ORIGINAL ARTICLE
Factors affecting the occurrence of high-frequency
positive occipital sharp transients of sleep
Masanori Saito, Tetsuhiro Ishida,1 Makoto Nakamura,1 Masahiro Ihara2 and Ikurou Murase3
Department of Psychiatry, Kitasato University School of Medicine, Sagamihara-shi, Kanagawa, 1Department of
Neuropsychiatry, Keio University School of Medicine, Shinjuku-ku, Tokyo, Department of 2Pediatrics, and3Neurosurgery, Saiseikai Utsunomiya Hospital, Utsunomiya-shi, Tochigi, JAPAN(Received for publication on September 18, 2002)

Abstract. To elucidate possible factors affecting the incidence of high-frequency positive occipital
sharp transients of sleep (H-POSTS) and to investigate whether H-POSTS are associated with any
speciﬁc pathophysiological conditions, we visually inspected on consecutive routine diurnal EEGs
(2705 records) using our criteria for identiﬁcation of H-POSTS. H-POSTS were observed in 164
records (subject age: 7.9 to 83.2 years). The incidence of H-POSTS was lower in subjects with highly
abnormal EEG ﬁndings. Female subjects had a signiﬁcantly higher incidence of H-POSTS than males
that could not be explained by other factors. Patients with either fainting or headache had a signiﬁcantly higher incidence of H-POSTS and lower grade of EEG abnormality. The precise mechanismof the gender difference in the incidence of H-POSTS could not be determined. The occurrence of
H-POSTS seemed to be associated with healthy sleep macrostructure, rather than with any speciﬁc
pathophysiological phenomena. (Keio J Med 52 (1): 25–29, March 2003)
Key words: electroencephalography (EEG), sleep, high-frequency POSTS (H-POSTS), gendernormal controls.4 Third, the frequency (around 6 Hz)
and the distribution (often posteriorly-distributed) of
H-POSTS resemble those of 6 Hz positive spikes, whichheve been reported to be associated with behavior disorders.5 However, because these data were collectedfrom groups containing small number of subjects, thepathophysiological signiﬁcance of H-POSTS has not yet
been determined.
Since EEG recordings from a large population areneeded, to strictly determine whether H-POSTS areassociated with any disorders or symptoms, we investigated a large EEG cohort of Keio University Hospitalpatiens to search for factors that might affect the occurrence of H-POSTS.

Introduction
Positive transient EEG deﬂections have been noted
during sleep, and Gibbs et al. referred to the waveforms
as ‘positive spike-like waves in the occipital areas’.1
Vignaendra et al. called them ‘positive occipital sharptransients of sleep (POSTS)’ and described similarities
in shape and topography to lambda waves during
awake records, suggesting that POSTS are related tothe ﬁxation of visual informations.2
POSTS occur singly or repetitively. To our knowledge, Blume and Kaibara3 were the ﬁrst to use theterm ‘high-frequency POSTS’ (H-POSTS) for frequently repeating POSTS and to describe them in detail. Although little is known about repetitive POSTS or
H-POSTS, some have proposed that they are related tocertain pathophysiological conditions. First, Vignaendra et al. presented an example of repetitive POSTS
recorded in a patient with fainting spells,2 not a healthy
subject. Second, dysthymic patients have been reported
to have a signiﬁcantly higher incidence of POSTS than

Methods
Consecutive routine diurnal EEGs recorded at Keio
University Hospital during October 1999 and June 2000
were visually inspected by one of the authors (M.S.).
Only the ﬁrst record during the study period of each

Reprint requests to: Dr. Masanori Saito, Department of Psychiatry, Kitasato University School of Medicine, 2-1-1 Asamizodai, Sagamihara-shi,
Kanagawa 228-8520, JAPAN, e-mail: 7n2ecx@jarl.com
25

26

Saito M, et al: High-frequency POSTS

Fig. 1 An example of high-frequency POSTS (H-POSTS) in a sleep record of a 51-year old male who complained of fainting spells. Simultaneous recordings with different time constants (0.3 sec and 0.1 sec) are shown in P3, P4, O1, O2, T5, and T6. Arrowheads: H-POSTS. Scale:
50 microvolts, 1 sec. Note that there are no obvious differences in the shape, duration, or amplitude of H-POSTS under different time constantconditions.

