LETTJZR TO THE EDITORS
OCULOMOTOR DURING
AND PERCEPTUAL LOCALIZATION SMOOTH EYE MOVEMENTS (Receired
12 June 19731
ISTRODUCTION
mislocation of brief flashes presented during smooth tracking eye movements has been known since the work of Hazelhoff and Wiersma (1924). Mateeff (19773 has criticized their explanation of this penomenon based on the concept of visual latency. In another study, Mitrani and Dimitrov (1978) showed that the smooth eye movement persisted for some time after the disappearance of the moving target pursued with the eyes. This is what we shall call “overtracking”. The present letter is aimed at revealing to what extent the overtracking and the mislocation of a disappearing moving target are interrelated and whether the misIocation can be determined by the ocutomotor behavior during performance of the localization task. The
%lETHOD Three subjects-one woman and two men-aged 27-32. with normal vision. participated in the experiments. The experimental set-up for stimulus presentation and eye and stimulus movement registration used is described in our previous paper (Mitrani and Dimitrov, 1978). The subject sat in front of an uniformly illuminated screen, 50’ x 30’ visual angle in size, at a distance of 40 cm from his eyes. The brightness of the screen was such as to create an illumination of 7 lx at the place of the subject’s eyes. A clearly visible horizontal scale was mounted on the screen. A bright light target subtending 0.4’ x 0.8'of visual angfe appeared on the left-hand border of the screen and moved to the right with a constant velocity. A movable opaque occluder behind the screen. invisible to the subject, was used to eclipse the light beam, thus causing the disappearance of the target. Eye and target movement was recorded by means of an ink recorder-Sat&-Socci, Japan-at a speed of 50 mm/set. Each presentation of the stimulus was preceded by a warning signal. The subject was instructed to look and to track the moving target as soon as it appeared at the left-hand edge of the screen. He was informed that the target should disappear somewhere above the scale and was asked to report the scale division at which the target had disappeared by typing it on a digit electromechanical calculator. Before each trial the experimenter adjusted the moving occluder according to a random number program. Thus the target disappeared with equal probability in the range from the 4th to the 75th division of the scale. Experiments were carried out with two target velocities -9.2”,&ec and 19.4’/sec. Each subject performed 14.4tracking eye movements for each target velocity.
RESULTS .-\.VD DISCCSSIOS Figure 1 illustrates two typical samples of the recorded eye movements (trace EM) and target move-
ment (trace TM). Vertical bars indicate (a) the coincidence of the target with the zero scale division. and (b) the moment (and location ) of target disappearance. It is clearly seen from Fig. 1 that after the disappearance of the target the smooth eye movement continues with the same velocity for some time (overtracking). Every time. almost immediately after the end of the smooth eye movement, a saccade in the opposite direction occurred. Often it was followed by a second and even by a third saccade. Then for some time the eyes remained immobile and afterwards the subject looked at the calculator in order to type his report. In general the reported place of target disappearance did not coincide with the actual one-it was mislocated in the direction of the eye movement. The following variables were obtained from the experiment: Ire-the angular distance between the position of the visual axis at the end of the overtracking and the actual place of target disappearance. %,--the angular distance between eye position after all saccades and the place of target disappearance. A-the angular distance between the reported and the actual place of target disappearance.
Fig. I. Typical samples of recorded eye mokcment (EM) and target movement (TM). Vertical bars mark the zero scale division-bar a-and the place (or moment) of target disappearance--bar b. z0 is the angular size of the overtracking and z,, the displacement of the final position of the eyes with respect to the place of target disappearance.
610
Letter to the Editors
L.H.
6” . a
b
Fig. 2. Dependence of the overtracking a,, and the mislocation A (the ordinate) on the place of stimulus disappearance over the scale (the abscissa). a-target velocity 9.2”/sec; h-target velocity 19.4’/sec. 9SY, confidence limits are given.
