On the delay in processing high spatial frequency visual information: reaction time and VEP latency study of the effect of local intensity of stimulation

Continuous psychophysics shows millisecond-scale visual processing delays are faithfully preserved in movement dynamics

Image differences between the eyes can cause interocular discrepancies in the speed of visual processing. Millisecond-scale differences in visual processing speed can cause dramatic misperceptions of the depth and three-dimensional direction of moving objects. Here, we develop a monocular and binocular continuous target-tracking psychophysics paradigm that can quantify such tiny differences in visual processing speed. Human observers continuously tracked a target undergoing Brownian motion with a range of luminance levels in each eye. Suitable analyses recover the time course of the visuomotor response in each condition, the dependence of visual processing speed on luminance level, and the temporal evolution of processing differences between the eyes. Importantly, using a direct within-observer comparison, we show that continuous target-tracking and traditional forced-choice psychophysical methods provide estimates of interocular delays that agree on average to within a fraction of a millisecond. Thus, visual processing delays are preserved in the movement dynamics of the hand. Finally, we show analytically, and partially confirm experimentally, that differences between the temporal impulse response functions in the two eyes predict how lateral target motion causes misperceptions of motion in depth and associated tracking responses. Because continuous target tracking can accurately recover millisecond-scale differences in visual processing speed and has multiple advantages over traditional psychophysics, it should facilitate the study of temporal processing in the future.

Do monocular myopia children need to wear glasses? Effects of monocular myopia on visual function and binocular balance

Objective

This study aims to compare the binocular visual functions and balance among monocular myopic adolescents and adults and binocular low myopic adolescents and explore whether monocular myopia requires glasses.

Methods

A total of 106 patients participated in this study. All patients were divided into three groups: the monocular myopia children group (Group 1 = 41 patients), the monocular myopia adult group (Group 2 = 26 patients) and the binocular low myopia children group (Group 3 = 39 patients). The refractive parameters, accommodation, stereopsis, and binocular balance were compared.

Results

The binocular refractive difference in Group 1, Group 2, and Group 3 was -1.37 ± 0.93, -1.94 ± 0.91, and -0.32 ± 0.27 D, respectively. Moreover, uncorrected visual acuity (UCVA), spherical equivalent (SE) and monocular accommodative amplitude (AA) between myopic and emmetropic eyes in Group 1 and Group 2 were significantly different (all P < 0.05). There was a significant difference in the accommodative facility (AF) between myopic and emmetropic eyes in Group 2 (t = 2.131, P = 0.043). Furthermore, significant differences were found in monocular AA (t = 6.879, P < 0.001), binocular AA (t = 5.043, P < 0.001) and binocular AF (t = -3.074, P = 0.003) between Group 1 and Group 2. The normal ratio of stereopsis according to the random dots test in Group 1 was higher than in Group 2 (χ2 = 14.596, P < 0.001). The normal ratio of dynamic stereopsis in Group 1 was lower than in Group 3 (χ2 = 13.281, P < 0.001). The normal signal-to-noise ratio of the binocular balance point in Group 1 was lower than Group 3 (χ2 = 4.755, P = 0.029).

Conclusion

First, monocular myopia could lead to accommodative dysfunction and unbalanced input of binocular visual signals, resulting in myopia progression. Second, monocular myopia may also be accompanied by stereopsis dysfunction, and long-term uncorrected monocular myopia may worsen stereopsis acuity in adulthood. In addition, patients with monocular myopia could exhibit stereopsis dysfunction at an early stage. Therefore, children with monocular myopia must wear glasses to restore binocular balance and visual functions, thereby delaying myopia progression.

Perceptual consequences of interocular differences in the duration of temporal integration

Temporal differences in visual information processing between the eyes can cause dramatic misperceptions of motion and depth. Processing delays between the eyes cause the Pulfrich effect: oscillating targets in the frontal plane are misperceived as moving along near-elliptical motion trajectories in depth (Pulfrich, 1922). Here, we explain a previously reported but poorly understood variant: the anomalous Pulfrich effect. When this variant is perceived, the illusory motion trajectory appears oriented left- or right-side back in depth, rather than aligned with the true direction of motion. Our data indicate that this perceived misalignment is due to interocular differences in neural temporal integration periods, as opposed to interocular differences in delay. For oscillating motion, differences in the duration of temporal integration dampen the effective motion amplitude in one eye relative to the other. In a dynamic analog of the Geometric effect in stereo-surface-orientation perception (Ogle, 1950), the different motion amplitudes cause the perceived misorientation of the motion trajectories. Forced-choice psychophysical experiments, conducted with both different spatial frequencies and different onscreen motion damping in the two eyes show that the perceived misorientation in depth is associated with the eye having greater motion damping. A target-tracking experiment provided more direct evidence that the anomalous Pulfrich effect is caused by interocular differences in temporal integration and delay. These findings highlight the computational hurdles posed to the visual system by temporal differences in sensory processing. Future work will explore how the visual system overcomes these challenges to achieve accurate perception.

Delayed Correction for Extrapolation in Amblyopia

Purpose

It has been suggested that amblyopes present impaired motion extrapolation mechanisms. In this study, we used the flash grab effect (FGE), the illusory mislocalization of a briefly flashed stimulus in the direction of a reversing moving background, to investigate whether the amblyopic visual system can correct overextrapolation.

