Effects of sustained, voluntary attention on amplitude and latency of steady-state visual evoked potential: a costs and benefits analysis

Effect of Cognitive Mental Load on Attended and Nonattended Visual Stimuli

SIGNIFICANCE

In the real word, visual tasks may be concurrent with other activity that imposes mental load. Although the brain's capacity to process information is limited, attention can improve visual performance by selectively allocating processing resources. Therefore, measuring visual performance under such circumstances can reflect patients' vision more accurately.

PURPOSE

The aim of this study was to evaluate the effect of nonvisual task-induced mental load on visual performance at both attended and unattended locations in stimulus-driven captured attention.

METHODS

Visual function was measured with an orientation discrimination task for Gabor patches with contrasts of 10, 15, 30, 50, and 80%. Three attentional conditions (valid-cue, invalid-cue, and neutral-cue) were randomly interleaved within runs. To modulate mental load, the visual task was performed either with or without a simultaneous auditory n-back task (two-back for maximum mental load and zero-back to control for the effect of having to perform a simultaneous task).

RESULTS

Our result showed that the effect of mental load on correct responses was significant ( P = .02). Correct responses decreased significantly during the two-back task when compared with the baseline condition ( P = .03), but there was no significant difference between baseline and zero-back conditions ( P = .06). The effect of attention and spatial frequencies on the percentage of correct responses was significant ( P < .001). There was no significant interaction between mental load and spatial frequency, contrast level, or attention ( P > .05).

CONCLUSIONS

Mental load had a similar decreasing effect on attended and unattended visual stimuli. This may be due to a generalized effect on processing resources upstream to where spatial attention is allocated.

Long-term effects of methamphetamine abuse on visual evoked potentials

PURPOSE

To compare visual evoked potential (VEP) components in normal individuals and those with long-term methamphetamine and crystal methamphetamine use.

METHODS

In this study, monocular pattern-reversal VEPs were recorded in 40 methamphetamine and crystal methamphetamine users and 38 normal individuals. Visual stimuli were high-contrast (99%) checkerboard patterns at 15 and 60 min of arc with a reversal rate of 1.53 reversals per second.

RESULTS

A significant difference was seen between the two groups for the P100 peak time for the 60 min of arc checks (p = 0.002, d = 0.75, 4.61% higher peak time in the addicted group) and the 15 min of arc checks (p = 0.004, d = 0.73, 4.78% higher peak time in the addicted group). However, other VEP components were not significantly different between the two groups.

CONCLUSIONS

The higher P100 peak time at both 15 and 60 min of arc in methamphetamine-dependent users reveals that VEPs are highly sensitive for the diagnosis of retinal and visual pathway lesions.

Optimization of Visual Stimulus Sequence in a Brain-Computer Interface Based on Code Modulated Visual Evoked Potentials

Brain-computer interfaces based on code-modulated visual evoked potentials provide high information transfer rates, which make them promising alternative communication tools. Circular shifts of a binary sequence are used as the flickering pattern of several visual stimuli, where the minimum correlation between them is critical for recognizing the target by analyzing the EEG signal. Implemented sequences have been borrowed from communication theory without considering visual system physiology and related ergonomics. Here, an approach is proposed to design optimum stimulus sequences considering physiological factors, and their superior performance was demonstrated for a 6-target c-VEP BCI system. This was achieved by defining a time-factor index on the frequency response of the sequence, while the autocorrelation index ensured a low correlation between circular shifts. A modified version of the non-dominated sorting genetic algorithm II (NSGAII) multi-objective optimization technique was implemented to find, for the first time, 63-bit sequences with simultaneously optimized autocorrelation and time-factor indexes. The selected optimum sequences for general (TFO) and 6-target (6TO) BCI systems, were then compared with m-sequence by conducting experiments on 16 participants. Friedman tests showed a significant difference in perceived eye irritation between TFO and m-sequence (p = 0.024). Generalized estimating equations (GEE) statistical test showed significantly higher accuracy for 6TO compared to m-sequence (p = 0.006). Evaluation of EEG responses showed enhanced SNR for the new sequences compared to m-sequence, confirming the proposed approach for optimizing the stimulus sequence. Incorporating physiological factors to select sequence(s) used for c-VEP BCI systems improves their performance and applicability.

