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ScienceWeek
2004 9 July C4 NEUROBIOLOGY: ON SACCADIC SUPPRESSION IN HUMANS
Notes by ScienceWeek:
In this context, the term "BOLD signal" refers to the blood oxygen level dependent signal in functional magnetic resonance imaging (fMRI) studies.
The following points are made by R. Kleiser et al (Current Biology 2004 14:386):
1) Upon looking into a mirror and moving your eyes left to right, you will see that you cannot observe your own eye movements. This demonstrates the phenomenon of "saccadic suppression": during saccadic eye movements, visual sensitivity is much reduced. Given that humans make more than 100,000 eye movements each day, it is clear why suppression is needed: without it, the motion on the retina would prevent us from seeing anything at all.
2) Many psychophysical studies have investigated saccadic suppression and have generally concluded that suppression of visibility starts approximately 75 ms before a saccade and returns back to normal 100 ms after saccade onset[1]). In laboratory setups, visibility is not reduced to zero [2-5], but it has been reported as tenfold poorer during saccades. Suppression is stimulus selective; several groups have shown that those visual stimuli typically processed by the magnocellular, dorsal visual stream are suppressed the most. Given the role of the dorsal stream in processing motion information, this fits well with the purported role of saccadic suppression to eliminate retinal image motion due to saccadic eye movements.
3) Electrophysiological studies have demonstrated that saccades affect single cell visual responses in the lateral geniculate nucleus (LGN), the middle temporal (MT), middle superior temporal (MST) [13], and ventral intraparietal area (VIP). These single-unit responses are at times either enhanced or suppressed when compared to fixation. In humans, positron emission tomography studies have shown that activity in human visual areas decreases when subjects make saccades in total darkness. This confirms that there is extraretinal (i.e., nonvisual) suppression of metabolic activity in the visual cortex, but this approach does not allow one to investigate the stimulus-specific, perceptual nature of the suppression.
4) The authors used event-related functional magnetic resonance imaging (fMRI) to find brain areas with a stimulus-selective suppression of the BOLD signal that matches the psychophysical data. The authors found such a neural correlate of saccadic suppression in the dorsal stream (hMT+, V7) and in ventral area V4. These areas receive magnocellular input; thus the findings are consistent with the magnocellular hypothesis. The range of effects in these data and in single cell data, however, argues against a single thalamic mechanism that suppresses all cortical input. Instead, the authors speculate that saccadic suppression relies on multiple mechanisms operating in different cortical areas.
References (abridged):
1. Ross, J., Morrone, M.C., Goldberg, M.E., and Burr, D.C. (2001). Changes in visual perception at the time of saccades. Trends Neurosci. 24, 113-121
2. Ilg, U.J. and Hoffmann, K.P. (1993). Motion perception during saccades. Vision Res. 33, 211-220
3. Castet, E. and Masson, G.S. (2000). Motion perception during saccadic eye movements. Nat. Neurosci. 3, 177-183
4. Garcia-Perez, M.A. and Peli, E. (2001). Intrasaccadic perception. J. Neurosci. 21, 7313-7322
5. Castet, E., Jeanjean, S., and Masson, G.S. (2002). Motion perception of saccade-induced retinal translation. Proc. Natl. Acad. Sci. USA 99, 15159-15163
Current Biology http://www.current-biology.com
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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
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Related Material:
FUNCTIONAL PROPERTIES OF NEURONS IN THE ANTERIOR BANK OF THE PARIETO-OCCIPITAL SULCUS OF THE MACAQUE MONKEY
The following points are made by C. Galletti et al (Eur J Neurosci 1991 3:452):
1) The authors report that extracellular recordings were made in the anterior bank of the parieto-occipital sulcus of two waking monkeys trained to perform fixation tasks in normal illumination or in complete darkness. Of the recorded neurons, 73% (251/343) were responsive to visual stimulation, but their overall organization did not conform to a simple, continuous retinotopic map.
2) Most of the visual neurons showed a high degree of orientation and direction sensitivity, higher than that found in areas V1, V2 and V3A under the same experimental conditions. Whether they had a resolvable receptive field or not, the discharge rate of many neurons in the anterior bank of the parieto-occipital sulcus was influenced by oculomotor activity.
3) The animals were required to execute pursuit or saccadic eye movements in darkness. Saccadic eye movements were found to influence 19% of the neurons tested (29/156); by contrast, pursuit eye movements were without effect (0/64). Saccade responses were direction-tuned and, in several cases, the neuronal discharge started before the onset of eye movement. The animals were also required to gaze, in darkness, at nine different positions on the screen they faced. Of the neurons tested, 59% (102/174) were affected by the direction of gaze. Higher discharge rates were generally observed when the animals looked towards the lower part of the field of view.
4) Given the functional properties of its neurons, its connections with area V3A -- where neural signals appropriate for building an objective map of the visual space are present (Galletti and Battaglini, 1989, J. Neurosci., 9, 1112-1125) --and its output to the visuomotor centers involved in the generation of saccades (frontal eye fields and superior colliculus), the authors infer that the cortex of the anterior bank of the parieto-occipital sulcus might be part of the network involved in the control of gaze in order to locate objects in visual space.
European J. Neuroscience http://www.blackwellpublishing.com/journals/ejn/
ScienceWeek http://scienceweek.com
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