Blindsight Reconsidered Author(S): Michael S

Home , Blindsight

Blindsight Reconsidered Author(s): Michael S. Gazzaniga, Robert Fendrich, C. Mark Wessinger Reviewed work(s): Source: Current Directions in Psychological Science, Vol. 3, No. 3 (Jun., 1994), pp. 93-96 Published by: Sage Publications, Inc. on behalf of Association for Psychological Science Stable URL: http://www.jstor.org/stable/20182276 . Accessed: 12/03/2012 15:12

Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

Sage Publications, Inc. and Association for Psychological Science are collaborating with JSTOR to digitize, preserve and extend access to Current Directions in Psychological Science.

http://www.jstor.org CURRENTDIRECTIONS INPSYCHOLOGICAL SCIENCE 93

systems such as the superior collic Blindsight Reconsidered ulus can mediate the transfer of vi sual information between the cere Michael S. Gazzaniga, Robert Fendrich, and C. Mark Wessinger bral hemispheres. Early studies of spatial attention indicated subcorti cal mechanisms would allow cues In subhuman primates, the major explorable in scientific terms. In ad presented to one hemisphere to di visual pathway projects from the ret dition, visual scientists were excited rect spatial attention in the other. ina to primary (striate) visual cortex. because various subcortical and par Some preliminary studies even pro However, monkeys with striate le allel pathways and centers could vided indications of interhem?spher sions are not only able to localize now be studied in the human brain. ic semantic priming (i.e., semantic objects in space, but also able to As the early reports settled in, our facilitation of a right-visual-field carry out color, luminance, orienta laboratory began to examine related stimulus by the prior presentation of tion, and pattern discriminations.1 issues in other types of patients. Pa a left-field stimulus).5 These abilities appear to depend on tients with intact visual cortex but In the light of reports of blind the integrity of a secondary visual neglect (i.e., a tendency to fail to sight, it hardly seemed surprising pathway, which projects from the acknowledge stimuli in the visual that subjects could make use of retina to the superior colliculus, and field opposite a damaged hemi visually presented information not then to extrastriate visual cortex. sphere) due to parietal lobe lesions accessible to consciousness. Sub Does this secondary visual path were asked to judge if two lateral cortical networks with their inter way operate in humans? A growing ized visual stimuli, one appearing in hemispheric connections provided a literature has argued that this may be each visual field, were the same or plausible anatomy to explain the be the case. Abilities ranging from sim different. (For both eyes, the left vi havioral results. In fact, itwould be ple detection to the extraction of se sual field projects to the right visual difficult to argue against the concept mantic content have been attributed cortex, and the right visual field pro that perceptual decisions or cogni to a human extrastriate visual path jects to the left visual cortex.) Sub tive activities routinely result from way. Weiskrantz coined the term jects were able to perform this task. processes outside conscious aware blindsight to describe these residual However, when they were ques ness. abilities, because they occur al tioned as to the nature of the stimuli though subjects deny that they actu after a trial, they could easily name see the stimuli to which are the stimulus in the visual field ally they right LIMITATIONSAND but denied seen the stimulus responding.2 having UNCERTAINTIES Ever since the original report of presented in the neglected left field.3 the phenomenon, philosophers, We observed another curious psychologists, and neuroscientists finding in split-brain patient J.W. In As our studies progressed, how have been fascinated with blind a simple two-choice task, his right ever, the data failed to support the sight. The original reports were in hemisphere was able to program the notion that higher order information triguing and sensible. Because these left hemisphere for a spoken or writ interacts between the hemispheres activities went on outside the realm ten response indicating which of two following surgical section of the cor of consciousness, it looked as stimuli had been presented to the pus callosum. When new reports of though an aspect of the silent dimen right hemisphere, or left visual field. high-level hemispheric interactions sion of cognitive life had been However, in a subsequent experi following full commissurotomy (sur tapped. The unconscious was now ment, the left hemisphere could not gical section of the corpus callo use a pointing response to identify sum and anterior commissure)6 which of two stimuli in the vi us to reexamine our Michael S. Gazzaniga is Director right prompted split of the Center for Neuroscience at sual field matched the stimulus in brain patients carefully, we could the left visual the left no the University of California, Davis. field.4 Thus, find interhemispheric interac Robert Fendrich is an Assistant hemisphere did not know the iden tions of this kind. This was the case Professor of Neurology at Davis, tity of the stimulus presented to the even when we used stabilized im a and C. Mark Wessinger is grad right hemisphere. It seemed that the ages to permit extended stimulus uate student in neuroscience there. verbal responses could occur in the presentations. Moreover, we could Address to correspondence absence of conscious awareness of not reproduce our original findings Michael S. Gazzaniga, Center for the stimuli. on semantic priming, and have been Neuroscience, University of Cali Data from split-brain patients forced to conclude that this report fornia, Davis, CA 95616. have also shown that subcortical was in error.

