The cutaneous rabbit revisited

In the cutaneous rabbit effect (CRE), a tactile event (so-called attractee tap) is mislocalized toward an adjacent attractor tap. The effect depends on the time interval between the taps. The authors delivered sequences of taps to the forearm and asked participants to report the location of one of the taps. The authors replicated the original CRE findings and observed a smaller but significant mislocalization when the attractor tap preceded the attractee tap. These results are consistent with the CRE arising from spatiotemporal interactions between the sensory codes for each individual tap. In subsequent experiments, the authors showed that the CRE was not affected by either gaze direction or concurrent auditory temporal information. The authors propose a model that explains the CRE by the spatiotemporal dynamics of an early, unimodal, sensory map.     

On the hand transport component of prehensile movements

Jeannerod (1981) proposed that prehensile movements involve two independent visuomotor channels that are responsible for hand transport and hand aperture. In many studies, the movement of a marker placed on the wrist has been used as an index of hand transport because wrist movement is unaffected by the movements of the digits responsible for hand aperture. In the present study, the spatial paths of the wrist, index finger, and thumb of 5 adults, each performing 50 reaching movements, were measured with a WATSMART movement tracking system, and their variability was analyzed. The measures of movement variability suggest that the motor system is more concerned with thumb position than with wrist position during hand transport. Although the wrist is a technically convenient index of hand transport, the thumb may be a more appropriate index from the point of view of motor control     

Book reviews

POULTON, E. C. Environment and Human Eficiency. Springfield: Charles C .Thomas. 1970. Pp. 328. $15.50.

TOBIAS, J. V. (Ed.). Foundations of Modern Auditory Theory. Vol. II. New York: Academic Press Inc. 1972. Pp. xviii + 508. $24.00.

JAMES L. MCGAUCH (Ed.). Psychobiology: Behaviour from a Biological Viezupoint. London: Academic Press. 1971. Pp. 366. £6.75.

BAKKER, D. J. Temporal Order in Disturbed Reading. Rotterdam: Rotterdam University Press. 1972. Pp. 100     

Task coordination in human prehension

Movement patterns may be complex in the sense of being composed of separable component tasks. These components may be coordinated at some level by the voluntary motor system, in order to combine tasks into appropriate actions. This study describes the use of task interference methods and phase transition curves (PTCs) to quantify task interference in tasks that may have two components. Comparison of the effects of task interference on the different components suggests how these may be coordinated during normal movements. These techniques can be applied to the coordination of hand transport and grasp aperture components in the reaching and grasping movements that people make in order to pick things up. Five subjects made cyclical movements that involved either composite reaching or just the transport or grasp component in isolation, according to condition. The cyclical movements were "perturbed" by requiring a rapid transport or grasping response to an auditory signal by the contralateral hand. The pattern of phase shifts, or changes in the timing of the cyclical task introduced by these perturbations was modeled using phase transition curves, in order to assess the nature of the functional linkage between transport and aperture in normal prehensile movement. The results suggest a functional linkage between grasp aperture and hand transport in normal prehensile movement.     

The Experience of Agency: Feelings, Judgments, and Responsibility

ABSTRACT— The experience of agency refers to the experience of being in control both of one's own actions and, through them, of events in the external world. Recent experimental studies have investigated how people recognise a particular event as being caused by their own action or by that of another person. These studies suggest that people match sensory inputs to a prediction based on the action they are performing. Other studies have contrasted voluntary actions to physically similar but passive body movements. These studies suggest that voluntary action triggers wide-ranging changes in the spatial and temporal experience not only of one's own body but also of external events. Prediction and monitoring of the consequences of one's own motor commands produces characteristic experiences that form our normal, everyday feeling of being in control of our life. We conclude by discussing the implications of recent psychological work for our notions of responsibility for action.     

