Flash suppression is a phenomenon of visual perception in which an image presented to one eye is removed from visual awareness by the abrupt presentation ("flash") of another image presented to the other eye.
Flash suppression is an example of a class 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 neural 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).
In flash suppression, a small image is presented to one eye for a second or shorter while the other eye looks at a blank field. When a different, small image is abruptly presented to the previously unstimulated eye at the location corresponding to the image in the stimulated eye, the image in the first eye disappears and only the novel image is perceived even though the original image remains present in the first eye. The more recent image perceptually dominates the earlier image which has now become perceptually suppressed.To observe flash suppression, one needs a device to deliver two different images to two eyes. Typically, scientists use a mirror stereoscope for this purpose. In the demo movie Anaglyph image). Note that the face is seen while the picture of the car is still present. If the first stimulus is swapped, the percept for the second image is also swapped.
Flash suppression was discovered by William McDougall (1901), rediscovered by Robert Lansing (1964), and researched intensively by Jeremy Wolfe (1984).
The effectiveness of flash suppression is dependent on a number of spatiotemporal parameters associated with both first and second stimuli (Wolfe 1984, Brascamp et al 2006).
Relationship with binocular rivalry
Flash suppression occurs due to the conflict between the inputs to the two eyes. When this conflict is sustained without abrupt presentations, binocular rivalry occurs. In both flash suppression and binocular rivalry, perceptual conflict between the two eyes is key to induction of 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 are probably 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 experienced with flash suppression is precisely controlled by the sequence of stimulation of the two eyes.
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 a second before the introduction of a new image. This requirement limits the usage of flash suppression for the study of nonconscious visual processing: the initial stimulus will be perceived for a fraction of a second.
Continuous Flash SuppressionA 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 (in this demo, a right tilted grating is presented through red filter) is completely suppressed by a stream of rapidly changing images flashed into the second eye (in this demo, through blue/green filter is presented a series of Mondrian-like patterns replaced every 0.1 sec by a new Mondrian)
Continuous flash suppression is a 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, visual 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).
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 opposite eyes (albeit to different regions), 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 the firing of neurons in early visual cortex was untouched by the perceptual effect, whereas neurons in higher visual areas altered their activity pattern during the illusion (Wilke et al. 2006). In addition, authors report that the disappearance of the stimulus triggered changes in the local field potentials even in the early visual cortex, suggesting that the perception during flash suppression is not only reflected in a large network but also by different types of electrical activity in the brain.
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Generalized Flash Suppression
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Continuous Flash Suppression
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