Pinpointing of an Upper Limb Prosthesis
Yoshihisa Tsukamoto, MD, PhD
ABSTRACT
The case of a 42-year-old right-handed man with a right forearm amputation who has made skillful use of his work arm in using a personal computer was reported. His work arm consists of a single wall socket, which envelops the stump of his arm near the mid-humerus, a nine-figure harness, and a stick. He can tap all the computer keys with his work arm without seeing the keyboard and feels touch sense at the tip of his work arm. I postulated that this ability depends on the neural mechanism coding for the formation of body schema involving stumps and prostheses.
Key Words: Amputation; perceptual assimilation of a tool and the hand; upper limb prosthesis; body schema; neurophysiological control
Introduction
A tool is an extension of the hand in both physical and perceptual senses. The presence of body schema has been postulated as the basis of the perceptual assimilation of a tool and the hand.1,2 Iriki et al.3 found that specific neurons code for the formation of body schema involving tools during tool use in the brains of monkeys. In a similar manner, amputees often recognize the localization of prostheses as an extension of their stump or stumps. In amputees, specific neurons may code for the formation of body schema involving prostheses as they do for tool use. In this report, I describe neural mechanisms involved in the perceptual assimilation of a stump and prosthesis.
Case Report
A 42-year-old right-handed man underwent a very short (7 cm; 29%) right forearm surgical amputation because of a work accident when he was 38 years old. After completion of a rehabilitation program, he was able to skillfully use an upper extremity prosthesis with a body-powered prosthetic hand. After recovery, he changed his occupation from construction worker to office worker, specifically an advertising designer. This job required him to use a personal computer. He immediately faced a serious problem in operating the computer keyboard because he could not tap both the shift key and certain other keys simultaneously using only his left hand. He tried to use his prosthesis for the right side of the keyboard, but the hand of the prosthesis was too heavy for tapping keys rapidly and correctly. Therefore, he created a work arm that would allow him to tap the shift key of the keyboard. This work arm consists of a single wall socket, a nine-figure-harness, and a stick. The socket envelops his stump near the mid-humerus and fixes the elbow joint. The stick is shorter than the left arm and is bent a little at the tip to tap the shift key easily. One month after he began using the work arm 8 hours a day, his efficiency in doing the tasks his work involves remarkably improved because he could tap the shift key as well as other keys on the keyboard accurately without seeing the keyboard. He could identify the position of the stick without seeing it (Figure 1
). He has been successful as an advertising designer using the work arm for the past 4 years.
Discussion
When we use a tool, it becomes an extension of the hand in both physical and perceptual senses. By holding a stick we can reach beyond the limit of our arm. Modification of the body schema has been postulated as the basis of the perceptual assimilation of a tool and the hand.1,2 The body schema is a functional element for perceiving visual images of the size, shape, location, and movement of the body in environmental space. Therefore, the neural mechanism that integrates somatosensory and visual information regarding our own body parts is necessary for forming the body schema.
Recently, the neural mechanism code for forming the body schema in the brain of monkeys was found. Neurons in the intraparietal sulcus of the brain responded to visual stimuli near the somatosensory receptive field most effectively.4,5 Iriki et al.3 trained a macaque monkey to retrieve distant objects by using a rake and recorded the activity of those neurons that code the hand. During tool use, the monkey's visual receptive field was altered from the hand to the rake. These findings represent neural correlates of the modified schema of the hand in which the tool was incorporated. They also found that schema modification can occur in the hand image on a real time video monitor.6 In humans, the modification must occur in a mental image because we can use common tools such as a cane and a toothbrush well without seeing them.
Many amputees can use a prosthesis well without seeing it. I am sure that modification of the body schema from the stump to a mental image of the prosthesis occurs in these amputees. As a result, they can recognize the localization of their prosthesis. In the present case, I suppose that the almost continuous use of the work arm for one month induced the above process and enabled the patient to pinpoint the tip of his work arm without seeing it. The successful use of a prosthesis in this case has been reported to contribute to a better understanding of the human ability to use prostheses.
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- Paillard J. The hand and the tool: The functional architecture of human technical skills. In: Berthelet A, Chavail J, eds. The Use of Tools by Human and Non-human Primates. New York: Oxford University Press; 1993:36-46.
- Iriki A, Tanaka M, Iwamura Y. Coding of modified body schema during tool use by macaque postcentral neurons. Neuroreport. 1996;7:2325-2330.
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- Iriki A, Tanaka M, Iwamura Y. Self image in the video monitor is coded by parietal neurons [abstract]. Soc Neurosci. 1997;23:211.
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