Generation of the VESPA response to rapid contrast fluctuations is dominated by striate cortex: evidence from retinotopic mapping

Steady state evoked potentials indicate changes in nonlinear neural mechanisms of vision in sight recovery individuals

Humans with a transient phase of congenital pattern vision deprivation have been observed to feature prevailing deficits, particularly in higher order visual functions. However, the neural correlates of these prevalent visual impairments remain unclear. To probe different visual processing stages, we measured steady state visual evoked potentials (SSVEPs) generated by luminance flicker stimuli at 6.1 Hz, with superimposed horizontal periodic motion at 2.1 Hz or 2.4 Hz. SSVEP responses at the fundamental and second harmonic of luminance flicker frequency, and at their intermodulation frequencies with motion information, were analyzed. Three groups were tested: (1) 15 individuals who had suffered a lack of pattern vision from birth due to the presence of bilateral total congenital cataracts (CC group), which were surgically removed between 4 months and 22 years of age, (2) 13 individuals with reversed developmental i.e., later developing cataracts (DC group), and (3) normally sighted control participants (SC group; n = 13) matched in age and sex to the CC individuals. SSVEPs at the second harmonic frequency (i.e., 12.2 Hz) and at the intermodulation frequencies (8.2 Hz, and 8.5 Hz) were attenuated in the CC group. In contrast, fundamental frequency responses (i.e., at 6.1 Hz) were not significantly altered in the CC group compared to the control groups (SC and DC groups). Based on previous evidence on the role of striate vs. extrastriate generators of fundamental vs. second harmonics of SSVEPs, these results provide evidence for a stronger experience dependence of extrastriate than striate cortical processing, and furthermore, suggest a sensitive period for the development of putative nonlinear neural mechanisms hypothesized to mediate visual feature binding.

Visual crowding involves delayed frontoparietal response and enhanced top-down modulation

Crowding, the disrupted recognition of a peripheral target in the presence of nearby flankers, sets a fundamental limit on peripheral vision perception. Debates persist on whether the limit occurs at early visual cortices or is induced by top-down modulation, leaving the neural mechanism for visual crowding largely unclear. To resolve the debate, it is crucial to extract the neural signals elicited by the target from that by the target-flanker clutter, with high temporal resolution. To achieve this purpose, here we employed a temporal response function (TRF) approach to dissociate target-specific response from the overall electroencephalograph (EEG) recordings when the target was presented with (crowded) or without flankers (uncrowded) while subjects were performing a discrimination task on the peripherally presented target. Our results demonstrated two components in the target-specific contrast-tracking TRF response-an early component (100-170 ms) in occipital channels and a late component (210-450 ms) in frontoparietal channels. The late frontoparietal component, which was delayed in time under the crowded condition, was correlated with target discrimination performance, suggesting its involvement in visual crowding. Granger causality analysis further revealed stronger top-down modulation on the target stimulus under the crowded condition. Taken together, our findings support that crowding is associated with a top-down process which modulates the low-level sensory processing and delays the behavioral-relevant response in the high-level region.

Delayed P100-Like Latencies in Multiple Sclerosis: A Preliminary Investigation Using Visual Evoked Spread Spectrum Analysis

Conduction along the optic nerve is often slowed in multiple sclerosis (MS). This is typically assessed by measuring the latency of the P100 component of the Visual Evoked Potential (VEP) using electroencephalography. The Visual Evoked Spread Spectrum Analysis (VESPA) method, which involves modulating the contrast of a continuous visual stimulus over time, can produce a visually evoked response analogous to the P100 but with a higher signal-to-noise ratio and potentially higher sensitivity to individual differences in comparison to the VEP. The main objective of the study was to conduct a preliminary investigation into the utility of the VESPA method for probing and monitoring visual dysfunction in multiple sclerosis. The latencies and amplitudes of the P100-like VESPA component were compared between healthy controls and multiple sclerosis patients, and multiple sclerosis subgroups. The P100-like VESPA component activations were examined at baseline and over a 3-year period. The study included 43 multiple sclerosis patients (23 relapsing-remitting MS, 20 secondary-progressive MS) and 42 healthy controls who completed the VESPA at baseline. The follow-up sessions were conducted 12 months after baseline with 24 MS patients (15 relapsing-remitting MS, 9 secondary-progressive MS) and 23 controls, and again at 24 months post-baseline with 19 MS patients (13 relapsing-remitting MS, 6 secondary-progressive MS) and 14 controls. The results showed P100-like VESPA latencies to be delayed in multiple sclerosis compared to healthy controls over the 24-month period. Secondary-progressive MS patients had most pronounced delay in P100-like VESPA latency relative to relapsing-remitting MS and controls. There were no longitudinal P100-like VESPA response differences. These findings suggest that the VESPA method is a reproducible electrophysiological method that may have potential utility in the assessment of visual dysfunction in multiple sclerosis.