The range of target disappearance was divided into equal subintervals subtending 3.15’ of visual angle. Data for Q,. rl and A were averaged inside each subinterval and 95% confidence limits were calculated. Experimental data for one of the subjects are given in Fig. 2. The averages of the two variables CL,,and A are plotted on the ordinate; the angular distance between the zero scale division and the center of each subinterval, on the abscissa. The results for the other two subjects were very similar. Figure 2 shows that the overtracking depends on the place of target disappearance. It diminishes sig nificantly towards the right-hand end of the scale. The value of the mislocation A also depends on the place of target disappearance on the scale. In general the saceades occurring after the smooth eye movement bring the line of sight back towards the point of target disappearance. However, in most of the cases even after a third saccade the gaze is
directed to a point displaced up to several degrees to the right with respect to the actual place of target disappearance. Figure 3 illustrates the dependencxs of z, on the place of target disappearance for the three subjects tested. aI seems not to depend on the place of target disappearance. After the last saccade the position of the line of sight is almost equally displaced to the right from the place of disappearance all over the scale. As the mislocation was always in the direction of the target and eye movement the inevitable overtracking might be thought to determine the mislocation. The subjects simply might report the scale division at which their eyes have stopped after the end of the overtracking. Figure 2 clearly shows this not be be the case. Great differences between the values of the overtracking (ae) and the mislocation (A) were observed all over the scale for the three subjects tested. Both variables zo and A. however, exhibit similar dependences on the place of target disappearance
611
Letter to the Editors
____ _._._
L.H. T.M. H.S.
b
L? 2”-
__.e- -.-.-.-.*.-*
5”
-%
loo
‘..+A
15O
2o”
H-‘-w
2s”
Fig. 3. Dependences of the displacement of the line of sight after the last saccade with respect to the place of target disappearance (xx,)on the place of target disappearance over the scale. Data obtained from all three subjects are given for the two target velocities. The confidence limits are from kO.4’
to kO.8”.
over the scale. As far as mislocation is concerned, this is a confirmation of our earlier results (Mateeff et al., 1977). Hazelhoff and Wiersma (1924) considered the mislocation during tracking eye movements as manifestation of the signal latency in the visual system. They introduced the “perception time” (Wahrnehmungszeit) concept and stated that the magnitude of the ratio between the value of the mislocation and the velocity of tracking is the value of the visual latency. However. if pure visual latency determined the mislocation of the target disappearance in our experiments, the data obtained should have been independent on the place of disappearance over the scale. It is quite probable that visual latency plays some role in the phenomenon of mislocation but, no doubt, other factors are also involved. Mitrani and Dimitrov (1978) have shown that the overtracking is a function of the eye movement duration. The greater the duration the shorter the overtracking, tending asymptotically towards a constant value. The curve of the overtracking obtained in the present experiments is different from the curves of the “drift time” demonstrated in their study. A pronounced decrease in the overtracking at the end of the scale is observed. This difference might be explained by the different tasks. Here the subject’s task is not only to track the moving target but also to report the place of its disappearance over a limited
scale. Some psychological factors such as expectancy, evaluation of the probability for disappearance of the target over the scale, etc. might influence the smooth eye movement control. The fact that immediately (without any delay) after the end of the overtracking a saccade toward the place of target disappearance takes place means that somehow the subject “knows” that his eyes have surpassed this place. Nevertheless, the saccading back is not accurate. The final position of the eyes is systematically displaced in the direction of the target movement. In general, the position of the line of sight after the saccades does not coincide with the perceived place of stimulus disappearance. Similar results were reported by Ward (1976) under somewhat different experimental conditions. This curious fact may be explained bearing in mind that “central vision” meati an area of about 4”-5” around the fovea centralis. Different subjects under different conditions may take the risk of announcing a scale division which is not in the very center of their vision. Other explanations may, however, exist. That saccades aimed at a target are not accurate is a well-known fact (Mateeff et al., 1972; Fuchs, 1976). Systematic undershoots are usually observed. The dissociation between the reported place of disappearance and the final position of the line of sight might be accounted for by this phenomenon. Further-
