Contrast dependency of VEPs as a function of spatial frequency: the parvocellular and magnocellular contributions to human VEPs

Fixation-related potentials during a virtual navigation task: The influence of image statistics on early cortical processing

Historically, electrophysiological correlates of scene processing have been studied with experiments using static stimuli presented for discrete timescales where participants maintain a fixed eye position. Gaps remain in generalizing these findings to real-world conditions where eye movements are made to select new visual information and where the environment remains stable but changes with our position and orientation in space, driving dynamic visual stimulation. Co-recording of eye movements and electroencephalography (EEG) is an approach to leverage fixations as time-locking events in the EEG recording under free-viewing conditions to create fixation-related potentials (FRPs), providing a neural snapshot in which to study visual processing under naturalistic conditions. The current experiment aimed to explore the influence of low-level image statistics-specifically, luminance and a metric of spatial frequency (slope of the amplitude spectrum)-on the early visual components evoked from fixation onsets in a free-viewing visual search and navigation task using a virtual environment. This research combines FRPs with an optimized approach to remove ocular artifacts and deconvolution modeling to correct for overlapping neural activity inherent in any free-viewing paradigm. The results suggest that early visual components-namely, the lambda response and N1-of the FRPs are sensitive to luminance and spatial frequency around fixation, separate from modulation due to underlying differences in eye-movement characteristics. Together, our results demonstrate the utility of studying the influence of image statistics on FRPs using a deconvolution modeling approach to control for overlapping neural activity and oculomotor covariates.

Modulation of the neuronal response in human primary visual cortex by re-entrant projections during retinal input processing as manifest in the visual evoked potential

Initial deflections in the visual evoked potential (VEP) reflect the neuronal process of extracting features from the retinal input; a process not modulated by re-entrant projections. Later deflections in the VEP reflect the neuronal process of combining features into an object, a process referred to as 'object closure' and modulated by re-entrant projections. Our earlier work indicated that the VEP reflects independent neuronal responses processing temporal - and spatial luminance contrast and that these responses arise from an interaction between forward and re-entrant input. In this earlier work, changing the temporal luminance contrast property of a stimulus altered its spatial luminance contrast property. We recorded the VEP in 12 volunteers viewing image pairs of a windmill, regular dartboard or an RMS dartboard rotated by either Π/4, Π/2, 3Π/4 or Π radians with respect to each other. The windmill and regular dartboard had identical white to black ratio, while the two dartboards identical contrast edges per unit area. Rotation varied temporal luminance contrast of a stimulus without affecting its spatial luminance contrast. N75, P100, N135 and P240 amplitude and latency were compared and a source localisation and temporal frequency analysis performed. P100 amplitude signals a neuronal response processing temporal luminance contrast that is modulated by re-entrant projections with fast axonal conduction velocities. N135 and P240 signal the neuronal response processing spatial luminance contrast and is modulated by re-entrant projections with slow axonal conduction velocities. The dorsal stream is interconnected by fast axonal conduction velocities, the ventral stream by slow axonal conduction velocities.

Electrophysiological signatures of visual temporal processing deficits in developmental dyslexia

Developmental dyslexia (DD) is a common neurodevelopmental disorder that affects reading ability despite normal intelligence and education. In search of core deficits, previous evidence has linked DD with impairments in temporal aspects of perceptual processing, which might underlie phonological deficits as well as inefficient graphemic parsing during reading. However, electrophysiological evidence for atypical temporal processing in DD is still scarce in the visual modality. Here, we investigated the efficiency of both temporal segregation and integration of visual information by means of event-related potentials (ERPs). We confirmed previous evidence of a selective segregation deficit in dyslexia for stimuli presented in rapid succession (<80 ms), despite unaffected integration performance. Importantly, we found a reduced N1 amplitude in DD, a component related to the allocation of attentional resources, which was independent of task demands (i.e., evident in both segregation and integration). In addition, the P3 amplitude, linked to working memory and processing load, was modulated by task demands in controls but not in individuals with DD. These results suggest that atypical attentional sampling in dyslexia might weaken the quality of information stored in visual working memory, leading to behavioral and electrophysiological signatures of atypical temporal segregation. These results are consistent with some existing theories of dyslexia, such as the magnocellular theory and the "Sluggish Attentional Shifting" framework, and represent novel evidence for neural correlates of decreased visual temporal resolution in DD.

Visual Neurophysiological Biomarkers for Patient Stratification and Treatment Development Across Neuropsychiatric Disorders

The human visual system begins in the retina and projects to cortex through both the thalamocortical and retinotectal visual pathways. The thalamocortical system is divided into separate magnocellular and parvocellular divisions, which engage separate layers of the lateral geniculate nucleus (LGN) and project preferentially to the dorsal and ventral visual streams, respectively. The retinotectal system, in contrast, projects to the superior colliculus, pulvinar nucleus of the thalamus and amygdala. The pulvinar nucleus also plays a critical role in the integration of information processing across early visual regions.The functions of the visual system can be assessed using convergent EEG- and functional brain imaging approaches, increasingly supplemented by simultaneously collected eye-tracking information. These approaches may be used for tracing the flow of information from retina through early visual regions, as well as the contribution of these regions to higher-order cognitive processing. A pathway of increasing interest in relationship to neuropsychiatric disorders is the primate-specific "third visual pathway" that relies extensively on motion-related input and contributes preferentially to social information processing. Thus, disturbances in the brain's responsiveness to motion stimuli may be especially useful as biomarkers for early visual dysfunction related to impaired social cognition.Visual event-related potentials (ERPs) can be collected with high-fidelity and have proven effective for the study of neuropsychiatric disorders such as schizophrenia and Alzheimer's disease, in which alterations in visual processing may occur early in the disorder, andautism-spectrum disorder (ASD), in which abnormal persistence of early childhood patterns may persist into adulthood, leading to impaired functioning of visual social pathways. The utility of visual ERPs as biomarkers for larger clinical studies is limited at present by the need for standardization of visual stimuli across laboratories, which requires specialized protocols and equipment. The development of optimized stimulation protocols as well as newer headset-based systems may increase the clinical utility of present stimulation approaches.

