The electroretinogram, the visual evoked potential, and the area-luminance relation

Cyfip2 controls the acoustic startle threshold through FMRP, actin polymerization, and GABAB receptor function

Animals process a constant stream of sensory input, and to survive they must detect and respond to dangerous stimuli while ignoring innocuous or irrelevant ones. Behavioral responses are elicited when certain properties of a stimulus such as its intensity or size reach a critical value, and such behavioral thresholds can be a simple and effective mechanism to filter sensory information. For example, the acoustic startle response is a conserved and stereotyped defensive behavior induced by sudden loud sounds, but dysregulation of the threshold to initiate this behavior can result in startle hypersensitivity that is associated with sensory processing disorders including schizophrenia and autism. Through a previous forward genetic screen for regulators of the startle threshold a nonsense mutation in Cytoplasmic Fragile X Messenger Ribonucleoprotein (FMRP)-interacting protein 2 (cyfip2) was found that causes startle hypersensitivity in zebrafish larvae, but the molecular mechanisms by which Cyfip2 establishes the acoustic startle threshold are unknown. Here we used conditional transgenic rescue and CRISPR/Cas9 to determine that Cyfip2 acts though both Rac1 and FMRP pathways, but not the closely related FXR1 or FXR2, to establish the acoustic startle threshold during early neurodevelopment. To identify proteins and pathways that may be downstream effectors of Rac1 and FMRP, we performed a candidate-based drug screen that indicated that Cyfip2 can also act acutely to maintain the startle threshold branched actin polymerization and N-methyl D-aspartate receptors (NMDARs). To complement this approach, we used unbiased discovery proteomics to determine that loss of Cyfip2 alters cytoskeletal and extracellular matrix components while also disrupting oxidative phosphorylation and GABA receptor signaling. Finally, we functionally validated our proteomics findings by showing that activating GABAB receptors, which like NMDARs are also FMRP targets, restores normal startle sensitivity in cyfip2 mutants. Together, these data reveal multiple mechanisms by which Cyfip2 regulates excitatory/inhibitory balance in the startle circuit to control the processing of acoustic information.

An investigation into the relationship between stimulus property, neural response and its manifestation in the visual evoked potential involving retinal resolution

The visual evoked potential (VEP) has been shown to reflect the size of the neural population activated by a processing mechanism selective to the temporal - and spatial luminance contrast property of a stimulus. We set out to better understand how the factors determining the neural response associated with these mechanisms. To do so we recorded the VEP from 14 healthy volunteers viewing two series of pattern reversing stimuli with identical temporal-and spatial luminance contrast properties. In one series the size of the elements increased towards the edge of the image, in the other it decreased. In the former element size was congruent with receptive field size across eccentricity, in the later it was incongruent. P100 amplitude to the incongruent series exceeded that obtained to the congruent series. Using electric dipoles due the excitatory neural response we accounted for this using dipole cancellation of electric dipoles of opposite polarity originating in supra- and infragranular layers of V1. The phasic neural response in granular lamina of V1 exhibited magnocellular characteristics, the neural response outside of the granular lamina exhibited parvocellular characteristics and was modulated by re-entrant projections. Using electric current density, we identified areas of the dorsal followed by areas of the ventral stream as the source of the re-entrant signal modulating infragranular activity. Our work demonstrates that the VEP does not signal reflect the overall level of a neural response but is the result of an interaction between electric dipoles originating from neural responses in different lamina of V1.

VEP estimation of visual acuity: a systematic review

Purpose

Visual evoked potentials (VEPs) can be used to measure visual resolution via a spatial frequency (SF) limit as an objective estimate of visual acuity. The aim of this systematic review is to collate descriptions of the VEP SF limit in humans, healthy and disordered, and to assess how accurately and precisely VEP SF limits reflect visual acuity.

Methods

The protocol methodology followed the PRISMA statement. Multiple databases were searched using “VEP” and “acuity” and associated terms, plus hand search: titles, abstracts or full text were reviewed for eligibility. Data extracted included VEP SF limits, stimulus protocols, VEP recording and analysis techniques and correspondence with behavioural acuity for normally sighted healthy adults, typically developing infants and children, healthy adults with artificially degraded vision and patients with ophthalmic or neurological conditions.

