Spontaneous eyelid movements during human sleep:

a possible ponto-geniculo-occipital analogue?

RUSSELL CONDU IT 1 , SHE I LA G ILLARD CREWTHER 2 ,

DOROTHY BRUCK 3 and GRAHAME COLEMAN 1

1Department of Psychology, Monash University, Caulfield, Victoria, Australia, 2School of Psychological Science, Faculty of Science and

Technology, La Trobe University, Victoria, Australia, and 3Department of Psychology, Victoria University, St Albans, Victoria, Australia.

Accepted in revised form 6 January 2002; received 9 March 2001

INTRODUCTION

The ponto-geniculo-occipital (PGO) wave is a physiological

event occurring during sleep that is controversially claimed to

form the physiological basis of dream mentation (Hobson and

McCarley 1977; Hobson et al. 2000). However, one of the

most problematic aspects of testing any hypothesis regarding

PGO activity and mentation is that PGO spikes can be studied

extensively in animals with indwelling electrodes, but cannot

be recorded directly in humans. From this, hypothesized

analogues of PGO activity that can be non-invasively meas-

ured in humans, such as phasic integrated potentials (PIPs;

Rechtschaffen et al. 1970) and middle ear muscle activity

(MEMA; Pessah and Roffwarg 1972), have been investigated.

However, previous reviews by Pivik (1991, 1994) and Rechts-

chaffen (1973) critically examining data regarding PIP and

MEMA as indicators of PGO activity and sleep mentation

have argued that these PGO analogues have failed to provide a

distinctive relationship to dream mentation recalled from

sleep. The PGO analogues to date have been considered

inadequate, despite a frequency during sleep reflecting that of

PGO activity, with high activity during rapid eye movement

(REM) and low frequency during non-rapid eye movement

(NREM), peaking just before REM onset (Callaway et al.

1987). This is possibly the result of a ‘less than 1-to-1’

correspondence with PGO activity in animals (Pivik 1991).

More peripheral, but relevant investigations of PGO ana-

logues involve the notion that all phasic muscle activity

represents the activation of a central PGO generator

(Slegel et al. 1991). Most PIPs (Rechtschaffen 1973) and

Correspondence: Russell Conduit, Department of Psychology, Monash

University, 900 Dandenong Road, Caulfield, Victoria, Australia, 3045.

Tel.: +61 39903 2217; fax: +61 39903 2501; e-mail: russell.conduit@

med.monash.edu.au

J. Sleep Res. (2002) 11, 95–104

SUMMARY The aim of the present study was to investigate whether eye lid movements (ELMs)

were temporally related to the activity of other skeletal musculature and to proposed

analogues of ponto-geniculo-occipital (PGO) waves during human sleep. Electroen-

cephalogram (EEG), laryngeal-masseter electromyogram (EMG), electrooculgram

(EOG), peri-orbital integrated potentials (PIPs), middle ear muscle activity (MEMA),

ankle flexion (AF) and respiration (RESP) were monitored with ELMs during one

night’s sleep. Results showed that ELMs always occurred during full arousal and

movement time. The ELMs that occurred during sleep were most prominent during

rapid eye movement (REM) sleep, occurred at higher frequency just before REM, and

were observed synchronously with other PGO analogues, supporting the notion that

ELMs may be an indicator of PGO activity in humans. Of the ELMs observed during

sleep, 16% showed changes in EOG, PIP, MEMA, AF and RESP simultaneously,

suggesting generalized muscle activation. This coactivation of muscle activity suggested

that the relationship between the muscular measures and PGO activity might be an

indirect one, possibly mediated by alerting mechanisms, previously shown to be related

to PGO waves in animals. Such an interpretation is consistent with the use of ELM as a

widely accepted measure of the eye-blink startle response in awake human subjects.

KEYWORDS alerting, eyelid movements, MEMA, PGO waves, phasic activity, PIP,

startle

� 2002 European Sleep Research Society 95

approximately 50% of MEMA (Slegel et al. 1991) have been

shown to occur with EM activity. Additionally, 75% of

MEMA has been shown to occur with EMs and/or other

motor events (Slegel et al. 1991).

