| | Analysis of photo-pattern sensitivity in patients with Pokemon-related symptomsReceived 22 January 2002; accepted 11 June 2002. Abstract This study was designed to analyze photo-pattern sensitivity in patients who developed acute neurologic symptoms associated with watching an animated television program, “Pokemon.” The 18 patients (13 females and five males) underwent electroencephalograms and photo-pattern stimulation testing, including special stimulation test batteries (strobe-pattern test and cathode ray tube-pattern test). Photo-pattern sensitivity was confirmed in 16 patients with and without seizure episodes. The strobe-pattern test including a white flickering light test (with eyes open, closed, and open or closed), and the cathode ray tube-pattern test each induced a photo-paroxysmal response in more than 80% of patients. However, with the eyes closed only, as is common in Japan, the photo-paroxysmal response induction rate with a white flickering light stimulus was significantly lower (43%). In the cathode ray tube-pattern test, higher spatial frequencies produced higher rates of photo-paroxysmal response induction. It was demonstrated that underlying photo-pattern sensitivity is more accurately investigated by our method than by standard intermittent photic stimulation alone. By characterizing underlying photo-pattern sensitivity and identifying predisposing factors more precisely, we can develop better guidelines for prevention of a second “Pokemon” incident. According to the results of the present cathode ray tube-pattern test, pattern sensitivity (especially spatial resolution) appears to also be involved in Pokemon-related symptoms, in addition to chromatic sensitivity.
Introduction  During the broadcasting of an animated television program, “Pocket Monster” on December 16, 1997, many in the viewing audience (mostly children) developed acute symptoms, including seizures, and were brought to medical facilities throughout the country. According to a report by the investigative team of Television Tokyo, the company responsible for broadcasting the program, the putative image, consisting of continuous alternately flickering red and blue lights that occupied almost the entire screen (60-100%) and lasted for approximately 4.5 seconds, was responsible for the incident [1]. As the red and blue colors were alternated at intervals of 1/20 and 1/30 of a second, respectively, the reversal frequency of “red-blue” was approximately equivalent to a 12-Hz flickering light. Subsequent studies have implicated these alternating color images, especially long wavelength reds, as being primarily responsible for provocation of photosensitive seizures [2]. For the past several years, we have been conducting a specially designed electroencephalographic test using computer-generated pattern elements on patients who experienced an electronic screen game-induced seizure to elucidate the pathophysiology of this condition and establish preventive measures [3]. In this study, we analyzed patients who experienced seizures and other symptoms associated with watching the aforementioned television program, using the same methodology, and we discuss herein the recently compiled guidelines [4] for picture techniques involved with animation programs. Patients and methods Patients Eighteen patients who experienced seizures or other symptoms while watching the aforementioned television program and were brought to the outpatient clinic of our hospital were included in this study. At the onset of these Pokemon-related symptoms, their ages ranged from 8 years, 3 months to 24 years, 6 months (mean, 12 years, 8 months ± 3 years, 7 months). Of these 18 patients, 13 were females and five were males (male to female ratio, 1:2.6). Before the test, the details of the procedure were thoroughly explained to the patients and their families, and informed consent was obtained. Clinical analysis A detailed clinical evaluation, including previous history of afebrile seizures was obtained, and the clinical manifestations of the acute neurologic symptoms were investigated. The clinical and electroencephalogram findings were separately analyzed in patient groups with and without a history of afebrile seizures, and were also compared between the two groups. Electroencephalogram analysis Routine electroencephalogram examinations, including standard intermittent photic stimulation, in the awake and sleeping states, were conducted in all patients before the following special