Electrophysiological estimate of human cortical magnification

Alpha and SSVEP power outperform gamma power in capturing attentional modulation in human EEG

Attention typically reduces power in the alpha (8-12 Hz) band and increases power in gamma (>30 Hz) band in brain signals, as reported in macaque local field potential (LFP) and human electro/magneto-encephalogram (EEG/MEG) studies. In addition, EEG studies often use flickering stimuli that produce a specific measure called steady-state-visually-evoked-potential (SSVEP), whose power also increases with attention. However, effectiveness of these neural measures in capturing attentional modulation is unknown since stimuli and task paradigms vary widely across studies. In a recent macaque study, attentional modulation was more salient in the gamma band of the LFP, compared to alpha or SSVEP. To compare this with human EEG, we designed an orientation change detection task where we presented both static and counterphasing stimuli of matched difficulty levels to 26 subjects and compared attentional modulation of various measures under similar conditions. We report two main results. First, attentional modulation was comparable for SSVEP and alpha. Second, non-foveal stimuli produced weak gamma despite various stimulus optimizations and showed negligible attentional modulation although full-screen gratings showed robust gamma activity. Our results are useful for brain-machine-interfacing studies where suitable features are used for decoding attention, and also provide clues about spatial scales of neural mechanisms underlying attention.

Interaction between spatial perception and temporal perception enables preservation of cause-effect relationship: Visual psychophysics and neuronal dynamics

INTRODUCTION

Visual perception of moving objects is integral to our day-to-day life, integrating visual spatial and temporal perception. Most research studies have focused on finding the brain regions activated during motion perception. However, an empirically validated general mathematical model is required to understand the modulation of the motion perception. Here, we develop a mathematical formulation of the modulation of the perception of a moving object due to a change in speed, under the formulation of the invariance of causality.

METHODS

We formulated the perception of a moving object as the coordinate transformation from a retinotopic space onto perceptual space and derived a quantitative relationship between spatiotemporal coordinates. To validate our model, we undertook the analysis of two experiments: (i) the perceived length of the moving arc, and (ii) the perceived time while observing moving stimuli. We performed a magnetic resonance imaging (MRI) tractography investigation of subjects to demarcate the anatomical correlation of the modulation of the perception of moving objects.

RESULTS

Our theoretical model shows that the interaction between visual-spatial and temporal perception, during the perception of moving object is described by coupled linear equations; and experimental observations validate our model. We observed that cerebral area V5 may be an anatomical correlate for this interaction. The physiological basis of interaction is shown by a Lotka-Volterra system delineating interplay between acetylcholine and dopamine neurotransmitters, whose concentrations vary periodically with the orthogonal phase shift between them, occurring at the axodendritic synapse of complex cells at area V5.

CONCLUSION

Under the invariance of causality in the representation of events in retinotopic space and perceptual space, the speed modulates the perception of a moving object. This modulation may be due to variations of the tuning properties of complex cells at area V5 due to the dynamic interaction between acetylcholine and dopamine. Our analysis is the first significant study, to our knowledge, that establishes a mathematical linkage between motion perception and causality invariance.

On the cortical mapping function - Visual space, cortical space, and crowding

The retino-cortical visual pathway is retinotopically organized: Neighbourhood relationships on the retina are preserved in the mapping. Size relationships in that mapping are also highly regular: The size of a patch in the visual field that maps onto a cortical patch of fixed size follows, along any radius and over a wide range, simply a linear function with retinal eccentricity. As a consequence, the mapping of retinal to cortical locations follows a logarithmic function along that radius. While this has already been shown by Fischer (1973, Vision Research, 13, 2113-2120), the link between the linear function - which describes the local behaviour by the cortical magnification factor M - and the logarithmic location function for the global behaviour, has never been made explicit. The present paper provides such a link as a set of ready-to-use equations using Levi and Klein's E₂ nomenclature, and examples for their validity and applicability in the mapping literature are discussed. The equations allow estimating M in the retinotopic centre; values thus derived from the literature show enormous, hitherto unnoticed, variability. A new structural parameter, d₂, is proposed to characterize the cortical map, as a counterpart to E₂; it shows much more stability. One pitfall is discussed and spelt out, namely the common myth that a pure logarithmic function, without constant term, will give an adequate map. The correct equations are finally extended to describe the cortical map of Bouma's law on visual crowding. The result contradicts recent suggestions that critical crowding distance corresponds to constant cortical distance.

The striate cortex and hemianopia

We begin with the functions of the striate cortex (area V1 of the visual cortex) and end with a review of the effects of damage to striate cortex or its inputs; namely, homonymous hemifield defects. Clinical and anatomical studies accrued over the past 25 years have modified our understanding of the role of V1 in vision. We discuss the evidence that V1 is not the sole recipient of visual signals; is not the earliest recipient of visual signals; and is not essential for conscious vision. In the second section, we give a brief history of how the visual field was found to be represented in striate cortex, then cover the work that has demonstrated the overrepresentation of the central region of vision in humans. The common patterns of visual field disturbance caused by damage to the retrochiasmal visual system are discussed, with some less common examples shown as brief case studies.

