Research paper for PSY 506 Fall 2004 (Return to classweb page)
Much like the search for aliens or ghost hunting, theoxymoronically phrase "blindsight", may seem more reminiscent of anX-files episode than an actual perceptual condition. As defined bythe Concise Oxford Dictionary, the term blindsight refers to a"medical condition in which the sufferer responds to visual stimuliwithout consciously perceiving them" and is associated with damage tothe primary visual cortex (V1). These claims can be backed up withexperimental data; subjects that for all intensive purposes should beblind have retained the extraordinary ability to discriminate stimuliwithout the compliment of a primary visual cortex or awareness thatan event has occurred. How is this possible? Many psychologists havelong hypothesized that most of the stimuli that we encounter neverreach our conscious awareness. However, it can be seen in blindsightresearch that behavior can be directed in a zombie-like fashion dueto the unconscious processing of these stimuli indicating thatconscious awareness is not a sufficient requirement to behave in theworld. While the true depths of the unconscious may never be known,blindsight does provide a crude measure of the unconscious' transferof perception to behavior.
This seemingly supernatural ability to discriminate withoutacknowledged awareness is demystified to some degree when it isconsidered that the V1 is not the only depository of retinalinformation. Although the V1 receives the lion's share of theinformation generated in the retina, the superior colliculus of thebrain stem (prominent in rodents, reptiles, birds and insects) alsoreceives information from where it can be relayed to higher braincenters (NOVA | Transcripts | Secrets of the Mind | PBS, 2001). Thispathway is concerned with directing eye and head movements, orientingthe animal toward important events within the environment.
This is not to say that blindsight subjects are limited to themere detection of stimuli, but rather have been able to discriminatestimuli on multiple dimensions as described in Weiskrantz (2003).These discriminations (including detection) are generally not asaccurate as is found with in the intact visual field; however theresults are still compelling. For example, several subjects have beenable to localize stimuli either by making saccadic eye movements orby pointing to the stimuli locus. In fact the use of pointinggestures to localize stimuli is hypothesized to tap into theunconscious representation of the stimuli in the brain (Robichaud& Stelmach, 2003), however this technique was unsuccessful indemonstrating blindsight in normal observers.
When asked to grasp an object, researchers have reported that asubject's hand will adopt the appropriate arrangement for shape priorto grasping it within the blind field as opposed to clumsily fumblingabout. This capability indicates that unconscious perceptions aretuned to the form or structural elements of the stimuli and not justthe presence of the stimuli, which modify overt behavior accordingly.Exploiting this finding, researchers have been able to present wordsinto a subject's blind field which can influence the interpretationof ambiguous words subsequently displayed in the intact field
Researchers have also noted the blindsight patients' perplexingability to attend to stimuli without a conscious awareness of theattention. When a stimulus is presented in the blind field thatserves as a cue to the location of a target stimulus also locatedwithin the blind field, researchers have found a reaction timeadvantage without a loss of accuracy, indicating the participantattended to the cueing stimuli (Kentridge, Heywood & Weiskrantz,2004). Furthermore, this finding indicates that while illogical,attention is not a sufficient condition of awareness and cannot be ofthe same process.
Remarkably, other subjects lack the ability to detect stationarystimuli in their blind field however has retained the ability toidentify transient events. Even at slow onset rates, the blind fieldsof these subjects have remained attentive to movement. But perhapsthe most intriguing discriminative ability is that of color. Inmeasuring the spectral sensitivity of hemianopes (a blindness orreduction in vision in one half of the visual field due to damage ofthe optic pathways in the brain) researchers found the blind fieldsto be qualitatively normal although with reduced sensitivity.Although subdued, the spectral profiles within the blind field revealthe humps and troughs that reflect color opponency and these subjectseven displayed the Purkinje shift as a normal observer would.Interestingly however, the ability of the blind field is biasedtoward the red end of the spectrum.
It is of importance to note that the blindsight sight evidence isbased upon a small population of intensely studied subjects and notall subjects display all capabilities.
Methods of Measurement
Thus far the only noted response on the part of the subject toindicate a stimulus discrimination has been made is that of apointing gesture. Nonetheless, researchers have developed othermethods that have proven just as reliable. For example, the reflexiveactivity of the pupil (pupillometry) provides a quantitative measureof constriction due to exposure to various stimuli depending uponspatial frequency, wavelength and movement without a change instimuli luminance. This pupillary response has been shown to be areliable predictor of residual vision within the defective field(Trevethan & Sahraie, 2003).
