Transient VEPs Research Articles

VEP visual acuity in children with cortical visual impairment

Given improvements in neonatal care and the increased survival rates of infants born pre-term, Cortical Visual Impairment (CVI) is now the leading cause of visual impairment (VI) in the developed world. In this study, Step VEPS, transient VEPS and Vernier Sweep VEPs all demonstrated unbiased relationships with Preferential looking (PL) cards over the whole range of Visual Acuity (VA) in children with CVI, allowing equations for clinical use to be derived. The results also suggested that a slower, vernier steady-state stimulus of 80% contrast and presented with the Step VEP algorithm could further improve VA agreement with PL and optimise developmental sensitivity. An eye tracking device has proved very useful in the clinical assessment of this cohort. It is also now known that children can have good VA and CVI, and that sweep VEPS can highlight higher processing deficits. As well as negative findings, compensatory neuroplasticity is thought to occur during maturation and it is now realistic to study this mechanism, and other age-related changes across VI with functional tests and neuroimaging (including VEPS). A cross-sectional study of adults would highlight CVI’s ultimate functional limitations.

Classification of\xa0\u03b1-synuclein-induced changes\xa0in the AAV \u03b1-synuclein rat model of Parkinson\u2019s disease using electrophysiological measurements of visual processing

Biomarkers suitable for early diagnosis and monitoring disease progression are the cornerstone of developing disease-modifying treatments for neurodegenerative diseases such as Parkinson’s disease (PD). Besides motor complications, PD is also characterized by deficits in visual processing. Here, we investigate how virally-mediated overexpression of α-synuclein in the substantia nigra pars compacta impacts visual processing in a well-established rodent model of PD. After a unilateral injection of vector, human α-synuclein was detected in the striatum and superior colliculus (SC). In parallel, there was a significant delay in the latency of the transient VEPs from the affected side of the SC in late stages of the disease. Inhibition of leucine-rich repeat kinase using PFE360 failed to rescue the VEP delay and instead increased the latency of the VEP waveform. A support vector machine classifier accurately classified rats according to their `disease state’ using frequency-domain data from steady-state visual evoked potentials (SSVEP). Overall, these findings indicate that the latency of the rodent VEP is sensitive to changes mediated by the increased expression of α-synuclein and especially when full overexpression is obtained, whereas the SSVEP facilitated detection of α-synuclein across reflects all stages of PD model progression.

Transient brain activity explains the spectral content of steady-state visual evoked potentials.

Steady-state visual evoked potentials (SSVEPs) are widely used in the design of brain-computer interfaces (BCIs). A lot of effort has therefore been devoted to find a fast and reliable way to detect SSVEPs. We study the link between transient and steady-state VEPs and show that it is possible to predict the spectral content of a subject's SSVEPs by simulating trains of transient VEPs. This could lead to a better understanding of evoked potentials as well as to better performances of SSVEP-based BCIs, by providing a tool to improve SSVEP detection algorithms.

Orientation-reversal VEP: Comparison of phase and peak latencies in adults and infants

The peak latency of pattern-reversal (PR)-VEP has been found to develop rapidly, reaching the adult level around 15weeks of age. However, the development of orientation-reversal (OR)-VEP, reflecting the specific spatial organization of cortical receptive fields, still remains unknown.OR-VEP was tested in 81 adults at 1–12reversals/sec (r/s) and 94 infants (age 4–79weeks) at 2–8r/s. OR data at 4r/s from an additional 123 infants (age 4.0–20.3weeks) studied previously were also analyzed. In addition to peak transient latencies at 1–4r/s, latency values derived from the gradient of phase against temporal frequency in steady-state recording were also calculated.For both adults and infants, no significant latency differences in the initial positive peaks were found among the low reversal rates. The calculated latency was statistically longer than the transient latency in both groups. While the transient latency asymptoted to adult value of 102ms at around 50weeks of age, the calculated latency, unlike that for PR-VEP, showed little variation across the age span.The data suggest a dominant effect of transmission delay on the initial peak in infancy, which reduces with age. However, the overall timing of the cortical response to orientation change remains slower than for pattern reversal in the fully developed visual cortex. Upon reaching maturity, the latencies of the initial positive peak in both pattern and orientation VEPs may arise from the same level of cortical processing in V1, but the overall time course reflected in the steady-state phase continues to show a much more prolonged response to orientation change than the transmission delay seen in the transient VEPs.