Methods

Thirteen amblyopes and 13 control subjects participated in the experiment. We measured the monocular FGE magnitude for each subject. Two spatial frequency (2 and 8 cycles), two texture configurations (square wave or sine wave), and two speed conditions (270 degrees/s and 67.5 degrees/s) were tested. In addition, control subjects were further tested in reduced luminance conditions.

Results

Compared with controls, amblyopes exhibited a larger FGE magnitude both in their fellow eye (FE) and amblyopic eye (AE). The FGE magnitude of their AE was significantly larger than that of the FE. In a control experiment, we observed that the FGE magnitude increases with the decreasing of the luminance. The FGE magnitude of amblyopes fall into the same range as that of controls under reduced luminance conditions.

Conclusions

We observed a lager FGE in patients with amblyopia, which indicates that the amblyopic visual system does not accurately correct the overextrapolation when a moving object abruptly reverses its direction. This spatiotemporal processing deficit could be ascribed to delayed visual processing in the amblyopic visual system.

Temporal vision: measures, mechanisms and meaning

Time is largely a hidden variable in vision. It is the condition for seeing interesting things such as spatial forms and patterns, colours and movements in the external world, and yet is not meant to be noticed in itself. Temporal aspects of visual processing have received comparatively little attention in research. Temporal properties have been made explicit mainly in measurements of resolution and integration in simple tasks such as detection of spatially homogeneous flicker or light pulses of varying duration. Only through a mechanistic understanding of their basis in retinal photoreceptors and circuits can such measures guide modelling of natural vision in different species and illuminate functional and evolutionary trade-offs. Temporal vision research would benefit from bridging traditions that speak different languages. Towards that goal, I here review studies from the fields of human psychophysics, retinal physiology and neuroethology, with a focus on fundamental constraints set by early vision.

Summary: Simple measures of temporal vision such as the critical flicker frequency can be useful for modelling natural vision only if their relationship to photoreceptor responses and retinal processing is understood.

High-gamma oscillations precede visual steady-state responses: A human electrocorticography study

The robust steady-state cortical activation elicited by flickering visual stimulation has been exploited by a wide range of scientific studies. As the fundamental neural response inherits the spectral properties of the gazed flickering, the paradigm has been used to chart cortical characteristics and their relation to pathologies. However, despite its widespread adoption, the underlying neural mechanisms are not well understood. Here, we show that the fundamental response is preceded by high-gamma (55-125 Hz) oscillations which are also synchronised to the gazed frequency. Using a subdural recording of the primary and associative visual cortices of one human subject, we demonstrate that the latencies of the high-gamma and fundamental components are highly correlated on a single-trial basis albeit that the latter is consistently delayed by approximately 55 ms. These results corroborate previous reports that top-down feedback projections are involved in the generation of the fundamental response, but, in addition, we show that trial-to-trial variability in fundamental latency is paralleled by a highly similar variability in high-gamma latency. Pathology- or paradigm-induced alterations in steady-state responses could thus originate either from deviating visual gamma responses or from aberrations in the neural feedback mechanism. Experiments designed to tease apart the two processes are expected to provide deeper insights into the studied paradigm.

Contact lenses, the reverse Pulfrich effect, and anti-Pulfrich monovision corrections

Interocular differences in image blur can cause processing speed differences that lead to dramatic misperceptions of the distance and three-dimensional direction of moving objects. This recently discovered illusion-the reverse Pulfrich effect-is caused by optical conditions induced by monovision, a common correction for presbyopia. Fortunately, anti-Pulfrich monovision corrections, which darken the blurring lens, can eliminate the illusion for many viewing conditions. However, the reverse Pulfrich effect and the efficacy of anti-Pulfrich corrections have been demonstrated only with trial lenses. This situation should be addressed, for clinical and scientific reasons. First, it is important to replicate these effects with contact lenses, the most common method for delivering monovision. Second, trial lenses of different powers, unlike contacts, can cause large magnification differences between the eyes. To confidently attribute the reverse Pulfrich effect to interocular optical blur differences, and to ensure that previously reported effect sizes are reliable, one must control for magnification. Here, in a within-observer study with five separate experiments, we demonstrate that (1) contact lenses and trial lenses induce indistinguishable reverse Pulfrich effects, (2) anti-Pulfrich corrections are equally effective when induced by contact and trial lenses, and (3) magnification differences do not cause or impact the Pulfrich effect.

Low-contrast Pattern-reversal Visual Evoked Potential in Different Spatial Frequencies

Purpose

To evaluate the pattern-reversal visual evoked potential (PRVEP) in low-contrast, spatial frequencies in time, frequency, and time-frequency domains.

Methods

PRVEP was performed in 31 normal eyes, according to the International Society of Electrophysiology of Vision (ISCEV) protocol. Test stimuli had checkerboard of 5% contrast with spatial frequencies of 1, 2, and 4 cycles per degree (cpd). For each VEP waveform, the time domain (TD) analysis, Fast Fourier Transform(FFT), and discrete wavelet transform (DWT) were performed using MATLAB software. The VEP component changes as a function of spatial frequency (SF) were compared among time, frequency, and time–frequency dimensions.

Results

As a consequence of increased SF, a significant attenuation of the P100 amplitude and prolongation of P100 latency were seen, while there was no significant difference in frequency components. In the wavelet domain, an increase in SF at a contrast level of 5% enhanced DWT coefficients. However, this increase had no meaningful effect on the 7P descriptor.

