Blindsight: A Case Study and Implications

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1 University Press Scholarship Online You are looking at 1-10 of 12 items for: keywords : visual acuity Blindsight: A Case Study and Implications L. Weiskrantz Published in print: 1990 Published Online: ISBN: eisbn: Item type: book acprof:oso/ Damage to a particular area of the brain the neocortex is generally understood to result in blindness. Studies of some patients who have suffered from this form of blindness have nevertheless revealed that they can, in fact, discriminate certain types of visual events within their blind fields without being aware that they can do so: they think they are only guessing. This phenomenon has been termed blindsight by the author of this book and his collaborators who were among the first to describe it. It continues to attract considerable interest among neuroscientists, psychologists, and philosophers who see possible implications for theories of perception and consciousness. This book gives an account of the research into a particular case of blindsight, together with a discussion of the historical and neurological background. The empirical findings are followed by a review of other cases reported by other investigators, in which there is a dysjunction between clinical assessment of blindness and unexpected findings of residual function. Finally, a number of theoretical and practical issues and implications are discussed. This reissued version of the text includes a new Introduction summarizing some of the advances that have taken place in the field since the book was first published in Newborn vision Janette Atkinson in The Developing Visual Brain Published in print: 2002 Published Online: ISBN: eisbn: acprof:oso/ This chapter considers the visual capabilities of the normal newborn infant, starting with crude visual orienting behaviour towards visually Page 1 of 6

2 conspicuous objects. The evidence for a specific ability in newborns to recognize face configurations, and its possible neural underpinnings is reviewed. Studies that measure the development of acuity and contrast sensitivity from birth are discussed, including the likely optical, retinal, and neural factors limiting these processes, and the relationship between measurements based on preferential looking and those obtained by VEP/ VERP methods. It presents data from two unique intensive longitudinal studies of the author's daughters, Fleur and Ione, on development of contrast sensitivity and acuity. The results show that compared across many testing sessions, OKN (optokinetic nystagmus), VEPs, and preferential looking measures show broad agreement in the time course of development of contrast sensitivity. Paediatric vision testing Janette Atkinson in The Developing Visual Brain Published in print: 2002 Published Online: ISBN: eisbn: acprof:oso/ This chapter describes some of the methods used for testing vision in infants and young children in research and clinical assessment, including behavioural measures from observing eye movements (forced choice preferential looking /habituation), video/photorefraction, and visual evoked potentials (VEP) or visual event related potentials (VERP). It outlines how these methods have been used and compared, including the use of forced-choice preferential looking for acuity testing and acuity measures in preschool children using the Cambridge Crowding Cards, devised in the Visual Development Unit. It describes the subtests from the Atkinson Battery of Child Development for Examining Functional Vision (ABCDEFV), a behavioural battery which assesses children's functional use of sensory, perceptual, and cognitive vision, from birth to five years. The Rotterdam C-chart: visual acuity and interocular acuity differences in very low birth weight and/or very prematurely born children at the age of 5 years J. van Hof-van Duin and J. W. R. Pott in Infant Vision Published in print: 1996 Published Online: ISBN: eisbn: acprof:oso/ Page 2 of 6

3 It is generally accepted that children born very prematurely or with a very low birth weight (VLBW; #1500 g) are at risk for visual impairment, at least during the first few years after birth. Whether visual impairment such as low visual acuity is only a sign of delayed development, or is irreversible, is still questionable. Visual acuity development continues until puberty which possibly allows a catching up in this lag in development. The purpose is to determine whether VLBW and/or very prematurely born children are at risk. For permanent impairment of visual acuity by comparison of acuity values obtained in five-year-old children who are at risk for an impaired visual development because of a very low birth weight and/or a very short gestational duration, to those of healthy control five-year-olds. In this earlier study, it is demonstrated that with the Rotterdam C-chart acuity estimates of both five-year-old control children and adults show a normal distribution and no ceiling effect. Therefore, the Rotterdam C-chart seems suitable for acuity assessment in young and visually impaired children. Dioptric blur, grating visual acuity, and stereoacuity in infants Paulette P. Schmidt, Ivan C. J. Wood, Sarah Lewin, and Helen Davis in Infant Vision Published in print: 1996 Published Online: ISBN: eisbn: acprof:oso/ In the assessment of visual function, visual and stereo acuity measurement play important roles in diagnosing vision problems and in monitoring the effects of clinical treatments. Grating-acuity measurement determined by preferential-looking methods is the most widely available tool for assessment of visual function in infants and toddlers. Visual acuity may be measured binocularly and monocularly but requires monocular measurements to determine the presence of unilateral blur. Stereo acuity is the perception of depth as a consequence of cortical recognition of horizontal retinal disparity between the two eyes. It is a form of hyper acuity and is measured binocularly. Stereopsis is the result of activating cortical binocular cells with identical but disparate images. Disrupted or discordant binocular vision during human infancy results in amblyopia and/or suppression thereby interrupting stereopsis. The development of stereopsis has important implications for both understanding human visual development and the clinical treatment of visual anomalies. Page 3 of 6

