Apparatus

A custom-built apparatus consisting of an electric motor and slider-crank mechanism was used to control the movement of the stimulus objects in one of the two experimental conditions. An Apple MacBook computer was used to randomly order the presentation of the experimental stimuli and record the participants’ responses.

Experimental Stimuli

The stimulus objects were machine-milled polyvinyl chloride (PVC) plastic blocks (20 cm × 2 cm × ~5 cm) that have been used extensively in previous research7,15,16,17. The blocks featured convex or concave top surface curvatures, which ranged in magnitude from 0.2 to 2.2/m in increments of 0.4/m (see Fig. 1). Tactile gratings (JVP Domes, Stoelting, Inc.) with groove widths ranging from 0.75 to 6.0 mm were used to assess the participants’ tactile acuity. The 12 small objects used in the Moberg pick-up test of manual dexterity were the same as those used previously9,12.

Procedure

The basic procedures for the curvature discrimination task were similar to those used by Norman et al.7. On every trial, participants reached underneath an opaque curtain to feel the top surface of a stimulus block; they then judged its curvature to be either convex or concave. The participants performed this task using both active and passive touch. Each participant performed the touch conditions in an order that was counterbalanced across all participants (i.e., half of the participants used active touch first, while the remaining half used passive touch first). In the active touch condition (Fig. 2a), the blocks remained in a fixed position while the participants used their index finger to laterally explore their top surfaces. An aperture was used to limit exploration to the middle 10 cm extent of the blocks. The passive touch condition employed a procedure similar to that used by van der Horst et al.17 and Smith et al.18. As in the active touch condition, only the index finger contacted the blocks, but in this condition the blocks moved (i.e., translated) underneath and perpendicular to the long axis of the finger (see Fig. 2b). In order to prevent active manipulation, the participants’ hand, wrist and arm were restrained into a fixed position; the participants’ index finger could only move up and down (to maintain contact with the block). After the participant’s hand and arm were secured, the blocks then translated ±5 cm relative to the fingertip at an average rate of 10 cm per second. Under these circumstances, the index finger passively felt the same 10 cm extent of the block that could be actively felt in the active condition.

The testing in each condition began with a block of trials evaluating each participant’s ability to discriminate convex and concave curvature magnitudes of 2.2/m. Subsequent blocks of trials evaluated discrimination of curvature magnitudes in descending increments of 0.4/m (e.g., curvature magnitudes of 2.2, 1.8, 1.4, 1.0, 0.6 and 0.2/m). The order of presentation of concave and convex stimuli within each block was randomly determined and there was an equal probability of presenting a convex or concave stimulus on any given trial. For each individual participant, discrimination performances above and below a d′19 value of 1.35 were obtained; these two d′ values were then used to calculate a threshold estimate (i.e., the curvature magnitude needed to discriminate at a d′ value of 1.35) using linear interpolation. In order to reduce the total number of trials to a manageable number, the participants initially completed blocks of 12 trials for each curvature magnitude. If a participant made 10 or more correct judgments, testing would begin again with a new block of trials devoted to the next smaller curvature magnitude. If fewer than 10 of the 12 trials with a given curvature magnitude were judged correctly, however, the participants would then complete a new block of 40 trials with the current curvature magnitude and all subsequent curvature magnitudes would be tested with 40-trial blocks. This procedure was utilized for all curvature magnitudes except 0.2/m (i.e., the minimum curvature magnitude of the stimulus set), which was always tested with 40-trial blocks.

Given that aging is known to reduce participants’ tactile acuity7,9,10,11, it is certainly possible that this age-related reduction could influence performance for the current tactile curvature discrimination task. Tactile acuity was accordingly assessed for all participants using grating orientation discrimination20,21. Grating orientation discrimination is a widely used task that possesses significant advantages over traditional methods used to evaluate tactile acuity, such as the determination of two-point thresholds7,9,22,23. In our study, tactile gratings were applied to the distal pad of each participant’s index finger by the experimenter for one second; on each trial, the participants judged whether the orientation of the grooves of the grating was parallel or perpendicular to the long axis of the finger. The order of presentation (parallel vs. perpendicular) within a block was randomly determined and there was an equal probability of either stimulus orientation on any given trial. For younger participants, the first block of 40 trials utilized a groove width of 3.0 mm. Subsequent blocks of 40 trials used gratings with smaller and smaller groove widths (e.g., 2, 1.5, 1.2, & 1.0 mm) until a participant’s discrimination performance dropped below a d′ value of 1.35. Threshold estimates for tactile grating orientation discrimination were then calculated in the same manner (linear interpolation) as described for surface curvature discrimination. The procedure for determining tactile acuity for the older participants was identical, except that the initial groove width was larger (e.g., 4–6 mm), since it is well known that older adults possess higher thresholds7,9,24.

In addition to tactile acuity, we evaluated the participants’ manual dexterity–any reduction in dexterity could potentially affect a participant’s ability to actively explore the stimulus blocks while performing the curvature discrimination task. To evaluate manual dexterity, we required the participants to complete a modified version of the Moberg pick-up test25,26. This task has been used previously to evaluate manual dexterity and hand function in both younger and older adults9,12,27. In this task, the participants picked up 12 small metal objects (e.g., nail, paperclip, coins, flat-head screw, etc.) and placed them inside a container as rapidly as possible and the cumulative time required to place the objects in the container was recorded. People with no substantial deficits in manual dexterity can perform this task well without seeing the objects. The participants performed this picking-up task with and without vision. This test was performed twice, with the best performance (shortest overall time) being included in the analysis.

Participants

Twenty-eight younger and older adults participated in the experiment. Fourteen of the participants were older (M = 71.4 years of age, SD = 4.9, range = 67 to 80 years) and fourteen were younger (M = 23.1 years of age, SD = 1.5, range = 21 to 25 years). All participants were naive regarding the purpose of the experiment. The study was approved by the Institutional Review Board of Western Kentucky University and was conducted in accordance with the Code of Ethics of the World Medical Association (Declaration of Helsinki). Each participant signed an informed consent document prior to testing.