The effect of binocular stimulation on each component of transient and steady-state VEPs

Neural binocular summation and the effect of defocus on the pattern electroretinogram and visual evoked potentials for different pupil sizes

PURPOSE

To evaluate the influence of defocus and pupil size on subjective (visual acuity [VA]) and objective (electrophysiology) descriptors of human vision and their effect on binocular visual performance by means of neural binocular summation (BS).

METHODS

Fifteen healthy young subjects were recruited in this crossover study. Pattern electroretinogram (PERG) and visual evoked potentials (VEP) were measured under two levels of positive (+1.5 and +3.0 D) spherical and astigmatic defocus (axis 90°). Pupil size was controlled to reduce the inter-individual variability factor.

RESULTS

Low- and high-contrast VA showed poorer visual performance in the monocular versus the binocular condition. Positive BS (for VA) was higher with greater pupil size and higher levels of defocus. In the visual electrophysiology tests (i.e., VEP and PERG), peak time and amplitude were affected by pupil size and defocus. The increase in peak time was larger and the reduction in amplitude was more significant with greater levels of defocus and smaller pupil sizes. For the VEP, positive BS was found in all conditions, being stronger with larger amounts of defocus and pupil size (for the P100 amplitude). Significant negative correlations were observed between the P100 amplitude and VA BSs.

CONCLUSION

Smaller pupil size and levels of defocus produced greater changes in cortical activity as evidenced by both the PERG and VEP. Considering these changes and the obtained positive BS, the mechanism could be initiated as early as the retinal processing stage, then being modulated and enhanced along the visual pathway and within the visual cortex.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Appraisal of definition of baseline length for somatosensory evoked magnetic fields

BACKGROUND

The baseline (BL) segment in the prestimulus period is generally assigned as a reference of evoked activities. However, an experimenter empirically defines its length in each condition. So far, the criterion for the length of a BL segment has not been established.

NEW METHOD

We evaluated the effect of the length of the BL segment by recording somatosensory evoked magnetic fields (SEFs) under fixed stimulus onset asynchrony (SOA). For the evaluation of the length of the BL segment in the prestimulus period, five proportions in relation to SOA were used as the BL segment. In addition, we adopted other two types of BL segment which were the single data point measured from the value of stimulus onset (BL0) and the mean value of the whole raw data throughout the recording (DC mean). We investigated the influence of the BL segments on SEFs by utilizing two indicators: normalized N20 m amplitudes and estimated locations of corresponding equivalent current dipoles (ECDs).

RESULTS

Both indicators did not show any significant differences, based on the factor of BL segments, in any SOA conditions.

COMPARISON WITH EXISTING METHOD

The BL0 had by far the largest variation in the ECD locations.Therefore, utilizing stimulus onset as the BL segment should be avoided. In addition, considering that other BL segments provided comparable values by the two indicators, the DC mean can reasonably be adopted.

CONCLUSIONS

We suggest that utilizing the DC mean could be employed as the BL segment.

Human visual steady-state responses to amplitude-modulated flicker: Latency measurement

The response latency of steady-state visually evoked potentials (SSVEPs) is a sensitive measurement for investigating visual functioning of the human brain, specifically in visual development and for clinical evaluation. This latency can be measured from the slope of phase versus frequency of responses by using multiple frequencies of stimuli. In an attempt to provide an alternative measurement of this latency, this study utilized an envelope response of SSVEPs elicited by amplitude-modulated visual stimulation and then compared with the envelope of the generating signal, which was recorded simultaneously with the electroencephalography recordings. The advantage of this measurement is that it successfully estimates the response latency based on the physiological envelope in the entire waveform. Results showed the response latency at the occipital lobe (Oz channel) was approximately 104.55 ms for binocular stimulation, 97.14 ms for the dominant eye, and 104.75 ms for the nondominant eye with no significant difference between these stimulations. Importantly, the response latency at frontal channels (125.84 ms) was significantly longer than that at occipital channels (104.11 ms) during binocular stimulation. Together with strong activation of the source envelope at occipital cortex, these findings support the idea of a feedforward process, with the visual stimuli propagating originally from occipital cortex to anterior cortex. In sum, these findings offer a novel method for future studies in measuring visual response latencies and also potentially shed a new light on understanding of how long collective neural activities take to travel in the human brain.

