Jörg Trojan

Perceptual drifts of real and artificial limbs in the rubber hand illusion

In the rubber hand illusion (RHI), transient embodiment of an artificial hand is induced. An often-used indicator for this effect is the “proprioceptive drift”, a localization bias of the real hand towards the artificial hand. This measure suggests that the real hand is attracted by the artificial hand. Principles of multisensory integration, however, rather suggest that conflicting sensory information is combined in a “compromise” fashion and that hands should rather be attracted towards each other. Here, we used a new variant of the RHI paradigm in which participants pointed at the artificial hand. Our results indicate that the perceived positions of the real and artificial hand converge towards each other: in addition to the well-known drift of the real hand towards the artificial hand, we also found an opposite drift of the artificial hand towards the real hand. Our results contradict the notion of perceptual substitution of the real hand by the artificial hand. Rather, they are in line with the view that vision and proprioception are fused into an intermediate percept. This is further evidence that the perception of our body is a flexible multisensory construction that is based on integration principles.

Body posture affects tactile discrimination and identification of fingers and hands

It is an unresolved question whether the posture of single fingers relative to each other is represented in the brain within an external frame of reference. In two experiments, we investigated postural influences on the processing of tactile stimuli at fingers and hands. Healthy subjects received two simultaneous tactile stimuli at the fingertips while the fingers of both hands were either interleaved or not. In speeded response tasks, they were asked to discriminate (experiment 1) or to identify (experiment 2) the touched body parts, either regarding hand laterality or finger type. The results demonstrate that both finger discrimination and finger identification are influenced by body posture. We conclude that the assumption of a solely somatotopic representation of fingers is not tenable and that an external reference system must be available for the detection of single fingers. The results are discussed in terms of a mental segmentation of external space, based on body posture and task requirements.

Spatiotemporal integration in somatosensory perception: effects of sensory saltation on pointing at perceived positions on the body surface.

In the past, sensory saltation phenomena (Geldard and Sherrick, 1972) have been used repeatedly to analyze the spatiotemporal integration capacity of somatosensory and other sensory mechanisms by means of their psychophysical characteristic. The core phenomenon consists in a systematic mislocalization of one tactile stimulus (the attractee) toward another successive tactile stimulus (the attractant) presented at another location, increasing with shorter intervals. In a series of four experiments, sensory saltation characteristics were studied at the forearm and the abdomen. Participants reported the perceived positions of attractees, attractants, and reference stimuli by pointing. In general, saltation characteristics compared well to those reported in previous studies, but we were able to gain several new insights regarding this phenomenon: (a) the attractee–attractant interval did not exclusively affect the perceived attractee position, but also the perceived attractant position; (b) saltation characteristics were very similar at different body sites and orientations, but did show differences suggesting anisotropy (direction-dependency) in the underlying integration processes; (c) sensory saltation could be elicited with stimulation patterns crossing the body midline on the abdomen. In addition to the saltation-specific results, our experiments demonstrate that pointing reports of perceived positions on the body surface generally show pronounced systematic biases compared to veridical positions, moderate intraindividual consistency, and a high degree of inter-individual variability. Finally, we address methodological and terminological controversies concerning the sensory saltation paradigm and discuss its possible neurophysiological basis.

Independent psychophysical measurement of experimental modulations in the somatotopy of cutaneous heat-pain stimuli

Distortions of the body image have been repeatedly reported for various clinical conditions, but direct experimental analyses of the perceptual changes involved are still scarce. In addition, most experimental studies rely on cerebral activation patterns to assess neuroplastic changes in central representation, although the relationship between cerebral topography and the topology of the perceptual space is not clear. This study examines whether the direct psychophysical mapping approach we introduced recently (Trojan et al., Brain Res 2006;1120:106–113) is capable of tracking perceptual distortions in the somatotopic representation of heat-pain stimuli. Eleven healthy participants indicated the perceived positions of CO2 laser stimuli, repetitively presented to the dorsal forearm, with a 3D tracking system in two consecutive sessions, separated by the topical application of capsaicin cream. In line with earlier reports, we expected that the resulting individual perceptual maps (i.e., one-dimensional projections of the perceived positions onto the forearm surface) would be subject to modulation through the altered sensory input, to be measured in terms of altered topological parameters. We found that the topology and metrics of the somatotopic representation were well preserved in the second session, but that the perceptual map was compressed to a smaller range in 9 out of 11 participants. By providing dimensional measures of perceptual representations, perceptual maps constitute an independent, genuinely psychological complement to the topography of cortical activations measured with neuroimaging methods. In addition, we expect them to be useful in diagnosing pathological changes in body perception accompanying chronic pain and other disorders.

Tactile-auditory saltation: Spatiotemporal integration across sensory modalities

The perceptual phenomena of sensory saltation involve the systematic displacement of a target stimulus (the attractee) towards a subsequent stimulus (the attractant), which occurs closely in time and space. Here, we demonstrate the existence of cross-modal tactile-auditory saltation. Tactile stimuli were delivered to the forehead and spatially congruent stereoscopic auditory stimuli were presented via headphones to a total of 20 participants. After a reference stimulus at one of five spatial positions, the attractee was presented at a fixed position, followed by the attractant at a different fixed position with a delay of 81, 121, or 181 ms. Participants rated whether the attractee was perceived left or right of the reference in 2 uni-modal and 2 cross-modal (different reference/attractee vs. attractant mode) configurations. Saltation was present in all uni- and cross-modal configurations at an attractee-attractant delay of 81 ms. At delays of 81 ms the overall displacements were stronger than at delays of 121 ms, and tactile attractants generally induced stronger displacements than auditory attractants. The results indicated the existence of cross-modal tactile-auditory saltation, suggesting the application of the saltation phenomenon as a powerful approach for examining multi-modal sensory representations in future studies.

A dynamic neural model of localization of brief successive stimuli in saltation

Somatosensory saltation is an illusion robustly generated using short tactile stimuli [1,2]. There is a perceived displacement of a first stimulus if followed by a subsequent nearby stimulus with a short stimulus onset asynchrony (SOA). Experimental reports suggest that this illusion results from spatiotemporal integration in early processing stages, but the exact neural mechanism is unknown. The neuronal mechanism involved is probably quite generic as similar phenomena occur in other modalities, audition for example [3].

Dynamic Perceptual Maps - A Psychophysical Approach to Spatiotemporal Interactions and Plasticity in Body Perception

Tactile illusions like sensory saltation provide a tool to assess the spatiotemporal dynamics of perceptual representations of our body. Brain imaging has been used to analyze the topographical properties of central body representations, but it is unclear how cerebral topography relates to the perceptual domain, and temporal dynamics were not evaluated. Here we show, that psychophysical methods based on spatiotemporal 2D-stimulus arrays can be used to quantify the transformation rules mapping the physical (anatomical) space of the body surface (dynamic tactile maps) via cerebral space (somatosensory maps in SI/SII) into perceptual space. Characteristic spatio-temporal functions are used to parametrize the dynamics of the representational cascade. Combining this technique with brain imaging can be used for dynamic causal modelling of the intermediate steps of the cascade as well as to construct realistic virtual environments based on ones own body as reference space