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.