Effects of age and sex on pattern electroretinograms and visual evoked potentials

Altered Use of Context During Visual Perception in Psychotic Psychopathology: A Neurophysiological Investigation of Tuned and Untuned Suppression During Contrast Perception

BACKGROUND AND HYPOTHESIS

The human visual system streamlines visual processing by suppressing responses to textures that are similar to their surrounding context. Surround suppression is weaker in individuals with schizophrenia (ISZ); this altered use of visuospatial context may relate to the characteristic visual distortions they experience.

STUDY DESIGN

To understand atypical surround suppression in psychotic psychopathology, we investigated neurophysiological responses in ISZ, healthy controls (HC), individuals with bipolar disorder (IBP), and first-degree relatives (ISZR/IBPR). Participants performed a contrast judgment task on a circular target with annular surrounds, with concurrent electroencephalography. Orientation-independent (untuned) suppression was estimated from responses to central targets with orthogonal surrounds; the orientation-dependence of suppression was estimated by fitting an exponential function to the increase in suppression as surrounds became more aligned with the center.

RESULTS

ISZ exhibited weakened untuned suppression coupled with enhanced orientation-dependence of suppression. The N1 visual evoked potential was associated with the orientation-dependence of suppression, with ISZ and ISZR (but not IBP or IBPR) showing enhanced orientation-dependence of the N1. Collapsed across orientation conditions, the N1 for ISZ lacked asymmetry toward the right hemisphere; this reduction in N1 asymmetry was associated with reduced untuned suppression, real-world perceptual anomalies, and psychotic psychopathology. The overall amplitude of the N1 was reduced in ISZ and IBP.

CONCLUSIONS

Key measures of symptomatology for ISZ are associated with reductions in untuned suppression. Increased sensitivity for ISZ to the relative orientation of suppressive surrounds is reflected in the N1 VEP, which is commonly associated with higher-level visual functions such as allocation of spatial attention or scene segmentation.

Sensitivity and specificity of the uniform field electroretinogram in glaucoma detection in comparison to the pattern electroretinogram

PURPOSE

To determine the ability of the photopic negative response (PhNR) of the uniform field electroretinogram (UF-ERG) to identify early glaucomatous changes in comparison to the checkerboard and bar stimuli of the pattern electroretinogram (PERG).

METHODS

Forty-nine glaucoma patients were classified into two groups: glaucoma-suspect (23 eyes) and early to moderate glaucoma (30 eyes), based on their clinical examination and the results of standard automated perimetry. Thirty patients (30 eyes) with intraocular pressures (IOP) of 21 mmHg or less, with no history of reported high IOP, were included as controls. PERG and UF-ERG recordings were obtained on a Diagnosys D-341 Attaché-Envoy System. Visual field testing was done only for glaucoma-suspect and glaucoma patients.

RESULTS

All three tests (PERG bar stimulus, PERG checkerboard stimulus and PhNR) displayed significantly prolonged peak times for glaucoma and glaucoma-suspect patients, with delays ranging from 7.8 to 14.8%, depending on the test. The PERG bar stimulus also showed a significantly lower N95 amplitude for both glaucoma groups (with reductions of 26.0% and 33.0% for glaucoma-suspect and glaucoma groups, respectively). The PERG checkerboard N95 amplitude component had high sensitivity for detecting glaucoma patients but a low specificity (97% and 37%, respectively; AUC = 0.61). Overall, the PhNR peak time showed the highest sensitivity and specificity (77% and 90%, respectively; AUC = 0.87).

CONCLUSIONS

PERG bar stimuli and the PhNR of the UF-ERG can be used in the clinical setting to detect glaucoma-related changes in glaucoma-suspect and glaucoma patients. However, our data confirm that the PhNR peak time has the best combined sensitivity and specificity.

Relationship between function and structure in the visual cortex in healthy individuals and in patients with severe mental disorders

Patients with schizophrenia spectrum disorders (SCZ_spect) and bipolar disorders (BD) show impaired function in the primary visual cortex (V1), indicated by altered visual evoked potential (VEP). While the neural substrate for altered VEP in these patients remains elusive, altered V1 structure may play a role. One previous study found a positive relationship between the amplitude of the P100 component of the VEP and V1 surface area, but not V1 thickness, in a small sample of healthy individuals. Here, we aimed to replicate these findings in a larger healthy control (HC) sample (n = 307) and to examine the same relationship in patients with SCZ_spect (n = 30) or BD (n = 45). We also compared the mean P100 amplitude, V1 surface area and V1 thickness between controls and patients and found no significant group differences. In HC only, we found a significant positive P100-V1 surface area association, while there were no significant P100-V1 thickness relationships in HC, SCZ_spect or BD. Together, our results confirm previous findings of a positive P100-V1 surface area association in HC, whereas larger patient samples are needed to further clarify the function-structure relationship in V1 in SCZ_spect and BD.

Subsequent and simultaneous electrophysiological investigation of the retina and the visual cortex in neurodegenerative and psychiatric diseases: what are the forecasts for the medicine of tomorrow?

Visual electrophysiological deficits have been reported in neurodegenerative disorders as well as in mental disorders. Such alterations have been mentioned in both the retina and the cortex, notably affecting the photoreceptors, retinal ganglion cells (RGCs) and the primary visual cortex. Interestingly, such impairments emphasize the functional role of the visual system. For this purpose, the present study reviews the existing literature with the aim of identifying key alterations in electroretinograms (ERGs) and visual evoked potentials electroencephalograms (VEP-EEGs) of subjects with neurodegenerative and psychiatric disorders. We focused on psychiatric and neurodegenerative diseases due to similarities in their neuropathophysiological mechanisms. Our research focuses on decoupled and coupled ERG/VEP-EEG results obtained with black-and-white checkerboards or low-level visual stimuli. A decoupled approach means recording first the ERG, then the VEP-EEG in the same subject with the same visual stimuli. The second method means recording both ERG and VEP-EEG simultaneously in the same participant with the same visual stimuli. Both coupled and decoupled results were found, indicating deficits mainly in the N95 ERG wave and the P100 VEP-EEG wave in Parkinson’s, Alzheimer’s, and major depressive disorder. Such results reinforce the link between the retina and the visual cortex for the diagnosis of psychiatric and neurodegenerative diseases. With that in mind, medical devices using coupled ERG/VEP-EEG measurements are being developed in order to further investigate the relationship between the retina and the visual cortex. These new techniques outline future challenges in mental health and the use of machine learning for the diagnosis of mental disorders, which would be a crucial step toward precision psychiatry.

