NEWS AND VIEWS found that this protein is required for to be addressed. For example, it is also This was cleverly done by demonstrating sustained InsP3 production in permeabil­ intriguing that degeneration of photo­ detection in the 'blind' hemifield of ized HL60 cells. In Drosophila, the rdgB receptors deficient in CDS and Ptdins-TP monkeys with unilateral removal of Vl, gene encodes a Ptdins-TP that is ex­ requires light whereas degeneration of and then having monkeys classify trials 9 pressed in photoreceptors • Mutations photoreceptors deficient in DAG kinase in the 'blind' hemifield as a blank (no in this gene produce an abnormal photo­ occurs even in darkness. Taken together, object present) or a stimulus (object response that further deteriorates upon these findings highlight the importance of present) trial. Monkeys classified detect­ exposure to light, and rdgB mutants, like the enzymes of the phosphoinositide cycle ed objects as unseen (blank trials). cds mutants, undergo light-dependent that supply substrate on demand for a Given this apparent demonstration of retinal degeneration. signal-transduction pathway that is found vision without awareness in monkeys, the Wu et al. have shown that there is a in nearly all eukaryotic cells. D case for blindsight in humans becomes unique pathway for synthesis of the CDP­ more plausible. DAG used for phototransduction, and James B. Hurley, of the Howard Hughes The report of Cowey and Stoerig also their demonstration of the consequences Medical Institute Research Laboratories, reminds us of the modularity of processing of overexpressing and underexpressing University of Washington, Seattle, is cur• in our brains, and that we do not have CDS in vivo raises the intriguing possibil­ rently in the Biological Laboratories, Har• awareness of all modular functions. It ity that this reaction could have a regula­ vard Medical School, 16 Divinity Avenue, seems that Vl is critical for visual aware­ tory role in vision. Many issues now need Cambridge, Massachusetts 02138, USA. ness, even though some higher visual areas may be activated through the mid­ VISUAL PERCEPTION------brain pathway. Interestingly, monkeys with large lesions of non-visual cortex act 1 as if blind \ implying that visual cortex Vision without awareness alone is not sufficient for visual aware­ ness. Jon H. Kaas Recognizing a fundamental similarity in the role of Vl in visual perception in 1 ON page 247 of this issue , Cowey and has been challenged in two ways. First, it humans and monkeys does not mean that Stoerig tackle the difficult question of can be difficult to determine the extent of there are no species differences in visual what monkeys see if they don't have a lesions in humans. So it is possible that, in processing. Old World monkeys resemble visual cortex. Early studies of monkeys2 at least some cases, preserved but com­ humans in that Vl is profoundly impor­ and humans3 seemed to indicate that there promised remnants of Vl in humans per­ tant in visual processing; lesions of Vl is nothing to investigate. The loss of mit detection without awareness, much as alter many parts of the visual system, and primary visual cortex (visual area 1 or Vl) near-threshold stimuli can be detected by in humans and Old World monkeys such impaired vision to such an extent that the normal individuals without them being lesions result in the loss of 80 per cent of 12 loss was called 'cortical blindness' - certain or fully aware of the presence of the ganglion cells of the retina , making monkeys or people with this condition the stimulus. Indeed, in a study involving the preserved abilities seem even more 9 appeared to have no awareness of objects brain imaging , a patient demonstrating remarkable. In other adult mammals that or attributes of objects such as location, features of blindsight was found to have have been examined, including New form, size or brightness. possibly functional remnants of Vl. One World monkeys and prosimian primates, Researchers were therefore surprised current viewpoint is therefore that no such loss of ganglion cells occurs after by subsequent evidence that, after com­ blindsight in humans reflects suboptimal Vl lesions. For this reason alone, one plete, bilateral lesions of Vl, squirrel­ function in Vl. might expect greater sparing of visual like tree shrews4 and cats5 could avoid The second type of challenge has been function after lesions of Vl in many obstacles, follow moving objects and to question the relevance of animal studies mammals. Unlike humans and Old World discriminate between simple visual as support for the argument that humans monkeys, tree shrews appear to have patterns. About the same time, monkeys have blindsight. In monkeys and other nearly normal visual behaviour after deprived of Vl were found to be able mammals, it is possible to remove all of lesions to Vl (ref. 4), but they seem to accurately to reach out for visually pre­ Vl, test for remaining vision, and then be blind 13 after lesions of the visual mid­ 6 sented objects , and the absence of vision examine the histology of the brain to make brain, thus apparently suffering from in humans with such lesions came into certain the lesions were complete. Be­ 'midbrain blindness'. D question. cause monkeys deprived of Vl can locate Further studies in humans7 led to the objects in space and discriminate between Jon H. Kaas is in the Department of 6 8 conclusion that considerable vision is simple forms • , the remaining abilities of Psychology, Vanderbilt University, Nash• possible without Vl, so that objects can be human subjects after Vl damage do not ville, Tennessee 37240, USA. detected and visually followed, but not seem so surprising. Yet, despite many identified. Most remarkably, object de­ apparent similarities in at least early 1. Cowey, A. &Stoerig, P. Nature373, 247-249 (1995). tection is not accompanied by awareness. stages of visual processing, humans are 2. Kluver, H. Bioi. Symp. 7, 253-299 (1992). 3. Holmes, G. Proc. R. Soc. 8132, 34&--361 (1945). For impaired observers, it seems that not monkeys, and the possibly more com­ 4. Snyder, M., Hall, W. C. &Diamond, I. T. Psychon. Sci. 6, there is nothing to detect even though they pelling evidence for preserved vision in 243-244 (1966). 5. Winans, S. S. Science 158,944-946 (1971). perform well above chance when forced to monkeys after Vllesions can be dismissed 6. Humphrey, K. &Weiskrantz, L. Nature215, 595-597 make choices. The remaining ability to as irrelevant to the issue of human (1967). 10 7. Weiskrantz, L., Warrington, E. K., Sanders, M.D. & detect without awareness has been called blindsight . Marshaii,J. Brain97, 709-728 (1974). 7 1 'blindsight' . Because Vl provides most Cowey and Stoerig directly address the 8. Cowey, A. & Stoerig, P. Trends Neurosci. 14, 140-145 but not all of the visual input to higher possibility that monkeys as well as humans (1991). 9. Fendrich, R., Wessinger, C. M. &Gazzaniga, M.S. visual areas of the brain, an indirect have blindsight. They present evidence Science258, 1489-1491 (1992). visual pathway from the retina through that monkeys without Vl, like humans, 10. Barinaga, M. Science258, 1438-1439 (1992). 11. Nakamura, R. K. & Mishkin, M. Brain Res. 188,572-577 midbrain and thalamus to higher visual detect visual objects without awareness. (1980). cortical areas presumably mediates this The trick, of course, is to get monkeys to 12. Weller, R. E. & Kaas, J. H. Visual Neurosci. 3, 327-349 8 (1989). condition . tell you that they can visually detect 13. Casagrande, V. A. &Diamond, I. T.J. comp. Neurol.156, The validity of the blindsight concept something that they don't consciously see. 207-238(1974). 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