Exploring sensory sensitivity, cortical excitability, and habituation in episodic migraine, as a function of age and disease severity, using the pattern-reversal task

BACKGROUND

Migraine is a cyclic, neurosensory disorder characterized by recurrent headaches and altered sensory processing. The latter is manifested in hypersensitivity to visual stimuli, measured with questionnaires and sensory thresholds, as well as in abnormal cortical excitability and a lack of habituation, assessed with visual evoked potentials elicited by pattern-reversal stimulation. Here, the goal was to determine whether factors such as age and/or disease severity may exert a modulatory influence on sensory sensitivity, cortical excitability, and habituation.

METHODS

Two similar experiments were carried out, the first comparing 24 young, episodic migraine patients and 28 healthy age- and gender-matched controls and the second 36 middle-aged, episodic migraine patients and 30 healthy age- and gender-matched controls. A neurologist confirmed the diagnoses. Migraine phases were obtained using eDiaries. Sensory sensitivity was assessed with the Sensory Perception Quotient and group comparisons were carried out. We obtained pattern-reversal visual evoked potentials and calculated the N1-P1 Peak-to-Peak amplitude. Two linear mixed-effects models were fitted to these data. The first model had Block (first block, last block) and Group (patients, controls) as fixed factors, whereas the second model had Trial (all trials) and Group as fixed factors. Participant was included as a random factor in both. N1-P1 first block amplitude was used to assess cortical excitability and habituation was defined as a decrease of N1-P1 amplitude across Blocks/Trials. Both experiments were performed interictally.

RESULTS

The final samples consisted of 18 patients with episodic migraine and 27 headache-free controls (first experiment) and 19 patients and 29 controls (second experiment). In both experiments, patients reported increased visual hypersensitivity on the Sensory Perception Quotient as compared to controls. Regarding N1-P1 peak-to-peak data, there was no main effect of Group, indicating no differences in cortical excitability between groups. Finally, significant main effects of both Block and Trial were found indicating habituation in both groups, regardless of age and headache frequency.

CONCLUSIONS

The results of this study yielded evidence for significant hypersensitivity in patients but no significant differences in either habituation or cortical excitability, as compared to headache-free controls. Although the alterations in patients may be less pronounced than originally anticipated they demonstrate the need for the definition and standardization of optimal methodological parameters.

Decoding of EEG signals reveals non-uniformities in the neural geometry of colour

The idea of colour opponency maintains that colour vision arises through the comparison of two chromatic mechanisms, red versus green and yellow versus blue. The four unique hues, red, green, blue, and yellow, are assumed to appear at the null points of these the two chromatic systems. Here we hypothesise that, if unique hues represent a tractable cortical state, they should elicit more robust activity compared to other, non-unique hues. We use a spatiotemporal decoding approach to report that electroencephalographic (EEG) responses carry robust information about the tested isoluminant unique hues within a 100-350 ms window from stimulus onset. Decoding is possible in both passive and active viewing tasks, but is compromised when concurrent high luminance contrast is added to the colour signals. For large hue-differences, the efficiency of hue decoding can be predicted by mutual distance in a nominally uniform perceptual colour space. However, for small perceptual neighbourhoods around unique hues, the encoding space shows pivotal non-uniformities which suggest that anisotropies in neurometric hue-spaces may reflect perceptual unique hues.

The shift in sensory eye dominance from short-term monocular deprivation exhibits no dependence on test spatial frequency

Background

Studies have shown that short-term monocular deprivation induces a shift in sensory eye dominance in favor of the deprived eye. Yet, how short-term monocular deprivation modulates sensory eye dominance across spatial frequency is not clear. To address this issue, we conducted a study to investigate the dependence of short-term monocular deprivation effect on test spatial frequency.

Methods

Ten healthy young adults (age: 24.7 ± 1.7 years, four males) with normal vision participated. We deprived their dominant eye with a translucent patch for 2.5 h. The interocular contrast ratio (dominant eye/non-dominant eye, i.e., the balance point [BP]), which indicates the contribution that the two eyes make to binocular combination, was measured using a binocular orientation combination task. We assessed if BPs at 0.5, 4 or 6 cycles/degree (c/d) change as a result of monocular deprivation. Different test spatial frequency conditions were conducted on three separate days in a random fashion.

Results

We compared the BPs at 0.5, 4 and 6 c/d before and after monocular deprivation. The BPs were found to be significantly affected by deprivation, where sensory eye dominance shift to the deprived eye (F_{1.86, 16.76} = 33.09, P < 0.001). The changes of BP were consistent at 0.5, 4, and 6 c/d spatial frequencies (F_2,18 = 0.15, P = 0.57).

Conclusion

The sensory eye dominance plasticity induced by short-term deprivation is not dependent on test spatial frequency, suggesting it could provide a practical solution for amblyopic therapy that was concerned with the binocular outcome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40662-022-00303-4.

Luminance Contrast Drives Interactions between Perception and Working Memory

Visual working memory (WM) enables the use of past sensory experience in guiding behavior. Yet, laboratory tasks commonly evaluate WM in a way that separates it from its sensory bottleneck. To understand how perception interacts with visual memory, we used a delayed shape recognition task to probe how WM may differ for stimuli that bias processing toward different visual pathways. Luminance compared with chromatic signals are more efficient in driving the processing of shapes and may thus also lead to better WM encoding, maintenance, and memory recognition. To evaluate this prediction, we conducted two experiments. In the first psychophysical experiment, we measured contrast thresholds for different WM loads. Luminance contrast was encoded into WM more efficiently than chromatic contrast, even when both sets of stimuli were equated for discriminability. In the second experiment, which also equated stimuli for discriminability, early sensory responses in the EEG that are specific to luminance pathways were modulated by WM load and thus likely reflect the neural substrate of the increased efficiency. Our results cannot be accounted for by simple saliency differences between luminance and color. Rather, they provide evidence for a direct connection between low-level perceptual mechanisms and WM by showing a crucial role of luminance for forming WM representations of shape.