Early research investigating the elicitation of an electrically

induced blink reflex in humans during sleep found that

spontaneous eye-twitch activity occurred during REM sleep

(Ferrari and Messina 1972). Soon after, Orem and Dement

(1974) investigated spontaneous eyelid behaviour in sleeping

cats and its relationship to PGO activity measured at the

laternal geniculate nucleus (LGN). Phasic eyelid twitches were

observed in every REM period. These twitches appeared

throughout REM, with 80–90% associated with EMs. A total

of 28 REM periods were analysed in three cats to determine

the relationship between lid twitches and PGO waves. A

positive case was defined as the presence of a PGO spike either

1 s before or after the onset of a twitch. Using this criterion, an

average of 89% of twitches co-occurred with PGO waves.

Bowker and Morrison (1976) later found that PGO waves

could be elicited by presenting startling tones during sleep. As

the intensity of tones was increased, eye-blinks, neck electro-

myogram (EMG) twitches, body twitches, electroencephalo-

gram (EEG) desynchronization and arousal were also induced

with PGO waves. From this initial work, it was proposed that

PGO activity might be a product of both external and

internally generated startles, in both awake and sleeping

animals (Bowker and Morrison 1976). However, this was

followed by investigations demonstrating that PGO waves

could: (a) be elicited by lower intensity stimuli that did not

result in observable whole body startle activity (Ball et al.

1991), (b) habituate more rapidly than PGO activity (Sanford

et al. 1992), and (c) lacked tight temporal correspondence with

phasic muscular activity such as short-latency respiratory

fractionations (Hunt et al. 1998). Such findings led these

researchers to modify their position, stating that PGO waves

could be more accurately described as an indicator of

‘alerting’, preparing the brain for incoming sensory informa-

tion, rather than simple a reflexive response to a startling

stimulus (Hunt et al. 1998; Morrison et al. 1995; Sanford et al.

1993, 1994).

Previously, Hunt et al. (1998) alluded to the possibility that

discrepancies between PGO and their startle measures could

be produced by a minor activation of the acoustic startle

response (ASR) pathway, not enough to cause awakening or

a whole body response, which is the usual measure of startle.

As eye-blinks are a widely used and accepted measure of

startling in awake human subjects (Filion et al. 1998), this

provides the possibility that fine-motor eye lid movements

(ELMs) measured during sleep could have close correspon-

dence with brain alerting mechanisms by detecting minor

activations of the ASR pathway. Thus, such a measure might

provide a more sensitive external indicator of PGO activity in

humans than whole body startles or previously proposed

PGO indices, such as PIP and MEMA (Pivik 1994).

Currently, it is proposed that spontaneous ELMs in sleeping

subjects are related to arousal, and decreased ELMs in awake

subjects are related to decreased vigilance and sleep onset

(Ajilore et al. 1995). However, at odds with such claims are the

findings of Kralevski et al. (2000), who have found no

relationship between eye-blinks and microarousals (Atlas Task

Force 1992).

Stickgold et al. (1995) have proposed that activity of the

upper eyelid may be related to the activity of the reticular

formation. This relationship is based on the reasoning that the

upper eyelid is contracted by the levator superioris, which is

innervated by the oculomotor nucleus, which, in turn is

innervated from the midbrain reticular system and the pontine

reticular formation (Spencer and McNeer 1991; Stickgold

et al. 1995). However, this is also the same pathway that is

implicated in the activation of superior rectus muscles in the

generation of eye movements during waking (Spencer and

McNeer 1991) and presumably REM sleep (Hobson et al.

2001).