Results

A total of 155 studies are included. Commonly used stimulus, recording and analysis techniques are summarised. Average healthy adult VEP SF limits vary from 15 to 40 cpd, depend on stimulus, recording and analysis techniques and are often, but not always, poorer than behavioural acuity measured either psychophysically with an identical stimulus or with a clinical acuity test. The difference between VEP SF limit and behavioural acuity is variable and strongly dependent on the VEP stimulus and choice of acuity test. VEP SF limits mature rapidly, from 1.5 to 9 cpd by the end of the first month of life to 12–20 cpd by 8–12 months, with slower improvement to 20–40 cpd by 3–5 years. VEP SF limits are much better than behavioural thresholds in the youngest, typically developing infants. This difference lessens with age and reaches equivalence between 1 and 2 years; from around 3–5 years, behavioural acuity is better than the VEP SF limit, as for adults. Healthy, artificially blurred adults had slightly better behavioural acuity than VEP SF limits across a wide range of acuities, while adults with heterogeneous ophthalmic or neurological pathologies causing reduced acuity showed a much wider and less consistent relationship. For refractive error, ocular media opacity or pathology primarily affecting the retina, VEP SF limits and behavioural acuity had a fairly consistent relationship across a wide range of acuity. This relationship was much less consistent or close for primarily macular, optic nerve or neurological conditions such as amblyopia. VEP SF limits were almost always normal in patients with non-organic visual acuity loss.

Conclusions

The VEP SF limit has great utility as an objective acuity estimator, especially in pre-verbal children or patients of any age with motor or learning impairments which prevent reliable measurement of behavioural acuity. Its diagnostic power depends heavily on adequate, age-stratified, reference data, age-stratified empirical calibration with behavioural acuity, and interpretation in the light of other electrophysiological and clinical findings. Future developments could encompass faster, more objective and robust techniques such as real-time, adaptive control.

Registration

International prospective register of systematic reviews PROSPERO (https://www.crd.york.ac.uk/PROSPERO/), registration number CRD42018085666.

Shades of grey; Assessing the contribution of the magno- and parvocellular systems to neural processing of the retinal input in the human visual system from the influence of neural population size and its discharge activity on the VEP

INTRODUCTION

Retinal input processing in the human visual system involves a phasic and tonic neural response. We investigated the role of the magno- and parvocellular systems by comparing the influence of the active neural population size and its discharge activity on the amplitude and latency of four VEP components.

METHOD

We recorded the scalp electric potential of 20 human volunteers viewing a series of dartboard images presented as a pattern reversing and pattern on-/offset stimulus. These patterns were designed to vary both neural population size coding the temporal- and spatial luminance contrast property and the discharge activity of the population involved in a systematic manner.

RESULTS

When the VEP amplitude reflected the size of the neural population coding the temporal luminance contrast property of the image, the influence of luminance contrast followed the contrast response function of the parvocellular system. When the VEP amplitude reflected the size of the neural population responding to the spatial luminance contrast property the image, the influence of luminance contrast followed the contrast response function of the magnocellular system. The latencies of the VEP components examined exhibited the same behavior across our stimulus series.

CONCLUSIONS

This investigation demonstrates the complex interplay of the magno- and parvocellular systems on the neural response as captured by the VEP. It also demonstrates a linear relationship between stimulus property, neural response, and the VEP and reveals the importance of feedback projections in modulating the ongoing neural response. In doing so, it corroborates the conclusions of our previous study.

Stimuli to differentiate the neural response at successive stages of visual processing using the VEP from human visual cortex

BACKGROUND

Clarifying the enigmatic relationship between stimulus property, neural response and the evoked potential is essential if non-invasive functional imaging is to make a meaningful contribution to the understanding of how maturational or degenerative processes influence brain activity. Visual cortex has proven a favourite target to elucidate this relationship. However, to date most studies involving the visual system have yielded inconsistent results or have been strongly criticised.