It has also become apparent that drantanopia (quarter-field blindness) vealed an occipital lesion that subcortical systems may not explain with a peripheral rim of normal vi ablated primary visual cortex, but blindsight. For example, hemi sion. Moreover, an area of partially clearly spared prestriate regions as spherectomy patients (i.e., patients preserved vision approximately 10? well as the colliculus. Ifblindsight is with one hemisphere surgically re in diameter is embedded within the based on subcortical function, then moved) who have limited residual scotoma that remains. Thus, the ex these intact areas should have been vision in their blind field have been tent of D.B/s cortical damage ap able to support many of the blind as to held up evidence for subcortical pears have been overestimated sight phenomena commonly re mediation of residual vision.7 How initially. Many of the studies on D.B. ported. However, although this sub in were ever, the residual vision found conducted without monitoring ject performed accurately when to or so these patients may well be due controlling eye movement, that required to localize targets by look one must cortical reorganization, which might simply have faith that stim ing at them in her perceptually intact have occurred if the brain damage uli were placed properly.2 right visual field, her performance had taken place during prenatal or Finally, the preliminary mapping did not exceed chance levels for early postnatal development. Cer of a patient's visual fields is normally stimuli in her blind left visual field. tainly, these patients do not demon carried out with techniques that test These results support the notion that strate blindsight: They can generally for the detection of a stimulus with a localization of stimuli requires an in report the phenomenal basis for their simple "yes" response. As Campion tact geniculostriate system (the pro decisions when they perform above and his colleagues10 pointed out, jection from the lateral geniculate chance in visual tasks. G.Y., an ex this procedure assures the fields are nucleus to the striate cortex). tensively studied patient, also can be vulnerable to criterion effects (espe viewed as possibly having experi cially failures to respond when un enced cortical reorganization, be certain), so that the true sizes of IMPLICATIONS cause his traumatic to the left scotomas are to be overesti OF injury likely RECENTPROGRESS occipital lobe came early in life. mated. When such assessments are Like the hemispherectomy patients, followed by criterion-free, two aware he is frequently of stimuli alternative forced-choice measures Recently, we began to reexamine within his scotoma (i.e., blind of residual function, it is not surpris blindsight armed with a newly ac patch). It should also be pointed out ing that visual capacities are found quired image stabilizer, which al to un that, contrary published reports in regions where they remained lows us to keep images steady on the that all striate cortex was missing, reported during the initial visual retina despite the observer's eye mo now G.Y. has been shown to have field testing, or perimetry. tions, and an interesting patient. remaining striate cortex.8 In the early 1980s, Holtzman,11 C.L.T. is a robust, 55-year-old out a cau in our to Similarly, there is need for lab, began study blindsight doorsman who suffered a right oc se. tion in interpreting studies that dem per Accurately examining visual cipital stroke 6 years ago. Magnetic onstrate wavelength discrimination capacities following cortical dam resonance ?mages were used to as within scotomas. Such studies often age is difficult. Conventional man sess the extent of this patient's le make use of stimuli that are posi ual perimetric methods lack effi sion. They revealed a lesion that in tioned close to areas of intact vi ciency. Also, they often yield cludes damage to the calcarine sion,9 without controlling eye mo artifactual detections because of un cortex, although some tissue in the tions. In addition, large stimuli may stable observer fixation, which al region of the calcarine fissure is be employed in such studies,2 lows the target to stray into function spared, and the colliculus is intact. which increases the probability that ing parts of the visual field.12 In our Standard perimetry indicated areas we with spared, but not fully func lab, have made use of a purkinje C.L.T. had a left hemianopia with tional (i.e., dysfunctional), islands of ?mage eyetracker, which provides lower quadrant macular sparing. Us vision will be included in the visual very stable, high-resolution informa ing our more exacting methods, we field regions tested. tion on a subject's eye position. This replicated this finding, but found ad The possible inclusion of visual information allows us to compensate ditional regions of residual vision. field regions that project to areas of for eye position instabilities, so we Specifically, using high-contrast ret surviving but dysfunctional cortex is can present stimuli precisely within inally stabilized stimuli and an inter also a problem with the classic the scotoma. val two-alternative forced-choice blindsight case, D.B. Over the We first studied a 34-year-old procedure (which requires the sub years, D.B.'s scotoma has receded woman with a left hemianopia sub ject to report which of two temporal from a full hemianopia (half-field sequent to surgery. Magnetic reso intervals contained the stimulus pre blindness) to a partial lower left qua nance images of her visual cortex re sentation), we found an isolated is

