Rebekah C. White

A sensational illusion: Vision-touch synaesthesia and the rubber hand paradigm

For individuals with vision-touch synaesthesia, the sight of touch on another person elicits synaesthetic tactile sensation on the observers own body. Here we used the traditional rubber hand paradigm (Botvinick and Cohen, 1998) and a no-touch rubber hand paradigm to investigate and to authenticate synaesthetic tactile sensation. In the traditional rubber hand paradigm, the participant views a prosthetic hand being touched by the Examiner while the participants hand - hidden from view - is also touched by the Examiner. Synchronous stimulation of the prosthetic hand and the participants hidden hand elicits the rubber hand illusion. It may seem to the participant that she is feeling touch at the location of the viewed prosthetic hand - visual capture of touch, and that the prosthetic hand is the participants own hand - illusion of ownership. Thus, for participants who experience the traditional rubber hand illusion, tactile sensation on the participants hidden hand is referred to the prosthetic hand. In our no-touch rubber hand paradigm, the participant views a prosthetic hand being touched by the Examiner but the participants hand - hidden from view - is not touched by the Examiner. Questionnaire ratings indicated that only individuals with vision-touch synaesthesia experienced the no-touch rubber hand illusion. Thus, synaesthetic tactile sensation on the (untouched) hidden hand was referred to the prosthetic hand. These individuals also demonstrated proprioceptive drift (a change, from baseline, in proprioceptively perceived position) of the hidden hand towards the location of the prosthetic hand, and a pattern of increased proprioceptive drift with increased trial duration (60 sec, 180 sec, 300 sec). The no-touch rubber hand paradigm was an excellent method to authenticate vision-touch synaesthesia because participants were naïve about the rubber hand illusion, and they could not have known how they were expected to perform on either the traditional or the no-touch rubber hand paradigm.

Attention set for number: expectation and perceptual load in inattentional blindness.

Inattentional blindness is the failure to detect unexpected events when attention is otherwise engaged. Previous research indicates that inattentional blindness increases as perceptual demands intensify. The authors present 6 cuing experiments that manipulated both the perceptual demands of a primary letter-naming task and the expectations of the individual. Inattentional blindness was greatest for individuals who held a numerical expectation that was consistent with the number of primary-task items presented. Expectation also affected detection differentially at various levels of perceptual load: Detection at moderate and high perceptual load was significantly affected by expectation, whereas detection at low perceptual load was not. The authors suggest that at moderate to high levels of perceptual load, individuals whose numerical expectations are fulfilled terminate processing when the primary task is complete, at the expense of the unexpected visual event. These experiments provide compelling evidence that expectations do affect detection of an unexpected stimulus, and they are the first to demonstrate that individuals set their attention for the number of items to be detected and are vulnerable to inattentional blindness whenever their primary-task numerical expectation is fulfilled.

Tactile expectations and the perception of self-touch: An investigation using the rubber hand paradigm

The rubber hand paradigm is used to create the illusion of self-touch, by having the participant administer stimulation to a prosthetic hand while the Examiner, with an identical stimulus (index finger, paintbrush or stick), administers stimulation to the participants hand. With synchronous stimulation, participants experience the compelling illusion that they are touching their own hand. In the current study, the robustness of this illusion was assessed using incongruent stimuli. The participant used the index finger of the right hand to administer stimulation to a prosthetic hand while the Examiner used a paintbrush to administer stimulation to the participants left hand. The results indicate that this violation of tactile expectations does not diminish the illusion of self-touch. Participants experienced the illusion despite the use of incongruent stimuli, both when vision was precluded and when visual feedback provided clear evidence of the tactile mismatch.

Visual capture of action, experience of ownership, and the illusion of self-touch: a new rubber hand paradigm.

A new rubber hand paradigm evokes an illusion with three conceptually distinct components: (i) the participant experiences her/his hidden right hand as administering touch at the location of the examiners viewed administering hand (visual capture of action); (ii) the participant experiences the examiners administering hand as being the participants own hand (experience of ownership); and (iii) the participant experiences her/his two hands as being in contact, as if she/he were touching her/his own hand (illusion of self-touch). The presence of these illusory experiences was confirmed by questionnaire responses and proprioceptive drift data.

Spatial limits on the nonvisual self-touch illusion and the visual rubber hand illusion: Subjective experience of the illusion and proprioceptive drift

The nonvisual self-touch rubber hand paradigm elicits the compelling illusion that one is touching ones own hand even though the two hands are not in contact. In four experiments, we investigated spatial limits of distance (15 cm, 30 cm, 45 cm, 60 cm) and alignment (0°, 90° anti-clockwise) on the nonvisual self-touch illusion and the well-known visual rubber hand illusion. Common procedures (synchronous and asynchronous stimulation administered for 60s with the prosthetic hand at body midline) and common assessment methods were used. Subjective experience of the illusion was assessed by agreement ratings for statements on a questionnaire and time of illusion onset. The nonvisual self-touch illusion was diminished though never abolished by distance and alignment manipulations, whereas the visual rubber hand illusion was more robust against these manipulations. We assessed proprioceptive drift, and implications of a double dissociation between subjective experience of the illusion and proprioceptive drift are discussed.