The Predictive Role of Low Spatial Frequencies in Automatic Face Processing: A Visual Mismatch Negativity Investigation

Visual processing is thought to function in a coarse-to-fine manner. Low spatial frequencies (LSF), conveying coarse information, would be processed early to generate predictions. These LSF-based predictions would facilitate the further integration of high spatial frequencies (HSF), conveying fine details. The predictive role of LSF might be crucial in automatic face processing, where high performance could be explained by an accurate selection of clues in early processing. In the present study, we used a visual Mismatch Negativity (vMMN) paradigm by presenting an unfiltered face as standard stimulus, and the same face filtered in LSF or HSF as deviant, to investigate the predictive role of LSF vs. HSF during automatic face processing. If LSF are critical for predictions, we hypothesize that LSF deviants would elicit less prediction error (i.e., reduced mismatch responses) than HSF deviants. Results show that both LSF and HSF deviants elicited a mismatch response compared with their equivalent in an equiprobable sequence. However, in line with our hypothesis, LSF deviants evoke significantly reduced mismatch responses compared to HSF deviants, particularly at later stages. The difference in mismatch between HSF and LSF conditions involves posterior areas and right fusiform gyrus. Overall, our findings suggest a predictive role of LSF during automatic face processing and a critical involvement of HSF in the fusiform during the conscious detection of changes in faces.

Spatial frequency impacts perceptual and attentional ERP components across cultures

Culture impacts visual perception in several ways.To identify stages of perceptual processing that differ between cultures, we usedelectroencephalography measures of perceptual and attentional responses to simple visual stimuli.Gabor patches of higher or lower spatialfrequencywere presented at high contrast to 25 American and 31 East Asian participants while they were watching for the onset of aninfrequent, oddball stimulus. Region of interest and mass univariate analyses assessed how cultural background and stimuli spatial frequency affected the visual evoked response potentials. Across both groups, the Gabor of lower spatial frequency produced stronger evoked response potentials in the anterior N1 and P3 than did the higher frequency Gabor. The mass univariate analyses also revealed effects of spatial frequency, including a frontal negativity around 150 ms and a widespread posterior positivity around 300 ms. The effects of spatial frequency generally differed little across cultures; although there was some evidence for cultural differences in the P3 response to different frequencies at the Pz electrode, this effect did not emerge in the mass univariate analyses. We discuss these results in relation to those from previous studies, and explore the potential advantages of mass univariate analyses for cultural neuroscience.

Human Sensory Cortex Contributes to the Long-Term Storage of Aversive Conditioning

Growing animal data evince a critical role of the sensory cortex in the long-term storage of aversive conditioning, following acquisition and consolidation in the amygdala. Whether and how this function is conserved in the human sensory cortex is nonetheless unclear. We interrogated this question in a human aversive conditioning study using multidimensional assessments of conditioning and long-term (15 d) retention. Conditioned stimuli (CSs; Gabor patches) were calibrated to differentially activate the parvocellular (P) and magnocellular (M) visual pathways, further elucidating cortical versus subcortical mechanisms. Full-blown conditioning and long-term retention emerged for M-biased CS (vs limited effects for P-biased CS), especially among anxious individuals, in all four dimensions assessed: threat appraisal (threat ratings), physiological arousal (skin conductance response), perceptual learning [discrimination sensitivity (d') and response speed], and cortical plasticity [visual evoked potentials (VEPs) and cortical current density]. Interestingly, while behavioral, physiological, and VEP effects were comparable at immediate and delayed assessments, the cortical substrates evolved markedly over time, transferring from high-order cortices [inferotemporal/fusiform cortex and orbitofrontal cortex (OFC)] immediately to the primary and secondary visual cortex after the delay. In sum, the contrast between P- and M-biased conditioning confirms privileged conditioning acquisition via the subcortical pathway while the immediate cortical plasticity lends credence to the triadic amygdala-OFC-fusiform network thought to underlie threat processing. Importantly, long-term retention of conditioning in the basic sensory cortices supports the conserved role of the human sensory cortex in the long-term storage of aversive conditioning.SIGNIFICANCE STATEMENT A growing network of neural substrates has been identified in threat learning and memory. The sensory cortex plays a key role in long-term threat memory in animals, but such a function in humans remains unclear. To explore this problem, we conducted multidimensional assessments of immediate and delayed (15 d) effects of human aversive conditioning. Behavioral, physiological, and scalp electrophysiological data demonstrated conditioning effects and long-term retention. High-density EEG intracranial source analysis further revealed the cortical underpinnings, implicating high-order cortices immediately and primary and secondary visual cortices after the long delay. Therefore, while high-order cortices support aversive conditioning acquisition (i.e., threat learning), the human sensory cortex (akin to the animal homolog) underpins long-term storage of conditioning (i.e., long-term threat memory).

An investigation into the relationship between stimulus property, neural response and its manifestation in the visual evoked potential involving retinal resolution

The visual evoked potential (VEP) has been shown to reflect the size of the neural population activated by a processing mechanism selective to the temporal - and spatial luminance contrast property of a stimulus. We set out to better understand how the factors determining the neural response associated with these mechanisms. To do so we recorded the VEP from 14 healthy volunteers viewing two series of pattern reversing stimuli with identical temporal-and spatial luminance contrast properties. In one series the size of the elements increased towards the edge of the image, in the other it decreased. In the former element size was congruent with receptive field size across eccentricity, in the later it was incongruent. P100 amplitude to the incongruent series exceeded that obtained to the congruent series. Using electric dipoles due the excitatory neural response we accounted for this using dipole cancellation of electric dipoles of opposite polarity originating in supra- and infragranular layers of V1. The phasic neural response in granular lamina of V1 exhibited magnocellular characteristics, the neural response outside of the granular lamina exhibited parvocellular characteristics and was modulated by re-entrant projections. Using electric current density, we identified areas of the dorsal followed by areas of the ventral stream as the source of the re-entrant signal modulating infragranular activity. Our work demonstrates that the VEP does not signal reflect the overall level of a neural response but is the result of an interaction between electric dipoles originating from neural responses in different lamina of V1.

