|
ScienceWeek
COGNITIVE SCIENCE: ON THE VISUAL PERCEPTION OF MOTION
The following points are made by R.J. Snowden and T.C. Freeman (Current Biology 2004 14:R828):
1) Sensing the movements of the world and the objects within it appears to be a fundamental job for our visual system. In rare cases of brain damage, we find that individuals lacking motion perception live in a very different world of frozen images, where simple tasks like filling a kettle or crossing the road take on alarming difficulties.
2) That tasks such as driving a fast car down the freeway require a good sense of the movements of yourself and other objects are obvious, but motion information is used in many less obvious ways. For example, it may seem a trivial task to us to follow a moving object with our eyes, but without motion perception these smooth pursuit eye movements are not possible. One way to show this is to have people attempt to move their eyes smoothly along a line etched on a wall. At the same time we place a very bright light just under this line. The bright light burns an afterimage into the retina which can then be examined at leisure.
3) In attempting the eye movement we find that the afterimage is not a smooth line but a series of "dots". This is because you find it difficult to smoothly move yours eyes along the line --even more so if it is vertical -- instead flicking your eyes in a series of small, fast jumps known as "saccades". Each dot represents the alighting point of a saccade and each gap the distance moved by the saccade.
4) Now, if the line is replaced with a moving dot and we try to track this, the resulting afterimage is a smooth line. This shows that our eyes moved at a constant rate so that the very center of our vision, where it is best at seeing fine detail, remained focused on the target. Not surprisingly, damage to areas of the brain involved in analyzing the moving image destroys this ability.
5) The above illustration suggests we can sense the motion of an object as we follow it with our eyes. Of course, if we think about what is happening in the image during an eye movement the situation is somewhat different, because the moving object we are tracking is actually stationary on our retina and the stationary world around it moves across our retina in the opposite direction. Clearly the situation on the retina needs "interpretation" by the higher centers of the brain if we are to correctly deduce the actual motions about us.
References (abridged):
1. Sumnall, J.H., Freeman, T.C.A. and Snowden, R.J. (2003). Optokinetic potential and the perception of head-centred motion. Visi. Res. 43, 1709-1718
2. Hassenstein, B. and Reichardt, W. (1956). Systemtheoretische analyse der zeit-, reihenfolgen- und vorzeichenauswertung bei der bewegungspwezeption des rüsselkafers Chlorophanus. Z. Naturforsch 11b, 513-524
3. Borst, A. and Egelhaaf, M. (1989). Principles of visual motion detection. Trends Neurosci. 12, 297-306
4. Lennie, P. (2003). Receptive fields. Curr. Biol. 13, R216-R219
5. Newsome, W.T. and Paré, E.B. (1988). A selective impairment of motion perception following lesions of the middle temporal visual area (MT). J. Neurosci. 8, 2201-2211
Current Biology http://www.current-biology.com
--------------------------------
Related Material:
NEUROBIOLOGY: FORM VS. MOTION IN VISUAL SYSTEM ANALYSIS
The following points are made by D. Burr and J. Ross (Current Biology 2004 14:R381):
1) One of the major breakthroughs of the last few decades of vision research has been the discovery of two separate functional streams: a ventral stream for the analysis of form (the "what" stream), and a dorsal stream for the analysis of position and motion (the "where" stream) [1]. Of interest is that a similar division of labor has recently been described for the auditory system [2].
2) The visual system is usually thought to separate its processing of form from its processing of motion, and to subdivide these two main streams further into processing modules, each specialized for different aspects of "what" and "where": luminance, color, texture, depth, complex motion and so on [3]. Indeed, so widely accepted is the idea of separate modules for different visual attributes, many neurobiologists believe that there is a "binding problem" of how to link the different attributes together to recover a coherent holistic percept.
3) While this neat picture of separate paths of analysis is very appealing, and receives support from various lines of study, there are several lines of evidence suggesting the story is at best incomplete. One clear example of interaction between form and motion is the "biological motion" first described by Gunnar Johansson [4]: when point light sources are attached to an actor's joints, they are perceived as a meaningless jumble of lights when the actor is stationary, but give an immediate vivid impression of the actor when she or he is walking (see http://www.bml.psy.ruhr-uni-bochum.de/Demos/BMLwalker.html for demonstration). Motion reveals form. This phenomenon has been very well studied and is generally ascribed to the combination of information from the form and motion pathways [5].