On the Influence of Aging on Classification Performance in the Visual EEG Oddball Paradigm Using Statistical and Temporal Features

The utilization of a non-invasive electroencephalogram (EEG) as an input sensor is a common approach in the field of the brain-computer interfaces (BCI). However, the collected EEG data pose many challenges, one of which may be the age-related variability of event-related potentials (ERPs), which are often used as primary EEG BCI signal features. To assess the potential effects of aging, a sample of 27 young and 43 older healthy individuals participated in a visual oddball study, in which they passively viewed frequent stimuli among randomly occurring rare stimuli while being recorded with a 32-channel EEG set. Two types of EEG datasets were created to train the classifiers, one consisting of amplitude and spectral features in time and another with extracted time-independent statistical ERP features. Among the nine classifiers tested, linear classifiers performed best. Furthermore, we show that classification performance differs between dataset types. When temporal features were used, maximum individuals' performance scores were higher, had lower variance, and were less affected overall by within-class differences such as age. Finally, we found that the effect of aging on classification performance depends on the classifier and its internal feature ranking. Accordingly, performance will differ if the model favors features with large within-class differences. With this in mind, care must be taken in feature extraction and selection to find the correct features and consequently avoid potential age-related performance degradation in practice.

Retinal electroretinogram features can detect depression state and treatment response in adults: A machine learning approach

BACKGROUND

Major depressive disorder (MDD) is a major public health problem. The retina is a relevant site to indirectly study brain functioning. Alterations in retinal processing were demonstrated in MDD with the pattern electroretinogram (PERG). Here, the relevance of signal processing and machine learning tools applied on PERG was studied.

METHODS

PERG - whose stimulation is reversible checkerboards - was performed according to the International Society for Clinical Electrophysiology of Vision (ISCEV) standards in 24 MDD patients and 29 controls at the inclusion. PERG was recorded every 4 weeks for 3 months in patients. Amplitude and implicit time of P50 and N95 were evaluated. Then, time/frequency features were extracted from the PERG time series based on wavelet analysis. A statistical model has been learned in this feature space and a metric aiming at quantifying the state of the MDD patient has been derived, based on minimum covariance determinant (MCD) mahalanobis distance.

RESULTS

MDD patients showed significant increase in P50 and N95 implicit time (p = 0,006 and p = 0,0004, respectively, Mann-Whitney U test) at the inclusion. The proposed metric extracted from the raw PERG provided discrimination between patients and controls at the inclusion (p = 0,0001). At the end of the follow-up at week 12, the difference between the metrics extracted on controls and patients was not significant (p = 0,07), reflecting the efficacy of the treatment.

CONCLUSIONS

Signal processing and machine learning tools applied on PERG could help clinical decision in the diagnosis and the follow-up of MDD in measuring treatment response.

Fullfield and extrafoveal visual evoked potentials in healthy eyes: reference data for a curved OLED display

PURPOSE

Visual evoked potentials (VEP) present an important diagnostic tool in various ophthalmologic and neurologic diseases. Quantitative response data varied among patients but are also dependent on the recording and stimulating equipment. We established VEP reference values for our setting which was recently modified by using a curved OLED display as visual stimulator. Distinction is made between fullfield (FF) and extrafoveal (EF) conduction, and the effect of sex, age and lens status was determined.

METHODS

This prospective cross-sectional study included 162 healthy eyes of 162 test persons older than 10 years. A fullfield pattern-reversal visual evoked potential (FF-PR-VEP) with two stimulus sizes (ss) (20.4' and 1.4°) as well as an extrafoveal pattern onset-offset VEP (EF-P-ON/OFF-VEP) (ss 1.4° and 2.8°) was derived in accordance with the International Society for Clinical Electrophysiology of Vision guidelines. Amplitudes and latencies were recorded, and the mean values as well as standard deviations were calculated. Age- and sex-dependent influences and the difference between phakic and pseudophakic eyes were examined. A subanalysis of EF-P-ON/OFF-VEP and fullfield pattern onset-offset VEP (FF-P-ON/OFF-VEP) was performed. A 55-inch curved OLED display (LG55EC930V, LG Electronics Inc., Seoul, South Korea) was used as visual stimulator.

RESULTS

Mean P100 latency of the FF-PR-VEP was 103.81 ± 7.77 ms (ss 20.4') and 102.58 ± 7.26 ms (ss 1.4°), and mean C2 latency of the EF-P-ON/OFF-VEP was 102.95 ± 11.84 ms (ss 1.4°) and 113.58 ± 9.87 ms (ss 2.8°). For all stimulation settings (FF-PR-VEP, EF-P-ON/OFF-VEP), a significant effect of age with longer latencies and smaller amplitudes in older subjects and higher amplitudes in women was observed. We saw no significant difference in latency or amplitude between phakic and pseudophakic eyes and between EF-P-ON/OFF-VEP and FF-P-ON/OFF-VEP.

CONCLUSIONS

A curved OLED visual stimulator is well suited to obtain VEP response curves with a reasonable interindividual variability. We found significant effects of age and gender in our responses but no effect of the lens status. EF-P-ON/OFF-VEP tends to show smaller amplitudes.

Impact of Structural Changes on Multifocal Electroretinography in Patients With Use of Hydroxychloroquine

Purpose

To investigate the relationship between retinal structure and macular function in eyes screened for hydroxychloroquine (HCQ) toxicity.

Methods

Participants referred for hydroxychloroquine retinopathy screening with spectral domain optical coherence tomography (SD-OCT) and multifocal electroretinogram (mfERG) testing were included in the analysis. Amplitude and implicit time of mfERG N1 and P1 responses were included in the analysis. Ring ratios were computed for amplitude values as the ratio of rings 1–3:5 (R1–3:R5). A control group of healthy participants was included for comparison of SD-OCT metrics.

Results

Sixty-three eyes screened for HCQ retinopathy and 30 control eyes were analyzed. The outer nuclear layer (ONL) was significantly thinner in HCQ patients in the foveal (P = 0.008), parafoveal (P < 0.0001), and perifoveal (P < 0.0001) regions. The HCQ cohort was further divided into two subgroups according to the presence of structural clinically detectable retinopathy (i.e., structural damage as detected by multimodal imaging). HCQ eyes without retinopathy had a thinner ONL thickness in the foveal (P = 0.032), parafoveal (P < 0.0001), and perifoveal (P < 0.0001) regions and a thinner inner nuclear layer (INL) in the parafoveal region (P = 0.045 versus controls). Structural changes in HCQ patients without retinopathy were significantly associated with macular function as R2:R5 ring ratio of mfERG P1 amplitude was associated with INL (P = 0.002) and ONL (P = 0.044) thicknesses, and R3:R5 ring ratio of P1 amplitude was associated with ONL thickness (P = 0.004).

