Real-time ultra-high optical coherence tomography monitoring of optical tissue effects caused by selective retina therapy

Purpose: Selective retina therapy (SRT) has shown great promise compared to conventional retinal laser photocoagulation as it avoids collateral damage and selectively targets the retinal pigment epithelium (RPE). Its use, however, is challenging in terms of therapy monitoring and dosage because an immediate tissue reaction is not biomicroscopically discernibel. To overcome these limitations, real-time optical coherence tomography (OCT) might be useful to monitor retinal tissue during laser application. We have thus evaluated a proprietary OCT system for its capability of mapping optical changes introduced by SRT in retinal tissue. Methods: Freshly enucleated porcine eyes, covered in DMEM upon collection were utilized and a total of 175 scans from ex-vivo porcine eyes were analyzed. The porcine eyes were used as an ex-vivo model and results compared to two time-resolved OCT scans, recorded from a patient undergoing SRT treatment (SRT Vario, Medical Laser Center Lübeck). In addition to OCT, fluorescin angiography and fundus photography were performed on the patient and OCT scans were subsequently investigated for optical tissue changes linked to laser application. Results: Biomicroscopically invisible SRT lesions were detectable in OCT by changes in the RPE / Bruch's complex both in vivo and the porcine ex-vivo model. Laser application produced clearly visible optical effects such as hyperreflectivity and tissue distortion in the treated retina. Tissue effects were even discernible in time-resolved OCT imaging when no hyper-reflectivity persisted after treatment. Data from ex-vivo porcine eyes showed similar to identical optical changes while effects visible in OCT appeared to correlate with applied pulse energy, leading to an additional reflective layer when lesions became visible in indirect ophthalmoscopy. Conclusions: Our results support the hypothesis that real-time high-resolution OCT may be a promising modality to obtain additional information about the extent of tissue damage caused by SRT treatment. Data shows that our exvivo porcine model adequately reproduces the effects occurring in-vivo, and thus can be used to further investigate this promising imaging technique.

Larger visual stimuli are perceived to last longer from time to time: The internal clock is not affected by nontemporal visual stimulus size

Performance on interval timing is often explained by the assumption of an internal clock based on neural counting. According to this account, a neural pacemaker generates pulses, and the number of pulses relating to a physical time interval is recorded by a counter. Thus, the number of accumulated pulses is the internal representation of this interval. Several studies demonstrated that large visual stimuli are perceived to last longer than smaller ones presented for the same duration. The present study was designed to investigate whether nontemporal visual stimulus size directly affects the internal clock. For this purpose, a temporal reproduction task was applied. Sixty participants were randomly assigned to one of two experimental conditions with stimulus size being experimentally varied within either the target or the reproduction interval. A direct effect of nontemporal stimulus size on the pacemaker-counter system should become evident irrespective of whether stimulus size was experimentally varied within the target or the reproduction interval. An effect of nontemporal stimulus size on reproduced duration only occurred when stimulus size was varied during the target interval. This finding clearly argues against the notion that nontemporal visual stimulus size directly affects the internal clock. Furthermore, our findings ruled out a decisional bias as a possible cause of the observed differential effect of stimulus size on reproduced duration. Rather the effect of stimulus size appeared to originate from the memory stage of temporal information processing at which the timing signal from the pacemaker-counter component is encoded in reference memory.

Retinal microglia are activated by systemic fungal infection

Purpose: To determine whether systemic fungal infection could cause activation of retinal microglia and therefore could be potentially harmful for patients with retinal degenerative diseases. Methods: Activation of retinal microglia was measured in a model of sublethal invasive candidiasis in C57BL/6J mice by (i) confocal immunofluorescence and (ii) flow cytometry analysis, using anti-CD11b, anti-Iba1, anti-MHCII and anti-CD45 antibodies. Results: Systemic fungal infection causes activation of retinal microglia, with phenotypic changes in morphology, surface markers expression, and microglial re-location in retinal layers. Conclusions: As an excessive or prolonged microglial activation may lead to chronic inflammation with severe pathological side effects, causing or worsening the course of retinal dystrophies, a systemic infection may represent a risk factor to be considered in patients with ocular neurodegenerative diseases, such as diabetic retinopathy, glaucoma, age-related macular degeneration or retinitis pigmentosa.

