Retinotopic mapping of visual event-related potentials

Visual stimulation is frequently employed in electroencephalographic (EEG) research. However, despite its widespread use, no studies have thoroughly evaluated how the morphology ofthe visual event-related potentials (ERPs) varies according to the spatial location of stimuli. Hence, the purpose of this study was to perform a detailed retinotopic mapping of visual ERPs. We recorded EEG activity while participants were visually stimulated with 60 pattern-reversing checkerboards placed at different polar angles and eccentricities. Our results show five pattern-reversal ERP components. C1 and C2 components inverted polarity between the upper and lower hemifields. P1 and N1 showed higher amplitudes and shorter latencies to stimuli located in the contralateral lower quadrant. In contrast, P2 amplitude was enhanced and its latency was reduced by stimuli presented in the periphery of the upper hemifield. The retinotopic maps presented here could serve as a guide for selecting optimal visuo-spatial locations in future ERP studiesThis work was supported by the Spanish Ministry of Science and Innovation/Economy and Competitiveness (MICINN/MINECO) (PSI2011-26314, PSI2012-34558 and PSI2014-54853-P

A novel visual stimulation paradigm: exploiting individual primary visual cortex geometry to boost steady state visual evoked potentials (SSVEP)

The steady-state visual evoked potential (SSVEP) is an electroencephalographic response to flickering stimuli generated in significant part by activity in primary visual cortex (V1). SSVEP signal-to-noise ratio is generally low for stimuli that are located in the visual periphery, at frequencies higher than 20 Hz, or at low contrast. Because of the typical cruciform geometry of V1, large stimuli tend to excite neighboring cortical regions of opposite orientation, likely resulting in electric field cancellation. In Study 1, we explored ways to exploit V1 geometry in order to boost scalp SSVEP amplitude via oscillatory summation, by manipulating flicker-phase offsets among angular segments of a large annular stimulus. We found that by dividing the annulus into standard octants, flickering upper horizontal octants with opposite temporal phase to the lower horizontal ones, and left vertical octants opposite to the right vertical ones, the normalized SSVEP power was enhanced by 202% relative to the conventional condition with no temporal phase offsets. In two further conditions we individually customized the phase-segment boundaries based on early-latency topographical shifts in pattern-pulse multifocal visual-evoked potentials (PPMVEP) derived for each of 32 equal-sized segments. Adjusting the boundaries between 8 phase-segments by visual inspection resulted in significant enhancement of normalized SSVEP power of 383%, a further significant improvement over the standard octants condition. An automatic segment-phase assignment algorithm based on the relative strength of vertically- and horizontally-oriented multifocal VEP scalp potential amplitudes produced an enhancement of 300%. In Study 2, we applied the same principle to obtain more reliable measures of visual evoked activity to obtain surround suppression measures. Here we report for the first time, a novel vii paradigm that exploits simple signal processing, sensory physiology and psychophysical evidences in order to extract a direct index of surround suppression using EEG. Surround suppression effects were tested for low and high flickering frequencies in two different configurations of a flickering stimulus (foreground, FG) on a static surrounding pattern (background, BG): foveal, where the stimulus was a unique central disc, and peripheral, where four discs were presented at symmetrical locations around the horizontal meridian. We varied FG and BG contrast combinations and also evaluated the influence of differences in spatial phase and orientation between the surrounding pattern and the foreground. Across a population of sixteen healthy subjects, we found that the foreground contrast response function was significantly suppressed in proportion with the contrast of the background, and that, like psychophysical measures, this suppression effect was greater when the background was oriented in parallel with the foreground than when it was orthogonal. Suppression effects were also greater for the peripheral stimulus condition. This is the first demonstration of a clear surround suppression effect in the visual evoked potentials of humans, and paves the way for the first definitive measurement of the relative contributions of under-inhibition and over-excitation to hyperexcitability in epilepsy

Electrophysiological measurement of the effect of inter-stimulus competition on early cortical stages of human vision