Conclusions

Our results suggest that structural alterations secondary to HCQ toxicity may occur in the absence of clinically detectable retinopathy, and this may reflect in an impaired macular function.

Normative values of visual evoked potential in adults

PURPOSE

Visual evoked potentials (VEP) are used to determine the function of visual pathway from the optic nerve to visual cortex. Various factors may affect VEP response, viz., technical and environmental. The aim of this study is to obtain the normative value of VEP latency and amplitude parameters in adulthood in Indonesia, as well as the relationship of height, weight, body mass index (BMI), head circumference, and visual acuity with the variety of latency and amplitude values of VEP parameters.

METHODS

It is a cross-sectional study on 120 healthy subjects consisting of 60 males and 60 females between 18 and 65 years old. Height, weight, BMI, head circumference, and visual acuity were measured and continued with VEP examination using a 26' checkerboard pattern on the left and right eyes alternately. All data were collected and analyzed with the Shapiro-Wilk test using statistical software R version 3.5.2.

RESULTS

Mean value of P100 latency (interocular latency) of left and right eye were 104.6 ± 3.4 ms and 104.1 ± 3.4 ms, respectively, as well as 9.8 ± 4.7 μV and 10.3 ± 5.4 μV for the amplitude. There was no significant difference between the male and female group, as well as on the age group. Female significantly exhibited a higher P100 amplitude than male. The greater the age, the lower amplitude of P100 significantly.

CONCLUSION

Gender and age do not affect the P100 latency value but only affect P100 amplitude. Height, weight, BMI, head circumference, and visual acuity also do not affect the P100 latency and amplitude.

Task-dependent audiovisual temporal sensitivity is not affected by stimulus intensity levels

Flexibility and robustness of multisensory temporal recalibration is paramount for maintaining perceptual constancy of the surrounding natural world. Different environments impart various impediments, distances and routes that alter the propagation times of sight and sound cues comprising a multimodal event. One's ability to rapidly calibrate and account for these external variations allows for maintained perception of synchrony which is crucial for coherent and consistent perception. The two common paradigms used to compare precision of temporal processing between experimental and control groups, the simultaneity judgment (SJ) and temporal order judgment (TOJ) tasks, often use supra-threshold stimuli. However, few studies have specifically examined the effects of normalizing stimulus intensities to participant's unisensory detection thresholds. The current project presented multiple combinations of auditory and visual stimulus intensity levels, based on individual detection thresholds, during a TOJ and a SJ task. While no effect of stimulus intensity was found on temporal sensitivity or perceived temporal synchrony, there was a significant difference in point of subjective simultaneity (PSS) measures between tasks. In addition, PSS estimates were audio-leading, rather than visual-leading as previously reported, suggesting that exposure to the particular combinations of stimulus intensity levels used influenced temporal synchrony perception. Overall, these results support the use of supra-threshold stimuli in TOJ and SJ tasks as a way of minimizing the confound from differences in unisensory processing.

Developmental Effects on Pattern Visual Evoked Potentials Characterized by Principal Component Analysis

Purpose

Peak amplitude and peak latency in the pattern reversal visual evoked potential (prVEP) vary with maturation. We considered that principal component analysis (PCA) may be used to describe age-related variation over the entire prVEP time course and provide a means of modeling and removing variation due to developmental age.

Methods

PrVEP was recorded from 155 healthy subjects ages 11 to 19 years at two time points. We created a model of the prVEP by identifying principal components (PCs) that explained >95% of the variance in a “training” dataset of 40 subjects. We examined the ability of the PCs to explain variance in an age- and sex-matched “validation” dataset (n = 40) and calculated the intrasubject reliability of the PC coefficients between the two time points. We explored the effect of subject age and sex upon the PC coefficients.

Results

Seven PCs accounted for 96.0% of the variability of the training dataset and 90.5% of the variability in the validation dataset with good within-subject reliability across time points (R > 0.7 for all PCs). The PCA model revealed narrowing and amplitude reduction of the P100 peak with maturation, and a broader and smaller P100 peak in male subjects compared to female subjects.

Conclusions

PCA is a generalizable, reliable, and unbiased method of analyzing prVEP. The PCA model revealed changes across maturation and biological sex not fully described by standard peak analysis.

Translational Relevance

We describe a novel application of PCA to characterize developmental changes of prVEP in youths that can be used to compare healthy and pathologic pediatric cohorts.

Capsule Network for ERP Detection in Brain-Computer Interface

Event-related potential (ERP) is bioelectrical activity that occurs in the brain in response to specific events or stimuli, reflecting the electrophysiological changes in the brain during cognitive processes. ERP is important in cognitive neuroscience and has been applied to brain-computer interfaces (BCIs). However, because ERP signals collected on the scalp are weak, mixed with spontaneous electroencephalogram (EEG) signals, and their temporal and spatial features are complex, accurate ERP detection is challenging. Compared to traditional neural networks, the capsule network (CapsNet) replaces scalar-output neurons with vector-output capsules, allowing the various input information to be well preserved in the capsules. In this study, we expect to utilize CapsNet to extract the discriminative spatial-temporal features of ERP and encode them in capsules to reduce the loss of valuable information, thereby improving the ERP detection performance for BCI. Therefore, we propose ERP-CapsNet to perform ERP detection in a BCI speller application. The experimental results on BCI Competition datasets and the Akimpech dataset show that ERP-CapsNet achieves better classification performances than do the state-of-the-art techniques. We also use a decoder to investigate the attributes of ERPs encoded in capsules. The results show that ERP-CapsNet relies on the P300 and P100 components to detect ERP. Therefore, ERP-CapsNet not only acts as an outstanding method for ERP detection, but also provides useful insights into the ERP detection mechanism.

Visual N80 latency as a marker of neuropsychological performance in schizophrenia: Evidence for bottom-up cognitive models

OBJECTIVE

Cognitive deficits and visual impairment in the magnocellular (M) pathway, have been independently reported in schizophrenia. The current study examined the association between neuropsychological (NPS) performance and visual evoked potentials (VEPs: N80/P1 to M- and P(parvocellular)-biased visual stimuli) in schizophrenia and healthy controls.

METHODS

NPS performance and VEPs were measured in n = 44 patients and n = 34 matched controls. Standardized NPS-scores were combined into Domains and a PCA (Principal Component Analysis) generated Composite. Group differences were assessed via (M)ANOVAs, association between NPS and VEP parameters via PCA, Pearson's coefficient and bootstrapping. Logistic regression was employed to assess classification power.

RESULTS

Patients showed general cognitive impairment, whereas group differences for VEP-parameters were non-significant. In patients, N80 latency across conditions loaded onto one factor with cognitive composite, showed significant negative correlations of medium effect sizes with NPS performance for M/P mixed stimuli and classified low and high performance with 70% accuracy.