Dynamics of Contrast Decrement and Increment Responses in Human Visual Cortex

Purpose

The goal of the present experiments was to determine whether electrophysiologic response properties of the ON and OFF visual pathways observed in animal experimental models can be observed in humans.

Methods

Steady-state visual evoked potentials (SSVEPs) were recorded in response to equivalent magnitude contrast increments and decrements presented within a probe-on-pedestal Westheimer sensitization paradigm. The probes were modulated with sawtooth temporal waveforms at a temporal frequency of 3 or 2.73 Hz. SSVEP response waveforms and response spectra for incremental and decremental stimuli were analyzed as a function of stimulus size and visual field location in 67 healthy adult participants.

Results

SSVEPs recorded at the scalp differ between contrast decrements and increments of equal Weber contrast: SSVEP responses were larger in amplitude and shorter in latency for contrast decrements than for contrast increments. Both increment and decrement responses were larger for displays that were scaled for cortical magnification.

Conclusions

In a fashion that parallels results from the early visual system of cats and monkeys, two key properties of ON versus OFF pathways found in single-unit recordings are recapitulated at the population level of activity that can be observed with scalp electrodes, allowing differential assessment of ON and OFF pathway activity in human.

Translational Relevance

As data from preclinical models of visual pathway dysfunction point to differential damage to subtypes of retinal ganglion cells, this approach may be useful in future work on disease detection and treatment monitoring.

Can VEP-based acuity estimates in one eye be improved by applying knowledge from the other eye?

PURPOSE

It is desirable to make VEP-based acuity estimates match standard subjective acuity numerically, as the latter is familiar to ophthalmologists and optometrists. This is achieved by applying an empirical conversion factor, and previous studies found the resulting values to be within ±1 octave of subjective acuity. This leaves room for improvement. In the present study, we tested for the case of a monocular acuity deficit whether the known difference between subjective and objective acuity in the trusted fellow eye can be used to get a more precise objective estimate in the eye of which the acuity is to be estimated. In other words, we tested whether it would make sense to determine a patient-specific conversion factor.

METHODS

In 19 subjects, we obtained monocular objective and subjective acuity estimates with both eyes. Normal vision and artificially degraded vision were tested. Subjective acuity was taken as the veridical value. We computed the differences between objective and subjective acuity and reasoned that if these were correlated between eyes and acuity levels, the valid information from the trusted healthy eye could be used to improve the precision of the acuity estimate in the other, potentially impaired, eye.

RESULTS

The difference between objective and subjective acuity values was neither correlated significantly between eyes, nor was it correlated significantly between acuity levels.

CONCLUSIONS

Knowledge about the discrepancy between objective and subjective acuity values in one eye does not help improving the accuracy of acuity estimates in the other eye. The lack of a significant correlation between eyes even at the same acuity level suggests that a major part of the discrepancies between subjective acuity and VEP-based acuity is not the result of factors that would equally apply to both eyes, such as cortical morphology.

Steady-State Visually Evoked Potentials Elicited by Multifrequency Pattern-Reversal Stimulation

Purpose

It has been shown that multifrequency stimulation with multifocal electroretinography can reduce recording time without a loss in signal-to-noise ratio. Here, we studied the applicability of multifrequency stimulations for steady-state visually evoked potential (VEP) recordings.

Methods

Multifrequency VEPs were recorded monocularly from 10 healthy subjects using pattern-reversal stimuli. The reversal frequency varied between 5 and 30 Hz. Pattern-reversal checkerboard stimuli were generated using four square arrays, each containing 100 light-emitting diodes (LEDs), positioned in four quadrants. Each array had a temporal frequency that differed slightly from the nominal frequency. The long duration of the data acquisition ensured that the slightly different stimulus frequencies in the four LED arrays can be resolved and that the responses to the stimulus in each array can be distinguished (e.g., with a frequency resolution: 0.011 Hz at 12 Hz). The best response from the four recording electrode configuration, defined as the recording with the maximal signal-to-noise ratio, was used for further analysis. Algorithmic latencies were calculated from the ratio of phase data and frequencies in a range of 4 and 20 Hz.

