Scanning is a common selection technique used by persons with a severe disability for the operation of electronic aids. Setting the proper time for the scan delay is critical for successful operation of aids that scan. Previous tests compared the force used to operate a control switch with the user’s comfort level, but we found no discernable pattern. In this set of tests, we examined the changes in physical effort during each trial for each participant. We found that the switch activation force tended to decrease during each test; however, the force increased at the beginning of each subsequent test when the investigator decreased the scan delay.
computer access, augmentative and alternative communication, row/column scanning, force
Persons with a severe physical disability often operate electronic aids such as communication aids or computers by using the scanning selection technique along with a single control switch. Setting the proper scan delay, or amount of time that the device highlights each selection, is critical to the successful use of the scanning technique. Users are frustrated whenever the scan delay is set to be either too short or too long; however, no commercial aid currently monitors the user’s performance in order to adjust the scan delay automatically. Though researchers had previously attempted to develop a scanning technique that automatically adjusts the scan delay as the user’s ability to operate the scan changes (1) (2) (3), none had examined the relationships between the user’s perception of the scan delay and measures of their performance.
In earlier tests, we compared the user’s perception of the scan delay with measures of their performance such as throughput and error rates (4). We found that, in all cases, the users felt that the scan delay at which they achieved their best throughput was either “a little too fast” or “much too fast.” We were concerned that, even though users achieved better performance at shorter scan delays, the discomfort that they experienced at these faster scan delays could lead to inordinately high levels of physical effort. We conducted tests to compare physical effort used to operate a control switch with the user’s ability to operate a scanning aid (5). We had expected that the switch activation force would increase as the scan delay became shorter; however, we did not observe this change in performance in our tests. In analyzing our data, we did however note that patterns of forces appeared to emerge during individual tests, and decided to investigate further.
The objective of this study was to determine how the force used to operate the control switch changes as the scan delay decreases.
We ran a simulation of the row-column scan technique our lab had previously developed on Visual Basic.NET 2003. The computer platform for the simulation was a Dell Latitude C400 laptop computer running Windows Professional XP. The left mouse button of the computer trackpad simulated the action of the single switch. The selection set was a six by five matrix composed of 26 English letters in alphabetic order, a period character, and a space character.
We conducted these experiments with eight non-disabled participants, consisting of five males and three females ranging in age from 18 to 23. The task of each participant was to compose the sentence “The quick brown fox jumps over a lazy dog.” with the row-column scanning aid simulation. A box above the selection matrix displayed the next letter in the sentence the participant needed to enter. Each participant had the opportunity to practice using the scanning simulation until he or she felt ready to begin the tests.
The tests with each participant began with a scan delay of 800 ms. Testing continued with the scan delay decreasing by 20% from the previous trial until the participant judged that he or she would not able to finish the composing task.
During testing, the scanning aid simulation recorded the time, the number of the currently-highlighted row or column, and details on the accuracy of the selection (correct, incorrect due to anticipation or lag, etc.). The program automatically exported the data into an EXCEL spreadsheet.
During the tests a separate Dell Dimension 2350 PC collected force measurements using LabVIEW 7. We connected a FlexiForce Model A201 force sensor (maximum 1-lb load) to a National Instruments 6036 DAQCard with analog signal measurement and conditioning modules. We mounted the sensor on the left mouse button of the trackpad. A LabVIEW VI (Virtual Instrument) measured the force applied to the sensor at a sampling rate of 40 Hz. The LabVIEW VI stored the force data in a text file.
At the conclusion of the tests, both sets of data files were synchronized using MATLAB 7.0.1 (R14). The statistical tests were performed using Microsoft EXCEL 2003.
Figure 1 shows a comparison of the mean switch activation force for all participants between the first ten selections and the last ten selections of each test. For all participants, the mean switch activation force decreased from the beginning to the end of each set of tests. A statistical analysis using the Student’s t-test demonstrated that the differences are highly significant (P < 0.00001). This analysis demonstrated that, as each user adjusted to the scan delay set at the beginning of each test, the effort used to operate the control switch decreased.
Figure 2. Comparison of the switch activation force for each participant between the last ten selections at the longest scan delay (800 ms) and the last ten selections of the shortest scan delay, the value of which varied with each participant. (Click image for larger view)
Figure 2 shows a comparison of the mean switch activation force for all participants between the last ten selections at the longest scan delay (800 ms for each participant) and the last ten selections at the shortest scan delay (134 to 262 ms, depending on the ability of the participant). The mean force increased for three participants, and decreased for the other five. A statistical analysis using the Student’s t-test indicated that there was no significant difference between the switch activation forces at the end of the first test and the end of the last test. This analysis demonstrated that overall, participants tended to apply the same switch activation force at the end of each test, regardless of the duration of the scan delay.
Figure 3. Comparison of the mean switch activation force for each participant between the last ten selections of the previous test and the first ten selections of the next test. (Click image for larger view)
Figure 3 shows a comparison of the mean switch activation force for all participants between the last ten selections of one test and the first 10 selections of the next test. The mean switch activation force increased substantially for seven out of the eight participants; applying a Student’s t-test we found these results to be highly significant (P < 0.00001). This analysis demonstrated that participants tended to work harder to operate the control switch at the beginning of each test as they perceived the significant reduction in the scan delay from the previous test.
We believe that the reason the force always increases whenever scan rate increases is that the individual is aware of the change in the scan delay and assumes that he or she will need to work a little harder because less time is available per selection to activate the switch.
We conclude that any algorithm used to alter scan delays to match the ability of the user should avoid instantaneously large changes in the scan delay, because the user perceives the change and initially uses more force to operate the switch. This effect appears to be short-lived, as the switch activation force tends to decrease to previous levels after a small number of selections. However, any algorithm trying to match the length of the scan delay with the ability of the user should balance the need to make rapid adjustments in the scan delay (due to fatigue or medication) with the need to avoid increasing the effort that the user applies to the operation of the control switch.
Stan Cronk, PhD
Center for Biomedical Engineering and Rehabilitation Science
Louisiana Tech University
711 S. Vienna
Ruston, LA 71270
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