Eye Movements And Perception Research Paper

1. Types Of Eye Movements

One counterintuitive fact about the eyes is that they are almost constantly moving. Although one is generally not aware of it, the eyes frequently make very small involuntary movements often called physiological nystagmus. Consequently, the optical image that falls on the retina is constantly jittering. Rather than being an impediment to perception, however, these tiny movements are actually a necessity. If the eyes are somehow constrained from moving or equivalently if the image that falls on the back of the eyes is stabilized by some external means then after a few seconds perception of the visual image fades away and nothing is seen at all! Thus, in some sense, without eye movements visual perception would be impossible.

While nystagmus keeps a fresh image of the world on the retina, in many situations it is desirable to minimize retinal image displacement in order to extract high-resolution information from objects in the environment. One way this is achieved is via vestibular eye movements. Vestibular eye movements keep the eyes directed on an object in the world despite movements of the head and body. As the head moves in one direction, the eyes rotate in the opposite direction just enough to keep the fixated object in the center of gaze. Without this ability even small movements of the head would cause the optical images of objects in the world to be smeared across the retina, making their recognition difficult.

Optokinetic eye movements often occur in conjunction with vestibular eye movements and they serve the same purpose: maintaining fixation on an object in the visual world. The optokinetic reflex refers to the fact that when a large portion of the visual field moves across the retina, the eyes reflexively fixate one object in the world and track it slowly for some period of time before moving rapidly back in the opposite direction to begin fixation on another object in the visual world.

Smooth pursuit eye movements are used to track the position of a single moving object in the visual field. This enables one to examine the moving object for an extended time, aiding object identification. However, the eyes are able to keep up with the moving object only as long as it does not move too quickly (usually 20–30 degrees per second).

Vergence eye movements allow one to fixate objects at different distances so that their optical images fall in the center of gaze. The two eyes converge so that they both point at the object in the world, and the degree of convergence required is a cue to depth. Vergence eye movements also allow one to track objects moving in depth.

The major purpose of the eye movements described so far is to maintain the optical image of an object in the world in central gaze. However, another reason for moving the eyes is to acquire new information. This is necessary because the ability to resolve spatial detail (visual acuity) is best at the center of vision and drops off precipitously with increasing eccentricity. Thus, saccadic eye movements are made in order to bring new objects into the center of gaze. Saccades are made while reading, while viewing pictures, and while exploring the visual environment. Via saccades the point of gaze is rapidly and abruptly shifted from one position in space to another. It takes approximately 150–200 milliseconds (ms) to plan a single saccade, but the movements themselves are extremely rapid. Movement time depends on saccade length, but the average saccadic movement lasts approximately 30 ms. Between movements the eyes fixate for a variable period of time on an object of interest in the environment before moving on. The average fixation duration is approximately 250–300 ms. In sum, saccadic eye movements occur approximately three times each second when one is scanning the environment.

2. The Perception Of A Stable Visual World Across Saccades

One of the classic problems in visual perception concerns how it is that people ordinarily perceive the visual world as unified, stable, and continuous despite eye movements. Research addressing this question has concentrated primarily on the perceptual consequences of saccades. Although saccades facilitate the exploration of the visual environment they also create several problems for visual perception. One problem is that during a saccade visual stimulation is swept across the back of the eyes at a very high speed, producing a blur or a smear that is not ordinarily perceived because of saccadic suppression. Saccadic suppression is largely a consequence of visual masking: the clear, bright, long-duration fixations that precede and follow each saccade mask the perception of the low-contrast blur that is on the retina in much the same way that a bright flash of light would mask the perception of any low-contrast, brief-duration stimulus. Because of saccadic suppression visual information about the world is registered in isolated glimpses that are separated in time.

A second problem is that the retinal positions of objects in the world change suddenly and dramatically when the eyes move, so that an object that appears in the periphery in one fixation suddenly may appear at the center of gaze in the next fixation. Thus, saccades result in visual input that is temporally discontinuous and rapidly changing. Despite this, the visual world appears to be unified, stable, and continuous, with objects maintaining their positions in space. This quality of perception suggests that the perceptual system computes the spatial coordinates of objects in the world and maintains this information across eye movements to produce the experience of a stable and continuous visual world. How this is accomplished has intrigued psychologists and vision researchers for over a century (see Bridgeman et al. 1994 for a review).

Much of the research addressing this question has concentrated on why objects in the world appear to maintain a constant direction with respect to the viewer even though their retinal positions change when the eyes move. This phenomenon is usually called visual direction constancy. Traditionally, theories of visual direction constancy have assumed that some kind of oculomotor information about eye position is used in conjunction with retinal (i.e., visual) information to allow the viewer to compensate for the changes in visual stimulation that occur during an eye movement. Some have claimed that this oculomotor information consists of a copy of the efferent commands that are sent to the eyes to move them while others have argued that it consists of inflowing afferent information about eye position from muscle spindles in the oculomotor muscles. Hybrid solutions have been proposed as well in which direction constancy is achieved via an interaction between outflowing gamma efferent signals and inflowing afferent signals. Evidence for these theories comes largely from demonstrations that direction constancy breaks down when there is a mismatch between oculomotor information and visual stimulation. For example, people with oculomotor paralysis report that the perceived world seems to jump when they attempt to move their eyes, and passive displacements of the eyes (produced by pressing on the side of the eye, for example) produce a perception of retinal image movement.

These traditional theories of direction constancy have been called into question by experiments that have examined the spatial coding of objects across changes in eye position. Perceptual research examining people’s ability to localize visual flashes that are presented near the time of a saccade has shown that any oculomotor information that exists is too slow and too imprecise to account for the stability that is perceived across saccades. Other studies have shown that stimulus displacements that occur during saccades are often unnoticed, which suggests that the compensatory oculomotor signal accompanying the saccade must be rather inaccurate. Visual direction constancy is also greatly influenced by the presence of visual landmarks in the environment; for example, the spatial localization errors that occur when the eyes are paralyzed are experienced only in the dark and not in normally illuminated visual surrounds. Similarly, the illusory displacements of the visual world that are generated by passive displacements of the eyes are greatly reduced in the presence of a stable visual background. Based on the results of such studies, it appears that visual direction constancy relies not on oculomotor information alone, but rather on some combination of oculomotor information and relative visual position information, with the visual information dominating the oculomotor information in many circumstances.

More recent research has addressed the role of memory in the construction of a stable and continuous visual environment (see Irwin 1996 for a review). At one time it was hypothesized that the visible contents of successive eye fixations were spatially reconciled across eye movements (using the same sources of information that produce direction constancy) and superimposed according to environmental coordinates in a low-level memory buffer to produce a unified representation of the visual environment. However, extensive research has failed to find evidence for such a memory buffer. In fact, very little information appears to be remembered from one eye fixation to the next. Thus, the experience of a stable and continuous visual environment appears to arise from the contents of individual eye fixations, rather than from a detailed mental representation that is constructed across eye movements.

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