413
Letter to the Editors
more, the subjects acre given no special instruction to look at the place of target disappearance. One may choose another explanation for the dissociation between x1 and A. We may assume that there exist two visual systems in man. The first is mainly concerned with the answer of the question “Where?” It is more primitive and its functions are determined by the structures of the superior colliculi. This system plays an important role in the eye movement control and possesses a relative autonomy. A lot of data are available now (Sprague et al.. 1973) that support the notion that such a system exists. The second visual system identified as geniculo-striate system involves higher cortical mechanisms. Its functions are subject to many influences of much higher cortical origin that might be called “psychological”, such as set, expectancy. etc. It is possible that in some cases the functioning of the two systems will determine different and even contradictory behavioural responses to what seems to be a single visual task. Such an example was demonstrated in a previous work (Dimitrov et al., 1976). A well-known fact is that saccidic eye movements are goal directed. Wurtz and Goldberg (1971) have demonstrated that goal directed saccades are most probably controlled by collicular mechanisms. We may therefore assume that in our experiments the direction of the line of sight after the execution of the last saccade corresponds to the place of target disappearance as “perceived” by the primitive collicular system. As we have already pointed out, this system is relatively independent and probably not influenced by factors which might be referred to as “psychological”. The system simply localizes the event in the visual field and initiates a saccade aimed at that place. The present data demonstrate that the deviation of the final position of the eyes from the place of target disappearance is almost constant in contrast with the mislocation. which depends on the place of disappearance of the target. This means that the “primitive visual system” is not concerned with the “meaning” of the background and is only stimulated by the event of disappearance. It is the second or “higher” visual system that ellaborates a decision about the reported place of stimulus disappearance. This system might be influenced by several different factors: the inevitable
overtracking, the existence of a limited metric scale, the conditional probability of stimulus disappearance at a given place on the scale, the scale division numbers, etc. The shape of the curve for !J follows the shape of the overtracking curve. The smallest errors in estimating the place of stimulus disappearance are observed at the right-hand end of the scale where the conditional probability of disappearance is near to 1.0. At the left-hand end of the scale the uncertainty of the expected stimulus disappearance may lead to greater deviations, both in overtracking and estimated place of target disappearance. Institure of Physiolog! Bulgarian Academy of Sciences Sofia Bulgaria
L. MITRAN G. DWTROL N. Y.AKLMOFF S. MATEEFF
REFERENCES Dimitrov G.. Mitrani L.. Yakimoff N.. MateeK S.. RadilWeiss T. and Bozkov V. (1976) Saccadic eye movements on Bela Julesz figure. Vision Res. 16, 411-414. Fuchs A. F. (1976) The neurophysiology of saccades. In Eye Moremems and Ps.vchological Processes (Edited by R. A. Monty and J. W. Senders). pp. 39-54. Lawrence Erlbaum Ass.. Hillsdale. New Jersey. Hazelhoff F. and Wiersma H. (1924) Die Wahrnehmungszeit. Z. Psphol. 96. 181-188. .Llateeff S. (1977) Is the method for “perception time” measurement adequate’! C.r. Acad. buig. Sci. 30. 603-606. MateetTS.. Mitrani L. and Yakimoff N. (19771 Localization of disappearance of light target during tracking eye movements-I. Acta Phsriol. Pharmac. Bula. 3. 21-27. Xtateeff S.. Yakimoff N. and Mitrani L. (1972j Kinematic characteristics of voluntary saccadic eye movements. Bull. fnsr. Ph~siol., Bulg. Acad. Sci. XIV, 35-46. Mitrani I. and Dimitrov G. (19781 Pursuit eye movements of a disappearing moving target. Vision Res. 18, 537-539. Suraeue J. M.. Berlucchi G. and Rizzolatti G. (1973) The r role of superior colliculus and pretectum in vision and .visually guided behaviour. In Handbook of Sensor! Physiology, Vol. VII, 3. Central processing of visual information, part B (Edited by R. Jung). pp. 27-102. Springer-Verlag. Berlin. Ward F. (1976) Pursuit eye movements and visual localization. In Eye Mocemenrs and Psychological Processes (Edited by R. A. Monty and J. W. Sanders), pp. 289-297. Lawrence Erlbaum Ass.. Hillsdale, New Jersey. Wurtz R. H. and Goldberg M. E. (1971) Superior colliculus cell responses related to eye movements in awake monkeys. Science 171. 82-84. Y
