The experiment was conducted using two different computers. One was a Dell XPS700 desktop pc with an Intel Core2 Duo 6600 chipset, running at 2.4 GHz and with 2GB of RAM. The video card was a GeForce 7950 GX2 in dual SLi mode, with 1024MB of RAM. The other computer was a Dell XPS600, with an Intel Pentium 4 processor, running at 3.6 GHz and containing 1GB of internal RAM. This computer’s video card was a GeForce 7800GTX in dual SLi mode, with 256MB RAM. Both computers used a Toshiba TDP-P6 beamer to back-project the generated virtual environment on a semi-transparent (‘frosted’) screen 1.46 by 1.09 meters in dimensions, at a resolution of 1024 by 768 pixels, with a screen refresh rate of 75 Hz.

A total of 32 persons (15 males and 17 females) participated in the experiment in return for monetary compensation. Two persons had to forfeit prematurely because they surpassed the preset level of cybersickness that was adopted as an upper limit in this experiment. The age of the remaining 30 participants ranged from 20 to 65 years, with an average of 45.8 years, and a standard deviation of 15.2 years. All participants were healthy and had no significant susceptibility to motion sickness and related phenomena, as determined through self-report by means of a health questionnaire. Only 6 participants had some experience with first person shooter games. Two of those had some experience with HALF-LIFE 2, but none of them recognized any of the scenes shown in the course of the experiment. The experimental protocol was approved by TNO Human Factors internal review board on experiments with human participants. The participants gave their written informed consent prior to testing.

The experiment was performed in a dimly lit room, using two similar set-ups separated by black curtains. Participants were seated in a comfortable chair with their head against a raised headrest, at approximately 1.2 meters viewing distance from the projection screen, resulting in a geometrical field of view of 60 degrees. Their visual straight ahead was aligned with the centre of the screen. About 10% of the participants suffered from nausea at different stages during the experiment. When participants verbally informed the experimenter of their misery, they were allowed to sit slightly further away from the screen, as this probably would not significantly influence the mental representation of the virtual world [15].

Subjects viewed a simulated walk through a virtual environment. The virtual environment was generated with the graphics engine of the game HALF-LIFE 2 (the ‘Source’ engine) and comprises rural, mostly coastline, areas, an industrial vista and an urban area, depicting a city with mixed European architecture (Fig. 1). Due to the use of a contemporary game engine, the graphics are up to modern day standards.

Fig. (1) Screen shots from the virtual environment used in this study. (a) Coastline view. (b-d) Urban areas. (d) Target area.

The navigation through the virtual space was passive, i.e. without user control, in order to assure that every person had the same impression of the environment. The simulated walkthrough started at a beach and ended in a city, and lasted about 12 minutes. At the end of the first walkthrough, the presentation started over and ran for another 5 minutes, resulting in a total presentation duration of 17 minutes. The presentation stopped at a designated location in the city. The participant was then asked to appraise the environment displayed at that moment. The walk through the rural part of the environment, starting at the beach, served to focus the attention of the participants on the urban area once they arrived there, and prevented a monotonous experience. By duplicating the first part of the walkthrough the presentation duration could be extended, without the need to extend the simulated environment.

The virtual walk took place at a simulated speed of 13.6 km/h. Our aim was to induce cybersickness without hindering the formation of an affective appraisal. To increase the imagery’s potential to provoke sickness, we used a large geometrical field-of-view (60º) in combination with an erratic swaying camera motion about an axis at foot level and with a peak amplitude of 8º. This camera motion lasted for the entire duration of the simulation. These parameter settings were likely to induce cybersickness within a relatively small amount of time [16], before participants would get bored (a mood which might confound the affective appraisal).