4 Development of the visual field: results from human and animal studies Ruxandra Sireteanu in Infant Vision Published in print: 1996 Published Online: ISBN: eisbn: acprof:oso/ The extent of the adult visual field gives important clues on brain functioning. Visual field deficits often accompany, and sometimes predict, the onset of neurological problems. It is therefore of utmost importance to have reliable methods of early assessment of the visual field, in addition to the assessment of visual resolution. The behavioural function that is used for visual acuity assessment is the spontaneous orienting towards a salient visual stimulus in preference to a homogeneous field. In this chapter, an overview on the achievements and limitations of the methods of visual perimetry used with infants and children is given, including correlative data from cats and monkeys. Visual acuity L. Weiskrantz in Blindsight: A Case Study and Implications Published in print: 1990 Published Online: ISBN: eisbn: acprof:oso/ D. B. was randomly presented with either a grating of a given spatial frequency and constant contrast, or a plain stimulus matched in flux (actually a stimulus with ultra-high spatial frequency to guarantee a flux match) and asked to respond, lines or no lines. The spatial frequency was changed in groups of thirty trials and the cut-off spatial frequency determined for 75% performance. Two different apparatus were used for presenting sine-wave gratings: the Moiré fringes generated by light passing though a pair of diffraction gratings, or non-luminous nonprojected photographic prints mounted on cardboard. In his blind field, acuity actually improved with increasing eccentricity out to about 30. His spatial frequency acuity compared to his good field was reduced by about 2 octaves, a result similar to that reported by the Pasiks for the monkey without striate cortex. D. B. never reported any experience of the gratings and said he was just guessing. Page 4 of 6

5 Structure and Function of the Retina in Aquatic Tetrapods Tom Reuter and Leo Peichl in Sensory Evolution on the Threshold: Adaptations in Secondarily Aquatic Vertebrates Published in print: 2008 Published Online: ISBN: eisbn: Publisher: University of California Press DOI: / california/ This chapter discusses the anatomy and physiology of the retina in aquatic tetrapods. It focuses on the input and output stages of retinal processing: the photoreceptors and the ganglion cells. It describes the characteristics of rod and cone visual pigments, photoreceptor sensitivity, and the importance of temporal summation of photoreceptor signals. It also discusses ganglion cell densities and topographies, and examines how ganglion cells receive photoreceptor inputs and determine anatomical visual acuity. Visual Evoked Responses Jay A. Liveson and Dong M. Ma in Laboratory Reference for Clinical Neurophysiology Published in print: 1999 Published Online: ISBN: eisbn: acprof:oso/ Visual evoked potentials provide a quantitative measure of the visual system. The function measured includes that of the optic nerve, through the optic chiasm and tract, to the lateral geniculate bodies, and the geniculocalcarine projection to the visual cortex. The use of smallsized stimuli tests the foveal region, emanating primarily from the central fifteen degrees. The most reliable information relates to lesions of the optic nerve, and is derived by individual testing of each eye. An important application of visual evoked responses is as a screen for multiple sclerosis lesions. Abnormalities have also been shown in other conditions such as glaucoma, parkinsonism, and cortical blindness. It can also be used to measure visual acuity in infants. Since the response relies on a visual image reaching the retina, it is important to screen initially for any significant decrease in visual acuity. If possible, this should be corrected. If not, flash stimuli may yield some information, although this may or may not be as sensitive a test. Page 5 of 6

6 D.B.: Clinical history and early testing L. Weiskrantz in Blindsight: A Case Study and Implications Published in print: 1990 Published Online: ISBN: eisbn: acprof:oso/ D. B. is the subject of the book who underwent intensive tests of his blindsight. His visual cortex in one hemisphere was removed surgically to excise an invasive tumour when he was 33 years old. The findings of Pöppel et al. (1973) were confirmed, in which brain damaged subjects could move their eyes to the location of an unseen stimulus in their blind fields. The use of animal testing methods were then applied by asking D. B. to guess the location of stimuli by reaching for them. He was also asked to guess between two alternatives: whether a line was horizontal or not, and whether a stimulus was a particular shape or an alternative shape. Asking him to guess whether a sine-wave grating or a homogenous luminous-matched patch was present or not made it possible to measure his visual acuity and compare it with that of the intact visual hemifield. Following a battery of tests, his verbal commentaries were recorded in which he characteristically said he was just guessing and thought he was at chance because he could not see anything, although in some tests he had a feeling that something was there. Page 6 of 6