Cone-Opponent Ganglion Cells in the Primate Fovea Tuned to Noncardinal Color Directions

A long-standing question in vision science is how the three cone photoreceptor types-long (L), medium (M), and short (S) wavelength sensitive-combine to generate our perception of color. Hue perception can be described along two opponent axes: red-green and blue-yellow. Psychophysical measurements of color appearance indicate that the cone inputs to the red-green and blue-yellow opponent axes are M vs. L + S and L vs. M + S, respectively. However, the "cardinal directions of color space" revealed by psychophysical measurements of color detection thresholds following adaptation are L vs. M and S vs. L + M. These cardinal directions match the most common cone-opponent retinal ganglion cells (RGCs) in the primate retina. Accordingly, the cone opponency necessary for color appearance is thought to be established in the cortex. While neurons with the appropriate M vs. L + S and L vs. M + S opponency have been reported in the retina and lateral geniculate nucleus, their existence continues to be debated. Resolving this long-standing debate is necessary because a complete account of the cone opponency in the retinal output is critical for understanding how downstream neural circuits process color. Here, we performed adaptive optics calcium imaging to noninvasively measure foveal RGC light responses in the living Macaca fascicularis eye. We confirm the presence of L vs. M + S and M vs. L + S neurons with noncardinal cone opponency and demonstrate that cone-opponent signals in the retinal output are more diverse than classically thought.

Cone-Opponent Ganglion Cells in the Primate Fovea Tuned to Non-Cardinal Color Directions

A long-standing question in vision science is how the three cone photoreceptor types - long (L), medium (M) and short (S) wavelength sensitive - combine to generate our perception of color. Hue perception can be described along two opponent axes: red-green and blue-yellow. Psychophysical measurements of color appearance indicate that the cone inputs to the red-green and blue-yellow opponent axes are M vs. L+S and L vs. M+S, respectively. However, the "cardinal directions of color space" revealed by psychophysical measurements of color detection thresholds are L vs. M and S vs. L+M. The cardinal directions match the most common cone-opponent retinal ganglion cells (RGCs) in the primate retina. Accordingly, the cone opponency necessary for color appearance is thought to be established in cortex. However, small populations with the appropriate M vs. L+S and L vs. M+S cone-opponency have been reported in large surveys of cone inputs to primate RGCs and their projections to the lateral geniculate nucleus (LGN) yet their existence continues to be debated. Resolving this long-standing open question is needed as a complete account of the cone-opponency in the retinal output is critical for efforts to understand how downstream neural circuits process color. Here, we performed adaptive optics calcium imaging to longitudinally and noninvasively measurements of the foveal RGC light responses in the living macaque eye. We confirm the presence of L vs. M+S and M vs. L+S neurons with non-cardinal cone-opponency and demonstrate that cone-opponent signals in the retinal output are substantially more diverse than classically thought.

Contributed Session III: In vivo calcium imaging of macaque foveolar retinal ganglion cells reveals spatiochromatic receptive field properties

Here, we optically record responses to spatial and chromatic stimuli using a calcium indicator in the living macaque eye to characterize the receptive field (RF) properties of retinal ganglion cells (RGCs) serving the foveal center. GCaMP6s was expressed in three female macaques. Adaptive optics ophthalmoscopy was used to image fluorescence (488nm ex, 520/35nm em) from RGCs whose RF centers were driven by cones in the central 36 arcmin of the fovea and additional RGCs driven by cones in the central 6 arcmin of the foveola. Using cone isolating and luminance flicker (1.3deg, 0.15Hz, LED 420nm, 530nm, 660nm), we derived cone weights in over 250 RGCs. Using drifting gratings (1.9deg, 6Hz, 4-50c/deg, 561nm), we derived the spatial frequency responses of 15 L vs. M cone opponent RGCs at the foveolar center. Employing computational modeling (ISETbio), we inferred the full spatial difference of gaussians center and surround structure for those 15 cells. Of the 34 foveolar RGCs, 44% exhibited L vs. M cone opponency, 15% were L+M ON, 6% were -L-M OFF, and 35% showed only L or only M responses. The spatial frequency response functions of 12/15 L vs. M opponent cells peaked at high spatial frequencies (25-40c/deg) and had a strong bandpass characteristic. Our model indicates that the responses of all 15 L vs. M opponent cells are consistent with single cone input to their RF centers and that our data are consistent with extrafoveal data when the blurring of the optics is accounted for.

Optogenetic therapy restores retinal activity in primate for at least a year following photoreceptor ablation

All retina-based vision restoration approaches rely on the assumption that photoreceptor loss does not preclude reactivation of the remaining retinal architecture. Whether extended periods of vision loss limit the efficacy of restorative therapies at the retinal level is unknown. We examined long-term changes in optogenetic responsivity of foveal retinal ganglion cells (RGCs) in non-human primates following localized photoreceptor ablation by high-intensity laser exposure. By performing fluorescence adaptive optics scanning light ophthalmoscopy (AOSLO) of RGCs expressing both the calcium indicator GCaMP6s and the optogenetic actuator ChrimsonR, it was possible to track optogenetic-mediated calcium responses in deafferented RGCs over time. Fluorescence fundus photography revealed a 40% reduction in ChrimsonR fluorescence from RGCs lacking photoreceptor input over the 3 weeks following photoreceptor ablation. Despite this, in vivo imaging revealed good cellular preservation of RGCs 3 months after the loss of photoreceptor input, and histology confirmed good structural preservation at 2 years. Optogenetic responses of RGCs in primate persisted for at least 1 year after the loss of photoreceptor input, with a sensitivity index similar to optogenetic responses recorded in intact retina. These results are promising for all potential therapeutic approaches to vision restoration that rely on preservation and reactivation of RGCs.

Imaging Transplanted Photoreceptors in Living Nonhuman Primates with Single-Cell Resolution

Stem cell-based transplantation therapies offer hope for currently untreatable retinal degenerations; however, preclinical progress has been largely confined to rodent models. Here, we describe an experimental platform for accelerating photoreceptor replacement therapy in the nonhuman primate, which has a visual system much more similar to the human. We deployed fluorescence adaptive optics scanning light ophthalmoscopy (FAOSLO) to noninvasively track transplanted photoreceptor precursors over time at cellular resolution in the living macaque. Fluorescently labeled photoreceptors generated from a CRX+/tdTomato human embryonic stem cell (hESC) reporter line were delivered subretinally to macaques with normal retinas and following selective ablation of host photoreceptors using an ultrafast laser. The fluorescent reporter together with FAOSLO allowed transplanted photoreceptor precursor survival, migration, and neurite formation to be monitored over time in vivo. Histological examination suggested migration of photoreceptor precursors to the outer plexiform layer and potential synapse formation in ablated areas in the macaque eye.

