Probability Distributions.- Linear Models for Regression.- Linear Models for Classification.- Neural Networks.- Kernel Methods.- Sparse Kernel Machines.- Graphical Models.- Mixture Models and EM.- Approximate Inference.- Sampling Methods.- Continuous Latent Variables.- Sequential Data.- Combining Models.

Pattern Recognition and Machine Learning

Many current neurophysiological, psychophysical, and psychological approaches to vision rest on the idea that when we see, the brain produces an internal representation of the world. The activation of this internal representation is assumed to give rise to the experience of seeing. The problem with this kind of approach is that it leaves unexplained how the existence of such a detailed internal representation might produce visual consciousness. An alternative proposal is made here. We propose that seeing is a way of acting. It is a particular way of exploring the environment. Activity in internal representations does not generate the experience of seeing. The outside world serves as its own, external, representation. The experience of seeing occurs when the organism masters what we call the governing laws of sensorimotor contingency. The advantage of this approach is that it provides a natural and principled way of accounting for visual consciousness, and for the differences in the perceived quality of sensory experience in the different sensory modalities. Several lines of empirical evidence are brought forward in support of the theory, in particular: evidence from experiments in sensorimotor adaptation, visual \"filling in,\" visual stability despite eye movements, change blindness, sensory substitution, and color perception.

A sensorimotor account of vision and visual consciousness-Authors' Response-Acting out our sensory experience

State-of-the-art retinal optical coherence tomography

We have combined Fourier-domain optical coherence tomography (FD-OCT) with a closed-loop adaptive optics (AO) system using a Hartmann-Shack wavefront sensor and a bimorph deformable mirror. The adaptive optics system measures and corrects the wavefront aberration of the human eye for improved lateral resolution (~4 μm) of retinal images, while maintaining the high axial resolution (~6 μm) of stand alone OCT. The AO-OCT instrument enables the three-dimensional (3D) visualization of different retinal structures in vivo with high 3D resolution (4×4×6 μm). Using this system, we have demonstrated the ability to image microscopic blood vessels and the cone photoreceptor mosaic.

/pdf/adaptive-optics-optical-coherence-tomography-for-high-3r5d76hchu.pdf

Adaptive-optics optical coherence tomography for high-resolution and high-speed 3D retinal in vivo imaging

We demonstrate ultrahigh speed spectral / Fourier domain optical coherence tomography (OCT) using an ultrahigh speed CMOS line scan camera at rates of 70,000 - 312,500 axial scans per second. Several design configurations are characterized to illustrate trade-offs between acquisition speed, resolution, imaging range, sensitivity and sensitivity roll-off performance. Ultrahigh resolution OCT with 2.5 - 3.0 micron axial image resolution is demonstrated at approximately 100,000 axial scans per second. A high resolution spectrometer design improves sensitivity roll-off and imaging range performance, trading off imaging speed to 70,000 axial scans per second. Ultrahigh speed imaging at >300,000 axial scans per second with standard image resolution is also demonstrated. Ophthalmic OCT imaging of the normal human retina is investigated. The high acquisition speeds enable dense raster scanning to acquire densely sampled volumetric three dimensional OCT (3D-OCT) data sets of the macula and optic disc with minimal motion artifacts. Imaging with approximately 8 - 9 micron axial resolution at 250,000 axial scans per second, a 512 x 512 x 400 voxel volumetric 3D-OCT data set can be acquired in only approximately 1.3 seconds. Orthogonal registration scans are used to register OCT raster scans and remove residual axial eye motion, resulting in 3D-OCT data sets which preserve retinal topography. Rapid repetitive imaging over small volumes can visualize small retinal features without motion induced distortions and enables volume registration to remove eye motion. Cone photoreceptors in some regions of the retina can be visualized without adaptive optics or active eye tracking. Rapid repetitive imaging of 3D volumes also provides dynamic volumetric information (4D-OCT) which is shown to enhance visualization of retinal capillaries and should enable functional imaging. Improvements in the speed and performance of 3D-OCT volumetric imaging promise to enable earlier diagnosis and improved monitoring of disease progression and response to therapy in ophthalmology, as well as have a wide range of research and clinical applications in other areas.

Ultrahigh speed spectral / Fourier domain OCT ophthalmic imaging at 70,000 to 312,500 axial scans per second

We present the first scanning laser ophthalmoscope that uses adaptive optics to measure and correct the high order aberrations of the human eye. Adaptive optics increases both lateral and axial resolution, permitting axial sectioning of retinal tissue in vivo. The instrument is used to visualize photoreceptors, nerve fibers and flow of white blood cells in retinal capillaries.

Adaptive optics scanning laser ophthalmoscopy

Wavefront sensor noise and fidelity place a fundamental limit on achievable image quality in current adaptive optics ophthalmoscopes. Additionally, the wavefront sensor ‘beacon’ can interfere with visual experiments. We demonstrate real-time (25 Hz), wavefront sensorless adaptive optics imaging in the living human eye with image quality rivaling that of wavefront sensor based control in the same system. A stochastic parallel gradient descent algorithm directly optimized the mean intensity in retinal image frames acquired with a confocal adaptive optics scanning laser ophthalmoscope (AOSLO). When imaging through natural, undilated pupils, both control methods resulted in comparable mean image intensities. However, when imaging through dilated pupils, image intensity was generally higher following wavefront sensor-based control. Despite the typically reduced intensity, image contrast was higher, on average, with sensorless control. Wavefront sensorless control is a viable option for imaging the living human eye and future refinements of this technique may result in even greater optical gains.

