Chapter 2. Prehension

Chapter 6 - During Contact 253
higher rate, replicating Johansson and Westling. Peak grip and load
forces were achieved after liftoff, i.e., after the object broke physical
contact with the supporting table surface.
In marked contrast to the effects of dowel weight during the
loading phase, Weir (1991) found that during the unloading phase,
after (re)placement of the dowel, the rate of decrease of grip force
increased with object weight, mirroring the effects during the loading
phase. At the end of unloading and prior to release, subjects again
pushed the object into the table, only with a greater load force for the
lighter dowels (66 g dowel: -0.65 N, 155 g: -0.55 N) than for the
heavier dowel (423 g: -0.3 N). This may be reflecting the need to
stabilize the lighter object as it was replaced onto the table.
As well as the forces described above, Johansson and Westling
measured single tactile afferent units (using microneurographic
methods, after Vallbo & Hagbarth, 1968) and surface
electromyography. Of interest is their conclusion that Type I units
monitor the frictional demands for modulation of the grip during
interaction with the object. Johansson & Westling (1990) summarized
3 sets of evidence indicating that tactile afferents having endings in the
area of manipulative contact (Type I) are essential for proper
adaptation of the grip force/load force ratio to the friction between the
object and skin surfaces. They found: (1) when afferent signals were
abolished by local anaesthesia, adaptation failed (Johansson &
Westling, 1984b, Westling & Johansson, 1984); (2) electrical
stimulation of tactile afferents triggered changes in the grip forceboad
force ratio; and (3) signals in tactile afferents appeared well correlated
with subsequent alterations in the grip force/load force ratio
(Johansson & Westling, 1987, Westling & Johansson, 1987).
Johansson & Westling (1984b; 1990) further suggested that this
tactile afferent information is necessary for the phase transitions in the
sequential unfolding of the task. Table 6.3 indicates mechanoreceptive
afferent unit activity at critical points in the lifting task. Frictional
aspects (surface texture) appear to affect firing frequency, specifically
of FA1 units during the loading phase. Weight aspects are not
reported to affect unit firing systematically. Included in the table are
only reliable, obligatory responses, in the sense that mechanoreceptive
tactile afferents appear to provide these functionally related responses
consistently (Westling, 1986). Note that of the phases identified
above, there are corresponding afferent impulses in FAI, SAI, and
FA11 units, not SAII units. Activity corresponding to transitions at
contacting and releasing the object are found in all 3 unit types; in
contrast, activity corresponding to transitions at object lift and replace