Simeon Keates, P John Clarkson

University of Cambridge, Dept. of Engineering

Peter Robinson

University of Cambridge, Computer Laboratory


People with motion-impairments often find it difficult to perform many of the actions required to interact with a computer. This paper presents the results of an on-going series of experiments designed to understand how using force feedback affects interaction for motion-impaired users. Point and click tasks were analyzed using new cursor control measures. The results showed significant improvement in throughput for all users with force-feedback and the cursor control measures were effective in capturing the differences between the conditions.


force feedback, motion-impaired users, cursor studies


Computer use often involves interaction with a graphical user interface (GUI), typically using a keyboard and mouse. For people with reduced physical capabilities, this can be difficult, if not impossible [2]. Conditions such as cerebral palsy, muscular dystrophy, and spinal injuries can give rise to many symptoms, such as tremor, spasm, and reduced strength, and can restrict the user’s performance in typical GUI tasks, such as "pointing and clicking".

Previous studies have shown haptic feedback, to be beneficial for able-bodied users in point and click tasks, using both gravity wells [4] and haptic tunnels to targets [1]. Studies have shown that haptic feedback also offers noticeable improvement in point and click tasks for motion-impaired users [2].


Analyses of "point and click" interactions are common, but tend to focus on gross measures such as movement time and error rate. Although these measures may establish that differences exist, they are limited in explaining why they exist. MacKenzie et al. [3] proposed seven new accuracy measures to evaluate computer pointing devices. The measures are intended to elicit subtle differences through an analysis of the cursor movement along the cursor path:

Target re-entry (TRE) - occurs when the cursor enters a target region, leaves and then re-enters the target region.

Task axis crossing (TAC) - occurs when the cursor crosses the task axis, defined as the straight line from the start point to the target’s center.

Movement direction change (MDC) - occurs when the tangent to the cursor path is parallel to the task axis.

Orthogonal direction change (ODC) - occurs when the tangent to the cursor path is perpendicular to the task axis.

Movement variability (MV) - represents the extent to which the sample cursor points lie in a straight line along an axis parallel to the task axis.

Movement error (ME) - the mean of the absolute distances of the cursor sample points from the task axis, irrespective of whether the points are above or below the axis.

Movement offset (MO) - the overall mean distance of the cursor sample points from the task axis, allowing for whether the points are above or below the axis, unlike ME.

Supplementary measures of cursor movement include: movement time, effective target width and throughput:

Movement time (MT) - the time between the defined start point of the movement towards the target and the time when the target is selected.

Effective target width (We) - a modified value for the width of the target defined as:


where SDx is the standard deviation of the distance between the cursor location when the target is selected and the target center, resolved along the task axis.

Throughput (TP) - the throughput measure aims to capture both accuracy and speed and can be calculated from:


where IDe is the effective index of difficulty, based on We and the initial distance to the target, D:


Motion-impaired users are more likely to press the input device buttons prematurely or simply by accident. Consequently, an additional potential measure is:

Missed click (MCL) - occurs when a button click is registered outside of the target.


The study involved five volunteers (Table 1) with a range of motion impairments. For benchmark comparison, data from three able-bodied users (CU1-3) were also collected. The aim was to understand in detail the mechanisms by which gravity wells assist motion-impaired users in selecting targets.




Athetoid/ataxic Cerebral Palsy, wheelchair user


Athetoid Cerebral Palsy, spasm, wheelchair user


Athetoid Cerebral Palsy, deaf, non-speaking


Athetoid Cerebral Palsy


Friedrich’s Ataxia, tremor, wheelchair user

Table 1. Study participants.

Participants performed a series of multidirectional point and click tasks. 16 circular targets were arranged in an equidistant circular layout, Figure 1. A sequence of trials began when a participant clicked in the target at the top of the circle. The next selection was the target directly opposite and so on clockwise round the targets. Each sequence comprised 33 target selections, and was repeated both with and without haptic assistance, using a Logitech Wingman force-feedback mouse. Haptic assistance was provided by gravity wells around the targets. When the cursor entered the well, a spring force pulled the mouse toward the center of the target until the target was selected.


The cursor movements were recorded and analyzed using the cursor measures previously described. Figure 1 shows example cursor paths for (a) an able-bodied user, (b) a motion-impaired user, both without gravity well assistance and (c) the same motion-impaired user with assistance.

A 2 factor repeated measures ANOVA showed that there was a significant difference between the conditions with and without gravity well assistance (F=40.77, df=1,959, p<.001). There were significant decreases in means for missed clicks (MCL), target re-entries (TRE), movement direction changes (MDC), movement time (MT) and effective width (We), and an increase in throughput (TP), particularly for the motion-impaired users. The other measures were very similar for both conditions, suggesting that they were incapable of distinguishing between them.

The experiment has shown that gravity well assistance not only produces significant reductions in movement time for this task, but also significant increases in throughput. The cursor measures offer insights as to the mechanisms by which the throughput improves. The inherent circling of targets typical of motion-impaired users, Figure 1(b), is reduced and this is shown by the reductions in TRE and the ability to select a target accurately is shown by the reductions in CL and We. Thus the cursor measures are likely to be useful in future experiments.

Figure 1. Cursor traces and the target layout:

(a) CU1 unassisted; (b) PT7 unassisted; (c) PT7 assisted


Although the experiment described is suitable for demonstrating the use of the cursor measures for interpreting cursor movement for motion-impaired users, it is insufficient to provide the sole basis for more accessible computers. Further work is continuing looking at adding gravity to targets in more realistic tasks, such as multiple targets in close proximity to each other. Other methods of haptic support such as tunnels to targets [1] are also being studied. Finally, new measures of cursor control are being developed and evaluated throughout these experiments.


  1. Dennerlein, J.T., Martin, D.B., and Hasser, C. Force-feedback improves performance for steering and combined steering-targeting tasks, in Proceedings of CHI 2000 (The Hague, 2000) 423-429.

  2. Keates, S., Langdon, P., Clarkson, P.J., and Robinson, P. Investigating the use of force feedback for motion-impaired users, in Proceedings of the 6th ERCIM Workshop (Florence, Italy, 2000), 207-212.

  3. MacKenzie, I.S., Kauppinen, T., and Silfverberg, M. Accuracy measures for evaluating computer pointing devices, in Proceedings of CHI 2001, (Seattle, WA, 2001), 9-15.

  4. Oakley, I., McGee, M.R., Brewster, S. and Grey, P. Putting the feel in ‘look and feel’, in Proceedings of CHI 2000 (The Hague, 2000), 415-422.

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