CONCLUSION

The study provides no evidence for early visual pathway impairment but suggests a heightened association between early visual processing and cognitive performance in schizophrenia.

SIGNIFICANCE

Our results lend support to bottom-up models of cognitive function in schizophrenia and implicate visual N80 latency as a potential biomarker of cognitive deficits in schizophrenia.

Retinal layers and visual conductivity changes in a case series of microangiopathic ischemic stroke patients

Background

It is unknown whether microangiopathic ischemic strokes outside the visual pathway go along with subclinical changes of the retinal structure or the visual system. The objectives of this prospective non-interventional case series were to investigate if spectral-domain optical coherence tomography (SD-OCT) or multifocal visual evoked potentials (mfVEPs) can detect structural retinal changes or functional impairment of the visual system in patients with microangiopathic ischemic stroke.

Methods

We used SD-OCT to cross-sectionally analyze the retinal morphology of 15 patients with microangiopathic ischemic stroke according to the Trial of Org 10172 in Acute Stroke Treatment (TOAST) classification not affecting the visual pathway. We employed semi-automated segmentation of macular volume scans to analyze the thickness of the macular retinal layers and peripapillary ring scans to investigate the retinal morphology in comparison to a control group without stroke. Visual function was assessed by the mfVEP technique in 13 microangiopathic ischemic stroke patients.

Results

First peak latency of mfVEPs was significantly delayed in the microangiopathic ischemic stroke group compared to the control patients. Neither the retinal layers nor the mfVEPs’ amplitude differed between the microangiopathic ischemic stroke patients and the control group.

Conclusions

In conclusion, microangiopathic ischemic stroke patients presented a delayed first peak latency in mfVEPs as a sign of subclinical functional impairment of the visual pathway. However, our case series suggests no influence on retinal structure resulting from microangiopathic ischemic stroke outside the visual system. Larger and longitudinal studies are needed to confirm these mfVEP findings.

Aging Impairs Temporal Sensitivity, but not Perceptual Synchrony, Across Modalities

Encoding the temporal properties of external signals that comprise multimodal events is a major factor guiding everyday experience. However, during the natural aging process, impairments to sensory processing can profoundly affect multimodal temporal perception. Various mechanisms can contribute to temporal perception, and thus it is imperative to understand how each can be affected by age. In the current study, using three different temporal order judgement tasks (unisensory, multisensory, and sensorimotor), we investigated the effects of age on two separate temporal processes: synchronization and integration of multiple signals. These two processes rely on different aspects of temporal information, either the temporal alignment of processed signals or the integration/segregation of signals arising from different modalities, respectively. Results showed that the ability to integrate/segregate multiple signals decreased with age regardless of the task, and that the magnitude of such impairment correlated across tasks, suggesting a widespread mechanism affected by age. In contrast, perceptual synchrony remained stable with age, revealing a distinct intact mechanism. Overall, results from this study suggest that aging has differential effects on temporal processing, and general impairments with aging may impact global temporal sensitivity while context-dependent processes remain unaffected.

Normative values of visual evoked potentials in Northeastern of Iran

PURPOSE

Visual evoked potentials (VEPs) provide important diagnostic information related to the functional integrity of the visual pathways. The aim of this study was to establish normative values of different components of pattern reversal VEPs on Iranian normal adult subjects.

METHODS

Monocular and binocular pattern reversal VEPs were recorded on 59 healthy participants (22.55±3.79 years old) using the Roland RETI system for two check sizes of 15 and 60min of arc. The measured VEP components were the latencies of N75, P100, N135 and amplitude of N75-P100.

RESULTS

Repeated measures ANOVA showed that viewing eye condition has a significant impact on the amplitude of N75-P100 (P<0.001, F=13.89). Also, the effect of check size on the latencies of N75, P100, N135, amplitude of N75-P100 (P≤0.010), as well as the intraocular difference of P100 latency and amplitude N75-P100 (P=0.007) was significant. More specifically, the amplitude of N75-P100 in both check sizes significantly differed between gender groups (P<0.023).

CONCLUSION

According to the results of this study, VEPs components are affected by the stimulus size, monocular and binocular recording conditions and gender. Therefore, it is necessary to determine the normative values of VEPs in each population, so that the results could be used in clinical studies.

Age, Height, and Sex on Motor Evoked Potentials: Translational Data From a Large Italian Cohort in a Clinical Environment

Introduction

Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are known to be susceptible to several sources of variability. However, conflicting evidences on individual characteristics in relatively small sample sizes have been reported. We investigated the effect of age, height, and sex on MEPs of the motor cortex and spinal roots in a large cohort.

Methods

A total of 587 subjects clinically and neuroradiologically intact were included. MEPs were recorded during mild tonic contraction through a circular coil applied over the "hot spot" of the first dorsal interosseous and tibialis anterior muscles (TAs), bilaterally. Central motor conduction time (CMCT) was estimated as the difference between MEP cortical latency and the peripheral motor conduction time (PMCT) by cervical or lumbar magnetic stimulation. Peak-to-peak MEP amplitude to cortical stimulation and right-to-left difference of each parameter were also measured.

Results

After Bonferroni correction, general linear (multiple) regression analysis showed that both MEP cortical latency and PMCT at four limbs positively correlated with age and height. At lower limbs, an independent effect of sex on the same measures was also observed (with females showing smaller values than males). CMCT correlated with both age (negatively) and height (positively) when analyzed by a single regression; however, with a multiple regression analysis this significance disappeared, due to the correction for the multicollinearity within the dataset.

Conclusion

Physical individual features need to be considered for a more accurate and meaningful MEPs interpretation. Both in clinical practice and in research setting, patients and controls should be matched for age, height, and sex.

Age Related Decline in Cortical Multifocal Flash VEP: Latency Increases Shown to Be Predominately Magnocellular

As the visual system ages, flicker sensitivity decreases and the latencies of cortical visual evoked potentials (VEP) increase. However, the extent to which these effects reflect age-related changes in the magnocellular (M) and or parvocellular (P) pathways remain unclear. Here, we investigated the relation between flicker fusion frequencies and VEP non-linearities induced by rapid stimulation, as a function of age over 6 decades. The approach, using Wiener kernel analysis of multifocal flash (mf)VEP, allows the extraction of signatures of both M and P processing and hence establishing a neural basis of the known decline in flicker fusion threshold. We predicted that, in a sample of 86 participants, age would be associated with a latency increase in early mfVEP response components and that flicker fusion thresholds, for both low and high contrast stimuli, would relate to the temporal efficiency of the M-generated VEP component amplitudes. As expected, flicker fusion frequency reduced with age, while latencies of early second order peaks of the mfVEP increased with age, but M temporal efficiency (amplitude ratio of first to second order peaks) was not strongly age-related. The steepest increases in latency were associated with the M dominated K2.1 (second order first slice) N70 components recorded at low and high contrast (6.7 and 5.9 ms/decade, respectively). Interestingly, significant age-related latency shifts were not observed in the first order responses. Significant decreases in amplitude were found in multiple first and second order components up to 30 years of age, after which they remained relatively constant. Thus, aging and decline in visual function appears to be most closely related to the response latencies of non-linearities generated by the M pathway.