OCULOMOTOR DURING
AND PERCEPTUAL LOCALIZATION SMOOTH EYE MOVEMENTS (Receired
12 June 19731
ISTRODUCTION
mislocation of brief flashes presented during smooth tracking eye movements has been known since the work of Hazelhoff and Wiersma (1924). Mateeff (19773 has criticized their explanation of this penomenon based on the concept of visual latency. In another study, Mitrani and Dimitrov (1978) showed that the smooth eye movement persisted for some time after the disappearance of the moving target pursued with the eyes. This is what we shall call “overtracking”. The present letter is aimed at revealing to what extent the overtracking and the mislocation of a disappearing moving target are interrelated and whether the misIocation can be determined by the ocutomotor behavior during performance of the localization task. The
%lETHOD Three subjects-one woman and two men-aged 27-32. with normal vision. participated in the experiments. The experimental set-up for stimulus presentation and eye and stimulus movement registration used is described in our previous paper (Mitrani and Dimitrov, 1978). The subject sat in front of an uniformly illuminated screen, 50’ x 30’ visual angle in size, at a distance of 40 cm from his eyes. The brightness of the screen was such as to create an illumination of 7 lx at the place of the subject’s eyes. A clearly visible horizontal scale was mounted on the screen. A bright light target subtending 0.4’ x 0.8'of visual angfe appeared on the left-hand border of the screen and moved to the right with a constant velocity. A movable opaque occluder behind the screen. invisible to the subject, was used to eclipse the light beam, thus causing the disappearance of the target. Eye and target movement was recorded by means of an ink recorder-Sat&-Socci, Japan-at a speed of 50 mm/set. Each presentation of the stimulus was preceded by a warning signal. The subject was instructed to look and to track the moving target as soon as it appeared at the left-hand edge of the screen. He was informed that the target should disappear somewhere above the scale and was asked to report the scale division at which the target had disappeared by typing it on a digit electromechanical calculator. Before each trial the experimenter adjusted the moving occluder according to a random number program. Thus the target disappeared with equal probability in the range from the 4th to the 75th division of the scale. Experiments were carried out with two target velocities -9.2”,&ec and 19.4’/sec. Each subject performed 14.4tracking eye movements for each target velocity.
RESULTS .-\.VD DISCCSSIOS Figure 1 illustrates two typical samples of the recorded eye movements (trace EM) and target move-
ment (trace TM). Vertical bars indicate (a) the coincidence of the target with the zero scale division. and (b) the moment (and location ) of target disappearance. It is clearly seen from Fig. 1 that after the disappearance of the target the smooth eye movement continues with the same velocity for some time (overtracking). Every time. almost immediately after the end of the smooth eye movement, a saccade in the opposite direction occurred. Often it was followed by a second and even by a third saccade. Then for some time the eyes remained immobile and afterwards the subject looked at the calculator in order to type his report. In general the reported place of target disappearance did not coincide with the actual one-it was mislocated in the direction of the eye movement. The following variables were obtained from the experiment: Ire-the angular distance between the position of the visual axis at the end of the overtracking and the actual place of target disappearance. %,--the angular distance between eye position after all saccades and the place of target disappearance. A-the angular distance between the reported and the actual place of target disappearance.
Fig. I. Typical samples of recorded eye mokcment (EM) and target movement (TM). Vertical bars mark the zero scale division-bar a-and the place (or moment) of target disappearance--bar b. z0 is the angular size of the overtracking and z,, the displacement of the final position of the eyes with respect to the place of target disappearance.
610
Letter to the Editors
L.H.
6” . a
b
Fig. 2. Dependence of the overtracking a,, and the mislocation A (the ordinate) on the place of stimulus disappearance over the scale (the abscissa). a-target velocity 9.2”/sec; h-target velocity 19.4’/sec. 9SY, confidence limits are given.