test. Then, a specially designed photo-pattern stimulation test using a strobe light stimulus (strobe-pattern test, as described by Takahashi et al. [5], [6], [7]) was successively performed in 14 patients, and the test using computer-generated pattern elements displayed on a cathode ray tube display was performed in all cases. The test was conducted during the day, while the patients were fully awake after a good night’s sleep. In each procedure, electroencephalogram signs of a photo-paroxysmal response were monitored and the stimulation was immediately interrupted if a sign was recognized. The succeeding stimulation was resumed only after electroencephalogram recovery had been confirmed. The development of diffuse spike-and-wave complexes was defined as photo-paroxysmal response provocation (type 4 of Waltz classification) [8]. When the amplitudes of these spike-and-wave complexes were focally exaggerated despite the electroencephalogram pattern being diffuse or the spike-and-wave complexes locally preceded the diffuse pattern, the particular brain region was designated as photo-paroxysmal response-dominant. When spike-and-wave complexes were localized (type 2 or 3 of Waltz classification) rather than generalized, a designation of photo-paroxysmal response-negativity was assigned. The strobe-pattern test and cathode ray tube-pattern test methods were described in greater detail in our earlier report [3]. Strobe-pattern test In addition to the standard white intermittent photic stimulation, various filters that produce impressions of a deep red color (more than 600 nm, stimulates only red cones), polka dots, and oblique line patterns, were mounted on the front part of a flicker device (Nihon Kohden lamp, LS-703A, 0.6J, Tokyo, Japan) at a distance of 30 cm from the eye. The flicker frequency was set at 3-30 Hz. Each stimulation was delivered at 10-second intervals and lasted for 10 seconds. Stimulation with white light was also evaluated with the eyes closed, with the eyes open, and with the eyes open or closed (with the eyes closed when the flicker stimulation was began, open during the test, and closed again during stimulation). Reactions were also observed while the patients gazed at patterns produced by polka-dot and oblique-design filters. To elucidate the sensitivity of each procedure, the number of individuals exhibiting photo-paroxysmal response-positive responses was counted and the induction rate of each procedure was also calculated. Stimulation for each procedure was assessed as provocative when there was a single patient who demonstrated photo-paroxysmal response more than once at all frequencies. Cathode ray tube-pattern test The following computer-generated patterns were displayed on a 21-inch multimonitor and electroencephalogram responses were recorded simultaneously, to elucidate which pattern elements were most responsible for photo-paroxysmal response development in each patient. The elements of the patterns used for stimulation were divided and tested in the following sequence. (1) Spatial resolution was studied with (a) vertical stripes, (b) horizontal stripes, (c) checkered patterns, (d) grid patterns, and (e) concentric circles, the spatial frequencies of which were set at 1, 2, and 4 cycles per degree. For a comparative evaluation, well-defined patterns (a border consisting of square waves) and not so well-defined patterns (a border consisting of sine waves) were also prepared (Fig 1). (2) Brightness perception was examined with checkered patterns: one in yellow and blue, complementary to yellow with limited brightness change, and another in yellow and black with an exaggerated brightness change. (3) Pattern-movement recognition was achieved with lines moving in four directions (horizontally and vertically) at five speeds (3, 6, 12, 24, and 48 degrees/second), and circles moving in two directions (centripetally and centrifugally) at five speeds (1, 2, 4, 8, and 16 degrees/second). For the first two elements, a pattern-reversal stimulation method was used, at pattern-reversal frequencies of 5, 10, 20, and 30 Hz. The number of individuals demonstrating photo-paroxysmal response-positive responses was counted to calculate the induction rate of each procedure. Stimulation for each procedure was judged to be provocative when there was a single patient who demonstrated photo-paroxysmal response more than once for all stimuli. To determine the effects of spatial frequencies, pattern border definitions, changes in the