Dysfunction of the magnocellular stream in Alzheimer's disease evaluated by pattern electroretinograms and visual evoked potentials

Magnetoencephalographic brain activity evoked by the optic-flow task is correlated with β-amyloid burden and parahippocampal atrophy

Visuospatial perception is often impaired in people with Alzheimer's disease (AD). Because visuospatial information is thought to be processed in the visual dorsal stream, it is believed that brain activities in the dorsal stream will be altered in AD patients. In this study, we investigated whether regional brain activity related to visuospatial perception were associated with AD progression markers. An optic-flow task, which activates the dorsal stream associated with visuospatial perception, was performed, and the brain activities evoked by the task were evaluated using magnetoencephalography (MEG). First, we evaluated the responses to optic-flow stimuli in 21 cognitively unimpaired participants and determined the regions of interest (ROIs) where optic-flow activities were activated. Task-related activations were observed in 14 cortical regions including the dorsal stream: the right and left medial ventral occipital cortex (MVOcC), lateral occipital cortex (LOcC), precuneus (Pcun), inferior parietal lobule (IPL), superior parietal lobule (SPL), posterior superior temporal sulcus (pSTS), and fusiform gyri (FuG). Next, we performed correlation analyses between task-related activity in each ROI and two AD progression markers, global amyloid burden and parahippocampal gyrus (PHG) volume, for 25 participants who underwent amyloid positron emission tomography (PET) scans. We found that the global amyloid burden was negatively correlated with task-related activity in the left MVOcC and right SPL [r = -0.488 (p = 0.013) and r = -0.421 (p = 0.038), respectively]. Furthermore, significant positive correlations were observed between PHG volume and task-related activity in both the left and right SPL [r = 0.500 (p = 0.011) and r = 0.549 (p = 0.005), respectively]. Since the SPL is known to be responsible for visuospatial perception, these results suggest that MEG neuronal activity of patients performing the optic-flow activity can detect changes in brain activity associated with visuospatial impairment related to AD.

Age-Related Retinal Layer Thickness Changes Measured by OCT in APPNL-F/NL-F Mice: Implications for Alzheimer's Disease

In Alzheimer's disease (AD), transgenic mouse models have established links between abnormalities in the retina and those in the brain. APPNL-F/NL-F is a murine, humanized AD model that replicates several pathological features observed in patients with AD. Research has focused on obtaining quantitative parameters from optical coherence tomography (OCT) in AD. The aim of this study was to analyze, in a transversal case-control study using manual retinal segmentation via SD-OCT, the changes occurring in the retinal layers of the APPNL/F-NF/L AD model in comparison to C57BL/6J mice (WT) at 6, 9, 12, 15, 17, and 20 months of age. The analysis focused on retinal thickness in RNFL-GCL, IPL, INL, OPL, and ONL based on the Early Treatment Diabetic Retinopathy Study (ETDRS) sectors. Both APPNL-F/NL-F-model and WT animals exhibited thickness changes at the time points studied. While WT showed significant changes in INL, OPL, and ONL, the AD model showed changes in all retinal layers analyzed. The APPNL-F/NL-F displayed significant thickness variations in the analyzed layers except for the IPL compared to related WT. These thickness changes closely resembled those found in humans during preclinical stages, as well as during mild and moderate AD stages, making this AD model behave more similarly to the disease in humans.

Alzheimer's disease pathophysiology in the Retina

The retina is an emerging CNS target for potential noninvasive diagnosis and tracking of Alzheimer's disease (AD). Studies have identified the pathological hallmarks of AD, including amyloid β-protein (Aβ) deposits and abnormal tau protein isoforms, in the retinas of AD patients and animal models. Moreover, structural and functional vascular abnormalities such as reduced blood flow, vascular Aβ deposition, and blood-retinal barrier damage, along with inflammation and neurodegeneration, have been described in retinas of patients with mild cognitive impairment and AD dementia. Histological, biochemical, and clinical studies have demonstrated that the nature and severity of AD pathologies in the retina and brain correspond. Proteomics analysis revealed a similar pattern of dysregulated proteins and biological pathways in the retina and brain of AD patients, with enhanced inflammatory and neurodegenerative processes, impaired oxidative-phosphorylation, and mitochondrial dysfunction. Notably, investigational imaging technologies can now detect AD-specific amyloid deposits, as well as vasculopathy and neurodegeneration in the retina of living AD patients, suggesting alterations at different disease stages and links to brain pathology. Current and exploratory ophthalmic imaging modalities, such as optical coherence tomography (OCT), OCT-angiography, confocal scanning laser ophthalmoscopy, and hyperspectral imaging, may offer promise in the clinical assessment of AD. However, further research is needed to deepen our understanding of AD's impact on the retina and its progression. To advance this field, future studies require replication in larger and diverse cohorts with confirmed AD biomarkers and standardized retinal imaging techniques. This will validate potential retinal biomarkers for AD, aiding in early screening and monitoring.

Sensory processing deficits and related cortical pathological changes in Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder primarily affecting cognitive functions. However, sensory deficits in AD start to draw attention due to their high prevalence and early onsets which suggest that they could potentially serve as diagnostic biomarkers and even contribute to the disease progression. This literature review examines the sensory deficits and cortical pathological changes observed in visual, auditory, olfactory, and somatosensory systems in AD patients, as well as in various AD animal models. Sensory deficits may emerge at the early stages of AD, or even precede the cognitive decline, which is accompanied by cortical pathological changes including amyloid-beta deposition, tauopathy, gliosis, and alterations in neuronal excitability, synaptic inputs, and functional plasticity. Notably, these changes are more pronounced in sensory association areas and superficial cortical layers, which may explain the relative preservation of basic sensory functions but early display of deficits of higher sensory functions. We propose that sensory impairment and the progression of AD may establish a cyclical relationship that mutually perpetuates each condition. This review highlights the significance of sensory deficits with or without cortical pathological changes in AD and emphasizes the need for further research to develop reliable early detection and intervention through sensory systems.

Retinal electrophysiology in central nervous system disorders. A review of human and mouse studies

The retina and brain share similar neurochemistry and neurodevelopmental origins, with the retina, often viewed as a "window to the brain." With retinal measures of structure and function becoming easier to obtain in clinical populations there is a growing interest in using retinal findings as potential biomarkers for disorders affecting the central nervous system. Functional retinal biomarkers, such as the electroretinogram, show promise in neurological disorders, despite having limitations imposed by the existence of overlapping genetic markers, clinical traits or the effects of medications that may reduce their specificity in some conditions. This narrative review summarizes the principal functional retinal findings in central nervous system disorders and related mouse models and provides a background to the main excitatory and inhibitory retinal neurotransmitters that have been implicated to explain the visual electrophysiological findings. These changes in retinal neurochemistry may contribute to our understanding of these conditions based on the findings of retinal electrophysiological tests such as the flash, pattern, multifocal electroretinograms, and electro-oculogram. It is likely that future applications of signal analysis and machine learning algorithms will offer new insights into the pathophysiology, classification, and progression of these clinical disorders including autism, attention deficit/hyperactivity disorder, bipolar disorder, schizophrenia, depression, Parkinson's, and Alzheimer's disease. New clinical applications of visual electrophysiology to this field may lead to earlier, more accurate diagnoses and better targeted therapeutic interventions benefiting individual patients and clinicians managing these individuals and their families.

