Ординатура / Офтальмология / Английские материалы / Eye Movements A Window on Mind and Brain_Van Gompel_2007

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Figure 7. Left, Stratton’s camera for recording eye movements, and right, records for tracking a circle, square and an S shape (both from Stratton, 1902).

supposed to produce a given illusion are present when the illusion is absent, and absent when the illusion is present. One can hardly believe, then, that the supposed causal connection really exists. (Stratton, 1906, p. 93)

In his investigation of the eye movements of observers while looking at a variety of symmetrical and non-symmetrical figures, Stratton was again surprised by what he found when he examined the photographic negatives recorded: “one is struck by the almost grotesque unlikeness between the outline observed and the action of the eye in observing it. For the most part the eye moves irregularly over the figure, seeking certain points of vantage from which the best view of important features may be obtained. And these positions are marked by the eye’s momentary resting there” (1906, p. 94). Again, the path taken by the eye during movements did not relate to the form of the figure being viewed, but Stratton also observed that the positions fixated did not show any clear and consistent relationship with the figure:

Now these points of rest are evidently of more consequence to the observer than the path by which the eye reaches them; indeed the form of any single path between two stops usually bears no observable resemblance to the outline which the subject was taking in, and which in many cases he believes his eye to be accurately following. But even these points of rest are not so arranged as to supply of themselves a rough sense of the form perceived, after the manner of an outline pricked disconnectedly in paper. The points of the eye’s rest in the records are usually too few and too inexact to give any such clear and connected perception of the form as the observer regularly and readily obtains. (Stratton, 1906, p. 94)

The phrasing of the article suggests some degree of despair in Stratton’s reports of the lack of correspondence between the eye movements and the figure being observed. The degree of surprise and disbelief evident in this article highlights the fact that the saccade and fixate behaviour of the eye was still very much a new aspect of vision research.

This continued to surprise and intrigue researchers even nearly 30 years after Javal’s report of Lamare’s work and Hering’s article describing saccadic eye movements. Not only was there no clear relationship between the eye movements and form of the figure viewed, but there was also no clear relationship between the symmetry of the figure viewed and the symmetry of the eye movements made when viewing it.

Stratton’s work is significant because it attempts to bridge the gap between visual phenomena (illusions), cognition (aesthetic judgements), and the underlying mechanisms (eye movements). Thus, both Stratton and Dodge defined new directions for eye-movement research, highlighting the discrepancy between cognition and perception in vision. Stratton explicitly states the inadequacy of eye movements as an explanation for aesthetic judgements: “The sources of our enjoyment of symmetry, therefore, are not to be discovered in the form of the eye’s behaviour. A figure which has for us a satisfying balance may be brought to the mind by most unbalanced ocular motions” (1906, p. 95). It is clear from Stratton’s closing remarks in his article on eye movements and symmetry that he appreciated the gulf between perception and cognition and that he was aware that his work was defining new questions in vision that would be addressed in the future.

Charles Hubbard Judd (1873–1946) worked initially on geometrical illusions and was dismissive of interpretations based on angle expansion and perspective. Rather he favoured what he called “the movement hypothesis”: “The more intense the sensation of movement, the greater will be the estimation of the distance; conversely, the less the intensity of the sensations of movement, the shorter the estimated distance” (Judd, 1899, p. 253). He did not equate the hypothesis with actual movement nor did he, at this stage, provide any measurements of the extent of eye movements. Judd was impressed by the work of Dodge and Stratton, but felt that the eye-trackers used by both were somewhat limited in the range of tasks to which they could be applied: Dodge’s eye-tracker was designed for recording movements only along a straight line (as occurs in reading) and Stratton’s lacked temporal resolution and required a dark room in which the photographic plate was exposed during experimental recording. Judd developed a “kinetoscopic” eye-tracker in which a small fleck of Chinese white, which had been affixed to the cornea, was photographed; eye movements could thus be discerned in two dimensions, and did not require a dark room (Judd, McAllister & Steele, 1905). While Judd’s eye-tracker offered specific advantages over previous photographic devices, it was still somewhat limited in its temporal resolution, typically operating at about 8–9 frames per second (in comparison to Dodge’s 100 Hz Photochronograph). One impressive feature of Judd’s eye-tracker was that it allowed binocular recordings to be taken.

Having developed this new photographic eye-tracker, Judd and colleagues went on to use it to investigate eye movements between simple targets (McAllister, 1905) and the role of eye movements in the Müller-Lyer illusion (Judd, 1905a), the Poggendorff illusion (Cameron and Steele, 1905), and the Zöllner illusion (Judd & Courten, 1905). In the first of these reports (McAllister, 1905) observers were asked to alternate fixation between two targets. The nature of these targets was varied comprising different arrangements of dots and lines. In his investigation of the Müller-Lyer illusion, Judd (1905a) found that

there was some evidence for there being differences in the eye movements when looking at the overestimated and underestimated regions of the illusions, and also that practice influenced eye movements:

The positive outcome of our researches consists, therefore, in the exhibition of a consistent tendency on the part of five subjects to show restricted movements in looking across the underestimated Müller-Lyer figure and freer movements in looking across the overestimated figure. One of these subjects who performed a series of adjustments with sufficient frequency to overcome the illusion showed a marked change in the character of the movement. (Judd, 1905a, p. 78)

In the case of the Poggendorff illusion, eye movements again correlated with the experience of the illusion (Cameron and Steele, 1905). However, this was not the case for the Zöllner illusion: “The usual fact is that the eye movement, and consequently the sensations from these movements, are in the wrong direction to account for the illusion” (Judd & Courten, 1905, p. 137). It is interesting to note that Stratton and Judd appeared to disagree in their investigations of the role of eye movements in visual illusions. While Judd offered eye-movement explanations for the Müller-Lyer and Poggendorff illusions, Stratton, as we have seen, proposed that no such explanations were possible.

Judd’s interest in interpreting the results of the studies on illusions was in addressing the relationship between movement and perception (Judd, 1905b). Judd sought to use his results to dismiss the notion that perceptions might arise directly from movements themselves; rather he stressed the importance of visual information from the retina both in forming perceptions and in coordinating the movements:

When the eye moves toward a point and the movement does not at first suffice to bring the point in question on the fovea, the retinal sensations which record the failure to reach the desired goal will be a much more powerful stimulus to new action than will any possible muscle sensation. (Judd, 1905b, p. 218)

While much of Judd’s discussion of his eye-movement studies focused upon the discussion of their relation to theories of movement sensation, he did also notice that the pattern of eye movements was likely to be influenced by the instructions given to the observers during the experiments. The recognition that eye movements were not entirely directed by the stimulus being viewed echoed the opinion expressed by Stratton at the same time, but it was not investigated in any great depth by either Judd or Stratton.

A more thorough treatment of this question was not conducted until some years later when Judd’s former doctoral student, Guy Thomas Buswell (1891–1994), conducted an impressive series of eye movement studies. While Buswell’s work on reading is probably his most renowned work, it was from his study of eye movements when viewing pictures that one of his major contributions to eye-movement research arises. The latter work was published in his impressive monograph How people look at pictures (Buswell, 1935). In the monograph, Buswell reports eye-movement data recorded from 200 participants each viewing multiple pictures, such that his data comprised almost 2000 eye-movement records each containing a large number of fixations. This volume of eye-movement data is

there could be quite large individual differences in where people fixate when viewing pictures:

The positions of the fixations indicate clearly that for certain pictures the center or centers of attention are much more limited than in other pictures. The fact that the density plots represent composite diagrams from a great many different subjects obviously results in a wider distribution of fixations on account of the varied interests of different individuals in looking at the same picture. However, the density plots do give a rather clear indication as to what parts of a given picture are likely to prove most interesting to a random selection of subjects. (Buswell, 1935, p. 24)

The issue of individual differences in the patterns of eye movements was explored in detail in a section at the end of the second chapter. The differences that were present in the locations fixated by individuals when viewing each image were also reflected in the durations of the fixations, with a large variation between observers in their average fixation duration on each picture. Buswell’s investigation of individual differences extended to exploring differences between artistically trained individuals and those without training; between children and adults; and between Western and Oriental participants. In all cases, differences between the groups were small: “The average differences between the groups were so much less than the individual differences within each group that the results cannot be considered significant” (Buswell, 1935, p. 131). Differences were found in the eye-movement data that emerged over the course of viewing a picture for some time. The regions fixated in the picture were more consistent between observers for the first few fixations than for the last few on each picture. Buswell also found that fixation duration increased over the course of viewing a picture for some time.

Buswell devoted a chapter of the book to looking at the influence of the characteristics of the picture upon where is fixated. This work is very reminiscent of that conducted some years earlier by Stratton although he did not cite Stratton’s work. In places Buswell appears to argue that eye movements do tend to follow lines in pictures. This is contrary to Stratton’s suggestion that eye movements do not appear to be particularly influenced by the form of the figure being viewed. However, other aspects of Buswell’s data suggest less concordance between eye movements and the characteristics of the picture. When Buswell showed participants more basic designs and patterns he found that:

The effect of different types of design in carrying the eye swiftly from one place to another is apparently much less than is assumed in the literature of art. The writer should emphasize that the data from eye movements are not to be considered as evidence either positively or negatively for any type of artistic interpretation. (Buswell, 1935, p. 115).

Like Stratton, Buswell felt that the pattern of eye movements was insufficient to explain our visual experience and so highlighted the need to appeal to cognitive explanations of vision.

Perhaps the most often overlooked aspect of Buswell’s work was his investigation of how different instructions given to observers prior to viewing an image can influence

the eye movements made during viewing. For example, when presented with a picture of Chicago’s Tribune Tower, eye movements were first recorded while the participant viewed the picture “without any special directions being given. After that record was secured, the subject was told to look at the picture again to see if he could find a person looking out of one of the windows of the tower” (Buswell, 1935, p. 136). Very different eye-movement records were obtained in these two situations, demonstrating that cognitive factors such as the viewer’s task can have a strong effect upon how a picture is inspected. Such descriptions of influences played by high-level factors upon eye movements are not typically associated with Buswell, but rather it is Yarbus who is generally regarded to be the first to have offered such an observation (Yarbus, 1967).