Reduced Visual Magnocellular Event-Related Potentials in Developmental Dyslexia

(1) Background—the magnocellular hypothesis proposes that impaired development of the visual timing systems in the brain that are mediated by magnocellular (M-) neurons is a major cause of dyslexia. Their function can now be assessed quite easily by analysing averaged visually evoked event-related potentials (VERPs) in the electroencephalogram (EEG). Such analysis might provide a useful, objective biomarker for diagnosing developmental dyslexia. (2) Methods—in adult dyslexics and normally reading controls, we recorded steady state VERPs, and their frequency content was computed using the fast Fourier transform. The visual stimulus was a black and white checker board whose checks reversed contrast every 100 ms. M- cells respond to this stimulus mainly at 10 Hz, whereas parvocells (P-) do so at 5 Hz. Left and right visual hemifields were stimulated separately in some subjects to see if there were latency differences between the M- inputs to the right vs. left hemispheres, and these were compared with the subjects’ handedness. (3) Results—Controls demonstrated a larger 10 Hz than 5 Hz fundamental peak in the spectra, whereas the dyslexics showed the reverse pattern. The ratio of subjects’ 10/5 Hz amplitudes predicted their reading ability. The latency of the 10 Hz peak was shorter during left than during right hemifield stimulation, and shorter in controls than in dyslexics. The latter correlated weakly with their handedness. (4) Conclusion—Steady state visual ERPs may conveniently be used to identify developmental dyslexia. However, due to the limited numbers of subjects in each sub-study, these results need confirmation.

The Dominant Eye: Dominant for Parvo- But Not for Magno-Biased Stimuli?

Eye dominance is often defined as a preference for the visual input of one eye to the other. Implicit in this definition is the dominant eye has better visual function. Several studies have investigated the effect of visual direction or defocus on ocular dominance, but there is less evidence connecting ocular dominance and monocular visual thresholds. We used the classic “hole in card” method to determine the dominant eye for 28 adult observers (11 males and 17 females). We then compared contrast thresholds between the dominant and non-dominant eyes using grating stimuli biased to be processed more strongly either by the magnocellular (MC) or parvocellular (PC) pathway. Using non-parametric mean rank tests, the dominant eye was more sensitive overall than the non-dominant eye to both stimuli (z = −2.54, p = 0.01). The dominant eye was also more sensitive to the PC-biased stimulus (z = −2.22, p = 0.03) but not the MC-biased stimulus (z = −1.16, p = 0.25). We discuss the clinical relevance of these results as well as the implications for parallel visual pathways.

Differences in early and late pattern-onset visual-evoked potentials between self- reported migraineurs and controls

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Migraineurs had an enhanced N2 evoked by gratings with a spatial frequency of 13 cpd.

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Migraineurs had an attenuated occipital late negativity (LN) for viewing all gratings.

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Hyperexcitable controls showed similar VEP pattern compared to migraineurs.

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Enhanced N2 deflection could be driven by cortical hyperexcitation.

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LN reduction could reflect inhibitory control during processing of aversive stimuli.

Striped patterns have been shown to induce strong visual illusions and discomforts to migraineurs in previous literature. Previous research has suggested that these unusual visual symptoms to be linked with the hyperactivity on the visual cortex of migraine sufferers. The present study searched for evidence supporting this hypothesis by comparing the visual evoked potentials (VEPs) elicited by striped patterns of specific spatial frequencies (0.5, 3, and 13 cycles-per-degree) between a group of 29 migraineurs (17 with aura/12 without) and 31 non-migraineurs. In addition, VEPs to the same stripped patterns were compared between non-migraineurs who were classified as hyperexcitable versus non-hyperexcitable using a previously established behavioural pattern glare task. We found that the migraineurs had a significantly increased N2 amplitude for stimuli with 13 cpd gratings but an attenuated late negativity (LN: 400 – 500 ms after the stimuli onset) for all the spatial frequencies. Interestingly, non-migraineurs who scored as hyperexcitable appeared to have similar response patterns to the migraineurs, albeit in an attenuated form. We propose that the enhanced N2 could reflect disruption of the balance between parvocellular and magnocellular pathway, which is in support of the cortical hyperexcitation hypothesis in migraineurs. In addition, the attenuation of the late negativity could reflect a top-down feedback mechanism to suppress visual processing of an aversive stimulus.

Evidence for intact stimulus-specific neural adaptation for visual objects in schizophrenia and bipolar disorder: An ERP study

People with schizophrenia (SZ) or bipolar disorder (BD) experience dysfunction in visual processing. Dysfunctional neural tuning, in which neurons and neuronal populations are selectively activated by specific features of visual stimuli, may contribute to these deficits. Few studies have examined this possibility and there are inconsistent findings of tuning deficits in the literature. We utilized an event-related potential (ERP) paradigm to examine neural adaptation for visual objects, a measure of neural tuning whereby neurons respond less strongly to the repeated presentation of the same stimulus. Seventy-seven SZ, 53 BD, and 49 healthy comparison participants (HC) were examined. In three separate conditions, pictures of objects were presented repeatedly: the same object (SS), different objects from the same category (e.g., two different vases; SD), or different objects from different categories (e.g., a barrel and a clock, DD). Mass-univariate cluster-based permutation analyses identified electrodes and time-windows in which there were significant differences between the SS vs. DD and the SD vs. DD conditions. Mean ERP amplitudes were extracted from these clusters and analyzed for group differences. Results revealed a significant condition difference over parieto-occipital electrodes for the SS-DD comparison between 109–164 ms and for the SD-DD comparison between 78–203 ms, with larger amplitudes in the DD compared to either SS or SD condition. However, there were no significant differences in the pattern of results between groups. Thus, while we found neural adaptation effects using this ERP paradigm, we did not find evidence of group differences. Our results suggest that people with SZ or BD may not exhibit deficits in neural tuning for processing of visual objects using this EEG task with rapidly presented stimuli. However, the results are inconsistent with other studies using different methodologies (e.g., fMRI, behavioral tasks) that have found tuning deficits in people with schizophrenia.