subject was investigated. The time constant of the ampliﬁer was set at either 0.1 sec or 0.3 sec according to
the situation.
The criteria for identiﬁcation of H-POSTS were:(1) occurs in sleep-pattern EEG, (2) consists of threeor more consecutive positively-peaking deﬂections with
ﬁve to seven waveforms/sec, (3) amplitude of eachwaveform is 30 mV or more, (4) duration of each waveform is 80 msec to 200 msec, (5) distribution is limited
to the occipital area, and to any one of the neighboring
electrode sites, if extended.
The clinical data and EEG ﬁndings were stored in a
Lotus 1-2-3 ﬁle. The EEG abnormality was gradedaccording to the descriptions of Ichijo and Saunders.6
Statistical analyses were performed on ‘Dr. SPSS for
Windows’ (SPSS Japan Co.).
All data management was carefully conducted
exclusively by the authors to protect the subjects’ privacy. No clinical data was registered, if a candidate
refused any EEG recordings.

Results
An example of H-POSTS in the occipital area
recorded simultaneously with two different time constants is shown in Figure 1. No obvious differences inthe shape, duration, or amplitude of the H-POSTS wereobserved out between the two time-constant conditions.
During the investigation period, the 1627 sleep EEG
records among the 2705 total contained 164 records
with H-POSTS (subject age: 7.9 to 83.2 years).
Two subjects (1 male with retinitis pigmentosa who
was legally blind with light perception and 1 male lacking the right eyeball) had disturbances of the retina,
which have been reported to reduce the occurrence rateof POSTS.7 Neither subject slept during the EEG session.
Table 1 shows the age-distribution of the records
containing H-POSTS. The incidence of H-POSTS in
1000 sleep records (age-stratiﬁed and adjusted to theJapanese population of the year 20008) was 96.19.

Keio J Med 2003; 52 (1): 25–29

27

Table 1 Number of Routine EEG Records with Sleep Pattern andthe Incidence of H-POSTS. Diurnal Sleep was Recorded in 1627 of the2705 EEG Records. Note That Children (Under 7.9 Years Old) and
Elderly Subjects (Above 83.2 Years Old) did not Exhibit H-POSTS
Age

0–4
5–9
10–14
15–19
20–24
25–29
30–34
35–39
40–44
45–49
50–54
55–59
60–64
65–69
70–74
75–79
80–84
85–89
90–94
95–99
100 ¼<

Number of sleep
records

Number of records
with H-POSTS

109
54
71
116
188
143
142
95
74
78
137
102
71
93
69
50
25
9
0
1
0

0
1
5
21
25
19
19
11
4
10
12
11
8
9
3
5
1
0
0
0
0

1627

164

Table 2 Number of Sleep Records and Records with H-POSTS
According to the Purpose of the EEG Recording
Number of
sleep records

Number of records
with H-POSTS

638
166
149
75
60
35
33
23
448

62
25
25
7
5
4
0
0
36

1627

164

Epilepsy
Headache
Fainting
Convulsion NOS
Dizziness/Vertigo
Depression
Brain infarction
Febrile convulsion
Others

NOS: not otherwisely speciﬁed.

The number of sleep records with H-POSTS according to the purpose of the EEG recording (classiﬁcationof diagnoses or chief complaints) is shown in table 2.
None of the values in the Table was independent of theothers (df ¼ 8, w 2 ¼ 20:673, p < 0:01). The incidence of
H-POSTS in ‘fainting’ (df ¼ 1, w 2 ¼ 8:12, p < 0:005)
and ‘headache’ (df ¼ 1, w 2 ¼ 5:06, p < 0:05) was signiﬁcantly higher.

Table 3 Relationship Between Incidence Rate of H-POSTS and
Deepest Sleep Stage in 1506 Sleep Records (Subject Age: 5 to 84
Years)
Deepest sleep
stage
Stage 1
Stage 2
Stage 3/4

Number of
sleep records

Incidence rate of
H-POSTS

480
997
29

0.025
0.150
0.100

Table 4 EEG Abnormality and Incidence Rate of H-POSTS. Fiveof the Records Showed a Grade 5 EEG Abnormality during theRegistration Period (2 Isoelectric, 3 Hypsarrhythmia), and 25 Records(Including 8 Records with Sleep Patterns) Showed a Grade 4 Abnormality
Grade of EEG
abnormality
0
1
2
3
4
5