Localized Photoreceptor Ablation Using Femtosecond Pulses Focused With Adaptive Optics

Purpose

The development of new approaches to human vision restoration could be greatly accelerated with the use of nonhuman primate models; however, there is a paucity of primate models of outer retina degeneration with good spatial localization. To limit ablation to the photoreceptors, we developed a new approach that uses a near-infrared ultrafast laser, focused using adaptive optics, to concentrate light in a small focal volume within the retina.

Methods

In the eyes of eight anesthetized macaques, 187 locations were exposed to laser powers from 50 to 210 mW. Laser exposure locations were monitored for up to 18 months using fluorescein angiography (FA), optical coherence tomography (OCT), scanning laser ophthalmoscopy (SLO), adaptive optics scanning laser ophthalmoscope (AOSLO) reflectance imaging, two-photon excited fluorescence (TPEF) ophthalmoscopy, histology, and calcium responses of retinal ganglion cells.

Results

This method produced localized photoreceptor loss with minimal axial spread of damage to other retinal layers, verified by in-vivo structural imaging and histologic examination, although in some cases evidence of altered autofluorescence was found in the adjacent retinal pigment epithelium (RPE). Functional assessment using blood flow imaging of the retinal plexus and calcium imaging of the response of ganglion cells above the photoreceptor loss shows that inner retinal circuitry was preserved.

Conclusions

Although different from a genetic model of retinal degeneration, this model of localized photoreceptor loss may provide a useful testbed for vision restoration studies in nonhuman primates.

Translational Relevance

With this model, a variety of vision restoration methods can be tested in the non-human primate.

In vivo-directed evolution of adeno-associated virus in the primate retina

Efficient adeno-associated virus-mediated (AAV-mediated) gene delivery remains a significant obstacle to effective retinal gene therapies. Here, we apply directed evolution - guided by deep sequencing and followed by direct in vivo secondary selection of high-performing vectors with a GFP-barcoded library - to create AAV viral capsids with the capability to deliver genes to the outer retina in primates. A replication-incompetent library, produced via providing rep in trans, was created to mitigate risk of AAV propagation. Six rounds of in vivo selection with this library in primates - involving intravitreal library administration, recovery of genomes from outer retina, and extensive next-generation sequencing of each round - resulted in vectors with redirected tropism to the outer retina and increased gene delivery efficiency to retinal cells. These viral vectors expand the toolbox of vectors available for primate retina, and they may enable less invasive delivery of therapeutic genes to patients, potentially offering retina-wide infection at a similar dosage to vectors currently in clinical use.

Optogenetic restoration of retinal ganglion cell activity in the living primate

Optogenetic therapies for vision restoration aim to confer intrinsic light sensitivity to retinal ganglion cells when photoreceptors have degenerated and light sensitivity has been irreversibly lost. We combine adaptive optics ophthalmoscopy with calcium imaging to optically record optogenetically restored retinal ganglion cell activity in the fovea of the living primate. Recording from the intact eye of a living animal, we compare the patterns of activity evoked by the optogenetic actuator ChrimsonR with natural photoreceptor mediated stimulation in the same retinal ganglion cells. Optogenetic responses are recorded more than one year following administration of the therapy and two weeks after acute loss of photoreceptor input in the living animal. This in vivo imaging approach could be paired with any therapy to minimize the number of primates required to evaluate restored activity on the retinal level, while maximizing translational benefit by using an appropriate pre-clinical model of the human visual system.

Non-human primate models are important for the development of high quality vision restoration therapies for blindness. Here, the authors demonstrate restoration of light responses in foveal retinal ganglion cells of the living macaque following optogenetic gene therapy.

Imaging Retinal Activity in the Living Eye

Retinal function has long been studied with psychophysical methods in humans, whereas detailed functional studies of vision have been conducted mostly in animals owing to the invasive nature of physiological approaches. There are exceptions to this generalization, for example, the electroretinogram. This review examines exciting recent advances using in vivo retinal imaging to understand the function of retinal neurons. In some cases, the methods have existed for years and are still being optimized. In others, new methods such as optophysiology are revealing novel patterns of retinal function in animal models that have the potential to change our understanding of the functional capacity of the retina. Together, the advances in retinal imaging mark an important milestone that shifts attention away from anatomy alone and begins to probe the function of healthy and diseased eyes.

Cellular-scale evaluation of induced photoreceptor degeneration in the living primate eye

Progress is needed in developing animal models of photoreceptor degeneration and evaluating such models with longitudinal, noninvasive techniques. We employ confocal scanning laser ophthalmoscopy, optical coherence tomography (OCT) and high-resolution retinal imaging to noninvasively observe the retina of non-human primates with induced photoreceptor degeneration. Photoreceptors were imaged at the single-cell scale in three modalities of adaptive optics scanning light ophthalmoscopy: traditional confocal reflectance, indicative of waveguiding; a non-confocal offset aperture technique visualizing scattered light; and two-photon excited fluorescence, the time-varying signal of which, at 730 nm excitation, is representative of visual cycle function. Assessment of photoreceptor structure and function using these imaging modalities revealed a reduction in retinoid production in cone photoreceptor outer segments while inner segments appeared to remain present. Histology of one retina confirmed loss of outer segments and the presence of intact inner segments. This unique combination of imaging modalities can provide essential, clinically-relevant information on both the structural integrity and function of photoreceptors to not only validate models of photoreceptor degeneration but potentially evaluate the efficacy of future cell and gene-based therapies for vision restoration.

Functional architecture of the foveola revealed in the living primate

The primate foveola, with its high cone density and magnified cortical representation, is exquisitely specialized for high-resolution spatial vision. However, uncovering the wiring of retinal circuitry responsible for this performance has been challenging due to the difficulty in recording receptive fields of foveal retinal ganglion cells (RGCs) in vivo. In this study, we use adaptive optics scanning laser ophthalmoscopy (AOSLO) to image the calcium responses of RGCs in the living primate, with a stable, high precision visual stimulus that allowed us to localize the receptive fields of hundreds of foveal ganglion cells. This approach revealed a precisely radial organization of foveal RGCs, despite the many distortions possible during the extended developmental migration of foveal cells. By back projecting the line connecting RGC somas to their receptive fields, we have been able to define the ‘physiological center’ of the foveola, locating the vertical meridian separating left and right hemifields in vivo.