Rectus extraocular muscles have previously been shown to

be closely related to PGO activity in cats and rats (Fredrickson

et al. 1972; Rechtschaffen et al. 1972). Also, the levator

superioris is an extraocular muscle innervated by similar

oculomotor pathways to that of the superior rectus muscle

(Spencer and McNeer 1991). Together, these findings suggest

that an external measure of upper eyelid activity might provide

a useful indicator of PGO activity in humans. If this were the

case, it would be expected that ELMs would occur during

REM. However, Hobson and colleagues (Ajilore et al. 1995;

Pace-Schott et al. 1994; Rowley et al. 1998; Stickgold and

Hobson 1994; Stickgold et al. 1994, 1995) have always

monitored both ELMs and REMs by a small adhesive-backed

piezo-electric film placed directly on the eyelid. If one is

interested in whether ELMs occur during REM, these data

then become problematic because differentiation of ELMs and

REMs during REM sleep using this placement method is

technically difficult.

Using a new technique of measuring ELM activity avoiding

EM artefact, the aim of the present study was to investigate

whether ELMs show a similar distribution across sleep to that

of PGO activity, with high activity during REM and low

frequency during NREM, peaking just before REM onset

(Callaway et al. 1987). A second aim was to determine whether

ELMS were temporally related to proposed indicators of PGO

activity (EMs, MEMA, PIPs) and other phasic muscle activity

(leg movements, laryngeal–masseter EMG) presumably related

to whole body startles during sleep.

It was hypothesized that ELMs, as an indicator of PGO

activity, would occur significantly more frequently during

REM compared with NREM sleep and the frequency of

ELMs during NREM 1 min before REM onset would be

significantly higher than the frequency across NREM in

general. Also, ELMs would occur concurrently with

MEMA, PIP and EM activity more often than expected

by chance. Finally, it was predicted that phasic muscle

activation across all the measures taken during sleep, if

related, would also occur simultaneously at a rate greater

than expected by chance alone.

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METHOD

Subjects

Five female and five male normal, healthy adults aged 18–39

participated as paid volunteers in this experiment. All had

participated in previous sleep experiments and were familiar

with the laboratory conditions. Subjects were told that the aim

of the experiment was to investigate the incidence of various

forms of muscle activity over the course of a single, uninter-

rupted night’s sleep. All procedures were approved by the

La Trobe University human ethics committee.

Apparatus and materials

Sleep was monitored using Grass model 8–16 and model 7

polygraphs (Grass Instrument Co., Quincy, MA, USA). The

output from the model 7 polygraph was interfaced with the

chart output of the model 8–16 polygraph in order to provide a

synchronized output of all measures. These data were then

converted to digital form (Data Translation Inc., model

DT-2801 A, Marlboro, MA, USA) and saved on a PC using

non-commercial graphing software (Trinder 1994).

The EEG, laryngeal–masseter EMG, differential EOG,

PIPs, MEMA, ankle flexion (AF) and respiration (RESP)

made up the recording montage.

Electroencephalograph placements were made to C4-A1 and

C3-A2 according to the international 10–20 placement system

(Jasper 1958).

To record differential EOG, electrodes placed above the

outer canthus of the left eye and at the inner canthus of the

right eye were combined to form one input. Electrodes placed

below the outer canthus of the right eye and the inner cathus of

the left eye were combined to form the second input of the

differential EOG channel. This procedure was adapted from

Slegel et al. (1991), but with outer-upper left canthus and

outer-lower right canthus placements, allowed both horizontal

and vertical EMs to be registered on the one differential EOG

trace.

For laryngeal–masseter EMG recording, one electrode

was attached at the right masseter muscle and the other was

attached at the second laryngeal notch of the throat. This was

carried out to establish an EMG measure for sleep scoring

purposes and to additionally serve as a measure of head

movement and Eustachian tube artefact on the MEMA record

(Slegel et al. 1992).

