, or, unexpectedly, even show a clear reverse effect. These findings provide a novel perspective on a long standing debate in behavioral cognitive neuroscience.

Perception-action Dissociations as a Window into Consciousness.

Understanding the neural correlates of unconscious perception stands as a primary goal of experimental research in cognitive psychology and neuroscience. In this Perspectives paper, we explain why experimental protocols probing qualitative dissociations between perception and action provide valuable insights into conscious and unconscious processing, along with their corresponding neural correlates. We present research that utilizes human eye movements as a sensitive indicator of unconscious visual processing. Given the increasing reliance on oculomotor and pupillary responses in consciousness research, these dissociations also provide a cautionary tale about inferring conscious perception solely based on no-report protocols.

Sensorimotor adaptation and cue reweighting compensate for distorted 3D shape information, accounting for paradoxical perception-action dissociations.

Visually guided movements can show surprising accuracy even when the perceived three-dimensional (3D) shape of the target is distorted. One explanation of this paradox is that an evolutionarily specialized "vision-for-action" system provides accurate shape estimates by relying selectively on stereo information and ignoring less reliable sources of shape information like texture and shading. However, the key support for this hypothesis has come from studies that analyze average behavior across many visuomotor interactions where available sensory feedback reinforces stereo information. The present study, which carefully accounts for the effects of feedback, shows that visuomotor interactions with slanted surfaces are actually planned using the same cue-combination function as slant perception and that apparent dissociations can arise due to two distinct supervised learning processes: sensorimotor adaptation and cue reweighting. In two experiments, we show that when a distorted slant cue biases perception (e.g., surfaces appear flattened by a fixed amount), sensorimotor adaptation rapidly adjusts the planned grip orientation to compensate for this constant error. However, when the distorted slant cue is unreliable, leading to variable errors across a set of objects (i.e., some slants are overestimated, others underestimated), then relative cue weights are gradually adjusted to reduce the misleading effect of the unreliable cue, consistent with previous perceptual studies of cue reweighting. The speed and flexibility of these two forms of learning provide an alternative explanation of why perception and action are sometimes found to be dissociated in experiments where some 3D shape cues are consistent with sensory feedback while others are faulty.NEW & NOTEWORTHY When interacting with three-dimensional (3D) objects, sensory feedback is available that could improve future performance via supervised learning. Here we confirm that natural visuomotor interactions lead to sensorimotor adaptation and cue reweighting, two distinct learning processes uniquely suited to resolve errors caused by biased and noisy 3D shape cues. These findings explain why perception and action are often found to be dissociated in experiments where some cues are consistent with sensory feedback while others are faulty.

The value of cognitive neuropsychology: The case of vision research

ABSTRACTCognitive neuropsychological evidence is widely viewed as inherently flawed or weak, despite well-reasoned arguments to the contrary by many theorists. Rather than attempting yet another defence of cognitive neuropsychology on logical grounds, we point out through examples that in practice, cognitive neuropsychological evidence is widely accepted as valid and important, and has had a major impact on cognitive theory and research. Objections offered in the abstract rarely arise in the context of actual studies. We develop these points through examples from the domain of vision, discussing cerebral achromatopsia and akinetopsia, selective impairment and sparing of face recognition, perception–action dissociations, and blindsight.

Adaptation effects in grasping the Müller-Lyer illusion

Recent results have shown that effects of pictorial illusions in grasping may decrease over the course of an experiment. This can be explained as an effect of sensorimotor learning if we consider a pictorial size illusion as simply a perturbation of visually perceived size. However, some studies have reported very constant illusion effects over trials. In the present paper, we apply an error-correction model of adaptation to experimental data of N=40 participants grasping the Müller-Lyer illusion. Specifically, participants grasped targets embedded in incremental and decremental Müller-Lyer illusion displays in (1) the same block in pseudo-randomised order, and (2) separate blocks of only one type of illusion each. Consistent with predictions of our model, we found an effect of interference between the two types when they were presented intermixed, explaining why adaptation rates may vary depending on the experimental design. We also systematically varied the number of object sizes per block, which turned out to have no effect on the rate of adaptation. This was also in accordance with our model. We discuss implications for the illusion literature, and lay out how error-correction models can explain perception-action dissociations in some, but not all grasping-of-illusion paradigms in a parsimonious and plausible way, without assuming different illusion effects.

