Somatosensory Perception of Objects Requires Active Manipulation

In monkeys, lesions of somatosensory cortex impaired the ability of the animal to discriminate the form, size, and roughness of tactile objects (Norsell, 1980). Each of these impairments could be localized to a subregion of somatosensory cortex: lesions in one area affected mainly the discrimination of texture, lesions in a second area impaired mainly the discrimination of angles, and lesions in a third area affected all forms of tactile discrimination (Randolph and Semmes, 1974).

Scientists have recorded the activity of neurons in somatosensory cortex of monkeys while exposing the animal’s hands to complex stimuli (Darian-Smith et al., 1980). Metal strips of varied widths and spacing were moved at different rates of speed under the fingertips of monkeys while recordings were made from sensory nerves. No single fiber could give an accurate record of each ridge and depression in the stimulus, but the ensemble of fibers provided an accurate representation.

Some somatosensory cortical cells could not be activated when the experimenter stimulated either skin or joints but responded strongly when the animal grasped an object and manipulated it. Some of these “active touch” cells had highly specific response characteristics (Iwamura and Tanaka, 1978). For example, one unit responded actively when the monkey felt a straight-edged ruler or a small rectangular block but did not respond when the monkey grasped a ball or bottle. The presence of two parallel edges appeared to be crucial for effective activation of this cell. Another cell responded best when the monkey grasped a ball or a bottle, but did not respond at all when the monkey manipulated a rectangular block.

PET studies have shown that, in humans, the posterior part of the parietal cortex is activated during exploration of objects by touch (E. Roland and Larson, 1976). Only when the subject explored an object by touch was the posterior parietal cortex specifically activated, suggesting that this region is particularly involved in active touch.