Age, Height, and Sex on Motor Evoked Potentials: Translational Data From a Large Italian Cohort in a Clinical Environment

INTRODUCTION

Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) are known to be susceptible to several sources of variability. However, conflicting evidences on individual characteristics in relatively small sample sizes have been reported. We investigated the effect of age, height, and sex on MEPs of the motor cortex and spinal roots in a large cohort.

METHODS

A total of 587 subjects clinically and neuroradiologically intact were included. MEPs were recorded during mild tonic contraction through a circular coil applied over the "hot spot" of the first dorsal interosseous and tibialis anterior muscles (TAs), bilaterally. Central motor conduction time (CMCT) was estimated as the difference between MEP cortical latency and the peripheral motor conduction time (PMCT) by cervical or lumbar magnetic stimulation. Peak-to-peak MEP amplitude to cortical stimulation and right-to-left difference of each parameter were also measured.

RESULTS

After Bonferroni correction, general linear (multiple) regression analysis showed that both MEP cortical latency and PMCT at four limbs positively correlated with age and height. At lower limbs, an independent effect of sex on the same measures was also observed (with females showing smaller values than males). CMCT correlated with both age (negatively) and height (positively) when analyzed by a single regression; however, with a multiple regression analysis this significance disappeared, due to the correction for the multicollinearity within the dataset.

CONCLUSION

Physical individual features need to be considered for a more accurate and meaningful MEPs interpretation. Both in clinical practice and in research setting, patients and controls should be matched for age, height, and sex.

Impact of Choriocapillaris Flow on Multifocal Electroretinography in Intermediate Age-Related Macular Degeneration Eyes

Purpose

To investigate the relationship between perfusion of the choriocapillaris (CC) and macular function in eyes with intermediate age-related macular degeneration.

Methods

In this prospective, observational, cross-sectional study, macular optical coherence tomography angiography images and multifocal electroretinograms were obtained in 20 eyes with intermediate age-related macular degeneration from 20 patients. The main outcome measures were (1) the percent nonperfused choriocapillaris area (PNPCA), which represents a measure of the total area of CC vascular dropout, and (2) the average size of the CC signal voids, which represent contiguous regions of CC dropout. Furthermore, amplitude and implicit time of multifocal electroretinograms N1 and P1 waves in the two central rings (R1 and R2) were included in the analysis.

Results

Of the 20 patients enrolled, only 17 eyes from 17 patients (13 women) were included in this analysis. Three patients were excluded because of poor scan quality. Mean ± SD age was 75.1 ± 7.9 years (range, 62-89 years). The best corrected visual acuity was 0.17 ± 0.13 logarithm of the minimum angle of resolution. In univariate analysis, both the PNPCA and average signal void size were found to have a significant direct relationship with N1 implicit time in the R2 ring (P = 0.006 and P = 0.035, respectively). Neither PNPCA nor the average signal void size was associated with P1 or N1 implicit times in R1.

Conclusions

In intermediate age-related macular degeneration eyes, PNPCA and average signal void size are related to N1 multifocal electroretinogram implicit times, which suggests an association between CC perfusion and photoreceptor function.

The pre-stimulus oscillatory alpha phase affects neural correlates of early visual perception

A growing number of studies suggest the phase of ongoing alpha oscillations in the brain influences visual perception. However, it remained largely unconsidered if this is associated with a phase dependence of neurophysiological processes especially in the visual cortex. Therefore, this study investigated the link between the pre-stimulus oscillatory alpha phase and neural correlates of early visual perception. In 64 subjects a 64-channel EEG system was used to examine the phase dependence of pattern-reversal visual evoked potentials (VEP) in a visual perception experiment. The pre-stimulus oscillatory phase over the primary visual cortex was determined for the individual alpha peak frequency (iAPF) as well as the frequency of maximal phase locking (PLF_fmax). The phase dependence of VEP latency was determined using single-trial phase sorting. The results indicate a significantly shorter latency for the N75 and P100 components of the VEP between 40°-100° (p < 0.05) and 90°-120° (p < 0.05), respectively when trials were phase-sorted based on the iAPF. In contrast, the PLF_fmax phase did not affect the N75 or P100 latency. The study indicates a link between the pre-stimulus alpha phase and neural correlates of early visual perception. These results extend previous behavioral findings to the neurophysiological level and support current models suggesting visual perception is modulated by ongoing alpha oscillations.

Early isolated optic neuropathy caused by cyclosporine

PURPOSE

To examine, for the first time, whether cyclosporine intake has an early isolated effect on the optic nerve.

MATERIALS AND METHODS

This observational case series consisted of 192 eyes of 98 patients treated with cyclosporine. Patient age and duration and dosage of cyclosporine were recorded, and visual acuity, optic nerve function, visual fields, and visual evoked potential (VEP) were tested. Fundus examination was also performed. Patients with glaucoma, vascular retinopathies, and deep amblyopia were excluded.

RESULTS

Mean patient age was 46 years, average duration of treatment was 6 years, and median dosage of cyclosporine was 200 mg daily. VEP was tested in 73 patients (142 eyes) and yielded a delayed P100 wave in 9 (12.32%) (14 eyes). Among these 9 patients, abnormal findings were also noted on the Ishihara colour test in 42.86% of the eyes, and on the visual field test in 64.3% of the eyes. Abnormal VEP showed a significant correlation (p < 0.05) with older age (> 46 years) and a non-significant correlation with longer duration of treatment. Higher abnormal VEP potential was not correlated with higher cyclosporine dose, and there was no correlation between abnormal VEP and blood level of cyclosporine.

CONCLUSION

Optic neuropathy was significantly associated with older age in cyclosporine-treated patients. A correlation between optic neuropathy with longer duration of cyclosporine treatment was noted but was not statistically significant. We suggest that tests of optic nerve function, including VEP, be a part of the follow-up of patients receiving cyclosporine.

