How does an image become invisible?
Bahrami, et. al. used the same method to render images invisible as did a previous team, Fang and He. This method is called flash suppression. It goes as follows: a low-contrast, simple image (of a tool such as a wrench in this study) is shown to one eye while simultaneously, a high-contrast, dynamic noise image is flashed repeatedly to the other eye. In each eye's visual field, the image locations are the same. Subjects of the study were unable to detect the fainter, simpler image, "confirming their complete lack of awareness" that they'd seen it at all - hence, invisibility.
Invisible, but still affecting your brain:
Fang and He found that despite their lack of awareness of the tool images, subjects were nevertheless affected at the neurological level. Using function Magnetic Resonance Imaging (fMRI), they found that the tool images "activated dorsal visual areas along the intraparietal sulcus at almost normal levels, but did not activate ventral visual areas of the lateral occipital complex." This affirms the 'conventional notion' that conscious visual perception is supported by the ventral visual cortex, while 'subconscious' vision (see image below) is supported by the dorsal visual cortex. Essentially, Fang and He found that a subject could 'see' an object with his brain, with almost a completely normal neurological effect on the dorsal side, while remaining completely unaware that he has done so - ventral side unaffected.
Ventral vs. Dorsal pathways illustrated:
Bahrami, et. al. investigate attention’s effect on awareness:
The study noted in the original ScienceDaily article, conducted at Univerity College of London, used the same model as the Fang and He study with low-contrast ‘invisible’ tools and high-contrast noise images. However, they added a focus of attention for their subjects as well. The invisible images and the noise images were both shown in the periphery of the visual field while in the center of the visual field, subjects were asked to observe a stream of letters. They had subjects perform simple tasks in picking certain letters to assure that their attention remained there while taking using fMRI to map the brain’s responses. Bahrami, et. al. had subjects perform a ‘low attention task,’ such as ‘report all Zs’ and a ‘high attention task,’ such as ‘report blue Zs and white Ns.’ See the diagram below for a basic illustration of the study:
Figure 1. A schematic representation of the display used by Bahrami et al. [6].At peripheral locations, one eye views high-contrast, dynamic noise images that are visible to the observer. At some of the same locations, the other eye views low-contrast tool objects that remain invisible. Both eyes view identical letters at the center of the visual field. The complexity of a letter-monitoring task controls the extent to which peripheral images are attended.
The results:
Bahrami, et. al. found that the peripheral ‘invisible’ images still registered in fMRI when subjects were performing low-attention tasks, but not when their attention was drawn away more insistently. In the low-attention task trials, the subjects were able to tell where the noise-images were flashing, and the invisible tool images had a greater affect on the visual cortex. By contrast, the high-attention task rendered the subjects less able to place the location of the peripheral images and showed less fMRI activation from the invisible tool images. So, Bahrami, et. al. found that ‘a neuronal response need not contribute to visual awareness, even though it is enhanced by visual attention. In short, attention does not guarantee awareness.”
Why is this important?
Subliminal messaging (brief flashing images hidden in other media, such as a picture of a Coca-Cola flashed during a projected movie) is illegal in Britain, but not in the USA. There is continued debate as to whether subliminal messaging is an effective advertising tool, or even possibly a tool for controlling the thoughts and opinions of the general public – imagine if subliminal messages could be used to get people to vote for a political candidate, or to encourage the public to approve of an unpopular war. Subliminal messaging has never been proven to ‘work,’ but it remains a fascinating hypothesis. The Fang and He study showed that there is a neurological response to some images even when the viewer remains unaware of having seen them. Bahrami, et. al. showed that attention affects this brain activity. Essentially, if the subject is focusing attentively on one task, even if it is a visually-based task, the brain will register less about an invisible flashed image. This implies that too much attention paid to the ‘overt message’ could conceivably cancel out the subliminal message. Maybe advertisers should only flash images of their products during ‘low-attention’ viewing tasks (such as the boring part of the movie?) instead of the ‘high-attention’ times (extremely complicated chase sequence?). This is simply speculation – these scientific studies cannot be said to solve the question of subliminal advertising once and for all. As Bahrami says, “What our study doesn't address is whether this would then influence you to go out and buy a product. I believe that it's likely that subliminal advertising may affect our decisions -- but that is just speculation at this point."
Bibliography:
F. Fang and S. He, Cortical responses to invisible objects in the human dorsal and ventral pathways, Nat. Neurosci. 8 (2005), pp. 1380–1385.
B. Bahrami, N. Lavie and G. Rees, Attentional load modulates responses of human primary visual cortex to invisible stimuli, Curr. Biol. 17 (2007), pp. 509–513
J. Braun, Vision: Attending the Visible, Curr. Biol. 17 (2007), pp. R202-R203
University College London (2007, March 9). Subliminal Advertising Leaves Its Mark On The Brain. ScienceDaily. Retrieved February 17, 2009, from http://www.sciencedaily.com /releases/2007/03/070308121938.htm
C. Koch and N. Tsuchiya, Attention and consciousness: two distinct brain processes, Trends Cogn. Sci. 11 (2007), pp. 16–22.
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