The Influence of the Stimulus Design on the Harmonic Components of the Steady-State Visual Evoked Potential

Steady-state visual evoked potentials (ssVEPs) are commonly used for functional objective diagnostics. In general, the main response at the stimulation frequency is used. However, some studies reported the main response at the second harmonic of the stimulation frequency. The aim of our study was to analyze the influence of the stimulus design on the harmonic components of ssVEPs. We studied 22 subjects (8 males, mean age ± SD = 27 ± 4.8 years) using a circular layout (r₁ = 0–1.6°, r₂ = 1.6–3.5°, r₃ = 3.5–6.4°, r₄ = 6.4–10.9°, and r₅ = 10.9–18°). At a given eccentricity, the stimulus was presented according to a 7.5 Hz square wave with 50% duty cycle. To analyze the influence of the stimulus eccentricity, a background luminance of 30 cd/m² was added to suppress foveal stray light effects; to analyze the influence of simultaneous foveal and peripheral stimulations, stimulations are performed without stray light suppression. For statistical analysis, medians M of the amplitude ratios for amplitudes at the second harmonic to the first harmonic and the probability of the occurrence of the main response at the second harmonic P(MCSH) are calculated. For stimulations with foveal stray light suppression, the medians were M_0–1.6° = 0.45, M_1.6–3.5° = 0.45, M_3.5–6.4° = 0.76, M_6.4–10.9° = 0.72, and M_10.9–18° = 0.48, and the probabilities were P_0–1.6°(MCSH) = 0.05, P_1.6–3.5°(MCSH) = 0.05, P_3.5–6.4°(MCSH) = 0.32, P_6.4–10.9°(MCSH) = 0.29, and P_10.9–18°(MCSH) = 0.30. For stimulations without foveal stray light suppression, the medians M were M_0–1.6° = 0.29, M_1.6–3.5° = 0.37, M_3.5–6.4° = 0.98, M_6.4–10.9° = 1.08, and M_10.9–18° = 1.24, and the probabilities were P_0–1.6°(MCSH) = 0.09, P_1.6–3.5°(MCSH) = 0.05, P_3.5–6.4°(MCSH) = 0.50, P_6.4–10.9°(MCSH) = 0.55, and P_10.9–18°(MCSH) = 0.55. In conclusion, the stimulus design has an influence on the harmonic components of ssVEPs. An increase in stimulation eccentricity during extrafoveal stimulation leads to a transition of the main response to the second harmonic. The effect is enhanced by a simultaneous foveal stimulation.

In vivo functional localization of the temporal monocular crescent representation in human primary visual cortex

The temporal monocular crescent (TMC) is the most peripheral portion of the visual field whose perception relies solely on input from the ipsilateral eye. According to a handful of post-mortem histological studies in humans and non-human primates, the TMC is represented visuotopically within the most anterior portion of the primary visual cortical area (V1). However, functional evidence of the TMC visuotopic representation in human visual cortex is rare, mostly due to the small size of the TMC representation (~6% of V1) and due to the technical challenges of stimulating the most peripheral portion of the visual field inside the MRI scanner. In this study, by taking advantage of custom-built MRI-compatible visual stimulation goggles with curved displays, we successfully stimulated the TMC region of the visual field in eight human subjects, half of them right-eye dominant, inside a 3 ​T MRI scanner. This enabled us to localize the representation of TMC, along with the blind spot representation (another visuotopic landmark in V1), in all volunteers, which match the expected spatial pattern based on prior anatomical studies. In all hemispheres, the TMC visuotopic representation was localized along the peripheral border of V1, within the most anterior portion of the calcarine sulcus, without any apparent extension into the second visual area (V2). We further demonstrate the reliability of this localization within/across experimental sessions, and consistency in the spatial location of TMC across individuals after accounting for inter-subject structural differences.

Integrating Brain and Biomechanical Models-A New Paradigm for Understanding Neuro-muscular Control

To date, realistic models of how the central nervous system governs behavior have been restricted in scope to the brain, brainstem or spinal column, as if these existed as disembodied organs. Further, the model is often exercised in relation to an in vivo physiological experiment with input comprising an impulse, a periodic signal or constant activation, and output as a pattern of neural activity in one or more neural populations. Any link to behavior is inferred only indirectly via these activity patterns. We argue that to discover the principles of operation of neural systems, it is necessary to express their behavior in terms of physical movements of a realistic motor system, and to supply inputs that mimic sensory experience. To do this with confidence, we must connect our brain models to neuro-muscular models and provide relevant visual and proprioceptive feedback signals, thereby closing the loop of the simulation. This paper describes an effort to develop just such an integrated brain and biomechanical system using a number of pre-existing models. It describes a model of the saccadic oculomotor system incorporating a neuromuscular model of the eye and its six extraocular muscles. The position of the eye determines how illumination of a retinotopic input population projects information about the location of a saccade target into the system. A pre-existing saccadic burst generator model was incorporated into the system, which generated motoneuron activity patterns suitable for driving the biomechanical eye. The model was demonstrated to make accurate saccades to a target luminance under a set of environmental constraints. Challenges encountered in the development of this model showed the importance of this integrated modeling approach. Thus, we exposed shortcomings in individual model components which were only apparent when these were supplied with the more plausible inputs available in a closed loop design. Consequently we were able to suggest missing functionality which the system would require to reproduce more realistic behavior. The construction of such closed-loop animal models constitutes a new paradigm of computational neurobehavior and promises a more thoroughgoing approach to our understanding of the brain's function as a controller for movement and behavior.

Several studies with significant C1 attention effects survive critical analysis

In a discussion paper (Slotnick, this issue), I conducted a selective review of spatial attention studies to compare experimental parameters and determine whether particular stimulus, task, or analysis conditions were more likely to produce significant attentional modulation of the event-related potential (ERP) C1 component. It was concluded that to maximize C1 attention effects, stimuli should be in the upper visual field, there should be distractors, conditions should be high perceptual or attentional load, there should be exogenous cuing, and effects should be measured at midline parietal-occipital electrodes POz, Pz, and CPz. Commentaries were received by Fu (this issue), Qu and Ding (this issue), Zani and Proverbio (this issue), Pitts and Hillyard (this issue), Di Russo (this issue), and Mohr and Kelly (this issue). Comments included additional ideas to amplify C1 attention effects, support for some conclusions, and challenges to some conclusions. The challenges led to a more in depth analysis of many issues pertaining to C1 attention effects including optimal electrode and stimulus locations, null V1 source localization attention effects, whether all significant C1 attention effects can be discounted, and the number of studies with null versus significant C1 attention effects. Analysis of the studies that survived critical analysis, which included several that observed significant C1 attention effects, led to the same conclusions as Slotnick (this issue). Lines of future research include replicating studies that have observed C1 attention effects using identical experimental parameters and systematically manipulating parameters to determine the impact of each parameter on C1 spatial attention effects.

