Chromatic Flicker Research Articles

Temporal Sensitivity for Achromatic and Chromatic Flicker across the Visual Cortex.

The retinal ganglion cells (RGCs) receive different combinations of L, M, and S cone inputs and give rise to one achromatic and two chromatic postreceptoral channels. The goal of the current study was to determine temporal sensitivity across the three postreceptoral channels in subcortical and cortical regions involved in human vision. We measured functional magnetic resonance imaging (fMRI) responses at 7 T from three participants (two males, one female) viewing a high-contrast, flickering, spatially uniform wide field (∼140°). Stimulus flicker frequency varied logarithmically between 2 and 64 Hz and targeted the L + M + S, L - M, and S - (L + M) cone combinations. These measurements were used to create temporal sensitivity functions of the primary visual cortex (V1) across eccentricity and spatially averaged responses from the lateral geniculate nucleus (LGN), and the V2/V3, hV4, and V3A/B regions. fMRI responses reflected the known properties of the visual system, including higher peak temporal sensitivity to achromatic versus chromatic stimuli and low-pass filtering between the LGN and V1. Peak temporal sensitivity increased across levels of the cortical visual hierarchy. Unexpectedly, peak temporal sensitivity varied little across eccentricity within area V1. Measures of adaptation and distributed pattern activity revealed a subtle influence of 64 Hz achromatic flicker in area V1, despite this stimulus evoking only a minimal overall response. The comparison of measured cortical responses to a model of the integrated retinal output to our stimuli demonstrates that extensive filtering and amplification are applied to postretinal signals.

Brain stimulation with 40 Hz heterochromatic flicker extended beyond red, green, and blue

Alzheimer’s disease (AD) is associated with electrophysiological changes in the brain. Pre-clinical and early clinical trials have shown promising results for the possible therapy of AD with 40 Hz neurostimulation. The most notable findings used stroboscopic flicker, but this technique poses an inherent barrier for human applications due to its visible flickering and resulting high level of perceived discomfort. Therefore, alternative options should be investigated for entraining 40 Hz brain activity with light sources that appear less flickering. Previously, chromatic flicker based on red, green, and blue (RGB) have been studied in the context of brain-computer interfaces, but this is an incomplete representation of the colours in the visual spectrum. This study introduces a new kind of heterochromatic flicker based on spectral combinations of blue, cyan, green, lime, amber, and red (BCGLAR). These combinations are investigated by the steady-state visually evoked potential (SSVEP) response from the flicker with an aim of optimising the choice of 40 Hz light stimulation with spectrally similar colour combinations in BCGLAR space. Thirty healthy young volunteers were stimulated with heterochromatic flicker in an electroencephalography experiment with randomised complete block design. Responses were quantified as the 40 Hz signal-to-noise ratio and analysed using mixed linear models. The size of the SSVEP response to heterochromatic flicker is dependent on colour combinations and influenced by both visual and non-visual effects. The amber-red flicker combination evoked the highest SSVEP, and combinations that included blue and/or red consistently evoked higher SSVEP than combinations only with mid-spectrum colours. Including a colour from either extreme of the visual spectrum (blue and/or red) in at least one of the dyadic phases appears to be more important than choosing pairs of colours that are far from each other on the visual spectrum. Spectrally adjacent colour pairs appear less flickering to the perceiver, and thus the results motivate investigations into the limits for how alike the two phases can be and still evoke a 40 Hz response. Specifically, combining a colour on either extreme of the visual spectrum with another proximal colour might provide the best trade-off between flickering sensation and SSVEP magnitude.

Poster Session I: Effect of flicker adaptation on perception of small spots presented with AOSLO.

Previous work suggests the detection of small spots is mediated by a mixture of chromatic and achromatic mechanisms. We tested whether exposure to spatially-uniform chromatic or luminance flicker affected detection thresholds for 543 nm increments delivered through an AOSLO. Heterochromatic flicker photometry was used to determine isoluminant settings for the red and green primaries of a DLP display; this isoluminant red-green mixture provided the 2.1° background upon which 23 arcmin (N=4) or 3 arcmin (N=2) stimuli were presented for 100ms. The projector background was modulated to produce isoluminant chromatic flicker or isochromatic luminance flicker at 3.75 or 30 Hz. The time-averaged luminance and chromaticity for all adaptation conditions were equivalent. For each condition, data collection was preceded by 2 minutes of preadaptation, followed by alternating windows of stimulus delivery (1 sec, steady background) and top-up adaptation (3 sec). Thresholds for all flicker conditions were compared to data obtained on a static background. For 23 arcmin spots, we found reduced sensitivity in the 3.75 Hz chromatic and luminance flicker conditions, but no adaptation effect was observed for 3 arcmin flashes or for 30 Hz flicker of either type. Our data suggest that raster-scanned, AO-corrected stimuli are susceptible to flicker adaptation, but that proximity to a flickering edge may be an important factor governing the effects of contrast adaptation on small spot detection.