Wavefront sensorless adaptive optics ophthalmoscopy in the human eye

Glaucoma is a multifaceted group of eye diseases that results in the degeneration of retinal ganglion cell axons and the death of retinal ganglion cells (RGCs). The mechanisms of glaucomatous damage (e.g., mechanical, vascular, and glial) are not fully understood. Although several of these factors likely contribute to RGC death, substantial evidence suggests that the initial site of axonal injury likely occurs at the level of the lamina cribrosa in the optic nerve head (ONH).1–11 In humans and nonhuman primates, the lamina cribrosa is a three-dimensional porous structure consisting of flexible beams of collagenous tissue that support and nourish the RGC axons passing through it from the retina to the brain.12 Increases in intraocular pressure (IOP) impart stress and strain on the lamina,13–15 resulting in a posterior bowing and stretching of the load-bearing laminar beams12,16 and potential increases in laminar pore area and elongation (or more elliptically shaped pores). Stretching and deformation of the laminar beams (and associated pores) could sheer or damage encompassed axons and laminar capillaries, thereby hampering axonal transport, blood flow and the diffusion of nutrients,17 and/or the neurotrophic support provided to the axons by alterations in glial cells.18

ONH tissues have been examined in human glaucomatous eyes and nonhuman primate models of experimental glaucoma. Postmortem histologic studies have shown significant changes in prelaminar and laminar tissues within the ONH in early experimental glaucoma.16,19,20 Additional ex vivo studies in glaucomatous human and nonhuman primate eyes have described early changes in laminar morphology and position,16,21 laminar pore geometry,22 and the composition and architecture of laminar connective tissues.20,23 In light of this work, there is a growing consensus that in vivo studies are necessary to validate ex vivo results and examine longitudinal changes during glaucoma.24,25

The lamina cribrosa has been examined in vivo. Fontana et al.26 imaged anterior laminar pores in living human glaucomatous eyes using a modified confocal scanning laser ophthalmoscope (SLO) and showed that pore area and elongation were larger in eyes with more severe glaucoma. However, images were taken using a wide field of view (20°) and were not of high resolution because of the presence of ocular aberrations. Therefore, the size and total number of pores that could be resolved and the number of subjects in which laminar pores could be successfully imaged were limited. Vilupuru et al.27 used a confocal adaptive optics scanning laser ophthalmoscope (AOSLO), an instrument that noninvasively provides cellular-level images through a correction of the eye's aberrations, to improve the resolution and contrast of laminar images in vivo in nonhuman primates with experimental glaucoma. This study quantified laminar pore geometries in three diseased monkey eyes, each at a single time point. However, changes in pore geometry with disease were not investigated. Recent studies have combined adaptive optics with optical coherence tomography (OCT) to image the human lamina cribrosa in three spatial dimensions,28,29 but a quantitative analysis of laminar geometry and pore structure has not been reported.

We have assessed the repeatability of imaging and quantifying laminar pores in vivo in two normal macaque and three normal human eyes using an AOSLO at different time points. Statistical tests were performed to investigate whether significant differences were present in any analyzed laminar pore parameters within and across imaging sessions. This study provides an important first step in understanding the feasibility of using in vivo AOSLO imaging to assess laminar structure in normal eyes, human glaucomatous eyes, and nonhuman primate eyes with experimentally induced glaucoma.

/pdf/reproducibility-of-measuring-lamina-cribrosa-pore-geometry-1ds4hq405x.pdf

Reproducibility of measuring lamina cribrosa pore geometry in human and nonhuman primates with in vivo adaptive optics imaging.

Attenuation of short wavelengths alters sleep and the ipRGC pupil response

We present a direct slope-based correction algorithm to simultaneously control two deformable mirrors (DMs) in a woofer-tweeter adaptive optics system. A global response matrix was derived from the response matrices of each deformable mirror and the voltages for both deformable mirrors were calculated simultaneously. This control algorithm was tested and compared with a 2-step sequential control method in five normal human eyes using an adaptive optics scanning laser ophthalmoscope. The mean residual total root-mean-square (RMS) wavefront errors across subjects after adaptive optics (AO) correction were 0.128 ± 0.025 μm and 0.107 ± 0.033 μm for simultaneous and 2-step control, respectively (7.75-mm pupil). The mean intensity of reflectance images acquired after AO convergence was slightly higher for 2-step control. Radially-averaged power spectra calculated from registered reflectance images were nearly identical for all subjects using simultaneous or 2-step control. The correction performance of our new simultaneous dual DM control algorithm is comparable to 2-step control, but is more efficient. This method can be applied to any woofer-tweeter AO system.

Hope M Queener

Papers

Adaptive optics scanning laser ophthalmoscopy

Wavefront sensorless adaptive optics ophthalmoscopy in the human eye

Reproducibility of measuring lamina cribrosa pore geometry in human and nonhuman primates with in vivo adaptive optics imaging.

Attenuation of short wavelengths alters sleep and the ipRGC pupil response

A correction algorithm to simultaneously control dual deformable mirrors in a woofer-tweeter adaptive optics system