Predictive value of N95 waveforms of pattern electroretinograms (PERGs) in children with optic nerve hypoplasia (ONH)

PURPOSE

As part of a long-term, prospective study of prenatal and clinical risk factors for optic nerve hypoplasia (ONH) at Children's Hospital Los Angeles, pattern ERGs (PERGs) were evaluated for prognostic value using an automated objective and robust analytical method.

METHODS

Participants were 33 children with ophthalmoscopically diagnosed ONH [disc diameter-to-disc macula ratio (DD/DM) less than 0.35 in one or both eyes on fundus photographs]. Using cycloplegia and chloral hydrate sedation in one session before 26 months of age, we recorded PERGs to checkerboard reversal using five check sizes. Participants were followed with clinical and psychometric testing until 5 years of age. PERGs were analysed using automated robust statistics based on magnitude-squared coherence and bootstrapping optimized to objectively quantify PERG recovery in the challenging recordings encountered in young patients. PERG measures in the fixating or better-seeing eyes were compared with visual outcome data.

RESULTS

PERG recording was complete to at least three check sizes in all eyes and to all five sizes in 79%. Probability of recording a PERG that is significantly different from noise varied with check size from 73% for the largest checks to 30% for the smallest checks (p = 0.002); smaller waveforms were associated with earlier implicit times. The presence of significant PERGs in infancy is associated with better visual outcomes; the strongest association with visual outcome was for the threshold check size with a significant N95 component (ρ = 0.398, p = 0.02).

CONCLUSIONS

Automated statistically robust signal-processing techniques reliably and objectively detect PERGs in young children with ONH and show that congenital deficits of retinal ganglion cells are associated with diminished or non-detectable PERGs. The later negativity, N95, was the best indicator of visual prognosis and was most useful to identify those with good visual outcomes (≤0.4 LogMAR). Although PERGs reflect function of the inner layers of the central retina, they lack the specificity required to determine prognosis reliably in individual cases.

From evoked potentials to cortical currents: Resolving V1 and V2 components using retinotopy constrained source estimation without fMRI

Despite evoked potentials' (EP) ubiquity in research and clinical medicine, insights are limited to gross brain dynamics as it remains challenging to map surface potentials to their sources in specific cortical regions. Multiple sources cancellation due to cortical folding and cross-talk obscures close sources, e.g. between visual areas V1 and V2. Recently retinotopic functional magnetic resonance imaging (fMRI) responses were used to constrain source locations to assist separating close sources and to determine cortical current generators. However, an fMRI is largely infeasible for routine EP investigation. We developed a novel method that replaces the fMRI derived retinotopic layout (RL) by an approach where the retinotopy and current estimates are generated from EEG or MEG signals and a standard clinical T1-weighted anatomical MRI. Using the EEG-RL, sources were localized to within 2 mm of the fMRI-RL constrained localized sources. The EEG-RL also produced V1 and V2 current waveforms that closely matched the fMRI-RL's (n = 2) r(1,198)  = 0.99, P < 0.0001. Applying the method to subjects without fMRI (n = 4) demonstrates it generates waveforms that agree closely with the literature. Our advance allows investigators with their current EEG or MEG systems to create a library of brain models tuned to individual subjects' cortical folding in retinotopic maps, and should be applicable to auditory and somatosensory maps. The novel method developed expands EP's ability to study specific brain areas, revitalizing this well-worn technique. Hum Brain Mapp 37:1696-1709, 2016. © 2016 Wiley Periodicals, Inc.

Estimation of cortical magnification from positional error in normally sighted and amblyopic subjects

We describe a method for deriving the linear cortical magnification factor from positional error across the visual field. We compared magnification obtained from this method between normally sighted individuals and amblyopic individuals, who receive atypical visual input during development. The cortical magnification factor was derived for each subject from positional error at 32 locations in the visual field, using an established model of conformal mapping between retinal and cortical coordinates. Magnification of the normally sighted group matched estimates from previous physiological and neuroimaging studies in humans, confirming the validity of the approach. The estimate of magnification for the amblyopic group was significantly lower than the normal group: by 4.4 mm deg(-1) at 1° eccentricity, assuming a constant scaling factor for both groups. These estimates, if correct, suggest a role for early visual experience in establishing retinotopic mapping in cortex. We discuss the implications of altered cortical magnification for cortical size, and consider other neural changes that may account for the amblyopic results.

Infants' visual system nonretinotopically integrates color signals along a motion trajectory

Whereas early visual processing has been considered primarily retinotopic, recent studies have revealed significant contributions of nonretinotopic processing to the human perception of fundamental visual features. For adult vision, it has been shown that information about color, shape, and size is nonretinotipically integrated along the motion trajectory, which could bring about clear and unblurred perception of a moving object. Since this nonretinotopic processing presumably includes tight and elaborated cooperation among functional cortical modules for different visual attributes, how this processing matures in the course of brain development is an important unexplored question. Here we show that the nonretinotopic integration of color signals is fully developed in infants at five months of age. Using preferential looking, we found significantly better temporal segregation of colors for moving patterns than for flickering patterns, even when the retinal color alternation rate was the same. This effect could be ascribed to the integration of color signals along a motion trajectory. Furthermore, the infants' color segmentation performance was comparable to that of human adults. Given that both the motion processing and color vision of 5-month-old infants are still under development, our findings suggest that nonretinotopic color processing develops concurrently with basic color and motion processing. Our findings not only support the notion of an early presence of cross-modal interactions in the brain, but also indicate the early development of a purposive cross-module interaction for elegant visual computation.