Middle ear muscle activity was measured using a modified

pressure transducer technique (Slegel et al. 1992). When

adopting this technique, a piezo-resistive pressure transducer

is mounted in a project box and attached to the bed headboard

or wall. A piece of tubing is inserted into the project box and

the other end is inserted into a custom made ear-mould, which

is worn by the subject and then secured with latex glue. In the

present experiment a miniaturized piezo-resistive pressure

transducer (Radio Spares Australia, model 286-658, Sydney,

Australia) was mounted in one arm of a modified audio

headset. Reusable auditory probe tips (Madsen brand;

Feldman and Wilber 1976) fitted over the transducer to

provide a seal within the ear canal. The audiometric tips were

sterilized after use with a chlorhexidine ethanol based disin-

fectant. Various sized and type audiometric tips were used to

maximize comfort and seal. A small amount of petroleum jelly

was applied before the tip was inserted into the ear canal to

ease insertion, prevent chafing and maintain a seal within the

ear canal. This new technique had the advantage of portability,

less chance of artefact, as the tubing to the patient box is

eliminated, and cheaper, more general application, as individ-

ual ear moulds were not necessary. Figure 1 depicts the

modified MEMA measurement device.

PIPs were recorded from a Grass Model 7P3A integrator

preamplifier. Electrodes were placed above and below the

midline of the right orbit, 1 cm above and below the orbital

ridges (Wyatt et al. 1972).

AF was recorded from a piezo-ceramic vibration sensor

(NTK Japan, model EB-T-320) placed just below the external

lateral ankle joint.

Respiration was measured using a twin-pronged nasal

thermister (Radio Spares Australia, model 286-658).

Eye lid movements were measured using a piezo-ceramic

vibration sensor (NTK Japan, model EB-T-320, Tokyo,

Japan), attached with double-sided tape and medical tape just

below the eyebrow on the supra-orbital ridge of the right orbit.

All subjects were situated in a sound-attenuated sleep

laboratory. There was no visual contact between the subject

and experimenter. However, communication was maintained

via intercom.

Procedure

Each subject chose the night as well as the start and end times

of the experiment. Subjects were encouraged to try to mimic

their usual sleep–wake patterns.

Electrodes for EEG, laryngeal–masseter EMG, differential

EOG, PIPs, MEMA, AF and RESP were attached to each

subject for sleep recording.

Calibration procedures

ELM and EOG. Subjects were asked to lightly blink. The

ELM sensitivity was then adjusted to ensure the trace showed

at least 1-cm pen deflection. Subjects were then asked to look

left, right, up and down to ensure clear EOG traces were

present and no artefacts were present on the ELM channel.

AF and EMG. Subjects were asked to move their feet to ensure

clear AF traces were observable. Subjects were then asked to

gently move their head (side-to-side, up-and-down) and then

swallow, to ensure clear laryngeal–masseter EMG traces were

present.

MEMA. As performed previously (Slegel et al. 1992) MEMA

was not calibrated per se. The transducer was determined to be

operative if a baseline pulse resulting from the interior carotid

Spontaneous ELMs during human sleep 97

� 2002 European Sleep Research Society, J. Sleep Res., 11, 95–104

artery moving the tympanic membrane was present (Slegel

et al. 1992). Amplifier sensitivity was adjusted so that the

baseline pulse was at least 5 mm.

PIP and EEG. For PIP, the preamplifier was set at its highest

sensitivity with the lowest threshold and full rectification. The

half (½) low frequency time constant was set at 10 and the

integrator time constant at 0.02 (Wyatt et al. 1972). The EEG

tracings were calibrated at 50 lV ¼ 1 cm.

Sleep and sleep scoring. After the calibration procedures,

subjects were allowed to sleep undisturbed until their predes-

ignated waking time. Sleep scoring was carried out manually in

30-s epochs according to the criteria of Rechtschaffen and

Kales (1968). Microarousals were scored according to Ameri-

can Sleep Disorders Association criteria (Atlas Task Force

1992).

Scoring phasic muscle events. For an EMG, PIP or AF

phasic event to be considered valid in the current study, it

had to show a minimum increase in activity of twice the

baseline amplitude (Slegel et al. 1991). These criteria for

scoring phasic events also applied for MEMA. However, in

addition, MEMA had to independently show phasic activity

for at least 0.3 s, before or after any simultaneous EMG

event, to demonstrate it was not an artefact (Slegel et al.

1991). In order to maintain a strictly conservative count of

ELM activity, an ELM event had to show a minimum

increase in activity of 10 times the baseline amplitude.