Number of
sleep records

Incidence rate of
H-POSTS

824
88
336
250
8
0

0.110
0.140
0.140
0.060
0.000
–

Because H-POSTS were observed among records
from small age-stratiﬁed groups (5 to 84 years old, 1506
sleep records), the following analysis were performedon the 1506 records.
Logistic regression analysis of the occurrence of
H-POSTS extracted deepest sleep stage (p < 0:001),
degree of EEG abnormality (P ¼ 0:0158), and gender(p < 0:001) as signiﬁcant factors.
Table 3 shows the incidence of H-POSTS according
to deepest sleep stages. The Stage 1 group had a signiﬁcantly lower rate than the other groups (KruskalWallis test, p < 0:001).
Table 4 shows the relationship between the incidenceof H-POSTS and grade of EEG abnormality. Records
with grade 3 or 4 abnormality contained signiﬁcantly
fewer H-POSTS (Kruskal-Wallis test, p ¼ 0:032).
The incidence of H-POSTS in male subjects (54 of
785 sleep records) was signiﬁcantly lower than that infemale subjects (110 of 721; df ¼ 1, w 2 ¼ 27:496,
p < 0:001). No gender difference was observed indeepest sleep stage or EEG abnormality. Records fromsubjects with fainting (df ¼ 2, w 2 ¼ 13:145, p < 0:002)
and headache (df ¼ 2, w 2 ¼ 31:262, p < 0:001) showedsigniﬁcantly less EEG abnormality.
Six-Hz positive spikes were observed in 24 cases(15 of epilepsy, 3 of headache, 2 of dizziness, 1 of convulsion of unknown etiology, 1 of anorexia nervosa, 1 ofdisorientation, 1 of amnesia of unknown etiology). No
signiﬁcant differnces in age, gender, or diagnosis, were28

Saito M, et al: High-frequency POSTS

detected among patients with 6 Hz positive spikes. One
subject exhibited H-POSTS and 6 Hz positive spikes
within the same record.
Discussion
Diurnal EEG and POSTS
Ideally sleep microstructures, such as POSTS, should
be investigated by recording whole-night EEGs orpolysomnograms. However, it is impossible to perform
whole-night studies on hundreds of consecutive subjects
in an EEG laboratory. Diurnal EEG has the advantage
of allowing large numbers of records to be obtained.
Since the incidence of POSTS has been reported tobe highest during the initial 30-minute period of NREM
sleep after sleep onset in both nocturnal and diurnal
sleep,9 the incidence rate of POSTS in routine diurnal
sleep EEGs was expected to be nearly as high as inwhole-night records.
For this reason diurnal routine EEGs are the best
choice for studying POSTS in a large population.
Deﬁnition and detection of H-POSTS
The deﬁnition of H-POSTS must be based on thedeﬁnition of POSTS.2
Repetition can be deﬁned as the presence of two or
more waveforms, however, it is sometimes difﬁcult todistinguish two consecutive POSTS from POSTS anda non-speciﬁc waveform. Since identiﬁcation of threeor more POSTS is deﬁnitely easier and more reliable,
investigations of ‘three or more’ consecutive POSTS
may provide higher quality data than ‘two or more’consecutive POSTS.
Repetitive POSTS has been of interest because it
resembles 6 Hz positive spikes. Thus, the deﬁnition of
the frequency of H-POSTS must be around 6 Hz. Positive repetitive waveforms occurring at around 8 Hz,
which resemble not only POSTS but traces of alphaactivity, rarely occur present during stage 1 NREM
sleep. The frequency restriction of H-POSTS must be
under 7 Hz to exclude this unidentiﬁed waveform.
The incidence of Japanese subjects showing POSTS
has been reported to be 64% (35 of 55 normal volunteers) and 53% (71 of 134 patients with tension
headache, migraine, or neurosis).9 The incidence of
H-POSTS in our study (164 of 1627 records, 96.19 of1000 age-adjusted Japanese population) was lower, possibly because of the strict criteria we used to identify
H-POSTS.
Since inter-inspector reliability for detecting HPOSTS has never been investigated, all EEGs should
be inspected by the same clinical neurophysiologist,
even when the number of EEGs is large.

