Download Adaptive Optics and the cone mosaic

Adaptive Optics and the
cone mosaic
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What can you use it for?
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What is it, what are its limitations
and where is it going?
Why is it interesting for color
vision science?
Early AO for imaging cone mosaic
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Adaptive optics was first envisioned by Horace W.
Babcock in 1953, but it did not come into common
usage until advances in computer technology (1990).
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Adaptive optics (AO) is a technology used to improve
the performance of optical systems by reducing the
effect of wavefront distortions. It is used in astronomical
telescopes and laser communication systems to remove
the effects of Earth’s atmospheric distortion.
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Adaptive optics works by measuring the distortions in a
wavefront and compensating for them with a device that
corrects those errors such as a deformable mirror or a
liquid crystal array.
Sodium beacons are created by using a laser specially tuned to 589.2
nanometers to energize a layer of sodium atoms which are naturally
present in the mesosphere at an altitude of around 90 kilometers.
This laser creates a wavefront reference source in order to correct
atmospheric distortion of light. The distortions detected by the reflected
light from this laser source from the mesophere is used to guide the
wavefront distortion of the deformable mirror.
Laser Guide Stars:
Adaptive optics (AO) systems require a wavefront reference source in order
to correct atmospheric distortion of light (called astronomical seeing).
Sufficiently bright stars are not available in all parts of the sky, which greatly
limits the usefulness of natural guide star adaptive optics. Instead, one can
create an artificial guide star by shining a laser into the atmosphere.
Because the laser beam is deflected by astronomical seeing on the way up,
the returning laser light does not move around in the sky as astronomical
sources do. In order to keep astronomical images steady, a natural star
nearby in the sky must be monitored in order that the motion of the laser
guide star can be subtracted using a tip–tilt mirror. However, this star can
be much fainter than is required for natural guide star adaptive optics,
because it is only used to measure tip and tilt and all higher order
distortions are measured with the laser guide star.
Sodium beacons are created by using a laser specially tuned to 589.2
nanometers to energize a layer of sodium atoms which are naturally
present in the mesosphere at an altitude of around 90 kilometers. The
sodium atoms then re-emit the laser light, producing a glowing artificial
star.
Schematic of an AO system
Schematic illustration of an adaptive optics (AO) system. Light is shown
with dotted lines, control connections with dashed lines. The wavefront
enters the AO system at the top. The light first hits a tip–tilt (TT) mirror
and is then directed to a deformable mirror (DM). The wavefront is
corrected and part of the light is tapped off by a beamsplitter (BS). The
residual errors (due to system latency, finite number of actuators/sensors
etc.) are measured by a wavefront sensor (Shack-Hartmann in this case)
and the control hardware then sends updated signals to the DM and the
TT mirror. The two filterwheels (FW1 and FW2) are used during
calibration only.
Atmospheric Chromatic Dispersion
-- Light coming through the atmosphere (and most other media) suffers dispersion,
where different wavelengths of light are refracted by different amounts.
-- Shorter wavelengths are refracted more, and longer wavelengths are refracted
less, so the image of an object which suffers a high refraction and associated
dispersion actually consists of a series of images, each of a different wavelength,
slightly shifted relative to each other, with the shorter wavelength images at higher
altitudes, and the longer wavelength images at lower altitudes.
When light hits the lens it is refracted, ie, it changes direction. As most
people probably know just by looking at the rainbow, visible light is
comprised of different wavelengths which are perceived as different
colors. What happens in this situation is that the different wavelengths are
bent differently by the lens, causing them to diverge (a phenomenon
known as dispersion) and hit different planes, as shown in the following
diagram:
The “adaptive” part
Measurements or pictures come out of the beam pointing to
science.
The “adaptive” part
Measurements or pictures come out of the beam pointing to
science.
