A comfortable brain-interface to video displays

Abstract

Recent progress in and the popularization of computer graphics mean we now see many images that are composed artificially and include a lot of flicker to add to their impact. These highly flickering images, however, cause fatigue that affects our brain rather than our eyes. This is a content-dependent video display terminal (VDT) hazard that is unlike conventional VDT hazards. This paper shows that content-dependent VDT hazards are a genuine threat based on physiological evidence concerning the temporal response of visual cortical cells, and proposes a quantitative measure to estimate the risk of the hazard, and also provides an adaptive filtering method to reduce the risk. Images from the critical part of the “Pocket Monsters” TV animation episode were studied to confirm the effectiveness of this method.

Introduction

A wide variety of video display terminal (VDT) hazards have been recognized so far; musculoskeletal disorders, eye fatigue caused by the reflection of ceiling lights, dry eyes, and possible health hazards from electromagnetic radiation are some well known examples (US Department of Labor, 1977). Along with these conventional hazards, this paper proposes that content-dependent VDT hazards that include photosensitive seizures should also be considered.

Photosensitive seizure was believed to occur only for people who are light-sensitive epileptic or subclinically light-sensitive epileptic. The recent “Pokemon incident” in Japan (TV Tokyo, 1997), however, called this into question. A surveillance committee of the Japanese Ministry of Health and Welfare reported that 10.6% of elementary, junior, and high-school students who watched that TV episode felt sick, and 7.2% of these suffered convulsions (Japanese Ministry of Health and Welfare, 1998; Yamaguchi et al., 1998). This percentage is 30 times larger than the proportion of light-sensitive epileptics in the general population (1/4000) suggesting that this hazard is more significant than previously thought (Harding, 1998; Japanese Ministry of Posts and Telecommunications, 1998).

The evidence suggests, at the very least, that there is a content-dependent VDT hazard that many people might suffer from. The content-dependent VDT hazard is most likely visual, but the mechanism of the hazard differs from that of conventional VDT hazards. Conventional VDT hazards affect the user’s body or eyes, and the risk of the hazards can be reduced by improving the physical specifications or the configuration of the VDT environment. However, the mechanism of content-dependent VDT hazards is directly related to the neural nature of the brain, and to reduce the risk of these hazards may require a comfortable brain-interface to the content provided over video displays.

In this paper, we review physiological evidence suggesting that flicker with a temporal frequency of around 10 Hz will simultaneously facilitate most visual cortical cells. That will cause enormous neural fatigue, or may trigger seizure in very rare cases. The paper describes a mathematical model of the temporal cell response. This response model is used to detect the flicker of retina images. Based on the response model, we propose an adaptive inter-frame temporal filter that reduces the risk of content-dependent VDT hazards. This filter adaptively reduces the temporal frequency component of image input above 10 Hz. We have confirmed the effectiveness of this filter by conducting a computer simulation based on the “Pocket Monsters” video image sequence that caused the widespread incidence of what appeared to be photosensitive seizures.

Visual cortical cells show a wide variety of preferences for visual stimuli. For example, some cells show selectivity to the orientation of a bar, and other cells are selective to color, spatial frequency, motion direction, or speed (Livingstone and Hubel, 1988). Among these preferences, the temporal frequency preference, however, varies much less than other preferences.

Mikami et al. (1986) reported the distribution of the most preferred spatial or temporal intervals of monkey V1 and MT cells for a two-step flashing bar apparent motion stimuli (Fig. 1). There is a great deal of variation, but with a systematic increase in the most preferred spatial intervals depending on the eccentricity. The means and the deviations of the preferred spatial interval of area V1 are smaller than those of area MT. The deviations of the most preferred temporal interval, however, do not depend on the eccentricity, even tuned to about 10 Hz, and are very similar for the two areas. Since the most preferred speeds are much faster with MT cells than with V1 cells, the wide variation in the most preferred speeds in the V1 and MT cells are based on the variation in the most preferred spatial frequency, not on that of the most preferred temporal frequency.

Nevertheless, the variation in the preferred speed is greatest in area MT among the visual areas (Logothetis, 1994), the most preferred temporal frequency for area MT cells is about 10 Hz, which is the same as for area V1 cells. Therefore, the most preferred temporal frequency is likely to be the same in a cell population taken from the other visual areas. This physiological evidence suggests that visual stimuli flickering at around 10 Hz might highly excite many cells simultaneously, or, in other words, resonate. Such resonance will cause neural fatigue in the brain at least, and is likely to be a principal cause of content-dependent VDT hazards.