Marieke Rohde

The Rubber Hand Illusion: Feeling of Ownership and Proprioceptive Drift Do Not Go Hand in Hand

In the Rubber Hand Illusion, the feeling of ownership of a rubber hand displaced from a participants real occluded hand is evoked by synchronously stroking both hands with paintbrushes. A change of perceived finger location towards the rubber hand (proprioceptive drift) has been reported to correlate with this illusion. To measure the time course of proprioceptive drift during the Rubber Hand Illusion, we regularly interrupted stroking (performed by robot arms) to measure perceived finger location. Measurements were made by projecting a probe dot into the field of view (using a semi-transparent mirror) and asking participants if the dot is to the left or to the right of their invisible hand (Experiment 1) or to adjust the position of the dot to that of their invisible hand (Experiment 2). We varied both the measurement frequency (every 10 s, 40 s, 120 s) and the mode of stroking (synchronous, asynchronous, just vision). Surprisingly, with frequent measurements, proprioceptive drift occurs not only in the synchronous stroking condition but also in the two control conditions (asynchronous stroking, just vision). Proprioceptive drift in the synchronous stroking condition is never higher than in the just vision condition. Only continuous exposure to asynchronous stroking prevents proprioceptive drift and thus replicates the differences in drift reported in the literature. By contrast, complementary subjective ratings (questionnaire) show that the feeling of ownership requires synchronous stroking and is not present in the asynchronous stroking condition. Thus, subjective ratings and drift are dissociated. We conclude that different mechanisms of multisensory integration are responsible for proprioceptive drift and the feeling of ownership. Proprioceptive drift relies on visuoproprioceptive integration alone, a process that is inhibited by asynchronous stroking, the most common control condition in Rubber Hand Illusion experiments. This dissociation implies that conclusions about feelings of ownership cannot be drawn from measuring proprioceptive drift alone.

Defining Agency: Individuality, Normativity, Asymmetry, and Spatio-temporality in Action

The concept of agency is of crucial importance in cognitive science and artificial intelligence, and it is often used as an intuitive and rather uncontroversial term, in contrast to more abstract and theoretically heavily weighted terms such as intentionality , rationality, or mind. However, most of the available definitions of agency are too loose or unspecific to allow for a progressive scientific research program. They implicitly and unproblematically assume the features that characterize agents, thus obscuring the full potential and challenge of modeling agency. We identify three conditions that a system must meet in order to be considered as a genuine agent: (a) a system must define its own individuality, (b) it must be the active source of activity in its environment (interactional asymmetry), and (c) it must regulate this activity in relation to certain norms (normativity). We find that even minimal forms of proto-cellular systems can already provide a paradigmatic example of genuine agency. By abstracting away some specific details of minimal models of living agency we define the kind of organization that is capable of meeting the required conditions for agency (which is not restricted to living organisms). On this basis, we define agency as an autonomous organization that adaptively regulates its coupling with its environment and contributes to sustaining itself as a consequence. We find that spatiality and temporality are the two fundamental domains in which agency spans at different scales. We conclude by giving an outlook for the road that lies ahead in the pursuit of understanding, modeling, and synthesizing agents.

The human touch: skin temperature during the rubber hand illusion in manual and automated stroking procedures.

A difference in skin temperature between the hands has been identified as a physiological correlate of the rubber hand illusion (RHI). The RHI is an illusion of body ownership, where participants perceive body ownership over a rubber hand if they see it being stroked in synchrony with their own occluded hand. The current study set out to replicate this result, i.e., psychologically induced cooling of the stimulated hand using an automated stroking paradigm, where stimulation was delivered by a robot arm (PHANToMTM force-feedback device). After we found no evidence for hand cooling in two experiments using this automated procedure, we reverted to a manual stroking paradigm, which is closer to the one employed in the study that first produced this effect. With this procedure, we observed a relative cooling of the stimulated hand in both the experimental and the control condition. The subjective experience of ownership, as rated by the participants, by contrast, was strictly linked to synchronous stroking in all three experiments. This implies that hand-cooling is not a strict correlate of the subjective feeling of hand ownership in the RHI. Factors associated with the differences between the two designs (differences in pressure of tactile stimulation, presence of another person) that were thus far considered irrelevant to the RHI appear to play a role in bringing about this temperature effect.

