SACCADE AUTOMATICITY AND PURSUING EYE MOVEMENTS.

V.Filin

Abstract

Adult subjects’ eye movements were recorded by photoelectronic method (highlight of subject’s eye by infrared light and projection of the eye image on photoreceiver) when fixing immovable point (IP) and during smooth pursuit of the point motion (MP). It was shown that the average intervals when fixing a point and during smooth pursuit of its motion through the data received for 6 subjects had very close values: 0.50 ± 0.29s and 0.51±0.27s accordingly. It points to the identical number of saccades in both cases, in spite of the fact that there were only involuntary saccades in the first case and involuntary and reflex ones in the second case. Formally it is possible to write down: ∑ S = RS + IS = Const The numbers of saccades were also identical under different pursuing velocity. When reflex saccades interval is equal to 0.4 – 0.6s, involuntary saccades disappear completely. The reflex saccade, in essence, replaces the involuntary one. Therefore saccadic centre generates anyone saccades in native mode that is in the mode of saccadic automation.

Keywords: saccade automaticity, smooth pursuit, fixation, constancy of saccade.

On the basis of our previous researches of eye movements in norm and in pathology (1967 – 1997) the saccade automaticity concept was formulated, according to which the basic part of saccades is generated as automaticity without any external and internal stimuli (Filin & Ananin, 1973; Filin & Filina, 1989; Filin, 1997). The purpose of the present work was a study of smooth pursuing eye movements on the saccade automaticity. Eye movements were recorded by photoelectronic method at which subject’s eye was lighted up by infrared light and the eye reflected image was projected on the photoreceiver. A “line” of luminodiodes was used as an object of shift; the velocity of the object motion was set manually from the control console. In the first series of experiments (6 subjects) we compared the number of saccades in the case when fixing an immovable point and when it moved at 3ang.deg. It was found that the total number of saccades under two absolutely different conditions was in comparable values. So, when fixing an immovable point 26 involuntary saccades of automatic character were recorded for 20 seconds; and when pursuing 27 saccades were recorded. In the second case there were 5 reflex saccades of large amplitude (3 ang.deg.) and 22 involuntary saccades of automatic character of small amplitude (up to 1 angular degree). Average intersaccadic intervals were alike in both cases: 0.38 and 0.37s. Similarly, total data by results of 6 subjects inspection, when 2496 and 2566 intervals were processed, when fixing an immovable point and during smooth pursuit of its shift accordingly, had very close values: 0.50 ± 0.29 and 0.51 ± 0.27.

On the basis of the received data we concluded that the number of saccades is a certain constant. Only an orientation and an amplitude of saccade change, and the interval stays invariable. On the assumption of stipulated in the formal state it is possible to write down:
∑ S = RS + IS = Const,
where is a sum of saccades, RS and IS are reflex saccades and involuntary ones accordingly.

Fig.1. Samples of records of the same subject’s eye movements when fixing an immovable luminous point (1) and during smooth pursuit of luminous point motion (2) for 3 ang.deg. The point motion velocity smooth changed from 0.65 to 0.29 ang.deg. per a second. The ray deviation upward corresponds to eye movement to the right, that one downward – to the left. The tracking was to the right, and return to original position was carried out by reflex saccade. The bottom is a mark of the point motion. Calibration is 1s.

Earlier, in 1965 Yarbus (1965) paid his attention to the identical number of saccades under different conditions, in particular when fixing motionless point and when looking at an art picture, and Cunitz and Steinman (1969) were revealed identical number of saccades when fixing motionless point and reading.