All-optical recording and stimulation of retinal neurons in vivo in retinal degeneration mice

Here we demonstrate the application of a method that could accelerate the development of novel therapies by allowing direct and repeatable visualization of cellular function in the living eye, to study loss of vision in animal models of retinal disease, as well as evaluate the time course of retinal function following therapeutic intervention. We use high-resolution adaptive optics scanning light ophthalmoscopy to image fluorescence from the calcium sensor GCaMP6s. In mice with photoreceptor degeneration (rd10), we measured restored visual responses in ganglion cell layer neurons expressing the red-shifted channelrhodopsin ChrimsonR over a six-week period following significant loss of visual responses. Combining a fluorescent calcium sensor, a channelrhodopsin, and adaptive optics enables all-optical stimulation and recording of retinal neurons in the living eye. Because the retina is an accessible portal to the central nervous system, our method also provides a novel non-invasive method of dissecting neuronal processing in the brain.

In Vivo Functional Imaging of Retinal Neurons Using Red and Green Fluorescent Calcium Indicators

Adaptive optics retinal imaging of fluorescent calcium indicators is a minimally invasive method used to study retinal physiology over extended periods of time. It has potential for discovering novel retinal circuits, tracking retinal function in animal models of retinal disease, and assessing vision restoration therapy. We previously demonstrated functional adaptive optics imaging of retinal neurons in the living eye using green fluorescent calcium indicators; however, the use of green fluorescent indicators presents challenges that stem from the fact that they are excited by short-wavelength light. Using red fluorescent calcium indicators such as jRGECO1a, which is excited with longer-wavelength light (~560 nm), makes imaging approximately five times safer than using short-wavelength light (~500 nm) used to excite green fluorescent calcium indicators such as GCaMP6s. Red fluorescent indicators also provide alternative wavelength imaging regimes to overcome cross talk with the sensitivities of intrinsic photoreceptors and blue light-activated channelrhodopsins. Here we evaluate jRGECO1a for in vivo functional adaptive optics imaging of retinal neurons using single-photon excitation in mice. We find that jRGECO1a provides similar fidelity as the established green indicator GCaMP6s.

Vision science and adaptive optics, the state of the field

Adaptive optics is a relatively new field, yet it is spreading rapidly and allows new questions to be asked about how the visual system is organized. The editors of this feature issue have posed a series of question to scientists involved in using adaptive optics in vision science. The questions are focused on three main areas. In the first we investigate the use of adaptive optics for psychophysical measurements of visual system function and for improving the optics of the eye. In the second, we look at the applications and impact of adaptive optics on retinal imaging and its promise for basic and applied research. In the third, we explore how adaptive optics is being used to improve our understanding of the neurophysiology of the visual system.

Calibration-free sinusoidal rectification and uniform retinal irradiance in scanning light ophthalmoscopy

Sinusoidal rectification (i.e., desinusoiding) is necessary for scanning imaging systems and is typically achieved by calculating a rectification transform from a calibration image such as a regular grid. This approach is susceptible to error due to electronic or mechanical instability that can alter the phase of the imaging window with respect to the calibration transform. Here, we show a calibration-free rectification method implemented from live video of a scanning light ophthalmoscope (SLO) with or without adaptive optics (AO). This approach, which capitalizes on positional differences in the images obtained in the forward and backward scan directions, dynamically keeps the imaging window in phase with the motion of the sinusoidal resonant scanner, preventing errors from signal drift over time. A benefit of this approach is that it allows the light power across the field-of-view (FOV) to be modulated inversely to achieve uniform irradiance on the retina, a feature desirable for functional imaging methods and light safety in SLOs.

Antibody neutralization poses a barrier to intravitreal adeno-associated viral vector gene delivery to non-human primates

Gene delivery vectors based on adeno-associated viruses (AAV) have exhibited promise in both preclinical disease models and human clinical trials for numerous disease targets, including the retinal degenerative disorders Leber's congenital amaurosis and choroideremia. One general challenge for AAV is that preexisting immunity, as well as subsequent development of immunity following vector administration, can severely inhibit systemic AAV vector gene delivery. However, the role of neutralizing antibodies (NABs) in AAV transduction of tissues considered to be immune privileged, such as the eye, is unclear in large animals. Intravitreal AAV administration allows for broad retinal delivery, but is more susceptible to interactions with the immune system than subretinal administration. To assess the effects of systemic anti-AAV antibody levels on intravitreal gene delivery, we quantified the anti-AAV antibodies present in sera from non-human primates before and after intravitreal injections with various AAV capsids. Analysis showed that intravitreal administration resulted in an increase in anti-AAV antibodies regardless of the capsid serotype, transgene or dosage of virus injected. For monkeys injected with wild-type AAV2 and/or an AAV2 mutant, the variable that most significantly affected the production of anti-AAV2 antibodies was the amount of virus delivered. In addition, post-injection antibody titers were highest against the serotype administered, but the antibodies were also cross-reactive against other AAV serotypes. Furthermore, NAB levels in serum correlated with those in vitreal fluid, demonstrating both that this route of administration exposes AAV capsid epitopes to the adaptive immune system and that serum measurements are predictive of vitreous fluid NAB titers. Moreover, the presence of preexisting NAB titers in the serum of monkeys correlated strongly (R=0.76) with weak, decaying or no transgene expression following intravitreal administration of AAV. Investigating anti-AAV antibody development will aid in understanding the interactions between gene therapy vectors and the immune system during ocular administration and can form a basis for future clinical studies applying intravitreal gene delivery.

In vivo-directed evolution of a new adeno-associated virus for therapeutic outer retinal gene delivery from the vitreous

Inherited retinal degenerative diseases are a clinically promising focus of adeno-associated virus (AAV)-mediated gene therapy. These diseases arise from pathogenic mutations in mRNA transcripts expressed in the eye's photoreceptor cells or retinal pigment epithelium (RPE), leading to cell death and structural deterioration. Because current gene delivery methods require an injurious subretinal injection to reach the photoreceptors or RPE and transduce just a fraction of the retina, they are suitable only for the treatment of rare degenerative diseases in which retinal structures remain intact. To address the need for broadly applicable gene delivery approaches, we implemented in vivo-directed evolution to engineer AAV variants that deliver the gene cargo to the outer retina after injection into the eye's easily accessible vitreous humor. This approach has general implications for situations in which dense tissue penetration poses a barrier for gene delivery. A resulting AAV variant mediated widespread delivery to the outer retina and rescued the disease phenotypes of X-linked retinoschisis and Leber's congenital amaurosis in corresponding mouse models. Furthermore, it enabled transduction of primate photoreceptors from the vitreous, expanding its therapeutic promise.