Behavioral evidence for attention selection as entrained synchronization without awareness.

Animal physiological and human neuroimaging studies have established a link between attention and γ-band (30-90 Hz) oscillations and synchronizations. However, a behavioral link between entrained γ-band oscillations and attention has been fraught with technical challenges. In particular, while entrainment at mid-γ band (40-70 Hz) has been claimed to be privileged in evoking attentional modulations without awareness, the effect may be attributed to display artifacts. Here, by exploiting isoluminant chromatic flicker without luminance modulation and not subject to these artifacts, we tested attentional attraction by chromatic flicker too fast to perceive. Awareness of flicker was subjectively and objectively tested with a high-powered design and evaluated with traditional and Bayesian statistics. Across 2 experiments in human participants, we observed-and also replicated-that 30-Hz chromatic flicker outside mid-γ band attracted attention, resulting in a facilitation effect at a 50 ms interstimulus interval (ISI) and an inhibition effect at a 500 ms ISI. The attention test was confirmed to be more sensitive to the cue than the direct cue-localization task was. We further showed that these attention effects were absent for 50-Hz chromatic flicker. These results provide strong direct evidence against a privileged role of mid-γ band in unconscious attention, but are consistent with known cortical responses to chromatic flicker in early visual cortex. Taken together, our findings provide behavioral evidence that entrained synchronization may serve as a mechanism for bottom-up attention selection and that chromatic flicker may offer a fruitful avenue for investigating unconscious processing. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Visual function deficits in eyes with resolved endophthalmitis

To evaluate the changes in functional vision in patients with resolved endophthalmitis. This was a cross-sectional study. The study included 20 patients with resolved endophthalmitis and best-corrected visual acuity of 20/100 or better. Visual acuity (VA), contrast threshold (CT), red/green (RG) and yellow/blue (YB) colour vision and 15 Hz flicker modulation threshold (FMT) were assessed using standard psychophysical techniques. The median age was 54 years. The median visual acuity was 0.27 (~ 20/40—Snellen Equivalent) ((interquartile range [IQR]), 0.30) logMAR). The median log contrast threshold (CT) was − 1.13 (IQR, 0.36) log units (normative value for age-matched CT: − 1.61 log units). The median red/green (RG) and yellow/blue (YB) thresholds were 11.52 (IQR, 26.19) and 9.45 (IQR, 16.20) CAD units respectively, which were at least 5 times higher than age-matched normative RG and YB thresholds. The median central cone- mediated FMT was 17.64% (IQR, 23.40%), which was much higher compared to age-matched FMT (5.48% [IQR, 3.47]). Linear regression revealed significant relationship between contrast thresholds and foveal thickness (y = 0.001x−1.47, R2 = 0.20, p = 0.048). Though endophthalmitis may resolve with a good visual acuity, deficits in visual functions like chromatic discrimination, cone-mediated flicker and contrast sensitivity persist.

In vivo classification of macaque foveal ganglion cells through optical recording of responses to chromatic and luminance flicker

<i>In vivo</i> classification of macaque foveal ganglion cells through optical recording of responses to chromatic and luminance flicker

Modelling Contrast Sensitivity for Chromatic Temporal Modulations

The temporal contrast sensitivity to isoluminant chromatic flicker was measured for three observers using the method of adjustment. The isoluminant stimuli were created for each observer individually, based on a technique similar to heterochromatic flicker photometry. The chromatic flicker stimuli were sinusoidal modulations, defined in the CIE 1976 UCS (u', v') chromaticity diagram. The chromaticity varied around a base color along a certain modulation direction with a certain amplitude at a certain frequency. Nine base colors, four modulation directions and seven frequencies were used, resulting in thirty-six temporal contrast sensitivity curves per observer. An exponential model was fitted to the resulting contrast sensitivity expressed as 1/Δ(u', v'), 1/ΔLMS and 1/Δlms. The model resulted in an average R2 value higher than 0.93 for the three different measures of contrast sensitivity. The two parameters of the model (i.e. the slope β1 and intercept β0) were found to significantly depend on the base color and direction of the chromatic modulation. This means that Δ(u', v'), ΔLMS and Δlms are not suitable measures to predict the sensitivity to temporal chromatic modulations in different locations of the color space.

