|
ScienceWeek
NEUROBIOLOGY: VISION AND THE BRAIN
The following points are made by Frank Sengpiel (Current Biology 2004 14:997):
1) At first glance, it seems obvious that what the visual cortex does is an analysis of the information conveyed by the retinal images in the two eyes. One would therefore expect that there should be a rather strict relationship between retinal input and the output of the visual cortex -- the spiking activity of its neurons. But researchers in the field know that many neurons display a high variability of response when the same stimuli are shown repeatedly. This variability is attributed to the spontaneous activity of cortical neurons, which in turn is usually dismissed as "noise". A recent study by Fiser et al[1], however, provides evidence that spontaneous activity may shape the response to sensory stimuli, which systematically improves during early postnatal development.
2) That spontaneous activity might not be as random as previously thought has been demonstrated in a number of studies of its spatio-temporal pattern at various levels of the visual pathway. Patterns of locally correlated bursting activity were originally discovered in the developing retina, where they became known as "retinal waves" [2]. Later they were also observed in the lateral geniculate nucleus [3] and the primary visual cortex, V1 [4,5]. Correlated spontaneous activity in V1 shows a similar kind of patchy organization as exhibited by horizontal intracortical connections [5], and periodic fluctuations in correlated activity match with periodic changes in ocular dominance.
3) Even more surprising than the mere presence of spatio-temporal patterns in spontaneous activity was the finding that those patterns often match those observed in response to actual visual stimuli. Using voltage-sensitive dye imaging, Kenet et al (2003) recorded and compared patterns of spontaneous activity and those elicited with full-field gratings of various orientations. Spontaneous activity patterns were highly dynamic, but at any moment in time they often corresponded quite closely to an orientation map in response to visual stimulation. This finding raises the question, addressed by Fiser et al[1], whether spontaneous activity is not just "noise" but plays an important role in how neurons respond during natural viewing.
References (abridged):
1. Fiser, J., Chiu, C. and Weliky, M. (2004). Small modulation of ongoing cortical dynamics by sensory input during natural vision. Nature 431, 573-578
2. Meister, M., Wong, R.O., Baylor, D.A. and Shatz, C.J. (1991). Synchronous bursts of action potentials in ganglion cells of the developing mammalian retina. Science 252, 939-943
3. Weliky, M. and Katz, L.C. (1999). Correlational structure of spontaneous neuronal activity in the developing lateral geniculate nucleus in vivo. Science 285, 599-604
4. Tsodyks, M., Kenet, T., Grinvald, A. and Arieli, A. (1999). Linking spontaneous activity of single cortical neurons and the underlying functional architecture. Science 286, 1943-1946
5. Chiu, C. and Weliky, M. (2001). Spontaneous activity in developing ferret visual cortex in vivo. J. Neurosci. 21, 8906-8914
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:
NEUROSCIENCE: ON TUNING IN VISUAL CORTEX
The following points are made by Mark Georgeson (Current Biology 2004 14:R751):
1) If you stare at a rotating disc for a little while and then stop the rotation, the disc will appear to be rotating backward, even though it is actually stationary. Similar illusory movement can be seen after looking at a waterfall, or the credits rolling at the end of a movie. This striking phenomenon -- the "motion aftereffect" -- has been known for hundreds of years [1], and is one of many visual aftereffects that have intrigued students and scholars of perception. Aftereffects reveal a gap between appearance and reality, and remind us that what we see is determined by how visual information is coded in the brain, and not simply by how things "really are".
2) Aftereffects also provide an opportunity for psychologists and neuroscientists to understand the way in which populations of visually selective cells encode information about visual dimensions, such as movement, orientation, size and color. For orientation, a few seconds or minutes of exposure to tilted lines will make vertical lines seem tilted the opposite way -- the "tilt aftereffect" [2]. Analogous effects are obtained when the adapting and test patterns are moving in different directions. Adapting to dots or gratings drifting for example -30 deg from vertical will make vertical movement appear shifted by about +20 deg -- the "directional aftereffect" [3,4]. The aftereffects are therefore "repulsive": neighboring test stimuli appear to be shifted away from the adaptor in orientation or direction of movement.
3) A major goal of research is to understand how and why the response properties of cells in visual areas of the brain change, both during and after a period of exposure to an adapting stimulus, and how these neural changes are related to the perceptual changes experienced in the aftereffects. Recent studies on the cat and monkey brain [5] have begun to shed new light on these questions, but also to imply that neural dynamics are more complex than we previously supposed.
4) Cells in the visual cortex are "tuned" or selective, such that individual cells respond best to a particular orientation and/or direction of motion, and across the population different cells respond best to different orientations and directions of motion. It is the pattern of activation across the population -- which cells are most active to a given stimulus -- that is likely to represent the perceived orientation or direction. Twenty or thirty years ago, it seemed reasonable to suppose that when exposed to, say, a pattern moving to the right, the cells most responsive to rightward motion would become adapted or desensitized, while other cells, less responsive to this stimulus, would be little affected. This simple "fatigue" model has been at the heart of much thinking about adaptation and aftereffects. It correctly predicts that individual cells -- and the whole observer -- should be less sensitive to the adapting stimulus after a period of exposure, and it broadly accounts for the perceptual distortions that result.
5) It has become increasingly clear, however, that visual cortical cells also adapt in ways not captured by the "fatigue" model. Several studies of V1 cells found that if a cell is exposed to stimulus X -- say, a left tilted grating -- then its sensitivity and responsiveness to X are indeed reduced, but its responses to stimulus Y -- say, a right tilted grating -- may be unaffected, or even enhanced. The adaptation effect can thus be selective at the single-cell level, and the tuning curve of the cell may be shifted, not merely scaled down [5].
References (abridged):
1 In The Motion Aftereffect: A Modern Perspective. (1998). Mather, G.X Verstraten, F. and Anstis, S. eds. (Cambridge, Mass: MIT Press)
2 Greenlee, M.W. and Magnussen, S. (1987). Saturation of the tilt aftereffect. Vis. Res. 27, 1041-1043
3 Levinson, E. and Sekuler, R. (1976). Adaptation alters perceived direction of motion. Vis. Res. 16, 779-781
4 Schrater, P.R. and Simoncelli, E.P. (1998). Local velocity representation: evidence from motion adaptation. Vis. Res. 38, 3899-3912
5 Dragoi, V.X Sharma, J. and Sur, M. (2000). Adaptation-induced plasticity of orientation tuning in adult visual cortex. Neuron 28, 287-298
Current Biology http://www.current-biology.com
ScienceWeek http://scienceweek.com
|