Imaging light responses of retinal ganglion cells in the living mouse eye

This study reports development of a novel method for high-resolution in vivo imaging of the function of individual mouse retinal ganglion cells (RGCs) that overcomes many limitations of available methods for recording RGC physiology. The technique combines insertion of a genetically encoded calcium indicator into RGCs with imaging of calcium responses over many days with FACILE (functional adaptive optics cellular imaging in the living eye). FACILE extends the most common method for RGC physiology, in vitro physiology, by allowing repeated imaging of the function of each cell over many sessions and by avoiding damage to the retina during removal from the eye. This makes it possible to track changes in the response of individual cells during morphological development or degeneration. FACILE also overcomes limitations of existing in vivo imaging methods, providing fine spatial and temporal detail, structure-function comparison, and simultaneous analysis of multiple cells.

In vivo two-photon imaging of the mouse retina

Though in vivo two-photon imaging has been demonstrated in non-human primates, improvements in the signal-to-noise ratio (SNR) would greatly improve its scientific utility. In this study, extrinsic fluorophores, expressed in otherwise transparent retinal ganglion cells, were imaged in the living mouse eye using a two-photon fluorescence adaptive optics scanning laser ophthalmoscope. We recorded two orders of magnitude greater signal levels from extrinsically labeled cells relative to previous work done in two-photon autofluorescence imaging of primates. Features as small as single dendrites in various layers of the retina could be resolved and predictions are made about the feasibility of measuring functional response from cells. In the future, two-photon imaging in the intact eye may allow us to monitor the function of retinal cell classes with infrared light that minimally excites the visual response.

Adaptive optics retinal imaging in the living mouse eye

Correction of the eye's monochromatic aberrations using adaptive optics (AO) can improve the resolution of in vivo mouse retinal images [Biss et al., Opt. Lett. 32(6), 659 (2007) and Alt et al., Proc. SPIE 7550, 755019 (2010)], but previous attempts have been limited by poor spot quality in the Shack-Hartmann wavefront sensor (SHWS). Recent advances in mouse eye wavefront sensing using an adjustable focus beacon with an annular beam profile have improved the wavefront sensor spot quality [Geng et al., Biomed. Opt. Express 2(4), 717 (2011)], and we have incorporated them into a fluorescence adaptive optics scanning laser ophthalmoscope (AOSLO). The performance of the instrument was tested on the living mouse eye, and images of multiple retinal structures, including the photoreceptor mosaic, nerve fiber bundles, fine capillaries and fluorescently labeled ganglion cells were obtained. The in vivo transverse and axial resolutions of the fluorescence channel of the AOSLO were estimated from the full width half maximum (FWHM) of the line and point spread functions (LSF and PSF), and were found to be better than 0.79 μm ± 0.03 μm (STD)(45% wider than the diffraction limit) and 10.8 μm ± 0.7 μm (STD)(two times the diffraction limit), respectively. The axial positional accuracy was estimated to be 0.36 μm. This resolution and positional accuracy has allowed us to classify many ganglion cell types, such as bistratified ganglion cells, in vivo.

The susceptibility of the retina to photochemical damage from visible light

The photoreceptor/RPE complex must maintain a delicate balance between maximizing the absorption of photons for vision and retinal image quality while simultaneously minimizing the risk of photodamage when exposed to bright light. We review the recent discovery of two new effects of light exposure on the photoreceptor/RPE complex in the context of current thinking about the causes of retinal phototoxicity. These effects are autofluorescence photobleaching in which exposure to bright light reduces lipofuscin autofluorescence and, at higher light levels, RPE disruption in which the pattern of autofluorescence is permanently altered following light exposure. Both effects occur following exposure to visible light at irradiances that were previously thought to be safe. Photopigment, retinoids involved in the visual cycle, and bisretinoids in lipofuscin have been implicated as possible photosensitizers for photochemical damage. The mechanism of RPE disruption may follow either of these paths. On the other hand, autofluorescence photobleaching is likely an indicator of photooxidation of lipofuscin. The permanent changes inherent in RPE disruption might require modification of the light safety standards. AF photobleaching recovers after several hours although the mechanisms by which this occurs are not yet clear. Understanding the mechanisms of phototoxicity is all the more important given the potential for increased susceptibility in the presence of ocular diseases that affect either the visual cycle and/or lipofuscin accumulation. In addition, knowledge of photochemical mechanisms can improve our understanding of some disease processes that may be influenced by light exposure, such as some forms of Leber's congenital amaurosis, and aid in the development of new therapies. Such treatment prior to intentional light exposures, as in ophthalmic examinations or surgeries, could provide an effective preventative strategy.

Intravitreal injection of AAV2 transduces macaque inner retina

PURPOSE

Adeno-associated virus serotype 2 (AAV2) has been shown to be effective in transducing inner retinal neurons after intravitreal injection in several species. However, results in nonprimates may not be predictive of transduction in the human inner retina, because of differences in eye size and the specialized morphology of the high-acuity human fovea. This was a study of inner retina transduction in the macaque, a primate with ocular characteristics most similar to that of humans.

METHODS

In vivo imaging and histology were used to examine GFP expression in the macaque inner retina after intravitreal injection of AAV vectors containing five distinct promoters.

RESULTS

AAV2 produced pronounced GFP expression in inner retinal cells of the fovea, no expression in the central retina beyond the fovea, and variable expression in the peripheral retina. AAV2 vector incorporating the neuronal promoter human connexin 36 (hCx36) transduced ganglion cells within a dense annulus around the fovea center, whereas AAV2 containing the ubiquitous promoter hybrid cytomegalovirus (CMV) enhancer/chicken-β-actin (CBA) transduced both Müller and ganglion cells in a dense circular disc centered on the fovea. With three shorter promoters--human synapsin (hSYN) and the shortened CBA and hCx36 promoters (smCBA and hCx36sh)--AAV2 produced visible transduction, as seen in fundus images, only when the retina was altered by ganglion cell loss or enzymatic vitreolysis.

CONCLUSIONS

The results in the macaque suggest that intravitreal injection of AAV2 would produce high levels of gene expression at the human fovea, important in retinal gene therapy, but not in the central retina beyond the fovea.

