Visual Masking Parameters for Virtual Environments

Jason Drummond*, Anthony Steed University College London

ABSTRACT Visual masking is a technique whereby the rapid presentation of an image can render another image unavailable to conscious perception. Many of its parameters have been studied but the stimuli have traditionally tended to be two dimensional in form, including those used in masked priming experiments. Recently, however, studies have begun to look at visual masking within stereoscopic, virtual environments (VEs). This paper outlines two experiments that alter various properties of visually masked, virtual objects. The first experiment found a significant interaction between surface-colour saturation and texture fluency. The second found an exposure effect modulating affective, visually masked stimuli. Using visually masked, affect-laden stimuli within a VE holds out the promise of creating a potent way to alter affective processes and behaviour. The potential of such effects is widespread, including use within psychotherapeutic interventions, training scenarios and video games.

Keywords: Virtual environments, affective psychology, visual masking.

Index Terms: Virtual reality, psychology, perception.

1 INTRODUCTION Virtual reality technology offers more than simulation of the real world. We can use it to embed phenomena within virtual environments (VEs) that would be either impractical or impossible to achieve otherwise. An example of this lies with a collection of phenomena that fall under the term visual masking. Such perceptual effects have been known within psychology for over a century [1].

The phenomena usually involve a graphic figure proceeding or replacing another on some form of display. This is followed by an inter-stimulus gap, after which the cycle returns to the display of the first figure and the process repeats. One figure is displayed very rapidly, in the order of a few milliseconds. This is termed the target. The other figure, the mask, is displayed for longer, often in the order of several hundred milliseconds. Constantly repeated, this cycle leads to the perception of a flickering or blinking mask, the target being rendered imperceptible to explicit awareness. The mask can precede the target, known as forwards masking, or replace it, known as backwards masking. Masks can entirely cover the visual area of the target or can sit adjacent to the target.

Visual masking is studied for its own sake but is often used as a means to an end by eliciting effects useful in other fields of visual perception. For instance, priming experiments have shown that people will respond to masked primes whilst claiming no conscious awareness of those primes. In one study, Winkielman and Berridge [8] asked participants to rate a particular drink on

several aspects including monetary value. In one condition, participants were exposed to masked primes consisting of angry faces, in another smiling faces were used. The control condition used a neutral face as both mask and target. The results showed the smiling prime scores were significantly higher than the angry prime scores and appeared to show the direct influence of non-conscious, affective processing. Participant thirst ratings were also shown to be influential, acting as a modulating factor. Participants also tended to drink more when exposed to the masked smiling expression. Yet another study, by Dimberg and colleagues [4], also exposed participants to masked facial expressions but measured their facial muscle activity. Results showed that participants tended to mimic the faces of the masked primes whilst, again, claiming no conscious perception of them.

VEs have often been used within visual perception research. However, to date there is little research that links visually masked primes with virtual environments, particularly with regard to affective processes. Previous work had shown visual masking to be possible within a VE, and that priming effects can follow [5] but parameter manipulation was limited.

2 THE CURRENT EXPERIMENTS The two experiments outlined here were designed to look at the interplay between various parameters found within a visual masking VE, including factors which would have an effect on affect cognition. Both experiments used backwards masked, 3D objects within a stereoscopic VE. Both experiments used a stereoscopic, desktop VE consisting of a series of open-topped rooms through which the participants had to pass, see figure 1.

The rooms terminated in a pair of doors, each of which lead to the next room and so on. The doors acted as planar masks, visually masking various non-planar objects placed behind them, within each arch. Although the masked, target objects were large enough to block each doorway, participants could pass through them. In the main experiment, participants could navigate through either doorway, the second experiment allowed only one exit per room. Each target was displayed for approximately 16 ms, the masking door for 250 ms with an inter-stimulus gap of 250 ms. The mask and targets had the same surface texture and colouring.

Figure 1: Image of the VE.

* e-mail: j.drummond@cs.ucl.ac.uk

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IEEE Virtual Reality 201316 - 20 March, Orlando, FL, USA978-1-4673-4796-9/13/$31.00 ©2013 IEEE

3 MAIN EXPERIMENT This experiment used static, non-planar faces and household objects as targets, investigating the interaction between the three, bi-valent target parameters: surface-colour saturation, surface-texture fluency and affective salience of target form. Would changing these parameters render targets more or less susceptible to conscious perception in a VE? Here, fluency refers to the ease with which an image or text can be perceived by an observer [7]. The sharpness of a texturing image on a virtual surface is one example of a fluency factor and was used for this experiment.