REMs of at least 50 lV were scored as phasic EM activity.

When ELM events occurred during microarousals according

to ASDA criteria (Atlas Task Force 1992), these events were

scored as occurring during microarousals from sleep. If

concurrent EEG arousal was longer, the event was classed

as occurring either during movement time (MT) or while

awake according to Rechtschaffen and Kales (1968) criteria.

These events were not scored as occurring during sleep.

These procedures were adopted to clearly distinguish ELM

events during sleep from those during wake and MT.

RESULTS

ELM frequency

All subjects slept an average of 277.3 min in NREM and 75.3

min in REM sleep. When scoring ELM events, the most clear

and consistent result was that ELMs always occurred during

full arousals and MT (Rechtschaffen and Kales 1968). Eye lid

movements were also observed during sleep. Table 1 is a

summary of the frequency of ELMs during REM, NREM,

and 1 min before REM onset during NREM sleep (pre-

REM). The frequency of ELMs during microarousals (Atlas

Figure 1. An illustration of the modified middle ear muscle activity measurement device. A miniaturized piezo-resistive pressure transducer was

mounted in one arm of a modified auditory headset. Reusable auditory probe tips were fitted over the transducer to provide a seal within the ear

canal.

98 R. Conduit et al.

� 2002 European Sleep Research Society, J. Sleep Res., 11, 95–104

Task Force 1992) across the total sleep period is also

included.

Owing to correlation between the mean values and stand-

ard deviations (SD) of the ELM frequencies across condi-

tions, a log10 transformation of this data was conducted

(Kirk 1982, p. 83). Planned contrast tests found that REM

ELM frequency was significantly higher than NREM

(F1,18 ¼ 46.57, P < 0.0001) and pre-REM (F1,18 ¼ 20.24,

P < 0.001) frequency. Also, pre-REM ELM frequency was

higher than overall NREM frequency (F1,18 ¼ 5.37, P <

0.05). Figure 2 is an example polygraph record showing pre-

REM ELM activity. This record was chosen as it demon-

strates ELM events independent of EOG activity, yet

coincident with AF.

Table 1 A summary of the frequency of eye lid movements (ELMs) during rapid eye movement (REM), non-REM (NREM), and 1 min before

REM onset during NREM sleep (pre-REM). The frequency of ELMs during microarousals across the total sleep period is also included

REM 1 min pre-REM NREM Microarousal(ASDA 1992)

Subject

Time

(min)

No. of

ELMs ELMs h)1Time

(min)

No. of

ELMs ELMs h)1Time

(min)

No. of

ELMs ELMs h)1 Arousals h)1 ELMs h)1

1 30 21 42 5 1 12 273.5 32 7 12 2.1

2 83 35 25.3 6 1 10 320 23 4.3 7 1

3 44 12 16.4 3 0 0 246.5 24 5.9 11 2.3

4 57 60 63.2 2 1 30 211 21 6 6 1.3

5 91.5 71 46.8 6 5 50 236 37 9.4 11 2

6 95 87 54.9 10 3 18 263.5 36 8.2 16 2.7

7 64 24 22.5 4 1 15 424 12 1.7 8 1

8 82 75 54.9 5 1 12 269 24 5.4 17 2.9

9 96.5 172 106.9 5 1 12 220 40 10.9 18 3.4

10 110 132 72 7 3 25.7 312 94 18 19 2.7

Average 75.3 68.9 50.49 5.3 1.7 18.47 277.5 34.3 7.68 12.5 2.14

Figure 2. An example record from Stage 2 sleep, within 1 min before rapid eye movement (REM) sleep (pre-REM), showing simultaneous AF and

ELM activity. Key: PIP: phasic integrated potential; EOG: differential electrooculargram; MEMA: middle ear muscle activity; AF: ankle flexion;

EMG: laryngeal–masseter electromyograph; RESP: respiration.

