Unexplained Progressive Visual Field Loss in the Presence of Normal Retinotopic Maps.

Lesions of primary visual cortex or its primary inputs typically result in retinotopically localized scotomas. Here we present an individual with unexplained visual field loss and deficits in visual perception in the absence of structural damage to the early visual pathway or lesions in visual cortex. The subject, monocular from an early age, underwent repeated perimetry tests over 8 years demonstrating severe anopia of the lower hemifield, and a clockwise progression of the loss through her upper left visual field. Her visual impairment was evident in a number of standardized tests and psychophysics, especially in tasks assessing spatial integration using illusory contours. However, her intellectual ability was intact and her performance in some other tasks assessing color vision or object detection in scenes was normal. We employed functional magnetic resonance imaging (fMRI), electroretinography and visually evoked potentials. Surprisingly, in contrast to the participant's severe anopia, we found no evidence of abnormal function of her early visual pathways. Specifically, we performed retinotopic mapping using population receptive field (pRF) analysis to map the functional organization of visual cortex in the anopic participant and three control participants on two occasions three and a half years apart. Despite the behavioral visual field loss, her retinotopic maps and pRF parameters in visual areas V1-V3 were qualitatively normal. Further behavioral experiments confirmed that this discrepancy was not trivially explained by the difference between stimuli used for retinotopic mapping and perimetry. Structural T1 scans were normal at both time points, and volumetric analysis of white and gray matter tissue on the segmented T1 volumes did not reveal any abnormalities or deterioration over time. Our findings suggest that normal functional organization of early visual cortex without evident structural damage to the early visual pathway as disclosed by the techniques employed in this study does not necessarily guarantee conscious perception across the visual field.

Visual Dysfunction in Posterior Cortical Atrophy.

Posterior cortical atrophy (PCA) is a syndromic diagnosis. It is characterized by progressive impairment of higher (cortical) visual function with imaging evidence of degeneration affecting the occipital, parietal, and posterior temporal lobes bilaterally. Most cases will prove to have Alzheimer pathology. The aim of this review is to summarize the development of the concept of this disorder since it was first introduced. A critical discussion of the evolving diagnostic criteria is presented and the differential diagnosis with regard to the underlying pathology is reviewed. Emphasis is given to the visual dysfunction that defines the disorder, and the classical deficits, such as simultanagnosia and visual agnosia, as well as the more recently recognized visual field defects, are reviewed, along with the evidence on their neural correlates. The latest developments on the imaging of PCA are summarized, with special attention to its role on the differential diagnosis with related conditions.

Lateralized occipital degeneration in posterior cortical atrophy predicts visual field deficits.

BACKGROUND: Posterior cortical atrophy (PCA), the visual variant of Alzheimer's disease, leads to high-level visual deficits such as alexia or agnosia. Visual field deficits have also been identified, but often inconsistently reported. Little is known about the pattern of visual field deficits or the underlying cortical changes leading to this visual loss. METHODS: Multi-modal magnetic resonance imaging was used to investigate differences in gray matter volume, cortical thickness, white matter microstructure and functional activity in patients with PCA compared to age-matched controls. Additional analyses investigated hemispheric asymmetries in these metrics according to the visual field most affected by the disease. RESULTS: Analysis of structural data indicated considerable loss of gray matter in the occipital and parietal cortices, lateralized to the hemisphere contralateral to the visual loss. This lateralized pattern of gray matter loss was also evident in the hippocampus and parahippocampal gyrus. Diffusion-weighted imaging showed considerable effects of PCA on white matter microstructure in the occipital cortex, and in the corpus callosum. The change in white matter was only lateralized in the occipital lobe, however, with greatest change in the optic radiation contralateral to the visual field deficit. Indeed, there was a significant correlation between the laterality of the optic radiation microstructure and visual field loss. CONCLUSIONS: Detailed brain imaging shows that the asymmetric visual field deficits in patients with PCA reflect the pattern of degeneration of both white and gray matter in the occipital lobe. Understanding the nature of both visual field deficits and the neurodegenerative brain changes in PCA may improve diagnosis and understanding of this disease.

Progressive cortical visual failure associated with occipital calcification and coeliac disease with relative preservation of the dorsal 'action' pathway.

We describe the first reported case of a patient with coeliac disease and cerebral occipital calcification who shows a progressive and seemingly selective failure to recognise visual stimuli. This decline was tracked over a study period of 22 years and occurred in the absence of primary sensory or widespread intellectual impairment. Subsequent tests revealed that although the patient was unable to use shape and contour information to visually identify objects, she was nevertheless able to use this information to reach, grasp and manipulate objects under central, immediate vision. This preservation of visuo-motor control was echoed in her day-to-day ability to navigate and live at home independently. We conclude that occipital calcification following coeliac disease can lead to prominent higher visual failure that, under prescribed viewing conditions, is consistent with separable mechanisms for visual perception and action control.

Severe, persistent visual impairment associated with occipital calcification and coeliac disease.

While coeliac disease is primarily a disease of the digestive system, there have been several reports of neurological effects, both motor and cognitive. Here, we present the case of a woman with coeliac disease, under dietary control, in whom there is profound long-standing visual disturbance including reduction of visual fields, loss of rapid flicker and colour sensitivity and severe deficits in acuity. Structural magnetic resonance imaging (MRI) indicates large regions of calcification and abnormal tissue that is restricted to the occipital cortex, particularly the posterior region. Functional MRI indicates an absence of normal visual activation in the primary visual cortex, but at least in one hemisphere, there is neural activity to moving stimuli in visual motion area hMT+. White matter microstructure in the pathway between the lateral geniculate nucleus and hMT+ is normal compared to healthy control subjects, but is severely abnormal between the lateral geniculate nucleus and primary visual cortex. This case study illustrates the very specific nature of cortical deficit that can arise in association with coeliac disease, and highlights the importance of early dietary control for the disease.