Conclusion

At a low contrast level of 5%, SF-dependent changes in PRVEP parameters can be better identified with the TD and DWT approaches compared to the Fourier approach. However, specific visual processing may be seen with the wavelet transform.

Visual Perception: Monovision Can Bias the Apparent Depth of Moving Objects

'Monovision' - using one eye for near work and one for distance - is a common alternative to reading glasses. New work shows that monovision can cause the distance of moving objects to be misestimated, with potentially serious consequences.

Interocular Differences in Spatial Frequency Influence the Pulfrich Effect

The Pulfrich effect is a stereo-motion phenomenon. When the two eyes are presented with visual targets moving in fronto-parallel motion at different luminances or contrasts, the perception is of a target moving-in-depth. It is thought that this percept of motion-in-depth occurs because lower luminance or contrast delays the speed of visual processing. Spatial properties of an image such as spatial frequency and size have also been shown to influence the speed of visual processing. In this study, we use a paradigm to measure interocular delay based on the Pulfrich effect where a structure-from-motion defined cylinder, composed of Gabor elements displayed at different interocular phases, rotates in depth. This allows us to measure any relative interocular processing delay while independently manipulating the spatial frequency and size of the micro elements (i.e., Gabor patches). We show that interocular spatial frequency differences, but not interocular size differences of image features, produce interocular processing delays.

Monovision and the Misperception of Motion

Monovision is a common prescription lens correction for presbyopia [1]. Each eye is corrected for a different distance, causing one image to be blurrier than the other. Millions of people have monovision corrections, but little is known about how interocular blur differences affect motion perception. Here, we report that blur differences cause a previously unknown motion illusion that makes people dramatically misperceive the distance and three-dimensional direction of moving objects. The effect occurs because the blurry and sharp images are processed at different speeds. For moving objects, the mismatch in processing speed causes a neural disparity, which results in the misperceptions. A variant of a 100-year-old stereo-motion phenomenon called the Pulfrich effect [2], the illusion poses an apparent paradox: blur reduces contrast, and contrast reductions are known to cause neural processing delays [3-6], but our results indicate that blurry images are processed milliseconds more quickly. We resolve the paradox with known properties of the early visual system, show that the misperceptions can be severe enough to impact public safety, and demonstrate that the misperceptions can be eliminated with novel combinations of non-invasive ophthalmic interventions. The fact that substantial perceptual errors are caused by millisecond differences in processing speed highlights the exquisite temporal calibration required for accurate perceptual estimation. The motion illusion-the reverse Pulfrich effect-and the paradigm we use to measure it should help reveal how optical and image properties impact temporal processing, an important but understudied issue in vision and visual neuroscience.

Towards a state-space geometry of neural responses to natural scenes: A steady-state approach

Our understanding of information processing by the mammalian visual system has come through a variety of techniques ranging from psychophysics and fMRI to single unit recording and EEG. Each technique provides unique insights into the processing framework of the early visual system. Here, we focus on the nature of the information that is carried by steady state visual evoked potentials (SSVEPs). To study the information provided by SSVEPs, we presented human participants with a population of natural scenes and measured the relative SSVEP response. Rather than focus on particular features of this signal, we focused on the full state-space of possible responses and investigated how the evoked responses are mapped onto this space. Our results show that it is possible to map the relatively high-dimensional signal carried by SSVEPs onto a 2-dimensional space with little loss. We also show that a simple biologically plausible model can account for a high proportion of the explainable variance (~73%) in that space. Finally, we describe a technique for measuring the mutual information that is available about images from SSVEPs. The techniques introduced here represent a new approach to understanding the nature of the information carried by SSVEPs. Crucially, this approach is general and can provide a means of comparing results across different neural recording methods. Altogether, our study sheds light on the encoding principles of early vision and provides a much needed reference point for understanding subsequent transformations of the early visual response space to deeper knowledge structures that link different visual environments.

Time course of spatial frequency integration in face perception: An ERP study

Face perception is based on the processing and integration of multiple spatial frequency (SF) ranges. However, the temporal dynamics of SF integration to form an early face representation in the human brain is still a matter of debate. To address this issue, we recorded event-related potentials (ERPs) during the presentation of spatial frequency-manipulated facial images. Twenty-six participants performed a gender discrimination task on non-filtered, low-, high-, and band-pass filtered face images, corresponding, respectively, to the full range, spatial frequencies up to 8 cycles/image, above 32 cycles/image, and from 8 to 16 cycles/image. Behaviorally, the task related-performance was more accurate and faster for non-filtered (NF) and mid-range SF (MSF) than for low SF (LSF) and high SF (HSF) stimuli. At both behavioral and electrophysiological levels, response to MSF contained in faces did not differ from the responses to full spectrum non-filtered (NF) facial images. In ERPs, LSF facial images evoked the largest P1 amplitude while HSF facial images evoked the largest N170 amplitude compared with the other three conditions. Since LSFs and HSFs would transmit global and local information respectively, our observations lend further support to the "coarse-to-fine" processing theory of faces. Furthermore, they offer original evidence of the effectiveness and adequacy of the mid-range spatial frequency in face perception. Possible theoretical interpretations of our findings are discussed.