The range of target disappearance was divided into equal subintervals subtending 3.15’ of visual angle. Data for Q,. rl and A were averaged inside each subinterval and 95% confidence limits were calculated. Experimental data for one of the subjects are given in Fig. 2. The averages of the two variables CL,,and A are plotted on the ordinate; the angular distance between the zero scale division and the center of each subinterval, on the abscissa. The results for the other two subjects were very similar. Figure 2 shows that the overtracking depends on the place of target disappearance. It diminishes sig nificantly towards the right-hand end of the scale. The value of the mislocation A also depends on the place of target disappearance on the scale. In general the saceades occurring after the smooth eye movement bring the line of sight back towards the point of target disappearance. However, in most of the cases even after a third saccade the gaze is
directed to a point displaced up to several degrees to the right with respect to the actual place of target disappearance. Figure 3 illustrates the dependencxs of z, on the place of target disappearance for the three subjects tested. aI seems not to depend on the place of target disappearance. After the last saccade the position of the line of sight is almost equally displaced to the right from the place of disappearance all over the scale. As the mislocation was always in the direction of the target and eye movement the inevitable overtracking might be thought to determine the mislocation. The subjects simply might report the scale division at which their eyes have stopped after the end of the overtracking. Figure 2 clearly shows this not be be the case. Great differences between the values of the overtracking (ae) and the mislocation (A) were observed all over the scale for the three subjects tested. Both variables zo and A. however, exhibit similar dependences on the place of target disappearance
611
Letter to the Editors
____ _._._
L.H. T.M. H.S.
b
L? 2”-
__.e- -.-.-.-.*.-*
5”
-%
loo
‘..+A
15O
2o”
H-‘-w
2s”
Fig. 3. Dependences of the displacement of the line of sight after the last saccade with respect to the place of target disappearance (xx,)on the place of target disappearance over the scale. Data obtained from all three subjects are given for the two target velocities. The confidence limits are from kO.4’
to kO.8”.
over the scale. As far as mislocation is concerned, this is a confirmation of our earlier results (Mateeff et al., 1977). Hazelhoff and Wiersma (1924) considered the mislocation during tracking eye movements as manifestation of the signal latency in the visual system. They introduced the “perception time” (Wahrnehmungszeit) concept and stated that the magnitude of the ratio between the value of the mislocation and the velocity of tracking is the value of the visual latency. However. if pure visual latency determined the mislocation of the target disappearance in our experiments, the data obtained should have been independent on the place of disappearance over the scale. It is quite probable that visual latency plays some role in the phenomenon of mislocation but, no doubt, other factors are also involved. Mitrani and Dimitrov (1978) have shown that the overtracking is a function of the eye movement duration. The greater the duration the shorter the overtracking, tending asymptotically towards a constant value. The curve of the overtracking obtained in the present experiments is different from the curves of the “drift time” demonstrated in their study. A pronounced decrease in the overtracking at the end of the scale is observed. This difference might be explained by the different tasks. Here the subject’s task is not only to track the moving target but also to report the place of its disappearance over a limited
scale. Some psychological factors such as expectancy, evaluation of the probability for disappearance of the target over the scale, etc. might influence the smooth eye movement control. The fact that immediately (without any delay) after the end of the overtracking a saccade toward the place of target disappearance takes place means that somehow the subject “knows” that his eyes have surpassed this place. Nevertheless, the saccading back is not accurate. The final position of the eyes is systematically displaced in the direction of the target movement. In general, the position of the line of sight after the saccades does not coincide with the perceived place of stimulus disappearance. Similar results were reported by Ward (1976) under somewhat different experimental conditions. This curious fact may be explained bearing in mind that “central vision” meati an area of about 4”-5” around the fovea centralis. Different subjects under different conditions may take the risk of announcing a scale division which is not in the very center of their vision. Other explanations may, however, exist. That saccades aimed at a target are not accurate is a well-known fact (Mateeff et al., 1972; Fuchs, 1976). Systematic undershoots are usually observed. The dissociation between the reported place of disappearance and the final position of the line of sight might be accounted for by this phenomenon. Further-