Visual stimulus eccentricity affects human gamma peak frequency

The peak frequency of neuronal gamma-band synchronization has received much attention in recent years. Gamma peak frequency shifts to higher frequency values for higher contrast, faster moving, and attended stimuli. In monkey V1, gamma peak frequency for a drifting grating is higher for a parafoveal as compared to an eccentric stimulus (Lima et al., 2010). This effect might be due to the cortical magnification factor: the higher cortical magnification for parafoveal stimuli increases the velocity with which the cortical representations of the moving grating stripes move across the cortical surface. Since faster moving stimuli lead to higher gamma frequency, a faster moving cortical representation might do the same. This explanation predicts that the eccentricity effect on gamma peak frequency is absent for stationary stimuli. To test this, we investigated the effect of eccentricity on gamma peak frequency by recording magnetoencephalography in human subjects while they viewed moving or stationary gratings. We found that both the moving and the stationary stimuli induced lower peak frequencies for larger eccentricities, arguing against an explanation based on the cortical magnification factor. We further investigated whether this eccentricity effect was explained by differences in the size or the spatial frequency of the expected cortical activation. Neither of those explained the eccentricity effect. We propose that the different stimulus and top-down factors leading to higher gamma peak frequency all result in higher stimulus salience, that salience is translated into gamma peak frequency, and that gamma peak frequency might subserve the preferential processing of neuronal activity induced by salient stimuli.

Visual spatial attention enhances the amplitude of positive and negative fMRI responses to visual stimulation in an eccentricity-dependent manner

Endogenous visual spatial attention improves perception and enhances neural responses to visual stimuli at attended locations. Although many aspects of visual processing differ significantly between central and peripheral vision, little is known regarding the neural substrates of the eccentricity dependence of spatial attention effects. We measured amplitudes of positive and negative fMRI responses to visual stimuli as a function of eccentricity in a large number of topographically-organized cortical areas. Responses to each stimulus were obtained when the stimulus was attended and when spatial attention was directed to a stimulus in the opposite visual hemifield. Attending to the stimulus increased both positive and negative response amplitudes in all cortical areas we studied: V1, V2, V3, hV4, VO1, LO1, LO2, V3A/B, IPS0, TO1, and TO2. However, the eccentricity dependence of these effects differed considerably across cortical areas. In early visual, ventral, and lateral occipital cortex, attentional enhancement of positive responses was greater for central compared to peripheral eccentricities. The opposite pattern was observed in dorsal stream areas IPS0 and putative MT homolog TO1, where attentional enhancement of positive responses was greater in the periphery. Both the magnitude and the eccentricity dependence of attentional modulation of negative fMRI responses closely mirrored that of positive responses across cortical areas.

Predicted selective increase of cortical magnification due to cortical folding

The cortical magnification matrix M is introduced founded on a notion similar to that of the scalar cortical magnification factor M. Unlike M, this matrix is suitable to describe anisotropy in cortical magnification, which is of particular interest in the highly gyrified human cerebral cortex. The advantage of our tensor method over other surface-based 3D methods to explore cortical morphometry is that M expresses cortical quantities in the corresponding sensory space. It allows us to investigate the spatial relation between sensory function and anatomical structure. To this end, we consider the calcarine sulcus (CS) as an anatomical landmark for the primary visual cortex (V1). We found that a stereotypically formed 3D model of V1 compared to a flat model explains an excess of cortical tissue for the representation of visual information coming from the horizon of the visual field. This suggests that the intrinsic geometry of this sulcus is adapted to encephalize a particular function along the horizon. Since visual functions are assumed to be M-scaled, cortical folding can serve as an anatomical basis for increased functionality on the horizon similar to a retinal specialization known as visual streak, which is found in animals with lower encephalization. Thus, the gain of surface area by cortical folding links anatomical structure to cortical function in a previously unrecognized way, which may guide sulci development.

Functional heterogeneity in posterior parietal cortex across attention and episodic memory retrieval

While attention is critical for event memory, debate has arisen regarding the extent to which posterior parietal cortex (PPC) activation during episodic retrieval reflects engagement of PPC-mediated mechanisms of attention. Here, we directly examined the relationship between attention and memory, within and across subjects, using functional magnetic resonance imaging attention-mapping and episodic retrieval paradigms. During retrieval, 4 functionally dissociable PPC regions were identified. Specifically, 2 PPC regions positively tracked retrieval outcomes: lateral intraparietal sulcus (latIPS) indexed graded item memory strength, whereas angular gyrus (AnG) tracked recollection. By contrast, 2 other PPC regions demonstrated nonmonotonic relationships with retrieval: superior parietal lobule (SPL) tracked retrieval reaction time, consistent with a graded engagement of top-down attention, whereas temporoparietal junction displayed a complex pattern of below-baseline retrieval activity, perhaps reflecting disengagement of bottom-up attention. Analyses of retrieval effects in PPC topographic spatial attention maps (IPS0-IPS5; SPL1) revealed that IPS5 and SPL1 exhibited a nonmonotonic relationship with retrieval outcomes resembling that in the SPL region, further suggesting that SPL activation during retrieval reflects top-down attention. While demands on PPC attention mechanisms vary during retrieval attempts, the present functional parcellation of PPC indicates that 2 additional mechanisms (mediated by latIPS and AnG) positively track retrieval outcomes.

Peripheral vision and pattern recognition: a review

We summarize the various strands of research on peripheral vision and relate them to theories of form perception. After a historical overview, we describe quantifications of the cortical magnification hypothesis, including an extension of Schwartz's cortical mapping function. The merits of this concept are considered across a wide range of psychophysical tasks, followed by a discussion of its limitations and the need for non-spatial scaling. We also review the eccentricity dependence of other low-level functions including reaction time, temporal resolution, and spatial summation, as well as perimetric methods. A central topic is then the recognition of characters in peripheral vision, both at low and high levels of contrast, and the impact of surrounding contours known as crowding. We demonstrate how Bouma's law, specifying the critical distance for the onset of crowding, can be stated in terms of the retinocortical mapping. The recognition of more complex stimuli, like textures, faces, and scenes, reveals a substantial impact of mid-level vision and cognitive factors. We further consider eccentricity-dependent limitations of learning, both at the level of perceptual learning and pattern category learning. Generic limitations of extrafoveal vision are observed for the latter in categorization tasks involving multiple stimulus classes. Finally, models of peripheral form vision are discussed. We report that peripheral vision is limited with regard to pattern categorization by a distinctly lower representational complexity and processing speed. Taken together, the limitations of cognitive processing in peripheral vision appear to be as significant as those imposed on low-level functions and by way of crowding.