Fig.2. Samples of pursuing eye movements records for 2 ang.deg. of the same subject when smooth change of tracking velocity: 1 – from 1.10 up to 1.22. ang.deg./s; 2 – from 2.0 up to 3.5 ang.deg./s; 3 – from 4.4 up to 2.3. ang.deg./s. Other designations are the same as in fig.1

In other series of experiments we compared the number of saccades under different smooth pursuit velocity. The pursuit velocity was set manually regulated from the control console. The researches once again confirmed the saccades number constancy. In particular, at smooth increase of pursuit velocity the sum of saccades was constant. For example a tested person had only 17 saccades (5 reflex saccades and 12 involuntary ones) when pursuit velocity was close to 1 ang.deg. per second (fig.2, record 1). At the bigger velocity the reflex saccades number increased (8), the involuntary saccades number decreased (7), but the sum was almost the same, there were 15 saccades (record 2). At last, when pursuit velocity changed from 4.4 up to 2.3 ang.deg. per a second there were also 15 saccades, 3 of which were only involuntary saccades (record 3). Others 12 saccades can be conditionally called as reflex ones taking into account their direction (to the right accordingly the main saccade direction). Though, strictly speaking, in the beginning of record just the smooth pursuit was absent. The subject hadn’t time to watch over luminous points shifts. For 12 shifts he had only 6 saccades. In essence, he operated in his native mode close to the saccade automaticity mode. As a whole on 3 records the number of involuntary saccades reduced from 12 up to 3, and the number of reflex saccades increased from 5 up to 12, but the sum of saccades (RS + IS) was similar: 17, 15 and 15.

On the data obtained for 8 subjects the diagram was plotted (fig.3). When analysing the data the number of reflex (RS) and involuntary saccades (IS) for 5 seconds of record was computed and their sum was accepted to be 100%: (RS + IS = 100%). The number of RS and IS also was expressed de corresponding rate. The envelope curves were constructed over the results.

Fig.3. Dependence of saccade quantity (%) from a subject pursuit frequency (f,Hz) of a point shift for 2 ang.deg. Dots are reflex saccades, daggers are involuntary ones; 1, 2 are envelope curves of reflex and involuntary saccades accordingly. Pursuit velocity in ang.deg/s is on an abscissa axis and the number of saccades in percents is on an ordinate axis.

As it is clear from the diagram, these two curves cross when the intersaccadic interval is close to 1s. In process of smooth pursuit velocity increasing the number of subjects reflex saccades increases and the number of involuntary saccades reduces; the last ones completely disappear when the intersaccadic interval is equal to 0.5s, but at the same time the reflex saccades are recorded in pure form. Enough clear appropriateness is observed: as soon as the interval between reflex saccades approximates to 0.4 – 0.6 seconds, so the involuntary saccades completely disappear. So it doesn’t matter what is the nature of saccades; but it’s very important that they are generated in saccade automaticity native mode. In essence, reflex saccade replaces the next involuntary one. The constant number of saccades under different conditions, such as when fixing a point and during pursuit, and also when pursuit with different velocity once again testifies saccadic centre automaticity character.

According to its physiological character the saccade automaticity resembles automaticity of heart, automaticity of breathing and that of digestive system.

We consider the saccadic automation to be a native mode of saccadic centre operation, some basic frequency, which is modulated in dependence of external and internal conditions. In the main the modulation concerns saccadic amplitudes and orientation, and in less extent it concerns intersaccadic interval. Automatic saccades “running out” by themselves are the background for all the variety of oculomotor activity. In our opinion in natural surrounding up to 80% of saccades have automaticity character and the rest 20% have reflex and involuntary character. Other types of saccades such as microsaccades, macrosaccades, saccades during sleep and also nystagmus are special cases of saccade automaticity.

References

Cunitz R.J., Steinman R.M. (1969) Comparison of saccadic eye movements during fixation and reading. Vision research, 9, 683 – 693
Filin V.A. (1997). Automation of saccades. Perception, 26, 96 (Abstract)
Filin V.A. (1997) Videoecology. Good and bad for eye, Moscow, TASS_advertisement, 317 p.
Filin V.A., Ananin V.F. (1973) Involuntary eye movements with and without fixed point. Physiologicheskiy jurnal SSSR, V.59, 4, 547 – 550
Filin V.A., Filina T.F. (1989) Saccadic automation in babies during rapid sleep. Journal vysshey nervnoy deyatelnosti. Moscow, Nauka, V.39, 4, 603 – 608
Yarbus A.L. (1965) Role of eye movements in vision process. Moscow, Nauka, 166 p.