brightness of pattern elements, pattern-reversal frequencies, pattern types and pattern movements on photo-paroxysmal response provocation, the photo-paroxysmal response frequencies were compiled for each stimulation in a single test patient and averaged for each individual. Statistical analysis Data were expressed as means ± standard deviation. Student t test was used to compare the means of the two groups. Statistical significance was set at P < 0.05 for the two-tailed test. Results Clinical analysis Among the 18 patients, 11 patients (61%) had no history of afebrile seizures, whereas the remaining seven patients had previously experienced afebrile seizures. Six of the seven patients had been treated with antiepileptic drugs under a diagnosis of epilepsy. With the exception of two patients (a brother and sister), no patient had a family history of photosensitivity. The clinical details are separately described below. Group A Group A included patients with no history of afebrile seizures (Patients 1-11 in Table 1). The mean age at the event was 12 years, 8 months ± 1 year, 6 months, and 73% of the patients were female. With the exception of Patient 4, all patients had experienced seizures at the event. Patient 4 suffered only discomfort but later had a seizure while viewing another program and was thus included in this study. Six (55%) patients reported a family history of seizure disorders. Four patients demonstrated partial elements suggesting a “partial origin” at the seizure onset. The mean duration of the seizure was 4.2 ± 1.8 minutes. All but three patients suffered symptoms while watching the television program, and the remaining 27% were symptomatic while watching a video recording of the program in question. | | |  | Patient | Age at Onset | Sex | Family History | Past History | Antiepileptic Drugs at Onset/Examination | Symptoms | Seizure Duration | Relation to TV-program | |
|---|
| 1 | 10 yr 1 mo | F | – | – | – | – | 2nd GTCS | 2.5 min | During | |
| 2 | 10 yr 4 mo | F | aFS | – | – | – | GTCS | 1.5 min | Video | |
| 3 | 11 yr 3 mo | F | – | – | – | – | 2nd.GTCS | 5 min | During | |
| 4 | 12 yr 3 mo | F | Epi | – | – | – | Discomfort | | During | |
| 5 | 12 yr 9 mo | F | FS | – | – | – | CPS | 5 min | Video | |
| 6 | 13 yr 5 mo | F | – | – | – | – | GTCS | 5 min | During | |
| 7 | 13 yr 10 mo | F | – | – | – | – | GTCS | 7.5 min | During | |
| 8 | 13 yr 11 mo | F | Epi | Visual disturbance (TV) | – | VPA | 2nd GTCS | 2.5 min | During | |
| 9 | 13 yr 3 mo | M | FS | FS | – | – | GTCS | 2.5 min | During | |
| 10 | 13 yr 9 mo | M | FS | – | – | – | GTCS | 5 min | Video | |
| 11 | 14 yr 4 mo | M | – | – | – | – | GTCS | 5 min | During | |
| 12 | 8 yr 5 mo | F | FS | Epi | VPA | VPA | Visual disturbance | | During | |
| 13 | 8 yr 8 mo | F | aFS | aFS (TV) | – | VPA | 2nd GTCS | 30 min | During | |
| 14 | 12 yr | F | – | Epi | VPA+ | VPA+ | Eyelid myoclonia | | During | |
| 15 | 14 yr 5 mo | F | – | Epi | VPA | VPA | GTCS | 4 min | During | |
| 16 | 24 yr 6 mo | F | – | Epi | VPA+ | VPA+ | Absence | 10 seconds | During | |
| 17 | 8 yr 3 mo | M | FS | Epi | VPA | VPA | Discomfort | | During | |
| 18 | 13 yr 1 mo | M | FS, aFS | Epi | Other | Other | Consciousness↓ | | During | | | | |
Two patients, a brother and sister (Patients 5 and 10), had been watching the video together and developed symptoms simultaneously. Two other patients (Patients 4 and 8) watched other television programs later, which apparently provoked a seizure. At the time of examination, only Patient 8 had been taking valproate. Group B Group B included patients with a history of afebrile seizures (Patients 12-18 in Table 1). The mean age at the event was 12 years, 9 months ± 5 years, 9 months. Female sex was predominant in this group (females, 71%). Five patients (71%) had been on valproate alone or were taking multiple agents, including valproate, whereas the other patients had been treated with multiple agents not including valproate at the time of symptom onset. A family history of seizure disorders was reported in four patients (57%) in this group. The epileptic seizures had been provoked by visual stimuli other than the Pokemon program in six (86%) patients. These visual stimuli included television programs (five patients), television games (two patients), and pattern stimulation (one patient). In addition to seizures (three patients) the symptoms were variable and included visual disturbance, eyelid myoclonia, discomfort, and consciousness impairment. At the