Each EEG in our study was recorded with a time
constant of 0.1 sec or 0.3 sec. It is generally known that
identical electrical activity may be represented by different waveforms when different time constant settings
are used, thereby possibly inﬂuencing the detection of
H-POSTS. However, as shown in Figure 1, since waveforms with the duration of 200 ms or less (such as
POSTS) were hardly affected at all by the difference
between time constant of 0.1 sec and 0.3 sec, it waspossible to use records with either time constant settingin the study.
Classiﬁcation of the purpose of EEG recording
As shown in Table 2, the purposes of the EEG
recordings could be categorized into according to clinical diagnoses and symptoms. Determining the incidenceof H-POSTS in patients with a certain established diagnosis was one of the purposes of this study. The incidence of H-POSTS in subjects without an establisheddiagnosis was also of interest, because H-POSTS may
be related to phenomena other than clinical entities. In
order search for possible factors (diagnoses, symptoms,
or complaints that) affecting the incidence of H-POSTS,
we did not eliminate the data of subjects in whom a diagnosis had not yet been made.
Sleep macrostructure, EEG abnormality, and H-POSTS
The incidence of POSTS has been reported to behigher in healthy subjects in their 20’s to 40’s who
exhibit well-organized sleep macrostructure, especiallystage 2 or 3 NREM sleep. Conversely, children (inwhom sleep structure is immature10), elderly (in whom
sleep structure changes with age), and patients withbrain disorders that affect sleep structures have beenreported to exhibit fewer POSTS.11 Thus, age andgrade of EEG abnormality are known to be factors affecting the stability of sleep macrostructure.
The characteristics of POSTS described above werealso exhibited by H-POSTS. The occurrence of HPOSTS clearly depended on sleep macrostructure inwhich stage 2 or deeper NREM sleep lasted for tens ofminutes (Table 3).
Theoretically, the occurrence of H-POSTS might berelated to pathophysiological phenomena in three possible ways. (1) The incidence of H-POSTS might increase as brain damage becomes severer. (2) H-POSTS
might represent certain brain reactions to disease, andthus the incidence of H-POSTS might be higher whenbrain damage is mild and lower when brain damage isseverer (because the brain reaction may be weak or
absent) or absent (because there is no need to react).
(3) The incidence of H-POSTS might decrease as brain
damage becomes severer. The ﬁrst (1) or the second (2)Keio J Med 2003; 52 (1): 25–29

models would be the basis of the view that repetitive
POSTS are a marker of certain pathophysiologicalphenomena, however, no evidence of (1) or (2) was
obtained in this study. Our data in Table 4 seem tosupport the third (3) model, in which H-POSTS are
thought to be of no pathophysiological signiﬁcance.
We previously reported that fainting patients tend toshow more H-POSTS than other patients,12 and thepresent study for the ﬁrst time showed a signiﬁcantly
higher incidence rate of H-POSTS in patients withheadache. Nevertheless, at present there is no direct
evidence that either fainting or headache is the cause of
the increased incidence of H-POSTS. One possible explanation is that the incidence of H-POSTS is higher
in patients with fainting or headache because suchpatients have signiﬁcantly lower grade of EEG abnormalities.
Taken together, the above ﬁndings suggest that theoccurrence of H-POSTS might not provide any evidence of a speciﬁc pathophysiological condition, butinstead reﬂect the stability of sleep structure with lesser
grades or no EEG abnormalities.
Gender and H-POSTS
The gender difference in the incidence of H-POSTS
was the most unexpected ﬁnding in this study and could
not be explained by the difference in age or EEG abnormality in each gender population. Only a small partof the difference could possibly be explained by thegender difference in several diseases (e.g., eating disorder, no males and 13 females, 3 H-POSTS; brain tumor,
13 males and 6 females, no H-POSTS, etc.). In othersmaller consecutive EEG cohorts (Saiseikai Utsunomiya Hospital with 281 sleep records and Akebono
Clinic with 117 sleep records, Oct. 1999–Jun. 2000), no
gender difference in the incidence of H-POSTS wasobserved (data not shown), suggesting that the genderdifference of H-POSTS might have arisen from possible
selection bias. To determine whether male subjects tend
to exhibit fewer H-POSTS than females, the incidenceof H-POSTS should be tested in other large populations.
Conclusions
The incidence of H-POSTS was affected by age, degree of EEG abnormality, deepest sleep stage, and29

gender. The incidence of H-POSTS was signiﬁcantly
higher in female patients, but the precise mechanism of
this gender difference has yet to be determined.
The incidence of H-POSTS was signiﬁcantly higher
in patients with fainting or headache. This could possibly be explained by signiﬁcantly lower degree of EEG
abnormality in these subjects.
The occurrence of H-POSTS appears to be associated with a lesser degree of EEG abnormality, rather
than to represent any speciﬁc pathophysiological phenomena.
Acknowledgments: We would like to express special appreciation forthe comments of Tsunekatsu Hara, MD, Rin-no-suke Ryu, MD, thelate Sumio Hara, MD, and Shigeki Takei, MD.

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