An inability to learn to read caused by shaken baby syndrome.

We report a 12-year-old boy who suffered from shaken baby syndrome at the age of 4 months and has been unable to learn to read even high-frequency, three-letter words, despite slow but accurate letter naming. He had a right homonymous hemianopia and evidence of impaired higher visual function, but not at a severe enough level to account for his inability to read. Speech production and reception of language were impaired for his age but at least of an order of magnitude better than his reading performance. MRI scanning revealed focal damage to the dorsal and ventral reading pathways. This case challenges the Kennard principle, a widely accepted assumption which claims that the earlier a brain injury occurs, the better the recovery. It also adds to the growing literature suggesting that early damage to multiple parts of the language learning network can result in relatively selective impairments later in life.

Evidence for Non-Opponent Coding of Colour Information in Human Visual Cortex: Selective Loss of "Green" Sensitivity in a Subject with Damaged Ventral Occipito-Temporal Cortex.

Damage to ventral occipito-temporal extrastriate visual cortex leads to the syndrome of prosopagnosia often with coexisting cerebral achromatopsia. A patient with this syndrome resulting in a left upper homonymous quadrantanopia, prosopagnosia, and incomplete achromatopsia is described. Chromatic sensitivity was assessed at a number of locations in the intact visual field using a dynamic luminance contrast masking technique that isolates the use of colour signals. In normal subjects chromatic detection thresholds form an elliptical contour when plotted in the Commission Internationale d'Eclairage, (x-y), chromaticity diagram. Because the extraction of colour signals in early visual processing involves opponent mechanisms, subjects with Daltonism (congenital red/green loss of sensitivity) show symmetric increase in thresholds towards the long wavelength ("red") and middle wavelength ("green") regions of the spectrum locus. This is also the case with acquired loss of chromatic sensitivity as a result of retinal or optic nerve disease. Our patient's results were an exception to this rule. Whilst his chromatic sensitivity in the central region of the visual field was reduced symmetrically for both "red/green" and "yellow/blue" directions in colour space, the subject's lower left quadrant showed a marked asymmetry in "red/green" thresholds with the greatest loss of sensitivity towards the "green" region of the spectrum locus. This spatially localized asymmetric loss of "green" but not "red" sensitivity has not been reported previously in human vision. Such loss is consistent with selective damage of neural substrates in the visual cortex that process colour information, but are spectrally non-opponent.

'Hemispherical asymmetry in the Meyer's Loop': a prospective study of visual-field deficits in 105 cases undergoing anterior temporal lobe resection for epilepsy.

OBJECTIVES: Visual-field deficits following temporal lobe surgery have been reported in the literature. In this prospective study, the authors analyse their experience of visual-field deficits in 105 consecutive cases undergoing temporal-lobe surgery performed by a single surgeon, with particular consideration to the laterality of the deficit and its functional implications. METHODS: 105 consecutive patients undergoing an anterior temporal lobe resection for epilepsy, between March 1998 and June 2004, were selected. The patient population had a mean age of 35 years (range 19-60 years); 53 had a left-sided resection and 52 a right-sided resection. 91 patients had mesial temporal sclerosis, three gangliogliomas, four dysembryoplastic neuroepithelial tumours (DNETs), two neurocytomas and two cavernomas, and in three cases the histology was inconclusive. Pre- and postoperative visual-field tests were obtained using the Humphrey Esterman binocular functional test for all cases. The test was set to stimulus white III, with a single intensity of 10 DB on the background of 31.5 ASB for all patients. A minimum follow-up period of 12 months postsurgery was employed. Postoperative MRI scans were carried out on all patients. 60 scans were randomly selected, and the extent of temporal lobe resection calculated manually for each. RESULTS: Of the 105 cases, 16 patients had a visual-field deficit postoperatively which was not present preoperatively: 12 following a left and four following a right-sided resection. The OR for incurring a postoperative visual-field defect following left versus right-sided surgery was 3.51 (95% CI 1.05 to 11.73, p=0.04). In four patients, the deficit was severe enough to preclude them from driving in the UK (three left- and one right-sided resection). There was no association between the extent of tissue resection and the incidence of postoperative visual-field deficits. CONCLUSIONS: This study suggests left-/right-hemispherical asymmetry in the Geniculocalcarine tracts with field deficits being 3.5 times more likely following left-sided anterior temporal lobe resections compared with right-sided resections. This has significant implications on counselling patients for these procedures. MR tractography may provide an anatomical substrate for these clinical findings, perhaps revealing a more anterior course of the optic radiations within the temporal lobe in one hemisphere versus the other.

Form completion across a hemianopic boundary: behindsight?

Patients with homonymous hemianopia may report the completion of forms that overlap the vertical meridian of their field defects. While previous investigations of "hemianopic completion" have variously attributed to the disorder to inattention, residual vision or unstable fixation, we believe that our investigation has controlled for such potentially confounding factors. We report patient P.O.V. who experienced hemianopic completion in everyday life following a surgical lesion of his left occipital lobe. He showed normal spatial attention and normal spatial orienting: hemianopic completion can therefore occur in the absence of inattention. His completion was retinotopic and affected partial as well as complete forms: his hemianopic completion cannot be attributed to residual visual input or poor fixation. P.O.V.'s completion was also systematically affected by varying stimulus contrast and pattern masking. We argue that while other explanations may be appropriate for different cases, P.O.V.'s hemianopic completion reflects normal "constructive" visual processes and can be attributed to the unconstrained operation of visual routines that are normally involved in the perception of partially occluded forms. As such, this disorder has the potential to shed light on some of the most basic aspects of visual perception.