Alzheimer's disease: a continuum with visual involvements

Introduction

Alzheimer's disease (AD) is the most common form of dementia affecting the central nervous system, and alteration of several visual structures has been reported. Structural retinal changes are usually accompanied by changes in visual function in this disease. The aim of this study was to analyse the differences in visual function at different stages of the pathology (family history group (FH+), mild cognitive impairment (MCI), mild AD and moderate AD) in comparison with a control group of subjects with no cognitive decline and no family history of AD.

Methods

We included 53 controls, 13 subjects with FH+, 23 patients with MCI, 25 patients with mild AD and, 21 patients with moderate AD. All were ophthalmologically healthy. Visual acuity (VA), contrast sensitivity (CS), colour perception, visual integration, and fundus examination were performed.

Results

The analysis showed a statistically significant decrease in VA, CS and visual integration score between the MCI, mild AD and moderate AD groups compared to the control group. In the CS higher frequencies and in the colour perception test (total errors number), statistically significant differences were also observed in the MCI, mild AD and moderate AD groups with respect to the FH+ group and also between the control and AD groups. The FH+ group showed no statistically significant difference in visual functions compared to the control group. All the test correlated with the Mini Mental State Examination score and showed good predictive value when memory decline was present, with better values when AD was at a more advanced stage.

Conclusion

Alterations in visual function appear in subjects with MCI and evolve when AD is established, being stable in the initial stages of the disease (mild AD and moderate AD). Therefore, visual psychophysical tests are a useful, simple and complementary tool to neuropsychological tests to facilitate diagnosis in the preclinical and early stages of AD.

Disruption of early visual processing in amyloid-positive healthy individuals and mild cognitive impairment

BACKGROUND

Amyloid deposition is a primary predictor of Alzheimer's disease (AD) and related neurodegenerative disorders. Retinal changes involving the structure and function of the ganglion cell layer are increasingly documented in both established and prodromal AD. Visual event-related potentials (vERP) are sensitive to dysfunction in the magno- and parvocellular visual systems, which originate within the retinal ganglion cell layer. The present study evaluates vERP as a function of amyloid deposition in aging, and in mild cognitive impairment (MCI).

METHODS

vERP to stimulus-onset, motion-onset, and alpha-frequency steady-state (ssVEP) stimuli were obtained from 16 amyloid-positive and 41 amyloid-negative healthy elders and 15 MCI individuals and analyzed using time-frequency approaches. Social cognition was assessed in a subset of individuals using The Awareness of Social Inference Test (TASIT).

RESULTS

Neurocognitively intact but amyloid-positive participants and MCI individuals showed significant deficits in stimulus-onset (theta) and motion-onset (delta) vERP generation relative to amyloid-negative participants (all p < .01). Across healthy elders, a composite index of these measures correlated highly (r =  - .52, p < .001) with amyloid standardized uptake value ratios (SUVR) and TASIT performance. A composite index composed of vERP measures significant differentiated amyloid-positive and amyloid-negative groups with an overall classification accuracy of > 70%.

DISCUSSION

vERP may assist in the early detection of amyloid deposition among older individuals without observable neurocognitive impairments and in linking previously documented retinal deficits in both prodromal AD and MCI to behavioral impairments in social cognition.

Hemispheric Patterns of Recruitment of Object Processing Regions in Early Alzheimer's Disease: A Study Along the Entire Ventral Stream

BACKGROUND

Investigation of neural response patterns along the entire network of functionally defined object recognition ventral stream regions in Alzheimer's disease (AD) is surprisingly lacking.

OBJECTIVE

We aimed to investigate putative functional reorganization along a wide-ranging network of known regions in the ventral visual stream in mild AD.

METHODS

Overall we investigated 6 regions of interest (5 of which were not investigated before), in 19 AD patients and 19 controls, in both hemispheres along the ventral visual stream: Fusiform Face Area, Fusiform Body Area, Extrastriate Body Area, Lateral Occipital Cortex, Parahippocampal Place Area, and Visual Word Form Area, while assessing object recognition performance.

RESULTS

We found group differences in dprime measures for all object categories, corroborating generalized deficits in object recognition. Concerning neural responses, we found region dependent group differences respecting a priori expected Hemispheric asymmetries. Patients showed significantly decreased BOLD responses in the right hemisphere-biased Fusiform Body Area, and lower left hemisphere responses in the Visual Word Form Area (with a priori known left hemispheric bias), consistent with deficits in body shape and word/pseudoword processing deficits. This hemispheric dominance related effects were preserved when controlling for performance differences. Whole brain analysis during the recognition task showed enhanced activity in AD group of left dorsolateral prefrontal cortex, left cingulate gyrus, and in the posterior cingulate cortex- a hotspot of amyloid-β accumulation.

CONCLUSION

These findings demonstrate region dependent respecting hemispheric dominance patterns activation changes in independently localized selective regions in mild AD, accompanied by putative compensatory activity of frontal and cingular networks.

Altered visual entrainment in patients with Alzheimer’s disease: magnetoencephalography evidence

Recent research has indicated that rhythmic visual entrainment may be useful in clearing pathological protein deposits in the central nervous system of mouse models of Alzheimer’s disease. However, visual entrainment studies in human patients with Alzheimer’s disease are rare, and as such the degree to which these patients exhibit aberrations in the neural tracking of rhythmic visual stimuli is unknown. To fill this gap, we recorded magnetoencephalography during a 15 Hz visual entrainment paradigm in amyloid-positive patients on the Alzheimer’s disease spectrum and compared their neural responses to a demographically matched group of biomarker-negative healthy controls. Magnetoencephalography data were imaged using a beamformer and virtual sensor data were extracted from the peak visual entrainment responses. Our results indicated that, relative to healthy controls, participants on the Alzheimer’s disease spectrum exhibited significantly stronger 15 Hz entrainment in primary visual cortices relative to a pre-stimulus baseline period. However, the two groups exhibited comparable absolute levels of neural entrainment, and higher absolute levels of entertainment predicted greater Mini-mental Status Examination scores, such that those patients whose absolute entrainment amplitude was closer to the level seen in controls had better cognitive function. In addition, 15 Hz periodic activity, but not aperiodic activity, during the pre-stimulus baseline period was significantly decreased in patients on the Alzheimer’s disease spectrum. This pattern of results indicates that patients on the Alzheimer’s disease spectrum exhibited increased visual entrainment to rhythmic stimuli and that this increase is likely compensatory in nature. More broadly, these results show that visual entrainment is altered in patients with Alzheimer’s disease and should be further examined in future studies, as changes in the capacity to entrain visual stimuli may prove useful as a marker of Alzheimer’s disease progression.

