The aim of Experiment 1 was to decouple cutaneous from kinesthetic stimulation. This was achieved by providing a cutaneous stimulus to one finger without kinesthetic information. Specifically, a raised line was yoked to the movement of the moving finger and presented against a stationary finger. The moving finger had kinesthetic stimulation with and without cutaneous information (i.e., the finger was moved by the participant, but the raised line was absent in some conditions). The trick was that, although the patterns were identical, each hand explored a different orientation of the pattern, potentially providing a different percept, as in the angles < and >, for example (see Fig. 1). Based on the findings of Symmons et al. (2007), we hypothesized that cutaneous inputs alone (i.e., those from the stationary finger) would be weighted more heavily than kinesthetic inputs alone (i.e., those from the moving finger). As such, simply moving a raised-line stimulus under a stationary finger without kinesthetic information would be enough to result in a report of that angle in preference to the rotated version of the angle experienced at the moving fingertip when this finger was deprived of cutaneous input.
The data from 16 volunteers (i.e., university undergraduates and members of the general public) 18 to 46 years of age (M = 22.3 years, SD = 7.1), 15 of whom were female and one male, were used. The responses of three additional participants (i.e., not counted among the 16 participants mentioned here) were removed prior to the analysis because of a failure to accurately identify one or more of the stimuli. Specifically, one participant felt that she was exploring circles, another felt as though the stimuli formed a straight line, and one felt that one of the angles was facing upward when, in fact, it was pointing to the right. None of the participants had any long-term or extensive experience with Braille, raised-line drawings, or tactile devices, nor did they report any motor or neurological abnormalities. All experiments presented here were carried out in accordance with the regulations of the Monash University Human Research Ethics Committee.
Pairs of raised-line stimuli were prepared by drawing on sheets used in raised-line drawing kits in a manner similar to that of Magee and Kennedy (1980). Each arm of the raised-line stimulus was approximately 7.5 cm long, and the angle between the arms was 90º. The stimulus pairs were >/< and ∧/∨, where the two stimuli of each pair were identical but rotated 180º. A sheet of unscored raised-line paper was used for conditions in which no tangible line was present (i.e., the shear condition).
The tactile display system (TDS) has two finger holders and two associated stimulus trays (see Fig. 1). The lower finger holder can be used to either passively guide a participant’s finger over a stationary stimulus or, as was the case in this experiment, to grip a participant’s finger while the participant actively explores a stationary stimulus. The upper finger holder keeps a finger stationary, while a stimulus is moved underneath it. The fingers are, on average, ~16 cm from each other. The upper stimulus tray is attached to the lower finger holder so that they both move at the same time. However, if the lower finger moves in the shape of a ∨, then, relative to the stationary upper finger holder, the upper stimulus traces out the shape of a ∧. Owing to the physical construction of the device, the shape experienced by one finger is a 180º rotation of what is felt at the other. Even so, each participant felt the raised line of any part of the stimulus at both fingers simultaneously. For example, in Fig. 1, the lower finger is at the intersection of the ∨. Simultaneously, the upper finger is at the intersection of the ∧—the corresponding position. If the next movement for the lower finger were to follow the line on the left-hand side of the ∨ to its endpoint (i.e., moving away from the body), the top finger would simultaneously sense the line on the right-hand side of the ∧ to its endpoint.
The reader can experience the effect by tracing out one of the stimuli mentioned with the index finger of the left hand (i.e., let this finger move in space toward and away from your body) and placing a finger from their right hand, and keeping it stationary in space (i.e., it is always the same distance from the body and in front, say, of the body midline) on the center of the back of the left hand. If one does this, it follows that moving the left hand forward and to the left, with the index finger of the stationary right hand in contact with the top of it, the skin of the right index fingertip will feel a shear force similar to that which would occur if the right index finger moved across a stationary surface toward the body and to the right. The shear forces at the moving fingertip indicate motion in a ∨ trajectory, whereas the shear forces experienced at the stationary right fingertip indicate the inverse (i.e., a ∧). Hence, a ∧ on the upper display connected to one hand can be paired with its inverse on the lower display, connected to the other hand. These two shapes, one the inverse of the other, are the distal objects, and the issue in the present study was which of the distal shapes the observer detected.
The participants were shown the TDS and how the device worked in both upper (stationary finger holder) and lower (moving finger holder) modes prior to the commencement of testing. The participants were also shown how the device worked in the dual-finger mode (i.e., a moving finger in the lower finger holder can move a stimulus underneath a stationary finger in the top finger holder) but, as a stimulus was not placed in the upper tray, reference to 180º rotations was avoided. Participants were also given an opportunity to feel a raised-line stimulus. At the conclusion of the instructions, participants were blindfolded and asked to place an index finger in each finger holder. They were told which finger (left or right) should be used in each finger holder—this first positioning of the fingers was counterbalanced across participants. This variable (i.e., finger positioning) was administered on a within-subjects basis and had two levels (i.e., left vs. right finger in the upper finger holder).
The participants were told that they would explore an angle with both fingers at the same time, but that one finger would be moving and the other would be stationary, with the stimulus moving underneath it. They were then told that their task was to identify “which way is the angle pointing?” by way of verbal report; no mention was made by the experimenter of the 180º rotation of the stimuli. Participants were not directed to focus their attention on either hand in particular, nor were they given any indication that there might be a difference between the percepts arising from each hand. Each participant explored each pair of stimuli, with one of each pair in the upper tray and the other in the lower tray. Thus, each participant underwent a total of six trials in this experiment (i.e., a single presentation of each condition), examining the same pair of stimuli on each trial—the goal of which was to improve performance and avoid floor effects in a novel task. As an example, a participant got only a ∨ in the upper tray and its inverse in the lower tray; he or she would never get a different pair (e.g., a ∧ in the upper tray and its inverse in the lower tray). Thus, stimulus combination was a between-subjects variable with four levels.