time of examination, all seven patients were being treated with antiepileptic drugs. Among them, six (86%) patients were taking valproate alone or multiple agents including valproate. Routine electroencephalogram findings (Table 2) Group A Paroxysmal epileptic discharges were evident on electroencephalogram while awake in seven (64%) patients. These discharges were generalized in five patients, and focal (occipital dominant) in two patients. Three (43%) of the seven patients demonstrated generalized epileptic discharges on their electroencephalogram while asleep. During the acute stage, electroencephalogram recording was performed in nine patients (Patients 3-11) at another medical facility, and the standard intermittent photic stimulation provoked photo-paroxysmal response in only two patients (Patients 8 and 9). In the remaining seven patients, photosensitivity was confirmed for the first time at our electroencephalogram facility. Patient 8 had already began valproate treatment at another hospital, and photo-paroxysmal response had become negative before the test was conducted at our department. | | | | Patient | Awake | Sleep | HV | IPS (Previous) | Strobe-Pattern | CRT-Pattern | |
|---|
| 1 | A(G) | A(G) | NE | NE | A(OC, O, dot) | N | |
| 2 | A(F) | NE | NE | NE | A(C, O, red, dot) | A(Oc) | |
| 3 | A(F) | N | N | N | A(OC, O, dot, obl.) | A(Oc) | |
| 4 | N | N | NE | N | A(C) | A(Fr) | |
| 5 | A(G) | NE | NE | N | NE | A(Oc) | |
| 6 | N | N | N | N | A(OC, O) | A(Oc) | |
| 7 | N | NE | NE | N | A(OC, O, dot) | A(Oc) | |
| 8 | N | N | N | A→N | NE | N | |
| 9 | A(G) | A(G) | A | A | A(OC, C, red) | A(Oc) | |
| 10 | A(G) | NE | NE | N | NE | A(Oc) | |
| 11 | A(G) | A(G) | N | N | A(OC, O, C, red) | A(Fr) | |
| 12 | N | N | N | A | A(OC) | A(Oc) | |
| 13 | N | N | N | A→N | NE | A(Oc) | |
| 14 | N | N | N | A→N | N | N | |
| 15 | A(G) | A(G) | A | A→N | A(obl.) | A(Oc) | |
| 16 | N | A(F) | A | A→N | A(OC, O, C, obl.) | A(Oc) | |
| 17 | A(G) | A(G) | N | A→N | A(O, C, dot) | A(Oc) | |
| 18 | A(G) | A(F) | N | A | A(OC, red, dot, obl.) | A(Fr) | | | | |
Group B Generalized epileptic discharges were evident on the electroencephalogram in three patients (43%) while awake. Paroxysmal epileptic discharges were also evident in four patients (57%) while asleep. These discharges were generalized in two patients, and focal (frontal dominant) in two patients. Photo-paroxysmal response had previously been confirmed in all patients by standard intermittent photic stimulation during routine electroencephalogram examinations. However, photo-paroxysmal response became negative in five patients (Patients 13-17) who had been receiving treatment at the time of the most recent standard intermittent photic stimulation examinations. Photo-paroxysmal response induction by strobe-pattern test and cathode ray tube-pattern test (Fig 2) In Group A, photosensitivity was recognized in all but one patient (Patient 8) (by strobe-pattern test in 8 of 8 (100%) and by cathode ray tube-pattern test in 9 of 11 (82%)). In Group B, all but one patient (Patient 14) exhibited photosensitivity (by strobe-pattern test in 5 of 6 (83%) and by cathode ray tube-pattern test in 6 of 7 (86%)). There was no significant difference in the incidence of photosensitivity between the two groups (P > 0.05). Although photosensitivity was not elicited in two patients (Patients 8 and 14) by the present special tests, both patients had a photosensitive response during routine electroencephalogram examinations at some point before these tests (before the introduction of valproate). Thus, photosensitivity was recognized in all 18 patients but was confirmed only in 16 patients (89%) in the present study. In comparing the percentage of photo-paroxysmal response induction by various stimuli, the strobe-pattern test (all tests) induced a response in 93% (13/14); white flickering light (all tests with eyes open, closed, and open or closed) in 86% (12/14); white flickering light (with eyes open or closed) in 64% (9/14); white flickering light (with eyes open) in 57% (8/14); white flickering light (with eyes closed) and polka dot flickering in 43% (6/14) each; red light flickering and oblique line flickering in 29% (4/14) each; pattern gazing in 7% (1/14); and the cathode ray tube-pattern test (all tests) in 83% (15/18). Significant differences in the photo-paroxysmal response induction percentage were evident between the white flickering light (with eyes closed) and three other tests: the strobe-pattern