Effects of Stimulus Size and Contrast on the Initial Primary Visual Cortical Response in Humans

Decades of intracranial electrophysiological investigation into the primary visual cortex (V1) have produced many fundamental insights into the computations carried out in low-level visual circuits of the brain. Some of the most important work has been simply concerned with the precise measurement of neural response variations as a function of elementary stimulus attributes such as contrast and size. Surprisingly, such simple but fundamental characterization of V1 responses has not been carried out in human electrophysiology. Here we report such a detailed characterization for the initial "C1" component of the scalp-recorded visual evoked potential (VEP). The C1 is known to be dominantly generated by initial afferent activation in V1, but is difficult to record reliably due to interindividual anatomical variability. We used pattern-pulse multifocal VEP mapping to identify a stimulus position that activates the left lower calcarine bank in each individual, and afterwards measured robust negative C1s over posterior midline scalp to gratings presented sequentially at that location. We found clear and systematic increases in C1 peak amplitude and decreases in peak latency with increasing size as well as with increasing contrast. With a sample of 15 subjects and ~180 trials per condition, reliable C1 amplitudes of -0.46 µV were evoked at as low a contrast as 3.13% and as large as -4.82 µV at 100% contrast, using stimuli of 3.33° diameter. A practical implication is that by placing sufficiently-sized stimuli to target favorable calcarine cortical loci, robust V1 responses can be measured at contrasts close to perceptual thresholds, which could greatly facilitate principled studies of early visual perception and attention.

The Effect of Spatial-Frequency Filtering on the Visual Processing of Global Structure

In three experiments we measured reaction times (RTs) and error rates in identifying the global structure of spatially filtered stimuli whose spatial-frequency content was selected by means of three types of 2-D isotropic filters (Butterworth of order 2, Butterworth of order 10, and a filters with total or partial Gaussian spectral profile). In each experiment, low-pass (LP), band-pass (BP), and high-pass (HP) filtered stimuli, with nine centre or cut-off spatial frequencies, were used. Irrespective of the type of filter, the experimental results showed that: (a) RTs to stimuli with low spatial frequencies were shorter than those to stimuli with medium or high spatial frequencies, (b) RTs to LP filtered stimuli were nearly constant, but they increased in a non-monotonic way with the filter centre spatial frequency in BP filtered stimuli and with the filter cut-off frequency in HP filtered stimuli, and (c) the identification of the global pattern occurred with all visible stimuli used, including BP and HP images without low spatial frequencies. To remove the possible influence of the energy, a fourth experiment was conducted with Gaussian filtered stimuli of equal contrast power ( crms = 0.065). Similar results to those described above were found for stimuli with spatial-frequency content higher than 2 cycles deg−1. A model of isotropic first-order visual channels collecting the stimulus spectral energy in all orientations explains the RT data. A subsequent second-order nonlinear amplitude demodulation process, applied to the output of the most energetic first-order channel, could explain the perception of global structure of each spatially filtered stimulus, including images lacking low spatial frequencies.

Correlation between electrical and hemodynamic responses during visual stimulation with graded contrasts

Brain functional activity involves complex cellular, metabolic, and vascular chain reactions, making it difficult to comprehend. Electroencephalography (EEG) and functional near infrared spectroscopy (fNIRS) have been combined into a multimodal neuroimaging method that captures both electrophysiological and hemodynamic information to explore the spatiotemporal characteristics of brain activity. Because of the significance of visually evoked functional activity in clinical applications, numerous studies have explored the amplitude of the visual evoked potential (VEP) to clarify its relationship with the hemodynamic response. However, relatively few studies have investigated the influence of latency, which has been frequently used to diagnose visual diseases, on the hemodynamic response. Moreover, because the latency and the amplitude of VEPs have different roles in coding visual information, investigating the relationship between latency and the hemodynamic response should be helpful. In this study, checkerboard reversal tasks with graded contrasts were used to evoke visual functional activity. Both EEG and fNIRS were employed to investigate the relationship between neuronal electrophysiological activities and the hemodynamic responses. The VEP amplitudes were linearly correlated with the hemodynamic response, but the VEP latency showed a negative linear correlation with the hemodynamic response.

"Slight" of hand: the processing of visually degraded gestures with speech

Co-speech hand gestures influence language comprehension. The present experiment explored what part of the visual processing system is optimized for processing these gestures. Participants viewed short video clips of speech and gestures (e.g., a person saying “chop” or “twist” while making a chopping gesture) and had to determine whether the two modalities were congruent or incongruent. Gesture videos were designed to stimulate the parvocellular or magnocellular visual pathways by filtering out low or high spatial frequencies (HSF versus LSF) at two levels of degradation severity (moderate and severe). Participants were less accurate and slower at processing gesture and speech at severe versus moderate levels of degradation. In addition, they were slower for LSF versus HSF stimuli, and this difference was most pronounced in the severely degraded condition. However, exploratory item analyses showed that the HSF advantage was modulated by the range of motion and amount of motion energy in each video. The results suggest that hand gestures exploit a wide range of spatial frequencies, and depending on what frequencies carry the most motion energy, parvocellular or magnocellular visual pathways are maximized to quickly and optimally extract meaning.

