Posted by Yesterday’s post brings up an interesting question: How can you be unaware of having even seen an image, and yet be able to make reliable judgments about that image? That article is just one example of a variety of situations in which people can be unaware of seeing something, even immediately after being given a quick glimpse of it, yet behave as if they have seen it.
We discussed how visual images can be “masked” — flashed quickly and then followed by another image which is displayed for a longer period. Though observers had no conscious recollection of seeing faces, they still could make accurate judgments about the attractiveness of the faces they had seen. Earlier experiments have found that the ability of the skin to conduct electricity as well as responses in the amygdala region of the brain can be affected by these masked images, again, with no conscious knowledge on the part of the viewer of having seen the image.
So how can our skin and brain respond to the image without our being conscious of seeing it? A team led by J.S. Morris developed a procedure to find out. Since the amygdala is activated during a fear response, they first conditioned volunteers to be afraid of a black and white photo of man with an angry facial expression. They used photos of four different men, two angry, and two neutral. These photos flashed randomly on the screen at intervals of around 20 seconds. When one (and only one) of the angry photos was displayed, a 1-second burst of white noise was played at a level of 100 decibels (loud enough to make you jump, but not to hurt your ears). Each face was shown six times.
Next, the volunteers were placed in an MRI scanner, and shown the faces they had seen before. Half the time, the faces were masked, so they were not conscious of seeing them. The other half the time, the faces were flashed quickly without masking, so they were consciously perceived. Skin conductance was greater when the participants saw the masked images which they had previously been conditioned to fear. The brain scan revealed differences in blood flow in the right amygdala:
Significantly more blood flow was observed in this region when the fear-inducing masked angry face was shown, compared to the non-fear-inducing masked angry face. So though participants were not aware of a difference in the images they were shown, their brains responded differently.
What Morris’s team found is that although the amygdala showed more activity during unseen fear-inducing faces, other areas of the brain associated with conscious visual activity were more active when the faces were seen. The mask appeared to disrupt the conscious visual process, but not the process that led to the fear reaction. The masked images were sensed — detected by the eye — but not perceived. The researchers identified a separate neural pathway which activates the amygdala, independently of visual cognition.
Morris’s group says that the pathway they identified is similar to the fear response in animals, which has been studied in more detail since more invasive procedures (surgery which systematically disables various neural pathways) can be used.
What’s especially striking about this experiment is how it shows two critical aspects of brain functioning: its modularity and its interconnectedness. Without the connection of the visual system to the amygdala, the fear response could not be activated as quickly. But the pathway doesn’t work in reverse: knowing we are fearful doesn’t tell us what we’ve seen. In that sense, the visual system and the amygdala are independent. This web of interconnections and dead ends is seamless — when we’re missing something, we don’t notice it at all, but when a feeling or thought such as fear or a visual representation is present, it is indistinguishable from any other.
Morris, J.S., Öhman, A., & Dolan, R.J., (1999, February). A subcortical pathway to the right amygala mediating “unseen” fear. Proceedings of the National Academy of Science, 96, 1680-1685.
April 12, 2006
April 12, 2006
April 14, 2006