Experimental Design

To investigate the effect of stimulus onset asynchrony (SOA) on the behavioral performance and differences on brain activities between single- and dual- task conditions in a virtual environment, we designed two tasks: unexpected car deviation and calculation of mathematical equations. The combinations of these two tasks provided different distraction effects to the subjects.

We developed a VR highway environment with a monotonic scene and eliminated all unnecessary visual stimuli as shown in Fig. 2-5. The four lanes from left to right were separated by a median strip in the VR-based scene. Our design was that the car must be kept in the third lane form the most left. The distance from the left side to the right side of the road was equally divided for outputting digital signal from WTK program, and the width of each lane and the car was 61 units and 32 units, respectively (as showed in Fig. 2-5). In the VR scene, the simulated driving speed was controlled by a scheduled program, thus subjects need not to step on paddles, to prevent large muscle activity on the throttle or brake.

Table-1: The Specification of driving simulator Screen Number or Location Dimension

Screen Number 1, 2, 3, 4 (FOV 42°) (W)×(H) = (300 cm)×(225 cm) Driver Head Height Relate to Screen 1 120 cm

Fig. 2-5: The illustration of the monotonic high way scene.

The monotonous scene was designed to reduce the visual disturbance. The width of highway from the left to right side was equally divided into 256 units and the width of the car was 32 units.

We designed four sessions in one complete driving simulation experiment for each subject and the session duration was 15 minutes. In each session, the subject sat in front of the monitor with their hands on the steering wheel to control the car in the center of the third lane (from the most left lane). Among these four-session experiments, the subjects were forced to rest for ten minutes between every two sessions to avoid getting tried. On the other hand, to avoid anticipative effect for subjects the events were presented to the subjects randomly [38], as shown in Fig. 2-6.

The inter-trial intervals were set from 6 to 8 seconds and the independent trials were not interaction to affect the subject. Thus a total of 100 trials could be presented to the subject in each session to ensure the number of events is enough for statistical analysis. There were about 80 trials in each case during one entire experiment.

Fig. 2-6: The illustration of the experimental paradigm.

Five cases were randomly appeared and the inter-trial intervals were varied from six to eight seconds. There were four sessions (15 minutes / per session) in each experiment.

Since the main purpose of this experiment was to investigate the distraction effect in dual-task conditions. Therefore, two tasks were designed including the car unexpected deviation and the mathematical equations. The car would randomly drift from the third lane of the road in the deviation task. When this event was occurred, subjects had to operate the steering wheel to keep car in the original third lane. The detail descriptions of this driving task were shown in the Fig. 2-7.

Response time

Fig. 2-7: The illustration of the deviation event.

There are four steps in one complete deviation event. (A) Vehicle moving in straight line; (B) the onset of deviation event; (C) response to the deviation and (D) vehicle back to middle lane.

Two-digit addition equations were presented to the subjects in the mathematics task as shown in Fig. 2-8. The answers to the equations were already designed to present with the equations but they could be either right or wrong. The subjects were asked to press the buttons on the steering wheel as soon as they can. When the equation is correct, the subjects must press the right button. On the other hand, they would press the left button for the wrong mathematic equation. The event allotment ratios were 50% and 50% for right and wrong equations, respectively.

Fig. 2-8: The illustration of the mathematic equation.

The mathematic equation would be showed in front of the windscreen and the subjects were asked to response the answer. If the equation was correct, the right button on the steering wheel (as showed in Fig. 2.2(B)) might be pressed. On the other hand, the subject might press the left button when the equation was wrong.

The combinations of these two tasks were used to provide different distraction effect to the subjects. Five conditions were developed to study the interaction of the two tasks, they are: (A) math was presented at 400ms before deviation (math-400ms-deviaiton), (B) two tasks were presented at the same time

(deviation-400ms-math), (D) only math presented (single-math) and (E) only deviation occurred (single-deviation). The illustrations of the five conditions were shown in Fig. 2-9. A pilot study was designed to determine the time of stimulus onset asynchrony, and the result suggested the interaction between tasks is significant with 400ms time interval. Thus, we adopted 400 ms as the time of stimulus onset asynchrony.

Fig. 2-9: The illustration of five cases in our experiment.

The 5 cases were randomly appeared in the whole experiment. Each sub-figure shows the relationship between the deviation onset and math occurred. M is the mathematic equation and D is the task of car deviation. (A) Case 1: math was presented at 400ms before the deviation onset. (B) Case 2: math and deviation occurred at the same time. (C) Case 3: math presented at 400ms after the deviation onset. (D) Case 4: only math presented. (E) Case 5: only deviation occurred.