Preserved Extra-Foveal Processing of Object Semantics in Alzheimer's Disease

Alzheimer's disease (AD) patients underperform on a range of tasks requiring semantic processing, but it is unclear whether this impairment is due to a generalised loss of semantic knowledge or to issues in accessing and selecting such information from memory. The objective of this eye-tracking visual search study was to determine whether semantic expectancy mechanisms known to support object recognition in healthy adults are preserved in AD patients. Furthermore, as AD patients are often reported to be impaired in accessing information in extra-foveal vision, we investigated whether that was also the case in our study. Twenty AD patients and 20 age-matched controls searched for a target object among an array of distractors presented extra-foveally. The distractors were either semantically related or unrelated to the target (e.g., a car in an array with other vehicles or kitchen items). Results showed that semantically related objects were detected with more difficulty than semantically unrelated objects by both groups, but more markedly by the AD group. Participants looked earlier and for longer at the critical objects when these were semantically unrelated to the distractors. Our findings show that AD patients can process the semantics of objects and access it in extra-foveal vision. This suggests that their impairments in semantic processing may reflect difficulties in accessing semantic information rather than a generalised loss of semantic memory.

Altered Visual Cortical Excitability Is Associated With Psychopathological Symptoms in Major Depressive Disorder

Previous studies suggest that in people with major depressive disorder (MDD), there exists a perturbation of the normal balance between the excitatory and inhibitory neurotransmitter systems in the visual cortex, indicating the possibility of altered visual cortical excitability. However, investigations into the neural activities of the visual cortex in MDD patients yielded inconsistent findings. The present study aimed to evaluate the visual cortical excitability utilizing a paired-pulse stimulation paradigm in patients with MDD and to access the paired-pulse behavior of recording visual evoked potentials (VEPs) as a marker of MDD. We analyzed the amplitudes of VEPs and paired-pulse suppression (PPS) at four different stimulus onset asynchronies (SOAs) spanning 93 ms to 133 ms. Further, the relationship between PPS and the symptom severity of depression was investigated using Spearman's correlation. We found that, whereas the first VEP amplitude remained unchanged, the second VEP amplitude was significantly higher in the MDD group compared to the healthy controls. As a result, the amplitude ratio (second VEP amplitude/first VEP amplitude) increased, indicating reduced PPS and thus increased excitability in the visual cortex. Moreover, we found the amplitude ratios had a significantly positive correlation with the symptom severity of depression in MDD, indicating a clinically useful biomarker for MDD. Our findings provide new insights into the changes in the excitation-inhibition balance of visual cortex in MDD, which could pave the way for specific clinical interventions.

Past, present and future role of retinal imaging in neurodegenerative disease

Retinal imaging technology is rapidly advancing and can provide ever-increasing amounts of information about the structure, function and molecular composition of retinal tissue in humans in vivo. Most importantly, this information can be obtained rapidly, non-invasively and in many cases using Food and Drug Administration-approved devices that are commercially available. Technologies such as optical coherence tomography have dramatically changed our understanding of retinal disease and in many cases have significantly improved their clinical management. Since the retina is an extension of the brain and shares a common embryological origin with the central nervous system, there has also been intense interest in leveraging the expanding armamentarium of retinal imaging technology to understand, diagnose and monitor neurological diseases. This is particularly appealing because of the high spatial resolution, relatively low-cost and wide availability of retinal imaging modalities such as fundus photography or OCT compared to brain imaging modalities such as magnetic resonance imaging or positron emission tomography. The purpose of this article is to review and synthesize current research about retinal imaging in neurodegenerative disease by providing examples from the literature and elaborating on limitations, challenges and future directions. We begin by providing a general background of the most relevant retinal imaging modalities to ensure that the reader has a foundation on which to understand the clinical studies that are subsequently discussed. We then review the application and results of retinal imaging methodologies to several prevalent neurodegenerative diseases where extensive work has been done including sporadic late onset Alzheimer's Disease, Parkinson's Disease and Huntington's Disease. We also discuss Autosomal Dominant Alzheimer's Disease and cerebrovascular small vessel disease, where the application of retinal imaging holds promise but data is currently scarce. Although cerebrovascular disease is not generally considered a neurodegenerative process, it is both a confounder and contributor to neurodegenerative disease processes that requires more attention. Finally, we discuss ongoing efforts to overcome the limitations in the field and unmet clinical and scientific needs.

Retinal ganglion cell dysfunction in preclinical Alzheimer's disease: an electrophysiologic biomarker signature

The current study evaluated retinal function using electroretinography (ERG) in cognitively healthy (CH) participants with preclinical Alzheimer's disease (AD), as classified by cerebral spinal fluid (CSF) Aβ₄₂/Tau ratio. Individuals with normal retinal morphology ascertained by spectral-domain optical coherence tomography were enrolled. Full-field ERG, pattern PERG, and photopic negative response (PhNR) were performed in 29 adult participants (58 eyes). Amplitude and implicit times of the ERG wave components were analyzed. Preclinical AD participants showed marked retinal ganglion cell dysfunction relative to controls. The PhNR was significantly diminished in preclinical AD relative to controls. PhNR amplitude and N95 implicit time differentiated CH individuals with CSF biomarkers of AD pathology with 87% sensitivity and 82% specificity. These quantitative electrophysiologic findings expand our understanding of early retinal functional changes that precede cognitive decline in AD. Retinal ganglion cell dysfunction, as detected by ERG, may be a clinically useful, non-invasive in vivo biomarker for early disease detection, which is necessary for ultimately pursuing early intervention.

SSVEP phase synchronies and propagation during repetitive visual stimulation at high frequencies

Steady-state visual evoked potentials (SSVEPs), the brain response to visual flicker stimulation, have proven beneficial in both research and clinical applications. Despite the practical advantages of stimulation at high frequencies in terms of visual comfort and safety, high frequency-SSVEPs have not received enough attention and little is known about the mechanisms behind their generation and propagation in time and space. In this study, we investigated the origin and propagation of SSVEPs in the gamma frequency band (40-60 Hz) by studying the dynamic properties of EEG in 32 subjects. Using low-resolution brain electromagnetic tomography (sLORETA) we identified the cortical sources involved in SSVEP generation in that frequency range to be in the primary visual cortex, Brodmann areas 17, 18 and 19 with minor contribution from sources in central and frontal sites. We investigated the SSVEP propagation as measured on the scalp in the framework of the existing theories regarding the neurophysiological mechanism through which the SSVEP spreads through the cortex. We found a progressive phase shift from posterior parieto-occipital sites over the cortex with a phase velocity of approx. 8-14 m/s and wavelength of about 21 and 24 cm. The SSVEP spatial properties appear sensitive to input frequency with higher stimulation frequencies showing a faster propagation speed.

