Proceedings of the 26th Annual International Conference of the IEEE EMBS
San Francisco, CA, USA • September 1-5, 2004

Relation between Human Alertness, Velocity Wave Profile of Saccade, andPerformance of Visual Activities
A. Ueno 1 , Y. Uchikawa1
1

Department of Electronic and Computer Engineering, Tokyo Denki University, Saitama, Japan

Abstract— Practical method for assessing human alertness
is desired for preventing traffic accidents caused by drowsinessin the drivers. In this article, we examined relation between
saccade profile (PV/D), which was reported to havedependency on the alertness, and visual activities to investigate
an applicability of the saccade for assessing the alertness invision-dependent circumstances such as driving. We defined anew parameter of visual activity as “bit rate of visualperception (BRVP)”, and analyzed relations of saccade velocity
profile to the BRVP, relative pupil diameter, and critical fusion
frequency. The results showed significant high positive
correlations between PV/D, BRVP, and relative pupil diameter.

Target Controller
(PC1)
CRT1

ImageCapturing
Card (PC3)

CRT2

CRT3
CCD
Camera1.03 m

A/D Converter(PC2)
Oscilloscope
Eye Movement
MonitorKeywords—Bit rate of visual perception, critical fusion
frequency, pupil diameter

SubjectFig. 1. A scheme of experimental setup .

I. INTRODUCTION
1 set

Practical method for assessing human alertness isdesired for preventing traffic accidents caused bydrowsiness in the drivers. Although bioelectric signals suchas EEG [1] or skin potential [2] are commonly used for theassessment of the alertness fluctuation, most of conventional
methods have a disadvantage in requiring attachment ofelectrodes or devices on the body surface for detecting thesignal. Whereas we have shown in the previous researches
that sharpness of the velocity wave profile of saccade has adependency on human alertness [3], [4]. This phenomenon
may have applicability to the practical alertness assessment,
because photometric measuring device of saccadic eye
movements is likely to be developed using image sensors
located in front of the face in the future [5]. Howeverfeasibility investigation to apply saccade dynamics forpreventing the traffic accidents is insufficient from theviewpoint that variations in spectrum power of EEG weremainly referred to indices of alertness fluctuations in ourprevious studies. Since the accidents are not directly caused
by decline in brain activities but by depression in visualfunctions due to decreased brain activities, it is necessary toanalyze the relation between alertness, saccade dynamics,
and performance of visual activities.
In this paper, we examined relation between velocity
wave profile of saccades, which was reported to have a

CFF Task Task Task Task Calibration Task Task Task Task CFF
06911Rest1525[min]

Blinking is forbidden
Fixation10Fix2 Numerical Target Comparison
6.5

9Fixation12430[s]Pupil Image
RecordingFig. 2. Time chart of the experimental protocol.

dependency on alertness, and performance of visualactivities. We defined a new parameter “bit rate of visualperception (BRVP)” as an index of the performance. Also
we measured pupil diameter and critical fusion frequency(CFF) as the references of the visual activities.
II. M ETHODS
A. Subject and Apparatus for Measuring
One male, aged 22, participated in the experiments.
Informed consent was obtained from the subject. The
subject was seated at the distance of 1.03m from the CRT1
as shown in Fig. 1. Horizontal eye movements are detectedby the infrared limbus reflection system. Eye movement
data were sampled at 1kHz. Ocular images were recordedwith a CCD camera and captured at 15fps. CFF was tested
with the dedicated device.

This work was addressed as part of the 21 st Century COE (Center ofExcellence) Research by the Ministry of Education, Culture, Sports,
Science and Technology, and partially supported by the Ministry ofEducation, Science, Sports and Culture, Grant-in-Aid for Young
Scientists (B), 13780694, 2002.

0-7803-8439-3/04/$20.00©2004 IEEE

4933

(a)

TABLE I
CORRELATION COEFFICIENTS BETWEEN THE P ARAMETERS OF
P V/D, BRVP, RELATIVE P UPIL DIAMETER, AND CFF

The center mark for initial fixation

20 bits

20 bits

11010100111101110011

11001110111100111011-10deg

0deg

PV/D
BRVP
Relative pupil
diameter
CFF+10degthe false pair(b)

PV/D

BRVP

—
—

0.73*
—

Relative pupil
diameter
0.92**
0.72*

—

—

—

0.49

—

—

—

—

A pair of 20-bit binary codes for the comparison task

CFF
0.46
0.73*

* p < 0.05, ** p < 0.01Fig. 3. A scheme of experimental setup .

researches. Mean PV/D was computed for every set of theexperiment.

TCMP

107.5

D. Analysis of Visual Activities

52.5

N

BRVP was computed by equation (1).

N

BRVP =

R

0-2.5-5

L-7.5
-10
91215
18
Time (seconds)

2124

NR + NLTCMP - åi =1 Din(1)where NR and NL are numbers of the figures compared in theright and the left codes in the task as shown in Fig. 4. TCMP
is the time required to fixate the center mark in the task.
Thus the BRVP means the time rate at which information isperceived within the brain through the retina during the task.
The BRVP has the unit of bps.
Maximum pupil diameter in each “Task” session was
manually measured using an image processing software.
The maximum diameters were divided by the diameter inthe first session for normalization. Mean relative pupil
diameter was calculated for each experimental set. Alsomean CFF was computed for the each set.

Fig. 4. A sample record of eye movement and the parameters forcalculating the BRVP .