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.

Contrast and spatial frequency modulation for diagnosis of amblyopia: An electrophysiological approach

Purpose

To evaluate the diagnostic value of visual evoked potentials (VEPs) and to find out which test setting has the most sensitivity and specificity for amblyopia diagnosis.

Methods

Thirty-three adult anisometropic amblyopes were intended in this study and were tested for visual evoked potentials with different stimulus conditions including three spatial frequencies [1, 2, and 4-cycles-per-degree (cpd)] at four contrast levels (100, 50, 25, and 5%). We also tested psychophysical contrast sensitivity and compared the results with electrophysiological ones. We plotted Receiver Operating Characteristic (ROC) curve for each VEP recording and psychophysical contrast sensitivity to evaluate the area under the curve, sensitivity, specificity, and cut-point value of each test stimulus for detecting amblyopic eyes.

Results

Thirty-three amblyopic and 33 non-amblyopic eyes were examined for psychophysical contrast sensitivity and VEPs. Area under the ROC curve (AURC) findings showed that VEP with different stimulus settings can significantly detect amblyopic eyes, as well as psychophysical contrast sensitivity test. We found that P100 amplitudes had the largest AURC in response to stimuli of 2-cpd spatial frequency at 50 (P < 0.001) and 25% (P < 0.001) contrast levels, respectively. Cut-off amplitudes for these stimuli were 8.65 and 4.50 μV, which had a sensitivity of 0.758 and 0.697 and a specificity of 0.788 and 0.848, respectively. The sensitivity and specificity of VEP P100 amplitude in response to the stimuli with 2 cpd spatial frequency and 50 and 25% contrast were greater than the findings obtained from psychophysical contrast sensitivity test.

Conclusion

According to our findings, assessment of VEP amplitudes in response to stimuli of 2-cpd spatial frequency at 50 and 25% contrast levels can best detect amblyopia with highest sensitivity and specificity and thus, are the protocols of choice for detection of amblyopic eyes.

Tracking changes in spatial frequency sensitivity during natural image processing in school age: an event-related potential study

Several studies have shown that behavioral and electrophysiological correlates of processing visual images containing low or high spatial frequency (LSF or HSF) information undergo development after early childhood. However, the maturation of spatial frequency sensitivity during school age has been investigated using abstract stimuli only. The aim of the current study was to assess how LSF and HSF features affect the processing of everyday photographs at the behavioral and electrophysiological levels in children aged 7-15 years and adults. We presented grayscale images containing either animals or vehicles and their luminance-matched modified versions filtered at low or high spatial frequencies. Modulations of classification accuracy, reaction time, and visual event-related potentials (posterior P1 and N1 components) were compared across five developmental groups and three image types. We found disproportionately worse response accuracies for LSF stimuli relative to HSF images in children aged 7 or 8 years, an effect that was accompanied by smaller LSF-evoked P1 amplitudes during this age period. At 7 or 8 years of age, P1 and N1 amplitudes were modulated by HSF and LSF stimuli (P1: HSF > LSF; N1: LSF > HSF), with a gradual shift toward the opposite pattern (P1: LSF > HSF; N1: HSF > LSF) with increasing age. Our results indicate that early cortical processing of both spatial frequency ranges undergo substantial development during school age, with a relative delay of LSF analysis, and underline the utility of our paradigm in tracking the maturation of LSF versus HSF sensitivity in this age group.

Magnocellular Bias in Exogenous Attention to Biologically Salient Stimuli as Revealed by Manipulating Their Luminosity and Color

Exogenous attention is a set of mechanisms that allow us to detect and reorient toward salient events—such as appetitive or aversive—that appear out of the current focus of attention. The nature of these mechanisms, particularly the involvement of the parvocellular and magnocellular visual processing systems, was explored. Thirty-four participants performed a demanding digit categorization task while salient (spiders or S) and neutral (wheels or W) stimuli were presented as distractors under two figure–ground formats: heterochromatic/isoluminant (exclusively processed by the parvocellular system, Par trials) and isochromatic/heteroluminant (preferentially processed by the magnocellular system, Mag trials). This resulted in four conditions: SPar, SMag, WPar, and WMag. Behavioral (RTs and error rates in the task) and electrophysiological (ERPs) indices of exogenous attention were analyzed. Behavior showed greater attentional capture by SMag than by SPar distractors and enhanced modulation of SMag capture as fear of spiders reported by participants increased. ERPs reflected a sequence from magnocellular dominant (P1p, ≃120 msec) to both magnocellular and parvocellular processing (N2p and P2a, ≃200 msec). Importantly, amplitudes in one N2p subcomponent were greater to SMag than to SPar and WMag distractors, indicating greater magnocellular sensitivity to saliency. Taking together, results support a magnocellular bias in exogenous attention toward distractors of any nature during initial processing, a bias that remains in later stages when biologically salient distractors are present.

Electrophysiological correlates of top-down effects facilitating natural image categorization are disrupted by the attenuation of low spatial frequency information

The modulatory effects of low and high spatial frequencies on the posterior C1, P1 and N1 event-related potential (ERP) amplitudes have long been known from previous electrophysiological studies. There is also evidence that categorization of complex natural images relies on top-down processes, probably by facilitating contextual associations during the recognition process. However, to our knowledge, no study has investigated so far how such top-down effects are manifested in scalp ERPs, when presenting natural images with attenuated low or high spatial frequency information. Twenty-one healthy subjects participated in an animal vs. vehicle categorization task with intact grayscale stimuli and images predominantly containing high (HSF) or low spatial frequencies (LSF). ERP scalp maps and amplitudes/latencies measured above occipital, parietal and frontocentral sites were compared among the three stimulus conditions. Although early occipital components (C1 and P1) were modulated by spatial frequencies, the time range of the N1 was the earliest to show top-down effects for images with unmodified low spatial frequency spectrum (intact and LSF stimuli). This manifested in ERP amplitude changes spreading to anterior scalp sites and shorter posterior N1 latencies. Finally, the frontocentral N350 and the centroparietal LPC were differently influenced by spatial frequency filtering, with the LPC being the only component to show an amplitude and latency modulation congruent with the behavioral responses (sensitivity index and reaction times). Our results strengthen the coarse-to-fine model of object recognition and provide electrophysiological evidence for low spatial frequency-based top-down effects within the first 200 ms of visual processing.