Different spatial frequency bands selectively signal for natural image statistics in the early visual system

Early visual evoked potentials (VEPs) measured in humans have recently been observed to be modulated by the image statistics of natural scene imagery. Specifically, the early VEP is dominated by a strong positivity when participants view minimally complex natural scene imagery, with the magnitude of that component being modulated by luminance contrast differences across spatial frequency (i.e., the slope of the amplitude spectrum). For scenes high in structural complexity, the early VEP is dominated by a prominent negativity that exhibits little dependency on luminance contrast. However, since natural scene imagery is broad band in terms of spatial frequency, it is not known whether the above-mentioned modulation results from a complex interaction within or between the early neural processes tuned to different bands of spatial frequency. Here, we sought to address this question by measuring early VEPs (specifically, the C1, P1, and N1 components) while human participants viewed natural scene imagery that was filtered to contain specific bands of spatial frequency information. The results show that the C1 component is largely unmodulated by the luminance statistics of natural scene imagery (being only measurable when such stimuli were made to contain high spatial frequencies). The P1 and N1, on the other hand, were observed to exhibit strong spatial frequency-dependent modulation to the luminance statistics of natural scene imagery. The results therefore suggest that the dependency of early VEPs on natural image statistics results from an interaction between the early neural processes tuned to different bands of spatial frequency.

The effect of mean luminance change and grating pedestals on contrast perception: model simulations suggest a common, retinal, origin

The percept of a contrast target is substantially affected by co-occurring changes in mean luminance or underlying ("pedestal") contrast elements. These two types of modulatory effects have traditionally been studied as separate phenomena. However, regardless of different higher-level mechanisms, both classes of phenomena will necessarily also depend on shared mechanisms in the first stages of vision, starting with the primary responses of photoreceptors. Here we present model simulations showing that important aspects of both classes may be explained by the temporal dynamics of photoreceptor responses read by integrate-and-fire operators. The model is physiologically justified and all its parameters are constrained by experimental evidence. Although there remains plenty of room for additional mechanisms to shape the exact quantitative realization of the perceptual functions in different situations, we suggest that signature features may be inherited from primary retinal signaling.

1/f Noise in human color vision: the role of S-cone signals

We examine the functional role of S-cone signals on reaction time (RT) variability in human color vision. Stimuli were selected along red-green and blue-yellow cardinal directions and at random directions in the isoluminant plane of the color space. Trial-to-trial RT variability was not statistically independent but correlated across experimental conditions and exhibited 1/f noise spectra with an exponent close to unity in most of the cases. Regarding contrast coding, 1/f noise for random chromatic stimuli at isoluminance was similar to that for achromatic stimuli, thus suggesting that S-cone signals reduce variability of higher order color mechanisms. If we regard spatial coding, the effect of S-cone density in the retina on RT variability was investigated. The magnitude of 1/f noise at 16 min of arc (S-cone free zone) was higher than at 90 min of arc in the blue-yellow channel, and it was similar for the red-green channel. The results suggest that S-cone signals are beneficial and they modulate 1/f noise spectra at postreceptoral stages. The implications related to random multiplicative processes as a possible source of 1/f noise and the optimal information processing in color vision are discussed.

Basic abnormalities in visual processing affect face processing at an early age in autism spectrum disorder

BACKGROUND

A detailed visual processing style has been noted in autism spectrum disorder (ASD); this contributes to problems in face processing and has been directly related to abnormal processing of spatial frequencies (SFs). Little is known about the early development of face processing in ASD and the relation with abnormal SF processing. We investigated whether young ASD children show abnormalities in low spatial frequency (LSF, global) and high spatial frequency (HSF, detailed) processing and explored whether these are crucially involved in the early development of face processing.

METHODS

Three- to 4-year-old children with ASD (n = 22) were compared with developmentally delayed children without ASD (n = 17). Spatial frequency processing was studied by recording visual evoked potentials from visual brain areas while children passively viewed gratings (HSF/LSF). In addition, children watched face stimuli with different expressions, filtered to include only HSF or LSF.

RESULTS

Enhanced activity in visual brain areas was found in response to HSF versus LSF information in children with ASD, in contrast to control subjects. Furthermore, facial-expression processing was also primarily driven by detail in ASD.

CONCLUSIONS

Enhanced visual processing of detailed (HSF) information is present early in ASD and occurs for neutral (gratings), as well as for socially relevant stimuli (facial expressions). These data indicate that there is a general abnormality in visual SF processing in early ASD and are in agreement with suggestions that a fast LSF subcortical face processing route might be affected in ASD. This could suggest that abnormal visual processing is causative in the development of social problems in ASD.

Effect of glare on simple reaction time

We systematized the study of the effect of glare on reaction time (RT), for visual conditions similar to the ones found during night driving: Mesopic range of adaptation (0.14 cd/m2), glare levels of the order of those produced by car headlights (E(G)=15, 60 lx), suprathreshold luminance contrasts, and a variety of spatial frequencies covering the selected range of visibility (1, 2, 4, and 8 c/deg). We found that for the no-glare situation, RT increases with decreasing contrast and increasing spatial frequency, which agrees with previous findings. When data are plotted as a function of the inverse of contrast, RT varies linearly, with k--the RT-contrast factor of Pieron's law--representing the slope of the lines. The effect of glare on RT is an increase in the slope of these lines. This effect is different for each spatial frequency, which cannot be accounted for in the classic approach considering that glare can be replaced by a single veiling luminance. We show that the effect of glare on RT must be modeled by an equivalent glare luminance that depends on spatial frequency.