Results

Quadrant-VEPs with simultaneous pattern-reversal stimulation yielded a significant dependency on temporal frequency and stimulus location. The frequency range leading to the maximal response amplitude was between 10 and 12 Hz. Response phases decreased approximately linearly, with increasing temporal frequency suggesting a mean algorithmic latency between 112 and 126 ms.

Conclusions

Multifrequency stimulation using LED arrays is an efficient method for recording pattern-reversal VEPs while all stimuli are presented at the same time.

Translational Relevance

Simultaneously recorded VEPs as performed by the multi-frequency method can be used for objective measurements of visual field defects.

Temporal Modulation of Steady-State Visual Evoked Potentials

Electroencephalographic responses to periodic stimulation are termed steady-state visual evoked potentials (SSVEP). Their characteristics in terms of amplitude, frequency and phase are commonly assumed to be stationary. In this work, we tested this assumption in 30 healthy participants submitted to 50 trials of 60 s flicker stimulation at 15 Hz frequency. We showed that the amplitude of the first and second harmonic frequency components of SSVEP signals were in general not stable over time. The power (squared amplitude) of the fundamental component was stationary only in 30% the subjects, while the power at the second harmonic frequency was stationary in 66.7% of the group. The phases of both SSVEP frequency components were more stable over time, but could exhibit small drifts. The observed temporal changes were heterogeneous across the subjects, implying that averaging results over participants should be performed carefully. These results may contribute to improved design and analysis of experiments employing prolonged visual stimulation. Our findings offer a novel characterization of the temporal changes of SSVEP that may help to identify their physiological basis.

Gamma Band Phase Delay in Schizophrenia

BACKGROUND

In 1999, Kwon et al. reported several electroencephalographic gamma band auditory steady-state response (ASSR) abnormalities in schizophrenia, spawning approximately 100 subsequent studies. While many studies replicated the finding of reduced 40-Hz ASSR power in schizophrenia and extended this by showing that 40-Hz phase synchrony (phase-locking factor [PLF]) was also reduced, none attempted to replicate the original phase delay finding of Kwon et al. Accordingly, we measured the 40-Hz ASSR phase-locking angle (PLA) to assess phase delay and examined its differential sensitivity to schizophrenia, relative to power and PLF measures.

METHODS

To obtain ASSRs, electroencephalography data were recorded from 28 patients with schizophrenia and 25 healthy control subjects listening to repeated 40-Hz 500-ms click trains. Evoked power, total power, PLF, and PLA were calculated after Morlet wavelet time-frequency decomposition of single trial data from electrode Fz.

RESULTS

In patients with schizophrenia, 40-Hz PLA was significantly reduced (i.e., phase delayed) (p < .0001) and was unrelated to reductions in their 40-Hz power or PLF. PLA discriminated patients from healthy control subjects with 85% accuracy compared with 67% for power and 65% for PLF.

CONCLUSIONS

Consistent with the original Kwon et al. study, 40-Hz click train-driven gamma oscillations were phase delayed in schizophrenia. Importantly, this phase delay abnormality was substantially larger than the gamma power and phase synchrony abnormalities that have been the focus of prior 40-Hz ASSR studies in schizophrenia. PLA provides a unique neurobiological measure of gamma band abnormalities in schizophrenia, likely reflecting a distinct pathophysiological mechanism from those underlying PLF and power abnormalities.

Visual Development and Neuropsychological Profile in Preterm Children from 6 Months to School Age

The aim of this semilongitudinal study was to investigate the development of central visual pathways in children born preterm but without major neurologic impairments and to establish their cognitive and behavioral profile at school age. Ten children born preterm were assessed at 6 months and at school age, using visual evoked potentials at both time points and cognitive and behavioral tests at school age. We also tested 10 age-matched children born full-term. At 6 months' corrected age, we found no significant differences between preterm and full-term groups for either amplitude or latency of N1 and P1 components. At school age, the preterm group manifested significantly higher N1 amplitudes and tended to show higher P1 amplitudes than the full-term group. We found no significant differences in cognitive and behavioral measures at school age. These results suggest that preterm birth affects visual pathways development, yet the children born preterm did not manifest cognitive problems.