4) Biological motion, however, is not a unique example. The visual system is capable of extracting complex form information from translating patterns. Furthermore, by taking advantage of the spatio-temporal information available only to a system tuned to motion, we can pick up information not available from any static view or any collection of views.
5) In summary: Our visual system must allow us to see the form of objects in motion. Tracking objects of interest stabilizes their images on the retina, but is not sufficient, as untracked images move on the retina. This problem is solved by nerve cells tuned in both space and time, combining information about form with information about motion.
References (abridged):
1. Mishkin, M., Ungerleider, L.G., and Macko, K.A. (1983). Object vision and spatial vision: two cortical pathways. Trends Neurosci. 6, 414-417
2. Rauschecker, J.P. and Tian, B. (2000). Mechanisms and streams for processing of what and where in auditory cortex. Proc. Natl. Acad. Sci. U.S.A. 97, 11800-11806
3. Zeki, S. (1993). A vision of the brain. (Oxford: Blackwell Scientific)
4. Johansson, G. (1973). Visual perception of biological motion and a model for its analysis. Percept. Psychophy 14, 201-211
5. Giese, M.A. and Poggio, T. (2003). Neural mechanisms for the recognition of biological movements. Nat. Rev. Neurosci. 4, 179-192
Current Biology http://www.current-biology.com
--------------------------------
Related Material:
ON NEURAL MECHANISMS OF SACCADIC SUPPRESSION
The following points are made by A. Thiele et al (Science 2002 295:2460):
1) During and after rapid shifts of gaze we are usually unaware of any image motion or image displacement. Comparable external image motion does not go unnoticed -- on the contrary, it has a startling effect. This phenomenon, known as "saccadic suppression" (1), has been extensively investigated psychophysically, but the underlying neuronal mechanisms remain elusive. Some researchers have suggested that vision is actively suppressed during saccades (2-4), whereas others have proposed that the visual system may simply be insensitive to the high image velocities (5). The latter is true for small features and objects, but the visual system can process image motion of 300 to 800 /s (as it occurs during saccadic eye movements), provided the features or objects are sufficiently large. Saccadic suppression predominantly affects the magnocellular visual system. It is particularly powerful in the motion domain and may have evolved to blunt the startling effect of rapid visual motion that saccades would otherwise induce.
2) The authors report they investigated correlates of saccadic suppression in the middle temporal (MT/V5) and middle superior temporal (MST) area of the primate brain, both of which have been linked to the perception of visual motion. The procedure allowed comparison of activity from directionally selective cells when visual motion was saccade induced (active condition) and when identical visual motion was induced externally (passive condition with eyes held stationary). Thirty-four of 51 (66%) cells in MT and 79 of 116 (68.1%) in MST showed significant differences between the two types of image motion. The remainder of the cells (34% in MT and 31.9% in MST) could be subdivided into two groups: (a) cells that did not respond to high-velocity image motion at all (5 MT, 15 MST cells); and (b) cells that responded equally well to the two types of motion, although with small but statistically significant latency differences.
3) In summary: In normal vision our gaze leaps from detail to detail, resulting in rapid image motion across the retina. Yet we are unaware of such motion, a phenomenon known as saccadic suppression. The authors recorded neural activity in the middle temporal and middle superior temporal cortical areas during saccades and identical image motion under passive viewing conditions. Some neurons were selectively silenced during saccadic image motion, but responded well to identical external image motion. In addition, a subpopulation of neurons reversed their preferred direction of motion during saccades. Consequently, oppositely directed motion signals annul one another, and motion percepts are suppressed.
References (abridged):
1. B. Bridgeman, D. Hendry, L. Stark, Vision Res. 15, 719 (1975)
2. E. Holt, Harvard Psychol. Stud. 1, 3 (1903)
3. E. von Holst and H. Mittelstaedt, Naturwissenschaften 20, 464 (1950)
4. R. W. Sperry, Vision Res. 16, 1185 (1950)
5. D. MacKay, Nature 255, 90 (1970)
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
|