An Event-related Potential Study on the Interaction between Lighting Level and Stimulus Spatial Location

Due to heterogeneous photoreceptor distribution, spatial location of stimulation is crucial to study visual brain activity in different light environments. This unexplored issue was studied through occipital event-related potentials (ERPs) recorded from 40 participants in response to discrete visual stimuli presented at different locations and in two environmental light conditions, low mesopic (L, 0.03 lux) and high mesopic (H, 6.5 lux), characterized by a differential photoreceptor activity balance: rod > cone and rod < cone, respectively. Stimuli, which were exactly the same in L and H, consisted of squares presented at fixation, at the vertical periphery (above or below fixation) or at the horizontal periphery (left or right). Analyses showed that occipital ERPs presented important L vs. H differences in the 100 to 450 ms window, which were significantly modulated by spatial location of stimulation: differences were greater in response to peripheral stimuli than to stimuli presented at fixation. Moreover, in the former case, significance of L vs. H differences was even stronger in response to stimuli presented at the horizontal than at the vertical periphery. These low vs. high mesopic differences may be explained by photoreceptor activation and their retinal distribution, and confirm that ERPs discriminate between rod- and cone-originated visual processing.

Developmental changes in ERP responses to spatial frequencies

Social interaction starts with perception of other persons. One of the first steps in perception is processing of basic information such as spatial frequencies (SF), which represent details and global information. However, although behavioural perception of SF is well investigated, the developmental trajectory of the temporal characteristics of SF processing is not yet well understood. The speed of processing of this basic visual information is crucial, as it determines the speed and possibly accuracy of subsequent visual and social processes. The current study investigated developmental changes in the temporal characteristics of selective processing of high SF (HSF; details) versus low SF (LSF; global). To this end, brain activity was measured using EEG in 108 children aged 3–15 years, while HSF or LSF grating stimuli were presented. Interest was in the temporal characteristics of brain activity related to LSF and HSF processing, specifically at early (N80) or later (P1 or N2) peaks in brain activity. Analyses revealed that from 7–8 years onwards HSF but not LSF stimuli evoked an N80 peak. In younger children, aged 3–8 years, the visual manipulation mainly affected the visual N2 peak. Selective processing of HSF versus LSF thus occurs at a rather late time-point (N2 peak) in young children. Although behavioural research previously showed that 3–6 year-olds can perceive detailed information, the current results point out that selective processing of HSF versus LSF is still delayed in these children. The delayed processing in younger children could impede the use of LSF and HSF for emotional face processing. Thus, the current study is a starting point for understanding changes in basic visual processing which underlie social development.

Miscoded Visual Processing in Degenerative Retinal Disorder?

Standard electrophysiological procedures for visual testing were applied to record the retinal and cortical electrophysiological responses to contrast stimulation from 35 subjects with unambiguously diagnosed retinitis pigmentosa and severe impairment of visual acuity and field. Stimuli (central 9° of visual field) were sinusoidal bars with spatial frequencies of 0.6–1.2 cycle/degree and 1.3–5.0 cycle/degree for the retinal (pattern-ERG) and cortical (pattern-VEP) responses, respectively; contrast was 80%; reversal at 2.13 Hz. Structured pattern-ERG above noise level was recorded from 29 subjects at 0.6 cycle/degree and from 24 subjects at 1.2 cycle/degree; latencies were increased and amplitude reduced. Pattern-VEP responses above noise level, with increased latencies and reduced amplitude, were observed in 92% of subjects with unilateral and in all subjects with bilateral retinal response. Both responses were phase-locked to stimulus. No correlation with the residual visual acuity or field was detected. The observation is consistent with evidence of the disease sparing the neuroretina and with unconscious visual processing and suggests miscoding of visual information processing.

Visual evoked cortical potential (VECP) elicited by sinusoidal gratings controlled by pseudo-random stimulation

The contributions of contrast detection mechanisms to the visual cortical evoked potential (VECP) have been investigated studying the contrast-response and spatial frequency-response functions. Previously, the use of m-sequences for stimulus control has been almost restricted to multifocal electrophysiology stimulation and, in some aspects, it substantially differs from conventional VECPs. Single stimulation with spatial contrast temporally controlled by m-sequences has not been extensively tested or compared to multifocal techniques. Our purpose was to evaluate the influence of spatial frequency and contrast of sinusoidal gratings on the VECP elicited by pseudo-random stimulation. Nine normal subjects were stimulated by achromatic sinusoidal gratings driven by pseudo random binary m-sequence at seven spatial frequencies (0.4–10 cpd) and three stimulus sizes (4°, 8°, and 16° of visual angle). At 8° subtence, six contrast levels were used (3.12–99%). The first order kernel (K1) did not provide a consistent measurable signal across spatial frequencies and contrasts that were tested–signal was very small or absent–while the second order kernel first (K2.1) and second (K2.2) slices exhibited reliable responses for the stimulus range. The main differences between results obtained with the K2.1 and K2.2 were in the contrast gain as measured in the amplitude versus contrast and amplitude versus spatial frequency functions. The results indicated that K2.1 was dominated by M-pathway, but for some stimulus condition some P-pathway contribution could be found, while the second slice reflected the P-pathway contribution. The present work extended previous findings of the visual pathways contribution to VECP elicited by pseudorandom stimulation for a wider range of spatial frequencies.

Early spatial frequency processing of natural images: an ERP study

The present study examined the role of spatial stimulus frequencies in the early visual processing of natural scenes. The content of initially degraded (low- or high-pass filtered) pictures was progressively revealed in a sequence of steps by adding high or low spatial frequencies. Event Related Potentials (ERPs) were used to track the early stages of visual processing. Picture degradation modulated the topography of the P1, with an occipital midline distribution for the most degraded pictures, which became progressively more laterally distributed as pictures became more complete. Picture degradation also modulated the amplitude of the P2. For both low-passed and high-passed scenes, a linear relationship between the spectral power and the amplitude of the P1 and P2 was observed. These results are likely to reflect the progressive engagement of the lateral occipital complex as the amount of information in both the low and high portions of the frequency spectrum increased.