Contralateral Bias of High Spatial Frequency Tuning and Cardinal Direction Selectivity in Mouse Visual Cortex

Binocular mechanisms for visual processing are thought to enhance spatial acuity by combining matched input from the two eyes. Studies in the primary visual cortex of carnivores and primates have confirmed that eye-specific neuronal response properties are largely matched. In recent years, the mouse has emerged as a prominent model for binocular visual processing, yet little is known about the spatial frequency tuning of binocular responses in mouse visual cortex. Using calcium imaging in awake mice of both sexes, we show that the spatial frequency preference of cortical responses to the contralateral eye is ∼35% higher than responses to the ipsilateral eye. Furthermore, we find that neurons in binocular visual cortex that respond only to the contralateral eye are tuned to higher spatial frequencies. Binocular neurons that are well matched in spatial frequency preference are also matched in orientation preference. In contrast, we observe that binocularly mismatched cells are more mismatched in orientation tuning. Furthermore, we find that contralateral responses are more direction-selective than ipsilateral responses and are strongly biased to the cardinal directions. The contralateral bias of high spatial frequency tuning was found in both awake and anesthetized recordings. The distinct properties of contralateral cortical responses may reflect the functional segregation of direction-selective, high spatial frequency-preferring neurons in earlier stages of the central visual pathway. Moreover, these results suggest that the development of binocularity and visual acuity may engage distinct circuits in the mouse visual system.SIGNIFICANCE STATEMENT Seeing through two eyes is thought to improve visual acuity by enhancing sensitivity to fine edges. Using calcium imaging of cellular responses in awake mice, we find surprising asymmetries in the spatial processing of eye-specific visual input in binocular primary visual cortex. The contralateral visual pathway is tuned to higher spatial frequencies than the ipsilateral pathway. At the highest spatial frequencies, the contralateral pathway strongly prefers to respond to visual stimuli along the cardinal (horizontal and vertical) axes. These results suggest that monocular, and not binocular, mechanisms set the limit of spatial acuity in mice. Furthermore, they suggest that the development of visual acuity and binocularity in mice involves different circuits.

Abnormal visual processing in migraine with aura: a study of steady-state visual evoked potentials

BACKGROUND

Although a number of studies reported different interictal findings between migraine with aura (MA) and migraine without aura (MO), the pathophysiology of the visual aura in migraine remains unclear.

OBJECTIVE

To investigate the visual processing in patients who experience MA between attacks using steady-state visual evoked potentials (SSVEPs).

METHODS

SSVEPs to high (98%) and low (29%) contrast black and white checkerboard gratings with two spatial frequencies (0.5 and 2.0 cpd) at 5 and 10 Hz (10 and 20 reversal/s) were recorded binocularly from 10 patients with MA, 10 patients with MO between attacks and 20 healthy controls (HC). The SSVEPs were Fourier analyzed to obtain the amplitude and phase of the second (2F) and fourth (4F) harmonic response.

RESULTS

In the amplitude of 2F, at 0.5 cpd, there was significant increased amplitude in both MA and MO in comparison to HC at 5 Hz in high and low contrast. However, no significant differences were detected at 2.0 cpd in both 5 and 10 Hz in high and low contrast. In the amplitude of 4F, at 2.0 cpd, there was significant increased amplitude in MA in comparison to MO and HC at 10 Hz in high contrast. However, there were no significant differences at 0.5 cpd at both 5 and 10 Hz in high and low contrast. There were no significant phase differences between MA, MO, and HC.

CONCLUSION

The high amplitude of the SSVEPs suggests that interictally migraine patients have abnormal excitability in the primary visual cortex, and this change in excitability may exist, at least partially, in the visual association cortex in MA.

