Sunday, January 15, 2012
If there were ever a perfect example of an oxymoron, the term blindsight would be it.
Other than the best oxymoron ever, what is blindsight? Alan Cowey, in his 2010 review article, The blindsight saga, describes it as such:
It is the ability of patients with absolute, clinically established, visual field defects caused by occipital cortical damage to detect, localize, and discriminate visual stimuli despite being phenomenally visually unaware of them.
In simpler terms, it's the ability to sense the presence of objects in one's visual field without consciously seeing them the way normal sighted people do. While it would be a stretch to call blindsight a superpower, the visual capabilities of a person with blindsight could be considered akin to those of Dare Devil. Blindsighted people often are able to identify visual stimuli, although they deny having a conscious experience of actually seeing anything.
In this video, a blindsighted patient is able to navigate a field of obstacles successfully, even though he can't see them.
How does blindsight happen? It is a fairly unique condition; not all people who are blind possess blindsight. It occurs in patients who become blind in part or all of their visual field after suffering damage to the primary visual cortex, known as V1.
|The primary visual cortex (V1) highlighted in yellow. The bottom view is from a mid-section of the brain, the top view is from the outside. In both views, your eyes would be on the left. Source.
Becoming totally "cortically blind," as is the case for the patient in the video above, is actually pretty rare (thankfully), so most patients with blindsight are only blind in part of their visual field, while the rest of the field remains normally sighted.
A controversial subject
Blindsight has drawn a lot of controversy among researchers. Since the hallmark trait of blindsight is responsiveness to visual stimuli without the conscious experience of perceiving it, it makes animal studies a little difficult. You can observe where a cortically blind animal turns its attention when presented a stimulus, but you can't ask it if it actually saw anything. So researchers interested in getting at the question of what people with blindsight actually do or do not experience have a relatively small pool of subjects they can perform research on.
Then, of course, there's the fact that whatever blindsighted patients report as their experiences of blindsight must be taken with the grain of salt as all experience is subjective. Some patients report that, even though they didn't see anything, they have a feeling that "something happened." The question, then is, is this feeling essentially visual in nature? Did it come about due to light scattering from the stimulus into their seeing field? Or did the subject report a feeling of something happening because that's what he or she felt that the researchers were looking for? The answers to these questions have proven to be extremely difficult to tease apart. Regardless, the fact that some patients have "feelings" when presented a stimulus and others do not requires that blindsight be split into two categories: type 1, no awareness at all; and type 2, awareness without visual experience.
Even in patients with type 2 blindsight, the awareness of the stimuli isn't always consistent. But we can still learn something here. In one task, patient GY was asked to discriminate between the presence or absence of a stimulus and then wager money on his answer. Incorrect wagers would be subtracted from his winnings, thus prompting him to only wager high when he was very confident of his answer. When he felt that he was aware of the stimulus, despite not seeing anything, he wagered high and was correct more than 90% of the time. When he didn't report any awareness, he wagered low, as would be expected. But he was correct more often than could be explained by chance, suggesting that even without awareness, he was still able to perceive visual stimuli.
Total cortical blindness
Let's re-visit the patient in the video, TN. After successive strokes, TN's primary visual cortex was completely destroyed, as was later verified by both structural and functional MRIs. Something helpful at least came of TN's misfortune, in that he gave researchers De Gelder et al a rare chance to study a person with total cortical blindness. The video above is truly astonishing because, at first glance, it looks like he is able to avoid obstacles without the use of a cane or any outside guidance.
Nevertheless, this video is not without its critics. The first that springs to mind is echolocation, a capability that has been shown in other blind people
.* The researchers recognized that this could not be ruled out, but many critics of the research suggested that not enough attention was paid to the possibility of echolocation anyway. Other critics say that TN could have been unconsciously processing auditory signals from the researcher shadowing him (to make sure he didn't stumble or fall), and that those aided in his navigation.
TN's contributions to blindsight research don't just end with that video. TN also demonstrated affective
blindsight, or the ability to discriminate between emotional stimuli in a physiological sense. Researchers Gonzalez-Andino et al
showed TN pictures of various facial expressions and monitored his brain activity via an EEG
. Without any awareness of the stimuli whatsoever, the researchers were able to localize changes in the electrical activity of parts of the brain associated with emotional stimuli, such as the right amygdala, in TN. In these trials, TN was also not asked to guess at the nature of the stimuli, either, so the results suggest some perceptual ability without any conscious awareness at all.
Conclusions: what blindsight can tell us about consciousness
It is extremely important to note that all studies on blindsight and consciousness are done with very small sample sizes. Most studies are case studies that focus on the abilities of only one patient. Because of this, everything that we can say about blindsight must be taken with a grain of salt. There simply aren't enough subjects with blindsight to tell us very much with any certainty about the nature of consciousness and vision. Nevertheless, some results are so provocative that they can at least give us clues and ideas about where consciousness in vision lies in the brain, and can give us leads for further and more focused studies in the future.
It has become clear that blindsighted patients' vision is certainly unlike normal vision, and obviously unlike total blindness. There is definitely some ability among patients to perceive objects, even if the true nature of that ability remains murky. And it's the murkiness of that ability that makes blindsight so tantalizing to researchers interested in the neural mechanisms of consciousness. How can one see without actually seeing? That is the big question. The visual pathway is certainly very complex, but the blindsighted just may be able to tell us what aspects of that pathway give rise to the conscious experience of sight, and wouldn't that be so cool?
*While looking for the video about Ben Underwood, I found out that just a few years after the original news spot was filmed, he sadly died of cancer. You can read more about him and his life here
Cowey, A. (2010). The blindsight saga. Experimental Brain Research 200:3-24.
de Gelder, B.; Tamietto, M.; van Boxtel, G.; Goebel, R.; Sahraie, A.; van den Stock, J.; Stienen, B.; Weiskrantz, L.; Pegna, A. (2008). Intact navigation skills after bilateral loss of striate cortex. Current Biology 18,24:1128-1129.
Gonzalez-Andino, S.L.; de Perlata Menendez, R.G.; Khateb, A.; Landis, T.; Pegna, A. (2009). Electrophysiological correlates of affective blindsight. NeuroImage 44:581-589.