test (all tests), white flickering light (all tests), and the cathode ray tube-pattern test (all tests) (P = 0.0067, P = 0.0461, and P = 0.0265, respectively). No significant difference was evident between white flickering light (with eyes closed) and red light flickering or oblique line flickering (P = 0.6946). In the cathode ray tube-pattern test, repeated exposure to stimuli caused some patients a certain amount of discomfort or signs heralding neurologic symptoms simultaneously with photo-paroxysmal response provocation. Photo-paroxysmal response production and each element of the cathode ray tube-pattern test The details of photo-paroxysmal response production elicited by each element of the pattern were assessed in the 15 cathode ray tube-pattern-positive patients (Fig 3). Pattern-reversal test Spatial resolution: (a) Comparison of spatial frequencies (fineness of the patterns): The number of individuals demonstrating photo-paroxysmal response-positive responses was nine patients for 1 cpd, 14 patients for 2 cpd, and 15 patients for 4 cpd, with induction rates of 60%, 93%, and 100%, respectively. It was demonstrated in all patients that the higher the spatial frequency applied, the higher the photo-paroxysmal response induction percentage. Specifically, the mean photo-paroxysmal response induction in each individual for each spatial frequency was 1.5 ± 2.4 for 1 cpd, 16.2 ± 11.5 for 2 cpd, and 21.5 ± 15 for 4 cpd, all after delivery of 48 stimuli. Statistical evaluation revealed significant differences between stimuli of 4 and 1 cpd, 2 and 1 cpd, and 4 and 2 cpd (P = 0.0001, P < 0.0001, and P = 0.02, respectively). (b) Comparison of pattern border definition: The number of individuals demonstrating photo-paroxysmal response-positive responses was 15 each for square waves and sine waves, each with an induction rate of 100%. When patterns with more clearly defined borders (square waves) were used, the patients were more likely to experience photo-paroxysmal response provocation, although the difference was not marked (P = 0.27). (c) Comparison of pattern types: Vertical and horizontal stripes, concentric circles, and checkered patterns were found to produce photo-paroxysmal response more frequently than grid patterns (P < 0.05). Specifically, the mean photo-paroxysmal response induction in each individual was 8.9 ± 4.3 for vertical stripes, 7.9 ± 4.9 for concentric circles, 7.1 ± 4.8 for horizontal stripes, 6.4 ± 6.2 for checkered patterns, and 3.5 ± 3.2 for grid patterns, all after delivery of 24 stimuli. Brightness perception (comparison of changes in the brightness of pattern elements): There was no noteworthy difference in the photo-paroxysmal response induction percentage after stimulation with a yellow-blue pattern vs a yellow-black pattern (P = 0.14). Photo-paroxysmal response induction by different pattern-reversal frequencies in the spatial resolution test and brightness perception test: Pattern-reversal frequencies at 10 and 30 Hz were found to effectively induce photo-paroxysmal response provocation (P = 0.002 between 10 and 5 Hz, P = 0.08 between 20 and 5 Hz, and P = 0.004 between 30 and 5 Hz). Pattern movement test (movement recognition) Photo-paroxysmal response was provoked in only three patients by pattern movement stimulation. Specifically, photo-paroxysmal response was induced only once in Patient 4 by a centrifugally moving circle (8 degrees/second), in Patient 10 by ascending lines (3 degrees/second), and in Patient 11 by lines moving from right to left (48 degrees/second). The frequency of photo-paroxysmal response induction was unaffected by the speed or direction of the movement, although statistical evaluation was difficult because of the small number of inductions. Evaluation of anatomic sites favoring photo-paroxysmal response development (Table 2) The 15 patients who developed photo-paroxysmal response during the cathode ray tube-pattern test were roughly divided according to photo-paroxysmal response dominance into two groups: one group demonstrated occipital dominance and comprised the majority of patients (Patient 2, 3, 5-7, 9, 10, 12, 13, and 15-17), the other group demonstrated frontal dominance and represented one fifth of the patients (Patients 4, 11, and 18). Interestingly, pattern movements provoked photo-paroxysmal response in only three patients (Patients 4, 10, and 11), with two of the three patients belonging to the frontal dominance group. Summary of the cathode ray tube-pattern test