Images of photoreceptors in living primate eyes using adaptive optics two-photon ophthalmoscopy

In vivo two-photon imaging through the pupil of the primate eye has the potential to become a useful tool for functional imaging of the retina. Two-photon excited fluorescence images of the macaque cone mosaic were obtained using a fluorescence adaptive optics scanning laser ophthalmoscope, overcoming the challenges of a low numerical aperture, imperfect optics of the eye, high required light levels, and eye motion. Although the specific fluorophores are as yet unknown, strong in vivo intrinsic fluorescence allowed images of the cone mosaic. Imaging intact ex vivo retina revealed that the strongest two-photon excited fluorescence signal comes from the cone inner segments. The fluorescence response increased following light stimulation, which could provide a functional measure of the effects of light on photoreceptors.

Short-component multiple schedules: effects of relative reinforcement duration

Pigeons were exposed to multiple variable-interval 2-min variable-interval 2-min schedules of food presentation in which relative duration of food presentation was manipulated. When components alternated every 5 sec and were scheduled on separate response keys, relative response rates closely matched relative reinforcement duration in three of four pigeons. On the other hand, relative response rates were insensitive to relative reinforcement duration when components scheduled on a single response key alternated every 5 sec, and when components scheduled on separate response keys alternated every 2 min. Thus, both rapid alternation and spatial separation of components were necessary to produce approximate matching of relative responding to relative reinforcement duration. This finding contrasts with previous findings that only rapid component alternation is necessary for matching when relative rate of reinforcement is manipulated.

In-vivo imaging of retinal nerve fiber layer vasculature: imaging histology comparison

Background

Although it has been suggested that alterations of nerve fiber layer vasculature may be involved in the etiology of eye diseases, including glaucoma, it has not been possible to examine this vasculature in-vivo. This report describes a novel imaging method, fluorescence adaptive optics (FAO) scanning laser ophthalmoscopy (SLO), that makes possible for the first time in-vivo imaging of this vasculature in the living macaque, comparing in-vivo and ex-vivo imaging of this vascular bed.

Methods

We injected sodium fluorescein intravenously in two macaque monkeys while imaging the retina with an FAO-SLO. An argon laser provided the 488 nm excitation source for fluorescence imaging. Reflectance images, obtained simultaneously with near infrared light, permitted precise surface registration of individual frames of the fluorescence imaging. In-vivo imaging was then compared to ex-vivo confocal microscopy of the same tissue.

Results

Superficial focus (innermost retina) at all depths within the NFL revealed a vasculature with extremely long capillaries, thin walls, little variation in caliber and parallel-linked structure oriented parallel to the NFL axons, typical of the radial peripapillary capillaries (RPCs). However, at a deeper focus beneath the NFL, (toward outer retina) the polygonal pattern typical of the ganglion cell layer (inner) and outer retinal vasculature was seen. These distinguishing patterns were also seen on histological examination of the same retinas. Furthermore, the thickness of the RPC beds and the caliber of individual RPCs determined by imaging closely matched that measured in histological sections.

Conclusion

This robust method demonstrates in-vivo, high-resolution, confocal imaging of the vasculature through the full thickness of the NFL in the living macaque, in precise agreement with histology. FAO provides a new tool to examine possible primary or secondary role of the nerve fiber layer vasculature in retinal vascular disorders and other eye diseases, such as glaucoma.

The reduction of retinal autofluorescence caused by light exposure

PURPOSE

A prior study showed that long exposure to 568-nm light at levels below the maximum permissible exposure safety limit produces retinal damage preceded by a transient reduction in the autofluorescence of retinal pigment epithelial (RPE) cells in vivo. The present study shows how the effects of exposure power and duration combine to produce this autofluorescence reduction and find the minimum exposure causing a detectable autofluorescence reduction.

METHODS

Macaque retinas were imaged using a fluorescence adaptive optics scanning laser ophthalmoscope to resolve individual RPE cells in vivo. The retina was exposed to 568-nm light over a square subtending 0.5 degrees with energies ranging from 1 to 788 J/cm(2), where power and duration were independently varied.

RESULTS

In vivo exposures of 5 J/cm(2) and higher caused an immediate decrease in autofluorescence followed by either full autofluorescence recovery (exposures <or= 210 J/cm(2)) or permanent RPE cell damage (exposures >or= 247 J/cm(2)). No significant autofluorescence reduction was observed for exposures of 2 J/cm(2) and lower. Reciprocity of exposure power and duration held for the exposures tested, implying that the total energy delivered to the retina, rather than its distribution in time, determines the amount of autofluorescence reduction.

CONCLUSIONS

That reciprocity held is consistent with a photochemical origin, which may or may not cause retinal degeneration. The implementation of safe methods for delivering light to the retina requires a better understanding of the mechanism causing autofluorescence reduction. Finally, RPE imaging was demonstrated using light levels that do not cause a detectable reduction in autofluorescence.

In vivo autofluorescence imaging of the human and macaque retinal pigment epithelial cell mosaic

PURPOSE

Retinal pigment epithelial (RPE) cells are critical for the health of the retina, especially the photoreceptors. A recent study demonstrated that individual RPE cells could be imaged in macaque in vivo by detecting autofluorescence with an adaptive optics scanning laser ophthalmoscope (AOSLO). The current study extended this method to image RPE cells in fixating humans in vivo and to quantify the RPE mosaic characteristics in the central retina of normal humans and macaques.

METHODS

The retina was imaged simultaneously with two light channels in a fluorescence AOSLO; one channel was used for reflectance imaging of the cones while the other detected RPE autofluorescence. The excitation light was 568 nm, and emission was detected over a 40-nm range centered at 624 nm. Reflectance frames were registered to determine interframe eye motion, the motion was corrected in the simultaneously recorded autofluorescence frames, and the autofluorescence frames were averaged to give the final RPE mosaic image.

RESULTS

In vivo imaging demonstrated that with increasing eccentricity, RPE cell density, and mosaic regularity decreased, whereas RPE cell size and spacing increased. Repeat measurements of the same retinal location 42 days apart showed the same RPE cells and distribution.

CONCLUSIONS

The RPE cell mosaic has been resolved for the first time in alert fixating human subjects in vivo using AOSLO. Mosaic analysis provides a quantitative database for studying normal and diseased RPE in vivo. This technique will allow longitudinal studies to track disease progression and assess treatment efficacy in patients and animal models of retinal disease.

Light-induced retinal changes observed with high-resolution autofluorescence imaging of the retinal pigment epithelium

PURPOSE

Autofluorescence fundus imaging using an adaptive optics scanning laser ophthalmoscope (AOSLO) allows for imaging of individual retinal pigment epithelial (RPE) cells in vivo. In this study, the potential of retinal damage was investigated by using radiant exposure levels that are 2 to 150 times those used for routine imaging.