Time course of suppression by surround gratings: highly contrast-dependent, but consistently fast

Timing is critical for the effectiveness of a modulating surround signal. In this study, the optimal timing of a suppressing surround signal was measured psychophysically in human subjects. The perceived contrast of a fixated 1-deg circular patch of vertical sinusoidal grating (the target: 4 cpd, Michelson contrast 0.2) was measured as a function of the onset asynchrony between the target and an annular "surround" grating with the same orientation and spatial frequency. The contrast and area of the surround stimulus were varied parametrically. The suppressive signal peaked at earlier times the higher the surround contrast (0.1-0.4), following a function consistent with the contrast-dependence of retinal response dynamics. Increasing the area of the surround grating also moved peak suppression to earlier times. At ca. 2 deg annulus outer diameter the time to peak of the suppressive signal was shortest, although its amplitude grew with annulus area even beyond that. When both the contrast and the area of the centre and surround gratings were equal, suppression was maximal if the surround stimulus was presented ca. 5 ms before the target. Such a short delay of suppression is consistent with a neural implementation based on feedforward-feedback connections, but not with horizontal connections.

Stimulus intensity affects early sensory processing: Visual contrast modulates evoked gamma-band activity in human EEG

We studied the effect of different contrast levels on the visual evoked gamma-band response (GBR) in order to investigate whether the GBR is modulated in a similar manner as previously reported for visual evoked potentials. Previous studies showed that the GBR can be modulated by individual characteristics (age) and experimental conditions (task difficulty, attention). However, stimulus properties, such as size and spatial frequency, also have a large impact on the GBR, which necessitates identification and control of relevant stimulus properties for optimal experimental setups. Twenty-one healthy participants were investigated during a forced-choice discrimination task. Sinusoidal gratings were presented at three contrast levels with a constant spatial frequency of 5 cycles per degree visual arc (cpd). The present data replicate the results reported for visual evoked potentials and exhibit a contrast dependent modulation of the GBR. Gamma activity is increased for higher contrast levels. These results demonstrate the importance of stimulus contrast for evoked gamma activity. Thus, it appears meaningful to control the contrast of stimuli in experiments investigating the role of gamma activity in perception and information processing.

EEG oscillations in the gamma and alpha range respond differently to spatial frequency

Physical properties of visual stimuli affect electrophysiological markers of perception. One important stimulus property is spatial frequency (SF). Therefore, we studied the influence of SF on human alpha (8-13 Hz) and gamma (>30 Hz) electroencephalographic (EEG) responses in a choice reaction task. Since real world images contain multiple SFs, an SF mixture was also examined. Event related potentials were modulated by SF around 80 and 300 ms. Evoked gamma responses were strongest for the low SF and the mixture stimulus; alpha responses were strongest for high SFs. The results link evoked and induced alpha and evoked gamma responses in human EEG to different modes of stimulus processing.

Two mechanisms underlying the effect of angle of motion direction change on colour-motion asynchrony

Under appropriate stimulus conditions, judgments about the degree of temporal synchrony in sequences containing rapid alternations of colour and motion direction imply a large apparent delay of motion perception relative to colour perception. Whether this colour-motion asynchrony results from the relative processing delay of different visual attributes, or from inappropriate matching of time markers assigned to first-order change of colour and position has been the subject of recent debate. Colour-motion asynchrony is significantly weakened when the angle of direction change is reduced from 180 degrees (direction reversal) to a smaller change in direction. Although this finding has been interpreted to favour the processing delay hypothesis, here we show that it is consistent with the time marker account. First, the reported dependence on the motion direction angle was particularly strong for random-dot stimuli, but our results indicate that this may reflect the introduction of an artefact, motion streaks, that allows subjects to make a colour-orientation synchrony judgement rather than a colour-motion synchrony judgment for direction change angles other than 180 degrees. Second, when we used streak-free plaid stimuli, a certain amount of angle dependence remained regardless of whether we asked the observers to judge the apparent binding or synchrony of colour and motion direction changes. The degree of direction change also affected reaction times, but the effect of apparent asynchrony for a direct comparison of sequences of 90 degrees and 180 degrees motion direction changes was very small, if at all present. These findings with plaid stimuli are consistent with the time marker account; in that we allow that the direction change angle can affect the time course of the recruitment of neural responses to the new direction of motion, which will have a consequential effect on the temporal location of salient features in the sequence of motion changes.

Estimation of the timing of human visual perception from magnetoencephalography

To explore the timing and the underlying neural dynamics of visual perception, we analyzed the relationship between the manual reaction time (RT) to the onset of a visual stimulus and the time course of the evoked neural response simultaneously measured by magnetoencephalography (MEG). The visual stimuli were a transition from incoherent to coherent motion of random dots and an onset of a chromatic grating from a uniform field, which evoke neural responses in different cortical sites. For both stimuli, changes in median RT with changing stimulus strength (motion coherence or chromatic contrast) were accurately predicted, with a stimulus-independent postdetection delay, from the time that the temporally integrated MEG response crossed a threshold (integrator model). In comparison, the prediction of RT was less accurate from the peak MEG latency, or from the time that the nonintegrated MEG response crossed a threshold (level detector model). The integrator model could also account for, at least partially, intertrial changes in RT or in perception (hit/miss) to identical stimuli. Although we examined MEG-RT relationships mainly for data averaged over trials, the integrator model could show some correlations even for single-trial data. The model predictions deteriorated when only early visual responses presumably originating from the striate cortex were used as the input to the integrator model. Our results suggest that the perceptions for visual stimulus appearances are established in extrastriate areas [around MT (middle temporal visual area) for motion and around V4 (fourth visual area) for color] approximately 150-200 ms before subjects manually react to the stimulus.