Impairment of Intrinsically Photosensitive Retinal Ganglion Cells Associated With Late Stages of Retinal Degeneration

Purpose. To evaluate quantitative and qualitative age-related changes in intrinsically photosensitive melanopsin-containing retinal ganglion cells (ipRGCs) in transgenic P23H rats, an animal model of autosomal dominant retinitis pigmentosa (RP) was examined. Methods. ipRGC density, morphology, and integrity were characterized by immunohistochemistry in retinas extracted from P23H and Sprague–Dawley (SD) rats aged 4, 12, and 18 months. Differences between SD and P23H rats throughout the experimental stages, as well as the interactions among them, were morphologically evaluated. Results. In rat retinas, we have identified ipRGCs with dendrites stratifying in either the outer margin (M1) or inner side (M2) of the inner plexiform layer, and in both the outer and inner plexuses (M3). A small group of M1 cells had their somas located in the inner nuclear layer (M1d). In SD rats, ipRGCs showed no significant changes associated with age, in terms of either mean cell density or the morphologic parameters analyzed. However, the mean density of ipRGCs in P23H rats fell by approximately 67% between 4 and 18 months of age. Moreover, ipRGCs in these animals showed a progressive age-dependent decrease in the dendritic area, the number of branch points and terminal neurite tips per cell, and the Sholl area. Conclusions. In the P23H rat model of retinitis pigmentosa, density, wholeness, and dendritic arborization of melanopsin-containing ganglion cells decrease in advanced stages of the degenerative disease.

Changes in the Photoreceptor Mosaic of P23H-1 Rats During Retinal Degeneration: Implications for Rod-Cone Dependent Survival

Purpose. To investigate the spatiotemporal relationship between rod and cone degeneration in the P23H-1 rat. Methods. Control Sprague-Dawley (SD) and P23H-1 rats of ages ranging from P30 to P365 were used. Retinas were processed for whole mounts or cross sections and rods and cones were immunodetected. We used newly developed image analysis techniques to quantify the total population of L/M cones (the most abundant cones in the rat) and analyzed the rings of rod-cone degeneration. Results. In P23H-1 rats, rod degeneration occurs rapidly: first the rod outer segment shortens, at P30 there is extensive rod loss, and by P180 rod loss is almost complete except for the most peripheral retina. The numbers of L/M cones are, at all postnatal ages, lower in P23H-1 rats than in control SD rats, and decrease significantly with age (by P180). Rod and cone degeneration is spatiotemporally related and occurs in rings that appear already at P90 and spread throughout the entire retina. At P180, the rings of rod-cone degeneration are more abundant in the equatorial retina and are larger in the dorsal retina. Conclusions. This work describes for the first time that in the P23H-1 rat, rod and cone degeneration is spatiotemporally related and occurs in rings. Cone loss follows rod loss and starts very soon, even before P30, the first age analyzed here. The characteristics of the rings suggest that secondary cone degeneration is influenced by retinal position and/or other intrinsic or extrinsic factors.

Phagocytosis of Photoreceptor Outer Segments by Transplanted Human Neural Stem Cells as a Neuroprotective Mechanism in Retinal Degeneration