A survey on stimuli for visual cortical function assessment in infants

Visual processing, as a significant and complex functionality of the human brain, changes during the life span with the most developmental changes in the infancy. Different types of visual stimuli are needed for evaluating different functionalities of the infants' visual system. Selecting appropriate visual stimuli is an important issue in evaluating visual cortical functions in infants. Properties of stimulation influence responses of visual system and must be adjusted according to the age and specific function which is going to be investigated. In this review, the most commonly used stimuli to elicit visual evoked potentials (VEPs) are evaluated and characteristics of VEPs extracted by these stimulations are studied. Furthermore, various studies investigating different functionalities such as selectivity for orientation and directional motion are presented. Valuable results regarding emerging and maturation times of different functions and normative data for clinical diagnosis are provided by these studies.

Sex differences in the human visual system

This Mini-Review summarizes a wide range of sex differences in the human visual system, with a primary focus on sex differences in visual perception and its neural basis. We highlight sex differences in both basic and high-level visual processing, with evidence from behavioral, neurophysiological, and neuroimaging studies. We argue that sex differences in human visual processing, no matter how small or subtle, support the view that females and males truly see the world differently. We acknowledge some of the controversy regarding sex differences in human vision and propose that such controversy should be interpreted as a source of motivation for continued efforts to assess the validity and reliability of published sex differences and for continued research on sex differences in human vision and the nervous system in general. © 2016 Wiley Periodicals, Inc.

Gender-based normative values for pattern-reversal and flash visually evoked potentials under binocular and monocular stimulation in healthy adults

PURPOSE

To determine gender-based normative values for pattern-reversal (PR) and flash (F) visually evoked potentials (VEP) under binocular and monocular stimulation in healthy adults.

METHODS

Healthy adults (age ≥18 years) were recruited among university employees and students. Inclusion criteria were absence of abnormalities in fundoscopy, tracking ability, stereopsis and pupillary reflexes; best-corrected visual acuity ≤.00 logMAR; and refractive error (spherical equivalent) from -6.00 to +6.00. Exclusion criteria were previous intraocular surgery, systemic and/or neurological disorders. Binocular and monocular tests were performed according to International Society for Clinical Electrophysiology of Vision standards for PRVEP (reversal rate = 1.9 Hz, checkerboard stimuli 15' and 60' at 100% contrast) and FVEP (3 cd s/m², rate = 1 Hz). VEP parameters of amplitude (µV) and peak times (ms) were measured. Inter-ocular differences, inter-peak intervals (N135-N75) and binocular summation were determined.

RESULTS

Fifty-four subjects (28 females; mean age = 40.4 ± 13.7 years; median = 40.0 years) were included. Mean P100 latencies for 15' and 60' stimuli were, respectively, 94.6 ± 4.7 ms and 96.1 ± 4.2 for women. Mean values of P100 latency for men were 97.4 ± 4.9 for 15' and 97.7 ± 4.2 for 60' stimuli. Larger mean P100 for 15' checks was observed in women (12.8 ± 5.7 µV) than men (8.6 ± 2.5 µV) in PRVEP. Similar results were found for FVEP N2-P2 amplitudes (mean = 14.6 ± 4.9 for women and 9.8 ± 4.0 for men).

CONCLUSIONS

Gender-based normative values for PRVEP and FVEP were determined, with women disclosing higher responses than men for smaller stimuli in the visual pathway. The use of gender-based normative values in the analysis of clinical VEP data for diagnostic and therapeutic purposes is recommendable. Additional analysis including inter-peak intervals and binocular summation ratio might improve the diagnostic power of VEP.

Visual Evoked Potentials: Normative Values and Gender Differences

INTRODUCTION

Visual evoked potentials (VEP) are used to assess the visual pathways through the optic nerves and brain. A normal VEP response to a pattern-reversal stimulus is a positive mid occipital peak that occurs at a mean latency of 100 ms. VEP may be affected by variety of physiological factors including age, sex, visual acuity and pupillary size.

AIMS AND OBJECTIVES

The present study was performed on healthy medical students to determine the normative values and to investigate the effect of sex and anthropometric parameters on visual evoked potentials.

MATERIALS AND METHODS

The study was conducted on 100 healthy medical students of Government Medical College, Patiala in the age group of 17-20 years, in which there were 50 males and 50 females. The anthropometric parameters including age, height, weight, BMI, BSA and Head circumference were recorded in all the subjects. VEP was recorded with a PC based, 2 channel, RMS EMG EP mark II machine and standard silver-silver chloride disc electrodes. A VEP monitor displaying checker board was used to give the pattern reversal stimulus. The VEP parameters recorded were latencies to N70, P100 and N155 waves, and peak to peak amplitude of P100 wave.

RESULTS

Our results showed that the latencies of N70, P100 and N155 waves were significantly longer in males as compared to females. The amplitude of P100 wave was higher in females in both left and right eye as compared to males. No significant correlation was found between VEP parameters and head circumference in both male and female subjects in our study.

CONCLUSION

Gender is an important variable affecting the VEP. The exact reason of gender difference is not clear, but it may be related to anatomical or endocrinal differences in the two sexes.

Cone-rod dysfunction is a sign of early-onset high myopia

PURPOSE

High myopia (HM) is one of the leading causes of irreversible blindness because of its associated complications. Early-onset HM (eoHM) that presents before primary school age may be genetically determined, whereas late-onset HM (loHM) is affected by genetic and environmental factors. Identification of signs that differentiate eoHM from loHM may provide valuable clues toward understanding the molecular basis of HM.

METHODS

In this study, 42 subjects with HM were recruited, including 32 eoHM (onset age, ≤5 years) and 10 loHM (onset age, 12.4 ± 2.5 years). Clinical data from these two groups were compared, including best visual acuity, refraction, axial length, and electroretinography.

RESULTS

The mean amplitudes of rod b-wave, maximum a-wave, maximum b-wave, cone a-wave, and cone b-wave of patients with eoHM were 179 ± 72 μv, -158 ± 63 μv, 345 ± 105 μv, -20 ± 11 μv, and 42 ± 21 μv, respectively, whereas those of patients with loHM were 252 ± 77 μv, -235 ± 60 μv, 464 ± 65 μv, -36 ± 9 μv, and 104 ± 26 μv, respectively (p < 0.001). Patients with eoHM had significantly reduced responses of cones and rods, especially cones, as compared with patients with loHM. The cone-rod responses in patients with loHM were within normal range.

CONCLUSIONS

The findings suggest that cone-rod dysfunction may be a sign for eoHM compared with loHM.

Low luminance/eyes closed and monochromatic stimulations reduce variability of flash visual evoked potential latency

CONTEXT

Visual evoked potentials are useful in investigating the physiology and pathophysiology of the human visual system. Flash visual evoked potential (FVEP), though technically easier, has less clinical utility because it shows great variations in both latency and amplitude for normal subjects.

