Circular Stimuli Research Articles

Bilberry Extract in Macular Pigment Density Improvement

Macular Pigments (MPs), by absorbing potentially toxic short-wavelength (400–500 nm) visible light, provide protection against photo-chemical damage. A new method of MPs levels screening analyzes polarization-dependent absorption of blue light by MPs, which results in the entoptic phenomenon called Haidinger’s Brushes (HB). Subjects identify the direction of rotation of HB when presented with a circular stimulus illuminated with an even intensity of polarized white light in which the electric field vector was rotating either clockwise or anti-clockwise. The examination was performed at baseline and after 3 months of daily intake of 320 mg bilberry extract, from Vaccinium myrtillus L. and standardized to contain more than 36% of anthocyanins (anthocyanosides), in normal subjects and MPs improvement was recorded after 3 months of dietary supplements. The results suggest a challenging role of daily intake of 320 mg bilberry extract in macular pigment density improvement possibly due to a macular protection from oxidative damage by bilberry.

Improving SSVEP-BCI System Interaction Efficiency: Design Recommendations for Shape of Visual Stimuli and Number of Auxiliary Stimuli

Currently, brain–computer interfaces (BCIs) have become an important part of advanced human-computer interactions. Its essence is to infer people’s ideas or purposes through brain signals to realize human-computer communication. BCI systems based on steady-state visual-evoked potentials (SSVEP) are widely used in the field of BCI because of their high information transfer rate (ITR), low training requirements, and simple system structure. To explore a reasonable design method of visual stimulation in an SSVEP-BCI interactive interface, this study completed two experiments based on the shape and presentation of visual stimulation. Experiment 1 explored the optimal shapes of different visual stimuli. The results indicated that there were significant differences in task completion times for different stimulus shapes. Combined with ergonomics experiment and subjective evaluation results, the circular was the optimal stimulus shape. Experiment 2 used different numbers of auxiliary stimuli under the condition of circular stimulus. The results showed significant differences in task completion time for various auxiliary stimuli, and the number of auxiliary stimuli negatively correlated with task completion time. Through subjective evaluation, it was found that all the median scores of the usability index increased when there were more auxiliary stimuli. The recommended number of auxiliary stimuli was eight. This research provides significant evidence for the design of visual stimulation for SSVEP-BCI systems. The experimental conclusions are of great value for improving the interaction efficiency of SSVEP-BCI systems, enhancing user experience, and expanding application fields. They will also provide new directions and ideas for the interface design of BCI systems, visual stimulation design, and application of advanced interaction technology.

Using colour pattern edge contrast statistics to predict detection speed and success in triggerfish (Rhinecanthus aculeatus).

Edge detection is important for object detection and recognition. However, we do not know whether edge statistics accurately predict the detection of prey by potential predators. This is crucial given the growing availability of image analyses and their application across non-human visual systems. Here, we investigated whether Boundary Strength Analysis (BSA), Local Edge Intensity Analysis (LEIA) and the Gabor Ratio (GabRat) could predict the speed and success with which triggerfish (Rhinecanthus aculeatus) detected patterned circular stimuli against a noisy visual background, in both chromatic and achromatic presentations. We found various statistically significant correlations between edge statistics and detection speed depending on treatment and viewing distance; however, individual pattern statistics only explained up to 2% of the variation in detection time, and up to 6% when considering edge statistics simultaneously. We also found changes in fish response over time. While highlighting the importance of spatial acuity and relevant viewing distances in the study of visual signals, our results demonstrate the importance of considering explained variation when interpreting colour pattern statistics in behavioural experiments. We emphasize the need for statistical approaches suitable for investigating task-specific predictive relationships and ecological effects when considering animal behaviour. This is particularly important given the ever-increasing dimensionality and size of datasets in the field of visual ecology.

Robust natural depth for anticorrelated random dot stereogram for edge stimuli, but minimal reversed depth for embedded circular stimuli, irrespective of eccentricity.