Shack Hartmann wavefront sensor
Schematic illustration of a Shack Hartmann wavefront sensor using a socalled lenslet array. The perturbed wavefront coming in at the top (black
solid lines) is imaged by an array of small lenses. Each of these lenses
only samples a small portion of the full wavefront. The position of the
image created by each of these lenses on the camera depends on the
local slope of the wavefront (red dashed lines). Thus, by measuring this
image displacement Δx for all lenslets, the local wavefront slopes can be
obtained and the full wavefront can be reconstructed.
From Star maps or retinal mosaic maps:
From extraterrestrial exploration to
intercorporeal exploration
Early AO for imaging
cone mosaic (~1996)
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Main Problems: Eye motion & optical
imperfections making imaging individual cone cells
(typically 40–50 µm long; diameter between 0.5
to 4.0 µm, being smallest in the fovea. S cones
are larger than the M/L cones. (fyi: a human hair
width ranges 17-181 µm)
AO in retinal imaging
Distribution of cone cells in the fovea of a individual with normal
color vision (left), and a color blind (protanopic) retina. The center
of the fovea holds very few blue-sensitive cones.
AO in retinal imaging
~2.5 dg.s
Distribution of cone cells in the fovea of a individual with normal
color vision (left), and a color blind (protanopic) retina. The center
of the fovea holds very few blue-sensitive cones.
AO in retinal imaging
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~1996. D. Willams et al. Using a Shack Hartmann wavefront
sensor.
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In microscopy and retinal imaging systems it is used to reduce
optical aberrations.
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Ocular aberrations are distortions in the wavefront passing
through the pupil of the eye. These are the same aberrations
diminish the quality of the image formed on the retina,
sometimes necessitating the wearing of spectacles or contact
lenses.
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In the case of retinal imaging, light passing out of the eye carry
similar wavefront distortions, leading to an inability to resolve
the microscopic structure (cells and capillaries) of the retina.
AO in retinal imaging
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Spectacles and contact lenses correct "low-order aberrations",
such as defocus and astigmatism, which tend to be stable in
humans for long periods of time (months or years).
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While correction of these is sufficient for normal visual
functioning, it is generally insufficient to achieve microscopic
resolution.
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Additionally, "high-order aberrations", such as coma, spherical
aberration, and trefoil, must also be corrected in order to
achieve microscopic resolution. High-order aberrations, unlike
low-order, are not stable over time, and may change with
frequencies between 10 Hz and 100 Hz. The correction of these
aberrations requires continuous, high-frequency measurement
and compensation.
In optics (especially telescopes), the coma, or comatic aberration, in an
optical system refers to aberration inherent to certain optical designs or
due to imperfection in the lens or other components that results in offaxis point sources such as stars appearing distorted, appearing to have
a tail (coma) like a comet.
Coma is common in patients with decentred corneal grafts, keratoconus,
and decentred laser ablations.
Trefoil produces less degradation in image quality compared with coma
of similar RMS magnitude.[6]
Taxonomy of Optical abberations
Plots of Zernike polynomials in the unit disk
In optics (especially telescopes), the coma, or comatic aberration, in an
optical system refers to aberration inherent to certain optical designs or
due to imperfection in the lens or other components that results in offaxis point sources such as stars appearing distorted, appearing to have
a tail (coma) like a comet.
Coma is common in patients with decentred corneal grafts, keratoconus,
and decentred laser ablations.
Trefoil produces less degradation in image quality compared with coma
of similar RMS magnitude.[6]
AOSLO & AO-OCT
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Adaptive Optics Scanning Laser Ophthalmoscopy (AOSLO)
- AO flood illumination and scanning laser ophthalmoscopy
(SLO) have been used to study many properties of the
cones, such as: arrangement and packing in normal and
defective retinas; changes in reflectance over time; and
sampling of the ocular image.
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AOSLO handles eye movement way better than gaze
tracking tech.
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Uses real-time retinal image motion signals in combination
with high speed modulation of a scanning laser. Show in
good fixation stability Subjects to yield stimulus location
accuracy averaged 0.26 arcminutes or approximately 1.3
microns, which is smaller than the cone-to-cone spacing at
the fovea (2007. Arathorn, Yang, Vogel, Zhang,
Tiruveedhula & Roorda.).