A separate variable (i.e., mode of delivery) was administered on a within-subjects, pseudorandomized basis. It had three levels:
Kinesthesis + Line + Shear: The participants could feel a line under both the active, participant-controlled moving index finger and the stationary index finger simultaneously.
Kinesthesis + Shear: The participants could feel a line under the stationary index finger but the active, participant-controlled moving index finger could feel only a lightly textured plastic surface—there was no line but cutaneous shear forces (i.e., signals produced when the skin and a surface are laterally shifted in relation to each other) remained due to friction between the blank raised-line sheet and the fingertip. In this condition and the next, the feedback from the stationary finger (i.e., the finger in contact with the line) was relied upon to guide the action of the moving finger.
Kinesthesis: The participants could feel a line under the stationary index finger but the actively moving index finger received no cutaneous information—it moved through air only. Thus, the guide for the moving finger was the line in contact with the stationary finger. So, although the participant-controlled moving finger could move in any direction, it was guided by the contact between the stationary finger and the line. To clarify, the upper stimulus tray was attached to the lower finger holder so that they both moved at the same time. If the upper stimulus traced out the shape of a ∧, the lower finger moved in the shape of a ∨ (see Table 1 for a full list of conditions, which were randomized within subjects). Though this is a complicated setup, all of the participants understood that they were required to use input to the stationary finger to control the moving finger, and thereby to cause the raised line to move under the stationary finger. They all maneuvered this stimulus smoothly and with ease.
In all conditions, the participants had control over the movement of the finger in the lower finger holder. In a bid to determine whether participants were aware of the discrepancy between what they experienced at the moving and stationary fingers, the participants were asked whether they had noticed anything odd at the completion of the experiment. Latencies were not recorded, as each participant was given sufficient time to provide a response, though this never took more than a minute. We debriefed participants at the end of the experiment.
The primary question of interest was which of each stimulus pair the participant would name. As is shown in Table 2, nearly all participants reported the stimulus present at both fingers when the line was present under the moving finger; they made a statement akin to “under my left finger is an arrow pointing up, while under my right finger is an arrow pointing down.” This suggested that kinesthetic and cutaneous stimuli together were as effective as a cutaneous stimulus alone. The vast majority of the participants reported the stimulus present at the stationary finger (i.e., the finger that received only cutaneous information) when the raised line was not present at the moving finger (i.e., when either kinesthesis + shear or kinesthesis only was presented at this finger), and thus they failed to notice the kinesthetic and/or shear stimuli present at the moving finger.
When cutaneous information was present at both the moving and stationary fingers (i.e., kinesthesis + line + shear in Table 2), the odds in favor of identifying both stimuli (i.e., the stimuli at the stationary and moving fingers) were significantly higher than the odds in favor of identifying both stimuli when a raised line was present at the stationary finger and kinesthesis + shear information was present at the moving finger (odds ratio = 960.99, z = 3.92, p < .001). In the latter condition, participants usually identified the shape of the cutaneous stimulus.
A second odds-ratio analysis found that the odds in favor of identifying both stimuli if the participant was provided with a stimulus composed of kinesthesis + line + shear at the moving finger were significantly higher than the odds in favor of identifying both stimuli in the kinesthesis-only condition (odds ratio = 3,100, z = 7.81, p < .001). Again, participants usually identified only the cutaneously presented angle.
When participants had a kinesthesis + shear stimulus at the moving finger, the likelihood of a response identifying both stimuli was not significantly higher than that of identifying both stimuli if the participant were provided with a kinesthetic-only stimulus (odds ratio = 3.00, z = 0.77, p = .20).
Cutaneous capture of kinesthetic information was indicated in this experiment. Participants were significantly more likely to acknowledge their movements if those movements were paired with cutaneous information. However, the participants seemed unaware of the pattern of their self-generated finger movements when a raised line was present at the stationary finger and kinesthesis + shear or kinesthesis-only was present at the moving finger. The participants appeared to attribute the kinesthetic input (as secondary information) to the cutaneous input (which was primary for control, since it indicated whether one was on or off the target line).
The participants reported the directions in which both stimuli pointed when the raised line was present at both fingers. This contrasts with findings reported by Symmons et al. (2007), who found that only one stimulus was reported. Given that postexperiment questioning by Symmons et al. (2007) revealed that active explorers were aware of a discrepancy between the stimuli (although not necessarily of what the discrepancy was), it seems likely that their participants responded to what they deemed to be the most salient stimulus. The difference between Symmons et al.’s (2007) findings and those reported here may be due to the difference in stimuli. Symmons et al. (2007) used more complex letters and numbers, whereas simple angles were used here. We speculate that simple stimuli may ease cognitive load and allow attention to be focused on both stimuli in this condition.
It remains a possibility that response confusion confounded this effect. Perhaps, because of the instruction to identify the direction in which the arrow was pointing, participants mistakenly thought to primarily report the raised line and, consequently, reported the raised line at the stationary finger when no raised line was present at the moving finger. The next experiment was designed to address this concern about response confusion; a fortunate consequence of altering the instructions was that we were also able to assess the role that endogenous attention plays in intrahaptic capture.