Both vision-for-perception and vision-for-action follow Weber's law at small object sizes, but violate it at larger sizes

According to a previous report, the visual coding of size does not obey Weber's law when aimed at guiding a grasp (Ganel et al., 2008a). This result has been interpreted as evidence for a fundamental difference between sensory processing in vision-for-perception, which needs to compress a wide range of physical objects to a restricted range of percepts, and vision-for-action when applied to the much narrower range of graspable and reachable objects. We compared finger aperture in a motor task (precision grip) and perceptual task (cross modal matching or "manual estimation" of the object's size). Crucially, we tested the whole range of graspable objects. We report that both grips and estimations clearly violate Weber's law with medium-to-large objects, but are essentially consistent with Weber's law with smaller objects. These results differ from previous characterizations of perception-action dissociations in the precision of representations of object size. Implications for current functional interpretations of the dorsal and ventral processing streams in the human visual system are discussed.

Differential effects of forward and backward masks on the relationship between perception and action.

A recent series of experiments has shown that the effects of near-threshold stimuli on perceptual and motor responses are highly dependent on experimental conditions. In particular, motor influences of near-threshold distractors were observed when using low-contrast unmasked stimuli and high-contrast masked stimuli although only the latter affected motor responses in the absence of stimulus awareness. These results are compatible with neurophysiological models of visual masking, suggesting that early neural responses to a visual stimulus can be decomposed in feedforward activations to-and feedback activations from-higher visual areas, correlating respectively with the actual presence of the stimulus and its conscious perception. We tested the compatibility between these neurophysiological models and the behavioural data obtained in near-threshold experiments. We recorded fast reaching movements directed to a highly visible target followed by a report of the presence of a near-threshold distractor presented either at low contrast without mask or at high contrast with a backward or forward mask. Analysis of hand trajectories revealed that deviations toward the distractor were observed in the no-mask condition when the distractor was present and reported, and when it was present but not reported in the backward and forward mask conditions, although the effect was weaker in the latter condition. These data reveal that the presence or absence of perception-action dissociations in behavioural studies are well accounted for by neurophysiological models of visual masking and that behavioural effects of near-threshold distractors cannot result merely from on a dichotomic visual system for perception and action.

The Two-Wrongs model explains perception-action dissociations for illusions driven by distortions of the egocentric reference frame

Several studies have demonstrated a dissociation of the effects of illusion on perception and action, with perception generally reported to be susceptible to illusions, while actions are seemingly immune. These findings have been interpreted to support Milner and Goodale's Two Visual Systems model, which proposes the existence of separate visual processing streams for perception and action. However, an alternative interpretation suggests that this type of behavioral dissociation will occur for any illusion that is caused by a distortion of the observer's egocentric reference frame, without requiring the existence of separate perception and action systems that are differently affected by the illusion. In this scenario, movements aimed at illusory targets will be accurate if they are guided within the same distorted reference frame used for target encoding, since the error of motor guidance will cancel with the error of encoding (hence, for actions, two wrongs do make a right). We further test this Two-Wrongs model by examining two illusions for which the hypothesis makes very different predictions: the rod-and-frame illusion (which affects perception but not actions) and the simultaneous-tilt illusion (which affects perception and actions equally). We demonstrate that the rod-and-frame illusion is caused by a distortion of the observer's egocentric reference frame suitable for the cancellation of errors predicted by the Two-Wrongs model. In contrast, the simultaneous-tilt illusion is caused by local interactions between stimulus elements within an undistorted reference frame, precluding the cancellation of errors associated with the Two-Wrongs model such that the illusion is reflected in both perception and actions. These results provide evidence for a class of illusions that lead to dissociations of perception and action through distortions of the observer's spatial reference frame, rather than through the actions of functionally separate visual processing streams.

Using scissors to bisect a line: A perception–action dissociation in complex tool use

Previous research has shown that unilateral visual neglect is improved when patients are required to actively grasp an object at its center, rather than only pointing to its center. A similar dissociation between pointing and grasping responses has been reported for pseudoneglect, a spatial bias toward the left side of space that is normally exhibited by healthy participants. Among other theories, the two-visual-streams hypothesis has been offered as an explanation for these dissociations. According to this hypothesis, highly practiced actions that are performed under easily available visual input (e.g., grasping an object) are controlled by the dorsal stream and can resist perceptual judgment biases. We investigated whether, in pseudoneglect, dissociations between perceptual judgments and motor-manipulative responses occur when participants have to respond with complex tools that are highly practiced but that have been shown to rely on both ventral- and dorsal-stream processing. In a standard line bisection task, participants had to either mark the center of a line with a pencil or cut the line in two halves using a pair of scissors. The results indicated the typical leftward bias (pseudoneglect) in the pencil task, whereas performance was much more accurate in the scissors task. These results show that actions featuring complex tool use can resist perceptual judgment biases, and the findings are discussed with reference to existing accounts of perception-action dissociations.