The small differences between the images formed in our left and right eyes are an important cue to the three-dimensional structure of scenes. These disparities are encoded by binocular neurons in the visual cortex. At the earliest stage of processing, these respond to binocular correlation between images. We assessed the perception of depth in anticorrelated stimuli, in which the contrast polarity in one eye is reversed, as a function of their location in the retinal image, and their depth configuration (a horizontal edge or a circle surrounded by an annulus) We found that, regardless of stimulus eccentricity, participants perceived depth in the natural direction for edge stimuli, and weakened, reversed depth for circular stimuli.

Brightness appearance of self‐luminous stimuli on a non‐uniform background

AbstractColor appearance models (CAM) generate a set of visual correlates attributed to a stimulus such as hue, brightness, and saturation. However, most of their applicability is limited to stimuli perceived on a uniform background. Additionally, when focusing on brightness, multiple studies have been performed to model the mechanisms of brightness perception of a stimulus on a non‐uniform background. In this paper, experimental data are gathered and the insights of both approaches are combined. The influence of adding a neutral ring‐shaped luminous area in the background of a neutral circular stimulus has been investigated via a series of psychophysics experiments. The ring is presented at three luminance levels (90, 335, 1200 cd/m2) with three thicknesses (0.33, 0.67, and 1.00 cm) at four angular distances to the edge of the stimulus (1.2°, 6.4°, 11.3° and 16.1°). It is observed that when the ring is closer to the stimulus, the brightness inhibition becomes larger; obviously, the impact is also related to the ring's area and luminance. Inspired by the classical CAM, the cone excitations corresponding to the stimulus and the background are inserted in a compression‐adaptation formula to obtain a cone response proportional to the perceived brightness. The semi‐saturation constant occurring in this formula is modelled as a function of the cone excitation, the distance and the width of the ring. The results are overall promising, yet the model shows some shortcomings and more complex weighting functions might be needed. In the future, image‐based approaches should get more attention.

Spatial Relationship Perception is Not Affected by Short-term Cortical Reorganization

Aims: Long-term cortical reorganization after cortical damage can induce abnormal spatial relationship perception (spatial anisotropy) but there is also evidence of short-term, reversible cortical re-modulation even in the absence of cerebral damage: simulated hemianopic deprivation, in fact, is found to affect the positional judgment. This study investigates if the same occurs for spatial relationship perception.
 Study Design: Case series.
 Place and duration of Study: Service of Neuro-Ophthalmology, University of Turin, Italy, from January 2020 to July 2020.
 Methodology: Spatial relationship perception (SRP) of three subjects was measured in the presence of simulated homonymous hemianopia with a psychophysical procedure that estimates the discrimination threshold between elliptical and circular stimuli centered to the fixation point. The extent of the deprivation was graded as the distance of the proximal border of the nonresponsive area from the fixation point.
 Results: Overall, spatial relationship perception is not affected by the hemifield deprivation in terms of distance of the scotoma from the fixation point (P= 0.26), laterality (P= 0.07), and distance X laterality (P= 0.15). However, a significant effect of distance and laterality (P= 0.01 and P= 0.02, respectively) was found in the sole observer who showed an anisotropic perception in normal (no-simulated) condition.
 Conclusion: SRP appears to be robust toward the reversible spatial remapping induced by simulated hemianopia along the deprived area. However, the response of the visual system to artificial visual deprivation seems conditioned by pre-existing anisotropy.

Non-cardinal color mechanism elicitation by stimulus shape: Bringing the S versus L+M color plane to the table

Neurons in the cortex typically respond best to elongated stimuli, or gratings, whereas neurons in the lateral geniculate nucleus (LGN) typically prefer circular stimuli, or spots. Further, neural mechanisms specifically tuned for non-cardinal colors largely do not emerge until the cortex; therefore, the use of gratings should better reveal non-cardinal color mechanisms. This hypothesis has been tested in the isoluminant color plane in macaque monkeys (Stoughton, Lafer-Sousa, Gagin, & Conway, 2012) and in the L–M versus L+M color plane in human subjects (Gegenfurtner & Kiper, 1992). Here, this hypothesis was tested in the third color plane, S versus L+M, in human subjects in two experiments. Experiment 1 tested 10 subjects across four directions in this color plane; Experiment 2 tested three subjects in eight to twelve color directions. Consistent with data from the other two color planes, in both experiments in the S versus L+M color plane, gratings revealed the presence of non-cardinal mechanisms more strongly than did spots.