Published by Cambridge University Press CURRENTDIRECTIONS INPSYCHOLOGICAL SCIENCE 95

? land of vision about 1 in diameter in visual processing pathways. But is 20 and 25 patients, respectively.17 the upper left quadrant of C.L.T.'s this formulation necessary or even In these studies, there were indica left visual field.13 Because C.L.T. re plausible? tions of blindsight in only 20% of the ports no awareness of stimuli pre It is commonplace to design de subjects. sented to this island, his vision at this manding perceptual tasks in which Nonetheless, the study of blind location is blindsight. The retinal re nonneurologic subjects routinely re sight does afford important insights. gions surrounding this island are to port low confidence values for tasks First, it underlines a general feature tally blind. This residual vision is re they are performing above chance. of human cognition: specifically, stricted to a very small retinal However, it is not necessary to pro that many perceptual and cognitive region, so the likely explanation is a pose secondary visual systems to ac activities can and do go on outside corresponding remnant of spared count for such data because the pri the realm of conscious awareness. striate cortex, rather than a more mary visual system is intact and fully However, our studies suggest this general secondary visual system. functional. feature need not be dependent on Thus, before one can assert that As previously noted, it is also the subcortical or secondary processing blindsight is due to subcortical or ex case that patients with parietal lobe systems. trastriate structures, one must first be damage but spared visual cortex can Second, the careful study of both extremely careful to rule out the pos carry out perceptual judgments out primates and humans with lesions of sibility of spared striate cortex. side conscious awareness. These the primary visual cortex serves as a When sufficiently careful mapping is subjects can compare two stimuli, reminder of the important differ carried out within a scotoma, re although they deny awareness of ences in the brain organization be gions of vision that would almost one of them. The failure of these pa tween species. It is clear that such certainly go undetected with con tients to consciously access the in lesions in the monkey allow for far ventional perimetry can be found. formation they use to compare the more visual capacity then do similar We have continued to examine ad stimuli should not be attributed to lesions in the human. ditional patients and have now iden processing within a secondary visual Finally, patients with residual vi tified other patients with islands of system because their geniculostriate sual capacities become an interest residual vision. pathway is still intact. ing source for studying the possible This aspect of our recent work, Many other examples can be modular nature of visual processes. however, speaks more to issues in found of phenomena involving, as In our recent study,13 the spared is the visual sciences than to the more Block says, impaired or nonexistent lands supported some visual capac integrative questions about the rela conscious access to brain events.15 ities and not others. While one is tionship of sensation and perception The vast staging for human mental land could process form and color to conscious awareness. Because it activities happens largely without information, itwas not able to elicit is common to find subjects remain monitoring. It is to be expected that a correct eye movement response. largely unaware of their capacities in this situation can be identified in Other points of residual capacity human performance tasks, this latter various experimental venues. were able to elicit eye movement re issue continues to be of considerable It is important to note that blind sponses but were unable to carry out interest. sight has been documented in a rel pattern discriminations. These in atively small number of patients. An triguing dissociations need further initial inspection of the literature study. conveys the impression that blind PERFORMANCEOUTSIDE a common sight is phenomenon, but Acknowledgments?This work was aided CONSCIOUS AWARENESS Grants NINDS 5 many of the reports are based on a by ROI, NS22626-08, and NINDS 5 P01 NS17778-11 from the few select patients. Two cases have National Institutes of Health and by a Blindsight demonstrates vision provided a wealth of reports: D.B., grant from the James S. McDonnell Foun dation. outside the realm of conscious who has been studied extensively by awareness. This point has been Weiskrantz and colleagues,2 and taken as support for the view that G.Y., who has been studied by a perception can occur in the absence number of investigators.16 Studies Notes of sensation,14 because sensations employing wavelength manipula are presumed to be experiences of tions have relied on three cases with 1. P. Pasik and T. Pasik, Visual functions in after total removal of visual cerebral cor stimuli. Advocates of this visual field defects due monkeys impinging incomplete tex, in Contributions to Sensory Physiology, Vol. 7, view have argued that sensations are to damage to visual cortex.9 The W.D. Neff, Ed. (Academic Press, New York, 1982). 2. L.Weiskrantz, A Case and visual cortex, overall of the is Blindsight: Study processed by primary rarity phenomenon Implications (Oxford University Press, Oxford, but attribute blindsight to alternative underscored by two group studies of 1986).