The rubber hand illusion: sensitivity and reference frame for body ownership

When subjects view stimulation of a rubber hand while feeling congruent stimulation of their own hand, they may come to feel that the rubber hand is part of their own body. This illusion of body ownership is termed 'Rubber Hand Illusion' (RHI). We investigated sensitivity of RHI to spatial mismatches between visual and somatic experience. We compared the effects of spatial mismatch between the stimulation of the two hands, and equivalent mismatches between the postures of the two hands. We created the mismatch either by adjusting stimulation or posture of the subject's hand, or, in a separate group of subjects, by adjusting stimulation or posture of the rubber hand. The matching processes underlying body ownership were asymmetrical. The illusion survived small changes in the subject's hand posture, but disappeared when the same posture transformations were applied to the rubber hand. Mismatch between the stimulation delivered to the subject's hand and the rubber hand abolished the illusion. The combination of these two situations is of particular interest. When the subject's hand posture was slightly different from the rubber hand posture, the RHI remained as long as stimulation of the two hands was congruent in a hand-centred spatial reference frame, even though the altered posture of the subject's hand meant that stimulation was incongruent in external space. Conversely, the RHI was reduced when the stimulation was incongruent in hand-centred space but congruent in external space. We conclude that the visual-tactile correlation that causes the RHI is computed within a hand-centred frame of reference, which is updated with changes in body posture. Current sensory evidence about what is 'me' is interpreted with respect to a prior mental body representation.     

Brain correlates of subjective freedom of choice

The subjective feeling of free choice is an important feature of human experience. Experimental tasks have typically studied free choice by contrasting free and instructed selection of response alternatives. These tasks have been criticised, and it remains unclear how they relate to the subjective feeling of freely choosing. We replicated previous findings of the fMRI correlates of free choice, defined objectively. We introduced a novel task in which participants could experience and report a graded sense of free choice. BOLD responses for conditions subjectively experienced as free identified a postcentral area distinct from the areas typically considered to be involved in free action. Thus, the brain correlates of subjective feeling of free action were not directly related to any established brain correlates of objectively-defined free action. Our results call into question traditional assumptions about the relation between subjective experience of choosing and activity in the brain’s so-called voluntary motor areas.     

Weber's illusion and body shape: anisotropy of tactile size perception on the hand

The perceived distance between touches on a single skin surface is larger on regions of high tactile sensitivity than those with lower acuity, an effect known as Weber's illusion. This illusion suggests that tactile size perception involves a representation of the perceived size of body parts preserving characteristics of the somatosensory homunculus. Here, we investigated how body shape is coded within this representation by comparing tactile distances presented in different orientations on the hand. Participants judged which of two tactile distances on the dorsum of their left hand felt larger. One distance was aligned with the proximodistal axis (along the hand), the other with the mediolateral axis (across the hand). Across distances were consistently perceived as larger than along ones. A second experiment showed that this effect is specific to the hairy skin of the hand dorsum and does not occur on glabrous skin of the palm. A third experiment demonstrated that this bias reflects orientation on the hand surface, rather than an eye- or torso-centered reference frame. These results mirror known orientational anisotropies of both tactile acuity and of tactile receptive fields (RFs) of cortical neurons. We suggest that the dorsum of the hand is implicitly represented as wider than it actually is and that the shape of tactile RFs may partly explain distortions of mental body representations.     

Visual distortion of body size modulates pain perception

Pain is a complex subjective experience that is shaped by numerous contextual factors. For example, simply viewing the body reduces the reported intensity of acute physical pain. In this study, we investigated whether this visually induced analgesia is modulated by the visual size of the stimulated body part. We measured contact heat-pain thresholds while participants viewed either their own hand or a neutral object in three size conditions: reduced, actual size, or enlarged. Vision of the body was analgesic, increasing heat-pain thresholds by an average of 3.2 °C. We further found that visual enlargement of the viewed hand enhanced analgesia, whereas visual reduction of the hand decreased analgesia. These results demonstrate that pain perception depends on multisensory representations of the body and that visual distortions of body size modulate sensory components of pain.