The next generation of AO is AOSLO & AO-OCT. These two new methods aim
to address the original problems Williams and colleagues faced in early retinal
AO.
AOSLO & AO-OCT
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AO was first attempted for SLO in the 1980s. This first
attempt did not use wavefront-detecting technology
with a deformable mirror but instead estimated
aberrations through pre-measured factors such as
astigmatism (not ideal).
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Now AOSLO the Shack-Hartmann wave-front detector
for the apparatus produced images of the retina with
much higher lateral resolution.The addition of
microelectricalmechanical (MEMs) mirrors instead of
larger, more expensive mirror deformable mirror
systems to the apparatus made AOSLO further usable
for a wider range of studies and for use in patients.[9]
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AOSLO & AO-OCT
Scanning laser SLO utilizes horizontal and vertical scanning
mirrors to scan a specific region of the retina and create raster
images viewable on a television monitor. While it is able to
image the retina in real time, it has issues with reflections from
eye astigmatism and the cornea. Eye movements additionally
can confound the data from SLO.
“Optomap” -- your eyedoc.
The next generation of AO is AOSLO & AO-OCT. These two new methods aim to address the original
problems Williams and colleagues faced in early retinal AO.
Once the subject is properly placed, wavefront correction and imaging takes place. A laser is
collimated and then reflected off of a beam-splitting mirror. As in confocal SLO, light must pass
through both a horizontal and a vertical scanning mirror before and after the eye is scanned to
align the moving beam for eventual retinal faster images of the retina. Additionally, the light is
reflected off of a deformable mirror before and after exposure to the eye to diffuse optical
aberrations. The laser enters the eye through the pupil to illuminate the region it has been
focused onto and light reflected back leaves the same way. Light returning from the mirrors
passes through the first beam splitter onto another beam splitter where it is directed
simultaneously toward a photomultiplier tube (PMT) and toward a Shark-Hartmann wavefront
sensor array. The light going toward the photomultiplier is focused through a confocal pinhole
to remove light not reflecting off of the plane of interest and then recorded in the PMT. Light
directed to the wavefront sensor array is split up by the lenslets in the array and then recorded
onto a Charge-coupled device (CCD) camera for detection of optical aberrations. These
aberrations are then subtracted from the images recorded in the PMT to vastly increase lateral
resolution.[3][5][8][10]
AOSLO & AO-OCT
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S
When Longitudinal Chromatic Abberation is corrected, retinal
images for all wavelengths (infrared, red, green) are in focus but
offset laterally due to TCA.
Chromatic dispersion in AOSLO imaging. (A) When Longitudinal Chromatic Aberation LCA is corrected, retinal
images
for all wavelengths (infrared, red, green) are in focus but offset laterally due to Transverse Chromatic Aberation
TCA. The
imaged area of the retina is outlined on the fundus photograph (top). TCA offsets between
colors are highlighted with example cone outlines. Dark regions in the AOSLO
microphotographs are caused by blood capillary shadows. Cone images are constructed by
referenced averaging of ~150 individual video frames, and are individually normalized for
display purposes. Scale bar: 2 deg (top), 2 arcmin (bottom). (B) Schematic explanation of
chromatic offsets in AOSLO imaging. Two aligned input beams (red and green) reach a
dispersing lens and land at different locations on the retina. The reflected light, passing back
through the lens, is realigned into one beam and is captured by two imaging devices. Although
the effects of TCA are cancelled on the second pass through the lens and the outward beams
are re-aligned, one can determine how much dispersion occurred between the two beams,
because the retina being imaged has spatial structure. This principle holds for our three
wavelength AOSLO setup.
AOSLO & AO-OCT
Two aligned input beams (red and
green) reach a dispersing lens and
land at different locations on the
retina.
