Oddball Paradigm in HMD Interface Evaluation

The oddball paradigm is one of the commonly used paradigms in ERP experiments. It refers to the random presentation of two stimuli of the same sensory channel in an experiment, and the probability of occurrence of the two stimuli is very different. The high-probability stimulus is called the standard stimulus, which is the background of the whole experiment. The low-probability stimulus is called deviant stimulus. The probability of the deviant stimulus is around 20% and the probability of the standard stimulus is around 80%. If the subject is asked to respond to the deviant stimulus, the deviant stimulus becomes the target stimulus at this time.Fig. 2

Indicating interface in the experiment. Target stimulus means that when subjects see the interface, they should click the left mouse button as soon as possible. Standard stimulus means that subjects do not have to react when they see the interface. The interface is only used as instruction at the beginning, and the corresponding text information will not appear during the experiment

In this work, the stimulus was the four different HMD interfaces shown in Fig. 1. The experiment was divided into four groups, and each participant completed all four groups of experiments.
At the beginning of the experiment, an indicating interface was presented (Fig. 2) which defines target stimulus and standard stimulus in each group. After participants confirmed that they remembered the difference between target stimulus and standard stimulus, they pressed the blank space key to enter the next step. The target stimulus and the standard stimulus will randomly appear in the middle of the screen. Each group of experimental target stimulus randomly appears 40 times (the probability is 0.2), the standard stimulus appears 160 times (the probability is 0.8), and the stimulus presentation time is 1000 ms. Subjects were asked to click the left mouse button as soon as they see the target stimulus. When the standard stimulus appeared, subjects will not need to make any response. From last stimulus disappear until next stimulus present, time interval length was 500ms. During this period, subjects were asked to gaze at the fixation cross presented in the middle of the screen.
Since the difference between the target stimulus and standard stimulus will directly affect P3b and P2 amplitude, each group of the experimental target stimulus and standard stimulus was different. To directly compare and analyze interface A and B, interface C and D, we exchanged the target and standard stimulus of the first and the third group, and set up the second and the fourth group of experimental. Table 1 shows the stimulus in each group.
The stimulus was presented in the center of the DELL P2314H LCD display on a black background with a refresh rate of 60 fps. We used psychology software E-prime 2.0 to control the presentation of the stimuli. The eyes of the subjects were fixed 70 cm away from the monitor. The horizontal and vertical angles of the stimulus relative to the subject’s field of view were both less than $6^{\circ }$ .Table 1

Standard stimulus and target stimulus in each group

	Standard stimulus	Target stimulus
First group	A	B
Second group	B	A
Third group	C	D
Fourth group	D	C

At the same time, there will be a rest after the completion of each group to avoid excessive fatigue of the subjects. The subjects must keep their head stable and close their eyes for rest. The subjects can define their own break time. There was a training stage before the formal experiment that aims to familiarize subjects with the task process. After the training stage, the formal experiment will begin and last about 25 min.