Circling around number: People can accurately extract numeric values from circle area ratios.

It has long been known that people have the ability to estimate numerical quantities without counting. A standard account is that people develop a sense of the size of symbolic numbers by learning to map symbolic numbers (e.g., 6) to their corresponding numerosities (e.g. :::) and concomitant approximate magnitude system (ANS) representations. However, we here demonstrate that adults are capable of extracting fractional numerical quantities from non-symbolic visual ratios (i.e., labeling a ratio of two circle areas with the appropriate symbolic fraction). Not only were adult participants able to perform this task, but they were remarkably accurate: linear regressions on median estimates yielded slopes near 1, and accounted for 97% of the variability. Participants also performed at least as well on line-estimation and ratio-estimation tasks using non-numeric circular stimuli as they did in earlier experiments using non-symbolic numerosities, which are frequently considered to be numeric stimuli. We discuss results as consistent with accounts suggesting that non-symbolic ratios have the potential to act as a reliable and stable ground for symbolic number, even when composed of non-numeric stimuli.

Towards an image-based brightness model for self-luminous stimuli.

Brightness is one of the most important perceptual correlates of color appearance models (CAMs) when self-luminous stimuli are targeted. However, the vast majority of existing CAMs adopt the presence of a uniform background surrounding the stimulus, which severely limits their practical application in lighting. In this paper, a study on the brightness perception of a neutral circular stimulus surrounded by a non-uniform background consisting of a neutral ring-shaped luminous area and a dark surround is presented. The ring-shaped luminous area is presented with 3 thicknesses (0.33 cm, 0.67 cm and 1.00 cm), at 4 angular distances to the edge of the central stimulus (1.2°, 6.4°, 11.3° and 16.1°) and at 3 luminance levels (90 cd/m2, 335 cd/m2, 1200 cd/m2). In line with the literature, the results of the visual matching experiments show that the perceived brightness decreases in presence of a ring and the effect is maximal at the highest luminance of the ring, for the largest thickness and at the closest distance. Based on the observed results, an image-based model inspired by the physiology of the retina is proposed. The model includes the calculation of cone-fundamental weighted spectral radiance, scattering in the eye, cone compression and receptive field post-receptor organization. The wide receptive field assures an adaptive shift determined by both the adaptation to the stimulus and to the background. It is shown that the model performs well in predicting the matching experiments, including the impact of the thickness, the distance and the intensity of the ring, showing its potential to become the basic framework of a Lighting Appearance Model.

Variation in antisaccadic response latencies investigated with the hierarchical LATER process model

The antisaccade paradigm is frequently applied to measure inhibitory control. Typically, simple, perceptually neutral stimuli are used as cues. Recently, emotional versions of this paradigm have also been employed. In our study, we used both versions of the paradigm. In addition, scrambled faces served to control for stimulus size and emotional valence. We applied a hierarchical extension to the Linear Approach to Threshold Ergodic Rate (LATER) process model, which allows the estimation of two latent cognitive parameters: speed of information accumulation (accretion rate) and the amount of information needed before a saccadic movement (caution threshold). We hypothesized a faster accretion rate and lower caution threshold for circular and scrambled compared to emotional face stimuli as well as meaningful differences between individual emotions. Our results showed a faster accretion rate and lower caution threshold for emotional compared to circular stimuli, though. In contrast, scrambled faces had a lower accretion rate and lower caution threshold. Furthermore, the LATER model uncovered subtle differences between different emotions. Happy faces tend to receive a faster accretion rate and higher caution threshold than neutral ones, while for fearful faces it was the other way around. Our results contradict earlier research on emotional stimuli interfering inhibitory control.