A robust index of lexical representation in the left occipito-temporal cortex as evidenced by EEG responses to fast periodic visual stimulation

Despite decades of research on reading, including the relatively recent contributions of neuroimaging and electrophysiology, identifying selective representations of whole visual words (in contrast to pseudowords) in the human brain remains challenging, in particular without an explicit linguistic task. Here we measured discrimination responses to written words by means of electroencephalography (EEG) during fast periodic visual stimulation. Sequences of pseudofonts, nonwords, or pseudowords were presented through sinusoidal contrast modulation at a periodic 10 Hz frequency rate (F), in which words were interspersed at regular intervals of every fifth item (i.e., F/5, 2 Hz). Participants monitored a central cross color change and had no linguistic task to perform. Within only 3 min of stimulation, a robust discrimination response for words at 2 Hz (and its harmonics, i.e., 4 and 6 Hz) was observed in all conditions, located predominantly over the left occipito-temporal cortex. The magnitude of the response was largest for words embedded in pseudofonts, and larger in nonwords than in pseudowords, showing that list context effects classically reported in behavioral lexical decision tasks are due to visual discrimination rather than decisional processes. Remarkably, the oddball response was significant even for the critical words/pseudowords discrimination condition in every individual participant. A second experiment replicated this words/pseudowords discrimination, and showed that this effect is not accounted for by a higher bigram frequency of words than pseudowords. Without any explicit task, our results highlight the potential of an EEG fast periodic visual stimulation approach for understanding the representation of written language. Its development in the scientific community might be valuable to rapidly and objectively measure sensitivity to word processing in different human populations, including neuropsychological patients with dyslexia and other reading difficulties.

Dissociation of part-based and integrated neural responses to faces by means of electroencephalographic frequency tagging

In order to isolate the repetition suppression effects for each part of a whole-face stimulus, the left and right halves of face stimuli were flickered at different frequency rates (5.88 or 7.14 Hz), changing or not changing identity at every stimulation cycle. The human electrophysiological (electroencephalographic) responses to each face half increased in amplitude when different rather than repeated face half identities were presented at every stimulation cycle. Contrary to the repetition suppression effects for whole faces, which are usually found over the right occipito-temporal cortex, these part-based repetition suppression effects were found on all posterior electrode sites and were unchanged when the two face halves were manipulated by separation, lateral misalignment, or inversion. In contrast, intermodulation components (e.g. 7.14-5.88 = 1.26 Hz) were found mainly over the right occipito-temporal cortex and were significantly reduced following the aforementioned manipulations. In addition, the intermodulation components decreased substantially for face halves belonging to different identities, which form a less coherent face than when they belong to the same face identity. These observations provide objective evidence for dissociation between part-based and integrated (i.e. holistic/configural) responses to faces in the human brain, suggesting that only responses to integrated face parts reflect high-level, possibly face-specific, representations.

A frequency-tagging electrophysiological method to identify central and peripheral visual field deficits

BACKGROUND

The aim of this study was to develop a fast and efficient electrophysiological protocol to examine the visual field's integrity, which would be useful in pediatric testing.

METHODS

Steady-state visual-evoked potentials (ssVEPs) to field-specific radial checkerboards flickering at two cycle frequencies (7.5 and 6 Hz for central and peripheral stimulations, respectively) recorded at Oz were collected from 22 participants from 5 to 34 years old and from 5 visually impaired adolescents (12-16 years old). Responses from additional leads (POz, O1, O2), and the impact of gaze deviation on the signals, were also investigated in a subgroup of participants.

RESULTS

Steady-state visual-evoked potentials responses were similar at all electrode sites, although the signal from the central stimulation was significantly higher at Oz and was highly sensitive in detecting gaze deviation. No effect of age or sex was found, indicating similar ssVEP responses between adults and healthy children. Visual acuity was related to the central signal when comparing healthy participants with four central visual impaired adolescents. Clinical validation of our electrophysiological protocol was also achieved in a 15-year-old adolescent with a severe peripheral visual deficit, as assessed with Goldmann perimetry.