Size matters: MEG empirical and simulation study on source localization of the earliest visual activity in the occipital cortex

While the relationship between sensory stimulation and tasks and the size of the cortical activations is generally unknown, the visual modality offers a unique possibility of an experimental manipulation of stimulus size-related increases of the spatial extent of cortical activation even during the earliest activity in the retinotopically organized primary visual cortex. We used magnetoecephalography (MEG), visual stimuli of increasing size, and numerical simulations on realistic cortical surfaces to explore the effects of increasing spatial extent of the activated cortical sources on the neuromagnetic fields, location estimation biases, and source resolution. Source localization was performed assuming multiple dipoles in a sphere model using an efficient, automatically restarted multi-start simplex minimizer within the Calibrated Start Spatio-Temporal (CSST) algorithm. We found size-related effects on amplitude and latencies and differences in relative locations of the earliest occipital sources evoked by stimuli of increasing size presented at the same eccentricity. This finding was confirmed by single patch simulations. Additionally, simulations of multiple extended sources demonstrated size-related increase in limits in source resolution for bilaterally simulated sources, biases in location estimates for a given separation of sources, and limits in source resolution due to source multiplicity within a hemisphere.

Effects of attentional load on early visual processing depend on stimulus timing

A growing number of studies suggest that early visual processing is not only affected by low-level perceptual attributes but also by higher order cognitive factors such as attention or emotion. Using high-density electroencephalography, we recently demonstrated that attentional load of a task at fixation reduces the response of primary visual cortex to irrelevant peripheral stimuli, as indexed by the C1 component. In the latter study, peripheral stimuli were always presented during intervals without task-relevant stimuli. Here, we use a similar paradigm but present central task stimuli and irrelevant peripheral stimuli simultaneously while keeping all other stimulus characteristics constant. Results show that rather than to suppress responses to peripheral stimulation, high attentional load elicits higher C1 amplitudes under these conditions. These findings suggest that stimulus timing can profoundly alter the effects of attentional load on the earliest stages of processing in human visual cortex.

Spatial attention improves reliability of fMRI retinotopic mapping signals in occipital and parietal cortex

Spatial attention improves visual perception and increases the amplitude of neural responses in visual cortex. In addition, spatial attention tasks and fMRI have been used to discover topographic visual field representations in regions outside visual cortex. We therefore hypothesized that requiring subjects to attend to a retinotopic mapping stimulus would facilitate the characterization of visual field representations in a number of cortical areas. In our study, subjects attended either a central fixation point or a wedge-shaped stimulus that rotated about the fixation point. Response reliability was assessed by computing coherence between the fMRI time series and a sinusoid with the same frequency as the rotating wedge stimulus. When subjects attended to the rotating wedge instead of ignoring it, the reliability of retinotopic mapping signals increased by approximately 50% in early visual cortical areas (V1, V2, V3, V3A/B, V4) and ventral occipital cortex (VO1) and by approximately 75% in lateral occipital (LO1, LO2) and posterior parietal (IPS0, IPS1, IPS2) cortical areas. Additionally, one 5-min run of retinotopic mapping in the attention-to-wedge condition produced responses as reliable as the average of three to five (early visual cortex) or more than five (lateral occipital, ventral occipital, and posterior parietal cortex) attention-to-fixation runs. These results demonstrate that allocating attention to the retinotopic mapping stimulus substantially reduces the amount of scanning time needed to determine the visual field representations in occipital and parietal topographic cortical areas. Attention significantly increased response reliability in every cortical area we examined and may therefore be a general mechanism for improving the fidelity of neural representations of sensory stimuli at multiple levels of the cortical processing hierarchy.

Attentional inhibition mediates inattentional blindness

Salient stimuli presented at unattended locations are not always perceived, a phenomenon termed inattentional blindness. We hypothesized that inattentional blindness may be mediated by attentional inhibition. It has been shown that attentional inhibition effects are maximal near an attended location. If our hypothesis is correct, inattentional blindness effects should similarly be maximal near an attended location. During central fixation, participants viewed rapidly presented colored digits at a peripheral location. An unexpected black circle (the critical stimulus) was concurrently presented. Participants were instructed to maintain central fixation and name each color/digit, requiring focused attention to that location. For each participant, the critical stimulus was presented either near to or far from the attended location (at the same eccentricity). In support of our hypothesis, inattentional blindness effects were maximal near the attended location, but only at intermediate task accuracy.

Exploring BOLD changes during spatial attention in non-stimulated visual cortex

Blood oxygen level-dependent (BOLD) responses were measured in parts of primary visual cortex that represented unstimulated visual field regions at different distances from a stimulated central target location. The composition of the visual scene varied by the presence or absence of additional peripheral distracter stimuli. Bottom-up effects were assessed by comparing peripheral activity during central stimulation vs. no stimulation. Top-down effects were assessed by comparing active vs. passive conditions. In passive conditions subjects simply watched the central letter stimuli and in active conditions they had to report occurrence of pre-defined targets in a rapid serial letter stream. Onset of the central letter stream enhanced activity in V1 representations of the stimulated region. Within representations of the periphery activation decreased and finally turned into deactivation with increasing distance from the stimulated location. This pattern was most pronounced in the active conditions and during the presence of peripheral stimuli. Active search for a target did not lead to additional enhancement at areas representing the attentional focus but to a stronger deactivation in the vicinity. Suppressed neuronal activity was also found in the non distracter condition suggesting a top-down attention driven effect. Our observations suggest that BOLD signal decreases in primary visual cortex are modulated by bottom-up sensory-driven factors such as the presence of distracters in the visual field as well as by top-down attentional processes.