Unconscious influence over executive control: Absence of conflict detection and adaptation

Executive control and its modulation of attentional mechanisms allow us to detect and adapt to conflicting information. According to recent studies, executive control functions may be modulated by unconsciously perceived information, although the available evidence is not consistent. In this study, we used a Flanker Task and employed Chromatic Flicker Fusion, a suppression technique that has been proposed as more adequate to elicit executive control functions, to assess conflict and conflict adaptation effects. Our results showed that, when suppressed, flankers did not evoke conflict related effects on performance. However, in trials where most flankers were incongruent, longer response times in congruent trials were observed, consistent with orienting responses. Our results help to support earlier theories regarding the inherent limitations of unconsciously perceived information, though future studies should further investigate why and under which conditions is the executive control system modulated by unconscious information.

Hue shifts produced by temporal asymmetries in chromatic signals depend on the alignment of the first and second harmonics.

When M- or L-cone-isolating sawtooth waveforms flicker at frequencies between 4 and 13.3 Hz, there is a mean hue shift in the direction of the shallower sawtooth slope. Here, we investigate how this shift depends on the alignment of the first and second harmonics of sawtooth-like waveforms. Below 4 Hz, observers can follow hue variations caused by both harmonics, and reliably match reddish and greenish excursions. At higher frequencies, however, the hue variations appear as chromatic flicker superimposed on a steady light, the mean hue of which varies with second-harmonic alignment. Observers can match this mean hue against a variable-duty-cycle rectangular waveform and, separately, set the alignment at which the mean hue flips between reddish and greenish. The maximum hue shifts were approximately frequency independent and occurred when the peaks or troughs of the first and second harmonics roughly aligned at the visual input-consistent with the hue shift's being caused by an early instantaneous nonlinearity that saturates larger hue excursions. These predictions, however, ignore phase delays introduced within the chromatic pathway between its input and the nonlinearity that produces the hue shifts. If the nonlinearity follows the substantial filtering implied by the chromatic temporal contrast-sensitivity function, phase delays will alter the alignment of the first and second harmonics such that at the nonlinearity, the waveforms that produce the maximum hue shifts might well be those with the largest differences in rising and falling slopes-consistent with the hue shift's being caused by a central nonlinearity that limits the rate of hue change.

Visual Cortical Activity Evoked by Unconscious Chromatic Flicker

Visual Cortical Activity Evoked by Unconscious Chromatic Flicker

Unconscious neural processing differs with method used to render stimuli invisible.

Visual stimuli can be kept from awareness using various methods. The extent of processing that a given stimulus receives in the absence of awareness is typically used to make claims about the role of consciousness more generally. The neural processing elicited by a stimulus, however, may also depend on the method used to keep it from awareness, and not only on whether the stimulus reaches awareness. Here we report that the method used to render an image invisible has a dramatic effect on how category information about the unseen stimulus is encoded across the human brain. We collected fMRI data while subjects viewed images of faces and tools, that were rendered invisible using either continuous flash suppression (CFS) or chromatic flicker fusion (CFF). In a third condition, we presented the same images under normal fully visible viewing conditions. We found that category information about visible images could be extracted from patterns of fMRI responses throughout areas of neocortex known to be involved in face or tool processing. However, category information about stimuli kept from awareness using CFS could be recovered exclusively within occipital cortex, whereas information about stimuli kept from awareness using CFF was also decodable within temporal and frontal regions. We conclude that unconsciously presented objects are processed differently depending on how they are rendered subjectively invisible. Caution should therefore be used in making generalizations on the basis of any one method about the neural basis of consciousness or the extent of information processing without consciousness.

Coherent Activity in Bilateral Parieto-Occipital Cortices during P300-BCI Operation.

