JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

FEBRUARY, 1962

VOLUME 52, NUMBER 2

Latency and Duration of Eye Movements in the Horizontal Plane CARROLLT. WHITE AND ROBERT G. EASON U. S. Navy Electronics Laboratory, San Diego, California AND

NEIL R. BARTLETT University of Arizona, Tucson, Arizona (Received September 22, 1960) The electrical method of eye movement recording (EOG) was used to study the reactions of the eyes when subjects attempted to fixate as quickly as possible on light stimuli appearing at various points along the horizontal plane. Each eye was recorded separately, allowing an assessment of the degree of independence of the eyes when reacting in this way. In addition to the data concerning latency of ocular reactions and the duration of eye movements, information regarding the relative speed of movements from periphery to center as compared to movements from center to periphery was obtained. It was also found that the two eyes tend to act independently in regard to latency and speed of movement when moving toward peripheral stimuli.

INTRODUCTION

THE study

of eye movements has long been a T favorite of workers in physiological optics and experimental psychology. The topic is treated in almost every standard textbook in both fields. Tinker has published a review of recent research on the subject.' One trend noted by Tinker was a renewed interest in the nature of eye movements themselves, as opposed, for example, to interest in the efficiency of movements for reading. In the course of a study of the delays involved in reacting to sequences of stimuli, a considerable amount of data was obtained concerning the latency and duration of eye movements as subjects attempted to fixate upon light stimuli appearing suddenly at various positions along the horizontal plane. This report presents those data. APPARATUS

AND METHOD

The electrical method of eye movement recording (electro-oculography, or EOG) was employed. This technique has been described in detail in an earlier paper.2 In the present case the horizontal movements of each eye were recorded separately. To accomplish this, electrodes were placed next to the canthus of each eye and on both sides of the bridge of the nose. The potential changes accompanying the movements of the eyes were suitably amplified and then recorded on two channels of a light-writing Visicorder).

oscillograph (Minneapolis-Honeywell

The subject sat in a darkened, electrically shielded room, with his head held in a relatively stationary position by means of a chin rest. Care was taken to insure that there would be no distracting flections within his field of view.

light sources or re-

In each trial two of the stimulus lights were presented in sequence, the time between the onset of each light being varied from 50-800 msec. At the instant the first light (SI) was activated, the fixation light was extinguished. Si remained on until the second light (S2) was activated, and then S2 remained on until the experimenter ended the trial. For the purpose of this paper it is not necessary to list all the stimulus sequences used or the exact separations between the onset of Si and S2 on the various trials, since we are primarily interested in aspects

of the eyes' behavior

which were not in-

fluenced by these variables. An experimental session consisted of 120 trials, with each of the six stimulus lights serving as Si 20 times. In each stimulus sequence, S2 was 10, 20, or 30 deg

visual angle either to the right or left of Si. Each of three subjects performed four such sessions of appropriately randomized trials. The data obtained from the oscillographic records which are pertinent to our present interests were: (1) time from the onset of Si until each eye began to move from the original fixation point toward Si; (2) time taken by each eye to complete the movement from the original fixation point to SI; and (3) time taken by each eye to complete the movement from S, to S2.

The visual stimuli were six small neon bulbs (type NE2H) placed both to the right and to the left of the fixation point such that they subtended visual angles of 10, 20, and 40 deg. Each light was 15 ft from the subject. The initial fixation point was a similar neon bulb mounted in line with the subject's position of forward gaze which remained on until a stimulus light was activated.

RESULTS

Latency

. A. 'Tiinker, Psycliol. Bull. 54, 215 (1958). A. Ford, C. T. White, and M. Lichtenstein, J. Opt. Soc. Am. 49, 287 (1959).

Latency data are presented in Table I. Means and standard deviations for right and left eyes are presented for each of the three subjects. The number of records on which the calculations are based varies between 74 and 80, the specific N depending on the readability of the records. The latency of reaction to SI was not influenced by the S1-S 2 int.rval. The mean latencies may appear

I

2

210

February

EYE MOVEMENTS IN HORIZONTAL PLANE

1962

to some as surprisingly long; but they are not surprising if it is remembered that in this situation the subjects were required to shift fixation to any one of six stimuli, with no knowledge of which stimulus would be presented. As Hackman 3 showed, knowledge of the stimulus

location is a major factor in determining latency. For the more peripheral stimuli, latency increases. In all likelihood, this effect is attributable to retinal factors, for the simple reaction time in responding in any way to visual stimuli shows it also. In any case, since all six stimuli were identical there is no basis other than retinal to which the change may be assigned. Each oscillographic tracing shows, independently, the response of the two eyes. In many cases such records show that the times for initiating the eye movements and the speeds of movements vary slightly from one eye to the other. Typically, when there is a difference, the left eye begins its motion a short time before the right eye when the stimulus is to the left, and the right eye begins before the left when the stimulus is to the right. The point is illustrated in Table I, showing the mean latencies of each eye to the six positions.