Relationship between structural and functional changes in glaucomatous eyes: a multifocal electroretinogram study

BackgroundThe nasal to temporal amplitudes ratio (N/T) of multifocal electroretinography (mfERG) scans measured within 5° of the macula can be used to detect glaucomatous change. The photopic negative response (PhNR) of mfERG elicited by a circular stimulus centered on the fovea was significantly reduced in eyes with glaucoma. The PhNR to B-wave ratio (PhNR/B) is the optimal measure of the PhNR. However, clinical superiority for evaluating glaucoma patients has not been determined between N/T and PhNR/B yet.MethodsFor morphological assessments, ganglion cell complex (GCC) in six regions and the average were measured by optical coherence tomography (OCT). For functional assessment, Humphrey visual fields (VF) with mean sensitivities (MT) and mfERG scans with parameters of N/T and the multifocal photopic negative response to B-wave ratio (mfPhNR/B) were measured. Sixty-nine eyes of 44 glaucoma patients were included and correlations between mfERG parameters and OCT or VF parameters were evaluated.ResultsThe mean age of patients was 59.4 years. The mean deviation for all eyes obtained with the VF 30–2 and VF 10–2 was − 7.00 and − 6.31 dB, respectively. Significant correlations between GCC thickness or VF parameter and the N/T were found, especially in the inferior and inforotemporal retinal areas corresponding to superior and superonasal VF sectors (GCC vs N/T; coefficient = − 7.916 and − 7.857, and MT vs N/T; coefficient = − 4.302 and − 4.437, in the inferior and inforotemporal retinal areas, respectively, all p values < 0.05). However, similar associations were not obtained between mfPhNR/B and OCT or VF parameters. The mfPhNR/B only in the inferotemporal sector was significantly correlated with the average thickness of GCC (coefficient = 4.823, P = 0.012).ConclusionsThe N/T was correlated with GCC and VF in more numbers of measurement areas than the mfPhNR/B in the current study, however, a future study modifying the stimuli and amplitudes to obtain the spatial correspondence to OCT and VF measurement will be required to evaluate the value of mfERG.

Intrasaccadic motion streaks jump-start gaze correction.

Rapid eye movements (saccades) incessantly shift objects across the retina. To establish object correspondence, the visual system is thought to match surface features of objects across saccades. Here, we show that an object's intrasaccadic retinal trace-a signal previously considered unavailable to visual processing-facilitates this match making. Human observers made saccades to a cued target in a circular stimulus array. Using high-speed visual projection, we swiftly rotated this array during the eyes' flight, displaying continuous intrasaccadic target motion. Observers' saccades landed between the target and a distractor, prompting secondary saccades. Independently of the availability of object features, which we controlled tightly, target motion increased the rate and reduced the latency of gaze-correcting saccades to the initial presaccadic target, in particular when the target's stimulus features incidentally gave rise to efficient motion streaks. These results suggest that intrasaccadic visual information informs the establishment of object correspondence and jump-starts gaze correction.

Faster Visual Information Processing in Video Gamers Is Associated With EEG Alpha Amplitude Modulation.

Video gaming, specifically action video gaming, seems to improve a range of cognitive functions. The basis for these improvements may be attentional control in conjunction with reward-related learning to amplify the execution of goal-relevant actions while suppressing goal-irrelevant actions. Given that EEG alpha power reflects inhibitory processing, a core component of attentional control, it might represent the electrophysiological substrate of cognitive improvement in video gaming. The aim of this study was to test whether non-video gamers (NVGs), non-action video gamers (NAVGs) and action video gamers (AVGs) exhibit differences in EEG alpha power, and whether this might account for differences in visual information processing as operationalized by the theory of visual attention (TVA). Forty male volunteers performed a visual short-term memory paradigm where they memorized shape stimuli depicted on circular stimulus displays at six different exposure durations while their EEGs were recorded. Accuracy data was analyzed using TVA-algorithms. There was a positive correlation between the extent of post-stimulus EEG alpha power attenuation (10–12 Hz) and speed of information processing across all participants. Moreover, both EEG alpha power attenuation and speed of information processing were modulated by an interaction between group affiliation and time on task, indicating that video gamers showed larger EEG alpha power attenuations and faster information processing over time than NVGs – with AVGs displaying the largest increase. An additional regression analysis affirmed this observation. From this we concluded that EEG alpha power might be a promising neural substrate for explaining cognitive improvement in video gaming.

Relationships between the vascular structure and neural function of the macula in patients with diabetes mellitus.

