A model of ganglion axon pathways accounts for percepts elicited by retinal implants.

Degenerative retinal diseases such as retinitis pigmentosa and macular degeneration cause irreversible vision loss in more than 10 million people worldwide. Retinal prostheses, now implanted in over 250 patients worldwide, electrically stimulate surviving cells in order to evoke neuronal responses that are interpreted by the brain as visual percepts ('phosphenes'). However, instead of seeing focal spots of light, current implant users perceive highly distorted phosphenes that vary in shape both across subjects and electrodes. We characterized these distortions by asking users of the Argus retinal prosthesis system (Second Sight Medical Products Inc.) to draw electrically elicited percepts on a touchscreen. Using ophthalmic fundus imaging and computational modeling, we show that elicited percepts can be accurately predicted by the topographic organization of optic nerve fiber bundles in each subject's retina, successfully replicating visual percepts ranging from 'blobs' to oriented 'streaks' and 'wedges' depending on the retinal location of the stimulating electrode. This provides the first evidence that activation of passing axon fibers accounts for the rich repertoire of phosphene shape commonly reported in psychophysical experiments, which can severely distort the quality of the generated visual experience. Overall our findings argue for more detailed modeling of biological detail across neural engineering applications

Irregular S-cone mosaics in felid retinas: spatial interaction with axonless horizontal revealed by cross-correlation

In most mammals short-wavelength-sensitive (S) cones are arranged in irregular patterns with widely variable intercell distances. Consequently, mosaics of connected interneurons either may show some type of correlation to photoreceptor placement or may establish an independent lattice with compensatory dendritic organization. Since axonless horizontal cells (A-HC’s) are supposed to direct all dendrites to overlying cones, we studied their spatial interaction with chromatic cone subclasses. In the cheetah, the bobcat, and the leopard, anti-S-opsin antibodies have consistently colabeled the A-HC’s in addition to the S cones. We investigated the interaction between the two cell mosaics, using autocorrelation and cross-correlation procedures, including a Voronoi-based density probe. Comparisons with simulations of random mosaics show significantly lower densities of S cones above the cell bodies and primary dendrites of A-HC’s. The pattern results in different long-wavelength-sensitive-L- and S-cone ratios in the central versus the peripheral zones of A-HC dendritic fields. The existence of a related pattern at the synaptic level and its potential significance for color processing may be investigated in further studies

Astigmatism and Pseudoaccommodation in Pseudophakic Eyes

noAdvanced IOLs with circumferential zones of different power provide pseudoaccommodation. We investigated the potential for power variation with meridian, namely astigmatism, to provide pseudo-accommodation. With appropriate power and axis orientations, acceptable pseudo-accommodation can be achieved

View-Invariant Object Category Learning, Recognition, and Search: How Spatial and Object Attention Are Coordinated Using Surface-Based Attentional Shrouds

Air Force Office of Scientific Research (F49620-01-1-0397); National Science Foundation (SBE-0354378); Office of Naval Research (N00014-01-1-0624

3D Image Reconstruction of Retina using Aplanats

Existing fundus cameras provide two-dimensional images for the fundus or retina which are curved in shape, thereby distorting the peripheral information. For a quality and aberration free image, a system of lenses need to be incorporated for imaging. But that comes with narrow field of view, high-cost, and makes it bulkier and difficult to operate. Retinal abnormalities outside the photo- graphic field cannot be captured with narrow field of view. Any compromise with the quality of imaging system will lead to poor fundus image quality. In this thesis, an attempt has been made to demonstrate that aplanats can be used to replace the lens system in fundus cameras with the benefit of acheiving high field of views of about 200o without any pupil dialation needed. Aplanats are aberration free and have high numerical apertures. Therefore, they can even collect highly di- verging rays. But due to their design, complete retina cannot be imaged using a single aplanat. So a three phase imaging procedure is proposed in which three annular regions of retina are imaged in each phase. In the first two phases imaging is done by the aplanats and for the third stage a simple mobile can also be used. This imaging covers the whole retina, with some overlaps so that the images collected can be stitched together post that. In first two phases, sensors are specially designed which capture the 3D information of retina. The 3D reconstruction of complete retina from sensor data is discussed later in detail. A developed ray tracer is talked about which was used as the tool to work towards the aim. Though aplanats are aberration free, the light reflected from retina has to pass through the eye-lens which makes the aggregate system aberrant. After the 3D retina image is constructed, all the details can be identified but with slight blur. With appropriate signal processing techniques, a sharp 3D image of retina can be obtained

Spatial Analysis of Retinal Pigment Epithelium Morphology

In patients with age-related macular degeneration, a monolayer of cells in the eyes called retinal pigment epithelium differ from healthy ones in morphology. It is therefore important to quantify the morphological changes, which will help us better understand the physiology, disease progression and classification. Classification of the RPE morphometry has been accomplished with whole tissue data. In this work, we focused on the spatial aspect of RPE morphometric analysis. We used the second-order spatial analysis to reveal the distinct patterns of cell clustering between normal and diseased eyes for both simulated and experimental human RPE data. We classified the mouse genotype and age by the k-Nearest Neighbors algorithm. Radially aligned regions showed different classification power for several cell shape variables. Our proposed methods provide a useful addition to classification and prognosis of eye disease noninvasively