AbstractCompetition between inputs in early visual cortex has been established as a key determinant in perception through decades of animal single cell and human fMRI research. We developed a novel ERP paradigm allowing this competition to be studied in humans, affording an opportunity to gain further insight into how competition is reflected at the neural level. Checkerboard stimuli were presented to elicit C1 (indexing processing in V1), C2 (hypothesized to reflect V1 after extrastriate feedback), and P1 (extrastriate) components. Stimuli were presented in three randomized conditions: single stimulus, near proximity pairs and far proximity pairs. Importantly, near stimuli (0.16° visual angle apart) were positioned to compete in primary visual cortex, whereas far stimuli (2° apart) were positioned to compete in extrastriate visual areas.As predicted, the degree and spatial range of competition increased from the C1 component to the C2 and P1 components. Specifically, competitive interactions in C1 amplitude were modest and present only for near-proximity pairs, whereas substantial competition was present for the P1, even for far-proximity pairs. To our knowledge, this is the first study to measure how competition unfolds over time in human visual cortex. Importantly, this method provides an empirical means of measuring competitive interactions at specific stages of visual processing, rendering it possible to rigorously test predictions about the effects of competition on perception, attention, and working memory

Temporal frequency dependence of the polarity inversion between upper and lower visual field in the pattern-onset steady-state visual evoked potential

Purpose: According to the cruciform model, the upper and lower halves of the visual field representation in the primary visual cortex are located mainly on the opposite sides of the calcarine sulcus. Such a shape would have consequences for the surface-recorded visual evoked potential (VEP), as V1 responses to stimulation of the upper and lower hemifield manifest with opposite polarity (i.e., polarity inversion). However, the steady-state VEP results from a complex superposition of response components from different cortical sources, which can obscure the inversion of polarity. The present study assesses the issue for different stimulation frequencies which result in different patterns of superposition in the steady-state response. Methods: Sequences of brief pattern-onset stimuli were presented at different stimulation rates ranging from 2 Hz (transient VEP) to 13 Hz (steady-state VEP). The upper and lower hemifields were tested separately and simultaneously. The data were assessed both in the time domain and in the frequency domain. Results: Comparing the responses to the stimulation of upper and lower hemifield, polarity inversion was present within a limited time interval following individual stimulus onsets. With increasing frequency, this resulted in an approximate inversion of the full steady-state response and consequently in a phase shift of approximately 180° in the time-domain response. Polarity inversion was more prominent at electrode Pz, also for transient responses. Our data also demonstrated that the sum of the hemifield responses is a good approximation of the full-field response. Conclusion: While the basic phenomenon of polarity inversion occurs irrespective of the stimulus frequency, its relative impact on the steady-state response as a whole is the largest for high stimulation rates. We propose that this is because longer-lasting response components from other visual areas are not well represented in the steady-state VEP at higher frequencies

Neurofeedback Therapy for Enhancing Visual Attention: State-of-the-Art and Challenges

We have witnessed a rapid development of brain-computer interfaces (BCIs) linking the brain to external devices. BCIs can be utilized to treat neurological conditions and even to augment brain functions. BCIs offer a promising treatment for mental disorders, including disorders of attention. Here we review the current state of the art and challenges of attention-based BCIs, with a focus on visual attention. Attention-based BCIs utilize electroencephalograms (EEGs) or other recording techniques to generate neurofeedback, which patients use to improve their attention, a complex cognitive function. Although progress has been made in the studies of neural mechanisms of attention, extraction of attention-related neural signals needed for BCI operations is a difficult problem. To attain good BCI performance, it is important to select the features of neural activity that represent attentional signals. BCI decoding of attention-related activity may be hindered by the presence of different neural signals. Therefore, BCI accuracy can be improved by signal processing algorithms that dissociate signals of interest from irrelevant activities. Notwithstanding recent progress, optimal processing of attentional neural signals remains a fundamental challenge for the development of efficient therapies for disorders of attention

When the Brain Plays a Game: Neural Responses to Visual Dynamics during Naturalistic Visual Tasks

