Illusory afterimages
Transcript of Illusory afterimages
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H I G H L I G H T S
NATURE REVIEWS | NEUROSCIENCE VOLUME 2 | OCTOBER 2001 | 679
A key question in the pathogenesis of Alzheimer’s disease is therelation between the accumulation of β-amyloid (Aβ) and theformation of neurofibrillary tangles. Are the two processesentwined or are they independent manifestations of the disease?Two recent papers provide convincing evidence that the twoarchetypal neuropathological changes seen in Alzheimer’s diseaseare linked. Both papers converge on the idea that the formation ofAβ deposits acts to promote the appearance of tangles.
Lewis et al. generated a long-awaited transgenic animal — amouse carrying mutant forms of both the amyloid precursorprotein (APP) and tau, the main component of neurofibrillarytangles. Mice bearing either one of the two mutant genes fail torecapitulate all of the neuropathological changes found inAlzheimer’s disease. It was therefore hoped that the double-transgenic mice would constitute a better model of the pathology.Indeed, these mice showed both amyloid plaques and tangles but,more importantly, the number of tangles in the double mutantwas higher than in mice expressing only tau, indicating that theaccumulation of Aβ might stimulate the formation of tangles.
But is Aβ responsible for the observed phenotype or is it APPitself? In an independent study, Götz et al. used transgenic miceexpressing tau but, instead of generating a double mutant, theyinjected Aβ directly into the cortex and hippocampus of theseanimals. They found that this treatment also increased thenumber of tangles, providing evidence that Aβ is indeed themolecule that stimulates tangle formation.
An intriguing aspect of both studies is that Aβ and tau werespatially segregated. Lewis et al. found that, in contrast to what isfound in Alzheimer’s disease, amyloid plaques were notsurrounded by tau-positive neurites in the double mutants, andGötz et al. found that tangle formation was stimulated in brainregions far from the Aβ injection sites. So, it seems that thesemodels also fail to reproduce every aspect of Alzheimer’s disease,although the two of them provide better tools to test morerigorously the benefit of new treatments for this pathology.
Juan Carlos López
References and linksORIGINAL RESEARCH PAPERS Lewis, J. et al. Enhanced neurofibrillary degeneration intransgenic mice expressing mutant tau and APP. Science 293, 1487–1491 (2001) | Götz, J.et al. Formation of neurofibrillary tangles in P301L tau transgenic mice induced by Aβ42fibrils. Science 293, 1487–1491 (2001)FURTHER READING Duff, K. & Rao, M. V. Progress in the modeling of neurodegenerativediseases in transgenic mice. Curr. Opin. Neurol. 14, 441–447 (2001)
We are all familiar with afterimages. Ifyou look at a bright light for a coupleof seconds, then close your eyes, youwill see a dark spot; similarly, gazing ata coloured shape can induce an after-image in the complementary colour— for example, a red square will pro-duce a green afterimage. This is usual-ly attributed to bleaching of the pig-ments in the retinal photoreceptors orto neural adaptation in the retina. ButShimojo et al. have found evidencethat some afterimages can result fromadaptation at a cortical level.
They used a type of visual stimu-lus that is designed to cause ‘filling-in’,where the visual system draws on ele-ments in a visual scene to ‘complete’ ashape that is not actually there. In theVarin configuration, for example,four circles like those depicted in thefigure are arrayed in a square layout.In each circle, the quadrant (orwedge) nearest the centre of thesquare is coloured differently fromthe rest of the circles, so that the fourcoloured quadrants appear to formthe corners of a smaller, colouredsquare seen through the circles orsuperimposed on them. Our visualsystem fills in the missing lines so thatwe perceive a complete square.
Looking at this stimulus for awhile leads to the perception of anafterimage of a square, the colour ofwhich is complementary to the per-ceived square’s colour, as well as after-images of the incomplete circles andthe wedges. There are two possiblemechanisms for the ‘global afterim-age’ of the square. It could arise fromthe perceived afterimages of the circlesand wedges (the ‘local afterimages’),just as the original perception arisesfrom the perception of the circlesthemselves; or it could be due to adap-tation of cortical neural circuits thatrepresent the filled-in square surface(the ‘surface-adaptation hypothesis’).
Shimojo and colleagues set out todiscover which of these hypotheseswas true. They used varying stimulidesigned to induce different degreesof filling-in and different strengths oflocal afterimage, and asked people torate the intensity and duration of the
global afterimage. The results showedthat the strength of the global after-image did not depend on the strengthof the local afterimages, but rather onthe degree of filling-in that had beeninduced by the stimulus. This resultwas predicted by the surface-adapta-tion hypothesis, but would not havebeen expected if the global afterimagearose from the local afterimages.
Another unexpected finding wasthat the local and global afterimagesappeared to rival each other, with mostsubjects reporting that they were seenseparately but not together. Thisargues against even a compromiseexplanation, in which both proposedmechanisms contribute to the globalafterimage. But the authors also reportthat some peripheral adaptation seemsto be necessary for the global afterim-age to be perceived — when only oneeye was adapted, for example, theglobal afterimage was not seen by theunadapted eye.
Undoubtedly, there are still ques-tions to be answered before we willfully understand the mechanisms ofadaptation. But studies such as thesewill also help us to understand howthe visual system represents surfaces,whether real or illusory.
Rachel Jones
References and linksORIGINAL RESEARCH PAPER Shimojo, S. et al. Afterimage of perceptually filled-in surface.Science 293, 1677–1680 (2001)FURTHER READING Pessoa, L. et al. Findingout about filling-in: a guide to perceptual comple-tion for visual science and the philosophy of per-ception. Behav. Brain Sci. 21, 723–748 (1998) |Wilson, H. R. A neural model of foveal light adap-tation and afterimage formation. Vis. Neurosci.14, 403–423 (1997)WEB SITE Shimojo’s lab: http://neuro.caltech.edu/
Aβ, τ and the fAβricaτion of τangles
N E U R O D E G E N E R AT I V E D I S O R D E R S
Illusory afterimages
V I S U A L P S Y C H O P H Y S I C S