Towards obtaining spatiotemporally precise responses to continuous sensory stimuli in humans: a general linear modeling approach to EEG

Noninvasive investigation of human sensory processing with high temporal resolution typically involves repeatedly presenting discrete stimuli and extracting an average event-related response from scalp recorded neuroelectric or neuromagnetic signals. While this approach is and has been extremely useful, it suffers from two drawbacks: a lack of naturalness in terms of the stimulus and a lack of precision in terms of the cortical response generators. Here we show that a linear modeling approach that exploits functional specialization in sensory systems can be used to rapidly obtain spatiotemporally precise responses to complex sensory stimuli using electroencephalography (EEG). We demonstrate the method by example through the controlled modulation of the contrast and coherent motion of visual stimuli. Regressing the data against these modulation signals produces spatially focal, highly temporally resolved response measures that are suggestive of specific activation of visual areas V1 and V6, respectively, based on their onset latency, their topographic distribution and the estimated location of their sources. We discuss our approach by comparing it with fMRI/MRI informed source analysis methods and, in doing so, we provide novel information on the timing of coherent motion processing in human V6. Generalizing such an approach has the potential to facilitate the rapid, inexpensive spatiotemporal localization of higher perceptual functions in behaving humans.

Understanding individual face discrimination by means of fast periodic visual stimulation

This paper reviews a fast periodic visual stimulation (FPVS) approach developed recently to make significant progress in understanding visual discrimination of individual faces. Displaying pictures of faces at a periodic frequency rate leads to a high signal-to-noise ratio (SNR) response in the human electroencephalogram, at the exact frequency of stimulation, a so-called steady-state visual evoked potential (SSVEP, Regan in Electroencephalogr Clin Neurophysiol 20:238-248, 1966). For fast periodic frequency rates, i.e., between 3 and 9 Hz, this response is reduced if the exact same face identity is repeated compared to the presentation of different face identities, the largest difference being observed over the right occipito-temporal cortex. A 6-Hz stimulation rate (cycle duration of ~170 ms) provides the largest difference between different and repeated faces, as also evidenced in face-selective areas of the ventral occipito-temporal cortex in functional magnetic resonance imaging. This high-level discrimination response is reduced following inversion and contrast-reversal of the faces and can be isolated without subtraction thanks to a fast periodic oddball paradigm. Overall, FPVS provides a response that is objective (i.e., at an experimentally defined frequency), implicit, has a high SNR and is directly quantifiable in a short amount of time. Although the approach is particularly appealing for understanding face perception, it can be generalized to study visual discrimination of complex visual patterns such as objects and visual scenes. The advantages of the approach make it also particularly well-suited to investigate these functions in populations who cannot provide overt behavioral responses and can only be tested for short durations, such as infants, young children and clinical populations.