Purpose. Transplantation of human central nervous system stem cells (HuCNS-SC) into the subretinal space of Royal College of Surgeons (RCS) rats preserves photoreceptors and visual function. To explore possible mechanism(s) of action underlying this neuroprotective effect, we performed a detailed morphologic and ultrastructure analysis of HuCNS-SC transplanted retinas. Methods. The HuCNS-SC were transplanted into the subretinal space of RCS rats. Histologic examination of the transplanted retinas was performed by light and electron microscopy. Areas of the retina adjacent to HuCNS-SC graft (treated regions) were analyzed and compared to control sections obtained from the same retina, but distant from the transplant site (untreated regions). Results. The HuCNS-SC were detected as a layer of STEM 121 immunopositive cells in the subretinal space. In treated regions, preserved photoreceptor nuclei, as well as inner and outer segments were identified readily. In contrast, classic signs of degeneration were observed in the untreated regions. Interestingly, detailed ultrastructure analysis revealed a striking preservation of the photoreceptor–bipolar–horizontal cell synaptic contacts in the outer plexiform layer (OPL) of treated areas, in stark contrast with untreated areas. Finally, the presence of phagosomes and vesicles exhibiting the lamellar structure of outer segments also was detected within the cytosol of HuCNS-SC, indicating that these cells have phagocytic capacity in vivo. Conclusions. This study reveals the novel finding that preservation of specialized synaptic contacts between photoreceptors and second order neurons, as well as phagocytosis of photoreceptor outer segments, are potential mechanism(s) of HuCNS-SC transplantation, mediating functional rescue in retinal degeneration.

Loss of Outer Retinal Neurons and Circuitry Alterations in the DBA/2J Mouse

Purpose. The DBA/2J mouse line develops essential iris atrophy, pigment dispersion, and glaucomatous age-related changes, including an increase of IOP, optic nerve atrophy, and retinal ganglion cell (RGC) death. The aim of this study was to evaluate possible morphological changes in the outer retina of the DBA/2J mouse concomitant with disease progression and aging, based on the reduction of both the a- and b-waves and photopic flicker ERGs in this mouse line. Methods. Vertically sectioned DBA/2J mice retinas were evaluated at 3, 8, and 16 months of age using photoreceptor, horizontal, and bipolar cell markers. Sixteen-month-old C57BL/6 mice retinas were used as controls. Results. The DBA/2J mice had outer retinal degeneration at all ages, with the most severe degeneration in the oldest retinas. At 3 months of age, the number of photoreceptor cells and the thickness of the OPL were reduced. In addition, there was a loss of horizontal and ON-bipolar cell processes. At 8 months of age, RGC degeneration occurred in patches, and in the outer retina overlying these patches, cone morphology was impaired with a reduction in size as well as loss of outer segments and growth of horizontal and bipolar cell processes into the outer nuclear layer. At 16 months of age, connectivity between photoreceptors and horizontal and bipolar cell processes overlying these patches was lost. Conclusions. Retinal degeneration in DBA/2J mice includes photoreceptor death, loss of bipolar and horizontal cell processes, and loss of synaptic contacts in an aging-dependent manner.

Binocular contrast vision at and above threshold

A fundamental problem for any visual system with binocular overlap is the combination of information from the two eyes. Electrophysiology shows that binocular integration of luminance contrast occurs early in visual cortex, but a specific systems architecture has not been established for human vision. Here, we address this by performing binocular summation and monocular, binocular, and dichoptic masking experiments for horizontal 1 cycle per degree test and masking gratings. These data reject three previously published proposals, each of which predict too little binocular summation and insufficient dichoptic facilitation. However, a simple development of one of the rejected models (the twin summation model) and a completely new model (the two-stage model) provide very good fits to the data. Two features common to both models are gently accelerating (almost linear) contrast transduction prior to binocular summation and suppressive ocular interactions that contribute to contrast gain control. With all model parameters fixed, both models correctly predict (1) systematic variation in psychometric slopes, (2) dichoptic contrast matching, and (3) high levels of binocular summation for various levels of binocular pedestal contrast. A review of evidence from elsewhere leads us to favor the two-stage model. © 2006 ARVO.

From filters to features:Scale-space analysis of edge and blur coding in human vision

To make vision possible, the visual nervous system must represent the most informative features in the light pattern captured by the eye. Here we use Gaussian scale-space theory to derive a multiscale model for edge analysis and we test it in perceptual experiments. At all scales there are two stages of spatial filtering. An odd-symmetric, Gaussian first derivative filter provides the input to a Gaussian second derivative filter. Crucially, the output at each stage is half-wave rectified before feeding forward to the next. This creates nonlinear channels selectively responsive to one edge polarity while suppressing spurious or "phantom" edges. The two stages have properties analogous to simple and complex cells in the visual cortex. Edges are found as peaks in a scale-space response map that is the output of the second stage. The position and scale of the peak response identify the location and blur of the edge. The model predicts remarkably accurately our results on human perception of edge location and blur for a wide range of luminance profiles, including the surprising finding that blurred edges look sharper when their length is made shorter. The model enhances our understanding of early vision by integrating computational, physiological, and psychophysical approaches. © ARVO.