Perception-action dissociations depend on the luminance contrast of the stimuli

The observation that near-threshold low-contrast visual distractors can equally influence perceptual state and goal-directed motor responses was recently taken as an argument against a sharp separation between a conscious vision for perception and an unconscious vision for action. However, data supporting the dual visual system theory have principally involved high-contrast stimuli. In the present study, we assessed the effect of varying the contrast of a near-threshold visual distractor while keeping its visibility constant with backward noise masks. Eight participants performed fast manual reaching movements toward a highly visible target while subsequently reporting the presence/absence of a near-threshold distractor appearing at the opposite location with respect to the body midline. For all distractor contrasts, hand trajectory deviations toward the distractor were observed when the distractor was present and detected. When the distractor remained undetected deviations also occurred, but for higher contrasts. The subliminal motor effect traditionally observed in visual masking studies may therefore primarily depend on the luminance contrast of the interfering stimuli. These results suggest that dissociations between perceptual and motor responses can be explained by a single-signal model involving differential thresholds for perception and action that are specifically modulated as a function of both the requirements of the task and the contrast level of the stimuli. Such modulation is compatible with neurophysiological accounts of visual masking in which feedforward activation to--and feedback activation from--higher visual areas are correlated with the actual presence of the stimulation and its conscious perception, respectively.

Motion by Nearby Players Biases Perception but Not Action in Judgments of Baseball Destination

In this research, the influence of irrelevant reference frames on estimates of ball destination was examined. In 3 experiments, confederate base runners and fielders served as distractor stimuli while balls were rolled from home plate to random locations along a barrier hidden under an elevated tarp between first and second base. Stationary participants estimated the position that the ball would exit from under the tarp if there were no barrier, whereas running participants ran along the back edge of the barrier and touched the top of the tarp above where they believed the ball would exit. Estimates of ball destination were significantly biased in the direction opposite to the confederates' motion for stationary participants, but were accurate for running participants. These findings are consistent with other perception-action dissociations, and show that relative motion effects can occur in a naturalistic setting.

Infants' Sensitivity to the Causal Features of Means-End Support Sequences in Action and Perception

Current work has yielded differential findings regarding infants' ability to perceptually detect the causal structure of a means-end support sequence. Resolving this debate has important implications for perception-action dissociations in this domain of object knowledge. In Study 1, 12-month-old infants' ability to perceive the causal structure of a cloth-pulling sequence was assessed via a habituation paradigm. After seeing an event in which a supported toy was moved by pulling a cloth that it sat on, 12-month-old infants demonstrated longer looking to events that violated the causal structure of this sequence than to events that preserved the causal structure but varied other perceptual features of the event. Studies 2 and 3 investigated 10-month-olds' interpretations of means-end support sequences using both a habituation paradigm and a task that assessed infants' own means-end actions. Whereas 10-month-olds failed to demonstrate an understanding of the causal structure when tested using a flat cloth as the support (Study 2), sensitivity to this structure was apparent when a rectangular box was the support. These patterns were evident in both action and perception (Study 3). Moreover, individual variation in action task performance was related to visual habituation performance. The results are discussed with respect to the relation between action and perception in infancy.

Perception-action dissociations of a walkable Müller-Lyer configuration.

These studies examined the role of spatial encoding in inducing perception-action dissociations in visual illusions. Participants were shown a large-scale Müller-Lyer configuration with hoops as its tails. In Experiment 1, participants either made verbal estimates of the extent of the Müller-Lyer shaft (verbal task) or walked the extent without vision, in an offset path (blind-walking task). For both tasks, participants stood a small distance away from the configuration, to elicit object-relative encoding of the shaft with respect to its hoops. A similar illusion bias was found in the verbal and motoric tasks. In Experiment 2, participants stood at one endpoint of the shaft in order to elicit egocentric encoding of extent. Verbal judgments continued to exhibit the illusion bias, whereas blind-walking judgments did not. These findings underscore the importance of egocentric encoding in motor tasks for producing perception-action dissociations.