Atypical cortical representation of peripheral visual space in children with an autism spectrum disorder

A key feature of early visual cortical regions is that they contain discretely organized retinotopic maps. Titration of these maps must occur through experience, and the fidelity of their spatial tuning will depend on the consistency and accuracy of the eye movement system. Anomalies in fixation patterns and the ballistics of eye movements are well documented in autism spectrum disorder (ASD), with off-center fixations a hallmark of the phenotype. We hypothesized that these atypicalities might affect the development of visuo-spatial maps and specifically that peripheral inputs might receive altered processing in ASD. Using high-density recordings of visual evoked potentials (VEPs) and a novel system-identification approach known as VESPA (visual evoked spread spectrum analysis), we assessed sensory responses to centrally and peripherally presented stimuli. Additionally, input luminance was varied to bias responsiveness to the magnocellular system, given previous suggestions of magnocellular-specific deficits in ASD. Participants were 22 ASD children (7-17 years of age) and 31 age- and performance-IQ-matched neurotypical controls. Both VEP and VESPA responses to central presentations were indistinguishable between groups. In contrast, peripheral presentations resulted in significantly greater early VEP and VESPA amplitudes in the ASD cohort. We found no evidence that anomalous enhancement was restricted to magnocellular-biased responses. The extent of peripheral response enhancement was related to the severity of stereotyped behaviors and restricted interests, cardinal symptoms of ASD. The current results point to differential visuo-spatial cortical mapping in ASD, shedding light on the consequences of peculiarities in gaze and stereotyped visual behaviors often reported by clinicians working with this population.

Visual sensory processing deficits in schizophrenia: is there anything to the magnocellular account?

Visual processing studies have repeatedly shown impairment in patients with schizophrenia compared to healthy controls. Electroencephalography (EEG) and, specifically, visual evoked potential (VEP) studies have identified an early marker of this impairment in the form of a decrement in the P1 component of the VEP in patients and their clinically unaffected first-degree relatives. Much behavioral and neuroimaging research has implicated specific dysfunction of either the subcortical magnocellular pathway or the cortical visual dorsal stream in this impairment. In this study, EEG responses were obtained to the contrast modulation of checkerboard stimuli using the VESPA (Visual Evoked Spread Spectrum Analysis) method. This was done for a high contrast condition and, in order to bias the stimuli towards the magnocellular pathway, a low contrast condition. Standard VEPs were also obtained using high contrast pattern reversing checkerboards. Responses were measured using high-density electrical scalp recordings in 29 individuals meeting DSM-IV criteria for schizophrenia and in 18 control subjects. Replicating previous research, a large (Cohen's d=1.11) reduction in the P1 component of the VEP was seen in patients when compared with controls with no corresponding difference in the VESPA response to high contrast stimuli. In addition, the low-contrast VESPA displayed no difference between patients and controls. Furthermore, no differences were seen between patients and controls for the C1 components of either the VEP or the high-contrast VESPA. Based on the differing acquisition methods between VEP and VESPA, we discuss these results in terms of contrast gain control and the possibility of dysfunction at the cortical level with initial afferent activity into V1 along the magnocellular pathway being intact when processing is biased towards that pathway using low contrast stimuli.

Isolating early cortical generators of visual-evoked activity: a systems identification approach

The VESPA (visual-evoked spread spectrum analysis) method estimates the impulse response of the visual system using a continuously varying stimulus. It has been used recently to address both basic cognitive and neurophysiologic questions as well as those surrounding clinical populations. Although the components of the average VESPA response are highly reminiscent of the early components of the visual-evoked potential (VEP) when measured over midline occipital locations, the two responses are acquired in different ways and, thus, they cannot be regarded as being equivalent. To further characterize the relationship between the VESPA and the VEP and the generative mechanisms underlying them, we recorded EEG from 31 subjects in response to checkerboard-based VEP and VESPA stimuli. We found that, across subjects, the amplitudes of the VEP C1 component and the VESPA C1 component were highly correlated, whereas the VEP P1 and the VESPA P1 bore no statistical relationship. Furthermore, we found that C1 and P1 amplitudes were significantly correlated in the VESPA but not in the VEP. We believe these findings point to the presence of common generators underlying the VESPA C1 and the VEP C1. We argue further that the VESPA P1, in light of its strong relationship to the VESPA C1, likely reflects further activation of the same cortical generators. Given the lack of correlation between the VEP P1 and each of these three other components, it is likely that the underlying generators of this particular component are more varied and widespread, as suggested previously. We discuss the implications of these relationships for basic and clinical research using the VESPA and for the assessment of additive-evoked versus phase-reset contributions to the VEP.