Lab 4: Fitts Reciprocal Tapping Task

Lab 4: Fitts’ Reciprocal Tapping

On page 157 of your text Schmidt and Wrisberg ask whether movement amplitude or target width (as discussed with Fitts’ law and the speed accuracy tradeoff) has a greater effect on movement time (MT). They invite the reader to tap reciprocally under three different conditions (a, b, & c). In condition a, the amplitude is 86 mm (A = 86 mm) and the circle diameter or target width is 8 mm (W = 8 mm). In condition b, the amplitude is still 86 mm, but the target width is doubled (W = 16 mm). Comparing condition a to condition b allows one to see what effect doubling target width has on MT. In condition c, the amplitude is 43 mm and the target width is 8 mm. So, comparing condition c to condition a, allows one to see what effect doubling movement amplitude has on MT.

In the present lab we will formally test the conclusion suggested by Schmidt and Wrisberg that target width has a greater effect on movement time than does movement amplitude.

Methodology

Participant

The participant for this experiment will be a right-handed male or female, 18-25 years of age, with normal or corrected 20/20 vision.

Equipment/Material

Students are to construct test sheets similar to the ones displayed on page 157 of the text. You can use any amplitudes and circle diameters you like with the provision that condition A and B have the same amplitude, but B has a circle diameter two times larger than A. Condition C will have the same circle diameter as condition A, but half the amplitude. Amplitude is measured from the centre of the circle on one side to the centre of the circle on the other side. Place each condition on a separate sheet of paper (probably easiest to have amplitude run along the 11” dimension of the paper) and photocopy five sheets for each condition so that five trials can be run under each condition. Suggested circle diameters might be 1 and 2 cm and amplitudes of 10 and 20 cm.

The experimenter will also need a stopwatch to time the trials and the participant will use a pen or pencil with which to tap.

Procedure

On a single trial the participant starts with his/her pen down on the right target. When the experimenter says, “go”, the participant begins tapping, and the experimenter starts the stopwatch. The participant continues to tap back and forth (hitting inside the circles/targets) as quickly as possible for 20s. At 20s the experimenter says, “stop”. During the tapping the experimenter counts the number of taps completed. This is done easiest by counting each time the pen touches down on the right circle and then multiplying by two to get the number of taps in 20s. If it is clear that the participant completed a last tap in the left circle, then multiply by two and add one to get the number of taps in 20s. If the participant makes more than three errors on a trial (the tap fell outside the circle) the trial must be redone. Checking for errors is done after each 20s trial by inspecting the pen marks at the circles. Marks on the line or marks that begin inside the circle but then cross the line are good taps (not errors). It is critical that the error rule be adhered to.

Participants are to complete five trials under each of the three conditions for a total of 15 trials. To control for potential practice effects, the order of testing across the 15 trials should be: a, b, c, a, b, c, a, b, c, a, b, c, a, b, c, a, b, c. The participant is allowed one practice trial on a unique condition at the beginning of testing to ensure he/she understands what is required. The experimenter should also practice counting taps prior to the testing.

Dependent Variable

First calculate the average number of taps in 20s over the five trials in each condition. Then to calculate the movement time per tap, divide 20s by this average. For example, if on average in condition A the participant completed 50 taps, then the movement time would be 20/50 = 0.40s/tap (or 400 ms per tap). Remember that a bigger number means that the person moved slower (so a movement time of 500 ms/tap is slower than a movement time of 350 ms/tap).

Results

Draw a single bar graph to show the effect of halving the circle diameter and doubling the amplitude. This can be done by plotting the difference in movement times between A and B compared to the difference between A and C (your graph should have two bars). Ensure that the graph is clearly labelled.

Discussion

1. Do your results agree with the suggested effect of target width and movement amplitude in your text (p 157)? What were the percentage differences in movement time of either doubling amplitude or halving the target width (both of these should make the movement time slower) 2. Using the concepts of open- and closed-loop motor control, what might explain the differential effect of target width and movement amplitude?

Form for write-up

1. Maximum word count for report is 300 words. 2. On top right (as a header) put your name, student number, and word count. 3. Your report will have the following parts:  title (centred at top of page)  1-2 sentences as introduction including your research hypothesis (what you think will happen.  in the methodology (titled and centred, “Methodology”) section describe the tapping conditions used (i.e., what were the amplitudes and target widths) and attach a copy of each to the back of your assignment labelling them condition A, B, & C.  results section (titled and centred “Results”) – ideally a computer generated bar graph – 1 sentence explanation if deemed necessary.  discussion section (titled and centred “Discussion”) – number your answers to the questions (see above).

1. Overall presentation of paper 20% 2. Results (accuracy & presentation/clarity of picture) 30% 3. Question one 20% 4. Question two 30%