Previous research indicated that motion sickness related symptoms are most prevalent when either a roll or pitch motion is performed [17, 18]. In this experiment we used a roll motion since this ensures less distortion of the scenery to be appraised (see (Fig. 2) for a pictorial representation of the roll axis). However, during a pilot experiment it became apparent that participants were able to accustom to the roll motion when a regular sinusoid was implemented. Therefore an unpredictable complex harmonic swaying motion was used, implemented as a linear combination of three sinusoids. The time (t, in s) dependent amplitude A (deg) of this motion ranged between -8 and 8 degrees, and was defined as

Fig. (2) Pictorial representation of the roll axis.

where φ represents the (dimensionless) phase, max the maximum operator, and a = 7.2 , b = 4.1 and c = 4 are constants. These constants were selected in order to achieve the highly erratic oscillation shown in Fig. (3).

Fig. (3) Schematical representation of the roll oscillation.

Most studies on motion, simulator and cybersickness use the Simulator Sickness Questionnaire (SSQ) [19], a subjective self-report scale first introduced by Kennedy and Lane [20]. However, the SSQ, while giving an accurate depiction of cybersickness, is a rather long questionnaire. As the average affective appraisal questionnaire used in this study is also comprehensive, we feared that cybersickness could subside over the course of the final appraisal task. Furthermore, this is an exploratory research study and the effect of cybersickness was deemed more important than the cybersickness itself. We therefore decided to use a short 0-10 index score scale, the Misery Scale (MISC), for the evaluation of cybersickness [21]. The MISC scores range from no symptoms (0), undeterminable symptoms (1) and slight determinable symptoms (2) gradually upwards to vomiting (10). We adopted a MISC score of 8 as a break-off criterion, i.e. subjects who reported a MISC score of 8 or higher were allowed to stop.

The affective appraisal of the virtual environment was rated using the eight point semantic differential scale developed by Russell and Pratt [22]. This scale describes the affective qualities of a molar environment as a 2D space, with the independent dimensions unpleasant-pleasant and arousing-sleepy on the two main axes. Diagonally through these axes are the dimensions exciting-gloomy and distressing-relaxing, which can optionally be inferred from the two main axes. Participants rate a list of 40 affective adjectives, which are then grouped into the two dimensions comprising the appraisal.

First, participants read and signed an informed consent.

Second, they filled in a questionnaire that assessed (a) their physical and mental state, (b) their past susceptibility to motion sickness and related phenomena, and (c) their proficiency with virtual environments and computer games, first person shooters and HALF-LIFE 2 in particular. This latter information is important, since participants who have previously played HALF-LIFE 2 may recall set pieces from the original game that were used in the construction of the virtual environment for the present study, and may associate these pieces with environments they previously encountered in the HALF-LIFE 2 game. Such associations could in turn influence their appraisal of the environments shown in the present study. Only two participants reported having played HALF-LIFE 2, but when prompted afterwards both stated they had not recognized any locale in the course of the experiment.

Third, participants rated the affective qualities of a picture of typical Dutch Neo-Classical / Neo- Renaissance style canal houses and a picture of 7^th avenue in New York, on Russell and Pratt’s environment description scale [22]. This was done primarily to accustom the participants with the measurement scale, but also to ascertain whether the appraisal of a Dutch cityscape was homogeneous among the participants, as the final environment to be appraised resembled these buildings.

After completing the aforementioned three questionnaires, the participants were delegated to the darkened experiment room and seated in the setup. There they watched the 17-minute tour of the virtual environment. During this presentation they were prompted to note down their MISC scores every five minutes, and at the end. At the end of the tour the camera froze on the area depicted in Fig. (1d). Then, the lights in the room were turned on dimly, and the participants appraised this particular part of the environment with Russell and Pratt’s affective scale.

Prior to the experiment nearly all participants found an exemplary Neo-Renaissance style architectural cityscape to be pleasant and, for the greater part, slightly sleepy (Fig. 4). Given the facts that cybersickness causes physical discomfort, and that people tend to misattribute their feelings to the environment they perceive, we hypothesized that cybersick participants would find this type of cityscape less pleasant.

Fig. (4) Scatterplot with 95% confidence ellipse detailing the affective appraisals of a picture of Neo-Renaissance buildings that resembled the target area of (Fig. 1d).