Rod pathway and cone pathway retinal dysfunction in the 5xFAD mouse model of Alzheimer's disease

To characterize rod- and cone-pathway function in the 5xFAD mouse model of Alzheimer’s disease (AD) using the full-field electroretinogram (ERG). Dark-adapted (DA; rod-pathway) and light-adapted (LA; cone-pathway) ERGs were recorded from three-month-old 5xFAD and wild type (WT) mice. ERGs were elicited by achromatic flashes (0.01–25 cd-s-m⁻²). Amplitude and implicit time (IT) of the a-wave, b-wave, and oscillatory potentials (OPs) were calculated according to convention. In addition, the amplitude and IT of the photopic negative response (PhNR) were measured from the LA recordings. Amplitude and IT differences between the 5xFAD and WT groups were evaluated using quantile regression models. Under DA conditions, there were significant differences between the 5xFAD and WT groups in post-receptor function, whereas photoreceptor function did not differ significantly. Specifically, the DA a-wave amplitude did not differ between groups (p = 0.87), whereas the b-wave amplitude was reduced in the 5xFAD mice (p = 0.003). There were significant OP (p < 0.001) and a-wave (p = 0.04) delays, but the a-wave delay may be attributable to a post-receptor abnormality. Under LA conditions, the only 5xFAD abnormalities were in the PhNR, which was reduced (p = 0.009) and delayed (p = 0.04). The full-field ERG can be abnormal in the 5xFAD model of AD, with the greatest effects on post-receptor rod pathway function. These results indicate that retinal electrophysiology may be a useful tool for evaluating neural dysfunction in AD.

RE-PERG in early-onset Alzheimer's disease: A double-blind, electrophysiological pilot study

PURPOSE

To evaluate the ability of re-test pattern electroretinogram (RE-PERG), a non-invasive and fast steady-state PERG, to detect inner retinal bioelectric function anomalies in patients with early-onset Alzheimer's disease (AD).

METHODS

The study population consisted of 17 patients with AD-related mild cognitive impairment (MCI), 16 patients with vascular dementia (VD)-related MCI, both assessed using the neuropsychological Mini-Mental State Examination (MMSE) and by structural magnetic resonance imaging, and 19 healthy, age-matched normal controls (NC). All participants were visually asymptomatic, had normal or near-normal general cognitive functioning and no or minimal impairments in daily life activities. Visual field (VF) test, optical coherence tomography (OCT) and RE-PERG, sampled in five consecutive blocks of 130 events, were performed.

RESULTS

There was no statistically significant difference among the three groups with respect to age, VF parameters (mean and pattern standard deviations) and OCT parameters (ganglion cell complex thickness and retinal nerve fiber layer thickness). The mean amplitude in the RE-PERG was significantly lower, but only weakly in the AD group than in NC (p = 0.1) whereas the intrinsic variability of the 2nd harmonic phase was significantly higher in the AD group than in either the VD or NC group (p<0.001).

CONCLUSIONS

RE-PERG is altered in early-stage AD, showing a reduced amplitude with high intrinsic phase variability. It also allows the discrimination of AD from VD. A high intrinsic variability in the PERG signal, determined using RE-PERG, may thus be a new promising test for neurodegenerative diseases.

Retinal Functional and Structural Changes in the 5xFAD Mouse Model of Alzheimer's Disease

Alzheimer’s disease is characterized by the aberrant deposition of protein in the brain and is the leading cause of dementia worldwide. Increasingly, there have been reports of the presence of these protein hallmarks in the retina. In this study, we assayed the retina of 5xFAD mice, a transgenic model of amyloid deposition known to exhibit dementia-like symptoms with age. Using OCT, we found that the retinal nerve fiber layer was thinner in 5xFAD at 6, 12, and 17 months of age compared with wild-type littermates, but the inner plexiform layer was thicker at 6 months old. Retinal function showed reduced ganglion cell responses to light in 5xFAD at 6, 12, and 17 months of age. This functional loss was observed in the outer retina at 17 months of age but not in younger mice. We showed using immunohistochemistry and ELISA that soluble and insoluble amyloid was present in the retina and brain at all ages. In conclusion, we report that amyloid is present in brain and retina of 5xFAD mice and that the pattern of neuronal dysfunction occurs in the inner retina at the early ages and progresses to encompass the outer retina with age. This implies that the inner retina is more sensitive to amyloid changes in early disease and that the outer retina is also affected with disease progression.

Variability, repeatability and test-retest reliability of equine flash visual evoked potentials (FVEPs)

Background

Visual evoked potentials (VEPs) are electrical potentials generated by neurons in the central nervous system in response to visual stimuli. A series of positive and negative wavelets in response to flash-stimuli (flash-VEP; FVEP) or reversing, iso-luminant patterns (pattern-VEP; PVEP) are recorded. Pathological conditions affecting the post-retinal pathways can alter overall waveform morphology, and also affect wavelet peak times and amplitudes. FVEPs have recently been described in horses, but more data on the variability within and between subjects is required, to adequately interpret results from clinical equine patients. Therefore, the purpose of this study was to describe the variability, repeatability and test-retest reliability of equine FVEPs in normal, adult horses.

Results

Equine FVEPs were recorded from one randomly selected eye in 17 horses, from both eyes in eight of these horses, and also at two separate recording sessions in six horses. N1, P2, N2 and P4 wavelets were present in 100% of the recordings in all horses, while P1, N2a, P3 and P5 were only present in some recordings. Coefficients of variation (CVs) were low for P2, N2 and P4 peak times, but higher for all amplitudes. There were no statistically significant differences comparing peak times and amplitudes between eyes or between sessions. Coefficients of repeatability (CRs) are reported for P2, N2 and P4 peak times between eyes (P2; 5 ms, N2; 18 ms, P4; 18 ms) and also between sessions (P2; 5 ms, N2; 16 ms, P4; 39 ms). Intraclass correlation coefficients (ICCs), as an estimate of test-retest reliability, was assessed to be fair to excellent for most parameters.

Conclusions

This study provides important data on variability, repeatability and test-retest reliability of FVEPs in normal, adult horses. We conclude that P2, N2 and P4 peak times should be included in the evaluation of equine FVEPs. The large inherent variability of FVEP amplitudes is likely to make them less suitable and useful for establishing a diagnosis on their own in most clinical patients, but they may occasionally provide support to a clinical diagnosis.

Detection of structural and electrical disturbances in macula and optic nerve in Alzheimer's patients and their correlation with disease severity

Aim: To evaluate and compare structural and functional changes in macula and optic nerve in Alzheimer disease (AD) patients and healthy subjects.Methods: Both eyes of 20 AD patients and 40 age-matched healthy controls were evaluated. All subjects were evaluated by cognitive testing and comprehensive ophthalmological examination, including visual acuity, visual fields, color vision, contrast sensitivity, anterior, and posterior segment examination, optical coherence tomography, multifocal electroretinography (mfERG), and pattern-reversal visual evoked potential (pVEP).Results: AD patients showed significantly reduced contrast sensitivity, thinner nerve fiber layer, ganglion cell layer andmacular volume. Multifocal ERG wave amplitudes were significantly reduced with delayed implicit times, which correlated significantly with the inner retinal layer thinning and poorer disease severity scores. The correlation with structural changes and disease severity was highest for pVEP, which showed significant derangement in AD patients.Conclusion: Subclinical visual dysfunction may be present in AD patients, which may be detected as inner retinal thinning. A probable photoreceptor abnormality may also form a part of the AD disease process.