The reflected light, passing back
through the lens, is realigned into
one beam and is captured by two
imaging devices.
Although the effects of TCA are
cancelled on the second pass
through the lens and the outward
beams are re-aligned, one can
determine how much dispersion
occurred between the two beams,
because the retina being imaged
has spatial structure. This principle
holds for a 3 wavelength AOSLO
setup.
AOSLO
Harmening, Tiruveedhula, Roorda, Sincich 2012. BioMed.Opt.Ex.
AOSLO & AO-OCT
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Adaptive Optics - Optical Coherence Tomography (AO-OCT)
(~2003)
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The combination of AO with optical coherence tomography
(OCT) has permitted cones to be resolved in depth as well,
facilitating study of cones’ three dimensional structure,
waveguiding properties, polarization behavior, and
volumetric changes over time.
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The technical benefits of adding AO to OCT (increased
lateral resolution, smaller speckle, and enhanced sensitivity)
increase the imaging capability of OCT in ways that make it
well suited for three-dimensional (3D) cellular imaging in the
retina. AO-OCT systems provide ultrahigh 3D resolution (3 ×
3 × 3 µm³) and ultrahigh speed (up to an order of magnitude
faster than commercial OCT). AO-OCT systems have been
used to capture volume images of retinal structures,
previously only visible with histology, and are being used for
studying clinical conditions.
emphasis is structural imaging
AOSLO & AO-OCT
AO-OCT
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To make an OCT scan of the retina, a beam of light is split: one
portion is scattered off the retina while the other is the reference
beam. The beams are interfered, and the resulting phase information
used to procure a precise measurement of a sample’s position.
(dealing with in vivo moving eyes). Instead of measuring the phase of
a single interference pattern, it measures phase differences between
patterns originating from two reference points within the retinal cells:
the top and bottom of the outer segment.
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E.g., Using this hidden phase information Ravi et al (2012) measured
microscopic changes in hundreds of cones, over a matter of hours, in
two test subjects with normal vision. They found they could resolve
the changes in length down to about 45 nm size, which is just slightly
longer than the thickness of a single one of the stacked discs that
make up the outer segment.
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FYI: Their work shows that the outer segments of the cone cells grow
at a rate of about 150 nm per hour. (Ravi S. Jonnal et al., Biomedical
Optics Express, Vol. 3, Issue 1, pp. 104-124 (2012)).
An interference filter or dichroic filter is an optical filter that reflects
one or more spectral bands or lines and transmits others, while
maintaining a nearly zero coefficient of absorption for all wavelengths of
interest. An interference filter may be high-pass, low-pass, bandpass, or
band-rejection.
Review: using AO Hofer et al. (2005) JOV.
Highly Variable Mosaicism in
Human Retinas
M-Rich
L-Rich
Example of a retinal sensitivity map and retinal stimulus profile used to model the
microstimulation of the mosaic.
Flashes were presented
without aberration correction
to a single location at
1.25 deg retinal eccentricity.
Hofer H et al. J Vis 2005;5:5
©2005 by Association for Research in Vision and Ophthalmology
Figure 5. Example of a retinal sensitivity map and retinal stimulus
profile used to model the microstimulation of the mosaic. L and
M cones have been colored red and green to aid in their identification.
The full width at half maximum of the retinal profile of the
spot imaged with adaptive optics is smaller than the radius of
individual cone inner segments near 1 deg.
Hofer et al. 2005.
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They achieved AO corrected cone size (a major feat), but
uncontrolled eye motion made it impossible to do repeat
assessment of stimulated cones, AND the “aim” of the
stimulation was off because of chromatic abberation.
Figure 4. The color sensations reported by subjects when viewing
a small spot of 550-nm light. At this wavelength only L and M
cones participate in detection. Shown are the percentages of
white and colored responses that were placed in each response
category, interpolated at 50% frequency of seeing. Percentages
for BS are white, 56%; red, 42%; yellow-green, 0.7%; green,
0.7%; blue-green, 0.3%; and blue, 0.8%. In addition to white,
each subject used at least five different hue categories.