Frequency-Following and Connectivity of Different Visual Areas in Response to Contrast-Reversal Stimulation

The sensitivity of visual areas to different temporal frequencies, as well as the functional connections between these areas, was examined using magnetoencephalography (MEG). Alternating circular sinusoids (0, 3.1, 8.7 and 14 Hz) were presented to foveal and peripheral locations in the visual field to target ventral and dorsal stream structures, respectively. It was hypothesized that higher temporal frequencies would preferentially activate dorsal stream structures. To determine the effect of frequency on the cortical response we analyzed the late time interval (220-770 ms) using a multi-dipole spatio-temporal analysis approach to provide source locations and timecourses for each condition. As an exploratory aspect, we performed cross-correlation analysis on the source timecourses to determine which sources responded similarly within conditions. Contrary to predictions, dorsal stream areas were not activated more frequently during high temporal frequency stimulation. However, across cortical sources the frequency-following response showed a difference, with significantly higher power at the second harmonic for the 3.1 and 8.7 Hz stimulation and at the first and second harmonics for the 14 Hz stimulation with this pattern seen robustly in area V1. Cross-correlations of the source timecourses showed that both low- and high-order visual areas, including dorsal and ventral stream areas, were significantly correlated in the late time interval. The results imply that frequency information is transferred to higher-order visual areas without translation. Despite the less complex waveforms seen in the late interval of time, the cross-correlation results show that visual, temporal and parietal cortical areas are intricately involved in late-interval visual processing.

Studies of human visual pathophysiology with visual evoked potentials

Visual evoked potentials (VEPs) offer reproducible and quantitative data on the function of the visual pathways and the visual cortex. Pattern reversal VEPs to full-field stimulation are best suited to evaluate anterior visual pathways while hemi-field stimulation is most effective in the assessment of post-chiasmal function. However, visual information is processed simultaneously via multiple parallel channels and each channel constitutes a set of sequential processes. We outline the major parallel pathways of the visual system from the retina to the primary visual cortex and higher visual areas via lateral geniculate nucleus that receive visual input. There is no best method of stimulus selection, rather visual stimuli and VEPs' recording should be tailored to answer specific clinical and/or research questions. Newly developed techniques that can assess the functions of extrastriate as well as striate cortices are discussed. Finally, an algorithm of sequential steps to evaluate the various levels of visual processing is proposed and its clinical use revisited.

Evaluating the effects of spatial frequency on migraines by using pattern-reversal visual evoked potentials

OBJECTIVE

To clarify the effects of contrast and spatial frequency in patients with migraine by means of pattern-reversal visual evoked potentials (PVEPs).

METHODS

PVEPs were obtained from 14 patients who had migraine without aura (MO), 11 patients who had migraine with aura (MA), and 25 age-matched, healthy controls (CO). PVEPs were binocularly recorded with a reversal rate of 1Hz (2 reversal/s) at 3 spatial frequencies (0.5, 1.0 and 4.0 cpd) at high (98%), medium (83%) and low (29%) contrast. N75, P100 and N135 latency and the amplitudes of P50-N75, N75-P100 and P100-N135 were analyzed.

RESULTS

Increased amplitude of PVEPs in patients with migraines were revealed at 3 different spatial frequencies in all components. The MO and the MA showed increased amplitudes mostly in high contrasts (98%). These findings were detected more at a high spatial frequency (4.0 cpd) than at a low spatial frequency (0.5 cpd). Increased amplitude with prolonged latency of N135 were found both in MO and MA at 4.0 cpd.

CONCLUSIONS

We conclude that pattern stimuli of high contrasts may be particularly effective in uncovering abnormal cortical reactivity which may be modified in the primary and secondary visual cortex in the interictal state of migraine.

SIGNIFICANCE

These findings indicate that there is abnormal visual cortex processing in patients with migraine.