METHODS

Macaque retinas were imaged in vivo with a fluorescence AOSLO. The retina was exposed to 568- or 830-nm light for 15 minutes at various intensities over a square (1/2) degrees per side. Pre- and immediate postexposure images of the photoreceptors and RPE cells were taken over a 2 degrees field. Long-term AOSLO imaging was performed intermittently from 5 to 165 days after exposure. Exposures delivered over a uniform field were also investigated.

RESULTS

Exposures to 568-nm light caused an immediate decrease in autofluorescence of RPE cells. Follow-up imaging revealed either full recovery of autofluorescence or long-term damage in the RPE cells at the exposure. The outcomes of AOSLO exposures and uniform field exposures of equal average power were not significantly different. No effects from 830-nm exposures were observed.

CONCLUSIONS

The study revealed a novel change in RPE autofluorescence induced by 568-nm light exposure. Retinal damage occurred as a direct result of total average power, independent of the light-delivery

METHOD

Because the exposures were near or below permissible levels in laser safety standards, these results suggest that caution should be used with exposure of the retina to visible light and that the safety standards should be re-evaluated for these exposure conditions.

Mechanisms of Sensitivity Loss due to Visual Cortex Lesions in Humans and Macaques

This study represents the first use of noise masking and signal detection theory to examine mechanisms of visual loss after lesions of visual cortex. Noise-masked contrast thresholds were increased in 2 macaques and 2 humans at lesion-affected, compared with control, regions of their visual fields. Experiments suggested by the organization of visual cortex examined possible mechanisms of the visual loss. Two experiments tested the hypothesis that damage to feedback connections might eliminate the benefit of comparing test stimuli with remembered representations but neither could account for the sensitivity loss. The third experiment found that extrastriate lesions did increase the trial-to-trial variability of sensory decisions, suggesting this as one mechanism of sensitivity loss. In addition to clarifying mechanisms of lesion-induced contrast sensitivity loss, this study also showed that elevated contrast thresholds, that are subtle in the absence of external noise, became dramatic when measured with masking noise.

Unilateral deficits in visual perception and learning after unilateral inferotemporal cortex lesions in macaques

This study adapted the method of partial lesions, combined with controlled fixation, to study the perceptual role of macaque inferotemporal (IT) cortex. Unilateral lesions were made in IT cortex of three monkeys, without section of the corpus callosum, and visual function was tested ipsilateral and contralateral to the lesion. The observed changes were compared to the effects of bilateral lesions of IT cortex in one monkey, the approach used in most previous studies. Unilateral lesions produced far less profound, although more selective, loss on the tested visual abilities than did bilateral lesions. All three monkeys with unilateral lesions showed decreased chromatic sensitivity, but sparing of achromatic sensitivity, and severely disrupted learning and performance of visual matching to sample, and in all cases, the visual loss was contralateral to the site of the lesion. Unexpectedly, the magnitude of the contralateral loss was not increased by later section of the corpus callosum and anterior commissure in one of the monkeys, a lesion that removes interhemispheric input to contralateral from ipsilateral temporal cortex neurons. These results support physiological findings that show that the response of IT cortex neurons is dominated by the contralateral visual field, despite the bilateral activation many IT neurons receive. Comparison to earlier studies of lesions of area V4, which provides input to IT cortex, shows that V4 and IT lesions produce qualitatively different effects.

Effect of parietal lobe lesions on saccade targeting and spatial memory in a naturalistic visual search task

The eye movements of two patients with parietal lobe lesions and four normal observers were measured while they performed a visual search task with naturalistic objects. Patients were slower to perform the task than the normal observers, and the patients had more fixations per trial, longer latencies for the first saccade during the visual search, and less accurate first and second saccades to the target locations during the visual search. The increases in response times for the patients compared to the normal observers were best predicted by increases in the number of fixations. In order to investigate the effects of spatial memory on search performance, in some trials observers saw a preview of the search display. The patients appeared to have difficulty using previously viewed information, unlike normal observers who benefit from the preview. This suggests a spatial memory deficit. The patients' deficits are consistent with the hypothesis that the parietal cortex has a role in the selection of targets for saccades, in memory for target location.

Cortical area V4 is critical for certain texture discriminations, but this effect is not dependent on attention

This study examined the question of which features of a complex grouping discrimination make it vulnerable to permanent elimination by V4 lesions. We first verified that the line element grouping discrimination, which we previously reported to be devastated by V4 lesions, was similarly affected in the monkeys of this study. The permanence of the deficit was established by mapping its visual field distribution and then testing this discrimination for an extended period at a locus on the border of the deficit. Also, a staircase procedure was used to provide the monkey with within session instruction in the grouping discrimination, but this did not improve V4 lesion performance. Grouping was then compared with several discriminations that shared some features with it, but which were found not to be permanently eliminated by V4 lesions. This comparison suggested that grouping (rather than segmentation or response to a single element) was one feature that made the discrimination vulnerable, a second was the similarity in shape of the texture elements to be grouped. Finally, we tested visual crowding, a property of peripheral vision that is thought to reflect neuronal interactions early in visual cortex, possibly in area V1, and found no effect of V4 lesions. A control experiment with human observers tested whether the elimination of grouping by V4 lesions might be due to an alteration of attention, but found no evidence to support this hypothesis. These results show that severe disruption of texture discriminations by V4 lesions depends on both the nature of the discrimination and the type of texture elements involved, but does not necessarily involve the disruption of attention.

Perceptual deficits after lesions of inferotemporal cortex in macaques

This study used a novel approach to examine a much studied question, the nature of visual deficits caused by lesions of the inferotemporal cortex (IT). Unlike many previous studies of IT lesions, we de-emphasized early, non-specific disruptions of testing caused by the lesions, and instead concentrated on permanent changes in thresholds. This approach produced unexpected results that suggest a re-evaluation of the traditional view of the role of the IT cortex in shape perception and such related visual abilities as perceptual invariances, visual grouping, the visibility of illusory contours and the performance of oddity discriminations. In addition, the measurement of stable, post-lesion hue discrimination thresholds gave us a different perspective on the severity of color vision deficits which result from lesions of the IT cortex. We found that shape distortion thresholds were not permanently elevated by IT lesions and, indeed, showed no greater transitory disruption than did other visual abilities. This result is inconsistent with the common view that IT is critical to shape discriminations. Two other visual abilities that would be expected to be disrupted by IT lesions - the visual grouping of misoriented line segments and shape invariances (failure of irrelevant stimulus changes to disrupt shape distortion thresholds) - were not affected by IT lesions. However, shape discriminations based on illusory contours and some oddity discriminations were severely and permanently affected. Our results also showed that IT lesions caused permanent, moderate to large impairments of color vision, but not color blindness. Bilateral damage to area TEO caused no disruption of performance on any of the abovediscriminations. Our results suggest that the IT cortex in macaques may be critical to the visibility of illusory contours and the performance of some oddity discriminations, that it plays some role in color perception, but that it is not essential for shape, grouping discriminations or perceptual shape invariances.