Cholinergic enhancement reduces spatial spread of visual responses in human early visual cortex

Animal studies have shown that acetylcholine decreases excitatory receptive field size and spread of excitation in early visual cortex. These effects are thought to be due to facilitation of thalamocortical synaptic transmission and/or suppression of intracortical connections. We have used functional magnetic resonance imaging (fMRI) to measure the spatial spread of responses to visual stimulation in human early visual cortex. The cholinesterase inhibitor donepezil was administered to normal healthy human subjects to increase synaptic levels of acetylcholine in the brain. Cholinergic enhancement with donepezil decreased the spatial spread of excitatory fMRI responses in visual cortex, consistent with a role of acetylcholine in reducing excitatory receptive field size of cortical neurons. Donepezil also reduced response amplitude in visual cortex, but the cholinergic effects on spatial spread were not a direct result of reduced amplitude. These findings demonstrate that acetylcholine regulates spatial integration in human visual cortex.

Two-Dimensional Mapping of the Central and Parafoveal Visual Field to Human Visual Cortex

Primate visual cortex contains a set of maps of visual space. These maps are fundamental to early visual processing, yet their form is not fully understood in humans. This is especially true for the central and most important part of the visual field—the fovea. We used functional magnetic resonance imaging (fMRI) to measure the mapping geometry of human V1 and V2 down to 0.5° of eccentricity. By applying automated atlas fitting procedures to parametrize and average retinotopic measurements of eight brains, we provide a reference standard for the two-dimensional geometry of human early visual cortex of unprecedented precision and analyze this high-quality mean dataset with respect to the 2-dimensional cortical magnification morphometry. The analysis indicates that 1) area V1 has meridional isotropy in areal projection: equal areas of visual space are mapped to equal areas of cortex at any given eccentricity. 2) V1 has a systematic pattern of local anisotropies: cortical magnification varies between isopolar and isoeccentricity lines, and 3) the shape of V1 deviates systematically from the complex-log model, the fit of which is particularly poor close to the fovea. We therefore propose that human V1 be fitted by models based on an equal-area principle of its two-dimensional magnification. 4) V2 is elongated by a factor of 2 in eccentricity direction relative to V1 and has significantly more local anisotropy. We propose that V2 has systematic intrinsic curvature, but V1 is intrinsically flat.

Localizing Sites of Activation in Primary Visual Cortex Using Visual-Evoked Potentials and Functional Magnetic Resonance Imaging

This study compared retinotopic map identification in primary visual cortex (V1) using: (i) functional magnetic resonance imaging (fMRI) and (ii) visual evoked potentials (VEPs) coupled with dipole source localization (DSL). A multielectrode array was used to record VEPs while subjects viewed a flickering dartboard pattern modulated by a 16-bit m-sequence. The stimulus preferentially activates V1. Using a common time function DSL algorithm, the primary source of each stimulus patch was found independent of the fMRI. The VEP/DSL and fMRI localization data for each subject were aligned by a rigid translation and rotation. The average distance between VEP and corresponding fMRI sources was 10.8 mm +/- 3.8 mm. To assess the significance of the results, fMRI and DSL solutions were scrambled so the comparisons were no longer for corresponding patches. The average distance between the noncorresponding data sets was 17.2 mm for 50 million scrambles. The probability of the scrambled data yielding a better fit than the real data was p < 10(-7). The combination of multielectrode recording, multiinput visual stimulation and common time function DSL analysis can provide a detailed retinotopic map of visual cortex that has high correspondence with independent fMRI localization analysis on the same subject.

Topographic change in ERP due to discrimination of geometric figures in the peripheral visual field

The present study examined the neurophysiological aspects of discrimination of geometric figures with different eccentricities. In the behavioral experiment, a circle and 3 geometric polygons (square, hexagon, or octagon) were presented for 300 ms at quadrant locations. The eccentricity was changed from 2 degrees to 16 degrees at 2 degrees intervals. The participants (10 adults) were instructed to fixate the center of the CRT and find the circle among the polygons. In the neurophysiological experiment, 4 identical stimuli (squares, hexagons, octagons, or circles) were presented at an eccentricity of 4 degrees, 8 degrees, or 12 degrees. Discrimination performance in the periphery declined when the angularity of the polygons increased. ERP components at 80 ms (P1 with posterior positivity) and 140 ms (N1 with posterior negativity) were observed regardless of the eccentricity or shape of the stimuli. N1 lasted longer as eccentricity increased. P2 with posterior positivity at 200 ms became blurred in the periphery, except for squares. The topography of N1 and P2 changed, depending on the retinal stimulus location; it was influenced by the angularity of the geometric figures in the peripheral visual field. Cerebral processing concerned with N1 and P2 might play an important role in the perception and recognition of visual objects in the peripheral visual field.

Spatial resolution for feature binding is impaired in peripheral and amblyopic vision

We measured spatial resolution for discriminating targets that differed from nearby distractors in either color or orientation or their conjunction. In the fovea of normal human observers, whenever both attributes are big enough to be individually visible, their conjunction is also visible. In the periphery, the two attributes may be visible, but their conjunction may be invisible. We found a similar impairment in resolving conjunctions for the fovea of deprived eyes of humans with abnormal visual development (amblyopia). These results are quantitatively explained by a model of primary visual cortex (V1) in which orientation and color maps are imperfectly co-registered topographically. Our results in persons with amblyopia indicate that the ability of the fovea to compensate for this poor co-registration is consolidated by visual experience during postnatal development.