To lead and to lag - forward and backward recalibration of perceived visuo-motor simultaneity

Studies on human recalibration of perceived visuo-motor simultaneity so far have been limited to the study of recalibration to movement-lead temporal discrepancies (visual lags). We studied adaptation to both vision-lead and movement-lead discrepancies, to test for differences between these conditions, as a leading visual stimulus violates the underlying cause-effect structure. To this end, we manipulated the temporal relationship between a motor action (button press) and a visual event (flashed disk) in a training phase. Participants were tested in a temporal order judgment task and perceived simultaneity (PSS) was compared before and after recalibration. A PHANToM©force-feedback device that tracks the finger position in real time was used to display a virtual button. We predicted the timing of full compression of the button from early movement onset in order to time visual stimuli even before the movement event of the full button press. The results show that recalibration of perceived visuo-motor simultaneity is evident in both directions and does not differ in magnitude between the conditions. The strength of recalibration decreases with perceptual accuracy, suggesting the possibility that some participants recalibrate less because they detect the discrepancy. We conclude that the mechanisms of temporal recalibration work in both directions and that there is no evidence that they are asymmetrical around the point of actual simultaneity, despite the underlying asymmetry in the cause-effect relation.

Predictability is necessary for closed-loop visual feedback delay adaptation

In case of delayed visual feedback during visuomotor tasks, like in some sluggish computer games, humans can modulate their behavior to compensate for the delay. However, opinions on the nature of this compensation diverge. Some studies suggest that humans adapt to feedback delays with lasting changes in motor behavior (aftereffects) and a recalibration of time perception. Other studies have shown little or no evidence for such semipermanent recalibration in the temporal domain. We hypothesize that predictability of the reference signal (target to be tracked) is necessary for semipermanent delay adaptation. To test this hypothesis, we trained participants with a 200 ms visual feedback delay in a visually guided manual tracking task, varying the predictability of the reference signal between conditions, but keeping reference motion and feedback delay constant. In Experiment 1, we focused on motor behavior. Only training in the predictable condition brings about all of the adaptive changes and aftereffects expected from delay adaptation. In Experiment 2, we used a synchronization task to investigate perceived simultaneity (perceptuomotor learning). Supporting the hypothesis, participants recalibrated subjective visuomotor simultaneity only when trained in the predictable condition. Such a shift in perceived simultaneity was also observed in Experiment 3, using an interval estimation task. These results show that delay adaptation in motor control can modulate the perceived temporal alignment of vision and kinesthetically sensed movement. The coadaptation of motor prediction and target prediction (reference extrapolation) seems necessary for such genuine delay adaptation. This offers an explanation for divergent results in the literature.

Perceptual crossing: the simplest online paradigm

Researchers in social cognition increasingly realize that many phenomena cannot be understood by investigating offline situations only, focusing on individual mechanisms and an observer perspective. There are processes of dynamic emergence specific to online situations, when two or more persons are engaged in a real-time interaction that are more than just the sum of the individual capacities or behaviors, and these require the study of online social interaction. Auvray et al.s (2009) perceptual crossing paradigm offers possibly the simplest paradigm for studying such online interactions: two persons, a one-dimensional space, one bit of information, and a yes/no answer. This study has provoked a lot of resonance in different areas of research, including experimental psychology, computer/robot modeling, philosophy, psychopathology, and even in the field of design. In this article, we review and critically assess this body of literature. We give an overview of both behavioral experimental research and simulated agent modeling done using the perceptual crossing paradigm. We discuss different contexts in which work on perceptual crossing has been cited. This includes the controversy about the possible constitutive role of perceptual crossing for social cognition. We conclude with an outlook on future research possibilities, in particular those that could elucidate the link between online interaction dynamics and individual social cognition.

Ascriptional and 'genuine' autonomy

Knowing that human judgment can be fallible, we propose to distinguish the subjective ascription of a property, such as autonomy, from the genuine fact that an entity is characterised by a certain property, i.e., it is autonomous. In this paper, we take a closer look at this distinction and what it is grounded on, taking a constructivist stance that sees the scientist as an observing subject. We arrive at a notion of fortified ascription, in which knowledge and scientific study of generative mechanisms play an important role, and look at some models of autonomy in the light of this distinction.