Twenty participants experienced both a high and low saliency conditions, in random order. In each condition, they navigated through twelve rooms containing targets with either high or low fluency texture surfaces and high or low saturation. The high saliency condition used affectively expressive faces as targets whereas the low saliency condition used household objects. At each doorway participants indicated what they saw. Each correct answer scored one point for that particular combination of saliency, fluency and saturation. Each incorrect answer scored zero. If nothing could be seen, the score was entered as zero. These scores formed the dependent variable measured for the three independent variables Salience, Fluency and Saturation.

3.1 Main Experiment Results Repeated measures ANOVA was used for the analysis. The results show that Saturation and Fluency interact: p<0.002, F = 13.8, partial eta squared was 0.42. Affective salience showed no effects, interactive or main.

4 SUPPLEMENTARY EXPERIMENT This experiment used non-planar faces as targets. The faces were given high fluency, low saturation textures, following the results of the main experiment. The faces displayed either an angry expression or a smile, each face being rendered as either a static or dynamic. Dynamic targets changed form with each target/mask cycle, beginning as a neutral expression. Every time it was re-rendered, the expression changed incrementally towards either fully angry or fully smiling, resembling frames of an animation. Dynamic expressions took seven increments to change from neutral to full. The VE was constructed so that at the maximum speed of travel the participants would experience at least fourteen target/mask render cycles (increments) before entering one of the doorways. The dynamic targets would therefore reach full expression at least twice per room visit. A comparison condition used static targets comprising of a full facial expressions. Each static target exposed either a smiling or an angry face at least 14 times per room visit.

Twenty-two participants navigated two trials of sixteen rooms apiece. For each pair of doors, in each room, one of the doors was blocked in-front by a large, non-masked (i.e. permanently visible) household object. This door was impassible. The other door masked the face but was passable, acting as the exit to the next room. Participants were instructed to navigate through each passable doorway, stop and give an attractiveness score for the household object that they had just passed. These scores formed the dependent variable and it was hypothesized that they would be influenced by dynamic/expression parameters of the faces, in line with evaluative conditioning effects [2][3].

This experiment arose from a proposal in an earlier paper [6].

4.1 Supplementary Experiment Results Repeated measures ANOVA was used for the analysis and the results showed an interaction between the dynamic vs. expression factors: p<0.05, F = 5.2, partial eta squared was 0.03.

5 DISCUSSION The results from the main experiment showed that saturation and fluency interact. When comparing low fluency to high fluency scores the largest range was shown by the high-saturation, colourful surfaces as opposed to low-saturation, greyscale. Similarly, when comparing low saturation scores with high-saturation, the greatest range was found with low fluency, blurred textures as opposed to the high fluency, sharp textures. Overall, however the results suggested that the best masking was to be had using high fluency, sharp texturing.

Saliency did not produce a significant effect, either interaction or main. This suggests that expression processing may be treated in a similar way to non-expression processing in so far as neither was made explicit.

The results from the supplementary experiment showed that there was a modulation effect but it was not internal to the dynamic condition. It arose due to the presence or absence of the dynamic form of the target itself i.e. static targets produce the stronger effects. This could indicate that some form of exposure effect was over-riding any dynamic target effects.

6 CONCLUSION Previous work had shown that visual masking was possible within a VE and that masked virtual objects could be non-planar. However, that work did not explore fully other parameters beyond depth. This current study begins that process by examining how surface qualities, target form and exposure timing can affect visual masking within a VE.

There is an interest in using masked, expressive faces to effect behavioural and cognitive change. The realism of those stimuli can be increased by building them as non-planar, virtual objects. The behavioural and affective responses elicited by such masked objects could have several areas of application, where stimuli should not be subject to the affect damping possibilities of top-down, cognitive interference. Before these or any other potential uses can be engineered, though, further research remains to be undertaken to ensure more robust, consistent and larger effects can be routinely elicited. If these goals are realised, then the field of potential uses for visual masking in VE is large.

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