CONCLUSIONS

A single electrode over Oz is sufficient to gather both central and peripheral visual signals and also to control for gaze deviation. Our method presents several advantages in evaluating visual fields integrity, as it is fast, reliable, and efficient, and applicable in children as young as 5 years old. However, a larger sample of healthy children should be tested to establish clinical norms.

Modulation of spontaneous alpha brain rhythms using low-intensity transcranial direct-current stimulation

Transcranial direct-current stimulation (tDCS) is a form of neurostimulation in which a constant, low current is delivered directly to the brain area of interest by small electrodes. The overall aim of this study was to examine and monitor the modulation of brain activity by electroencephalogram (EEG) in the frequency domain during tDCS in the resting state. To this end, we considered the modulation of spontaneous EEG to be a marker of the perturbation that was induced through the direct current (1.5 mA for 15 min). In all conditions (anodal, cathodal, and sham), an active electrode was placed over the right posterior parietal cortex, and a reference electrode was placed on the ipsilateral deltoid muscle. The EEG was recorded using a 64-channel system. The effect of tDCS was limited to the alpha rhythm, and the anodal stimulation significantly affected the alpha rhythm, whereas the cathodal stimulation did not elicit any modifications. Further, we observed modulation of alpha activity in areas that were stimulated directly through tDCS and in anterior noncontiguous areas. Finally, the anodal effect peaked 7.5 min after stimulation and decreased gradually over time. Our study demonstrates that in the resting brain, monocephalic anodal tDCS over posterior parietal areas alters ongoing brain activity, specifically in the alpha band rhythm. Our data can be used to fine-tune tDCS protocols in neurorehabilitation settings.

Transcutaneous electrical nerve stimulation effects on neglect: a visual-evoked potential study

We studied the effects of transcutaneous electrical nerve stimulation (TENS) in six right-brain-damaged patients with left unilateral spatial neglect (USN), using both standard clinical tests (reading, line, and letter cancelation, and line bisection), and electrophysiological measures (steady-state visual-evoked potentials, SSVEP). TENS was applied on left neck muscles for 15′, and measures were recorded before, immediately after, and 60′ after stimulation. Behavioral results showed that the stimulation temporarily improved the deficit in all patients. In cancelation tasks, omissions and performance asymmetries between the two hand-sides were reduced, as well as the rightward deviation in line bisection. Before TENS, SSVEP average latency to stimuli displayed in the left visual half-field [LVF (160 ms)] was remarkably longer than to stimuli shown in the right visual half-field [RVF (120 ms)]. Immediately after TENS, latency to LVF stimuli was 130 ms; 1 h after stimulation the effect of TENS faded, with latency returning to baseline. TENS similarly affected also the latency SSVEP of 12 healthy participants, and their line bisection performance, with effects smaller in size. The present study, first, replicates evidence concerning the positive behavioral effects of TENS on the manifestations of left USN in right-brain-damaged patients; second, it shows putatively related electrophysiological effects on the SSVEP latency. These behavioral and novel electrophysiological results are discussed in terms of specific directional effects of left somatosensory stimulation on egocentric coordinates, which in USN patients are displaced toward the side of the cerebral lesion. Showing that visual-evoked potentials latency is modulated by proprioceptive stimulation, we provide electrophysiological evidence to the effect that TENS may improve some manifestations of USN, with implications for its rehabilitation.

The dynamics of attention during free looking

Simple methods to study attention dynamics in challenging research and practical applications are limited. We explored the utility of examining attention dynamics during free looking with steady-state visual evoked potentials (SSVEPs), which reflect the effects of attention on early sensory processing. This method can be used with participants who cannot follow verbal instructions and patients without voluntary motor control. In our healthy participants, there were robust fluctuations in the strength of SSVEPs driven by the fixated and non-fixated stimuli (rapidly changing pictures of faces) in the seconds leading up to the moment they chose to shift their gaze to the next stimulus sequence. Furthermore, the amplitude of SSVEPs driven by the fixated stimuli predicted subsequent recognition of individual stimuli. The results illustrate how information about the temporal course of attention during free looking can be obtained with simple methods based on the attentional modulation of SSVEPs.