B. Procedures of the ExperimentFig. 2 shows time chart of the experimental protocol.
The experiment consisted of “CFF measurement”, “Task”,“Calibration” and “Rest”. In the “CFF measurement”
session, CFF was tested twice at ascending frequency
condition, and also at descending frequency condition
respectively. In the “Task” session, the subject performed anumerical comparison task, and monocular eye rotations
were recorded during the task. In the task, a pair of 20-bit
binary codes is presented horizontally and symmetricallyfrom the center mark on the CRT 1 (Fig. 1 and Fig. 3). The
binary codes are symmetrically almost the same except onebit. The subject was instructed to compare corresponding
binary digits alternately on the right and the left side in orderfrom the center to the exterior, and to fixate the center markafter recognizing the false pair of the digits. Presentation of
the codes continued 15s after recording pupil images for2.5s. The “Task” session was repeated 8 times for one set.
Totally 10 sets were conducted in the experiment. All
experiments were carried out in a darkened room.

III. RESULTS AND DISCUSSION
A. Correlation between PV/D and Visual Functions
As can be seen in Table I and Fig. 5, significant highcorrelations of the PV/D to the BRVP (r=+0.73, p<0.05) andto the relative pupil diameter (r=+0.92, p<0.01) wereobtained. Since decrease in the PV/D reflects decline inalertness of the subject according to our previous results, Fig.
5(a) indicate that the BRVP has a tendency to decrease withdecline in the alertness. This result implies that when thesubject is conducting slow saccade (i.e. small PV/D), theperceptual performance of the subject is in low activity, andvice versa. Since the previous study showed decline in PV/D
reflects decline in alertness, this results infer that when thesubject are in low alertness state, perceived information in aunit of time through retina is small.
On the other hand, pupil diameter is reported recently tohave a relevancy not only to alertness [6], but also toC. Eye Movement Analysis
The ratio of peak velocity and duration (PV/D) of thesaccade was calculated for each velocity waveform as anindex of sharpness of the velocity wave profile. PV/D wasreported to have a dependency on alertness in our previous934

3.9

y = 1.10x-0.28
r = 0.73

1.04

3.7

13.50.96
3.3

0.92

3.1
2.9
2.4

2.6

2.8

30.88
2.9

3.2

y = 0.11x-0.62
r = 0.72
3.1

PV/D of saccade (103deg/s 2)

3.9(a)(a)

1.04

3.3 3.5 3.7
BRVP (bps)36

y = 0.21x-0.39
r = 0.92

35
134
0.96

33

0.92
0.88
2.4

32

2.6

2.8

33.2

3231
2.9

PV/D of saccade (10 deg/s )y = 3.07x-2.35
r = 0.733.1

3.3 3.5 3.7
BRVP (bps)

3.9(b)(b)Fig. 6. Correlations of BRVP to relative pupil diameter and to CFF.

Fig. 5. Correlations between PV/D of saccade and visual activities of
BRVP and relative pupil diameter.

information capacity of retinal image [7]. According to thisarticle, decrease in the pupil diameter in sufficiently
darkened condition results in degrading the quality (i.e.
reducing the information capacity) of the retinal image.
Consequently, Fig. 5(b) infers that external image
information may be reduced at the retinal stage with the
reduced alertness level.

relative pupil diameter, and CFF. The results showedsignificant high positive correlations between PV/D, BRVP,and relative pupil diameter. This indicates an applicability ofsaccade dynamics for assessing alertness in visiondependent circumstances such as driving.
REFERENCES

B. Correlation between BRVP and Other Visual Activities[1] G. Ulrich and K. Frick, “A new quantitative approach to theassessment of stages of vigilance as defined by spatiotemporal
EEG patterning,” Perceptual and Motor Skills, vol. 62, pp. 567576, 1986.
[2] C. Nishimura and J. Nagumo, “ Feedback control of the level ofarousal using skin potential level as its index,” Ergonomic, vol.
28, pp. 905-913, 1984.
[3] A. Ueno, T. Tateyama, M. Takase, H. Minamitani, “ Dynamics ofsaccadic eye movement depending on diurnal variation in humanalertness,” System and Computers in Japan, vol. 33, no. 7, pp.
95-103, June 2002.
[4] A. Ueno, Y. Ota, M. Takase, H. Minamitani, “ Characteristics ofvisually triggered and internally guided saccade depending onvigilance states,” Trans. IEICE (in Japanese), vol. J81-D-II, no.
6, pp.1411-1420, June 1998.
[5] A. Tomono, F. Kishino, “ Gaze point detection algorithm basedon measuring 3D positions of face and pupil,” Trans. IEICE (inJapanese), vol. J75-D-II, no. 6, pp. 861-872, May 1992.
[6] Handbook of visual information processing (2.0) (in Japanese),
Asakura Shoten, 2001, p. 31.
[7] Y. Hirata, K. Yamaji, H. Sakai, S. Usui, “Function of the pupil invision and information capacity of retinal image,” Trans. IEICE(in Japanese), vol. J85-D-II, no. 4, pp. 659-667, April 2002.

Fig. 6 shows correlations of the BRVP to the relative
pupil diameter and to the CFF. These results also hadsignificant high correlations as in Table I. Fig. 6(a) indicatesmore directly the relation between the throughput of thevisual system and information capacity acquired throughretina as remarked above.
Since CFF value can be regarded as an index reflecting
time resolution of vision including higher central nerve’ssystem, Fig. 6(b) implies that decline in BRVP may becaused not only by decreased information capacity at retina
but also by decreased time resolution in the vis ion system.
IV. CONCLUSION
We examined the relation between velocity profile ofsaccades and performance of visual activities of BRVP,

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