The relative contributions of luminance contrast and task demands on saccade target selection

Luminance contrast and spatial frequency have a strong effect on when saccades are initiated. In this study, we ask to what extent the internal contrast response determines where saccades are directed to. Observers signalled, with a manual button press, which of two patterns was of higher (Experiment 1) or lower (Experiment 2) contrast. Even though the visual stimuli were identical in both experiments, the pattern of first fixated items was very different. Saccade target selection largely reflected the task instructions, suggesting that luminance contrast can be used to rapidly and effectively guide the eyes to task-relevant information.

Simultaneous EEG and fMRI in the macaque monkey at 4.7 Tesla

Simultaneous electroencephalography (EEG)/functional magnetic resonance imaging (fMRI) acquisition can identify the brain networks involved in generating specific EEG patterns. Yet, the combination of these methodologies is hampered by strong artifacts that arise due to electromagnetic interference during magnetic resonance (MR) image acquisition. Here, we report corrections of the gradient-induced artifact in phantom measurements and in experiments with an awake behaving macaque monkey during fMRI acquisition at a magnetic field strength of 4.7 T. Ninety-one percent of the amplitude of a 10 microV, 10 Hz phantom signal could successfully be recovered without phase distortions. Using this method, we were able to extract the monkey EEG from scalp recordings obtained during MR image acquisition. Visual evoked potentials could also be reliably identified. In conclusion, simultaneous EEG/fMRI acquisition is feasible in the macaque monkey preparation at 4.7 T and holds promise for investigating the neural processes that give rise to particular EEG patterns.

Dynamic properties of orientation discrimination assessed by using classification images

Recent physiological studies indicate that the tuning properties of neurons under acute preparation in primary visual cortex can change over time. We used a psychophysical reverse correlation paradigm to examine the potential repercussions of this neuronal property for human observers' ability to discriminate the orientation of targets over time. Observers were required to identify the orientation of a Gabor target presented within dynamic white noise. Frames from the noise movies were pooled to compute dynamic classification images (CIs) associated with the observers' discrimination performance, which then were fit with a weighted difference-of-Gabor function. Best-fitting templates were temporally bandpass, tuned to more oblique orientations than the stimulus but, crucially, did not change over time. The results suggest that the template for orientation discrimination is selected within the first 50 ms of stimulus onset and that, unlike the response of single cells, there is no measurable dynamic component to either orientation or spatial frequency tuning of human orientation discrimination.

Estimation of temporal resolution of object identification in human vision

The purpose of the study was to estimate the temporal processing capacity of human object identification under different stimulus conditions. Objects, either facial images or characters, were shown in a rapid sequence on a computer display using a rapid serial visual presentation (RSVP) method. One of the images was a target and the other images were distracters. The task of the observer was to identify the target. A staircase algorithm was used to determine the threshold frequency of image presentation in the RSVP sequence. The threshold frequency was determined as a function of image contrast, size, and mean luminance. The results showed that the threshold frequency, around 10 Hz for faces (100 ms per face) and about 25 Hz for characters (40 ms per character), was independent of contrast and size at medium and high contrast values, medium and large sizes, and high luminances, but decreased at very low contrasts or small sizes and medium or low levels of luminance. Computer simulations with a model, in which temporal integration limited perceptual speed, suggest that the experimentally found difference in processing time for faces and characters is not due to the physical differences of these stimulus types, but it seems that face-specific sites in the brain process facial information slower than object-specific areas process character information. Contrast, size, and luminance affect the signal-to-noise ratio and the temporal characteristics of low-level neural signal representation. Thus, the results suggest that at low contrasts, low luminances and small sizes, the processing speed of object identification is limited by low-level factors, while at high contrasts and luminances, and at large sizes, processing speed is limited by high-order processing stages. Processing speed seems to depend on stimulus type so that for faces processing is slower than for characters.

MEG responses correlated with the visual perception of velocity change

Magnetoencephalography (MEG) was used to find neural activities, in the human brain, involved in perception of velocity changes in visual motion. We recorded MEG responses evoked by the stimuli whose velocity increased by 40% or 80% of baseline velocities of 1.0, 2.0, 3.0, and 4.0 deg/s. The velocity increment threshold and the manual reaction time (RT) were also measured under similar stimulus conditions. To manipulate observer's sensitivity to velocity increments, the MEG responses and the psychophysical performances were measured after adaptation to motion in one direction (adapted condition) or alternating directions (control condition). MEG responses evoked by velocity increments peaked at 200-290 ms (M1), and the M1 amplitudes, especially those obtained for 40% increments, were correlated with the sensitivities, which are the reciprocal of velocity increment thresholds. Furthermore, motion adaptation enhanced sensitivity to velocity increments and increased the M1 amplitudes. These results suggest a close correlation between the perceptual velocity increment and the evoked MEG response. In other words, the results suggest that velocity increments are detectable when there is a constant increment in magnetic neural response. As for latencies, nearly constant value of M1 latency did not quantitatively match a large decrease in manual RT with the increase in the baseline velocity. Motion adaptation reduced neither the peak MEG latency nor the manual RT.

