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.

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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

Merging of ultrahigh-resolution optical coherence tomography (UHR OCT) and adaptive optics (AO), resulting in high axial (3 microm) and improved transverse resolution (5-10 microm) is demonstrated for the first time to our knowledge in in vivo retinal imaging. A compact (300 mm x 300 mm) closed-loop AO system, based on a real-time Hartmann-Shack wave-front sensor operating at 30 Hz and a 37-actuator membrane deformable mirror, is interfaced to an UHR OCT system, based on a commercial OCT instrument, employing a compact Ti:sapphire laser with 130-nm bandwidth. Closed-loop correction of both ocular and system aberrations results in a residual uncorrected wave-front rms of 0.1 microm for a 3.68-mm pupil diameter. When this level of correction is achieved, OCT images are obtained under a static mirror configuration. By use of AO, an improvement of the transverse resolution of two to three times, compared with UHR OCT systems used so far, is obtained. A significant signal-to-noise ratio improvement of up to 9 dB in corrected compared with uncorrected OCT tomograms is also achieved.

Adaptive Optics Ultrahigh Resolution Optical Coherence Tomography

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

We report root-mean-square (RMS) wavefront error (WFE) for individual aberrations and cumulative highorder (HO) RMS WFE for the normal human eye as a function of age by decade and pupil diameter in 1 mm steps from 3 to 7 mm and determine the relationship among HO RMS WFE, mean age for each decade of life, and luminance for physiologic pupil diameters. Subjects included 146 healthy individuals from 20 to 80 years of age. Ocular aberration was measured on the preferred eye of each subject (for a total of 146 eyes through dilated pupils; computed for 3, 4, 5, 6, and 7 mm pupils; and described with a tenth-radial-order normalized Zernike expansion. We found that HO RMS WFE increases faster with increasing pupil diameter for any given age and pupil diameter than it does with increasing age alone. A planar function accounts for 99% of the variance in the 3-D space defined by mean log HO RMS WFE, mean age for each decade of life, and pupil diameter. When physiologic pupil diameters are used to estimate HO RMS WFE as a function of luminance and age, at low luminance 9c d/m 2 HO RMS WFE decreases with increasing age. This normative data set details (1) the 3-D relationship between HO RMS WFE and age for fixed pupil diameters and (2) the 3-D relationship among HO RMS WFE, age, and luminance for physiologic pupil diameters. © 2007 Optical Society of America OCIS codes: 170.4460, 330.5510.

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Three-dimensional relationship between high-order root-mean-square wavefront error, pupil diameter, and aging

PURPOSE: To develop single-valued wavefront aberration metrics that correlate strongly with visual performance The purpose of this study is to explore whether photopic high contrast visual acuity (VA) is an appropriate visual performance reference and whether mesopic and/or low contrast testing provides any advantage METHODS: Subjects from the Texas Investigation of Cataract Optics study (N = 148) ranged in age from 216 to 838 years and from clear lens to dense nuclear cataract Visual acuity was measured under four conditions: photopic high (VA P H C ) and low (VA P L C ) contrast, mesopic high (VA M H C ) and low (VA M L C ) contrast (photopic = 280 cd/m 2 , mesopic = 0 cd/m2) Variables were tested for compliance with normality (-200 < skew and kurtosis < 200) and transformed if required Linear regression and Bland-Altman 95% limits of agreement (′196 SD) were used to examine relationships between VA variables and between VA and wavefront aberration metrics RESULTS The two photopic measures VA P H C (skew = 257, kurtosis = 122) and VA P L C (167, 541) were poorly distributed, but the two mesopic measures VA M H C (088, 167) and VA M L C (029, -018) were normally distributed Strong correlations existed between the (transformed) VA variables (R: 053 to 084) However, limits of agreement ranged over 030 to 042 logMAR, whereas retest data suggested a range of 015 All four were offered in stepwise multiple linear regression for 30 wavefront metrics: 20 included VA M L C alone, two included VA P L C , two included VA M H C and six included both VA M L C and VA M H C ; R 2 averaged 25% CONCLUSION: Although strongly correlated, low contrast and/or mesopic VA testing provides different information Wave aberration metrics correlate better with VA M L C making this the visual performance test of choice

Measuring visual acuity--mesopic or photopic conditions, and high or low contrast letters?

A computer model that incorporates the monochromatic aberrations of the eye is used to determine the optimal pupil size for axial and lateral resolution as it applies to retinal imaging instruments such as the confocal scanning laser ophthalmoscope. The optimal pupil size for axial resolution, based on the aberrations of 15 subjects, is 4.30 mm±1.19 mm standard deviation (sd), which is larger than that for lateral resolution [2.46 mm±0.66 mm (sd)]. When small confocal pinholes are used, the maximum detected light is obtained with a pupil size of 4.90 mm±1.04 mm sd. It is recommended to use larger pupil sizes in imaging applications where axial resolution is desired.

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Optimal pupil size in the human eye for axial resolution

PURPOSE: Quantify and localize lenticular forward scatter using Shack-Hartmann wavefront sensing (SHWS) as single-valued metrics and a scatter map, and to examine the relationships between forward scatter and backscatter metrics and visual acuity. METHODS: We obtained SHWS images from 148 patients in the Texas Investigation of Cataract Optics study. Patient age was 22 to 84 years, with Lens Opacities Classification System III (LOCS III) nuclear opalescence (NO) scores ranging from 0.8 to 5.6. Visual acuities were measured at photopic (280 cd/m 2 ) high (VA P H C ) and low contrast (VA P L C ) and mesopic (0.75 cd/m 2 ) high (VA M H C ) and low contrast (VA M L C ). Scattering was described in a scatter map and by five single-valued metrics characterizing SHWS lenslet point spread functions. The relationships between scatter and visual acuity were tested using linear regression. RESULTS: Visual acuities decreased proportional to both LOCS III NO (R 2 =up to 39%) and scatter metrics (R 2 =up to 21%). Stepwise multiple linear regression improved visual acuity prediction by including a backscatter and a forward scatter metric (R 2 up to 51.2%). For the subjects over age 60 years (N=46, 68.8′6.12 years), the forward scatter metrics explain as much variance in visual acuities (R 2 =up to 29%) as LOCS III NO (R 2 =up to 26%). Combined they accounted for up to 48.8% of visual acuity variance. CONCLUSION: Forward light scatter can be quantified using SHWS and the resulting metrics explain significant variance in visual acuity, especially in the aging eye. Together with a backscatter metric they explain approximately 50% of the variance in VA.

William J. Donnelly

Papers

Adaptive optics scanning laser ophthalmoscopy

Three-dimensional relationship between high-order root-mean-square wavefront error, pupil diameter, and aging

Measuring visual acuity--mesopic or photopic conditions, and high or low contrast letters?

Optimal pupil size in the human eye for axial resolution

Quantifying scatter in Shack-Hartmann images to evaluate nuclear cataract