Evaluation of Retinal Nerve Fiber Layer Thickness, Electroretinogram and Visual Evoked Potential in Patients with Alzheimer's Disease

OBJECTIVE

To study the retinal nerve fibre layer (RNFL) thickness and visual electrophysiology testing in patients with Alzheimer's disease (AD).

METHODS

A cross-sectional, hospital-based study: 25 AD subjects and 25 controls were recruited. Candidates who fulfil the criteria with normal ocular examinations were made to proceed with scanning laser polarimetry, pattern electroretinogram (PERG), and pattern visual evoked potential (PVEP) examinations of the right eye. RNFL thickness, PERG, and PVEP readings were evaluated.

RESULTS

In AD, the mean of average RNFL thickness was 45.28 μm, SD = 3.61, P < 0.001 (P < 0.05), while the superior RNFL thickness was 54.44 μm, SD = 2.85, P < 0.001 (P < 0.05) and inferior RNFL thickness was 47.11 μm, SD = 4.52, P < 0.001 (P < 0.05). For PERG, the mean P50 latency was 63.88 ms, SD = 7.94, P < 0.001 (P < 0.05) and the mean amplitudes of P50 waves were 1.79 μV, SD = 0.64, P < 0.001 (P < 0.05) and N95 waves were 2.43 μV, SD = 0.90, P < 0.001 (P < 0.05). For PVEP, the mean latency of P100 was 119.00 ms, SD = 9.07, P < 0.001 (P < 0.05), while the mean latency of N135 was 145.20 ms, SD = 8.53, P < 0.001 (P < 0.05). The mean amplitude of P100 waves was 3.71 μV, SD = 1.60, P < 0.001 (P < 0.05), whereas the mean amplitude of N135 waves was 3.67 μV, SD = 2.02, P < 0.001 (P < 0.05). RNFL thickness strongly correlates with PERG readings, with P50 latency R = 0.582, R2 = 0.339, P=0.002 (P < 0.05), amplitude of P50 wave at R = 0.749, R2 = 0.561, P ≤ 0.001 (P < 0.05), and amplitude of N95 wave at R = 0.500, R2 = 0.250, P=0.011 (P < 0.05). No significant difference and correlation were observed on PVEP readings.

CONCLUSION

The mean of the average, superior and inferior RNFL thickness were significantly lower in the AD group compared with control. There is also significant difference of PERG and PVEP parameters between AD and controls. Regression analysis showed average RNFL thickness having significantly linear relationship with the PERG parameters.

Retinal thinning of inner sub-layers is associated with cortical atrophy in a mouse model of Alzheimer's disease: a longitudinal multimodal in vivo study

BACKGROUND

It has been claimed that the retina can be used as a window to study brain disorders. However, concerning Alzheimer's disease (AD), it still remains controversial whether changes occurring in the brain and retina are associated. We aim to understand when changes start appearing in the retina and brain, how changes progress, and if they are correlated.

METHODS

We carried out a unique longitudinal study, at 4, 8, 12, and 16 months of age, in a triple transgenic mouse model of AD (3×Tg-AD), which mimics pathological and neurobehavioral features of AD, as we have already shown. Retinal structure and physiology were evaluated in vivo using optical coherence tomography and electroretinography. Brain visual cortex structure was evaluated in vivo using magnetic resonance imaging.

RESULTS

The retinal thickness of 3×Tg-AD decreased, at all time points, except for the outer nuclear layer, where the opposite alteration was observed. Amplitudes in scotopic and photopic responses were increased throughout the study. Similarly, higher amplitude and lower phase values were observed in the photopic flicker response. No differences were found in the activity of retinal ganglion cells. Visual cortex gray matter volume was significantly reduced.

CONCLUSIONS

Our results show that this animal model shows similar neural changes in the retina and brain visual cortex, i.e., retinal and brain thinning. Moreover, since similar changes occur in the retina and brain visual cortex, these observations support the possibility of using the eye as an additional tool (noninvasive) for early AD diagnosis and therapeutic monitoring.

The Retina as a Window or Mirror of the Brain Changes Detected in Alzheimer's Disease: Critical Aspects to Unravel

Alzheimer's disease is the most frequent cause of dementia worldwide, representing a global health challenge, with a massive impact on the quality of life of Alzheimer's disease patients and their relatives. The diagnosis of Alzheimer's disease constitutes a real challenge, because the symptoms manifest years after the first degenerative changes occurring in the brain and the diagnosis is based on invasive and/or expensive techniques. Therefore, there is an urgent need to identify new reliable biomarkers to detect Alzheimer's disease at an early stage. Taking into account the evidence for visual deficits in Alzheimer's disease patients, sometimes even before the appearance of the first disease symptoms, and that the retina is an extension of the brain, the concept of the retina as a window to look into the brain or a mirror of the brain has received increasing interest in recent years. However, only a few studies have assessed the changes occurring in the retina and the brain at the same time points. Unlike previous reviews on this subject, which are mainly focused on brain changes, we organized this review by comprehensively summarizing findings related with structural, functional, cellular, and molecular parameters in the retina reported in both Alzheimer's disease patients and animal models. Moreover, we separated the studies that assessed only the retina, and those that assessed both the retina and brain, which are few but allow establishing correlations between the retina and brain. This review also highlights some inconsistent results in the literature as well as relevant missing gaps in this field.

Visual Features in Alzheimer's Disease: From Basic Mechanisms to Clinical Overview

Alzheimer's disease (AD) is the leading cause of dementia worldwide. It compromises patients' daily activities owing to progressive cognitive deterioration, which has elevated direct and indirect costs. Although AD has several risk factors, aging is considered the most important. Unfortunately, clinical diagnosis is usually performed at an advanced disease stage when dementia is established, making implementation of successful therapeutic interventions difficult. Current biomarkers tend to be expensive, insufficient, or invasive, raising the need for novel, improved tools aimed at early disease detection. AD is characterized by brain atrophy due to neuronal and synaptic loss, extracellular amyloid plaques composed of amyloid-beta peptide (Aβ), and neurofibrillary tangles of hyperphosphorylated tau protein. The visual system and central nervous system share many functional components. Thus, it is plausible that damage induced by Aβ, tau, and neuroinflammation may be observed in visual components such as the retina, even at an early disease stage. This underscores the importance of implementing ophthalmological examinations, less invasive and expensive than other biomarkers, as useful measures to assess disease progression and severity in individuals with or at risk of AD. Here, we review functional and morphological changes of the retina and visual pathway in AD from pathophysiological and clinical perspectives.

Visual Evoked Potentials in Glaucoma and Alzheimer's Disease

Visual responses to gratings alternating in contrast have been studied in humans and several mammalian species. Previous evidence from human patients and animal models of neurodegeneration has highlighted the importance to record simultaneously the pattern electroretinogram (P-ERG) and visual evoked cortical potentials (VEPs) to investigate retinal and post-retinal sites of neurodegeneration.In view of the increasing importance of research on experimental models of neurodegenerative diseases, we present here the parametric properties of visual evoked responses in animal models of glaucoma and Alzheimer's disease. Glaucoma and Alzheimer's disease (AD) are two distinct multifactorial neurodegenerative and progressive diseases, primarily affecting the elderly.