Effects can precede their cause in the sense of agency

The sense of agency, i.e., the feeling that one׳s action is the cause of an external sensory event, involves causal inference based on the predicted sensory outcome of a motor act. Here, we investigated whether this inference process faithfully implements the physical principle that a cause (motor act) temporally precedes its effect (external sensory feedback). To this end, we presented participants with visual flashes that were temporally offset from voluntary button presses, including scenarios where the flash occurred shortly before the press. Participants then judged their experience of agency. As expected, cause-effect order is an important cue for this task: participants were far more likely to report agency for temporally lagging flashes than for leading flashes, even if very long sensory delays also disrupted the sense of agency (Experiment 1). This suggests that the temporal order between action and sensation is the dominant temporal cue for agency. However, when participants judged whether they had caused a first flash that occurred before the button press or a second flash that occurred afterwards, the temporal threshold for rejecting leading first flashes was relaxed proportionally to the delay of the second flash (Experiment 2). There was competition between different sensorimotor timing cues (temporal order favored the second flash and temporal proximity favored the first flash), and participants׳ tolerance for cause-effect inversions was modulated by the strength of the later, conflicting cue. We conclude that the perceived order of action and sensation is not used in a winner-take-all fashion in inference of agency. Instead, a probabilistic negotiation of the different timing cues favoring different flash events takes place postdictively, after presentation of the second flash.

Statistically Optimal Multisensory Cue Integration: A Practical Tutorial

Humans combine redundant multisensory estimates into a coherent multimodal percept. Experiments in cue integration have shown for many modality pairs and perceptual tasks that multisensory information is fused in a statistically optimal manner: observers take the unimodal sensory reliability into consideration when performing perceptual judgments. They combine the senses according to the rules of Maximum Likelihood Estimation to maximize overall perceptual precision. This tutorial explains in an accessible manner how to design optimal cue integration experiments and how to analyse the results from these experiments to test whether humans follow the predictions of the optimal cue integration model. The tutorial is meant for novices in multisensory integration and requires very little training in formal models and psychophysical methods. For each step in the experimental design and analysis, rules of thumb and practical examples are provided. We also publish Matlab code for an example experiment on cue integration and a Matlab toolbox for data analysis that accompanies the tutorial online. This way, readers can learn about the techniques by trying them out themselves. We hope to provide readers with the tools necessary to design their own experiments on optimal cue integration and enable them to take part in explaining when, why and how humans combine multisensory information optimally.

Asymmetries in visuomotor recalibration of time perception: Does causal binding distort the window of integration?

The recalibration of perceived visuomotor simultaneity to vision-lead and movement-lead temporal discrepancies is marked by an underlying causal asymmetry, if the movement (button press) is voluntary and self-initiated; a visual stimulus lagging the button press may be interpreted as causally linked sensory feedback (intentional or causal binding), a leading visual stimulus not. Here, we test whether this underlying causal asymmetry leads to directional asymmetries in the temporal recalibration of visuomotor time perception, using an interval estimation paradigm. Participants were trained to the presence of one of three temporal discrepancies between a motor action (button press) and a visual stimulus (flashed disk): 100 ms vision-lead, simultaneity, and 100 ms movement-lead. By adjusting a point on a visual scale, participants then estimated the interval between the visual stimulus and the button press over a range of discrepancies. Comparing the results across conditions, we found that temporal recalibration appears to be implemented nearly exclusively on the movement-lead side of the range of discrepancies by a uni-lateral lengthening or shortening of the window of temporal integration. Interestingly, this marked asymmetry does not lead to a significantly asymmetrical recalibration of the point of subjective simultaneity or to significant differences in discriminability. This seeming contradiction (symmetrical recalibration of subjective simultaneity and asymmetrical recalibration of interval estimation) poses a challenge to common models of temporal order perception that assume an underlying time measurement process with Gaussian noise. Using a two-criterion model of the window of temporal integration, we illustrate that a compressive bias around perceived simultaneity (temporal integration) even prior to perceptual decisions about temporal order would be very hard to detect given the sensitivity of the psychophysical procedures commonly used.