Simultaneous recordings of visual evoked potentials and BOLD MRI activations in response to visual motion processing

Visual motion processing in humans was studied by simultaneous 32-channel electroencephalography (EEG) recordings of visual evoked potentials and BOLD MRI activations at 2.9 T. The paradigms compared three different random dot patterns (12 s duration) with stationary random dots (18 s) or with each other. The stimuli represented pattern reversal (500 ms switches between two stationary patterns), motion onset (200 ms of starfield motion followed by 1000 ms of stationary dots) and motion reversal (reversal of moving starfield directions every 1000 ms). Whereas motion-evoked visual potentials, and in particular the N2 component in occipito-temporal channels, were most prominent for motion onset, the most extended BOLD MRI activations and strongest signal changes in V5/MT+ were obtained in response to motion reversal. These apparently contradictory findings most likely reflect different physiological aspects of the neural activity associated with visual motion processing. For example, desynchronized activity of subpopulations of cortical neurons inside V5/MT+ is expected to attenuate visual evoked potentials in scalp recordings while continuously driving metabolic demands that lead to sustained BOLD MRI responses. The understanding of the physiological correlates and neural processes underlying either technique is fundamental to exploring fully the potential of combined EEG-MRI for studying human brain function at both high temporal and spatial resolution.

Temporal summation and reaction times for detecting gratings

The effects of the duration of presentation of stimuli on the reaction time for detection of gratings of different spatial frequencies were studied. The product of contrast and duration (the contrast "energy") was kept constant for each duration. These experiments showed that at near-threshold "energies," reaction times did not change with increases in the duration to 15 and 30 msec at low and high spatial frequencies respectively. At high contrast "energies," this critical duration was identical (15 msec) for both low and high spatial frequencies. The effects of the duration of a linearly increasing front for grating presentation on detection reaction times were also studied. Increases in the duration of the front (to 60 msec) significantly lengthened the reaction time at low spatial frequencies at all contrast levels studied. Conversely, at high spatial frequencies, a significant increase in the reaction time was seen only at high contrast levels. These results suggest that reaction time depends on two types of mechanisms (phasic and tonic) at near-threshold contrast and one type (phasic) at high contrast.

How many pathways determine the speed of grating detection?

Reaction time (RT) for detection of gratings of different spatial frequencies (SF, 0.87-13.87 c/d) and contrast (2-32 times above the detection threshold) was measured. It was found that at low stimulus contrast, 2 and 4 times above the detection threshold, the dependence of mean RT on SF is an "S"-type function with the two plateaus 50-60 ms apart. The standard deviation (SD) increases significantly for SFs within the range 2.60-6.93 c/deg and this coincides with the transition of mean RT from one plateau to another. At higher contrast the dependence of RT on the SF is a monotonically increasing function without any plateau and SD does not demonstrate any extrema within the entire SF-range. The results suggest that RT is determined by two mechanisms (transient and sustained) at low contrast, and by one (transient) mechanism at higher contrasts.

Adaptive temporal integration of motion in direction-selective neurons in macaque visual cortex

Direction-selective neurons in the primary visual cortex (V1) and the extrastriate motion area MT/V5 constitute a critical channel that links early cortical mechanisms of spatiotemporal integration to downstream signals that underlie motion perception. We studied how temporal integration in direction-selective cells depends on speed, spatial frequency (SF), and contrast using randomly moving sinusoidal gratings and spike-triggered average (STA) analysis. The window of temporal integration revealed by the STAs varied substantially with stimulus parameters, extending farther back in time for slow motion, high SF, and low contrast. At low speeds and high SF, STA peaks were larger, indicating that a single spike often conveyed more information about the stimulus under conditions in which the mean firing rate was very low. The observed trends were similar in V1 and MT and offer a physiological correlate for a large body of psychophysical data on temporal integration. We applied the same visual stimuli to a model of motion detection based on oriented linear filters (a motion energy model) that incorporated an integrate-and-fire mechanism and found that it did not account for the neuronal data. Our results show that cortical motion processing in V1 and in MT is highly nonlinear and stimulus dependent. They cast considerable doubt on the ability of simple oriented filter models to account for the output of direction-selective neurons in a general manner. Finally, they suggest that spike rate tuning functions may miss important aspects of the neural coding of motion for stimulus conditions that evoke low firing rates.

The influence of spatial frequency and contrast on saccade latencies

We characterised the impact of spatial frequency and contrast on saccade latencies to single Gabor patches. Saccade latencies decreased as a function of contrast, and increased with spatial frequency. The observed latency variations are qualitatively similar to those observed for manual reaction times. For single target detection, our findings highlight the similarity in the visual processes that support both saccadic and manual responses.

Contrast coding and magno/parvo segregation revealed in reaction time studies

Reaction times (RTs) are obtained for a wide range of contrasts of vertical sinusoidal gratings. The data are plotted as a function of the reciprocal of contrast. In some conditions, a single linear function accounts for the data. In others a clear bi-linear function is obtained. The low and high contrast regions of the function are interpreted as representing magno and parvo activity, respectively. RT-based supra-threshold sensitivity functions are obtained for different luminances, stimulus durations and eccentricities and these are compared with conventional threshold-based sensitivities to establish the extent to which RTs and contrast sensitivity are constrained by the same sensory processes.