The retina as a window to early dysfunctions of Alzheimer's disease following studies with a 5xFAD mouse model

Alzheimer's disease (AD) is a progressive neurodegenerative disease leading to neuronal dysfunctions with cognitive impairment. AD can affect visual pathways and visual cortex and result in various visual changes and problems. However, how early the visual dysfunctions occur in AD is still a matter of discussion. Here, we used electrophysiological techniques to show the presence of early anomalies in AD visual system. To this aim, we used a familial AD (FAD) model, the 5xFAD transgenic mouse, characterized by severe progressive amyloid pathology and cognitive deficits. We investigated the retina and primary visual cortex responsivity together with behavioral assessment of the visual acuity. Visual tests and recordings were conducted at different ages in 5xFAD mice, corresponding to different phases of neurodegeneration and beta amyloid accumulation. We showed that the visual system is impaired in 5xFAD mice. In particular, we found that the inner retina impairment precedes neuronal disorders in other brain areas and cognitive deficits. Thus, noninvasive retinal electrophysiology can provide a support for assessing early visual dysfunctions in AD.

Vision in Alzheimer's disease: a focus on the anterior afferent pathway

Visual dysfunction has long been recognized as a manifestation of Alzheimer's disease (AD), particularly in the form of visuospatial impairment during all stages of disease. However, investigations have revealed findings within the anterior (i.e., pregeniculate) afferent visual pathways that rely on retinal imaging and electrophysiologic methodologies for detection. Here we focus on the anterior afferent visual pathways in AD and the measures used for assessment, including optical coherence tomography, electrophysiology, color vision testing and threshold visual field perimetry. A brief summary of higher order visual dysfunction is also included to allow the reader to keep in context the broader findings of afferent visual dysfunction in AD.

Neurosteroids and oxysterols as potential therapeutic agents for glaucoma and Alzheimer's disease

Glaucoma is one of the most frequent causes of visual impairment worldwide and involves selective damage to retinal ganglion cells (RGCs) resulting in degeneration of neural pathways connecting retina to visual cortex. It is of interest that similarities in pathological changes have been described in Alzheimer's disease (AD), the most common cause of progressive memory loss and dementia in older people. Accumulation of amyloid-beta (Abeta) and hyperphosphorylated tau is thought to contribute to apoptotic neuronal death in Alzheimer's disease, and similar changes have been linked to apoptotic RGC death in glaucoma. Both glaucoma and Alzheimer's disease also suffer from a lack of effective treatments prompting a search for novel therapeutic interventions. Neurosteroids (NSs) (including oxysterols) are endogenous molecules synthesized in the nervous system from cholesterol that can modulate glutamate and GABA receptors, the primary mediators of fast excitatory and inhibitory neurotransmission in the brain, respectively. Because changes in the glutamate and GABA neurotransmitter systems contribute to the pathogenesis of AD and glaucoma, NSs are possible therapeutic targets for these disorders. In this review, we present recent evidence supporting pathological links between Alzheimer's disease and glaucoma, and focus on the possible role of NSs in these diseases and how NSs might be developed for therapeutic purposes.

Visual Dysfunction in Posterior Cortical Atrophy

Posterior cortical atrophy (PCA) is a syndromic diagnosis. It is characterized by progressive impairment of higher (cortical) visual function with imaging evidence of degeneration affecting the occipital, parietal, and posterior temporal lobes bilaterally. Most cases will prove to have Alzheimer pathology. The aim of this review is to summarize the development of the concept of this disorder since it was first introduced. A critical discussion of the evolving diagnostic criteria is presented and the differential diagnosis with regard to the underlying pathology is reviewed. Emphasis is given to the visual dysfunction that defines the disorder, and the classical deficits, such as simultanagnosia and visual agnosia, as well as the more recently recognized visual field defects, are reviewed, along with the evidence on their neural correlates. The latest developments on the imaging of PCA are summarized, with special attention to its role on the differential diagnosis with related conditions.

Color perception differentiates Alzheimer's Disease (AD) from Vascular Dementia (VaD) patients

BACKGROUND

Alzheimer's Disease (AD) and Vascular Dementia (VaD) are the most common causes of dementia in older people. Both diseases appear to have similar clinical symptoms, such as deficits in attention and executive function, but specific cognitive domains are affected. Current cohort studies have shown a close relationship between αβ deposits and age-related macular degeneration (Johnson et al., 2002; Ratnayaka et al., 2015). Additionally, a close link between the thinning of the retinal nerve fiber (RNFL) and AD patients has been described, while it has been proposed that AD patients suffer from a non-specific type of color blindness (Pache et al., 2003).

METHODS

Our study included 103 individuals divided into three groups: A healthy control group (n = 35), AD (n = 32) according to DSM-IV-TR, NINCDS-ADRDA criteria, and VaD (n = 36) based on ΝΙΝDS-AIREN, as well as Magnetic Resonance Imaging (MRI) results. The severity of patient's cognitive impairment, was measured with the Mini-Mental State Examination (MMSE) and was classified according to the Reisberg global deterioration scale (GDS). Visual perception was examined using the Ishihara plates: "Ishihara Color Vision Test - 38 Plate."

RESULTS

The three groups were not statistically different for demographic data (age, gender, and education). The Ishihara color blindness test has a sensitivity of 80.6% and a specificity of 87.5% to discriminate AD and VaD patients when an optimal (32.5) cut-off value of performance is used.

CONCLUSIONS

Ishihara Color Vision Test - 38 Plate is a promising potential method as an easy and not time-consuming screening test for the differential diagnosis of dementia between AD and VaD.

Looking into the brain through the retinal ganglion cells in psychiatric disorders: A review of evidences

Psychiatry and neuroscience research need novel approaches to indirectly investigate brain function. As the retina is an anatomical and developmental extension of the central nervous system (CNS), changes in retinal function may reflect neurological dysfunctions in psychiatric disorders. The last and most integrated retinal relay before visual information transfer to the brain is the ganglion cell layer. Here, based on collected arguments, we argue that these cells offer a crucial site for indirectly investigating brain function. We describe the anatomical and physiological properties of these cells together with measurements of their functional properties named pattern electroretinogram (PERG). Based on ganglion cell dysfunctions measured with PERG in neurological disorders, we argue for the relevance of studying ganglion cell function in psychiatric research. We review studies that have evaluated ganglion cell function in psychiatric and addictive disorders and discuss how changes in PERG measurements could be functional markers of pathophysiological mechanisms of psychiatric disorders.

[Functional examinations of visual channels: clinical aspects]

The review discusses literature data on the clinical relevance of functional assessment of magno- (M), parvo- (P), and koniocellular (K) pathways. It also covers the differential contribution of the M, P, and K channels to visual impairments and how it determines the prognosis, early diagnosis, and treatment choice in patients with neurodegenerative diseases of the retina and brain. Selective changes in the performance of the visual channels are investigated by the example of glaucoma and optic neuritis in multiple sclerosis (MS) patients. A detailed analysis of pattern electroretinogram and visual evoked potentials in response to pattern stimuli of varying luminance and chromatic contrast in glaucoma and MS and characteristic functional alterations (objective markers of pathology of the visual pathways) are presented.