NEW METHOD

We developed a set of three visual stimuli, two of which either had the same low- or high spatial frequency characteristic. Adult volunteers viewed these as pattern reversing stimuli while the scalp electric potential was recorded using a 10-10 array of electrodes.

RESULTS

Established processing mechanisms of the primate visual system enabled us to link the amplitude of the N75 and P100 to the size of the neural population processing the temporal luminance contrast, and the amplitude of the N135 and P240 to the size of the neural processing the spatial luminance contrast in our stimuli. Calculating the distribution of current source density enabled us to identify the neural source of each VEP component.

CONCLUSIONS

Demonstrating a direct relationship between the temporal- and spatial luminance contrast properties of our stimuli and the size of the neural population involved provides a better understanding of the nature of the relationship between stimulus property, neural response and the VEP. It also shows that EEG can contribute in a significant manner to the study of the influence of maturational or degenerative processes on brain activity.

EEG in the classroom: Synchronised neural recordings during video presentation

We performed simultaneous recordings of electroencephalography (EEG) from multiple students in a classroom, and measured the inter-subject correlation (ISC) of activity evoked by a common video stimulus. The neural reliability, as quantified by ISC, has been linked to engagement and attentional modulation in earlier studies that used high-grade equipment in laboratory settings. Here we reproduce many of the results from these studies using portable low-cost equipment, focusing on the robustness of using ISC for subjects experiencing naturalistic stimuli. The present data shows that stimulus-evoked neural responses, known to be modulated by attention, can be tracked for groups of students with synchronized EEG acquisition. This is a step towards real-time inference of engagement in the classroom.

To see or not to see; the ability of the magno- and parvocellular response to manifest itself in the VEP determines its appearance to a pattern reversing and pattern onset stimulus

INTRODUCTION

The relationship between stimulus property, brain activity, and the VEP is still a matter of uncertainty.

METHOD

We recorded the VEP of 43 volunteers when viewing a series of dartboard images presented as both a pattern reversing and pattern onset/offset stimulus. Across the dartboard images, the total stimulus area undergoing a luminance contrast change was varied in a graded manner.

RESULTS

We confirmed the presence of two independent neural processing stages. The amplitude of VEP components across our pattern reversing stimuli signaled a phasic neural response based on a temporal luminance contrast selective mechanism. The amplitude of VEP components across the pattern onset stimuli signaled both a phasic and a tonic neural response based on a temporal- and spatial luminance contrast selective mechanism respectively. Oscillation frequencies in the VEP suggested modulation of the phasic neural response by feedback from areas of the dorsal stream, while feedback from areas of the ventral stream modulated the tonic neural response. Each processing stage generated a sink and source phase in the VEP. Source localization indicated that during the sink phase electric current density was highest in V1, while during the source phase electric current density was highest in extra-striate cortex. Our model successfully predicted the appearance of the VEP to our images whether presented as a pattern reversing or a pattern onset/offset stimulus.

CONCLUSIONS

Focussing on the effects of a phasic and tonic response rather than contrast response function on the VEP, enabled us to develop a theory linking stimulus property, neural activity and the VEP.

Retinocortical Gain in the Foveal Pathway: The Effect of Spatial Frequency and Stimulus Size

The amplitude and the phase of the simultaneously recorded steady-state pattern electroretinogram (PERG) and visual evoked potential (VEP) were evaluated in humans as a function of the vertical diameter (D) of unidimensional Gabor stimuli. In the other dimension, parallel to the horizontal gratings, the patterns all had constant diameter (see Methods and Materials). Spatial frequencies (SFs) of 1 cycle per degree (cpd) and 5.3 cpd were counterphase modulated at a rate of 6.8 Hz. After off-line artifact rejection, the response was subject to Fast Fourier Transformation (FFT). Amplitude and phase of the first and second harmonics of both ERG and VEP were displayed for each SF and stimulus diameter.