Buswell’s impressive monograph illustrates the rapid changes that were taking place in eye-movement research in the first half of the twentieth century. Understanding of saccadic eye movements was rapidly increasing, as was the technology with which they could be measured; with these changes came both new questions about vision and the ability to address them in increasingly realistic viewing conditions and with increasing flexibility and precision. Buswell’s book discussed a wide range of issues regarding the relationship between eye movements and visual experience and these questions have been reflected in much of the eye-movement research that had followed.

5. Conclusion

In an historical context, investigations of eye movements pose several paradoxes. On the one hand, they are a fundamental feature of our exploration of the world. We are aware that our own eyes move and we can readily detect movements in the eyes of those we observe. Indeed, eye movements are potent cues in social intercourse – we might see “eye to eye” with someone, we might determine honesty when we “look someone straight in the eye” or we might denigrate someone because they have a “shifty look”. On the other hand, throughout the long descriptive history of studies of eye movements a vital characteristic of them remained hidden from view, both in ourselves and in our observations of others. The rapid discontinuous nature of eye movements is a relatively recent discovery, as are the small involuntary movements that accompany fixation. For most of recorded history, the eyes were thought to glide over scenes to alight on objects of interest, which they would fix with unmoved accuracy.

Another paradox is that initial knowledge about eye movements derived from generating stimuli that did not move with respect to the eye when the eye moved. The first of these was the afterimage which, since the late eighteenth century, has been applied to determining the ways the eyes moved. More complex photographic recording devices appeared a century later, and the assistance of computers was incorporated three quarters of a century later still. Nonetheless, the insights derived from the skilful use of afterimages have tended to be overlooked in the histories of eye movements. One of the reasons that less attention has been paid to studies using afterimages is that they became tainted with other “subjective” measures of eye movements as opposed to the

“objective” recording methods of the twentieth century. A second factor was that the early studies were concerned with vertigo, which resulted in involuntary movements of the eyes. Moreover, histories of vertigo have been similarly blind to the early studies of it employing afterimages.

These points apply particularly to the experiments conducted by Wells (1792, 1794a, 1794b), both in the context of voluntary and involuntary eye movements. In the case of voluntary eye movements, Wells demonstrated that afterimages appeared to move with active eye movements whereas they appeared stationary when the eye was moved passively (by finger pressure); this procedure became one of the critical tests of afference vs efference, of inflow vs outflow. Thus, the essential aspects of visual stability during eye movements were examined using afterimages, and the procedure was repeated throughout the nineteenth century. There was nothing novel about afterimages themselves at that time. Wells produced them by the time-honoured technique of viewing a bright stimulus (like a candle flame) for many seconds; the afterimage remained visible for long enough for him to examine its apparent movement under a range of conditions. One of these was vertigo generated by rotating the body around a vertical axis.

Wells’ work was forgotten, but the techniques he pioneered were rediscovered by those scientists puzzling over the mechanisms of vestibular function. Mach, Breuer, and Crum Brown all employed afterimages to measure the movements of the eyes after body rotation. They were able to link vertigo with their hydrodynamic theory of semicircular canal activity. Crum Brown forged the link between eye movements during vertigo and normal scanning of scenes; again afterimages were the tool used to demonstrate how scanning proceeds via a sequence of rapid movements interspersed with brief stationary pauses.

Afterimages had been enlisted to attempt to examine eye movements during reading by Javal but the contrast between the printed letters and paper made the afterimage difficult to discern. Novel methods were required and he was instrumental in trying some, but with little success. Javal’s student, Lamare (1892), developed an indirect method that was more successful and yielded interesting and unexpected results. Lamare’s technique was to use a tube attached to the eyelid or conjunctiva, from which sounds could be heard by the observer whenever the eyes moved. Using this technique, Lamare observed that, contrary to expectation and introspection, eye movements during reading were discontinuous. A very similar acoustic technique had been employed by Hering (1879b) to observe eye movements. However, Hering’s technique had the added advantage of comparing the sounds thought to arise from eye movements to the movement of afterimages. Thus, Hering was able to confirm that the sounds coincided with movements of the afterimage and were therefore likely to be associated with eye movements.

The objective eye-trackers developed in the late nineteenth and early twentieth centuries allowed crucial new insights into the true nature of eye movements (see Venezky, 1977). Researchers were unanimously surprised by what they found; eye movements were not as smooth and continuous as they subjectively appeared. With these new devices for measuring eye movements so began a proliferation of interest in the nature of eye movements. The technological advancements allowed new questions to be addressed and

indeed identified new and unexpected questions in the psychology and physiology of eye movements and their relation to cognition and visual experience. Eye-tracking technology continues to evolve, and with it so do the range of questions that can be addressed. Increasingly, the eyes are measured when inspecting objects in three-dimensional space – the scene is being scanned!

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