Contributed articleA comfortable brain-interface to video displays
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.
Section snippets
Temporal cell response
The first step in developing an adaptive video display filter is to model the temporal cell response to visual stimuli. Although each neural cell activity is measured with spikes, we ignored the raw action potentials and incorporated the exchanged contribution of the mean firing rate of spikes to membrane potential over a short time range, and defined an analog variable that represents the cell states.
There are many processes in a cell response. An impulse input evokes a reverse potential
Simulation results
Recorded images of the latter half of the animated TV show “Pocket Monsters, the 38th episode” were analyzed to evaluate the effect of the filter, since this part of the animation is suspected to have caused the headaches, dizziness, and convulsions of many viewers. The TV episode was recorded with a normal-mode VHS home-video recorder, and digitized into 30-frames/s images of 320 × 240 pixels using a carefully tuned Apple Power Macintosh computer system with Radius PCI Video Vision Studio
Discussion
The most effective temporal frequencies to evoke a neural response in the brain have been reported from clinical studies. These reported critical frequencies range from 10 to 30 Hz, depending on conditions: the luminance or color of flashing light, the age of subjects, or whether the eyes are open or closed (Harding, 1998; Kasteleyn-Nolst, 1989). A clinical study using a luminance comparable to that of a TV display showed that the most effective frequency is 15-Hz flicker with red light (
Summary
The most preferred responses of the majority of visual cortical cells are obtained through visual stimuli that have a dominant temporal frequency of around 10 Hz. Evidence suggests that images that include a large amount of flicker at about 10 Hz may cause neural fatigue. This paper proposes that such neural fatigue is a content-dependent VDT hazard, and a comfortable brain-interface should be applied to reduce the risk. This paper provides a quantitative measure of the risk of the
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2010, Applied ErgonomicsCitation Excerpt :To achieve this protection for photosensitive individuals a device that modifies the TV picture at the receiver is required. To prevent visually induced seizures due to flash stimulation Nomura (1999) developed an adaptive temporal filter that able to reduce frame to frame flicker at 10–30 Hz. This was shown to be effective in preventing PPRs in eleven photosensitive patients (Nomura et al., 2000).
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