Other measures include the interactions of the intact and blindhemifield, such as the finding that the reaction time of a keyresponse to a stimulus in the intact field can be lengthened bypresenting a light just earlier to the blind hemifield. Functionalbrain imaging, namely fMRI, indicates an activation of the superiorcolliculus when presented with a red light but not a green light inthe blind hemifield; buttressing the red bias displayed indiscrimination ability. Animal research has played an important roleby indicating that other species are capable of blindsight as well.Monkeys that have had blindsight induced (V1 removal), respond tostimuli in the same manner that human participants do. When trainedto discriminate stimuli and behaviorally respond to it, the animalsubjects pressed a "no light" key (indicating no stimulus was "seen")when presented with a stimuli within the blind field. Amazingly,after indicating that a stimulus was not "seen", the monkeys are ableto reliably localize the stimuli just as their human counterparts areable to do.
Validation of the Blindsight Phenomenon
The topic of blindsight is understandably controversial. It iscounterintuitive to believe that one could reliably detect ordiscriminate a variety of visual stimuli, yet never purport anawareness of the event. This misunderstanding of the blindsightphenomenon has led skeptics to question the validity of theconclusions drawn by investigators, indicating alternativeexplanations that are more practical.
It has been suggested that following a traumatic injury to thevisual cortex, it is reasonable that most, but not all neurons maydie and become useless. In spite of this, the few surviving neuronscould theoretically thrive and permit vision, albeit at asubstantially reduced capacity, leaving the sufferer with "islands ofvision." Blindsight researchers have debunked the theory thatsubjects are utilizing these "islands" in a variety of ways, mostnotably with the use of an eye tracker. The eye tracker device issynchronized to the inadvertent eye movements that may allow for abrief fixation on an island and hence, "see" the stimuli. No islandscould be found in subject G.Y., the only subject noted (Kentridge,Heywood & Weiskrantz, 1997). More directly, the completeness ofthe visual cortex damage established in animal research would notpermit islands to exist, yet almost perfect accuracy is found in thedetection of visual events as is exhibited in human participants.
Factors Influencing the Occurrence of Blindsight
As stated above, the phenomenon of blindsight is understandablyrare. The sufferer has undergone some sort of trauma that hasobliterated the visual cortex which concurrently has left the "visualhardware" (eyes, retina and connections) intact. For example, Riddoch(1917) outlines a gunshot wound case where the participant was ableto detect moving but not stationary stimuli. This leads to the firstand perhaps most important factor moderating blindsight, variabledamage to the cortex. It is unlikely that naturally occurringoccipital lesions in humans would be restricted to the V1; animalresearch has indicated that damage extending beyond the V1 degradesthe degree of residual functioning and suggests that the same may betrue in humans.
Secondly, the age at which the damage occurs can arbitrateblindsight. Schärli, Harman & Hogben (1999) have foundevidence supporting this hypothesis signifying residual ability ispossible with an early onset rather than later. Indeed the two moststudied individuals (D.B. and G.Y.) both suffered incidents aschildren which left them with visual defects.
We already know that some aspects of behavior are controlled bythe unconscious such as breathing and salivation, but for whateverreason it is difficult to believe that our vision would also beprocessed at least in part by the unconscious. As can be seen fromthe blindsight subjects, visual processing can occur withoutawareness; as such, awareness is not a sufficient condition to "see".This has incited many skeptics to question the validity of thefindings, which thus far have withheld the scrutiny. While the topicof blindsight may never make an appearance on the "X-files", futuredevelopments will at the least shed more light on the behavior /unconscious connection, and at most revolutionize our concept of themind.
Kentridge, R. W., Heywood, C. A., & Weiskrantz, L. (1997).Residual Vision in multiple retinal locations within a scotoma:implications for blindsight. Journal of Cognitive Neuroscience, Vol.9, 1997, pp.191-202.
Kentridge, R. W., Heywood, C. A., & Weiskrantz, L. (2004).Spatial attention speeds discrimination without awareness inblindsight. Neuropsychologia, Vol. 42(6), 2004, pp. 831-835.
NOVA | Transcripts | Secrets of the Mind | PBS. (2001, Oct.).Retrieved October 11, 2004, fromhttp://www.pbs.org/wgbh/nova/transcripts/2812mind.html.
Riddoch, G. (1917). Dissociation of visual perceptions due tooccipital injuries, with reference to appreciation of movement. BrainVol. 40, 15-17.
Robichaud, L. & Stelmach, L. B. (2003). Inducing blindsight innormal observers. Psychonomic Bulletin & Review, Vol 10(1), Mar2003, pp. 206-209.
Schärli, H., Harman, A., & Hogben, J., (1999). Blindsightin subjects with homonymous visual field defects. Journal ofCognitive Neuroscience, Vol. 11(1), Jan 1999. pp.51.
Trevethan, C. T., & Sahraie, A. (2003). Spatial and temporalprocessing in a subject with cortical blindness following occipitalsurgery. Neuropsychologia, Vol. 41(10) 2004, pp. 1296-1306.
Weiskrantz, L. (2003). Unconscious perception: blindsight. In S.Soraci, & K. Murata-Soraci (Eds.), Visual Information Processing(pp. 283-306). Westport, CT: Praeger Publishers.