The time course of response inhibition in masked priming

In two experiments, we studied the temporal dynamics of the response time effects of masked visual prime stimuli, as a function of stimulus eccentricity and size. Experiment 1 factorially varied prime-target congruency, eccentricity, and mask-target stimulus onset asynchrony. Early facilitative and late inhibitory effects of congruency were observed at all eccentricities, with temporal dynamics modulated by eccentricity. To test whether this dependence on eccentricity is due to cortical magnification, Experiment 2 varied stimulus size as well. Response inhibition time courses were influenced by size and eccentricity jointly, with no discernible difference when stimuli were matched for cortical magnification. Analysis of the individual time course data revealed that the timescale of inhibition changes with the strength of the cortical representation of the prime stimulus. This imposes constraints on possible models.

Multifocal fMRI mapping of visual cortical areas

The multifocal mapping of electroretinograms and visual evoked potentials has established an important role in both basic research and in diagnostic procedures. We have developed a multifocal mapping method for fMRI, which allows detailed analysis of multiple local visual field representations in the cortex with excellent spatial resolution. Visual field was divided into 60 regions in a dartboard configuration, scaled according to the human magnification factor. Within blocks of 7 s, half of the regions were stimulated with checkerboard patterns contrast reversing at 8 reversals per second, while the other half remained inactive at uniform luminance. The subset of active regions changed with each 7-s block, according to an orthogonal design. Functional MRI was done with a 3-T GE Signa and analyzed with SPM2. A general linear model was fitted producing activation maps for each of the 60 regions, and local signal changes were quantified from V1. These activation maps were next assigned to 3D surface models of the cortical sheet, and then unfolded, using the Brain à la Carte software package. Phase-encoded retinotopic analysis of conventional design served as qualitative comparison data. With multifocal fMRI, all regions were mapped with good signal-to-noise ratio in V1, and subsets of regions showed activation in V2 and V3. This method allows rapid and direct exploration of multiple local visual responses, and is thus able to give complementary information to phase encoded mapping of retinotopic areas.

Normative ranges and specificity of the multifocal VEP

PURPOSE

To describe a normative database for the multifocal VEP (mfVEP) and to evaluate specificity for a range of cluster criteria.

METHODS

One hundred persons (62 females and 38 males) with normal visual fields and ranging in age from 21.6 to 92.4 years participated in this study. Self-reported race in 80 of these 100 persons was 'White or Caucasian,' eight were 'Black or African-American,' eight were 'Asian,' and four were 'Hispanic or Latino.' Pattern-reversal mfVEPs were obtained using a dartboard stimulus pattern in VERIS and two 8-min runs per eye were averaged. A bootstrap technique was used to estimate the normal range of mfVEP response signal-to-noise ratio (SNR) and inter-ocular amplitude ratio at each location. Specificity (1 - false alarm rate) was evaluated for a range of cluster criteria, whereby the number and probability level of the points defining a cluster were varied.

RESULTS

There was no overall effect of age on SNR (r2 = 0.16, p = 0.22) nor was the interaction between age and location significant (F = 0.83, p = 0.82, ANOVA). The location with the largest age effect had an r2 of only 0.13. There was a small but significant effect of sex (t = 2.1, p = 0.04) such that SNR was slightly (11%) larger in females than males, but there was no significant interaction between sex and age (t = 0.82, p = 0.41). There was a slight trend toward higher SNR in the Asian group and lower SNR in the African-American group, but the overall effect of race was not significant (F = 1.99, p = 0.12). Specificity depended on the number and probability level of the points defining a cluster. Specificity did not vary by age group in a simple monotonic manner. False positive rates were slightly higher in females than males, and slightly higher in the African-American group as compared with the Asian group.

CONCLUSIONS

Excellent specificity can be achieved for the mfVEP by using particular cluster criteria for monocular and inter-ocular tests. The effects of age, sex, and race were all very small and only the effect of sex was statistically significant. This normative database can be used for analyses of mfVEP results from individual patients with little risk that demographic factors such as age and sex will confound diagnostic accuracy.

Visual Mental Imagery Induces Retinotopically Organized Activation of Early Visual Areas

There is a long-standing debate as to whether visual mental imagery relies entirely on symbolic (language-like) representations or also relies on depictive (picture-like) representations. We sought to discover whether visual mental imagery could evoke cortical activity with precise visual field topography (retinotopy). Participants received three conditions: the perception condition consisted of a standard retinotopic mapping procedure, where two flickering checkerboard wedges rotated around a central fixation point. The imagery and attention conditions consisted of the same stimulus, but only the outer arcs of the wedges were visible. During imagery, participants mentally reproduced the stimulus wedges, using the stimulus arcs as a guide. The attention condition required either distributed attention or focused attention to where the stimulus wedges would have been. Event-related analysis revealed that the imagery (greater than either form of attention) retinotopic maps were similar to the perception maps. Moreover, blocked analysis revealed similar perception and imagery effects in human motion processing region MT+. These results support the depictive view of visual mental imagery.

Retinotopic organization and functional subdivisions of the human lateral geniculate nucleus: a high-resolution functional magnetic resonance imaging study

Functional magnetic resonance imaging (fMRI) has provided intriguing insights into the topography and functional organization of visual cortical areas in the human brain. However, little is known about the functional anatomy of subcortical nuclei. Here, we used high-resolution fMRI (1.5 x 1.5 x 2 mm3) at 3 tesla to investigate the retinotopic organization of the human lateral geniculate nucleus (LGN). The central 15 degrees of the visual field were mapped using periodic flickering checkerboard stimuli that evoked a traveling wave of activity. The contralateral visual hemifield was represented with the lower field in the medial-superior portion and the upper field in the lateral-inferior portion of each LGN. The horizontal meridian was significantly overrepresented relative to the vertical meridian. The fovea was represented in posterior and superior portions, with increasing eccentricities represented more anteriorly. The magnification of the fovea relative to the periphery was similar to that described for human primary visual cortex. The magnocellular regions of the LGN were distinguished based on their sensitivity to low stimulus contrast and tended to be located in its inferior and medial portions. Our results demonstrate striking similarities in the topographic organization of the macaque and human LGN and support accounts of a constant magnification from the retina through the cortex in both species.