Many day-to-day tasks involve processing of complex visual information in a continuous stream. While much of our knowledge on visual processing has been established from reductionist approaches in lab-controlled settings, very little is known about the processing of complex dynamic stimuli experienced in everyday scenarios. Traditional investigations employ event-related paradigms that involve presentation of simple stimuli at select locations in visual space and discrete moments in time. In contrast, visual stimuli in real-life are highly dynamic, spatially-heterogeneous, and semantically rich. Moreover, traditional experiments impose unnatural task constraints (e.g., inhibited saccades), thus, it is unclear whether theories developed under the reductionist approach apply in naturalistic settings. Given these limitations, alternative experimental paradigms and analysis methods are necessary. Here, we introduce a new approach for investigating visual processing, applying the system identification (SI) framework. We investigate the modulation of stimulus-evoked responses during a naturalistic task (i.e., kart race game) using non-invasive scalp recordings. In recent years, multivariate modeling approaches have become increasingly popular for assessing neural response to naturalistic stimuli. Encoding models use stimulus patterns to predict brain responses and decoding models use patterns of brain responses to predict stimulus that drove these responses. In this dissertation, we employ a hybrid method that “encodes” the stimulus to predict “decoded” brain responses. Using this approach, we measure the stimulus-response correlation (SRC), i.e. temporal correlation of neural response and dynamic stimulus. This SRC can be used to assess the strength of stimulus-evoked activity to uniquely experienced naturalistic stimulus. To demonstrate this, we measured the SRC during a kart race videogame. We find that SRC increased with active play of the game, suggesting that stimulus-evoked activity is modulated by the visual task demands. Furthermore, we analyzed the selectivity of neural response across the visual space. While it is well-established that neural response is spatially selective to discrete stimulus, it is unclear whether this is true during naturalistic stimulus presentation. To assess this, we measured the correlation of neural response with optical flow magnitude at individual locations on the screen during the videogame. We find that the SRC is greater for locations in space that are task-relevant, enhancing during active play. Moreover, the spatial selectivity differs across scalp locations, which suggest that individual brain regions are spatially selective to different visual dynamics. In summary, we leverage the SI framework to investigate visual processing during a naturalistic stimulus presentation, extending visual research to ecologically valid paradigms. Moreover, we demonstrate spatial selectivity of neural response that are task-relevant. Overall, our findings shed new insights about the stimulus-evoked neural response to visual dynamics during a uniquely experienced naturalistic visual task. Taken together, this dissertation work makes a significant contribution towards understanding how visual dynamics and task behavior affects neural responses in naturalistic conditions

Visual Impairment in the absence of ON-pathway signal

Congenital retinal diseases are a major cause of childhood and lifelong visual im- pairment. Such conditions can manifest a variable array of severe and subtle ef- fects on vision. Assessment of visual function in children can be challenging; yet, knowledge about phenotype, genotype and impact of these disorders is crucial for providing appropriate support, tailored diagnostics and for developing treatments. ON-and OFF-pathways are separately transmitting information on brightness and darkness from the retina to the cortex, where their interplay is crucial in visual perception. This project investigated the effects of retinal ON-pathway dysfunction on vision. A cohort of 109 patients with ON-pathway dysfunction was examined from four subgroups of visual electrophysiological phenotypes (incomplete and complete Congenital Stationary Night Blindness - CSNB, Duchenne Muscular Dystrophy - DMD, and congenital disorders of N-glycosylation - PMM2-CDG). Using spe- cialised visual evoked potential stimuli, designed to distinguish the ON-and OFF- pathway signal arrival at the striate cortex, marked ON system delays were revealed in patients with subtypes of CSNB, DMD mutations post exon 30 and PMM2-CDG. A child-friendly psychophysical software called LumiTrack was developed to assess motion and contrast perception, two important qualities conveyed by ON-and OFF-pathways. Patients with subtypes of CSNB and PMM2-CDG showed abnormalities in motion perception and subnormal contrast sensitivity, while patients with DMD performed at the level of healthy volunteers. These impairments may occur due to a delay of signal transmission through the retina, resulting in an ON/OFF signal asymmetry within the visual system. A genotype-phenotype comparison suggested a trend of increasing ON/OFF asymmetry associated with genetic defects affecting proteins placed later within the photoreceptor / ON bipolar cell signalling cascade. This systematic study of cortical and behavioural visual function in patients with ON-pathway dysfunction highlights the impairments encountered by patients in visual qualities important for everyday life