Contextual modulation involves suppression and facilitation from the center and the surround

In psychophysics, cross-orientation suppression (XOS) and cross-orientation facilitation (XOF) have been measured by investigating mask configuration on the detection threshold of a centrally placed patch of sine-wave grating. Much of the evidence for XOS and XOF comes from studies using low and high spatial frequencies, respectively, where the interactions are thought to arise from within (XOS) and outside (XOF) the footprint of the classical receptive field. We address the relation between these processes here by measuring the effects of various sizes of superimposed and annular cross-oriented masks on detection thresholds at two spatial scales (1 and 7 c/deg) and on contrast increment thresholds at 7 c/deg. A functional model of our results indicates the following (1) XOS and XOF both occur for superimposed and annular masks. (2) XOS declines with spatial frequency but XOF does not. (3) The spatial extent of the interactions does not scale with spatial frequency, meaning that surround-effects are seen primarily at high spatial frequencies. (4) There are two distinct processes involved in XOS: direct divisive suppression and modulation of self-suppression. (5) Whether XOS or XOF wins out depends upon their relative weights and mask contrast. These results prompt enquiry into the effect of spatial frequency at the single-cell level and place new constraints on image-processing models of early visual processing. © ARVO.

Size matters, but not for everyone:Individual differences for contrast discrimination

It is very well known that contrast detection thresholds improve with the size of a grating-type stimulus, but it is thought that the benefit of size is abolished for contrast discriminations well above threshold (e.g., Legge, G. E., & Foley, J. M. (1980)]. Here we challenge the generality of this view. We performed contrast detection and contrast discrimination for circular patches of sine wave grating as a function of stimulus size. We confirm that sensitivity improves with approximately the fourth-root of stimulus area at detection threshold (a log-log slope of -0.25) but find individual differences (IDs) for the suprathreshold discrimination task. For several observers, performance was largely unaffected by area, but for others performance first improved (by as much as a log-log slope of -0.5) and then reached a plateau. We replicated these different results several times on the same observers. All of these results were described in the context of a recent gain control model of area summation [Meese, T. S. (2004)], extended to accommodate the multiple stimulus sizes used here. In this model, (i) excitation increased with the fourth-root of stimulus area for all observers, and (ii) IDs in the discrimination data were described by IDs in the relation between suppression and area. This means that empirical summation in the contrast discrimination task can be attributed to growth in suppression with stimulus size that does not keep pace with the growth in excitation. © 2005 ARVO.

Grating and plaid masks indicate linear summation in a contrast gain pool

In human vision, the response to luminance contrast at each small region in the image is controlled by a more global process where suppressive signals are pooled over spatial frequency and orientation bands. But what rules govern summation among stimulus components within the suppressive pool? We addressed this question by extending a pedestal plus pattern mask paradigm to use a stimulus with up to three mask components: a vertical 1 c/deg pedestal, plus pattern masks made from either a grating (orientation = -45°) or a plaid (orientation = ±45°), with component spatial frequency of 3 c/deg. The overall contrast of both types of pattern mask was fixed at 20% (i.e., plaid component contrasts were 10%). We found that both of these masks transformed conventional dipper functions (threshold vs. pedestal contrast with no pattern mask) in exactly the same way: The dipper region was raised and shifted to the right, but the dipper handles superimposed. This equivalence of the two pattern masks indicates that contrast summation between the plaid components was perfectly linear prior to the masking stage. Furthermore, the pattern masks did not drive the detecting mechanism above its detection threshold because they did not abolish facilitation by the pedestal (Foley, 1994). Therefore, the pattern masking could not be attributed to within-channel masking, suggesting that linear summation of contrast signals takes place within a suppressive contrast gain pool. We present a quantitative model of the effects and discuss the implications for neurophysiological models of the process. © 2004 ARVO.