Poster #161 NEUROCOGNITIVE PROFILE AND ITS ASSOCIATION WITH PSYCHOPATHOLOGY IN FIRST-DEGREE RELATIVES OF PATIENTS WITH SCHIZOPHRENIA. A CASE-CONTROL STUDY

We report here the recent findings in transient and steady-state VEPs that were recorded in our laboratory.Firstly, we studied the neural generators of the pattern reversal transients VEPs (T-VEPs). The optic tract responses recorded during stereotactic pallidotomy showed a P50–N80 complex. Visual evoked magnetic fields (VEFs) showed that the equivalent current dipoles (ECDs) of the N75, P100 and N145 were located in the primary visual cortex (V1). Secondly, we have developed a technique using multimodality VEPs, which can evaluate the parvocellular (color and form) and magnocellular systems (motion). T-VEPs to isoluminant chromatic stimuli showed N120, which was absent or delayed in patients with acquired color deficits. T-VEPs to apparent motion (AM) showed P120. Visual evoked magnetic fields showed that the ECDs of N120 and P120 were located in V1. Abnormality rates to color and apparent motion stimuli were different in optic neuritis, suggesting that these potentials distinguish between the two parallel visual pathways. Thirdly, we studied the physiological properties of the second (2F) and fourth harmonic (4F) components of the steady-state VEPs (S-VEPs). It was found that the 2F and 4F were differently tuned to the spatial and temporal frequencies of achromatic and chromatic stimuli. The oblique effect was also clearly demonstrated in the S-VEPs. From the above findings, a combined use of T-VEPs and S-VEPs could allow us to explore the human visual function.

Maturation of steady-state flicker VEPs in infants: fundamental and harmonic temporal response frequencies

Transient flash VEPs allow objective assessment of visual function and are easily recorded in young infants. However, due to their high variability, they are an insensitive surrogate marker of visual development. The aim of our study was to investigate the early maturation of temporal characteristics of steady-state flicker VEPs. Data from 53 VEP sessions were analyzed in term-born infants between birth and 20 months of age. The stimulus was a square-wave modulated luminance flicker with 80% modulation depth at temporal frequencies of 4.7, 7.5, 12.5, and 19 Hz. A total of 18 healthy adults aged between 21 and 54 years served as controls. Contingent on the stimulus frequency, we found pronounced changes of the flicker VEP with age. Regression lines fit to the first harmonic VEP magnitude as a function of age between 3 and 88 weeks of age indicated increases at 7.5 (P = 0.004), 12.5 (P < 0.001), and 19 Hz (P = 0.07) and a non-significant decrease at 4.7 Hz (P = 0.3). The magnitude of the second harmonic increased for all frequencies (4.7 (P = 0.05), 7.5 (P = 0.01), 12.5 (P = 0.13), and 19 Hz (P = 0.18)). Over the whole infant age range, the flicker VEP was dominated by the first harmonic, in contrast to adults, where the response was typically shifted to a higher harmonic at low stimulus frequencies. The optimal stimulus frequency, defined as the frequency eliciting the highest magnitude for F1, shifted to higher rates with age. Due to the difference from adult responses, further developmental changes of the temporal properties must be assumed to occur after the age of 20 months. Changes in temporal characteristics of the flicker VEP with age may be useful as an indicator of visual system maturation and a useful tool to detect visual delay.

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.

Estimating chromatic contrast thresholds from the transient visual evoked potential

Chromatic contrast thresholds may be estimated from transient VEPs by measuring the peak-to-peak amplitude at a range of stimulus levels followed by extrapolation to zero amplitude. However, there have been reports of failure of this technique when applied to the transient chromatic VEP due to variability of amplitude, difficulties with component identification and poor correlation of amplitude with stimulus level. The aim of our study was to compare methods of transient VEP chromatic contrast threshold estimation in terms of success rate and comparison with psychophysical threshold. We found each of the methods we investigated to have a high success rate, and in most cases VEP and psychophysical thresholds did not differ significantly.

