Chapter 2. Prehension

258 THE PHASES OF PREHENSION
see Hepp-Reymond (199 l), Hepp-Reymond, Wannier, Maier and
Rufener (1989), Lemon, Muir and Mantel (1987), Muir and Lemon
(1983), Smith (1979, 198l), Smith, Frysinger, and Bourbonnais
(1983). As an example, Smith (1981) provides evidence that the
cerebellum plays an important role in switching between reciprocal
inhibition and coactivation of antagonist muscles in the hand during
grasping.
6.4.3 Slips, perturbations and magnets
In Westling and Johansson’s task, slips were rarely visible or
noticed by the subjects, but were inferred from accelerometer records.
Corresponding afferent slip responses were reliably observed from
FAI, FAII, and SAI afferent units, and were reliably absent from SAII
units. Occasionally, even in the absence of accelerometer evidence,
slips were inferred based on mechanoreceptive afferent activity in SAI
and FA1 units when the grip-forceboad-force ratio was dangerously
near the slip ratio. These were called localized afferent slip responses.
Johansson and Westling (1987) noted that 10% of the afferent slip
responses in FA1 and SAI units occurred prior to overt slips causing
accelerometer responses. Functionally, these slips resulted in a
consistent upgrading of the grip-force/load-force ratio, to ensure
stability. The latency between slip onset and the ‘automatic’ changes
in the force ratio were between 60 and 80 ms with a mean of 74 (+ 9)
ms. Interestingly, similar to the phase dependence of cutaneous input
during postural and locomotor activities, there is a phase dependent
contribution in grip and load forces to this upgrading of the force ratio.
During the loading phase (while the object is still supported by the
table), changes in the load force rate were involved, whereas during
the static phase (while the object is held stationary in the air), the grip
force increased to a higher stable value (Johansson & Westling,
1990).
In Chapter 5, the effects of visual and mechanical perturbations
on arm movement and hand configuration during the free motion
phase were examined. Similar experiments have been conducted
during a grasping mal, perturbing: the vertical load force of obiects.
Cole & Abbs (1988) showed that rapid, unexpected increases or
decreases in object load force (trapezoid pulse: 15 ms ramp, 50 ms
duration, increase of about 2.5 N to a 0.7 N object) bring about
corresponding increases or decreases in grip force with a latency of 60
- 90 ms with a mean of 74 (+ 11) ms. Phasic EMG bursts occurred