A Computer Input Device Selection
Methodology for Users with High-Level
Spinal Cord Injuries
R. Bates1
1 Matching Devices to Users
When choosing suitable computer input devices it is important for clinicians and
technologists to be aware of the characteristics of the intended user population.
Such knowledge of the specific characteristics of a group will help to define
suitable computer input device choices for that group and so reduce the number of
candidate devices that may be tested. This is particularly true of user groups with
disabilities, such as those with high-level spinal injuries, where the type and level
of disability greatly influences the range of usable input devices available.
The aim of this paper is to illustrate preliminary work on a classification of
input devices and high-level spinal injuries, together with an prototype input device
matching methodology that may allow rapid and accurate selection of potentially
suitable input devices for spinal-injured disabled user groups. It is hoped that such
a system will assist the user-device matching procedure.
2 Spinal Injuries and Input Devices
The spinal cord is the largest nerve in the human body and is responsible for the
passage of motor communication (muscle movement) and sensory communication
(skin touch and proprioception – the positional feeling of muscles and joints)
between the brain and the body below the head region. The spinal cord runs
through and branches from the vertebral, or spinal, column and consists of 8
cervical (neck), 12 thoracic (upper back), 5 lumber (low back) and 4 sacral (base of
the spine) branches. The number of the branch, running from cervical to sacral, and
a letter indicating the region are used to denote each nerve branch.
1
Human-Computer Interaction Research Group, De Montfort University,
Leicester, UK. Email: [email protected]
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Bates
Each branch supplies a defined set of muscles and sensory areas local to the
branch. Injury to the spine can cause damage to the spinal cord, resulting in a loss
of communication along the cord and hence a loss of neurologic (sensory and/or
motor) functions below the branch level of the injury. From this, spinal cord
injuries are classified according to the neurologic branch level of the injury. It is
important to note that the classification refers to the lowest level nerve branch
affected rather than the site of any vertebral damage. Injuries to the spinal cord can
be either complete or incomplete and are annotated with a “C” or “I” after the
neurological branch level classification (Grundy, 1993). Hence, for example, an
incomplete injury to the 5th cervical branch would be denoted as “C5I”. Incomplete
injuries result in partial and patchy loss of motor and/or sensory function below the
level of neurological injury whereas complete injuries result in a complete loss of
nerve function below the branch (Ditunno, 1994).
Note that this paper will only address high-level injuries that have the greatest
effect on the selection of computer input devices, (these are injuries to the cervical
branches and the first thoracic branch).
2.1 Sensory and Motor Abilities
In order for a computer input device to be used effectively it is important for the
user to be able to feel, or know that they are touching or holding, the device and to
be able to effectively manipulate, operate or move the device. The levels of spinal
injury affect neurological functions in a known way so that the effects of a highlevel injury on sensation and movement can be determined. Skin sensation, or
dermatome (“skin-division”) maps and motor, or myotome (“muscle division”)
maps can used to identify these skin areas and muscles supplied by the spinal cord
branches (Ditunno, 1994; Grundy, 1993; Hanak, 1983; Sutton, 1973). These
individual maps allow a detailed overall map of the sensory and motor capabilities
of a user to be built from the definition of their level of injury.
2.2 Input Device Requirements
The manipulation and control requirements of a computer input device may be
mapped using a design space (Card, 1990) in a similar manner to the mapping of
the sensory and motor abilities of a user. For example, to effectively manipulate a
standard desktop mouse, the device must be moved a given distance in two degrees
of freedom across a surface with a given level of spatial accuracy and requiring a
certain level of force. In addition, a button must be pressed and released with a
given level of temporal accuracy and force. Examining the manipulation
characteristics and requirements of a range of input devices allows a detailed map
to be built showing what sensory and motor characteristics are required to operate
differing computer input devices.
A Computer Input Device Selection Methodology
41
3 A Matching Map
Overlaying the spinal injury level dermatome and myotome maps and the input
device maps constructs a device-level map of spinal injury levels to possible
candidate input devices (Figure 1).
Complete spinal cord injury branch levels and candidate input devices
C1
C2
C3
C4
C5
C6
C7
C8
T1
Eye tracker
Eye wink switch
Brain wave
Facial movement
Bite switch
Tongue joystick
Tongue switches
Head switches – limited
movement directions
Head 6DOF position– limited
movement directions
Chin switch
Speech
Muscle EMG (available muscles)
Sip-puff switch
(diaphragmatic)
Chin joystick
Head mouse
Mouth stick
Shoulder switches
Shoulder 6DOF position
Elbow flexion switch (assisted return)
Shoulder joint – upper arm switches
Shoulder joint – upper arm 6DOF position
Wrist extension switch
(assisted return)
Mouse
Trackball
Joystick
Glidepoint
Touchscreen
Alternative keyboards
Wrist switches
Wrist position
Hand 6DOF position
Thumb switch
Thumb position
Standard keyboard
Finger switch
Finger position
Tablet / Pen
Spaceball
Space glove
Figure 1. Potential input devices with level of complete spinal cord injury, C1 to T1
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Bates
The device-level map suggests suitable input devices by level of complete
spinal cord injury. These cannot be definitive but do suggest a range of potentially
usable devices that should be tried by the user. To use the mapping methodology,
select the column for the injury reported and note the devices listed. Devices
appearing to the left of the column should be usable. Devices appearing in the
column or to the right of the column are unlikely to be usable. Finally, any device
that appears to the left of the column but also crosses to the right may be partially
usable.
For example, for a C3C injury:
•
•
•
Fully usable input devices: Eye tracker, eye wink switch, brain wave, facial
movement, bite switch, tongue switch, tongue joystick, muscle EMG (with
available muscles).
Partially usable input devices: Head switches, head 6 DOF position, chin
switch, speech.
Not usable input devices: Sip-puff switch, chin joystick, head mouse,
mouth stick, shoulder switches, shoulder 6 DOF movement, and all further
devices to the right of this level.
Note that incomplete injuries will present with a mixture of sensory and motor
abilities due to incomplete damage to the spinal cord, but the map will still give a
good indication of suitable devices.
4 Conclusions
This paper briefly shows a preliminary method of mapping levels of spinal cord
injury to computer input device requirements. It shows that it may be possible to
formalise the matching procedure and so reduce the number of devices that users
may be required to test. Future work will expand the mapping methodology to take
into account more detailed descriptions of disabilities and conduct user trials. It is
hoped that the paper will open a discussion into methodologies for matching users
with disabilities to suitable computer input devices.
5 References
Card SK, Mackinlay J, Robertson GG (1990) The design space of input devices. In:
Proceedings of CHI ‘90, ACM Press, pp 117-124
Ditunno JF, Young W, Donovan WH, Creasey G (1994) The international standards booklet
for neurological and functional classification of spinal cord injury. Paraplegia 32:70-80
Grundy D, Swain A (1993) ABC of spinal cord injury. BMJ Publishing
Hanak M, Scott A (1983) Spinal cord injury: an illustrated guide for healthcare
professionals. Springer Publishing Company
Sutton NG (1973) Injuries of the spinal cord: the management of paraplegia and tetraplegia.
Butterworths and Co.