413
Letter to the Editors
more, the subjects acre given no special instruction to look at the place of target disappearance. One may choose another explanation for the dissociation between x1 and A. We may assume that there exist two visual systems in man. The first is mainly concerned with the answer of the question “Where?” It is more primitive and its functions are determined by the structures of the superior colliculi. This system plays an important role in the eye movement control and possesses a relative autonomy. A lot of data are available now (Sprague et al.. 1973) that support the notion that such a system exists. The second visual system identified as geniculo-striate system involves higher cortical mechanisms. Its functions are subject to many influences of much higher cortical origin that might be called “psychological”, such as set, expectancy. etc. It is possible that in some cases the functioning of the two systems will determine different and even contradictory behavioural responses to what seems to be a single visual task. Such an example was demonstrated in a previous work (Dimitrov et al., 1976). A well-known fact is that saccidic eye movements are goal directed. Wurtz and Goldberg (1971) have demonstrated that goal directed saccades are most probably controlled by collicular mechanisms. We may therefore assume that in our experiments the direction of the line of sight after the execution of the last saccade corresponds to the place of target disappearance as “perceived” by the primitive collicular system. As we have already pointed out, this system is relatively independent and probably not influenced by factors which might be referred to as “psychological”. The system simply localizes the event in the visual field and initiates a saccade aimed at that place. The present data demonstrate that the deviation of the final position of the eyes from the place of target disappearance is almost constant in contrast with the mislocation. which depends on the place of disappearance of the target. This means that the “primitive visual system” is not concerned with the “meaning” of the background and is only stimulated by the event of disappearance. It is the second or “higher” visual system that ellaborates a decision about the reported place of stimulus disappearance. This system might be influenced by several different factors: the inevitable
overtracking, the existence of a limited metric scale, the conditional probability of stimulus disappearance at a given place on the scale, the scale division numbers, etc. The shape of the curve for !J follows the shape of the overtracking curve. The smallest errors in estimating the place of stimulus disappearance are observed at the right-hand end of the scale where the conditional probability of disappearance is near to 1.0. At the left-hand end of the scale the uncertainty of the expected stimulus disappearance may lead to greater deviations, both in overtracking and estimated place of target disappearance. Institure of Physiolog! Bulgarian Academy of Sciences Sofia Bulgaria
L. MITRAN G. DWTROL N. Y.AKLMOFF S. MATEEFF
REFERENCES Dimitrov G.. Mitrani L.. Yakimoff N.. MateeK S.. RadilWeiss T. and Bozkov V. (1976) Saccadic eye movements on Bela Julesz figure. Vision Res. 16, 411-414. Fuchs A. F. (1976) The neurophysiology of saccades. In Eye Moremems and Ps.vchological Processes (Edited by R. A. Monty and J. W. Senders). pp. 39-54. Lawrence Erlbaum Ass.. Hillsdale. New Jersey. Hazelhoff F. and Wiersma H. (1924) Die Wahrnehmungszeit. Z. Psphol. 96. 181-188. .Llateeff S. (1977) Is the method for “perception time” measurement adequate’! C.r. Acad. buig. Sci. 30. 603-606. MateetTS.. Mitrani L. and Yakimoff N. (19771 Localization of disappearance of light target during tracking eye movements-I. Acta Phsriol. Pharmac. Bula. 3. 21-27. Xtateeff S.. Yakimoff N. and Mitrani L. (1972j Kinematic characteristics of voluntary saccadic eye movements. Bull. fnsr. Ph~siol., Bulg. Acad. Sci. XIV, 35-46. Mitrani I. and Dimitrov G. (19781 Pursuit eye movements of a disappearing moving target. Vision Res. 18, 537-539. Suraeue J. M.. Berlucchi G. and Rizzolatti G. (1973) The r role of superior colliculus and pretectum in vision and .visually guided behaviour. In Handbook of Sensor! Physiology, Vol. VII, 3. Central processing of visual information, part B (Edited by R. Jung). pp. 27-102. Springer-Verlag. Berlin. Ward F. (1976) Pursuit eye movements and visual localization. In Eye Mocemenrs and Psychological Processes (Edited by R. A. Monty and J. W. Sanders), pp. 289-297. Lawrence Erlbaum Ass.. Hillsdale, New Jersey. Wurtz R. H. and Goldberg M. E. (1971) Superior colliculus cell responses related to eye movements in awake monkeys. Science 171. 82-84. Y