To determine the significance of the correlation between the vascular structure and neural function of the macula in patients with diabetes mellitus. Single-center observational study. Ninety eyes of 90 diabetic patients with an average (SD) age of 63.5 (3.8) years were studied. Fifty of the eyes had no clinically apparent diabetic retinopathy (non-DR), and 40 eyes had mild-to-moderate nonproliferative DR (NPDR). Thirty age-matched healthy individuals were also studied in the same way. Swept-source optical coherence tomography angiography (OCTA) was performed to obtain 3 × 3-mm en face images of the posterior pole of the eye. The vascular densities (VDs) of the superficial capillary plexus (SCP) and the deep capillary plexus (DCP) were determined. The focal macular electroretinograms (ERGs) elicited by a 15° circular stimulus centered on the fovea were recorded. The amplitudes of the a- and b-waves, sum of the oscillatory potentials (ΣOPs), photopic negative response (PhNR), and implicit times of the individual OPs (OP1-OP3) were measured. The VDs of the SCP and DCP were reduced in eyes with advanced DR (P < .01 for SCP). The implicit times of OP1-OP3 were significantly prolonged in eyes with a lower VD of the SCP and DCP in the non-DR group (P < .05). The amplitudes of the ΣOPs were significantly smaller in eyes with a reduced VD of the SCP and DCP in the NPDR group (P < .05). The correlation coefficients were higher for the OP implicit times than for the ΣOP amplitudes in the non-DR group. The OPs of the focal macular ERG are smaller with prolonged implicit times in association with capillary loss in the macula of diabetic patients. The implicit times are the most sensitive functional parameter that reflects the early changes of the microvasculature in the macula caused by diabetes.

Relationship between stimulus size and different components of the electroretinogram (ERG) elicited by flashed stimuli.

To investigate how light stimulus conditions of varying spatial sizes affect components of the flash and long-flash electroretinogram (ERG) in normal subjects. Three stimulus conditions were generated by a Ganzfeld stimulator: a white flash on white background (WoW), a red flash on a blue background (RoB) and an L+M-cone isolating on-off (long flash) stimulus (Cone Iso). ERGs were recorded from six subjects (5M, 1 F) with DTL electrodes to full-field (FF), 70°, 60°, 50°, 40°, 30° and 20° diameter circular stimuli. Amplitudes and peak times for a-, b-, d- and i-wave, and PhNR were examined. PhNR amplitudes were estimated in two different ways: from baseline (fB) and from preceding b-wave peak (fP). With decreasing stimulus size, amplitudes for all ERG waveform components attenuated and peak times increased, although the effect varied across different components. An exponential fit described the relationship between amplitudes and size of stimulated retinal area well for most components and conditions (R2= 0.75-0.99), except for PhNR(fB) (R2= - 0.16-0.88). For peak times, an exponential decay function also fitted the data well (R2= 0.81-0.97), except in a few cases where the exponential constant was too small and a linear regression function was applied instead (a-wave Cone Iso, b- and i-wave WoW). The exponential constants for RoB amplitudes (b-wave, PhNR(fB), PhNR(fP)) were larger compared to their counterparts under WoW (p < 0.05), while there was no difference between the constants for a-wave amplitudes and peak times and for PhNR peak times. The exponential constants of amplitudes vs. area under WoW and Cone Iso were remarkably similar, while under RoB PhNR(fB) showed larger constants compared to either a- or b-wave (p < 0.05). ERG components change in a predictable way with stimulus size and spectral characteristics of the stimulus under these conditions. This predictability could allow a modified version of these sets of stimuli to be tested for clinical applicability.

An electrophysiological biomarker for the classification of cataract-reversal patients: A case-control study.

