Tactile suppression of displacement

Abstract

Abstract. In vision, the discovery of the phenomenon of saccadic suppression of displacement has made important contributions to the understanding of the stable world problem. Here we report a similar phenomenon in the tactile modality. When scanning a single Braille dot with two fingers of the same hand, participants were asked to decide whether the dot was stationary or whether it was displaced from one location to another. The stimulus was produced by refreshable Braille devices which have dots that can be swiftly raised and recessed. In some conditions the dot was stationary. In others, a displacement was created by monitoring the participant's finger position and by switching the dot activation when it was not touched by either finger. The dot displacement was of either 2.5 mm or 5 mm. We found that in certain cases, displaced dots were felt to be stationary. If the displacement was orthogonal to the finger movements, tactile suppression occurred effectively when it was of 2.5 mm, but when the displacement was of 5 mm, the participants easily detected it. If the displacement was medial-lateral, the suppression effect occurred as well but less often when the apparent movement of the dot opposed the movement of the finger. In such cases, the stimulus appeared sooner than when the brain could predict it from finger movement, supporting a predictive rather than a postdictive differential processing hypothesis. Keywords: tactile suppression effects, stable world problem, suppression of displacement, tactile perceptionIn all sensing modalities, individuals have the ability to experience a continuous and stable world in spite of the fact that stimulation changes constantly. The problem faced by the haptic system in order to provide perceptual stability is analogous to the problem faced by the visual system. In the two cases, the primary sensory input arises from sensitive surfaces that can be displaced motorically with respect to the outside world. As Poincaré pointed out (1897), self-generated movement has the benefit of enlarging the portion of the world that is accessible to the senses, but has the downside that it introduces a fundamental ambiguity: It is easy to construct stimuli that are identical when generated by one's own movements or due to the movement of an external agent. This problem has intrigued many scientists: Descartes, Purkinje, Helmholtz, Von Holtz and Mittelstaedt, Sperry, and others; see (Bays and Husain, 2003) for a brief review. More recently Wertheim (1994) and O'Regan & Noe (2001) discussed the analogy between vision and touch from the viewpoint of perceptual stability. The general idea is that an organism needs to be aware of its own movements to resolve ambiguity and that assumptions must be made about the world, such as rigidity and immobility, to succeed at perceiving a stable world (Wexler et al., 2001). Some authors investigated other aspects of the somatosensory stability problem; one of them is object size perception in static contact with the skin for different body regions (...

Understanding human perception by human-made illusions

“…If we, however, film the same scene with a video camera, the whole procedure looks totally different: Now we clearly also see the fast movements; so we can directly experience the specific operation of the visual system in this respect by comparing the same scene captured by two differently working visual systems: our own, very cognitively operating, visual system and the rigidly filming video system which just catches the scene frame by frame without further processing, interpreting and tuning it 1 . We call this moment of temporary functional blindness phenomenon “saccade blindness” or “saccade suppression”, which again illustrates the illusionary aspects of human perception “saccadic suppression”, Bridgeman et al, 1975; “tactile suppression”, Ziat et al, 2010). We can utilize this phenomena for testing interesting hypotheses on the mental representation of the visual environment: if we change details of a visual display during such functional blind phases of saccadic movements, people usually do not become aware of such changes, even if very important details, e.g., the expression of the mouth, are changed (Bohrn et al, 2010).…”

Context-dependent changes in tactile perception during movement execution

“…This task normally involves specific, most often bi-manual, disjoint movements in order to extract useful information from the display (Hughes and Jansson, 1994). Nevertheless, under those conditions in which the participants are required to detect displacements in refreshable Braille displays, tactile suppression occurs (Ziat et al, 2010). Tactile suppression of displacement refers to an inability to detect tactual changes in a moving stimulus, when such changes appear while the fingers are no longer in contact with the specific stimulus, i.e., the tested Braille displays in the above example (Ziat et al, 2010; see also Keyson and Houtsma, 1995).…”

“…Nevertheless, under those conditions in which the participants are required to detect displacements in refreshable Braille displays, tactile suppression occurs (Ziat et al, 2010). Tactile suppression of displacement refers to an inability to detect tactual changes in a moving stimulus, when such changes appear while the fingers are no longer in contact with the specific stimulus, i.e., the tested Braille displays in the above example (Ziat et al, 2010; see also Keyson and Houtsma, 1995). Exploratory hand and arm movements are used in order to identify 3D objects, as well as to distinguish specific characteristics or features of objects in our proximal environment.…”

Weak spatial constancy in touch

“…[9], who found that under certain conditions participants failed to detect that Braille dots had changed spatial location as they moved between their fingers.…”