Both ERG and VEP amplitude were found to increase as a function of D. Using trend analysis we found that ERG amplitude increased linearly as a function of D. VEP amplitude was found to be both linear and cubic, as a function of D. We calculated the ratio of VEP amplitude and ERG amplitude at each D and termed it retinocortical gain (G). G normalized to stimulus area was high for small D and decreased with D independently of SF. Unity gain occurred at stimulus sizes of 6° to 7°.

ERG phase was found to be more negative at 5.3 cpd than at 1 cpd. Although no significant difference was found between VEP phases at the two SFs tested, neither ERG nor VEP phase changed as a function of D.

The results suggest that retinocortical gain is highest for the foveally centered low spatial frequency small patch Gabors. The results provide support for the notion of the “foveal window” in human vision.

Human flicker electroretinography using different temporal modulations at mesopic and photopic luminance levels

BACKGROUND

Electroretinographic measurement instruments allow the variation of several stimulation parameters enabling to study a wide range of retinal processes. The purpose of the present study was to measure human flicker electroretinograms (ERGs) varying temporal modulation, temporal frequency and mean luminance in the photopic and higher mesopic ranges where the change from cone to rod dominance occurs.

METHODS

Fourteen healthy subjects (mean age = 31 ± 6) participated in this study. ERG recordings were performed with the RetiPort system (Roland Consult, Germany). The stimuli were ON and OFF sawtooth waves, square wave and sine wave. The temporal frequencies were 4 and 8 Hz. The mean luminance varied from 1 to 60 cd/m(2).

RESULTS

The results confirmed the possibility to distinguish between rod- and cone-dominated retinal responses when using the flicker ERG at different temporal frequencies and luminances. We have also evaluated the responses at luminance levels at which the transition between rod- and cone-dominated responses occurs. This transition between rod- and cone-dominated flicker ERG responses is indicated by a significant change in the response characteristics between 4 and 8 cd/m(2) (between 200 and 400 phot Td).

CONCLUSIONS

The findings on the transition between rod- and cone-dominated ERGs along with the demonstration of ERG responses to different temporal flicker modulations might be informative for the electrophysiologists when setting up the stimulus at mesopic and photopic luminance levels.

Effect of competing stimuli on SSVEP-based BCI

Steady-state visual evoked potential (SSVEP)-based Brain-Computer Interface (BCI) works on the basis that an attended stimulus shows an enhanced visual evoked response. By examining EEG power at the frequency of the dominant evoked response, we are able to determine which stimulus the subject is attending. However, due to the limited processing capability of human visual system, when presented with multiple stimuli in the same visual field, the stimuli will compete for neural representations in the cortices. This study elucidates the effect of competing stimuli on SSVEP amplitudes by exploring the relationship between the number of stimuli and their inter-distance on the power spectra of attended stimuli. Results show that competing stimuli, when placed less than five degrees from the centre of the fovea, create a significant suppressive effect on the dominant frequency response. This result should guide how visual stimuli of SSVEP-based BCIs are spatially designed.

Effect of different stimulus configurations on the visual evoked potential (VEP)

The purpose of this study was to assess changes in the response profile of the pattern visual evoked potential (VEP) using three stimulus configurations simulating visual-field scotomas: central circular and central blank fields increasing incrementally in diameter from 1° to 15°, hemi-field, and quadrant patterns. Five visually normal adult subjects (ages 22-68 years) were tested binocularly at 1 m for each stimulus configuration on 5 separate days. A checkerboard test pattern (64 × 64 black-and-white checks, 85% contrast, 64 cd/m(2) luminance, 20 s of stimulus duration, 2-Hz temporal frequency) was used. The group mean VEP amplitude increased in a linear manner with increase in the central circular diameter (y = 0.805x + 2.00; r = 0.986) and decrease in central blank field diameter (y = -0.769x + 16.22; r = 0.987). There was no significant change in latency in nearly all cases. The group mean coefficient of variability results indicated that the VEP amplitude was repeatable for the different stimulus configurations. The finding of VEP response linearity for the circular stimulus fields, and repeatability for all stimulus configurations, suggests that the clinician may be able to use the VEP technique with the suggested test patterns as a rapid and simple tool for objective assessment for several types of visual-field defects for a range of abnormal visual conditions and special populations.