3. B.T. Volpe, J.E. LeDoux, and M.S. Gazzan of blindsight, Trends in Neurosciences, 14, MO 15. N. Block, Review of Consciousness Ex Information in an MS iga, processing of visual stimuli (1991). plained by Dennett, D.C, Journal of Philosophy, extinguished field, Nature, 282, 722-724 (1979). 10. J. Campion, R. Latto, and Y.M. Smith, Is 90, 181-193(1993). 4. M.S. Gazzaniga, J.D. Holtzman, and CS. blindsight an effect of scattered light, spared cortex, 16. J.L. Barbur, K.H. Ruddock, and V.A. Wa Smylie, Speech without conscious awareness, Neu and near-threshold vision? Behavioral and Brain Sci terfield, Human visual responses in the absence of rology, 35, 682-685 (1987). ences, 6, 423-486 (1983). the geniculo-calcarine projection, Brain, 103, 905 11. Interactions between corti 928 I.M. 5. M.S. Gazzaniga, Right hemisphere language J.D. Holtzman, (1980); Blythe, J.M. Bromley, C Kennard, cal and subcortical visual areas: Evidence from K.H. following brain bisection: A twenty year perspec hu and Ruddock, Visual discrimination of target man Vision tive, American Psychologist, 38, 525-547 (1983). commissurotomy patients, Research, 24, displacement remains after damage to the striate 801-813 (1984). cortex in humans, Nature, 320, 619-621 L. 6. J. Sergent, Furtive incursion into bicameral (1986); 12. R. Balliet, K. Mt Blood, and P. Bach-y-Rita, Weiskrantz, A. and J.L. Factors af minds: Integrating and coordinating role of subcor Harlow, Barbur, Visual field rehabilitation in the blind? visual in a tical structures, Brain, 113, 537-568 (1990). cortically fecting sensitivity hemianopic subject, Journal of Neurology, Neurosurgery, and Psychiatry, Brain, 114,2269-2282 (1991). 7. A. Ptito, F. Lepore, M. Ptito, and M. Las 48, 1113-1124(1985). 17. C.A. Marzi, G. Tassinaari, S. Aglioti, and L. sonde, detection and movement discrimina Target 13. R. Fendrich, CM. and M.S. summation across the vertical in Wessinger, Lutzemberger, Spatial tion the blind field of hemispherectomized pa a Gazzaniga, Residual vision in scotoma: Implica meridian in hemianopics: A test of blindsight, Neu tients, Brain, 114, 497-512 (1991). tions for blindsight, Science, 258, 1489-1491 ropsychologia, 24, 749-758 (1986); I.M. Blythe, C 8. C. Kennard, Blindsight Workshop (University (1992). Kennard, and K.H. Ruddock, Residual vision in pa of Oxford, Oxford, 1993). 14. N.K. Humphrey, A History of the Mind (Si tients with retrogeniculate lesions of the visual path 9. A. Cowey and P. Stoerig, The neurobiology mon & Schuster, New York, 1992). ways, Brain, 110, 887-905 (1987).

