Haptic Performance in Later Life Figure 6. Psychometric function for two-point discrimination performance. Example of the typical two-point discrimination performance of an older adult. Correct responses in percentages (blue squares) are plotted as a function of needle distances. Based on these results, a logistic regression is calculated (red squares). The 50% level of correct responses (dashed line) determines the individual 2-point discrimination threshold (arrow). doi:10.1371/journal.pone.0030420.g006 pairs of brass needles; in addition, zero distance was tested with a single needle. To overcome problems in the use of two-point measurements associated with hand-held probes, we used a specifically designed apparatus that secures a standardized form of testing (see figures in [45,67]). The apparatus allowed rapid switching between pairs of needles featuring different separations or 1 single needle (control condition). All tactile stimuli were applied to a fixed position on the skin of the fingertips for approximately 1 s. To account for the age-related decline in tactile acuity [41,38,44,45,67], we used different settings of the two-point discrimination set-up for subjects below 60 years of age (1.0, 1.4, 1.8, 2.2, 2.6, 3.2, and 4.0) and subjects aged 60 years and older (1.5, 2.3, 3.1, 3.9, 4.7, 5.6, and 7.0 mm). The diameter of the needles was 0.7 mm, and the diameter of the blunt endings was 200 mm. Application force was approximately 150 to 200 mN. Fixation of the tested fingers prevented explorative finger movements. All 8 test conditions were presented 8 times in a randomized order, resulting in a total of 64 tests per session. The subjects, who were not informed of the ratio of needle-pairs to single needles (i.e., 7:1), had to decide immediately whether they had the sensation of 1 or 2 needles. They were instructed to classify the percept of a single needle or doubtful stimuli as ‘‘1’’ but the distinct percept of 2 stimuli as ‘‘2.’’ The summed responses were plotted against the needle- distances, resulting in a psychometric function, which was fitted with a binary logistic regression (SPSS; SPSS Inc., USA). Threshold was taken from the fit where 50% correct responses were reached (fig. 6). All subjects had to attend 2 training sessions to get used to the testing procedure before the assessment was started in the third session. All subjects who participated in the present study knew the two-point discrimination test from previous studies. Raven Standard Progressive Matrices test The Raven Standard Progressive Matrices (RSPM) test is among the most widely used and researched non-verbal tests of References intelligence [98]. Compared to other tests, RSPM scores are recognized as reliable estimates of general intelligence (Spearman’s g factor) [99]. We applied the paper-and-pencil version of the RSPM to the subjects. The 5 sets (A, B, C, D, and E) of tasks are arranged according to the principles of increasing complexity [100]. In each task, a specific pattern or a number of geometrical structures are presented, with 1 part of the pattern or 1 component of the structures missing. On the basis of 6–8 presented solutions the subject has to decide which one is appropriate to complete the given pattern or set of structures. The RSPM was applied in a ‘‘speed-version,’’ which measures performance within a time limit of 30 minutes (remaining items were considered to be errors). Statistical analyses We investigated age-related changes in haptic, tactile, and cognitive performance using correlational analyses (one-tailed Pearson correlations and second order partial correlations, controlling for factor AGE) and repeated measures ANOVA for factors AGE and GENDER (2 by 2 factorial design). ANOVAs were calculated based on subsamples of the population. We allocated subjects aged 45 to 59 years to the group of ‘‘younger adults’’ (younger female adults ‘‘yf’’: N = 29, 60.2467.07 years; older female adults ‘‘of’’: N = 20, 76.9565.94 years) and subjects aged 60 to 94 years to the group of ‘‘older adults’’ (younger male adults ‘‘ym’’: N = 15, 58.53 years; older male ‘‘om’’: N = 17, 75.7666.71 years). Author Contributions Conceived and designed the experiments: TK MT HRD. Performed the experiments: TK J-CK RK. Analyzed the data: TK. Contributed reagents/materials/analysis tools: MT HRD. Wrote the paper: TK J-CK HRD. 1. Klatzky RL, Lederman SJ, Metzger VA (1985) Identifying objects by touch: an ‘‘expert system’’. Percept Psychophys 37(4): 299–302. 2. Lederman SJ, Klatzky RL (2009) Haptic perception: a tutorial. Atten Percept Psychophys 71(7): 1439–1459. PLoS ONE | www.plosone.org 9 January 2012 | Volume 7 | Issue 1 | e30420