Visual activation of auditory cortex reflects maladaptive plasticity in cochlear implant users

Cross-modal reorganization in the auditory cortex has been reported in deaf individuals. However, it is not well understood whether this compensatory reorganization induced by auditory deprivation recedes once the sensation of hearing is partially restored through a cochlear implant. The current study used electroencephalography source localization to examine cross-modal reorganization in the auditory cortex of post-lingually deafened cochlear implant users. We analysed visual-evoked potentials to parametrically modulated reversing chequerboard images between cochlear implant users (n = 11) and normal-hearing listeners (n = 11). The results revealed smaller P100 amplitudes and reduced visual cortex activation in cochlear implant users compared with normal-hearing listeners. At the P100 latency, cochlear implant users also showed activation in the right auditory cortex, which was inversely related to speech recognition ability with the cochlear implant. These results confirm a visual take-over in the auditory cortex of cochlear implant users. Incomplete reversal of this deafness-induced cortical reorganization might limit clinical benefit from a cochlear implant and help explain the high inter-subject variability in auditory speech comprehension.

Functional deficits resulting from laser-induced damage in the rat retina

BACKGROUND AND OBJECTIVES

To determine the threshold for electrophysiological detection of functional changes after laser photocoagulation in rats, and to correlate the functional damage with retinal morphology.

STUDY DESIGN/MATERIALS AND METHODS

Argon-laser lesions, covering a quarter or half of the retina, were produced in the right eyes of 25 rats. Eyes were evaluated by flash electroretinography (ERG) and histologically at 3, 21, and 60 days after lasering.

RESULTS

Lasering of half the retina, but not of a quarter, produced a significant decrease in signals at all time points. Some functional recovery was evident 60 days after injury, and was consistent with concurrent morphological healing.

CONCLUSIONS

In rats, lasering of half the retina, but not a quarter, leads to significant deficits in outer retinal functions over a 2-month period. Thereafter some recovery occurs, presumably as a result of migration of photoreceptors from undamaged areas and their retinal remodeling.

The effect of stimulus size on human cortical potentials evoked by chromatic patterns

The effect of stimulus size on the pattern onset-offset visual evoked potential elicited with stimuli of two different wavelengths is studied under intensive yellow adaptation: (1) The onset response obtained with a 460 nm pattern is of negative polarity (N1) and saturates in amplitude with a stimulus radius of 7 deg. The onset response obtained with a 550 nm pattern is of positive polarity and continues to increase up to the maximum size (32.2 deg). (2) The peak time of N1 (460 nm) decreases with increasing stimulus size, that of P1 (550 nm) remains constant. These results are discussed as reflecting either varying retinal and brain anatomy, or cone activity, color-opponent activity, or luminance contrast activity.

The human motion onset VEP as a function of stimulation area for foveal and peripheral vision

We studied amplitude of the wave N200 of the motion-onset VEP by varying the side length of a square stimulation field between 0.5 and 7 degrees. A significant increase in amplitude was obtained between 0.5 and 1 degree of side length in central stimulation and between 0.5 and 5 degrees in 10-degree peripheral stimulation. Variations of spatial frequency between 0.34 and 6.8c/deg did not modify the amplitude size, ie, no tuning effect could be found. The results of simultaneous and separate stimulation of foveal and parafoveal regions support the observation that the stimulation field size is a minor influence. Features of motion-sensitive cortical neurons, such as those found in monkeys, could account for this behavior.

Temporal spacing of pattern alternation and human visual response

The electroretinogram and the visual evoked potential were recorded in response to checkerboard stimuli presented in phase alternation. The times in which the pattern was in its two positions were changed within a fixed total cycle time of two seconds. Both the amplitude of the electroretinogram and the evoked potential were affected by the length of the time interval between alternations, but in opposite directions. An electroretinogram that closely followed a preceding response was reduced in size while an evoked potential was larger. The results indicate that there is an interaction between adjacent pattern areas at the retinal level.