Deficits in complex visual perception following unilateral temporal lobectomy

Although human temporal cortex is known to be important for short- and long-term memory, its role in visual perception is not well understood. In this study, we compared the performance of three patients with unilateral temporal lobectomies to that of normal controls on both "simple" and "complex" visual discriminations that did not involve explicit memory components. Two types of complex tasks were tested that involved discriminations secondary to texture segmentation. These were contrasted with simple discriminations using luminance-defined stimuli. Patients showed impaired thresholds only on tasks involving texture segmentation, performing as well as controls when the targets were defined by luminance rather than texture. The minimum stimulus presentation times for threshold performance were also measured for all tasks and found to be elevated in temporal lobectomy patients relative to controls. Although the magnitude of the deficits observed was substantial, loss was equivalent in ipsi- and contra-lesional regions of the visual field. Additional control experiments showed that the patients' perceptual deficits were not due, even in part, to disturbances of basic visual capacities such as acuity and contrast sensitivity. Our results indicate that temporal lobe damage disrupts complex, but not simple, visual discriminations throughout the visual field.

V4 lesions in macaques affect both single- and multiple-viewpoint shape discriminations

The role of cortical area V4 in complex shape discriminations was studied by testing the effects of V4 lesions in macaques on the ability to visually discriminate between images of three-dimensional (3D) objects from different viewpoints. Stimuli were presented in pairs in the lower left or lower right visual field quadrants about 4 deg from the fovea, and the monkeys judged on each trial whether the two views were of the same or of different objects. Object similarity was varied to determine a threshold shape difference. V4 lesions caused profound, retinotopic, and apparently permanent disruptions of discrimination, regardless of whether the images represented single or multiple viewpoints. In V4 lesioned portions of the visual field, monkeys could discriminate objects only when they differed much more grossly in shape than was true in control locations. These effects of the lesion were virtually identical for discriminations that had been learned before lesions were placed and for those learned afterwards. As in previous studies, V4 lesions elevated contrast thresholds by approximately a factor of two, but control observations showed that this was not the basis of the disruption of shape discrimination. Manipulation of cues to shape showed that in control locations, monkeys maintained excellent shape discrimination despite a variety of stimulus alterations, whereas in V4 lesioned areas their performance was easily disrupted. This finding suggests that V4 may support visual shape discriminations by facilitating the use of multiple visual cues. However, the fact that single-viewpoint and multiple-viewpoint discriminations were similarly affected indicates that the disruption was not specific to 3D shape discrimination, but may apply to a variety of subtle discriminations.

Basic visual capacities and shape discrimination after lesions of extrastriate area V4 in macaques

Ibotenic acid lesions were made in four macaque monkeys in a region of cortical area V4 that corresponds to the lower quadrant of one hemifield. For visual testing, fixation locus was monitored with scleral search coils and controlled behaviorally to place test stimuli either in the lesioned quadrant or in a control location in the opposite hemifield. Some basic visual capacities were slightly altered by the lesions; there was a two-fold reduction of luminance contrast sensitivity as well as red-green chromatic contrast sensitivity, both tested with stationary gratings. On the other hand, little or no loss was found when contrast sensitivity for detection or direction discrimination was tested with 10-Hz drifting gratings nor was there a reliable change in visual acuity. Hue and luminance matching were tested with a spatially more complex matching-to-sample task, but monkeys could not learn this task in the visual field locus of a V4 lesion. If previously trained at this locus, performance was not affected by the lesion. In contrast to the small effects on basic visual capabilities, performance on two form discrimination tasks was devastated by V4 lesions. The first involved discriminating the orientation of colinear groups of dots on a background of randomly placed dots. The second involved discriminating the orientation of a group of three line segments surrounded by differently oriented line segments. Some selectivity of the deficits for form discrimination was shown by the lack of an effect of the lesions on a global motion discrimination. These results show that while V4 lesions cause only slight disruptions of basic visual capacities, they profoundly disrupt form discriminations.

Motion perception following lesions of the superior temporal sulcus in the monkey

We examined the effect of bilateral ibotenic acid lesions, aimed at areas MT/MST in three macaques, on their perception of motion. The medial boundary of the lesions in the three monkeys was near the dorsal end of the STS, but the lesions extended different lengths ventrally along the STS. The lesions extended the shortest distance ventrally monkeys 1 and 2, covering most of MST but possibly sparing a portion of lateral MT. That in monkey 3 damaged all of MT and MST bilaterally and extended through most of FST. All three lesions caused a temporary disruption, followed by at least partial recovery, of most motion thresholds. Permanent effects of the lesions on visual sensitivity were graded with lesion extent. Contrast sensitivity for detecting low-spatial-frequency (1 cycle/degree) drifting gratings over a wide range of drift rates, as well as for identifying their direction of motion, was slightly affected only in monkey 3. Only monkeys 2 and 3 showed a deficit in discriminating stimulus speed, and the size of the loss was two- to fourfold. Discrimination of opposite directions of dot pattern motion, which required integration of local motion signals, was mildly affected in monkeys 2 and 3, and not affected in monkey 1. However, addition of directional noise to this discrimination caused the performance of all monkeys to be permanently disrupted, especially that of monkeys 2 and 3. Finally, direction difference thresholds were elevated by a factor of 2-4 after the lesions in all three monkeys. Many of these deficits were more pronounced during the first 2 months of testing following the lesion. Thus, our results demonstrate that areas within dorsal STS make an important contribution to the performance of various motion perception tasks including the discrimination of small differences in direction and speed, and the perception of global motion in the presence of directional noise. The residual motion perception, even in the monkey with virtually complete removal of areas MT/MST, may suggest either that these tasks are normally mediated in part by cortical areas outside of areas MT and MST, or that the disrupted functions were partially assumed by other cortical areas after lesions.