Transient and steady-state VEPs—reappraisal

We report here the recent findings in transient and steady-state VEPs that were recorded in our laboratory. Firstly, we studied the neural generators of the pattern reversal transients VEPs (T-VEPs). The optic tract responses recorded during stereotactic pallidotomy showed a P50–N80 complex. Visual evoked magnetic fields (VEFs) showed that the equivalent current dipoles (ECDs) of the N75, P100 and N145 were located in the primary visual cortex (V1). Secondly, we have developed a technique using multimodality VEPs, which can evaluate the parvocellular (color and form) and magnocellular systems (motion). T-VEPs to isoluminant chromatic stimuli showed N120, which was absent or delayed in patients with acquired color deficits. T-VEPs to apparent motion (AM) showed P120. Visual evoked magnetic fields showed that the ECDs of N120 and P120 were located in V1. Abnormality rates to color and apparent motion stimuli were different in optic neuritis, suggesting that these potentials distinguish between the two parallel visual pathways. Thirdly, we studied the physiological properties of the second (2F) and fourth harmonic (4F) components of the steady-state VEPs (S-VEPs). It was found that the 2F and 4F were differently tuned to the spatial and temporal frequencies of achromatic and chromatic stimuli. The oblique effect was also clearly demonstrated in the S-VEPs. From the above findings, a combined use of T-VEPs and S-VEPs could allow us to explore the human visual function.

Morphology of transient VEPs to luminance and chromatic pattern onset and offset

Characteristics of the visual evoked response to chromatic and luminance-modulated stimuli reflect the activity of underlying neural mechanisms, although selective neuronal activity depends upon stimulus parameters. In the present study, the behaviour of the transient visual evoked response to low spatial and temporal frequency chromatic stimuli is investigated at a range of colour luminance ratios. Our results show that the response to pattern-offset may be used in addition to the pattern-onset response as part of the signature of the evoked response to luminance-modulated or isoluminant chromatic stimuli.

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

We recorded the monocular and binocular VEPs to the alternation of sinusoidal gratings in order to evaluate the binocular interaction in each component of transient and steady-state VEPs in 13 normal subjects. Three spatial frequencies (1.3, 2.6 and 5.3 c/deg) with a 90% contrast were used as visual stimuli. The latencies and amplitudes of N70 and P100 of the transient VEPs were measured. The steady-state VEPs were Fourier analyzed, and both the phase and amplitude of the second (2F) and fourth (4F) harmonic responses were obtained. Binocular interaction was influenced by spatial frequency such that a binocular summation or even an inhibition occurred. For the transient VEPs, a binocular summation was more pronounced in the amplitude of N70 than in that of P100 at all spatial frequencies. There were no significant effects of binocular stimulation on latencies of N70 or P100. However, the latencies of N70 and P100 showed different spatial frequency characteristics. For the steady-state VEPs, the amplitude of 2F revealed a binocular summation that was more pronounced at 5.3 c/deg, whereas the 4F amplitude showed binocular inhibition at 2.6 and 5.3 c/deg. The 2F phase showed binocular inhibition at all spatial frequencies, whereas no such inhibition was observed in the 4F phase. These results suggest that individual components of transient and steady-state VEPs are physiologically distinct and may therefore be generated from different neuronal populations in striate cortex.

The Duffing oscillator: a model for the dynamics of the neuronal groups comprising the transient evoked potential

Thirty years ago Zeeman conjectured that the dynamics of the EEG might be modeled by the “equation of motion” of a Duffing oscillator, a pendulum with a nonlinear, cubic, restoring force. In this study the idea is extended to the evoked potential (EP). A transient sensory EP reflects activity from several neuronal groups, pools of neurons that fire in synchrony, with voltage-time curves that overlap appreciably. When the dynamics of each neuronal group is modeled by a Duffing oscillator, multi-electrode transient VEPs are well predicted. Predictions based on Duffing oscillator dynamics are substantially better than those based on the assumption that each neuronal group follows a simpler exponentially damped sinusoid or a function that simulates a post-synaptic potential. The component voltage-time curves are reasonably consistent over 7 subjects, suggesting sequential activation of neuronal groups with delays of several tens of milliseconds between them. The scalp topographies of the components suggest their origins in the occipital cortex.

