1995 IEEE-EMBC and CMBEC
Theme 6 Physiological Systems/Modellingand Identification

RELATIONSHIP BETWEEN VIGILANCE LEVELS AND
CHARACTERISTICS OF SACCADIC EYE MOVEMENTAkinori Uenot

Yoshihisa Otat

Moriichiro TakasettHaruyuki Minamitanit

t Institute of Biomedical Engineering, Faculty of Science and Technology,Keio University, Hiyoshi 3-14-1, Kohoku-ku, Yokohama, Kanagawa 223, JapantiSchool of Medicine, Keio University
Shinanomachi 35, Shinjuku-ku, Tokyo 160, Japan
Abstract-The purpose of this study was to investigate
the possibility for evaluating vigilance levels using a parameter of saccadic eye movement. EEG was measured
and analyzed by means of Fast Fourier Transform, and thespectral power of a! and /3 wave bands were obtained so as
to extract p]/[a+p]which was assumed to reflect vigilancelevels. Eye movement was measured so as to calculate saccadic peak velocity(SPV) and relative SPV(RSPV) which
was normalized by the mean SPV at the corresponding
amplitude of saccade. Relationship between Lf?]/[a+ p]
and RSPV was investigated and it revealed that RSPV
decreased according to the decline in the vigilance level.
This indicates the possibility for evaluating vigilance levels using RSPV.

I. INTRODUCTION
During VDT operation, motorcar driving and instrumentation monitoring, a decline in the vigilance level cancause a big accident, great misinterpretation, and so on.
For evaluating the vigilance levels, several methods suchas a method using EEG or galvanic skin reflex have been
investigated. To prevent such accidental events, it is necessary t o monitor the various vigilance states continuously. However, most of conventional methods has shown
disadvantage such as attachment of electrodes on the bodysurface for detecting physical electro-signals or interruption of the operation for the measurement. We took notice of the characteristic of saccadic eye movement which
provides a possibility for evaluating vigilance levels. Because measurement of eye movement using a photometricmethod has a possibility to recognize the vigilance levels
without any attachment and any interruption of working.
Saccadic eye movement consists of two different components: visually triggered saccade (VTSC) and internally
guided saccade (IGSC) [l]. VTSC is elicited reflexively
by a sudden presentation, while IGSC intentionally byown will. These shows different characteristics, that is,velocity of VTSC is relatively higher than that of IGSC.
Relationship between vigilance levels and characteristics
of both saccades remains unknown. Therefore we studied

0-7803-2475-7197 $1 0.00 0 1 997 IEEE1445

on this relationship between them as a basic research for
proposing a new method to evaluate vigilance levels.

11. METHODS
A subject was seated at the distance of 1.45m froma display system consisting of two personal computers.
Horizontal eye movements were detected by the infraredlimbus reflection system. EEG (Fz)were detected withstandard Ag/AgCl electrodes. Eye movement data andEEG data were amplified, and then sampled at 2kHz.
In order to induce VTSC and IGSC, experiments wereperformed under two conditions. VTSC was induced under a moving-target condition, where a target was presented at one of six possible locations f2.5, f5.0, f7.5deg,
and was controlled to shift the location repeatedly to theright and the left alternately at 2sec intervals. The subject were asked to follow the target presented. IGSC wasinduced under a non-target condition, where two targets
were presented at f2.5 or f5.0deg symmetrically before
each measurement. The subject memorized those locations, then the targets were extinguished. The subject
moved their eyes to memorized locations t o the right andthe left alternately at any timing. All measurements wereperformed for 3min and repeated two times under eachcondition.
EEG data were used to extract p]/[a+p]which may reflect vigilance levels. 2048 sample of EEG data obtained
just before the beginning of saccade were analyzed bymeans of Fast Fourier Transform, and the spectral powerof a (8-13H~,[a]) and /3 (13--10OHz,p]) wave bands wereobtained so as t o calculate @ ] / [ +a p]. The v]/[a+ p] decreases according to the decline in the vigilance level, since
the power of a! wave band ([a])
increases and the power of,fl wave band (PI) decreases.

111. RESULTS AND DISCUSSION
Fig. l(a) shows a relationship between amplitude andpeak velocity derived from the eye movement data accompanying with a saccade. It is well known that peak

velocity can be fitted by the following equation;

P V = PG{l- exp(-A/k)}(1)

where P V represents peak velocity, PVi the asymptotic
peak velocity for very large saccade, A an amplitude andIC a constant [2]. As shown in Fig. 1(a), not all experimental data are along the regression curve. This deviation of peak velocity from the regression curve is likely toattribute to some changes in the oculomotor control system during experiment. We supposed these changes werecaused by the changes in vigilance level which may affect a function of oculomotor control system. However wecan’t use peak velocity to compare with vigilance level,
because this parameter doesn’t quantitatively represent
the deviation from the regression curve. In order to clarify the degree of the deviation, we divided every peakvelocity by the value of regression curve at each amplitude, and named it relative saccadic peak velocity (RSPV,Fig. l(b)).
As shown in Fig. 2, RSPV of both saccades showed thesimilar tendency to [P]/la-t- p], which seem to mean that
RSPV reflects the changes in the vigilance level. Moreover, comparing the data between two types of saccade,we found that RSPV of VTSC is relatively higher than+ p ] . In other words,
that of IGSC, and so was the p]/[aunder the moving-target condition RSPV and vigilancelevel were relatively high, but under the non-target condition those were relatively low. In addition to thesefacts, Fig. 2 shows that scattering plots under two different conditions are overlapping each other and that thetrends of regression lines under each condition are almostsame. From these result, we could infer that difference ofsaccadic peak velocities under the two conditions might
depends on the difference between vigilance levels under
the different conditions, not on the difference in the typesof saccade with different cerebrophysiological processes of

VTSC and IGSC. This can be indirectly explained as follows; under the moving-target condition, differently fromunder the non-target condition, there exist a target to bea factor to maintain high vigilance level by giving out thelight and changing locations.
To determine whether or not our assumption is applicable, we performed two additional measurements undera fixed-target condition, where two fixed targets at f2.5
or f5.0deg were presented continuously and the subject
moved their eyes toward the targets to the right and theleft alternately at any timing. p]/[a+p] under this condition was in-between that of the non-target condition andthat of the moving-target condition, and so was RSPV.
This can be considered that the difference of three target
conditions of moving target, fixed target, *and no target
caused the difference of vigilance levels, and then this resulted in the difference of RSPV.

IV. CONCLUSION
This study has shown that vigilance level and relative
saccadic peak velocity have a positive correlation regardless of the type of saccade. This indicates a possibility forevaluating vigilance levels using saccadic parameter.

REFERENCES[l]C. Kennard, ”Higher control of mechanisms of saccadic eye movements,” Trum. Ophthulmol. Soc. U. K.,
vol. 105, pp. 705-708, 1986.
[2] A. C. Smit, J. A. M Van Gisbergen and A. R. Cools,”A parametric analysis of human saccades in different experimental paradigms,” Vision Res., vol. 27, no. 10, pp.

1745-1762,1987.

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Fig. 2 Relationship between [PI/[.! + p ] and RSPVFig. 1Noimalization of saccadic peak velocity

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