The medium (one-eighth volume) and small (one-twenty-seventh volume) sets of bell pepper replicas were printed in PLA plastic (polylactic acid) using a Bits From Bytes 3D Touch printer following Laser Scanning (NextEngine Laser Scanner) of the full-sized objects (see “Experimental stimuli” section). An Apple MacBook computer was used to randomly order the presentation of the experimental stimuli and record the participants’ responses.
The basic procedures for the shape discrimination task were similar to those used by Experiment 2 of Norman et al.7, Experiment 1 of Norman et al.8 and Norman et al.3. On each trial, participants reached behind an occluding curtain to haptically explore two stimulus objects sequentially (presented for 3 s each, separated by a 3-s interstimulus interval, ISI). The objects were presented in a random orientation each time, and the participants used both hands to actively explore each object’s shape. After both objects had been presented, the participants’ task was to indicate whether the objects possessed the same shape or had different shapes (regardless of size). Each participant made a total of 96 shape discrimination judgments (48 same trials, and 48 different trials), where the order of the same and different pairs of objects was determined randomly. As in some of our previous investigations11,12, only the most difficult (i.e., most confusable, see Fig. 1) object pairs (objects 1 and 3, objects 1 and 7, objects 2 and 11, objects 3 and 7, objects 3 and 8, and objects 5 and 12) were presented to the participants during different trials (each of these 6 different object pairs was presented 8 times throughout the entire block of 96 trials; during each of the 48 same trials, one of the 8 stimulus objects was randomly selected and presented twice).
There were three between-subjects’ conditions. For some participants, the two objects to be discriminated on each trial possessed the same size (both were of the medium size). For two other groups of participants, they had to discriminate object shape irrespective of differences in size: one group judged objects that differed in size by one step (either medium and large or small and medium) while another group judged objects that differed in size by two steps (small and large objects). For the groups that judged pairs of objects that differed in size, the larger of the two objects (e.g., medium size for people who judged small and medium objects) was sometimes presented first (with 50% probability) and sometimes presented second (also with 50% probability).
In addition to object shape discrimination, we evaluated the participants’ manual dexterities using a modified version of the Moberg pick-up test8,25,26,27,28,29. For this task, the younger and older participants picked up 12 small metal objects (e.g., nail, paperclip, coins, flat-head screw, a key, a wing nut, etc.) and placed them one at a time into a container as fast as possible; the total time required to place the objects in the container was recorded. The participants performed this test both with and without vision. The participants’ dexterity was assessed twice (twice with vision and twice without); their best performance (i.e., fastest times) was included in the analysis.
Seventy-two younger and older adults participated in the experiment (24 for each of the three between-subjects’ size conditions described earlier). Thirty-six of the participants were older (M = 73.5 years of age, SD = 4.9, range = 65 to 82 years) and 36 were younger (M = 21.5 years of age, SD = 2.5, range = 18 to 32 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 each participant signed an informed consent document prior to testing. Our research was carried out in accordance with the Code of Ethics of the World Medical Association (Declaration of Helsinki).