Standard deviations cited in Table I express the variability in a conventional statistical index. The character of the variability

is illustrated

in Fig. 1, showing the

distribution of the latencies for the right eye. In preparing this graph the data for all three subjects were pooled as were the right and left positions for each of the three stimulus locations. Each of the three distributions is skewed to the right; a small percentage of the latencies are so very long as to render untenable the hypothesis that all the latencies are drawn from a and standard deviations for latencies in milliseconds.

TABLE I. Mean

Position

Right eye Mean S.D.

Left eye Mean S.D.

40'L 20'L 10'L 10'R 20'R 40'R

Subject: NRB 284 51 256 56 247 48 238 51 246 47 279 50

281 253 243 242 256 278

53 59 48 50 47 51

40'L 20'L 10'L 10 0R 20'R 40'R

Subject: CTW 259 39 226 48 220 52 223 43 224 41 258 49

255 231 217 236 239 263

40 48 53 43 40 52

320 273 255 295 282 308

61 52 51 63 55 55

TABLE

II. Duration of ocular movements in milliseconds. Right eye

3

325 280 237 294 273 308

61 52 56 67 56 56

Left eye

Stimulus 40'L 20'L 10'L

B 125 89 72

Subject W E Mean 117 124 122 87 88 88 71 70 71

10'R 20'R 40'R

74 87 103

73 88 123

76 94 138

74 90 121

Subject W E Mean 124 139 134 94 93 93 74 71 73

B 140 93 74 68 78 94

66 82 114

67 81 111

Speed of Movement Table II shows the mean time occupied in executing the movements. The data are broken down by subject, eye, and position. Average velocity of movements increases the greater the extent of movement required. For these three subjects, however, there is a slightly greater mean speed for the right eye if the movement is to the left, and for the left eye if the movement is to the right. If the data for the two eyes are pooled (as is done electrically in typical eye-movement records that do not show separate recordings for the two eyes), the movements to the right appear to be faster than those to the left. Note, however, the differences among subjects. For the right eye, subject W shows faster movements to the left whereas B and E are faster to the right. Generalizations about speeds of movement in one direction or another must be made with caution. There is some variation, particularly for the wider excursions, in the time required for executing a movement. Table III expresses this variation for the right and left eyes of each subject for reaching each initial target from the central fixation position. In other words, Table III shows the standard durations for each of the 00

!

r-

W 0 I3

t!

60

40 2~

R. B. Hackman, J. Exptl. Psychol. 27, 546 (1940).

68 83 124

normal distribution about a mean time value. In such circumstances, it is often useful to discuss the average latency in terms of medians, and the variations in terms of percentiles. Figure 1 affords such an analysis.

Subject: RGE 40'L 20'L 10'L 10'R 20'R 40°R

211

STIMULUS

w WI~sTMULUS 20

C'

.150

.200

.250

.300 .350 .400 LATENCY IN SECONDS

.450

.500

FIG. 1. Cumulative distributions of latencies of reactions to stimuli located 10, 20, and 40 deg off the visual axis. Further details in text.

WHITE,

212

EASON, AND BARTLETT

means cited in Table II. There are significant differences among subjects, extents, and direction in this index, and moreover, for some positions there is a significant difference between the right and left eyes. The values given for the durations of the movements are considerably longer than those usually quoted in the literature. For example, Woodworth and Schlosberg4 give the following mean values: (1) for a 100 movement, 39 msec, (2) for a 20° movement, 55 msec, (3) for a 40°

movement, 100 msec. It can be seen that these values are approximately 30 msec less than the corresponding values reported in the present study. This difference could be due, in part, to each of three circumstances. First, the subjects knew the location of the point to which they were to move in the earlier studies, while they were to move to any one of six positions in the present study. It has already been shown that this situation leads to a considerable increase in the latency of the ocular reaction. It is doubtful that the speed of movement would be affected to any great degree by these circumstances, but the possibility exists that such may be the case. Second, the electrical technique of recording the eye movements may introduce time delays not present in the optical techniques used by the earlier workers. This is undoubtedly the case, but it certainly cannot account for differences as great as 30 msec. The frequency re-