[Functional examinations of visual channels: physiological basis]

In this paper, technical details of visual evoked potentials (VEP) assessment and pattern electroretinography (PERG) are reviewed. Both methods are used to perform an objective functional examination of visual channels and to clarify the level, at which they have been damaged. Contributions of parvo- (P), magno- (M) and koniocellular (K) systems to the morphology of PERG and VEP responses are discussed with account to test conditions, selectively supportive of the activity of particular cell populations. The review analyzes the physiological role of such stimulation parameters as brightness and color contrast of the pattern elements as well as spatial and temporal frequency in detecting dysfunction of color channels and mistuning of the P- and M- pathways. Different times taken for neuronal integration and signal conduction along the M- and P- pathways determine the timing of the P- and M- VEP components, allowing us to judge their contribution to VEP morphology from the same recording.

Different trajectories of decline for global form and global motion processing in aging, mild cognitive impairment and Alzheimer's disease

The visual processing of complex motion is impaired in Alzheimer's disease (AD). However, it is unclear whether these impairments are biased toward the motion stream or part of a general disruption of global visual processing, given some reports of impaired static form processing in AD. Here, for the first time, we directly compared the relative preservation of motion and form systems in AD, mild cognitive impairment, and healthy aging, by measuring coherence thresholds for well-established global rotational motion and static form stimuli known to be of equivalent complexity. Our data confirm a marked motion-processing deficit specific to some AD patients, and greater than any form-processing deficit for this group. In parallel, we identified a more gradual decline in static form recognition, with thresholds raised in mild cognitive impairment patients and slightly further in the AD group compared with controls. We conclude that complex motion processing is more vulnerable to decline in dementia than complex form processing, perhaps owing to greater reliance on long-range neural connections heavily targeted by AD pathology.

Visual steady state in relation to age and cognitive function

Neocortical gamma activity is crucial for sensory perception and cognition. This study examines the value of using non-task stimulation-induced EEG oscillations to predict cognitive status in a birth cohort of healthy Danish males (Metropolit) with varying cognitive ability. In particular, we examine the steady-state VEP power response (SSVEP-PR) in the alpha (8Hz) and gamma (36Hz) bands in 54 males (avg. age: 62.0 years) and compare these with 10 young healthy participants (avg. age 27.6 years). Furthermore, we correlate the individual alpha-to-gamma difference in relative visual-area power (ΔR_V) with cognitive scores for the older adults. We find that ΔR_V decrease with age by just over one standard deviation when comparing young with old participants (p<0.01). Furthermore, intelligence is significantly negatively correlated with ΔR_V in the older adult cohort, even when processing speed, global cognition, executive function, memory, and education (p<0.05). In our preferred specification, an increase in ΔR_V of one standard deviation is associated with a reduction in intelligence of 48% of a standard deviation (p<0.01). Finally, we conclude that the difference in cerebral rhythmic activity between the alpha and gamma bands is associated with age and cognitive status, and that ΔR_V therefore provide a non-subjective clinical tool with which to examine cognitive status in old age.

Synaptic Dysfunction in Alzheimer's Disease and Glaucoma: From Common Degenerative Mechanisms Toward Neuroprotection

Alzheimer’s disease (AD) and glaucoma are two distinct multifactorial neurodegenerative diseases, primarily affecting the elderly. Common pathophysiological mechanisms have been elucidated in the past decades. First of all both diseases are progressive, with AD leading to dementia and glaucoma inducing blindness. Pathologically, they all feature synaptic dysfunction with changes of neuronal circuitry, progressive accumulation of protein aggregates such as the beta amyloid (Aβ) and intracellular microtubule inclusions containing hyperphosphorylated tau, which belongs to microtubule associated protein family. During an early phase of degeneration, both diseases are characterized by synaptic dysfunction and changes of mitogen-activated protein kinases (MAPK). Common degenerative mechanisms underlying both diseases are discussed here, along with recent results on the potential use of the visual system as a biomarker for diagnosis and progression of AD. Common neuropathological changes and mechanisms in AD and glaucoma have facilitated the transfer of therapeutic strategies between diseases. In particular, we discuss past and present evidence for neuroprotective effects of brain-derived neurotrophic factor (BDNF).

Retinal biomarkers provide "insight" into cortical pharmacology and disease

The retina is an easily accessible out-pouching of the central nervous system (CNS) and thus lends itself to being a biomarker of the brain. More specifically, the presence of neuronal, vascular and blood-neural barrier parallels in the eye and brain coupled with fast and inexpensive methods to quantify retinal changes make ocular biomarkers an attractive option. This includes its utility as a biomarker for a number of cerebrovascular diseases as well as a drug pharmacology and safety biomarker for the CNS. It is a rapidly emerging field, with some areas well established, such as stroke risk and multiple sclerosis, whereas others are still in development (Alzheimer's, Parkinson's, psychological disease and cortical diabetic dysfunction). The current applications and future potential of retinal biomarkers, including potential ways to improve their sensitivity and specificity are discussed. This review summarises the existing literature and provides a perspective on the strength of current retinal biomarkers and their future potential.

Normal Amplitude of Electroretinography and Visual Evoked Potential Responses in AβPP/PS1 Mice

Alzheimer's disease has been shown to affect vision in human patients and animal models. This may pose the risk of bias in behavior studies and therefore requires comprehensive investigation. We recorded electroretinography (ERG) under isoflurane anesthesia and visual evoked potentials (VEP) in awake amyloid expressing AβPPswe/PS1dE9 (AβPP/PS1) and wild-type littermate mice at a symptomatic age. The VEPs in response to patterned stimuli were normal in AβPP/PS1 mice. They also showed normal ERG amplitude but slightly shortened ERG latency in dark-adapted conditions. Our results indicate subtle changes in visual processing in aged male AβPP/PS1 mice specifically at a retinal level.

The Eye As a Biomarker for Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder resulting in dementia and eventual death. It is the leading cause of dementia and the number of cases are projected to rise in the next few decades. Pathological hallmarks of AD include the presence of hyperphosphorylated tau and amyloid protein deposition. Currently, these pathological biomarkers are detected either through cerebrospinal fluid analysis, brain imaging or post-mortem. Though effective, these methods are not widely available due to issues such as the difficulty in acquiring samples, lack of infrastructure or high cost. Given that the eye possesses clear optics and shares many neural and vascular similarities to the brain, it offers a direct window to cerebral pathology. These unique characteristics lend itself to being a relatively inexpensive biomarker for AD which carries the potential for wide implementation. The development of ocular biomarkers can have far implications in the discovery of treatments which can improve the quality of lives of patients. In this review, we consider the current evidence for ocular biomarkers in AD and explore potential future avenues of research in this area.