results Among the conditions for the various pattern elements, those with higher spatial frequencies (i.e., finer patterns) resulted in higher photo-paroxysmal response induction frequencies. On the other hand, neither pattern border definition nor changes in the brightness of pattern elements had any effect on the photo-paroxysmal response induction frequency. The conditions provoking the highest photo-paroxysmal response induction rate were a vertical pattern with a spatial frequency of 4 cpd, and a pattern-reversal frequency of 10-30 Hz. All 15 patients who responded to the cathode ray tube-pattern test also responded to these conditions. Discussion Causes of symptoms other than seizures or loss of consciousness According to the survey (by questionnaire) on Pokemon-related incidents conducted by the authors in the Kanto Area, among 325 patients who developed various symptoms, photosensitivity was confirmed in only 41.1% of the group experiencing seizures or a loss of consciousness and in 23.8% of the group with other symptoms [9]. It appeared likely that both patient groups included a number of false-negative results dependent on the test methods or antiepileptic drug treatment. Enoki et al. [10] also conducted a detailed electroencephalogram study on 20 patients who developed acute symptoms while viewing the same television program. Among these 20 patients, photosensitivity to standard intermittent photic stimulation was confirmed in 92.3% of the group experiencing seizures and in 14.3% of the group with other symptoms. There was a significant difference in photo-paroxysmal response incidence between the groups. It was thus concluded that nonconvulsive symptoms, such as appetite loss, nausea, headache, vomiting, and feeling unwell, did not represent photosensitivity but rather autonomic dysfunctions caused by strong photic stimulation. These symptoms were believed to be closely linked to physiologic reactions. Of the five patients who developed symptoms other than seizures or loss of consciousness in the present series (Patients 4, 12, 14, 17, and 18), four demonstrated photo-paroxysmal response on the strobe-pattern test or the cathode ray tube-pattern test. Although the remaining patient was negative for photo-paroxysmal response in the present test, she had a history of photo-paroxysmal response during a previous routine electroencephalogram examination. Thus, photosensitivity was strongly suggested to be involved in these symptoms in all five patients. In two of the four patients, photo-paroxysmal response was initially negative with exposure to the standard intermittent photic stimulation and became positive when the present special tests were conducted, as indicated in Table 2. Thus it is suggested that underlying photo-pattern sensitivity can be more sensitively investigated by our method than by standard intermittent photic stimulation alone, even in those with nonconvulsive symptoms and presumably mild photo-pattern sensitivity. Because four of the five patients had already been treated with antiepileptic drugs, their seizures might have been camouflaged and may have appeared to be nonspecific autonomic symptoms. Therefore the etiology should be clarified in individuals who develop nonconvulsive symptoms for the first time while viewing television or playing an electronic screen game, by means of detailed tests such as those described herein, ideally in the untreated state. Comparison of photo-paroxysmal response induction percentages Panayiotopoulos [11] divided eye states on photic stimulation into three categories: with the eyes open, closed, and open or closed (closure), and compared the photo-paroxysmal response induction percentages among these three states. He reported the eye closure state during intermittent photic stimulation to be the most sensitive state for detecting photo-paroxysmal response. Kasteleijn-Nolst Trenite [12] also recognized the importance of this test in the aforementioned three different eye states and inferred that if the test is not conducted under these conditions, photo-paroxysmal response may be missed in 20-40% of patients with photosensitivity. In the strobe-pattern test conducted in the present study, there were no significant differences in photo-paroxysmal response induction percentages among the three eye states. However, combining the stimuli under these three eye states raised the photo-paroxysmal response induction percentage significantly and diminished false-negative results. In