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.

History of reading struggles linked to enhanced learning in low spatial frequency scenes

People with dyslexia, who face lifelong struggles with reading, exhibit numerous associated low-level sensory deficits including deficits in focal attention. Countering this, studies have shown that struggling readers outperform typical readers in some visual tasks that integrate distributed information across an expanse. Though such abilities would be expected to facilitate scene memory, prior investigations using the contextual cueing paradigm failed to find corresponding advantages in dyslexia. We suggest that these studies were confounded by task-dependent effects exaggerating known focal attention deficits in dyslexia, and that, if natural scenes were used as the context, advantages would emerge. Here, we investigate this hypothesis by comparing college students with histories of severe lifelong reading difficulties (SR) and typical readers (TR) in contexts that vary attention load. We find no differences in contextual-cueing when spatial contexts are letter-like objects, or when contexts are natural scenes. However, the SR group significantly outperforms the TR group when contexts are low-pass filtered natural scenes [F(3, 39) = 3.15, p<.05]. These findings suggest that perception or memory for low spatial frequency components in scenes is enhanced in dyslexia. These findings are important because they suggest strengths for spatial learning in a population otherwise impaired, carrying implications for the education and support of students who face challenges in school.

New visual information processing abnormality biomarker for the diagnosis of Schizophrenia

INTRODUCTION: Schizophrenia is currently diagnosed on the basis of patient reports and clinical observations. A diagnosis based on aetiology is inherently more reliable due to being closer to the disease process than the overt clinical manifestations. Accordingly, recent research in schizophrenia has focused on the development of biomarkers in a bit to improve the reliability and neurobiological relevance of the diagnosis. Visual information processing is one of these promising fields of recent biomarker research. AREAS COVERED: This article provides an overview of the available literature regarding deficits in schizophrenia detectable through psychophysical (contrast and motion sensitivity, visual backward-masking), ERP (P1 and N1 visual evoked potentials) and oscillatory (signal power and phase-locking factor of evoked oscilations) measures and their validity as trait or state biomarkers of the disease. The methodology included a search on articles related to visual information processing in schizophrenia on the PubMed database. EXPERT OPINION: Biomarker research in schizophrenia is a rapidly expanding area. Evidence exists to suggest that both psychotic and manic symptoms are associated with visual processing abnormalities. A specific impairment confined to the magnocellular component of the visual system might be a trait biomarker of schizophrenia.

Multisensory interactions in early evoked brain activity follow the principle of inverse effectiveness

A major determinant of multisensory integration, derived from single-neuron studies in animals, is the principle of inverse effectiveness (IE), which describes the phenomenon whereby maximal multisensory response enhancements occur when the constituent unisensory stimuli are minimally effective in evoking responses. Human behavioral studies, which have shown that multisensory interactions are strongest when stimuli are low in intensity are in agreement with the IE principle, but the neurophysiologic basis for this finding is unknown. In this high-density electroencephalography (EEG) study, we examined effects of stimulus intensity on multisensory audiovisual processing in event-related potentials (ERPs) and response time (RT) facilitation in the bisensory redundant target effect (RTE). The RTE describes that RTs are faster for bisensory redundant targets than for the respective unisensory targets. Participants were presented with semantically meaningless unisensory auditory, unisensory visual and bisensory audiovisual stimuli of low, middle and high intensity, while they were instructed to make a speeded button response when a stimulus in either modality was presented. Behavioral data showed that the RTE exceeded predictions on the basis of probability summations of unisensory RTs, indicative of integrative multisensory processing, but only for low intensity stimuli. Paralleling this finding, multisensory interactions in short latency (40-60ms) ERPs with a left posterior and right anterior topography were found particularly for stimuli with low intensity. Our findings demonstrate that the IE principle is applicable to early multisensory processing in humans.

From spatial frequency contrast to edge preponderance: the differential modulation of early visual evoked potentials by natural scene stimuli

The contrast response function of early visual evoked potentials elicited by sinusoidal gratings is known to exhibit characteristic potentials closely associated with the processes of parvocellular and magnocellular pathways. Specifically, the N1 component has been linked with parvocellular processes, while the P1 component has been linked with magnocellular processes. However, little is known regarding the response properties of the N1 and P1 components during the processing and encoding of complex (i.e., broadband) stimuli such as natural scenes. Here, we examine how established physical characteristics of natural scene imagery modulate the N1 and P1 components in humans by providing a systematic investigation of component modulation as visual stimuli are gradually built up from simple sinusoidal gratings to highly complex natural scene imagery. The results suggest that the relative dominance in signal output of the N1 and P1 components is dependent on spatial frequency (SF) luminance contrast for simple stimuli up to natural scene imagery possessing few edges. However, such a dependency shifts to a dominant N1 signal for natural scenes possessing abundant edge content and operates independently of SF luminance contrast.

A brief introduction to the use of event-related potentials in studies of perception and attention

Because of the precise temporal resolution of electrophysiological recordings, the event-related potential (ERP) technique has proven particularly valuable for testing theories of perception and attention. Here, I provide a brief tutorial on the ERP technique for consumers of such research and those considering the use of human electrophysiology in their own work. My discussion begins with the basics regarding what brain activity ERPs measure and why they are well suited to reveal critical aspects of perceptual processing, attentional selection, and cognition, which are unobservable with behavioral methods alone. I then review a number of important methodological issues and often-forgotten facts that should be considered when evaluating or planning ERP experiments.