AIM

To study the effect of eye closure, low luminance, and monochromatic stimulation on the variability of FVEPs.

SUBJECTS AND METHODS

Subjects in self-reported good health in the age group of 18-30 years were divided into three groups. All participants underwent FVEP recording with eyes open and with white light at 0.6 J luminance (standard technique). Next recording was done in group 1 with closed eyes, group 2 with 1.2 and 20 J luminance, and group 3 with red and blue lights, while keeping all the other parameters constant. Two trials were given for each eye, for each technique. The same procedure was repeated at the same clock time on the following day.

STATISTICAL ANALYSIS

Variation in FVEP latencies between the individuals (interindividual variability) and the variations within the same individual for four trials (intraindividual variability) were assessed using coefficient of variance (COV). The technique with lower COV was considered the better method.

RESULTS

Recording done with closed eyes, 0.6 J luminance, and monochromatic light (blue > red) showed lower interindividual and intraindividual variability in P2 and N2 as compared to standard techniques.

CONCLUSIONS

Low luminance flash stimulations and monochromatic light will reduce FVEP latency variability and may be clinically useful modifications of FVEP recording technique.

Gender impact on electrophysiological activity of the brain

Gender is presumed to be one of the factors causing interindividual variability in the brain's electrophysiological parameters. Our aim was to characterize the role of gender in visual evoked potentials (VEPs), event-related potentials (ERPs), visual mismatch negativity (vMMN) and the spectral characteristics of the EEG. We examined 42 healthy volunteers (21 women and 21 men, aged 20-29 years). We measured VEPs in response to pattern-reversal and motion-onset stimulation, ERPs in an oddball paradigm and vMMN in response to a combination of motion directions presented in the visual periphery. P100 peak latency for 40' reversal VEPs was significantly shorter in women than in men as determined using a non-parametric Wilcoxon signed-rank test. In addition, women showed higher relative EEG spectral power in the alpha band (p=0.023) and lower power in the theta band (p=0.004). Our results in this small but homogeneous group of subjects confirm previously reported gender influences on pattern-reversal VEPs and the EEG frequency spectrum. Gender should be taken into consideration in establishing norms on these measures. We found no statistically significant differences between women and men for any of the other stimuli presented.

The role of hemifield sector analysis in multifocal visual evoked potential objective perimetry in the early detection of glaucomatous visual field defects

Objective

The purpose of this study was to examine the effectiveness of a new analysis method of mfVEP objective perimetry in the early detection of glaucomatous visual field defects compared to the gold standard technique.

Methods and patients

Three groups were tested in this study; normal controls (38 eyes), glaucoma patients (36 eyes), and glaucoma suspect patients (38 eyes). All subjects underwent two standard 24-2 visual field tests: one with the Humphrey Field Analyzer and a single mfVEP test in one session. Analysis of the mfVEP results was carried out using the new analysis protocol: the hemifield sector analysis protocol.

Results

Analysis of the mfVEP showed that the signal to noise ratio (SNR) difference between superior and inferior hemifields was statistically significant between the three groups (analysis of variance, P < 0.001 with a 95% confidence interval, 2.82, 2.89 for normal group; 2.25, 2.29 for glaucoma suspect group; 1.67, 1.73 for glaucoma group). The difference between superior and inferior hemifield sectors and hemi-rings was statistically significant in 11/11 pair of sectors and hemi-rings in the glaucoma patients group (t-test P < 0.001), statistically significant in 5/11 pairs of sectors and hemi-rings in the glaucoma suspect group (t-test P < 0.01), and only 1/11 pair was statistically significant (t-test P < 0.9). The sensitivity and specificity of the hemifield sector analysis protocol in detecting glaucoma was 97% and 86% respectively and 89% and 79% in glaucoma suspects. These results showed that the new analysis protocol was able to confirm existing visual field defects detected by standard perimetry, was able to differentiate between the three study groups with a clear distinction between normal patients and those with suspected glaucoma, and was able to detect early visual field changes not detected by standard perimetry. In addition, the distinction between normal and glaucoma patients was especially clear and significant using this analysis.

Conclusion

The new hemifield sector analysis protocol used in mfVEP testing can be used to detect glaucomatous visual field defects in both glaucoma and glaucoma suspect patients. Using this protocol, it can provide information about focal visual field differences across the horizontal midline, which can be utilized to differentiate between glaucoma and normal subjects. The sensitivity and specificity of the mfVEP test showed very promising results and correlated with other anatomical changes in glaucomatous visual field loss. The intersector analysis protocol can detect early field changes not detected by the standard Humphrey Field Analyzer test.

Ambient intelligence for monitoring and research in clinical neurophysiology and medicine: the MIMERICA* project and prototype

Ambient Intelligence (AmI) provides extended but unobtrusive sensing and computing devices and ubiquitous networking for human/environment interaction. It is a new paradigm in information technology compliant with the international Integrating Healthcare Enterprise board (IHE) and eHealth HL7 technological standards in the functional integration of biomedical domotics and informatics in hospital and home care. AmI allows real-time automatic recording of biological/medical information and environmental data. It is extensively applicable to patient monitoring, medicine and neuroscience research, which require large biomedical data sets; for example, in the study of spontaneous or condition-dependent variability or chronobiology. In this respect, AML is equivalent to a traditional laboratory for data collection and processing, with minimal dedicated equipment, staff, and costs; it benefits from the integration of artificial intelligence technology with traditional/innovative sensors to monitor clinical or functional parameters. A prototype AmI platform (MIMERICA*) has been implemented and is operated in a semi-intensive unit for the vegetative and minimally conscious states, to investigate the spontaneous or environment-related fluctuations of physiological parameters in these conditions.

Resting state cortical EEG rhythms in Alzheimer's disease: toward EEG markers for clinical applications: a review

The human brain contains an intricate network of about 100 billion neurons. Aging of the brain is characterized by a combination of synaptic pruning, loss of cortico-cortical connections, and neuronal apoptosis that provoke an age-dependent decline of cognitive functions. Neural/synaptic redundancy and plastic remodeling of brain networking, also secondary to mental and physical training, promote maintenance of brain activity and cognitive status in healthy elderly subjects for everyday life. However, age is the main risk factor for neurodegenerative disorders such as Alzheimer's disease (AD) that impact on cognition. Growing evidence supports the idea that AD targets specific and functionally connected neuronal networks and that oscillatory electromagnetic brain activity might be a hallmark of the disease. In this line, digital electroencephalography (EEG) allows noninvasive analysis of cortical neuronal synchronization, as revealed by resting state brain rhythms. This review provides an overview of the studies on resting state eyes-closed EEG rhythms recorded in amnesic mild cognitive impairment (MCI) and AD subjects. Several studies support the idea that spectral markers of these EEG rhythms, such as power density, spectral coherence, and other quantitative features, differ among normal elderly, MCI, and AD subjects, at least at group level. Regarding the classification of these subjects at individual level, the most previous studies showed a moderate accuracy (70-80%) in the classification of EEG markers relative to normal and AD subjects. In conclusion, resting state EEG makers are promising for large-scale, low-cost, fully noninvasive screening of elderly subjects at risk of AD.