A steady-state visual evoked potential approach to individual face perception: effect of inversion, contrast-reversal and temporal dynamics

Presentation of a face stimulus for several seconds at a periodic frequency rate leads to a right occipito-temporal evoked steady-state visual potential (SSVEP) confined to the stimulation frequency band. According to recent evidence (Rossion and Boremanse, 2011), this face-related SSVEP is largely reduced in amplitude when the exact same face is repeated at every stimulation cycle as compared to the presentation of different individual faces. Here this SSVEP individual face repetition effect was tested in 20 participants stimulated with faces at a 4 Hz rate for 84 s, in 4 conditions: faces upright or inverted, normal or contrast-reversed (2×2 design). To study the temporal dynamics of this effect, all stimulation sequences started with 15s of identical faces, after which, in half of the sequences, different faces were introduced. A larger response to different than identical faces at the fundamental (4 Hz) and second harmonic (8 Hz) components was observed for upright faces over the right occipito-temporal cortex. Weaker effects were found for inverted and contrast-reversed faces, two stimulus manipulations that are known to greatly affect the perception of facial identity. Addition of the two manipulations further decreased the effect. The phase of the fundamental frequency SSVEP response was delayed for inverted and contrast-reversed faces, to the same extent as the latency delay observed at the peak of the face-sensitive N170 component observed at stimulation sequence onset. Time-course analysis of the entire sequence of stimulation showed an immediate increase of 4Hz amplitude at the onset (16th second) of different face presentation, indicating a fast, large and frequency-specific release to individual face adaptation in the human brain. Altogether, these observations increase our understanding of the characteristics of the human steady-state face potential response and provide further support for the interest of this approach in the study of the neurofunctional mechanisms of face perception.

Spatiotemporal brain mapping of spatial attention effects on pattern-reversal ERPs

Recordings of event-related potentials (ERPs) were combined with structural and functional magnetic resonance imaging (fMRI) to investigate the timing and localization of stimulus selection processes during visual-spatial attention to pattern-reversing gratings. Pattern reversals were presented in random order to the left and right visual fields at a rapid rate, while subjects attended to the reversals in one field at a time. On separate runs, stimuli were presented in the upper and lower visual quadrants. The earliest ERP component (C1, peaking at around 80 ms), which inverted in polarity for upper versus lower field stimuli and was localized in or near visual area V1, was not modulated by attention. In the latency range 80-250 ms, multiple components were elicited that were increased in amplitude by attention and were colocalized with fMRI activations in specific visual cortical areas. The principal anatomical sources of these attention-sensitive components were localized by fMRI-seeded dipole modeling as follows: P1 (ca. 100 ms-source in motion-sensitive area MT+), C2 (ca. 130 ms-same source as C1), N1a (ca. 145 ms-source in horizontal intraparietal sulcus), N1b (ca. 165 ms-source in fusiform gyrus, area V4/V8), N1c (ca. 180 ms-source in posterior intraparietal sulcus, area V3A), and P2 (ca. 220 ms-multiple sources, including parieto-occipital sulcus, area V6). These results support the hypothesis that spatial attention acts to amplify both feed-forward and feedback signals in multiple visual areas of both the dorsal and ventral streams of processing.

Voluntary attention changes the speed of perceptual neural processing

While previous studies in psychology demonstrated that humans can respond more quickly to the stimuli at attended than unattended locations, it remains unclear whether attention also accelerates the speed of perceptual neural activity in the human brain. One possible reason for this unclarity would be an insufficient spatial resolution of previous electroencephalography (EEG) and magnetoencephalography (MEG) techniques in which neural signals from multiple brain regions are merged with each other. Here, we addressed this issue by combining MEG with a novel stimulus-presentation technique that can focus on neural signals from higher visual cortex where the magnitude of attentional modulation is prominent. Results revealed that the allocation of spatial attention induces both an increase in neural intensity (attentional enhancement) and a decrease in neural latency (attentional acceleration) to the attended compared to unattended visual stimuli (Experiment 1). Furthermore, an attention-induced behavioural facilitation reported in previous psychological studies (Posner paradigm) was closely correlated with the neural 'acceleration' rather than 'enhancement' in the visual cortex (Experiment 2). In addition to bridging a gap between previous psychological and neurological findings, our results demonstrated a temporal dynamics of attentional modulation in the human brain.