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Coincidence of ELMs with proposed PGO analogues

and other muscle activity

For each of the 1157 ELMs observed during sleep across the

10 subjects studied, the closest occurring PIP, MEMA, EM,

EMG, AF and RESP event was tabulated. Of these, 84.3% of

PIPs, 40.3% of MEMA, 52.0% of EMs, 54.9% of EMG,

59.5% of AF and 43.8% of RESP events occurred within

15.5 s of an ELM event.

Sixty-three per cent of ELMs occurred simultaneously

(within 0.5 s) of phasic changes in EOG, PIP or MEMA. Of

these ELMs, 16% showed changes in EOG, EMG, PIP,

MEMA, AF and RESP simultaneously, suggesting generalized

muscle activation. Polygraph records representative of this

coincident muscle activity during Stage 2 and REM sleep are

shown in Figs 3 and 4. The distribution of these phasic muscle

events in every 31-s window around ELM activity during sleep

is graphed in Fig. 5.

Assuming phasic muscle events would have an equal

probability of occurring at any point within the 31-s window

around each ELM event, Chi-square analysis found a signi-

ficant difference between each of the present distributions and

one of equal distribution across the epoch, which would be

expected by chance (PIP: v230 ¼ 15261.8, P < 0.001; MEMA:

v230 ¼ 6752.7, P < 0.001; EM: v2

30 ¼ 6351.6, P < 0.001;

EMG: v230 ¼ 3920.3, P < 0.001; AF: v2

30 ¼ 3769.6, P <

0.001; RESP: v230 ¼ 3074.9, P < 0.001).

DISCUSSION

The data in the present study indicate that ELMs were

prominent during ongoing sleep. Furthermore, this activity

during sleep occurred in a distinctly disproportionate fashion,

with ELMs appearing much more frequently during REM

compared with NREM sleep, and peaking in NREM just

before the occurrence of REM. This is a pattern of occurrence

strikingly similar to that of PGO activity in the cat (Orem and

Dement 1974).

When considering the data of the present study, one must

remember that a direct measure of PGO activity was not

conducted and is not yet possible in humans. Furthermore, as

previously proposed PGO analogues used in this study, such

as MEMA and PIP, have also not been directly compared

with PGO activity in humans (Pivik 1994), evidence provided

from this study supporting a relationship between ELM and

PGO activity in humans is not based on direct evidence.

Figure 3. An example record from Stage 2 sleep showing simultaneous activation of all other muscle groups with eye lid movement activity. Key:

PIP: phasic integrated potential; EOG: differential electrooculargram; MEMA: middle ear muscle activity; AF: ankle flexion; EMG: laryngeal–

masseter electromyograph; RESP: respiration.

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However, combined with the previous animal results suggest-

ing a relationship between ELMs and direct recordings of

PGO waves (Orem and Dement 1974), the present results,

showing that ELMs are prominent during REM sleep, occur

at a higher NREM frequency just before REM, and occur

synchronously with other PGO analogues, provides conver-

ging evidence that ELMs could be a useful analogue of PGO

activity in humans.

The finding that ELMs concurrently occurred with all other

phasic muscle activity at a rate greater than expected by

chance, shows some support for the notion of a central phasic

generator of motor events during sleep. However, as the

concurrence was not a one-to-one relationship, the present

data can support only a partial synchrony of phasic events.

Further data related to the synchrony of phasic events was

observed in the AF recording. It was originally conceived at

the onset of this experiment that the AF measure would act as

a peripheral striate muscle measure in the observation of

whole-body startles. Indeed, AF was found to have high

concordance with ELM activity and other PGO analogues

(Fig. 5); however, AF was also observed co-occurring with

ELM activity independent of the activation of the other motor

activity measures (Fig. 2).

The explanation for a lack of complete synchrony between

MEMA and EM activity by Pessah and Roffwarg (1972) can

also be put forth as a possible explanation for the discrepancy

in synchrony of muscle measures found in the current findings:

‘…the partial asynchrony of REMs and MEMA does not

necessarily indicate that the primary brainstem dischar-

ges, which originate activation in the different systems are

not synchronous. It indicates only that the end motor

responses recordable with our transducers… are not

simultaneous.’ (p. 776)

At a more in-depth level of recording in animals, Chase et al.

