Imaging the visual system: from the eye to the brain.

Imaging technologies have revolutionized the study of human anatomy and physiology. Nowhere is this more evident than in the vision sciences, where imaging has provided unprecedented insights into the structure and function of the entire visual pathway in vivo. Ocular and retinal imaging techniques such as optical coherence tomography (OCT) have become established clinical tools, providing highly detailed images of ocular structures that are now used routinely to support the diagnosis and management of ocular disease. The expanding scope of measurements possible with ocular imaging technology is resulting in even more accurate diagnostic and prognostic clinical instruments and progressing our understanding of the eye’s structural and functional properties. On the other hand, brain imaging technologies such as functional magnetic resonance imaging and diffusion tensor tractography are not yet widely utilized in the clinical management of visual disorders. This is likely to change. There is increasing evidence that the impact of ocular disease on visual function cannot be fully understood without considering associated changes in the structure and function of the brain. Furthermore, attempts to restore vision using electrical prosthetics or regenerative medicine require an understanding of the entire visual pathway in patients with vision loss. For example, any neurodegenerative effects of long-term visual cortex deafferentation will limit the extent to which vision can be recovered when retinal input to the brain is restored. Therefore future advances in the field of vision restoration are likely to rely critically on information from a combination of both eye and brain imaging techniques. This feature issue had two main goals. The first was to identify new imaging technologies and recent progress in established imaging methodologies that can be applied to the visual system. The second was to highlight advances in our understanding of the visual system and visual disorders that have been achieved through the use of imaging techniques. These broad goals allowed us to assemble a collection of papers that span the entire visual system from the cornea to the extrastriate visual cortex. The feature issue begins with two invited reviews that together provide comprehensive overviews of both retinal and brain imaging. The first review, by Jessica Morgan, describes recent developments in retinal imaging resulting from the use of novel OCT techniques, adaptive optics and their combination. These techniques allow for detailed, cellular level imaging of retinal layers and vasculature, visualization of the photoreceptor mosaic (both cones and rods) and even perimetry at the level of individual photoreceptors. Morgan also describes the exciting possibility of high-resolution functional imaging of the retina. These cutting edge technological advances in retinal imaging have direct implications for the early detection of retinal dysfunction and improved assessment of treatment outcomes. The second review, by Brown et al., deals with the application of structural, functional and spectroscopic magnetic resonance imaging (MRI) techniques to the study of vision disorders. The visual system is particularly amenable to functional MRI (fMRI) because it is relatively straightforward to present participants with well-controlled stimuli for psychophysics within the scanner environment. In fact, the very first human functional magnetic resonance imaging (fMRI) studies involved measurements of visual cortex activation. It is perhaps not surprising then, that fMRI has been used extensively to study the functional organization of the human visual system. Precisely organized retinotopic maps have been visualized within the primary and extrastriate visual cortex as well as in thalamic areas such as the pulvinar. Recent advances in retinotopic mapping techniques have even allowed for the estimation of average receptive field size within specific regions of the visual cortex. In more general terms, structural and functional MRI has advanced our understanding of visual pathway disease mechanisms and the extent to which the brain can reorganize in response to impaired visual input. Brown et al. provide an introduction to structural and functional brain imaging techniques as well as a technique called magnetic resonance spectroscopy that enables the measurement of metabolite concentration in targeted brain areas. They then describe studies that have used these techniques to unveil the cortical and subcortical effects of anophthalmia, macular degeneration, retinitis pigmentosa, glaucoma, albinism and amblyopia in humans. It is clear from this review that the impact of ocular disease and dysfunction does not end at the eye. Both review papers highlight the remarkably rapid technical advances that imaging technologies have undergone, and the substantial contributions that imaging has made to vision science. It is also evident from both reviews that significant breakthroughs in the clinical application of imaging technologies are imminent. This theme is reflected in the original research papers included in the feature issue. The first two papers relate to anterior eye-imaging technologies. Iskander et al. report on the Eye Surface Profiler: a device that utilizes fourier transform profilome-