When What Meets Where puts of the earlier, spatially restricted analyses. Object-based constraints Maggie Shiffrar may be applied during some of these later stages so that information re garding entire objects can be recov ered uniquely. Object permanence How does the visual system de terpret inherently ambiguous visual (i.e., the tendency to assume the an termine inwhich direction object images? continued existence of an object moves? This question has fascinated Many researchers have argued even when it disappears momen generations of researchers because it that the visual system uses con tarily) is an example of a higher so encompasses many of the funda straints to limit the number of possi level, object-based constraint that mental issues in psychology. One of ble interpretations of a projected im the visual system appears to use in these concerns how the human per age.1'2 A constraint can be defined the interpretation of motion.1 ceptual system uniquely solves as an assumption or prior knowledge The proposal that the visual sys an patterns of stimulation having in about the nature of the physical tem uses a hierarchy of various con finite number of possible interpreta world. A visual system reduces the straints leads to numerous questions. tions. For example, in the case of number of possible ?mage interpre How do constraints interact within vision, a single two-dimensional tations by rejecting any interpreta and across levels of analysis? Are projected image is often consistent tion inconsistent with the system's some constraints applied to all im an with infinite number of different previously defined constraints. Most ages and others reserved for partic three-dimensional physical objects. current models of the visual system ular classes of images? To address are as a Nonetheless, human observers describe it multilayered, richly these questions, my colleagues and I un able to uniquely interpret such interconnected hierarchical struc have constructed dynamic stimuli derdetermined ?mages. What en ture.3 Because so much information that place different constraints in ables observers to systematically in is lost in the imaging process, con conflict with one another. By exam straints are required at many differ ining how people interpret these im ent levels within the visual system. ages, we can ascertain how visual Maggie Shiffrar is an Assistant Pro Some constraints are needed constraints are fessor in the Department of Psy during organized, invoked, the earliest to and The ar chology and the Center for Molec stages interpret small, applied. purpose of this ular & Behavioral Neuroscience at local regions of an image. These ticle is to examine when object Rutgers University. Address corre low-level constraints play a role in based constraints control the inter to spondence Maggie Shiffrar, the analysis of relatively small object pretation of moving images. I begin Center for Molecular & Behavioral segments or features. An edge detec with a discussion of how low-level, 197 Neuroscience, University tor that analyzes small parts of an feature-based motion constraints Ave., Rutgers University, Newark, ?mage is one example of a low-level compete with higher level, object NJ 07102; e-mail: mag@cortex. visual constraint. Later stages in the based motion constraints. I then ex rutgers.edu. visual system must combine the out amine the conditions under which

Published by Cambridge University Press