Transient VEP and psychophysical chromatic contrast thresholds in children and adults

It has been found that humans are able to distinguish colours without luminance cues by about 2-4 months of age and that sensitivity to colour difference develops during childhood, reaching a peak around adolescence. This prolonged period of maturation is reflected by improvements in psychophysical threshold measures and by the VEP characteristics of morphology, latency and amplitude. An intra-individual comparison of VEP and psychophysical responses to isoluminant colour stimuli has not been made in children, however, and this was the aim of the present study. VEPs were recorded from 49 subjects, children (age range: 4.8-12.6 years) and adults (age range: 25.7-33.2 years). Psychophysical and VEP thresholds were both measured in 40 of those subjects. Nominally isoluminant chromatic (L-M) sinewave gratings were presented in onset-offset mode and identical stimuli were used for psychophysical and VEP recordings to allow comparison. In agreement with previous reports, morphology of the transient VEP in response to this stimulus differed considerably between children and adults. There was a significant difference between psychophysical and VEP thresholds in children, but not in adults. Our findings support and expand on previous work on maturation of the L-M chromatic pathway and indicate a larger discrepancy between VEP and psychophysical chromatic thresholds in children than in adults.

The effect of selective attention on the gamma-band auditory steady-state response

Studies have demonstrated that selective attention can modulate the steady-state evoked potential to repetitive visual and tactile stimulation. However, examinations of the effect of attention on the auditory steady-state response (ASSR) have proven equivocal. The current experiment therefore utilized EEG to examine the effect of attention on the ASSR in healthy humans (n=15). Auditory click trains in the beta (20 Hz) and gamma (40 Hz) ranges were randomly presented binaurally in an oddball discrimination paradigm (each frequency served as the oddball (target) in each of two blocks). A Fast Fourier Transform was used to assess the effect of attention on the ASSR (signal power), and phase consistency across trials was assessed using the phase-locking factor (PLF). As expected, both 20 and 40 Hz targets elicited a robust P300 response, with maximal amplitudes over parietal regions. For the ASSR, it was found that EEG signal power was larger to 40 Hz targets compared to 40 Hz frequent stimuli across all frontocentral electrodes. No differences in signal power were observed during 20 Hz stimulation. Finally, increased PLF values were observed for 40 Hz targets compared to frequent trials. These results provide evidence that selective attention can enhance signal power and phase-locking of the ASSR, particularly to auditory stimulation in the gamma range.

Early selection of diagnostic facial information in the human visual cortex

There is behavioral evidence that different visual categorization tasks on various types of stimuli (e.g., faces) are sensitive to distinct visual characteristics of the same image, for example, spatial frequencies. However, it has been more difficult to address the question of how early in the processing stream this sensitivity to the information relevant to the categorization task emerges. The current study uses scalp event-related potentials recorded in humans to examine how and when information diagnostic to a particular task is processed during that task versus during a task for which it is not diagnostic. Subjects were shown diagnostic and anti-diagnostic face images for both expression and gender decisions (created using Gosselin and Schyns' Bubbles technique), and asked to perform both tasks on all stimuli. Behaviorally, there was a larger advantage of diagnostic over anti-diagnostic facial images when images designed to be diagnostic for a particular task were shown when performing that task, as compared to performing the other task. Most importantly, this interaction was seen in the amplitude of the occipito-temporal N170, a visual component reflecting a perceptual stage of processing associated with the categorization of faces. When participants performed the gender categorization task, the N170 amplitude was larger when they were presented with gender diagnostic images than with expression-diagnostic images, relative to their respective non-diagnostic stimuli. However, categorizing faces according to their facial expression was not significantly associated with a larger N170 when subjects categorized expression diagnostic cues relative to gender-diagnostic cues. These results show that the influence of higher-level task-oriented processing may take place at the level of visual categorization stages for faces, at least for processes relying on shared diagnostic features with facial identity judgments, such as gender cues.

