Flash suppression
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Flash suppression is a phenomenon of visual perception in which an image presented to one eye is suppressed by a flash of another image presented to the other eye.
To observe flash suppression, a small image is presented to one eye for a second or shorter while the second eye looks at a blank screen. If a different, small image is abruptly flashed into the second eye at the location corresponding to the image in the first eye, the image in the first eye disappear and only the novel image is perceived even though the original image is still present in the first eye. The novel image in the second eye perceptually dominates the former image which is perceptually suppressed. For example, if a vehicle is shown to the left eye for 1 second, and then a face is abruptly flashed to the right eye, the observer consciously sees first a vehicle and then a face. Note that the face is seen while the picture of the car is still present. If the order of presentation is reversed, the order of percept is reversed. Flash suppression was discovered by Jeremy Wolfe (1984).
Flash suppression is an example of perceptual illusions that render a highly visible image invisible and that are used to study the mechanisms of conscious and non-conscious visual processing (Koch, 2004). Related perceptual illusions include backward masking, binocular rivalry and motion-induced blindness.
The brain basis of flash suppression has been studied using microelectrode recordings in the visual brain of the macaque monkey (Logothetis, 1998) and in the human medial temporal lobe (Kreiman, Koch & Fried, 2002).
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[edit] Relationship with binocular rivalry
Flash suppression occurs due to the conflict between the inputs to the two eyes. When this conflict is sustained without any abrupt events, binocular rivalry occurs. In both flash suppression and binocular rivalry, perceptual conflict between the two eyes is key to induce the perceptual effect. If two similar images are used, fusion of the two images is experienced, rather than flash suppression or binocular rivalry. Despite some similarities in perceptual consequences, the neuronal mechanisms responsible for the two illusions can be different. For example, the strength (depth) of flash suppression seems much stronger than that of binocular rivalry. Comparative studies of the two methods are needed.
Flash suppression has certain methodological advantages over binocular rivalry as a tool to probe conscious vision. While the percept during binocular rivalry alternates stochastically, the percept during flash suppression is precisely controlled in time.
Although flash suppression allows one to present an image to the brain without seeing it consciously, it requires a to-be-erased image to be presented for a fraction of second before the introduction of a new image. This requirement limits the usage of flash suppression for the study of nonconscious visual processing.
[edit] Continuous Flash Suppression
A powerful variant of flash suppression is continuous flash suppression, discovered by Nao Tsuchiya and Christof Koch (2005). Here a small, fixed image in the first eye – say a grey-scale fearful face – is completely suppressed by a stream of constantly changing images flashed into the second eye (say a series of colorful Mondrian-scenes replaced every 0.1 sec by a new Mondrian pattern). This suppression can last for minutes, a remarkable testament to the fact that humans often do not see what is directly in front of their eyes.
Continuous flash suppression is an useful method for psychologists and neuroscientists interested in studying the mechanisms of conscious and nonconscious visual processing. While other visual illusions that render otherwise salient images invisible have their own shortcomings and advantages (Kim & Blake 2005), continuous flash suppression has a number of advantages for wiping images from conscious vision; it can erase an image presented at the fovea (which usually is much more resistant to perceptual suppression unlike, for example, crowding), in every trial (unlike binocular rivalry), for a longer duration (>1 sec, unlike backward masking), with an excellent control of timing (unlike binocular rivalry).
[edit] Generalized Flash Suppression
The differences between flash suppression and binocular rivalry have been further emphasized by the finding that stimulus conflict between the two eyes is not a requirement to achieve visual suppression. The novel paradigm of Generalized Flash Suppression (GFS) reported by Wilke, Logothetis and Leopold (2003) demonstrates that any visual stimulus can be rendered invisible when presented outside the fovea for a certain amount of time, followed by the addition of a distracting second stimulus in its vicinity.
This effect is strongest when the two stimuli are presented to (different regions in) opposite eyes, which suggests that it is somewhat related to binocular rivalry. At the same time, GFS shares similarities to Troxler's fading and motion induced blindness.
A recent study on the neuronal basis of GFS demonstrated that neuronal activity in early visual cortex was untouched by the perceptual effect, whereas neurons in higher areas altered their activity pattern during the illusion (Wilke et al. 2006). Authors report that the disappearance of the stimulus triggered changes in the local field potentials of all these areas, suggesting that the perception during flash suppression is reflected in large parts of the brain.
[edit] References
C. Koch (2004) The Quest for Consciousness: A Neurobiological Approach, Roberts, Englewood, Colorado
Kim, C.Y., and Blake, R. (2005) Psychophysical magic: rendering the visible 'invisible'. Trends Cogn Sci 9, 381-388
Logothetis, N.K. (1998) Single units and conscious vision. Philos Trans R Soc Lond B Biol Sci 353, 1801-1818
Flash suppression
J.M. Wolfe (1984) Reversing ocular dominance and suppression in a single flash. Vision Res 24, 471 478,
Sheinberg, D.L., and Logothetis, N.K. (1997) The role of temporal cortical areas in perceptual organization. Proc Natl Acad Sci U S A 94, 3408-3413
Kreiman, G., et al. (2002) Single-neuron correlates of subjective vision in the human medial temporal lobe. Proc Natl Acad Sci U S A 99, 8378-8383
Tsuchiya, N., et al. (2006) Depth of interocular suppression associated with continuous flash suppression, flash suppression, and binocular rivalry. J Vis 6, 1068-1078
Generalized Flash Suppression
Wilke, M., et al. (2003) Generalized flash suppression of salient visual targets. Neuron 39, 1043-1052
Wilke, M., et al. (2006) Local field potential reflects perceptual suppression in monkey visual cortex.. Proc Natl Acad Sci U S A 103, 17507-17512
Continuous Flash Suppression
Tsuchiya, N., and Koch, C. (2005) Continuous flash suppression reduces negative afterimages. Nat Neurosci 8, 1096-1101
Jiang, Y., et al. (2006) A gender- and sexual orientation-dependent spatial attentional effect of invisible images. Proc Natl Acad Sci U S A 103, 17048-17052
Jiang, Y., and He, S. (2006) Cortical Responses to Invisible Faces: Dissociating Subsystems for Facial-Information Processing. Curr Biol 16, 2023-2029
Kanai, R., et al. (2006) The scope and limits of top-down attention in unconscious visual processing. Curr Bioljp: フラッシュ抑制
