tional differences between the 5 classes of objects used are based on tiny details in the lower centimeter-range, which require at least basic tactile acuity. Sensory-cognitive link in later life The results reported herein support recent findings of a sensory- cognitive link found in the auditory, visual, and tactile domains in healthy aging adults [66,92,53]. Furthermore, our findings are in line with studies that investigated task complexity and the sensory- cognitive link in old age, as it is commonly found that more cognitively demanding tasks correlate more highly with measures of intelligence (‘‘g’’), than do tasks measuring simple sensory detection thresholds [93–95]. In the above-mentioned domains, basic measures of absolute sensory thresholds, such as pressure threshold sensitivity, hardly rely on cognitive resources. However, more cognitively demanding processes, such as the current haptic task, which involves processes of comparison, judgment, and mental rotation, show higher correlations with intelligence (‘‘g’’) [92]. Conclusion We have demonstrated that the age-related decline of cross- modal haptic performance, which occurs in late adulthood, is primarily related to cognitive functioning, rather than to tactile acuity. The results presented here support the view that the haptic object exploration process is structured in 2 phases: a first phase where a global understanding of the explored object is built up (‘‘general exploratory procedure,’’ [5]) and a second phase where specific object details are explored (‘‘specific exploratory proce- dure,’’ [5]). Tactile acuity seems to be a predictor of haptic performance, but only if the cognitively demanding first phase is completed successfully. Materials and Methods Subjects We tested 81 right handed volunteers aged 45 to 94 years (32 males, mean age 67.69611.22 years; 49 females, mean age 67.06610.58 years; t-test: p = 0.800). In all subjects, the educational level (number of school years and training) was balanced (males 12.0862.12 years; females 11.2062.52; t-test: p = 0.109). All subjects were neurologically healthy, as assessed by a neurologist. Individuals with polyneuropathy, peripheral nerve lesions, carpal tunnel syndrome, or other neurological disorders were excluded from the study. Eligibility criteria were lucidity, independence in activities of daily living, absence of motor handicaps such as functional impairments due to arthritis, or other causes of joint immobility. Furthermore, medication with central nervous effects in subjects’ present or recent reported history (past 5 years) was a criterion for exclusion. Tactile sensitivity of the subjects’ hands was checked prior to the experiments as a check for peripheral neuropathies [44]. Additionally, basic cognitive abilities were assessed using the ‘‘Mini Mental State Examination’’ [96]. Subjects with a score lower than 28 points were excluded. This study was approved by the Ethics Committee of the Ruhr- University of Bochum, and all subjects provided written informed consent before participating. Cross-modal haptic object recognition test The custom-made test consisted of 5 different sets of unfamiliar objects made from LEGOTM bricks [44,43]. The use of unfamiliar, instead of common objects, prevents the influence of prior knowledge of structural information, and creates a comparable initial test-situation for all subjects. Each object was Haptic Performance in Later Life constructed as a cuboid (1.5*2.7*4.7 cm) with an individual number and position of rectangular structures on the sides. These constructional differences were highlighted in terms of color. All objects had a smooth surface structure on the sides, a plain bottom side, and a nubby upper side that could be used as tactile cue for orientation during haptic exploration (fig. 5). To prevent the objects from falling apart, all components were glued together. One sample of each set was placed on a desk in front of the subject. The viewing conditions were full-cue, and the objects were binocularly viewed by observers under ample lighting [17]. In a familiarization phase, individual haptic and visual exploration of the objects was allowed. Afterwards, a total of 17 objects, hidden in a small fabric sac (20 * 30 cm), were explored by haptic perception only. For this aim, participants were asked to hold the sac with their left hand underneath the desk, while explorative movements were exclusively performed with the right hand. Each object had to be allocated to one of the visible samples on the desk, by removing it from the sac and placing it in a container behind the specific sample. No visual verification was permitted. The subject was instructed to perform as quickly and as accurately as possible (time limit of 4 minutes per session; remaining objects were considered to be errors). After a familiarization training (3 consecutive sessions), all subjects indicated good comprehension of the test. The estimation of individual performance was done by counting the number of errors occurring in the fourth test session. Two-point discrimination test Spatial two-point discrimination thresholds were assessed on the tips of all fingers of the right hand using the method of constant stimuli as described previously [44,45,97,77,76,78]. We tested 7 Figure 5. Objects used for the haptic object recognition test. Five groups of unfamiliar objects made from LEGOTM bricks were used for the haptic object recognition test (upper row of photo: view from the backside; lower row of photo: view from the front). In each group, the objects consisted of a cuboid (1.5 * 2.7 * 4.7 cm) with a specific number and position of rectangular structures (each 1.6 * 0.8 * 1 cm (4 in group a; 3 in group b and c, 2 in group d and e)) on the sides (marked in red, c.f. schematic drawing). A total of 17 objects (3 * a, 5 * b, 4 * c, 3 * d, 2 * e) were used for the test. doi:10.1371/journal.pone.0030420.g005 PLoS ONE | www.plosone.org 8 January 2012 | Volume 7 | Issue 1 | e30420