BackgroundUntreated congenital blindness through cataracts leads to lasting visual brain system changes, including substantial alterations of extrastriate visual areas. Consequently, late-treated individuals (> 5 months of age) with dense congenital bilateral cataracts (CC) exhibit poorer visual function recovery compared to individuals with bilateral developmental cataracts (DC). Reliable methods to differentiate between patients with congenital and developmental cataracts are often lacking, impeding efficient rehabilitation management and introducing confounds in clinical and basic research on recovery prognosis and optimal timing of surgery. A persistent reduction of the P1 wave of visual event-related potentials (VERPs), associated with extrastriate visual cortical activity, has been reported in CC but not in DC individuals. Using two experiments, this study developed and validated P1-based biomarkers for diagnosing a history of congenital blindness in cataract-reversal individuals.MethodsCongenital and developmental cataract-reversal individuals as well as typically sighted matched controls took part in a first experiment used for exploring an electrophysiological biomarker (NCC = 13, NDC = 13, NControl = 26). Circular stimuli containing gratings were presented in one of the visual field quadrants while visual event-related potentials (VERPs) were recorded. Two biomarkers were derived from the P1 wave of the VERP: (1) The mean of the normalized P1 amplitude at posterior electrodes, and (2) a classifier obtained from a linear support vector machine (SVM). A second experiment with partially new CC/DC individuals and their matched controls (NCC = 14, NDC = 15, NControl = 29) was consecutively used to validate the classification based on both biomarkers. Performance of the classifiers were evaluated using receiver operating characteristic (ROC) curve analyses. All cataract-reversal individuals were tested after at least one year of vision recovery.FindingsThe normalized P1 amplitude over posterior electrodes allowed a successful classification of the CC from the DC individuals and typically sighted controls (area under ROC curve, AUC = 0.803 and 0.929 for the normalized P1 amplitude and the SVM-based biomarker, respectively). The validation for both biomarkers in experiment 2 again resulted in a high classification success (AUC = 0.800 and 0.883, respectively for the normalized P1 amplitude and the SVM-based biomarker). In the most conservative scenario involving classification of CC from DC individuals in a group of only cataract-reversal individuals, excluding typically sighted controls, the SVM-based biomarker was found to be superior to the mean P1 amplitude based biomarker (AUC = 0.852 compared to 0.757 for the mean P1 based biomarker in validation). Minimum specificity obtained was 80% across all biomarkers.InterpretationA persistent reduction of the P1 wave provides a highly specific method for classifying cataract patients post-surgically as having suffered from bilateral congenital vs. bilateral developmental cataracts. We suggest that using the P1 based non-invasive electrophysiological biomarker will augment existing clinical classification criteria for individuals with a history of bilateral congenital cataracts, aiding clinical and basic research, recovery prognosis, and rehabilitation efforts.FundingGerman Research Foundation (DFG) and the European Research Council (ERC).

Macular function following intravitreal ranibizumab for macular edema associated with branch retinal vein occlusion: 12-month results.

To evaluate the 12-month effects of intravitral ranibizumab (IVR) injections on the physiology and morphology of the macula in eyes with a branch retinal vein occlusion with macular edema (BRVOME). We studied 13 eyes of 13 patients with a BRVOME. All patients were initially treated with IVR injections at 3 consecutive monthly intervals, the initiation phase. Additional treatments were done according to the pro re nata (PRN) regimen. The physiology of the macula was assessed by the focal macular electroretinograms (fmERGs) and the best-corrected visual acuity (BCVA). The morphology of the macular area was determined by spectral-domain optical coherence tomography. The retina was assessed at the baseline, and at 3, 6, and 12months after beginning the IVR injections. The fmERGs were elicited by a 15° circular stimulus, and also by 15° semicircular stimuli placed on the occluded or non-occluded side of the macula. The amplitudes of the a- and b-waves, photopic negative response (PhNR), and sums of the oscillatory potentials (ΣOPs) were measured. In addition, the implicit times of the a- and b-waves were measured. The BCVA was improved significantly relative to the baseline after the initiation phase (P < 0.01), and it was maintained with the PRN regimen for the 12-month experimental period. The foveal thickness was also significantly decreased after the initiation phase (P < 0.005) but was then worse at 12months during the maintenance phase (P < 0.05). The fmERGs on the occluded side were significantly reduced at the baseline, and the b-waves and ΣOPs amplitudes improved after the initiation phase. However, they decreased during the PRN period. On the non-occluded side, the amplitudes and implicit times of the a- and b-waves remained unchanged after the initiation phase, the PhNR and ΣOPs amplitudes significantly increased at 3months compared to the baseline (PhNR, P < 0.005; ΣOPs, P < 0.005), which was maintained throughout the 12month study period. The deterioration of the macular function on the occluded side during the maintenance phase suggests that there is a progression of the disease process during the PRN period in eyes with BRVOME.

Haidinger's brushes elicited at varying degrees of polarization rapidly and easily assesses total macular pigmentation.