The visual P300 brain–computer interface (BCI), a popular system for electroencephalography (EEG)-based BCI, uses the P300 event-related potential to select an icon arranged in a flicker matrix. In earlier studies, we used green/blue (GB) luminance and chromatic changes in the P300-BCI system and reported that this luminance and chromatic flicker matrix was associated with better performance and greater subject comfort compared with the conventional white/gray (WG) luminance flicker matrix. To highlight areas involved in improved P300-BCI performance, we used simultaneous EEG–fMRI recordings and showed enhanced activities in bilateral and right lateralized parieto-occipital areas. Here, to capture coherent activities of the areas during P300-BCI, we collected whole-head 306-channel magnetoencephalography data. When comparing functional connectivity between the right and left parieto-occipital channels, significantly greater functional connectivity in the alpha band was observed under the GB flicker matrix condition than under the WG flicker matrix condition. Current sources were estimated with a narrow-band adaptive spatial filter, and mean imaginary coherence was computed in the alpha band. Significantly greater coherence was observed in the right posterior parietal cortex under the GB than under the WG condition. Re-analysis of previous EEG-based P300-BCI data showed significant correlations between the power of the coherence of the bilateral parieto-occipital cortices and their performance accuracy. These results suggest that coherent activity in the bilateral parieto-occipital cortices plays a significant role in effectively driving the P300-BCI.

Hyperemic responses of the optic nerve head blood flow to chromatic equiluminant flicker are reduced by ocular hypertension and early glaucoma

We evaluated in ocular hypertension (OHT) and early glaucoma (EOAG) patients the optic nerve head (ONH) blood flow response (RFonh) to chromatic equiluminant flicker. This stimulus generates neural activity dominated by the parvo-cellular system. Eleven EOAG, 20 OHT patients, and 8 age-matched control subjects were examined. The blood flow (Fonh) at the neuroretinal rim was continuously monitored by laser Doppler flow- metry before, during, and after a 60-s exposure to a 4 Hz, red-green equiluminant flicker stimulus (30 deg field). RFonh was expressed as percentage Fonh-change during the last 20 s of flicker relative to baseline Fonh. Responses were collected at a number of temporal sites. The highest RFonh value was used for subsequent analysis. As compared to controls, both OHT and EOAG patients showed a decrease (p < 0.01) in mean RFonh. We conclude that RFonh elicited by chromatic equiluminant flicker is abnormally reduced in OHT and EOAG patients indicating an impairment of the parvo-cellular-mediated vasoactivity. This decrease of vascular response may occur independently of neural activity loss early in the disease process. © 2014 Society of Photo-

Alerting and orienting of attention without visual awareness

Two types of the attentional network, alerting and orienting, help organisms respond to environmental events for survival in the temporal and spatial dimensions, respectively. Here, we applied chromatic flicker beyond the critical fusion frequency to address whether awareness was necessary for activation of the two attentional networks. We found that high-frequency chromatic flicker, despite its failure to reach awareness, produced the alerting and orienting effects, supporting the dissociation between attention and awareness. Furthermore, as the flicker frequency increased, the orienting effect attenuated whereas the alerting effect remained unchanged. According to the systematic decline in temporal frequency sensitivity across the visual hierarchy, this finding suggests that unconscious alerting might be associated with activity in earlier visual areas than unconscious orienting. Since high-frequency flicker has been demonstrated to only activate early visual cortex, we suppose that neural activation in early visual areas might be sufficient to activate the two attentional networks.

Direction in the color plane as a factor in chromatic flicker and chromatic motion

A percept of motion results when a chromatic grating, formed from a spatial alternation between two isoluminant hues, drifts across the visual field. With hue pairs chosen to be equally subjectively dissimilar, the motion is greater for alternation along some directions in color space (orange/blue) than others (green/purple), suggesting a specific interaction between the (L-M) and S(0) chromatic opponent channels. This phenomenon was explored systematically by choosing 24 pairs of hues across the color circle and using the method of paired comparisons to scale their movement-inducing contrast. The flicker-inducing contrast observed from rapid alternation between the pairs was measured in the same way. Both phenomena consistently drew upon both chromatic channels, though in different proportions, as if chromatic and temporal variation information are multiplexed along motion-processing pathways. Border-distinctness data were also collected to isolate the (L-M) channel.