Effect of spatial frequency on transient and steady-state VEPs: stimulation with checkerboard, square-wave grating and sinusoidal grating patterns

We recorded VEPs to the alternation of checkerboard, square-wave grating and sinusoidal grating patterns to evaluate the contribution of the fundamental spatial frequency and higher harmonic components in 12 normal subjects. Their fundamental spatial frequencies were equated and ranged from 0.5 to 8.0 c/deg. Both the transient VEP (T-VEP) and steady-state VEP (S-VEP) were obtained. The latency and amplitude of P100 of T-VEPs were measured. S-VEPs were Fourier analyzed, and phase and amplitude of the second harmonic response were measured. The mean P100 latency and the mean phase had a U-shaped function with a peak at a fundamental spatial frequency of 2.0 c/deg irrespective of the stimulus patterns, while the mean P100 amplitude and the mean amplitude of S-VEPs did not show such spatial selectivity. At low and medium spatial frequencies, differences in P100 latency, phase and amplitude between pattern types were more pronounced. However, this difference became insignificant at high spatial frequencies. These results indicate that VEP responses are predominantly determined by the fundamental spatial frequency, and that the contribution of higher harmonics to VEP responses is not negligible. Our results are consistent with the concept that the human visual system may use spatial frequency-domain information.

Clinical feasibility of steady-state VEPs: Relationship between phase and the P100 latency of transient VEPs

Clinical feasibility of steady-state VEPs: Relationship between phase and the P100 latency of transient VEPs

Temporal frequency-dependent vep changes in Parkinson's disease

We recorded steady-state (4.19 and 8.41 Hz) VEPs in 17 Parkinson's Disease (PD) patients and 18 control observers using on off temporal modulation of a 2.3 c/deg sinusoidal grating. With 4.19 Hz. 20 out of 33 PD patient eyes had abnormal VEPs. However, only 8 of 33 eyes were abnormal with 8.41 Hz. “Routine” (counterphase) transient VEPs revealed delayed VEPs for only 7 out of 24 eyes of the patients. These findings suggest an “input” temporal frequency-dependent abnormality in the foveal pathway.

Visuell evozierte Potentiale: methodische Probleme - Variation der Reizparameter

The evaluation of visual potentials (VEP) evoked by pattern reversal includes the latency and amplitude of P100. An increase in the mean brightness or contrast of the checkerboard stimulus results in an increase in amplitude and a decrease in latency of P100. A square edge subtense of 20' in checkerboard pattern or a spatial frequency of 3-4 cyc/degrees in stripe pattern produce maximum responses. Transient VEPs are recorded at low repetition rates (less than 2 Hz), steady state VEPs at temporal frequencies above 8 Hz. In the latter condition the VEP takes the character of a sinusoidal train of waves which, compared to transient stimulation allows for a better measurement of VEP amplitude. The latency of P100 increases with the age of the subject. The influence of the different stimulus parameters and recording techniques must be known and have to be kept constant in order to record reliable and reproducible VEPs.

Visual evoked potential diagnosis of field defects in patients with chiasmatic and retrochiasmatic lesions.

Visual evoked potentials were studied in 18 patients with visual field defects assessed by perimetry. Stimuli were reversing checkerboard and grating patterns of 13', full field and hemifield. Two reversal rates were studied: 1 Hz (transient VEPs) and 8 Hz (steady-state VEPs). Using monocular hemifield stimulation the "paradoxical lateralisation" of transient but not of steady-state VEPs was confirmed. Diagnostic yield was 90% using transient, but less than 20% using steady-state stimuli.

Checkerboard reversal pattern and flash VEPs in dialysed and non-dialysed subjects

In forty-three patients suffering from chronic renal failure (CRF; 32 non-dialysed, 11 dialysed) the flash VEP and the pattern VEP to different spatial (7.5′, 15′, 30′, 60′ checks) and temporal (2, 10, 20, 40 Hz) frequencies were investigated. Abnormally delayed flash VEPs were obtained in 53.6% of the cases; transient pattern VEPs had a delayed major positive deflection in 46% of the tested eyes and altered steady-state response in 58% with a total of 65% abnormal responses. Chronically dialysed subjects showed higher levels of altered VEPs for both the techniques. The shorter latencies normally observed with coarser checks in healthy people were maintained in CRF patients, resulting in an abnormal prolongation of the shift between high and low spatial frequencies. Higher statistical correlations linked the pattern VEP with blood urea nitrogen ( P < 0.001) and other renal chemistry than the flash VEP. These and other features could be ascribed to a specific involvement of a maculo-calcarine channel devoted to ‘detailed’ vision and suggest the usefulness of this method to provide a tool for the early detection of an encephalopathic uraemia.