Adaptation, stimulus color, and the human electroretinogram

(1) The human electroretinogram was recorded using two procedures that were designed to change the ratio of photopic to scotopic activity. (2) With the first procedure, responses were recorded with patterns that alternated with an asymmetrical time period to produce two average responses, one of which was more adapted than the other. (3) The second employed symmetrical alternation, but red and blue stimulation were used. (4) The electroretinogram was larger in responses to patterns of low spatial frequency for all conditions. The evoked potential, which was recorded simultaneously, was larger at intermediate frequencies. (5) The electroretinogram must be mediated before the antagonistic receptive field organization. (6) Sensitivity of electroretinogram with blue stimuli was higher than those with red at low spatial frequency. The possible scotopic contribution was discussed.

The electrical response of the human eye to patterned stimuli: clinical observations

Following the first recording of electroretinographic responses in man to a barred pattern by Riggs and associates (1964) in normal and by Lawwill (1973, 1974) in clinical cases, the first striking observation of a complete loss of pattern electroretinogram (PERG) after injurious section of the optic nerve by Groneberg & Teping (1980) has led to the conclusion that the PERG originates from proximal retinal structures different from those responsible for the luminance electroretinogram (LERG). Typical changes of the PERG are seen during branch occlusion of the central retinal artery and vein. In ocular hypertension without visual field loss and glaucoma-related papillary changes the PERG is decreased at intraocular pressures above 26 mm Hg. In cases of primary glaucoma with regulated intraocular tension and without using miotics the amplitude of the PERG reflects the damage to the inner retinal layers. This favorably compares with the P100 latencies of the visual evoked cortical potential (VECP) which in primary glaucoma were partly within, partly outside the normal range. Other retinal diseases showing amplitude changes in the PERG are primary macular dystrophy, diabetic retinopathy, and the acute stage of optic neuritis. In all these cases the Ganzfeld LERG may be normal or nearly normal, whereas the PERG undergoes typical changes. On the contrary a highly preserved PERG can be recorded in cases of retinitis pigmentosa where the electrooculogram light rise and the LERG are already missing. In light of these findings the recording of PERG constitutes a new promising method of clinical electroretinography reflecting the activity of the hitherto omitted innermost retinal layers. It thereby contributes essentially to the location of disturbances within the visual system.

Visual evoked potential changes following renal transplantation

We have followed a group of 18 uremic patients through living-related donor renal transplantation (RTX) using pattern-reversal VEPs. Recordings were made prior to and 10 weeks after surgery at high, medium and low spatial frequencies. Prior to RTX, mean latency of the P100 component of the VEP was 107 msec. Individual values did not correlate with blood urea nitrogen or creatinine. Patients requiring hemodialysis did not differ from non-dialyzed patients. Ten weeks after RTX P100 latencies were significantly shortened while N75 latencies were unchanged. Several diabetic patients exhibited the appearance of previously unrecorded wave forms. P100 latency increased significantly with increasing spatial frequency before and after transplantation. Diabetic patients demonstrated a consistent increase in P100 amplitude while non-diabetic patients demonstrated a consistent decrease in P100 amplitude after RTX. The data indicate that renal transplantation has beneficial effects on the central nervous system of uremic patients not seen with chronic hemodialysis and that these effects may be quantitatively measured using the VEP. The data further suggest that electrophysiological effects of uremia and diabetes may be additive, but reversible after RTX. Alterations in the uremic and diabetic VEP may be related to retinal or more proximal central nervous system structures.

Spatial characteristics of the oscillatory potentials of the electroretinogram

The spatial properties of the trans- and intra-retinal oscillatory potentials (OPs), a- and b-waves of the mudpuppy electroretinogram (ERG) were analyzed. A comparison of the intra-retinal potentials was made with the proximal negative response (PNR), an extracellular response, predominantly from amacrine cells. The spatial characteristics of the OPs differed from the a- and b-waves and the PNR. The OPs integrated spatially up to full field illumination. They seem to represent summated neuronal activity over broader areas than that of the PNR and the a- and b-waves. The results indicate that the OPs reflect activity of second order neurons to which visual information converge from the whole retina. These neurons may be the bipolar cells. No evidence that the OPs orginate in the laterally extending amacrines was obtained. The b-wave data further suggested a centre-surround organization for the neurons indirectly involved in the b-wave generation.