Chromatic and achromatic transient VEPs in adults with Down syndrome

Oculo-visual abnormalities such as strabismus and high refractive error are common in people with Down syndrome, and account in large part for reduced visual function in this group. In the absence of such abnormalities, however, some spatial vision deficits persist, probably reflecting abnormal function of the neural visual pathway in this population. In addition, colour vision abnormalities are reportedly common in subjects with Down syndrome. We recorded transient visual evoked potentials in response to black-white and chromatic stimuli, in seven subjects with Down syndrome and 33 controls, to investigate function of the visual pathways underpinning spatial and chromatic visual function in Down syndrome. Our findings indicate, in agreement with previous studies, that retino-striate achromatic and chromatic processing in Down syndrome are abnormal. We find, however, that abnormal retino-striate processing of chromatic signals in this group may not give rise to colour vision deficits detected by the Colour Vision Test Made Easy or the City University test.

Effect of fixation tasks on multifocal visual evoked potentials

PURPOSE

This study investigated the effects of cognitive influence on the multifocal visual evoked potential (mVEP) at different levels of eccentricity. Three different foveal fixation conditions were utilized involving varying levels of task complexity. A more complex visual fixation task has been known to suppress peripheral signals in subjective testing.

METHODS

Twenty normal subjects had monocular mVEPs recorded using the AccuMap objective perimeter. This allowed simultaneous stimulation of 58 segments of the visual field to an eccentricity of 24 degrees. The mVEP was recorded using three different fixation conditions in random order. During task 1 the subject passively viewed the central fixation area. For task 2 alternating numbers were displayed within the fixation area; the subject on viewing the number '3' in the central fixation area indicated recognition by pressing a button. Throughout task 3, numbers were displayed as in task 2. The subject had the cognitive task of summating all the numbers.

RESULTS

Analysis revealed that the increased attention and concentration demanded by tasks 2 and 3 in comparison with task 1 resulted in significantly enhanced central amplitudes of 9.41% (Mann-Whitney P = 0.0002) and 13.45% (P = 0.0002), respectively. These amplitudes became reduced in the periphery and approached those of task 1, resulting in no significant difference between the three tasks. Latencies demonstrated no significant difference between each task nor at any eccentricity (P > 0.05). As the complexity of each task increased the amount of alpha rhythm was significantly reduced.

CONCLUSIONS

Our findings indicate that task 1 required a minimal demand of cognition and was associated with the greatest amount of alpha rhythm. It was also the most difficult to perform because of loss of interest. The other two tasks required a greater demand of higher order cognitive skills resulting in significantly enhanced amplitudes centrally and the attenuation of alpha rhythm. Therefore, amplitudes are increased around the area of attention.

Different attentional resources modulate the gain mechanisms for color and luminance contrast

We used an interference paradigm to investigate whether attention is attribute-specific at early levels of visual processing. We show that the peripheral increment thresholds for luminance contrast deteriorate when the observer is currently performing another luminance (form or contrast) discrimination task in central view, but not when he or she is performing a color discrimination task. Similar results were obtained for color increment thresholds, indicating that the interference is specific to contrast modality. The effects are strong and robust over primary task difficulties and perceptual learning levels. Modeling suggests that attention improves contrast sensitivity by modulating the gain of the neuronal response to contrast. These results suggest that attention is parceled in independent resources for luminance and color contrast.

Transient pattern Visual Evoked Potentials in children with Down's syndrome

Visual acuity and contrast sensitivity are lower in children with Down's syndrome than in those developing normally. In many cases, this difference might be accounted for by the relatively high incidence of ocular abnormalities (including refractive error and strabismus) in Down's syndrome. However, abnormal spatial vision persists in children with Down's syndrome in the absence of ocular abnormality, suggesting that abnormal retino-cortical visual processing explains reduced visual function in this group. The aim of the present study was to assess retino-cortical function in children with Down's syndrome by recording transient visual evoked potentials (VEPs) in response to pattern stimuli. Responses from children with Down's syndrome were compared with those recorded from children developing normally. Response latency is similar in the two groups, but morphology differs, with the N75 component being clearly present in the normal responses, but diminished or undetectable in responses from children with Down's syndrome. Our findings may suggest a cortical abnormality specific to the source of the N75 component of pattern-reversal achromatic VEPs.