Visual information processing deficits as biomarkers of vulnerability to schizophrenia: an event-related potential study in schizotypy

We aimed to clarify the importance of early visual processing deficits for the formation of cognitive deficits in the schizophrenia spectrum. We carried out an event-related potential (ERP) study using a computerised delayed matching to sample working memory (WM) task on a sample of volunteers with high and low scores on the Schizotypal Personality Questionnaire (SPQ). The amplitudes of the visual ERPs to the encoding and retrieval stimuli in the task were measured using the BESA software. The hypothesis was that the high schizotypes would have deficits in early visual processing (reduced P1 amplitude) and working memory similar to those observed in schizophrenia. The high schizotypy group identified fewer previously encoded target cues than the low schizotypy group in the WM task and their mean cue-evoked P1 amplitudes were significantly reduced, both in the encoding and the retrieval phases of the task. Accuracy on the WM task correlated with the P1 amplitude. None of the later components (N1 and P2) were significantly different between the groups, nor were there differences in performance on the CANTAB tests. The results are compatible with the hypothesis that trait vulnerability to schizophrenia is associated with impaired early visual processing which may contribute to impaired cognitive memory performance. However, the high schizotypes are apparently able to compensate for the visual processing deficits and perform normally when stimuli are presented for longer as in the CANTAB tasks. This study adds to growing evidence that the schizophrenia spectrum is characterized by early sensory abnormalities.

Differential maturation of brain signal complexity in the human auditory and visual system

Brain development carries with it a large number of structural changes at the local level which impact on the functional interactions of distributed neuronal networks for perceptual processing. Such changes enhance information processing capacity, which can be indexed by estimation of neural signal complexity. Here, we show that during development, EEG signal complexity increases from one month to 5 years of age in response to auditory and visual stimulation. However, the rates of change in complexity were not equivalent for the two responses. Infants’ signal complexity for the visual condition was greater than auditory signal complexity, whereas adults showed the same level of complexity to both types of stimuli. The differential rates of complexity change may reflect a combination of innate and experiential factors on the structure and function of the two sensory systems.

Improved visual sensitivity during smooth pursuit eye movements: Temporal and spatial characteristics

Recently, we showed that contrast sensitivity for color and high–spatial frequency luminance stimuli is enhanced during smooth pursuit eye movements (Schütz et al., 2008). In this study, we investigated the enhancement over a wide range of temporal and spatial frequencies. In Experiment 1, we measured the temporal impulse response function (TIRF) for colored stimuli. The TIRF for pursuit and fixation differed mostly with respect to the gain but not with respect to the natural temporal frequency. Hence, the sensitivity enhancement seems to be rather independent of the temporal frequency of the stimuli. In Experiment 2, we measured the spatial contrast sensitivity function for luminance-defined Gabor patches with spatial frequencies ranging from 0.2 to 7 cpd. We found a sensitivity improvement during pursuit for spatial frequencies above 2–3 cpd. Between 0.5 and 3 cpd, sensitivity was impaired by smooth pursuit eye movements, but no consistent difference was observed below 0.5 cpd. The results of both experiments are consistent with an increased contrast gain of the parvocellular retinogeniculate pathway.

Independent component analysis reveals atypical electroencephalographic activity during visual perception in individuals with autism

BACKGROUND

Individuals with autistic spectrum disorder (ASD) experience atypical visual perception, yet the etiology of this remains unknown. The aim of this study was to investigate the neural correlates of visual perception in individuals with and without ASD by carrying out a detailed analysis of the dynamic brain processes elicited by perception of a simple visual stimulus.

METHODS

We investigated perception in 20 individuals with ASD and 20 control subjects with electroencephalography (EEG). Visual evoked potentials elicited by Gabor patches of varying spatial frequency and stimulus-induced changes in alpha- and gamma-frequency bands of independent components were compared in those with and without ASD.

RESULTS

By decomposing the EEG data into independent components, we identified several processes that contributed to the average event related potential recorded at the scalp. Differences between the ASD and control groups were found only in some of these processes. Specifically, in those components that were in or near the striate or extrastriate cortex, stimulus spatial frequency exerted a smaller effect on induced increases in alpha- and gamma-band power, and time to peak alpha-band power was reduced, in the participants with ASD. Induced alpha-band power of components that were in or near the cingulate gyrus was increased in the participants with ASD, and the components that were in or near the parietal cortex did not differ between the two groups.

CONCLUSIONS

Atypical processing is evident in individuals with ASD during perception of simple visual stimuli. The implications of these data for existing theories of atypical perception in ASD are discussed.

Impairments of Gestalt perception in the intact hemifield of hemianopic patients are reflected in gamma-band EEG activity

Gamma-band responses (GBRs) are associated with Gestalt perception processes. In the present EEG study, we investigated the effects of perceptual grouping on the visual GBR in the perimetrically intact visual field of patients with homonymous hemianopia and compared them to healthy participants. All observers were presented either random arrays of Gabor elements or arrays with an embedded circular arrangement. For the hemianopic patients, the circle was presented in their intact hemifield only. For controls, the hemifield for the circle presentation was counterbalanced across subjects. The participants were instructed to detect the circle by pressing a corresponding button. A wavelet transform based on Morlet wavelets was employed for the calculation of oscillatory GBRs. The early evoked GBR exhibited a larger amplitude and shorter latency for the healthy group compared to hemianopic patients and was associated with behavioral measures. The late total GBR between 200 and 400ms after stimulus onset was significantly increased for Gestalt-like patterns in healthy participants. This effect was not manifested in patients. The present findings indicate deficits in the early and late visual processing of Gestalt patterns even in the intact hemifield of hemianopic patients compared to healthy participants.

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.

A few remarks on attention and magnocellular deficits in schizophrenia

In connection with schizophrenia, it has been proposed that the magnocellular system is specifically linked to the guiding of covert visual attention. The argument is that the magnocellular pathway provides input to the dorsal cortical stream which then projects back to area V1. We review problems with this model. (1) It requires that responses in the magnocellular system have a lead time over responses in the parvocellular system. However, measurements indicate that the actual response time difference between the two systems is small or negligible when entering the visual cortex. (2) Attention can be modified by stimuli that do not activate the magnocellular system. And, (3) lesions to area MT in the dorsal stream impair smooth pursuit eye movements, but not saccadic eye movements which are associated with shifts in attention. For these reasons, it is difficult to link attention defects in schizophrenia to potential magnocellular deficits.