(1994) have found that PGO activity is highly associated with

intracellular depolarization of masseter motor neurons; how-

ever, neural firing is not always observed. A likely reason why

these cells do not always reach threshold is because the

depolarizing currents occur amongst a concurrent hyperpolar-

ization. Thus, in the present experiment, a depolarizing signal

might be present at other motor sites coincident with ELMs,

but it is not observed as overt muscle movement. Although

both of these explanations might account for false negative

findings, they do not explain the occurrence of muscle activity

not related to PGO waves, or false positives (Pivik 1991;

Rechtschaffen 1973). An alternative explanation might be that

Figure 4. An example record of rapid eye movement sleep showing a high frequency of eye lid movement activity and other coincident muscle

activation. Key: EEG: electroencephalograph; EMG: laryngeal–masseter electromyograph; MEMA: middle ear muscle activity; AF: ankle flexion;

EOG: differential electrooculargram; ELM: eye lid movement; RESP: respiration; PIP: phasic integrated potential.

Spontaneous ELMs during human sleep 101

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Figure 5. The average number of phasic muscle events per 100 eye lid movements (ELM) within a 31-s window around each ELM event. Key:

EEG: electroencephalograph; EMG: laryngeal–masseter electromyograph; MEMA: middle ear muscle activity; AF: ankle flexion; EOG:

differential electrooculargram; ELM: eye lid movement; RESP: respiration; PIP: phasic integrated potential.

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the relationship between phasic muscle activity and PGO

waves is not causal, and that phasic muscle activity occurs

conjointly with PGO waves as part of some other form of

generalized body activation or arousal.

Taken together, the results suggest that ELM and all phasic

muscle activity observed during REM might covey an indirect

indication of PGO activity. Some investigators have proposed

spontaneous ‘alerting’ as an explanation for such coactivation

of phasic events during sleep (Hunt et al. 1998; Morrison et al.

1995; Sanford et al. 1992, 1993, 1994). Interestingly, ELM is a

widely used and accepted measure of eye-blink startle in awake

human subjects (Filion et al. 1998). This provides further

evidence to suggest that ELMs during sleep also represent such

spontaneous alerting activity. However, such ‘alerting’ might

simply be related to an arousal mechanism. It could be possible

that the coactivating muscle groups observed in this study

share an indirect relationship where they have common

activation related to arousal from sleep. Indeed, previous

researchers have proposed a relationship between ELM

activity and arousal (Ajilore et al. 1995; Stickgold et al.

1995). Recently, the present authors have proposed that such

spontaneous ‘alerting’ and arousal might be better conceptu-

alized as the activation of attention processes during sleep

(Conduit et al. 2000).

In the present study ELMs were present during all arousal

and movement time according to Rechtschaffen and Kales

(1968) criteria, but relatively infrequent during microarousals

(Atlas Task Force 1992). This result is interesting, as it

suggests that ELMs might not simply be an arousal artefact. It

also provides a possible explanation for the initial appearance

of discrepant results between Stickgold et al. (1995) showing a

relationship between ELMs and full arousal, and Kralevski

et al. (2000) who have found no relationship between blinks

and microarousals (Atlas Task Force 1992).

In concluding, the results of the present study are consistent

with proposals that: (a) ELMs could possibly be a useful

indicator of PGO waves during sleep in humans, (b) ELM

activity could be a product of internally generated ‘alerting’

during sleep or the associated arousal it produces, or (c) ELM

activity may simply be a product of arousal.

Future investigation of ELMs as an indicator of PGO

activity in humans will have to test for relationships that can

account for arousal artefacts. Thus, such studies could

investigate: (a) whether ELMs show PGO-like rebound pat-

terns during REM deprivation experiments (Duysan-Peyre-

thon et al. 1967; Vimont-Vicary et al. 1966) and (b) whether

ELMs show a relationship to dream recall (Pivik 1991, 1994).

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