TACTUAL DISCRIMINATION OF SOFTNESS

Citation
Ma. Srinivasan et Rh. Lamotte, TACTUAL DISCRIMINATION OF SOFTNESS, Journal of neurophysiology, 73(1), 1995, pp. 88-101
Citations number
13
Categorie Soggetti
Neurosciences,Physiology,Neurosciences,Physiology
Journal title
ISSN journal
00223077
Volume
73
Issue
1
Year of publication
1995
Pages
88 - 101
Database
ISI
SICI code
0022-3077(1995)73:1<88:TDOS>2.0.ZU;2-C
Abstract
1. We investigated the ability of humans to tactually discriminate the softness of objects, using novel elastic objects with deformable and rigid surfaces. For objects with deformable surfaces, we cast transpar ent rubber specimens with variable compliances. For objects with rigid surfaces (''spring cells'') we fabricated telescoping hollow cylinder s with the inner cylinder supported by several springs. To measure the human discriminability and to isolate the associated information-proc ressing mechanisms, we performed psychophysical experiments under thre e conditions: 1) active touch with the normal finger, where both tacti le and kinesthetic information was available to the subject; 2) active touch with local cutaneous anesthesia, so that only kinesthetic infor mation was available; and 3) passive touch, where a computer-controlle d mechanical stimulator brought down the compliant specimens onto the passive fingerpad of the subject, who therefore had only tactile infor mation. 2. We first characterized the mechanical behavior of the human fingerpad and the test objects by determining the relationship betwee n the depth and force of indentation during constant-velocity indentat ions by a rigid probe. The fingerpad exhibited a pronounced nonlinear behavior in the indentation depth versus force trace such that complia nce, as indicated by the local slope of the trace. decreased with incr eases in indentation depth. The traces for all the rubber specimens we re approximately linear, indicating a constant but distinct value of c ompliance for each specimen. The fingerpad was more compliant than eac h of the rubber specimens. 3. All the human subjects showed excellent softness discriminability in ranking the rubber specimens by active to uch, and the subjective perception of softness correlated one-to-one w ith the objectively measured compliance. The ability of subjects to di scriminate the compliance of spring cells was consistently poorer comp ared with that of the rubber specimen's. 4. For pairwise discriminatio n of a selected set of rubber specimens, kinesthetic information alone was insufficient. However, tactile information alone was sufficient, even when the velocities and forces of specimen application were rando mized. In contrast, for discriminating pairs of spring cells, tactile information alone was insufficient, and both tactile and kinesthetic i nformation were found to be necessary. 5. The differences in the suffi ciency of tactile information for the discrimination of the two types of objects can be explained by the mechanics of contact of the fingerp ad and its effect on tactile information. For objects with deformable surfaces, the spatial pressure distribution within the contact region depends on both the force applied and the specimen compliance. Consequ ently, for a given net force, skin deformation is dependent on specime n compliance and tactile information is able to encode the compliance of objects with deformable surfaces. For compliant objects with rigid surfaces, the pressure distribution and skin deformation for a given n et force are independent of object compliance and therefore tactile in formation alone is not sufficient to encode their compliance.