Two hands are better than one: a new assessment method and a new interpretation of the non-visual illusion of self-touch.

A simple experimental paradigm creates the powerful illusion that one is touching ones own hand even when the two hands are separated by 15 cm. The participant uses her right hand to administer stimulation to a prosthetic hand while the Examiner provides identical stimulation to the participants receptive left hand. Change in felt position of the receptive hand toward the prosthetic hand has previously led to the interpretation that the participant experiences self-touch at the location of the prosthetic hand, and experiences a sense of ownership of the prosthetic hand. Our results argue against this interpretation. We assessed change in felt position of the participants receptive hand but we also assessed change in felt position of the participants administering hand. Change in felt position of the administering hand was significantly greater than change in felt position of the receptive hand. Implications for theories of ownership are discussed.

When you fail to see what you were told to look for: inattentional blindness and task instructions.

Inattentional blindness studies have shown that an unexpected object may go unnoticed if it does not share the property specified in the task instructions. Our aim was to demonstrate that observers develop an attentional set for a property not specified in the task instructions if it allows easier performance of the primary task. Three experiments were conducted using a dynamic selective-looking paradigm. Stimuli comprised four black squares and four white diamonds, so that shape and colour varied together. Task instructions specified shape but observers developed an attentional set for colour, because we made the black-white discrimination easier than the square-diamond discrimination. None of the observers instructed to count bounces by squares reported an unexpected white square, whereas two-thirds of observers instructed to count bounces by diamonds did report the white square. When attentional set departs from task instructions, you may fail to see what you were told to look for.

Touch and feel? Using the rubber hand paradigm to investigate self-touch enhancement in right-hemisphere stroke patients

Following stroke, a patient may fail to report touch administered by another person but claim that she feels touch when it is self-administered. We investigated three explanations for self-touch enhancement: (1) proprioceptive information from the administering hand, (2) attentional modulation, and (3) temporal expectation. Tactile sensation was assessed with vision precluded, and with the affected hand positioned in the left and right hemispace. In four of six experiments, the somatic rubber hand paradigm was used: the Examiner administered stimulation to the patients affected left hand while guiding the patients right hand to administer synchronous stimulation to a prosthetic hand. Even though the patients two hands were not in contact, patients detected the same number of stimulations as when they touched their own hand directly (self-administered condition). Moreover, there was no decline in rates of detection when potentially informative movements of the administering hand were restricted. This demonstrates that patients feel rather than infer stimulation under conditions of self-touch. When patients received stimulation to the affected hand in the opposite hemispace to the hand administering touch to the prosthetic hand, all but one showed self-touch enhancement. Thus, neither proprioceptive information nor attentional modulation at the spatial region of the administering hand provided a sufficient explanation for self-touch enhancement. A follow-up experiment indicated an important role for temporal expectation: a delay, between the patients stimulation of the prosthetic hand and the Examiners stimulation of the patients affected hand, eliminated the self-touch enhancement effect.

Stimulus-parity synaesthesia (1893, 2014): introducing a 'forgotten' subtype.

When an individual with synaesthesia listens to a piece of music, she may see the notes as a kaleidoscope of colours dancing before her eyes, feel each note brushing against her skin, and swirl the notes around her mouth appreciating the rich flavours (Cytowic & Eagleman, 2011). The term synaesthesia e from syn(joining) and -aesthesia (sensation) e conjures images of cross-sensory experience, but many subtypes involve higher-order cognitive constructs as the inducing stimulus or concurrent experience (Simner, 2012). For example, in sequence-space synaesthesia, ordinal sequences (e.g., letters, months) are perceived as having an idiosyncratic twoor three-dimensional structure (Eagleman, 2009) in the minds eye or the space around the body (Jonas, Taylor, Hutton, Weiss, & Ward, 2011); and in ordinal linguistic personification (OLP), letters and numbers are experienced as having animate properties, such as gender and personality (Calkins, 1893; 1895; Simner & Holenstein, 2007). As of September 2014, at least 63 subtypes of synaesthesia have been identified (Day, 2014). Here we present a new subtype, in which a range of stimuli involuntarily elicit strong feelings of oddness or evenness. In our review of the contemporary literature, we did not find any references to this phenomenon, which we have tentatively termed “stimulusparity synaesthesia”. However, on examining Th eodore Flournoys 1893 book Des ph enom enes de synopsie [“Of synoptic phenomena”] e which details findings from a questionnaire completed by 371 synaesthetes e we discovered that he also encountered individuals who experienced many stimuli (e.g., weekdays, faces, types of food, “everything in the world”, p. 223) as odd or even. In a chapter dedicated to personification phenomena, Flournoy presented a footnote in which he noted that for such individuals:

Touching My Left Elbow: The Anatomical Structure of the Body Affects the Illusion of Self-Touch

A self-touch paradigm is used to create the illusion that one is touching ones own left elbow when one is actually touching the examiners arm. Our new self-touch illusion is sensitive to the anatomical structure of the body: you can touch your left elbow with your right index finger but not with your left index finger. Illusion onset was faster and illusion ratings were higher when participants administered touch using the plausible right index finger compared with the implausible left index finger.