Mechanism of binocular interaction in refraction errors: study using pattern-reversal visual evoked potentials

In this study we sought to determine whether a natural condition involving fine discrimination, for example moderately severe myopia, might yield interesting information regarding the binocular interaction expressed by visual evoked potentials (VEPs). We studied ten normal subjects with a mild refraction deficits. Transient VEPs were elicited by monocular and binocular stimulation under conditions of natural and lens-corrected vision. The visual stimulus was a pattern-reversal checkerboard consisting of 15' and 40' checks. VEPs in response to binocular stimulation were compared with monocular VEPs. We plotted the monocular 'better-VEP' and 'worse-VEP' response, since significant differences between individual eye stimulations were present. We found no significant difference between the mean N75 and P100 latencies of the binocular VEP and the better monocular VEP, regardless of the check size used and of natural or corrected vision. Under all stimulus conditions, the mean amplitude of the N75-P100 of the binocular VEPs was also larger than the better monocular VEP response. The difference proved more significant when we stimulated our subjects with smaller squares and left vision uncorrected. The mean P100-N145 amplitude obtained with binocular stimulation was larger than the better monocular VEP response only when using small checks (15') and uncorrected vision. Overlapping latencies are consistent with an earlier hypothesis that monocular and binocular VEPs originate postsynaptically from the binocular neurons in the primary visual cortex. The gain in amplitude achieved by binocular stimulation may depend upon the removal of 'tonic interocular inhibition' and/or on a cortical modulatory mechanism.

Steady-state vibration somatosensory evoked potentials: physiological characteristics and tuning function

OBJECTIVE

The steady-state somatosensory evoked potentials (S-SEPs) to vibratory stimulation were recorded to characterize their physiological properties.

METHODS

Vibratory stimuli were applied to the right palmar surface in 10 normal subjects. A total of 200 responses were recorded from electrodes at 2 cm posterior to C3, Cz and C4 and 2 cm anterior to C3. All responses were Fourier analyzed and the amplitudes of the first (1F) and second (2F) harmonic components were thus obtained. The effects of modulation frequency (5-30 Hz) and stimulus intensity (0.001-0.1 Newton (N)) on S-SEPs were studied.

RESULTS

The amplitudes of 1F and 2F were greatest at the electrode 2 cm posterior to C3, 1F being predominant. The mean 1F amplitudes as a function of modulation frequency showed a bimodal distribution with a trough at 14 Hz and a peak at 21 Hz. The mean 1F amplitudes showed a linear increase of up to 0.05 N and thereafter reached a plateau against the logarithmic stimulus intensity axis.

CONCLUSION

Vibratory S-SEPs may originate from the primary somatosensory cortex and provide information on the fast-adapting mechanoreceptive afferents. The temporal resonance at 21 Hz places the somatosensory system between the visual and auditory systems.

The effect of spatial frequency on chromatic and achromatic steady-state visual evoked potentials

OBJECTIVE

Little is known about the physiological properties of the major components of steady-state visual evoked potentials (VEPs). Based on the hypothesis that isoluminant color and high contrast pattern differentially activate the parvo- and magnocellular pathways, we studied difference in spatial frequency function between chromatic and achromatic VEPs to sinusoidal gratings.

METHODS

Steady-state VEPs to isoluminant chromatic (red-green) and high contrast (90%) achromatic (black-white) sinusoidal gratings with nine spatial frequencies (0.5 to 8.0 cycles/degrees (cpd)) at 4 Hz (8 reversals/s) were recorded in 13 normal subjects. VEPs were Fourier analyzed to obtain phase and amplitude of the second (2F) and fourth (4F) harmonic responses.

RESULTS

The 2F amplitude of chromatic VEPs decreased above 4.0 cpd in a low-pass function while that of achromatic VEPs showed a band-pass function with a peak at 4.0 cpd. The 4F amplitude of chromatic VEPs was not affected significantly by spatial frequency whereas that of achromatic VEPs exhibited a high-pass function. The phases of 2F and 4F showed a non-monotonic function of spatial frequency in both chromatic and achromatic stimuli with peaks at middle spatial frequencies.

CONCLUSION

Chromatic and achromatic visual stimuli differently affected 2F and 4F components, which thus suggests that 2F and 4F components are generated from different neuronal subgroups largely in the parvocellular pathway.

