We tested participants’ ability to accurately discriminate a single target vibration cue from a sequence of vibrations at different spatial locations on the forearm. We measured participant accuracy as the proportion of correct responses out of all trials presented in each condition while varying focus direction and agency. For this study, focus direction refers to the orientation of the participant’s focus, either spatially across the skin or across two sensory modalities (touch and sound). Agency refers to the participant’s ability to directly control the onset of stimulation or lack thereof. Our objective was to determine if these variations in focus direction and agency affected participants’ ability to complete the task accurately.
A total of 18 participants (7 female, 14 right-handed, 19–28 years old, average age 24) took part in this study. All participants in the study were healthy adults and did not suffer any cognitive or motor impairment that would affect their ability to perform the experiment. All participants gave informed consent and all procedures and methods of the experimental protocol were approved by the Rice University Institutional Review Board (IRB-FY2019-49).
2.2 Experimental Hardware
This experiment used a modified version of the Vibro-Tactile Sleeve (VT-Sleeve) used in Macklin et al. . The VT-Sleeve features 6 vibrotactile actuators (Compact Audio Exciter; Tectonic; Part No. TEAX14C02-8) embedded in a compression sleeve (Under Armour) with custom 3D-printed housings. The vibrotactors were each press-fit into their respective 3D-printed housing, which clipped into slits in the compression sleeve. The vibrotactors were placed 60mm apart on the sleeve when relaxed with 3 vibrotactors on the ventral side of the arm and 3 vibrotactors on the dorsal side. The sleeve held each vibrotactor snugly against the skin as shown in Fig. 1. The sleeve was adjusted on all participants such that the vibrotactors on each side of the forearm were sufficiently separated to satisfy the two-point discrimination threshold reported for successive touch stimuli . Wiring powering each vibrotactor was routed through another small slit in the compression sleeve directly next to the housing element, ensuring no direct contact between the wiring and skin. These wiring elements connected to a quick release ribbon cable connector, which in turn connected the tactors to a custom amplifier.
All tactile cues were 250 ms envelope sine waves presented 400 Hz for the target cue and 100 Hz for the distraction cues. Extensive pilot testing was done to verify that these characteristics resulted in clear and distinct vibrotactile cues. Tactile cues were rendered with Syntacts using a digital-to-analog converter (MOTU 24Ao USB) with signals amplified through the Syntacts V3.0 amplifier .
2.3 Experimental Conditions
The experimental task required participants to identify the location of the target cue out of a sequence of 6 vibrotactile cues delivered on the forearm. The cue sequence was randomized such that each vibrotactor vibrated once and the target cue could appear at anytime during the sequence. All sequences had one target cue and five distraction cues. All cue durations were 250 ms, with a 400 ms pause between the presentation of each cue. After each sequence, the participant used their mouse to indicate which vibrotactor they thought delivered the target cue to the forearm. This basic tactile task varied in each condition to shift focus direction or to adjust agency over sequence presentation in each trial. These variations made up the 6 conditions in this experiment to investigate the various combinations of focus direction and agency.
Focus direction was split into three categories: double focus, where participants were given a separate auditory task to perform simultaneously with the basic tactile task (as described above); wide focus, where the participant performed the basic task; and narrow focus, where the participant was told which side of the forearm the target would appear and therefore would only consider 3 of the 6 vibrotactors. The double focus task presented participants with the basic tactile task while simultaneously asking them to perform an auditory task that started in tandem with the tactile sequence. This auditory task presented a sequence of 6 auditory beeps 440 Hz each separated by 750 ms through the participant’s headphones. These beeps were presented with two perceptually distinct volumes: one quiet and one loud. The participant was asked to count the number of loud beeps out of the 6 in the sequence while simultaneously performing the basic tactile task. While each participant was instructed to perform both tasks to the best of their ability, we report participant accuracy in completing the tactile task only, as the purpose of the auditory task was only to direct focus across two tasks. The wide focus category presented the basic tactile task with 6 cues in sequence to the participant and asked them to identify the location of the target cue. The narrow focus task informed the participant about which side of the forearm the target cue was going to appear via arrow icons presented in the GUI. This directed the participant’s focus onto the 3 tactors on one side of the forearm. The participant then received the sequence of 6 cues and was asked where the target cue appeared.
Agency variations were split into only two categories: active, where the participants had a button to start the vibrotactile sequence; and passive, where the participants were allowed 30 s after each sequence was displayed to provide their response before the trial automatically advanced. The conditions in this experiment crossed these two categories such that there were 6 conditions for each participant presented in a random order: double focus passive, double focus active, wide focus passive, wide focus active, narrow focus passive, and narrow focus active.
Participants were seated in front of a computer screen with their left hand placed on the table and their right hand using a computer mouse. The participant wore the vibrotactile sleeve on their left forearm as described in Sect. 2.2. The participant wore noise canceling headphones playing pink noise throughout the experiment to isolate them from any outside auditory stimulation. After receiving instructions from the experimenter, the participant interacted with a GUI on the computer to begin the experiment. Participants started with 5 practice trials and received feedback on the correct response in each trial. After practice, participants responded to 20 identification trials in a randomly selected condition as described in Sect. 2.3. Participants repeated this process for all six conditions.