Thursday, January 29, 2009
Definition of Blindsight
The patient TN reported on in this NPR story is the latest in a series of cases of blindsight. This paradoxical and counterintuitive phenomenon refers to the ability of humans with a loss of primary visual cortex to make visual discriminations in their blind visual fields without awareness of the stimuli they are discriminating.
Measurement of Blindsight
To get around the lack of visual awareness of blind field stimuli researchers ask their patients to guess whether, where, or which one of a small number of stimuli has been presented within the blind visual field. The types of visual discriminations that have been reported are movement, orientation, wavelength (i.e. , color), spatial localization and combinations of these elementary visual features. Accuracy of responses sometimes reached 90% to 100% in various patients (Weiskrantz, 1995). In addition 'affective blindsight' has been demonstrated: patients can sometimes reliably detect the valence of emotional expressions in the absence of any visual awareness of the faces (Tamietto & deGelder, 2008).
This phenomenon was first studied in human patients the 1970s by Oxford University based researchers Lawrence Weiskrantz and Elizabeth Warrington. Their patient GY had extensive damage to his left visual cortex which rendered him functionally blind in his right visual field. They were able to demonstrate GY's capacity to perfectly discriminate the direction of motion within his right visual field. Figure 1 of Weiskrantz's review paper shows the different directions of motion that GY was able to accurately mimic with his arm. The grey area is the impaired hemi-field.
Controversies about the cause of blindsight:
Blindsight is most likely to be due to the use of visual pathways outside of the usual geniculostriate ones, connections that are either subcortical or that go directly to extrastriate areas bypassing primary visual cortex. Some brain researchers have objected that the residual visual function of blindsight could be subserved by fragments or islands of intact striate cortex rather than extrastriate cortex (Weiskrantz, 1995). This is unlikely to be the explanation for GY's motion, wavelength, and emotional expression discrimination capacities because a high-resolution MRI scan reveals only a small patch of striate cortex near the back of the brain on the left side, but he does have some remaining striate cortex.
Why TN is a notable case
The damage to TN's striate cortex is much more extensive than GY's. TN suffered two strokes 36 days apart; the first damaged his occipital cortex unilaterally, and the second destroyed the remaining primary visual cortex in the other hemisphere. Figure 1 of de Gelder et al's paper reporting the case shows the extensive primary visual cortex damage. TN is the only available case in the literature with selective bilateral occipital damage. Yet he can successfully navigate down a long corridor with various barriers set in his way, as demonstrated in the video. His blindsight despite total loss of primary visual cortex effectively refutes the remaining islands of functional visual cortex hypothesis. Extra-striate pathways in humans can sustain sophisticated visuo-spatial skills in the absence of perceptual awareness.
What is blindsight good for?
Blindsight is not demonstrated in every patient with loss of primary visual cortex, but when it is present then it the ability can be cultivated through training for rehabilitation. In the case of TN he was unaware of his residual ability to navigate obstacles using visual information. Behaviorally he was blind across the whole visual field. He walked like a blind man, using his stick to track obstacles and requiring guidance by another person when walking around the laboratory buildings during testing. The researchers were able to demonstrate navigation capacity that he did not know that he still retained in the face of such devastating visual loss. In their quick guide to blindsight for the journal Current Biology Stoerig and Cowey (2007) conclude with the speculation that implicit processes in many domains may always survive when explicit representations are damaged, and therefore that rehabilitation programs could always successfully harness the remaining implicit capacities for restitution.
de Gelder, B., Tamietto, M., van Boxtel, G., Goebel, R. Sahraie, A., van den Stock, J., Steinen, B.M.C., Weiskrantz, L. & Pegna, A. (2008). Intact navigation skills after bilateral loss of striate cortex. Current Biology, 18, 1128-1129. Link
Lamme, V.A.F. (2006). Zap! Magnetic tricks on conscious and unconscious vision. Trends in Cognitive Science, 10, 193-195.
Rees, G. (1999). Consciousness lost and found. Journal of Psychophysiology, 13, 56-60.
Stoerig, P. & Cowey, A. (2007). Blindsight quick guide. Current Biology, 17, 822-824.
Tamietto, M. & deGelder, B. (2008). Affective blindsight in the intact brain: Neural intrahemispheric summation for unseen facial expressions. Neuropsychologia, 46, 820-828.
Weiskrantz, L. ( 1995). Blindsight - Not an island unto itself. Current Directions in Psychological Science, 4, 146-151. Link to Academic Search Premier