The luminance origin of the pattern electroretinogram in man

Electroretinograms (e.r.g.s) and visually evoked potentials (v.e.p.s) to pattern stimuli were recorded simultaneously from healthy subjects. The stimuli were produced by a configuration in which the luminance of two sets of spatial elements (checks) could be modulated independently. Experiments were designed to distinguish between contrast responses and non-linear luminance responses. In the first of two basic experiments, the luminance of only one set of checks was modulated, at a constant level in every trial. The other set was not modulated, but its luminance was set at various levels. Under these conditions the local luminance stimulation was kept equal for every trial whereas the contrast stimulation varied. Therefore, local luminance responses in these experiments were expected to be constant and contrast responses were expected to vary. The e.r.g.s were identical for all luminance settings of the unmodulated checks, suggesting that luminance rather than contrast determines the response. The v.e.p.s showed, on the contrary, the behaviour expected for contrast responses. In the second basic experiment the local luminance stimulation was also kept constant, but the phase difference between the modulations of the two sets of checks was varied between 0 deg (pure luminance stimulation) and 180 deg (pattern reversal). In this type of experiment the second harmonic responses to local luminance modulation are expected to decrease to a minimum as phase difference goes from 0 to 90 deg and increase again as phase difference goes to 180 deg. Contrast responses are expected to increase monotonically from zero to maximal at phase difference shifts from 0 deg (no contrast stimulation) to 180 deg (contrast reversal). The e.r.g.s decreased to a minimum at 90 deg phase difference and increase again with phase difference going to 180 deg. At 0 and 180 deg the same value was recorded. Consequently, the e.r.g. behaviour suggests a luminance origin of the responses. The v.e.p.s monotonically increased as phase difference went from 0 to 180 deg, thus suggesting a contrast origin of the responses. Two additional control experiments were performed. The first experiment compared the responses to homogeneous field red/green exchange and pattern red/green exchange, with the luminances of the red and green sources matched by heterochromatic flicker photometry. The exchange of luminance-matched red and green checks (pattern reversal) did not produce different e.r.g. responses from those recorded in response to homogeneous field red/green exchange. The v.e.p. showed a significant increase for the pattern stimulus.(ABSTRACT TRUNCATED AT 400 WORDS)

Pattern electroretinogram: use of noncorneal skin electrodes

Pattern electroretinograms (ERGs) were recorded in three adults by a noncorneal electrode technique: silver-silver chloride skin electrodes attached to the inferior orbit of the left eye while the subject monocularly viewed a high contrast checker-board that reversed in contrast at a rate of 10 Hz. Recordings revealed pattern ERGs with an average amplitude of 1.5 microV. In addition, pattern ERGs were recorded as a function of spatial frequency and contrast of a square-wave grating. The results showed that an increase in spatial frequency or contrast yielded an increase in the amplitude of the pattern ERG. The results demonstrate that the pattern ERG can be recorded with a noncorneal electrode technique and that the noncorneal electrode technique is sensitive to changes in retinal function. Finally, it is demonstrated that the pattern ERG can transfer between the eyes such that the pattern ERG from a viewing eye can be recorded from an electrode placed below an occluded eye.

The pattern electroretinogram in optic nerve disease

Pattern evoked electroretinograms (PERG), diffuse flash electroretinograms (ERG) and visual evoked potentials were studied in patients with unilateral optic nerve disease. Patients with Snellen acuities of less than 6/30 did not have recordable PERGs in their affected eye, whereas their diffuse flash ERGs were normal. The VEPs were correspondingly reduced or absent when recorded from the poorer seeing eyes. A second group of patients with Snellen acuity between 6/6 and 6/30 in the involved eye showed reductions in the mean PERG amplitude of the affected as compared with the normal eyes. All affected eyes showed an abnormal contrast threshold measured with the PERG amplitude. Such results underscore the diagnostic value of the PERG in detecting even mildly affected cases of optic nerve disease.