Macular pigments (MPs), by absorbing potentially toxic short-wavelength (400-500nm) visible light, provide protection against photo-chemical damage thought to be relevant in the pathogenesis of age-related macular degeneration (AMD). A method of screening for low levels of MPs could be part of a prevention strategy for helping people to delay the onset of AMD. We introduce a new method for assessing MP density that takes advantage of the polarization-dependent absorption of blue light by MPs, which results in the entoptic phenomenon called Haidinger's brushes (HB). Subjects were asked to identify the direction of rotation of HB when presented with a circular stimulus illuminated with an even intensity of polarized white light in which the electric field vector was rotating either clockwise or anti-clockwise. By reducing the degree of polarization of the stimulus light, a threshold for perceiving HB (degree of polarization threshold) was determined and correlated (r2=0.66) to macular pigment optical density assessed using dual-wavelength fundus autofluoresence. The speed and ease of measurement of degree of polarization threshold makes it well suited for large-scale screening of macular pigmentation.

Retinotopic and topographic analyses with gaze restriction for steady-state visual evoked potentials

Although the mechanisms of steady-state visual evoked potentials (SSVEPs) have been well studied, none of them have been implemented with strictly experimental conditions. Our objective was to create an ideal observer condition to exploit the features of SSVEPs. We present here an electroencephalographic (EEG) eye tracking experimental paradigm that provides biofeedback for gaze restriction during the visual stimulation. Specifically, we designed an EEG eye tracking synchronous data recording system for successful trial selection. Forty-six periodic flickers within a visual field of 11.5° were successively presented to evoke SSVEP responses, and online biofeedback based on an eye tracker was provided for gaze restriction. For eight participants, SSVEP responses in the visual field and topographic maps from full-brain EEG were plotted and analyzed. The experimental results indicated that the optimal visual flicking arrangement to boost SSVEPs should include the features of circular stimuli within a 4–6° spatial distance and increased stimulus area below the fixation point. These findings provide a basis for determining stimulus parameters for neural engineering studies, e.g. SSVEP-based brain-computer interface (BCI) designs. The proposed experimental paradigm could also provide a precise framework for future SSVEP-related studies.

Rod- versus cone-driven ERGs at different stimulus sizes in normal subjects and retinitis pigmentosa patients.

To study how rod- and cone-driven responses depend on stimulus size in normal subjects and patients with retinitis pigmentosa (RP), and to show that comparisons between responses to full-field (FF) and smaller stimuli can be useful in diagnosing and monitoring disorders of the peripheral retina without the need for lengthy dark adaptation periods. The triple silent substitution technique was used to isolate L-cone-, M-cone- and rod-driven ERGs with 19, 18 and 33% photoreceptor contrasts, respectively, under identical mean luminance conditions. Experiments were conducted on five normal subjects and three RP patients. ERGs on control subjects were recorded at nine different temporal frequencies (between 2 and 60Hz) for five different stimulus sizes: FF, 70°, 60°, 50° and 40° diameter circular stimuli. Experiments on RP patients involved rod- and L-cone-driven ERG measurements with FF and 40° stimuli at 8 and 48Hz. Response amplitudes were defined as those of the first harmonic component after Fourier analysis. In normal subjects, rod-driven responses displayed a fundamentally different behavior than cone-driven responses, particularly at low temporal frequencies. At low and intermediate temporal frequencies (≤12Hz), rod-driven signals increased by a factor of about four when measured with smaller stimuli. In contrast, L- and M-cone-driven responses in this frequency region did not change substantially with stimulus size. At high temporal frequencies (≥24Hz), both rod- and cone-driven response amplitudes decreased with decreasing stimulus size. Signals obtained from rod-isolating stimuli under these conditions are likely artefactual. Interestingly, in RP patients, both rod-driven and L-cone-driven ERGs were similar using 40° and FF stimuli. The increased responses with smaller stimuli in normal subjects to rod-isolating stimuli indicate that a fundamentally different mechanism drives the ERGs in comparison with the cone-driven responses. We propose that the increased responses are caused by stray light stimulating the peripheral retina, thereby allowing peripheral rod-driven function to be studied using the triple silent substitution technique at photopic luminances. The method is effective in studying impaired peripheral rod- and cone- function in RP patients.