Multifocal stimulation techniques in ophthalmology -- Current knowledge and perspectives

The purpose of this paper is to give a brief overview on the application of multifocal stimulation techniques in ophthalmology. The use of m-sequences as a stimulus sequence allows a high-resolution topographic mapping of sensory function. Outer retinal dysfunction can readily be detected with the multifocal ERG (mfERG). When the inner retinal contribution to the mfERG response is enhanced through adapting the stimulation sequence, the sensitivity of the mfERG to detect retinal dysfunction in glaucoma can be increased. Testing of the entire visual pathway with multifocal cortical evoked visual potentials is also possible and recent studies have focused on reducing interindividual variability. The use of m-sequence stimulation in magnetic encephalography offers new ways to study visual processing without the need to apply electrodes.

Visual field anisotropy revealed by perceptual filling-in

Four experiments were performed to investigate how the time required for perceptual filling-in varies with the position of the target in the visual field. Conventional studies have revealed that filling-in is facilitated by a target with greater eccentricity, while no systematic studies have examined the effect of polar angle. Experiment 1 examined the effect of polar angle when the target and surround differed in luminance. Filling-in was facilitated as the target position changed from the horizontal to the vertical meridian. This dependency was more prominent in the upper field than in the lower, although no asymmetry was found between the left and right visual fields. These features were observed in both monocular and binocular viewing. These results were replicated in a modified stimulus configuration, in which the surround was a circular region concentric with the target (Experiment 2). Moreover, it was confirmed that the asymmetry was not due to fluctuation in the retinal image (i.e., eye movement) (Experiment 3). Finally, Experiment 4 examined whether this anisotropy was observed when two differently oriented gratings were presented in the target and surround regions. Again, filling-in was facilitated for a target close to the vertical meridian, irrespective of the relationship between the target and surround orientations. The underlying mechanism of this anisotropy is discussed from the viewpoints of cortical magnification and neural connections in the visual cortex.

Retinotopic mapping reveals extrastriate cortical basis of homonymous quadrantanopia

It has been conventionally assumed that cortically based quadrantic visual field deficits (homonymous quadrantanopias) are caused by lesions in striate cortex (V1), extending precisely to the horizontal meridian representation. A more recent model, supported by anatomic MRI evidence, consists of an exclusively extrastriate cortical basis (e.g. V2, V3, VP, V4v). Employing fMRI, we sought to distinguish between these models through retinotopic mapping of a patient with an upper right homonymous quadrantanopia. As expected, maps of the lower right quadrant and left hemifield were normal. The map corresponding to the impaired upper right quadrant was normal in V1 and V2, with little or no activity in VP and V4v. These results provide functional evidence that extrastriate cortical lesions can elicit homonymous quadrantanopias.

Efficient acquisition of human retinotopic maps

A bifield stimulation method for rapidly obtaining retinotopic maps in human occipital cortex using functional MRI was compared to conventional unifield stimulation. While maintaining central fixation, each participant viewed the conventional display, consisting of a single rotating checkerboard wedge and, in a separate run, the bifield display, consisting of two symmetrically placed rotating checkerboard wedges (a "propeller" configuration). Both stimulus configurations used wedges with 30 degree polar angle width, 6.8 degrees visual angle extension from fixation, and 8.3 Hz contrast polarity reversal rate. Retinotopic maps in each condition were projected onto a distortion corrected computationally flattened cortical surface representation obtained from a high-resolution structural MRI. An automated procedure to localize borders between early visual areas revealed, as expected, that map precision increased with duration of data acquisition for both conditions. Bifield stimulation required 40% less time to yield maps with similar precision to those obtained using conventional unifield stimulation.

Darkness beyond the light: attentional inhibition surrounding the classic spotlight

The aim of the present investigation was to determine the nature and spatial distribution of selective visual attention. Using cortical source localization of ERP data corresponding to 60 task-irrelevant stimuli across the visual field, we assessed attention effects on visual processing. Consistent with previous findings, visual processing was enhanced at the attended spatial location. In addition, this facilitation of processing extended from the attended location to the point of fixation resulting in a region of facilitation. Furthermore, a large region of inhibition was found surrounding this region of facilitation. The latter result is inconsistent with a simple facilitative spotlight model of attention and indicates that attention effects can be both facilitatory and inhibitory.

Interactions between thalamic and cortical rhythms during semantic memory recall in human

Human scalp electroencephalographic rhythms, indicative of cortical population synchrony, have long been posited to reflect cognitive processing. Although numerous studies employing simultaneous thalamic and cortical electrode recording in nonhuman animals have explored the role of the thalamus in the modulation of cortical rhythms, direct evidence for thalamocortical modulation in human has not, to our knowledge, been obtained. We simultaneously recorded from thalamic and scalp electrodes in one human during performance of a cognitive task and found a spatially widespread, phase-locked, low-frequency rhythm (7-8 Hz) power decrease at thalamus and scalp during semantic memory recall. This low-frequency rhythm power decrease was followed by a spatially specific, phase-locked, fast-rhythm (21-34 Hz) power increase at thalamus and occipital scalp. Such a pattern of thalamocortical activity reflects a plausible neural mechanism underlying semantic memory recall that may underlie other cognitive processes as well.

Large-scale cortical displacement of a human retinotopic map

The aim of the current study was to determine the retinotopic organization of a patient with congenital cortical dysgenesis and normal visual function. Using functional magnetic resonance imaging (fMRI), detailed retinotopic maps corresponding to the four visual field quadrants were projected onto cortical surfaces. Similar to control subjects, the upper right visual field mapped onto ventral left hemisphere and was retinotopically organized. The lower right visual field's cortical representation was also retinotopically organized, yet was displaced many centimeters anteriomedially. Moreover, the entire left visual field was represented in non-retinotopically organized islands in both hemispheres. These results indicate retinotopic maps can shift in both location and topography illustrating cortical reorganization presumably due to either cortical dysgenesis or functional displacement. NeuroReport