Japan it is customary to apply intermittent photic stimulation while the patient is resting with the eyes closed. Thus some of the photo-paroxysmal response induction results reported from Japan might be lower than the expected rate because of the difficulty in eliminating false-negative reactions because of the insufficiency of a single test. Takahashi et al. reported that low luminance red flickering stimulation or flickering geometric pattern stimulation, rather than the conventional high luminance strobe flickering stimulation, is more useful for analyzing patients with visually provoked seizures unearthed by the Pokemon incident [6], [7]. Tanabe et al. [13] examined 12 patients who reported symptoms that developed while viewing the Pokemon program and concluded that a flickering stimulus using a red filter more effectively induced photo-paroxysmal response than did the conventional white flickering stimulus. In the present series, five patients (Patients 1, 3, 7, 15, and 18) did not react to the conventional white flickering stimulus (with the eyes closed), and photo-paroxysmal response was induced with exposure to a red flickering stimulus or flickering pattern stimulation. As stated earlier, however, the percentage of photo-paroxysmal response induction can be significantly increased even with white flickering stimuli by combining different methods (such as utilizing the three eye states). In fact, using the white flickering stimulus and the three eye states, the induction rate was increased to more than those produced by a red flickering stimulus and flickering pattern stimulation. It is essential to be aware that photo-pattern sensitivity is heterogeneous, and various methods of administering stimuli should be combined to detect this sensitivity. Similarly, the percentage of photo-paroxysmal response induction was significantly higher with the cathode ray tube-pattern test (with all tests) than with the conventional white flickering stimulus (with the eyes closed). The disadvantages of the cathode ray tube-pattern test include the need for special equipment and the concentration required to watch the monitor. This test is also time consuming. According to the present study, all 15 patients who responded to the cathode ray tube-pattern test demonstrated photo-paroxysmal response with a vertical pattern with a spatial frequency of 4 cpd and a pattern-reversal frequency of 10 Hz or greater. Thus it is suggested that the pattern-reversal stimuli used under these conditions can be applied to screening patients for the cathode ray tube-pattern test. On the other hand the advantages of the cathode ray tube-pattern test should not be ignored. By simultaneously recording electroencephalogram while watching the cathode ray tube display (which is a conventional apparatus utilized routinely), the test patient can be informed and can recognize which pattern stimulus (appearing on the screen) has just triggered photo-paroxysmal response at the moment it happens. Through this experience the test patient can be taught how to avoid symptom recurrence. There was no marked difference in photo-paroxysmal response induction percentage between the strobe-pattern test (with all tests) or white flickering stimulus (with all tests) and the cathode ray tube-pattern test (with all tests). It was difficult to determine which test is more effective in detecting photo-paroxysmal response. However, there were differences in the stimulatory conditions, such as luminance and areas of stimulation. A simple comparison was thus believed to be inadequate. Etiologic mechanism Regarding the etiologic mechanism, the pattern stimuli used for the cathode ray tube-pattern test in the present study were divided into three elements for analysis, spatial resolution, brightness perception, and pattern movement recognition. These are major concepts often discussed in the field of information processing for visual perception. According to Livingstone and Hubel [14] the visual information processing pathway from the retinal level to the cerebral cortex beyond the primary field of visual perception is roughly divided into two major systems: the P cell system (blob and interblob) and the M cell system. Each participates in selective information processing tasks. As reported by Harding [2] and Tobimatsu et al. [15], it is highly likely that color stimulation is the cause of the Pokemon-related symptoms observed in the present study. Based on evidence from the present study that