Low spatial frequencies are suppressively masked across spatial scale, orientation, field position, and eye of origin

Masking is said to occur when a mask stimulus interferes with the visibility of a target (test) stimulus. One widely held view of this process supposes interactions between mask and test mechanisms (cross-channel masking), and explicit models (e.g., J. M. Foley, 1994) have proposed that the interactions are inhibitory. Unlike a within-channel model, where masking involves the combination of mask and test stimulus within a single mechanism, this cross-channel inhibitory model predicts that the mask should attenuate the perceived contrast of a test stimulus. Another possibility is that masking is due to an increase in noise, in which case, perception of contrast should be unaffected once the signal exceeds detection threshold. We use circular patches and annuli of sine-wave grating in contrast detection and contrast matching experiments to test these hypotheses and investigate interactions across spatial frequency, orientation, field position, and eye of origin. In both types of experiments we found substantial effects of masking that can occur over a factor of 3 in spatial frequency, 45° in orientation, across different field positions and between different eyes. We found the effects to be greatest at the lowest test spatial frequency we used (0.46 c/deg), and when the mask and test differed in all four dimensions simultaneously. This is surprising in light of previous work where it was concluded that suppression from the surround was strictly monocular (C. Chubb, G. Sperling, & J. A. Solomon, 1989). The results confirm that above detection threshold, cross-channel masking involves contrast suppression and not (purely) mask-induced noise. We conclude that cross-channel masking can be a powerful phenomenon, particularly at low test spatial frequencies and when mask and test are presented to different eyes. © 2004 ARVO.

Area summation and masking

At detection threshold, sensitivity improves as the area of a test grating increases, but not when the test is placed on a pedestal and the task becomes contrast discrimination (G. E. Legge, & J. M. Foley, 1980). This study asks whether the abolition of area summation is specific to the situation where mask and test stimuli have the same spatial frequency and orientation ("within-channel" masking) or is more general, also occurring when mask and test stimuli are very different ("cross-channel" masking). Threshold versus contrast masking functions were measured where the test and mask were either both small (SS), both large (LL), or small and large, respectively (SL). For within-channel masking, facilitation and area summation were found at low mask contrasts, but the results for SS and LL converged at intermediate contrasts and above, replicating Legge and Foley (1980). For all three observers, less facilitation was found for SL than for SS. For cross-channel masking, area summation occurred across the entire masking function and results for SS and SL were identical. The results for the entire data set were well fit by an extended version of a contrast masking model (J. M. Foley, 1994) in which the weights of excitatory and suppressive surround terms were free parameters. I conclude that (i) there is no empirical abolition of area summation for cross-channel masking, (ii) within-channel area summation can be abolished empirically without being disabled in the model, (iii) observers are able to restrict the area of spatial integration, but not suppression, (iv) extending a cross-channel mask to the surround has no effect on contrast detection, and (v) there is a formal similarity between area summation and contrast adaptation. © 2004 ARVO.

What is second-order vision for? Discriminating illumination versus material changes

The human visual system is sensitive to second-order modulations of the local contrast (CM) or amplitude (AM) of a carrier signal. Second-order cues are detected independently of first-order luminance signals; however, it is not clear why vision should benet from second-order sensitivity. Analysis of the first-and second-order contents of natural images suggests that these cues tend to occur together, but their phase relationship varies. We have shown that in-phase combinations of LM and AM are perceived as a shaded corrugated surface whereas the anti-phase combination can be seen as corrugated when presented alone or as a flat material change when presented in a plaid containing the in-phase cue. We now extend these findings using new stimulus types and a novel haptic matching task. We also introduce a computational model based on initially separate first-and second-order channels that are combined within orientation and subsequently across orientation to produce a shading signal. Contrast gain control allows the LM + AM cue to suppress responses to the LM-AM when presented in a plaid. Thus, the model sees LM -AM as flat in these circumstances. We conclude that second-order vision plays a key role in disambiguating the origin of luminance changes within an image. © ARVO.