Evoked potential contrast sensitivity in the parafovea: spatial organization

Visual evoked potential contrast sensitivity functions (VEP/CSFs) were determined for counterphase flickered sine-wave gratings in circular fields up to 8 degrees in diameter centered on the fovea. VEP sources responding to 16 c/deg gratings appeared to be concentrated in the central 2 degrees of the visual field while sources responding to lower spatial frequencies appeared to be distributed over progressively wider areas of the visual field as spatial frequency decreased. It was also found that independently determined VEP/CSFs for non-overlapping annular regions of the visual field centered on the fovea summed to equal the VEP/CSF obtained when both regions were stimulated simultaneously.

Pattern evoked retinal response (PERR) in human: effects of spatial frequency, temporal frequency, luminance and defocus

Spatial and temporal tuning were examined for both retinal and cortical signals using low contrast sinusoidol gratings. Maximum PERR and VECP responses occurred at the 4 Hz alternation rate and at 0.68 and 2.2 cpd, respectively. A 25% low spatial frequency attenuation (LSFA) was observed for the PERR. To test the possibility that prior inability to detect LSFA of the PERR was due to the masking of a pattern contrast response by a large amplitude local luminance response, mean luminance and optical power were varied in separate experiments. Greater relative LSFA was observed with a lower mean luminance and defocus differentially affected responses elicited by high and low spatial frequency gratings. In addition greater relative LSFA was observed when power at the second harmonic rather than amplitude was the dependent measure. We conclude that LSFA may be observed. The absence of spatial tuning in previous reports is explained by the masking of pattern contrast by local lumnance responses. Detection of LSFA in the present experiments is attributed to use of stimulus conditions which minimize local luminance responses and the use of an objective response measure with a high signal-to-noise ratio, i.e., power at the second harmonic of the stimulation frequency.

Effects of stimulus location and pattern upon the visually evoked cortical potential and the electroretinogram

The pattern properties of the visually evoked potential and the electroretinogram have been investigated for phase alternated patterns of checks presented to the near periphery of the retina. When the eye was light-adapted, coarse patterns became relatively more effective for eliciting the evoked potential as the stimulus was moved away from the fovea. Fine patterns were most effective in the center of the field. The electroretinogram responded best to coarse spatial frequencies at all retinal locations. At a lower level of light adaptation the sensitivity of the evoked potential shifted to coarse spatial frequencies in the center of the field, but did not change appreciably in the periphery. The results may reflect some of the dynamic properties of visual fields.

Stimulus alternation and fast retinal potentials: photopic anc scotopic contributions

Human retinal oscillatory potentials in response to an alternating checker-board pattern stimulus were studied in two subjects over an intensity range of 7.5 log units. Under scotopic conditions, two wavelets (S1 and S2) could be recorded. At an intensity of 1.9 photopic log Td four high frequency oscillations (O1--O4) were noticed and a discontinuity was observed in the corresponding luminance curve of the b-wave together with a sudden decrease in the magnitude of the standard deviation of the amplitude measures. The oscillations were noticed only on the ascending slope of the b-wave. With increasing stimulus intensity, their latency decreased at a slower rate than that of the b-wave and their number decreased. Each wavelet had an amplitude maximum at a certain stimulus intensity level. It was suggested that O1--O4 were generated by the activation of the photopic system and that S1 and S2 were of scotopic origin.

Visually evoked potentials: theory, techniques and clinical applications

The visually evoked potential (VEP), the recording of which has recently been made possible by the development of computer averaging techniques, is a gross electrical signal generated by the occipital region of the cortex in response to visual stimulation. It is more specific than the electroencephalogram (EEG) and more sensitive to changes in the visual stimulus; thus, it can provide ophthalmologists and vision researchers with information about the human visual system that is unavailable by other methods. Clinically, the VEP is of special value in the areas of refraction, infant acuity, diseases of the optic nerve, color blindness, amblyopia and field defects. Theory, techniques and instrumentation are described, and applications of the VEP to clinical situations and to vision research are discussed.