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Previous research showing systematic localisation errors in touch perception related to eye and head position has suggested that touch is at least partially localised in a visual reference frame. However, many previous studies had participants report the location of tactile stimuli relative to a visual probe, which may force coding into a visual reference. Also, the visual probe could itself be subject to an effect of eye or head position. Thus, it is necessary to assess the perceived position of a tactile stimulus using a within-modality measure in order to make definitive conclusions about the coordinate system in which touch might be coded. Here, we present a novel method for measuring the perceived location of a touch in body coordinates: the Segmented Space Method (SSM). In the SSM participants imagine the region within which the stimulus could be presented divided into several equally spaced, and numbered, segments. Participants then simply report the number corresponding to the segment in which they perceived the stimulus. The SSM represents a simple and novel method that can be easily extended to other modalities by dividing any response space into numbered segments centred on some appropriate reference point (e.g. the head, the torso, the hand, or some point in space off the body). Here we apply SSM to the forearm during eccentric viewing and report localisation errors for touch similar to those previously reported using a crossmodal comparison. The data collected with the SSM strengthen the theory that tactile spatial localisation is generally coded in a visual reference frame even when visual coding is not required by the task.

We investigated a technique that can be used to place tactile stimuli on a continuum between two points using two tactors placed several centimeters apart. As von Békésy first described in 1959, when two pieces of skin several centimeters apart are simultaneously stimulated, a single touch is perceived at an intermediate location. When the relative intensity of the vibration of the two tactors is slowly adjusted from 1:0 to 0:1 a sensation is created of a touch moving across the skin. This has come to be known as the funneling illusion. We have characterized the funneling illusion on a variety of body parts including the hand, arm, wrist, head, torso, and legs. We show that the illusion is robust to a variety of vibration parameters, and demonstrate how it may be used for within modality touch discrimination and adjustment procedures.

How the body is represented in the brain has been an important topic of investigation for over a hundred years, with neurological, philosophical and physiological implications. Tactile processing, at even the lowest level, including localization, detection thresholds and two-point discrimination thresholds, turns out to depend on factors other than the cutaneous receptors. How these properties are affected and by what, can give insight into how the body is represented in the brain. Distortions in localization related to gaze direction suggest spatial coding in multiple reference frames. Distortions in detection and two-point discrimination thresholds related to alteration in perceived body shape indicate distortions in the relative size of different parts of the representation — perhaps related to receptive field changes in a cortical map. I will review several recent studies that together demonstrate that the body’s representation in the brain can be matched to various task requirements and that can be dynamically updated to respond to changes in posture and perceived body dimensions.

Two-point discrimination threshold depends on the number and size of receptive fields between the touches. But what determines the size of the receptive fields? Are they anatomically fixed? Or are they related to perceived body size? To answer this question we manipulated perceived arm length using the Pinocchio illusion. The test arm was held at the wrist and the holding arm was made to feel perceptually more extended than it was by applying vibration to the tendon of the biceps (cf. de Vignemont et al., 2005). For control trials the holding arm was vibrated elsewhere. An array of tactors, separated by 3 cm, was placed on the upper surface of the arm and covered with a cloth. Vibro-tactile stimulation was applied to either one or two tactors in two periods. Subjects identified which period contained two stimuli. A psychometric function was drawn through the probability of correct response as a function of tactor separation to determine the threshold distance. In a separate experiment, subjects estimated the perceived location of each tactor against a scale laid on top of the cloth. The estimated locations of the tactors on the tested arm were displaced by tendon vibration of the holding arm compatible with a perceptual lengthening of the arm. The threshold for two-touch discrimination was significantly increased from 4.5 (±0.6) cm with no tendon stimulation to 5.7 (±0.5) cm when the arm was perceptually extended. We conclude that two-point touch discrimination depends on the size of central receptive fields that become larger when the arm is perceptually lengthened.

Eccentric gaze systematically biases touch localization on the arm and waist. These perceptual errors suggest that touch location is at least partially coded in a visual reference frame. Here we investigated whether touches to non-visible parts of the body are also affected by gaze position. If so, can the direction of mislocalization tell us how they are laid out in the visual representation? To test this, an array of vibro-tactors was attached to either the lower back or the forehead. During trials, participants were guided to orient the position of their head (90° left, right or straight ahead for touches on the lower back) or head and eyes (combination of ±15° left, right or straight ahead head and eye positions for touches on the forehead) using LED fixation targets and a head mounted laser. Participants then re-oriented to straight ahead and reported perceived touch location on a visual scale using a mouse and computer screen. Similar to earlier experiments on the arm and waist, perceived touch location on the forehead and lower back was biased in the same direction as eccentric head and eye position. This is evidence that perceived touch location is at least partially coded in a visual reference frame even for parts of the body that are not typically seen.

We have previously reported that head position affects the perceived location of touch differently depending on the dynamics of the task the subject is involved in. When touch was delivered and responses were made with head rotated touch location shifted in the opposite direction to the head position, consistent with body-centered coding. When touch was delivered with head rotated but response was made with head centered touch shifted in the same direction as the head, consistent with gaze-centered coding. Here we tested whether moving the head in-between touch and response would modulate the effects of head position on touch location. Each trial consisted of three periods, in the first arrows and LEDs guided the subject to a randomly chosen head orientation (90° left, right, or center) and a vibration stimulus was delivered. Next, they were either guided to turn their head or to remain in the same location. In the final period they again were guided to turn or to remain in the same location before reporting the perceived location of the touch on a visual scale using a mouse and computer screen. Reported touch location was shifted in the opposite direction of head orientation during touch presentation regardless of the orientation during response or whether a movement was made before the response. The size of the effect was much reduced compared to our previous results. These results are consistent with touch location being coded in both a gaze centered and body centered reference frame during dynamic conditions.

The perceived location of a touch shifts with eye or head position, suggesting touch is coded in a visual reference frame. Our lab has previously reported shifts in the direction of eccentricity, but Ho and Spence (2007, Brain Res. 1144: 136) reported head-related shifts in the opposite direction. These studies differed by body part touched (arm vs. torso), the nature of the touch (tap vs. vibration) and whether perceived touch location was recorded with head centered or eccentric. We performed a series of experiments applying vibration to the torso and measuring perceived location using a visual scale. We replicated Ho and Spence using trials blocked by head position and recording perceived position with the head displaced. A second experiment used randomized head displacements with the head centered before responding. In a control experiment head was centered during the touch and eccentric for response. Perceived touch location shifted in the direction opposite to head position under the blocked design, in the same direction as head position in the randomized design and not at all in the control. Results are discussed in terms of allocentric and egocentric coding mechanisms needed to complete each condition.