Simultaneous retinal and cortical visually evoked electrophysiological responses in between migraine attacks

PURPOSE

People with migraine often report aversion to flickering lights and show abnormal results on behavioural tasks that require the processing of temporal visual information. Studies have reported that the cortically evoked electrophysiological response to a flickering visual stimulus is abnormal; however, none have considered whether there is an underlying pre-cortical abnormality. In this cross-sectional study, we consider whether people with migraine have retinal and cortical electrophysiological abnormalities to flickering stimuli.

METHODS

Monocular transient (1 Hz) and steady-state (8.3 Hz) pattern reversal electroretinograms (PERGs) and pattern visual evoked responses (PVERs) were measured simultaneously in 45 people with migraine (26 without aura, 19 with aura) and 30 non-headache controls at a time between migraine attacks.

RESULTS

PERG amplitude and timing did not differ significantly between groups. Transient PVER amplitude was significantly reduced (28%) in the migraine with aura group compared to the controls F(2,72) = 3.6, p = 0.03). Both migraine groups showed significant reductions (32%, 39%) in steady-state PVER amplitude relative to controls (F(2,70) = 4.3, p = 0.02).

CONCLUSIONS

This study finds normal retinal processing of flickering stimuli in the presence of abnormal cortical function between migraine attacks.

Electroencephalographic rhythms in Alzheimer's disease

Physiological brain aging is characterized by synapses loss and neurodegeneration that slowly lead to an age-related decline of cognition. Neural/synaptic redundancy and plastic remodelling of brain networking, also due to mental and physical training, promotes maintenance of brain activity in healthy elderly subjects for everyday life and good social behaviour and intellectual capabilities. However, age is the major risk factor for most common neurodegenerative disorders that impact on cognition, like Alzheimer's disease (AD). Brain electromagnetic activity is a feature of neuronal network function in various brain regions. Modern neurophysiological techniques, such as electroencephalography (EEG) and event-related potentials (ERPs), are useful tools in the investigation of brain cognitive function in normal and pathological aging with an excellent time resolution. These techniques can index normal and abnormal brain aging analysis of corticocortical connectivity and neuronal synchronization of rhythmic oscillations at various frequencies. The present review suggests that discrimination between physiological and pathological brain aging clearly emerges at the group level, with suggested applications also at the level of single individual. The possibility of combining the use of EEG together with biological/neuropsychological markers and structural/functional imaging is promising for a low-cost, non-invasive, and widely available assessment of groups of individuals at-risk.

Healthy aging delays scalp EEG sensitivity to noise in a face discrimination task

We used a single-trial ERP approach to quantify age-related changes in the time-course of noise sensitivity. A total of 62 healthy adults, aged between 19 and 98, performed a non-speeded discrimination task between two faces. Stimulus information was controlled by parametrically manipulating the phase spectrum of these faces. Behavioral 75% correct thresholds increased with age. This result may be explained by lower signal-to-noise ratios in older brains. ERP from each subject were entered into a single-trial general linear regression model to identify variations in neural activity statistically associated with changes in image structure. The fit of the model, indexed by R², was computed at multiple post-stimulus time points. The time-course of the R² function showed significantly delayed noise sensitivity in older observers. This age effect is reliable, as demonstrated by test–retest in 24 subjects, and started about 120 ms after stimulus onset. Our analyses suggest also a qualitative change from a young to an older pattern of brain activity at around 47 ± 4 years old.

Visual sensitivity to a conspicuous male cue varies by reproductive state inPhysalaemus pustulosusfemales

The vocal sac is a visually conspicuous attribute of most male frogs, but its role in visual communication has only been demonstrated recently in diurnally displaying frogs. Here we characterized the spectral properties of the inflated vocal sac of male túngara frogs (Physalaemus pustulosus), a nocturnal species, and túngara visual sensitivity to this cue across reproductive state and sex. We measured the spectral and total reflectance of different male body regions, including inflated and non-inflated vocal sacs, along with samples of the visual background against which males are perceived. Inflated vocal sacs were the most reflective of all body parts, being one log unit more reflective than background materials. We utilized an optomotor drum with black stripes and stripes that mimicked the spectral reflectance of the inflated vocal sacs with various nocturnal light intensities to measure the visual sensitivity thresholds of males,non-reproductive females and reproductive females. All three groups exhibited visual sensitivities corresponding to intensities below moonless conditions in open habitats or at the edge of secondary tropical forests. Reproductive females exhibited the greatest visual sensitivity of all groups, and were significantly more sensitive than non-reproductive females. Though the mechanism for this physiological difference between reproductive and non-reproductive females is unknown, it is consistent with previously observed patterns of light-dependent phonotaxic behavior in túngaras. We suggest that the visual ecology of the vocal sac, especially in nocturnal frogs,offers a rich source for investigations of visual ecology and physiological regulation of vision.

Clinical neurophysiology of aging brain: from normal aging to neurodegeneration

Physiological brain aging is characterized by a loss of synaptic contacts and neuronal apoptosis that provokes age-dependent decline of sensory processing, motor performance, and cognitive function. Neural redundancy and plastic remodelling of brain networking, also secondary to mental and physical training, promotes maintenance of brain activity in healthy elderly for everyday life and fully productive affective and intellectual capabilities. However, age is the main risk factor for neurodegenerative disorders such as Alzheimer's disease (AD) that impact on cognition. Oscillatory electromagnetic brain activity is a hallmark of neuronal network function in various brain regions. Modern neurophysiological techniques including electroencephalography (EEG), event-related potential (ERP), magnetoencephalography (MEG), and transcranial magnetic stimulation (TMS) can accurately index normal and abnormal brain aging to facilitate non-invasive analysis of cortico-cortical connectivity and neuronal synchronization of firing and coherence of rhythmic oscillations at various frequencies. The present review provides a perspective of these issues by assaying different neurophysiological methods and integrating the results with functional brain imaging findings. It is concluded that discrimination between physiological and pathological brain aging clearly emerges at the group level, with applications at the individual level also suggested. Integrated approaches utilizing neurophysiological techniques together with biological markers and structural and functional imaging are promising for large-scale, low-cost and non-invasive evaluation of at-risk populations. Practical implications of the methods are emphasized.