Responses of the nervous system to low frequency stimulation and EEG rhythms: clinical implications

The present paper reviews literature data on the role of the non-specific central nervous system response mechanisms on the therapeutic effects of relatively weak external stimulations used in clinical practice. The factors affecting the stimulation efficiency and increased sensitiveness of living things to extra-low-frequency periodic stimulations (in the range of from less than 1 Hz to tens of Hz) are discussed. Among the factors determining such effects, the non-specific response mechanisms of the nervous system, the resonance phenomena in different organism systems, and the interaction of external stimulation with endogenous rhythmic processes are analyzed. Most attention is given to endogenous rhythms of the electrical brain activity reflected in the EEG rhythms. A high resolution EEG processing approach that is used to reveal the intrinsic oscillators in the individual EEG spectrum is described. Synchronization of sensory stimulation parameters with the frequencies of intrinsic EEG oscillators is supposed to be an appropriate way to enhance the therapeutic effects of various sensory stimulation treatments. Specific methods for utilizing resonance therapy via sensory stimulation with intrinsic EEG frequencies, and for automatic modulation of stimulation parameters by endogenous organism rhythms are delineated; some preliminary results are described.

Visual evoked cortical magnetic responses to checkerboard pattern reversal stimulation: a study on the neural generators of N75, P100 and N145

In an attempt to elucidate the neural generators of pattern reversal visual evoked potentials (PR-VEPs), we measured the visual evoked magnetic fields (PR-VEFs) using a 37-channel magnetoencephalography in six healthy young adults. A half-field checkerboard pattern was phase-reversed at a rate of 1 Hz to stimulate the right or left visual half-field, thus yielding 12 PR-VEFs in total from the six subjects. The simultaneously recorded scalp PR-VEPs showed three distinct components of N75, P100 and N145. Three corresponding components were also identified in the PR-VEFs with similar peak latencies (N75m, P100m and N145m). P100m and N145m were clearly identified in all 12 PR-VEFs, whereas N75m was observed in only nine of 12 PR-VEFs. The equivalent current dipoles (ECDs) of N75m, P100m and N145m were located closely to each other in the occipital cortex around the calcarine fissure contralateral to the stimulated visual field, when they were overlaid on the MRI. The reliability of dipole estimation was highest in P100m, followed by N145m while N75m showed the least reliability. The direction of the current flow of ECDs of N75m and N145m was from the medial to the lateral in the occipital cortex when viewed in a coronal section, whereas that for P100m was toward the medial. The ECD location of P100m changed according to the retinotopic organization when the upper or lower quadrant of the visual field was stimulated, with the ECDs being located in the lower or upper part, respectively, of the visual cortex. Our results therefore indicate that the neural origins of N75m, P100m and N145m of PR-VEFs are in the primary visual cortex on the contralateral side of the stimulated visual half-field, while the three components are physiologically distinct.

Visual evoked potentials in the vicinity of the optic tract during stereotactic pallidotomy

We recorded visual evoked responses in eight patients with Parkinson's disease, using a depth electrode either at or below the stereotactic target in the ventral part of the globus pallidus internus (GPi), which is located immediately dorsal to the optic tract. Simultaneously, scalp visual evoked potentials (VEPs) were also recorded from a mid-occipital electrode with a mid-frontal reference electrode. A black-and-white checkerboard pattern was phase reversed at 1 Hz; check size was 50 min of arc . Pallidal VEPs to full field stimulation showed an initial positive deflection, with a latency of about 50 ms (P50), followed by a negatively with a mean latency of 80 ms (N80). The mean onset latency of P50 was about 30 ms. P50 and N80 were limited to the ventralmost of the GPi and the ansa lenticularis. Left half field stimulation evoked responses in the right ansa lenticularis region while right half field stimulation did not, and vice versa. These potentials thus seemed to originate posterior to the optic chiasm. The scalp VEPs showed typical triphasic wave forms consisting of N75, P100 and N145. The location of the recording electrode in the ansa lenticularis region did not modify the scalp VEP. These results suggest that P50 and N80 are near-field potentials reflecting the compound action potentials from the optic tract. Therefore, N75 of the scalp VEPs may represent an initial response of the striate cortex but not of the lateral geniculate nucleus.