Measuring Color Vision on a Cellular Scale in an Adaptive
Optics Scanning Laser Ophthalmoscope
Sabesan, Tuten, Harmening, Carney, Klein, & Austin Roorda.
Optometry, University of California, Berkeley
Imaging and Applied Optics © OSA 2013
• They fix problems Hofer et al. and Brainard et al. encountered.
• Problems? Special challenges arise when pursuing multiwavelength imaging of retinal tissue in vivo, because the eye’s
optics must be used as the main focusing elements, and they
introduce significant chromatic dispersion.
• Using sophisticated high-speed retinal tracking, Roorda and
colleagues present an image-based method to measure and
correct for the eye’s transverse chromatic aberrations rapidly,
non-invasively, and with high precision. This method also
addresses ubiquitous eye movements.
Measuring Color Vision on a Cellular Scale in an Adaptive
Optics Scanning Laser Ophthalmoscope
Sabesan, Tuten, Harmening, Carney, Klein, & Austin Roorda.
Optometry, University of California, Berkeley
Imaging and Applied Optics © OSA 2013
• They validate the technique against hyperacute
psychophysical performance and the standard chromatic
human eye model.
• In vivo correction of chromatic dispersion enables confocal
multi-wavelength images of the living retina to be aligned, and
allows targeted chromatic stimulation of the photoreceptor
mosaic to be performed accurately with sub-cellular resolution.
Circular stimuli at green 543 nm wavelength and measuring 0.6
arc-min in diameter were delivered to 3 retinal locations at 4 deg
eccentricity and subjects were asked to report their color
percepts.
Figure 1: Single cone color naming. A) Circular stimuli at
green 543 nm wavelength and measuring 0.6 arc-min in
diameter were delivered to 3 retinal locations at 4 deg
eccentricity and subjects were asked to report their color
percepts. 30 trials per location were tested. The color of each
spot corresponds to the color percepts reported by the
subject, while its x,y coordinate represents the relative
location where it was delivered on the
retina. Note that the shown location does not correspond to
the actual cones that were stimulated in the experiment. B)
Histograms show the number of responses reported for each
color at the 3 locations.
Results: High-speed retinal tracking allowed repeatable targeted stimulation of the
three retinal locations. Mean stimulus delivery errors of 0.5 arc-min - within the size of a
single cone photoreceptor at 4 deg eccentricity.
Repeatable color percepts arising from the same retinal location were measured
psychophysically as shown in figure 1. The color of each spot corresponds to the color
percepts reported by the subject, while its x,y coordinate represents the relative location
where it was delivered on the retina. The histograms show the number of responses
reported for each color at the 3 locations. Mean purity of color percepts, calculated as
the percentage named either red or green, was 80 ± 3.33 %.
Future stuff AO may look at:
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Can we verify whether heterozygous opsin
genotypes give rise to retinas with 4 distinct
populations of cones?
Observe Four expressed cone types?
‘simulated’ 4-cone human retina Graphic from M. Neitz and J. Neitz (1998)
A remaining AO challenge? differentiating several
different M- & L-photopigment variants:
L-alanine
552nm
Figure from M. Neitz and J. Neitz (1998)
L-serine
557nm
Future stuff AO may look at:
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ala Brainard et al. (2008): does a single cone’s
percept also depend on it’s neighborhood
network (or the types of cones in the local
surround)?
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Waveguiding properties of the different cells
and pigments -- are they uniform?
Future stuff AO may look at:
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Test the idea that highly biased L/M cone ratios
in female CVD carriers, or in actual color
deficients, tracks chromatic discrimination
abilities.
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Ask if at isoluminance, relatively equal numbers
of L & M cones optimize detection of any
chromatic contrast.
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Ask, if cone ratios biased are does this favor
detection in one chromatic direction only -and therefore impair color discrimination in
other directions?