Changes to the lateral geniculate nucleus in Alzheimer's disease but not dementia with Lewy bodies

AIMS

Complex visual hallucinations occur in 70% of dementia with Lewy bodies (DLB) cases and significantly affect patient well-being. Visuo-cortical and retinal abnormalities have been recorded in DLB and may play a role in visual hallucinations. The present study aimed to investigate the lateral geniculate nucleus (LGN), a visual relay centre between the retina and visual cortex, to see if changes to this structure underlie visual hallucinations in DLB.

METHODS

Fifty-one [17 probable DLB, 19 control and 15 probable Alzheimer's disease (AD)] cases were recruited for a functional magnetic resonance imaging study, in which patients' response to a flashing checkerboard stimulus was detected and measured in the LGN, before comparison across experimental groups. Additionally, post mortem LGN tissue was acquired for a cross-sectional study using 20 (six DLB, seven control and seven AD) cases and analysed using stereology. α-Synuclein, phosphorylated tau and amyloid-β pathology was also assessed in all cases.

RESULTS

DLB cases did not significantly differ from controls on neuroimaging, morphometry or pathology. However, a significant increase in amyloid-β pathology, a reduction in number of parvocellular neurones and magnocellular gliosis was found in AD cases compared with control and DLB cases.

CONCLUSIONS

These findings suggest that the early visual system is relatively spared in DLB, which implies that upstream visual structures may be largely responsible for the generation of hallucinatory percepts. The significance of the degeneration of the LGN in AD cases is uncertain.

Perceiving Collision Impacts in Alzheimer's Disease: The Effect of Retinal Eccentricity on Optic Flow Deficits

The present study explored whether the optic flow deficit in Alzheimer's disease (AD) reported in the literature transfers to different types of optic flow, in particular, one that specifies collision impacts with upcoming surfaces, with a special focus on the effect of retinal eccentricity. Displays simulated observer movement over a ground plane toward obstacles lying in the observer's path. Optical expansion was modulated by varying [Formula: see text]. The visual field was masked either centrally (peripheral vision) or peripherally (central vision) using masks ranging from 10° to 30° in diameter in steps of 10°. Participants were asked to indicate whether their approach would result in "collision" or "no collision" with the obstacles. Results showed that AD patients' sensitivity to [Formula: see text] was severely compromised, not only for central vision but also for peripheral vision, compared to age- and education-matched elderly controls. The results demonstrated that AD patients' optic flow deficit is not limited to radial optic flow but includes also the optical pattern engendered by [Formula: see text]. Further deterioration in the capacity to extract [Formula: see text] to determine potential collisions in conjunction with the inability to extract heading information from radial optic flow would exacerbate AD patients' difficulties in navigation and visuospatial orientation.

Differential processing of natural scenes in posterior cortical atrophy and in Alzheimer's disease, as measured with a saccade choice task

Atrophy of the medial temporal lobe structures that support scene perception and the binding of an object to its context (i.e., the hippocampus and the parahippocampal cortex) appears early in the course of Alzheimer’s disease (AD). However, few studies have investigated scene perception in people with AD. Here, we assessed the ability to find a target object within a natural scene in people with AD and in people with posterior cortical atrophy (PCA, a variant of AD). Pairs of color photographs were displayed on the left and right of a fixation cross for 1 s. In separate blocks of trials, participants were asked to categorize the target (an animal) by either moving their eyes toward the photograph containing the target (the saccadic choice task) or pressing a key corresponding to the target’s location (the manual choice task). Isolated objects and objects within scenes were studied in both tasks. Participants with PCA were more impaired in detection of a target within a scene than participants with AD. The latter’s performance pattern was more similar to that of age-matched controls in terms of accuracy, saccade latencies and the benefit gained from contextual information. Participants with PCA benefited less from contextual information in both the saccade and the manual choice tasks—suggesting that people with posterior brain lesions have impairments in figure/ground segregation and are more sensitive to object crowding.

On using isoluminant stimuli to separate magno- and parvocellular responses in psychophysical experiments-a few words of caution

Isoluminant (or equiluminant) color stimuli (i.e., those that contain variations only in chromaticity) have been employed in attempts to separate magno- and parvocellular responses in psychophysical and noninvasive electrophysiological experiments. The justification for this has been the assumption that magnocellular cells, unlike parvocellular neurons, do not respond to stimuli varying only in hue. However, several problems are associated with this notion: (1) under many conditions, magnocellular neurons are not fully silenced at isoluminance, and (2) in many circumstances, parvocellular responses are substantially reduced at isoluminance. To rely upon isoluminant stimuli to "bias" stimuli toward the parvocellular system also faces obstacles. Therefore, caution is required when attempting to use isoluminant color to separate magno- and parvocellular responses.

The optic nerve: a "mito-window" on mitochondrial neurodegeneration

Retinal ganglion cells (RGCs) project their long axons, composing the optic nerve, to the brain, transmitting the visual information gathered by the retina, ultimately leading to formed vision in the visual cortex. The RGC cellular system, representing the anterior part of the visual pathway, is vulnerable to mitochondrial dysfunction and optic atrophy is a very frequent feature of mitochondrial and neurodegenerative diseases. The start of the molecular era of mitochondrial medicine, the year 1988, was marked by the identification of a maternally inherited form of optic atrophy, Leber's hereditary optic neuropathy, as the first disease due to mitochondrial DNA point mutations. The field of mitochondrial medicine has expanded enormously over the last two decades and many neurodegenerative diseases are now known to have a primary mitochondrial etiology or mitochondrial dysfunction plays a relevant role in their pathogenic mechanism. Recent technical advancements in neuro-ophthalmology, such as optical coherence tomography, prompted a still ongoing systematic re-investigation of retinal and optic nerve involvement in neurodegenerative disorders. In addition to inherited optic neuropathies, such as Leber's hereditary optic neuropathy and dominant optic atrophy, and in addition to the syndromic mitochondrial encephalomyopathies or mitochondrial neurodegenerative disorders such as some spinocerebellar ataxias or familial spastic paraparesis and other disorders, we draw attention to the involvement of the optic nerve in classic age-related neurodegenerative disorders such as Parkinson and Alzheimer disease. We here provide an overview of optic nerve pathology in these different clinical settings, and we review the possible mechanisms involved in the pathogenesis of optic atrophy. This may be a model of general value for the field of neurodegeneration. This article is part of a Special Issue entitled 'Mitochondrial function and dysfunction in neurodegeneration'.

Protein misfolding and retinal degeneration

The retina is a highly complex and specialized organ that performs preliminary analysis of visual information. Composed of highly metabolically active tissue, the retina requires a precise and well-balanced means of maintaining its functional activity during extended periods of time. Maintenance and regulation of a vast array of different structural and functional proteins is required for normal function of the retina. This process is referred to as protein homeostasis and involves a variety of activities, including protein synthesis, folding, transport, degradation, elimination, and recycling. Deregulation of any of these activities can lead to malfunctioning of the retina, from subtle subclinical signs to severe retinal degenerative diseases leading to blindness. Examples of retinal degenerative diseases caused by disruption of protein homeostasis include retinitis pigmentosa and Stargardt's disease. A detailed discussion of the role of disruption in protein homeostasis in these and other retinal diseases is presented, followed by examples of some existing and potential treatments.