Visual effects of lesions of cortical area V2 in macaques

Ibotenic acid lesions were placed in two monkeys in a portion of cortical area V2 that corresponds to a lower quadrant of the visual field extending approximately 3-7 degrees from the fovea. For purposes of comparison, another lesion was placed in area V1 in one animal. A wide range of visual capacities were then measured, using a discrimination between vertical and horizontal orientation, in and near the affected regions of the visual field. Visual acuity declined sharply as the test stimulus approached the visual field location corresponding to the V1 lesion, and no threshold could be measured at its center. In contrast, lesions of area V2 caused no measurable decrease in acuity, nor was there any substantial effect on several measures of contrast sensitivity. Subsequently, two types of more complex visual discriminations were measured (also using a vertical-horizontal discrimination), and these discriminations were severely disrupted by V2 lesions. The first discrimination was of the orientation of two parallel lines of five colinear dots each. We measured the number of background dots that would bring the discrimination to threshold, and this number of dots was greatly decreased by a V2 lesion. The second discrimination was of the orientation of a group of three distinctive texture elements embedded in a six by six element texture. This task could not be done in the visual field region affected by the V2 lesion when the distinctive elements differed in orientation from the others. Control experiments showed that the discrimination could be done when the three distinctive elements differed in size or color. These results suggest that cortical area V2 is not needed for some low-level discriminations, but may be essential for tasks involving complex spatial discriminations.

Visual effects of damage to P ganglion cells in macaques

Four indices of visual performance were measured in control macaques and in macaques that had been exposed to monomeric acrylamide, a neurotoxicant that preferentially damages P retinal ganglion cells. Morphological examination of the retina and visual pathways of these monkeys showed virtually complete loss of P ganglion cells over a region extending to at least 40 deg from the fovea, and relative sparing of M ganglion cells. The four tests examined visual functions for which the visual pathway from P ganglion cells might be of great importance: visual acuity, contrast discrimination, hyperacuity, and shape discrimination. In the acrylamide-dosed monkeys, visual acuity was reduced slightly more than fourfold, a somewhat larger reduction than that seen previously after ibotenic-acid lesions of the P pathway in the geniculate. The residual acuity was in good agreement with the Nyquist frequency calculated from the density of ON or OFF M ganglion cells. Contrast increment thresholds were elevated for the dosed monkeys only in one of the two conditions tested. The elevation was found only under those spatiotemporal conditions for which we have previously shown that contrast thresholds are increased by acrylamide exposure, and was most marked at low background contrasts. Vernier acuity was elevated in one dosed monkey, but not affected in a second monkey that also had severe loss of P ganglion cells. Finally, we found no effect of acrylamide exposure on the number of training trials required to learn simple or complex shape discriminations. These results support previous findings in showing that the P pathway mediates visual acuity, and they show that several other important aspects of visual perception are not exclusively dependent on the P pathway.

Does primate motion perception depend on the magnocellular pathway?

This study examined the importance of the primate magnocellular retinocortical pathway in the perception of moving stimuli. A portion of the magnocellular pathway was permanently and selectively interrupted by ibotenic acid injections in the LGN of macaque monkeys. We then tested contrast sensitivity for detecting moving stimuli, as well as two indices of motion perception, contrast sensitivity for opposite direction discrimination and speed difference thresholds, in the affected portion of the visual field. Magnocellular lesions greatly reduced detection contrast sensitivity at high temporal and low spatial frequencies and had a similar effect on contrast sensitivity for opposite direction discrimination under these same stimulus conditions. Consequently, opposite direction discriminations could be made at contrast threshold, suggesting that magnocellular lesions reduced the visibility of stimuli used to test direction perception, but did not act directly on direction perception. Magnocellular lesions also elevated speed difference thresholds under some stimulus conditions. However, this deficit was reduced or eliminated by raising the contrast of the test stimulus. Together, these findings suggest that magnocellular lesions reduce the visibility of stimuli used to test motion perception but that they do not appear to alter motion perception otherwise.

The effects of parvocellular lateral geniculate lesions on the acuity and contrast sensitivity of macaque monkeys

The effects of ablating the visual pathway that passes through the parvocellular (dorsal) LGN were tested in 2 macaque monkeys by measuring acuity and both luminance and chromatic contrast sensitivity. Thresholds were tested monocularly before and after ibotenic acid was used to lesion parvocellular layers 4 and 6 of the contralateral geniculate. The injections were centered at the representation of 6 degrees in the temporal field on the horizontal meridian, and vision was tested with localized stimuli at this location. In addition, in one of the monkeys, a lesion was made in magnocellular layer 1 of the opposite geniculate, and the same thresholds were tested. Physiological and anatomical reconstructions demonstrated complete destruction of the target layers in 1 parvocellular lesions and in the magnocellular lesion, and sparing of the nontarget layers in the tested region. Parvocellular lesions caused a 3-4-fold reduction in visual acuity within the affected part of the visual field, while the magnocellular lesion did not affect acuity. Both luminance and chromatic contrast sensitivity, tested with stationary gratings of 2 c/degree, were severely reduced by parvocellular lesions, but not affected by the magnocellular lesion. However, when luminance contrast sensitivity was tested with 1 c/degree gratings, counterphase modulated at 10 Hz, it was reduced by both parvocellular and magnocellular lesions. This study demonstrates that the parvocellular pathway dominates chromatic vision, acuity, and contrast detection at low temporal and high spatial frequencies, while the magnocellular pathway may mediate contrast detection at higher temporal and lower spatial frequencies.

Macaque vision after magnocellular lateral geniculate lesions

Ibotenic-acid lesions of the magnocellular portion of the macaque lateral geniculate nucleus were used to examine the role of the M-cell pathway in spatio-temporal contrast sensitivity. A lesion was placed in layer 1 of the lateral geniculate of each of two monkeys. Physiological mapping in one animal demonstrated that the visual-field locus of the lesion was on the horizontal meridian, approximately 6 deg in the temporal field. Visual thresholds were tested monocularly in the contralateral eye, and fixation locus was monitored with a scleral search coil to control the retinal location of the test target. Three threshold measures were clearly disrupted by the magnocellular lesions. Contrast sensitivity for a 1 cycle/deg grating that drifted at 10 Hz was reduced from about twofold greater than, to about the same as, that for 10-Hz counterphase modulated gratings. Sensitivity for a very low spatial frequency (Gaussian blob), 10-Hz flickering stimulus was reduced so severely that no threshold could be measured. In addition, flicker resolution was greatly reduced at lower modulation depths (0.22), but not at higher depths (1.0). Two of the measured thresholds were unaffected by the lesions. Contrast sensitivity for 2 cycle/deg stationary gratings remained intact, and little or no effect on sensitivity was found for 1 cycle/deg, 10-Hz counterphase modulated gratings. Together, these results suggest that the magnocellular pathway makes little contribution to visual sensitivity at low to moderate temporal frequencies. On the other hand, some contribution to detection sensitivity is evident at lower spatial and high temporal frequencies, especially for drifting stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)