sponse of the recording instrument used, the Visicorder, is supposed to be better than 1000 cps. The third, and by far the most likely, source of the difference between the present results and those reported earlier lies in the technique used in reading the time data from the records. The problem is that there is no exact way of determining when a movement begins and when it is completed. Each researcher establishes his own criteria. If these criteria are adhered to throughout the data reading, his data should be internally consistent. The absolute values reported by different investigators can be significantly different because of this, however, so it does not appear to be of much use to compare such results except for the purpose of establishing a general range of movement times. In the study which produced the data quoted by Woodworth and Schlosberg,4 for

Vol. 52

TAB3LEIV. Mean latencies, duration of movements, and

total response times to each of the six positions.

40'L

20'L

10'L

100R

20'R

40'R

247 90 337

276 121 397

254 81 335

283 111 394

Latency Duration Total

293 122 415

253 88 341

Right eye 242 249 71 74 313 323

Latency Duration Total

288 134 422

248 93 341

Left eye 236 253 73 67 309 320

example, the light beam used for recording the movement of the eyes was interrupted

100 times a second, and

the duration of the movements was obtained by counting the number of dashes between the points on the record which represented the positions of the eyes before and after the movement. This technique would obviously result in a low estimate since it takes no account of the very onset of movement or of the termination of movement when the eye moves relatively slowly to its final fixation position. Total Response Time The total time for fixating consists of the latent period and the movement time. Correlational analysis shows no relationship between the two; the movement to a target may be slow or fast irrespective of the latent time in initiating the movement. Thus the mean total time is the sum of the two component durations. Table IV shows the means for the pooled data for three subjects. An interesting feature of these means is that the difference in latencies between the two eyes is nearly matched by a counter difference in durations of movements,

TABLE V. Mean duration of movements in milliseconds.

Left

Center

40°

200

10°

0°

Right 100

40°

200

se___,ndmovments TABLE III. Standard deviations of movement

81

durations in milliseconds.

-0

70 Right eye B W E 40'L 200L 10'L 10'R 20k 40'R

14.5 12.6 7.7 8.8 9.6 53.9

Subject 20.3 4.5 7.8 6.3 4.4 6.0

25.3 13.4 8.2 11.0 9.2 24.7

B 20.0 9.5 6.5 10.0

Left eye W

E

Subject 19.4 30.8 7.4 14.6 8.9 8.5 3.3 10.5

13.3

6.9

7.9

49.1

8.2

10.6

4 R. S. Woodworth and . Schlosberg, Experimental Psychology (Henry Holt and Company, Inc., New York, 1954), p. 501.

so

that the mean time to fixate a target at any angle is very nearly the same for the two eyes.

75 65

65

first | 2.8

|

movements |lG

72T1T. 6 .91 0t

|l

-

February

1962

EYE MOVEMENTS IN HORIZONTAL PLANE

Inward Versus Outward Movements All excursions involved in the discussions above were

from the center to the periphery, and the eye was fixated upon the center for several seconds prior to the initiation of the movement. Immediately following this movement, however, the subjects were required to fixate upon a second target at some position either more central or more peripheral than the first, so there are data available to compare the speeds of movements toward the center with those toward the periphery. Table V presents the mean of the right and left eye movement times cited in Table IV and shows in addition the mean durations for the second movements. Clearly, second movements are faster when made toward the center than when to the periphery. Earlier observations made by Brockhurst and Lion5 and Dodge and Cline6 on initial movements 5 R. J. Brockhurst and K. S. Lion, A. M. A. Arch. Ophthalmol. 46, 311 (1951). 6 R. Dodge and T. S. Cline, Psychol. Rev. 8, 145 (1901).

213

indicated that there is greater speed when in the central direction; and these data corroborate the point by showing that it applies also to a movement following directly afterward. For the second movements, the similarity of mean durations in the right and left quadrants is impressive. DISCUSSION

These data are presented simply as empirical findings, and no attempt

will be made to explain some of the

phenomena which were noted. It must be remembered that these results were obtained with a specific type and value of visual stimulus, so the absolute values quoted will not hold for all other situations. The problem concerning the absolute value of the duration of eye movements has already been discussed. The interesting findings, however, deal with relative values of these durations, so are not affected by the uncertainty of our evaluation.