spatial frequencies and pattern types are major determinants of the photo-paroxysmal response induction percentage, although changes in the brightness of pattern elements are unimportant, it appears likely that the P cell system, especially the interblob system that excels in color and spatial resolution, is the most important visual information processing pathway. According to studies on pattern-sensitive epilepsy by Wilkins et al., oriented lines or oscillating patterns are considered to be more epileptogenic than checkerboards or static patterns. A spatial frequency of 2-4 cycles per degree and a reversal frequency of 10-20 Hz have also been found to be highly epileptogenic [16], [17], [18]. However, Binnie and Wilkins [18] also stated that the M cell pathway plays an important role in the two visual information processing systems described above in relation to the pathophysiology of pattern-sensitive epilepsy. They cited the following to illustrate the importance of this pathway (i.e., the patterns of stripes that differ in brightness are far more epileptogenic than those with stripes that differ only in color, strongly suggesting participation of the M cell pathway without color information). Thus, in this regard, the results of the present study are inconsistent with those of their experiments. However, the group with Pokemon-related symptoms also included three individuals in whom photo-paroxysmal response was induced not only by pattern-reversal stimuli but also by pattern movements. It was interesting that two patients (Patients 10 and 11) were males, although females predominated over the patients, and two patients (Patients 4 and 11) belonged to the frontal dominance group. In our previous study on electronic screen game-induced seizures, there were two patients (one male and one female, both belonging to the frontal dominance group) among 17 patients with electronic screen game-induced seizures, who demonstrated photo-paroxysmal response in response to both pattern-reversal stimuli and pattern movements, and in whom we considered the possibility of involvement of the M cell system [3]. Thus in the present study we could not establish the usefulness of pattern movement tests from the standpoint of the sensitivity of these tests for detecting photosensitivity. However, movement tests might be useful for identifying patients with some excitability of both P cell and M cell systems. It is necessary to compile such cases and to analyze the underlying pathophysiology . Preventive methods The “Guidelines for picture techniques involved with animation programs, etc.,” which was jointly prepared in 1999 by NHK (The Japan Broadcasting Corporation) and the National Association of Commercial Broadcasters in Japan, set restrictions on brightness inversions of high-contrast images, sudden scene changes, and the use of regular patterns, in addition to flickering images or lights, especially pure red color [4]. The results of the present study justify and support these guidelines. Specific explanations of patterns are provided through the description of their types or spatial frequencies, etc., in the commentary section (with high risk in the 2-4 cpd range). It appears that the results of the present study substantiate the statements in the guidelines. In the future, detailed regulations based on such scientific data should be added to the guidelines. To prevent recurrence of the incidents described herein, characterization of photo-pattern sensitivity by the medical profession and efforts at early discovery of predisposing conditions are important, in addition to self-regulation by the industry. As evidenced in the present study, it is possible to increase the sensitivity of photo-paroxysmal response induction and to allow test patients to provide a greater amount of pertinent information on how they are able to avoid visual stimuli, transmitted through a television (cathode ray tube) monitor, which might trigger neurologic events. Such steps will be essential for disseminating photosensitivity-related information. It will also be important to establish a special medical facility in which patients strongly suspected to be susceptible to these types of seizures may be examined using various specially designed photo-pattern stimulation methods, with consideration given to the heterogeneity of photosensitivity (e.g., with respect to wavelength, pattern, color, and flickering). References [1].
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