Effect of the Green/Blue Flicker Matrix for P300-Based Brain–Computer Interface: An EEG–fMRI Study

The visual P300-brain–computer interface, a popular system for EEG-based BCI, utilizes the P300 event-related potential to select an icon arranged in a flicker matrix. In the conventional P300-BCI speller paradigm, white/gray luminance intensification of each row/column in the matrix is used. In an earlier study, we applied green/blue luminance and chromatic change in the P300-BCI system and reported that this luminance and chromatic flicker matrix was associated with better performance and greater subject comfort compared with the conventional white/gray luminance flicker matrix. In this study, we used simultaneous EEG-functional magnetic resonance imaging (fMRI) recordings to identify brain areas that were more enhanced in the green/blue flicker matrix than in the white/gray flicker matrix, as these may highlight areas devoted to improved P300-BCI performance. The peak of the positive wave in the EEG data was detected under both conditions, and the peak amplitudes were larger at the parietal and occipital electrodes, particularly in the late components, under the green/blue condition than under the white/gray condition. fMRI data showed activation in the bilateral parietal and occipital cortices, and these areas, particularly those in the right hemisphere, were more activated under the green/blue condition than under the white/gray condition. The parietal and occipital regions more involved in the green/blue condition were part of the areas devoted to conventional P300s. These results suggest that the green/blue flicker matrix was useful for enhancing the so-called P300 responses.

Visual temporal processing in dyslexia and the magnocellular deficit theory: The need for speed?

A controversial question in reading research is whether dyslexia is associated with impairments in the magnocellular system and, if so, how these low-level visual impairments might affect reading acquisition. This study used a novel chromatic flicker perception task to specifically explore temporal aspects of magnocellular functioning in 40 children with dyslexia and 42 age-matched controls (aged 7-11). The relationship between magnocellular temporal resolution and higher-level aspects of visual temporal processing including inspection time, single and dual-target (attentional blink) RSVP performance, go/no-go reaction time, and rapid naming was also assessed. The Dyslexia group exhibited significant deficits in magnocellular temporal resolution compared with controls, but the two groups did not differ in parvocellular temporal resolution. Despite the significant group differences, associations between magnocellular temporal resolution and reading ability were relatively weak, and links between low-level temporal resolution and reading ability did not appear specific to the magnocellular system. Factor analyses revealed that a collective Perceptual Speed factor, involving both low-level and higher-level visual temporal processing measures, accounted for unique variance in reading ability independently of phonological processing, rapid naming, and general ability.

Flicker between equal-luminance colors examined with multidimensional scaling

Even when two rapidly alternating color stimuli are equated in luminance, the flicker between them is not always zero. By one hypothesis this residual chromatic flicker is tritanopic, like edge distinctness: dependent purely on L- and M-cone stimulation, with no contribution from S-cones. Judgments of flicker intensity between pairs of colors were analyzed with multidimensional scaling (MDS)--in effect treating them as an index of color dissimilarity. They reveal a systematic reduction of flicker when stimulus pairs differ along a chartreuse-magenta direction in the color plane, corresponding to an effective compression of color space along this axis or an equivalent elongation along a blue-orange axis. In contrast, judgments of edge distinctness between the same pairs reveal the expected tritanopic axis of compression. It follows that chromatic flicker does receive a contribution from S-cone stimulation, but this interacts with the contribution from L- and M-cones, perhaps due to phase delays.

Chromatic flicker perception in human peripheral vision under mental load

Chromatic flicker perception in human peripheral vision under mental load

Visual stimuli for the P300 brain–computer interface: A comparison of white/gray and green/blue flicker matrices

The white/gray flicker matrix has been used as a visual stimulus for the so-called P300 brain-computer interface (BCI), but the white/gray flash stimuli might induce discomfort. In this study, we investigated the effectiveness of green/blue flicker matrices as visual stimuli. Ten able-bodied, non-trained subjects performed Alphabet Spelling (Japanese Alphabet: Hiragana) using an 8 x 10 matrix with three types of intensification/rest flicker combinations (L, luminance; C, chromatic; LC, luminance and chromatic); both online and offline performances were evaluated. The accuracy rate under the online LC condition was 80.6%. Offline analysis showed that the LC condition was associated with significantly higher accuracy than was the L or C condition (Tukey-Kramer, p < 0.05). No significant difference was observed between L and C conditions. The LC condition, which used the green/blue flicker matrix was associated with better performances in the P300 BCI. The green/blue chromatic flicker matrix can be an efficient tool for practical BCI application.