Vittorio Porciatti

Normative data for onset VEPs to red-green and blue-yellow chromatic contrast

OBJECTIVEnTo better characterize the properties of chromatic VEPs to onset-offset of red-green and blue-yellow equiluminant patterns, and establish normative values for a set of stimuli able to elicit robust and reliable responses, suitable for the clinical application.nnnMETHODSnChromatic VEPs have been recorded (Oz lead) from 28 normal subjects (age range 20-53 years) in response to monocular presentation of both red-green and blue-yellow equiluminant sinusoidal gratings. Stimuli were generated by a Cambridge VSG/2 card and displayed on a Barco CCID monitor (14x14 deg field size). Spatial frequency, chromaticity, contrast and onset-offset duration were varied.nnnRESULTSnFor both red-green and blue-yellow equiluminant stimuli, robust responses have been obtained with gratings of 2 c/deg, presented in onset (300 ms) offset (700 ms) mode, at contrasts ranging from 90 to 6%. In all observers, the VEP waveform consisted mainly of a negative wave at stimulus onset, with a latency rapidly increasing with decreasing contrast. For both red-green and blue-yellow stimuli, the VEP contrast threshold coincided with the psychophysical threshold.nnnCONCLUSIONSnThe results complement previous studies aimed at characterizing the properties of chromatic VEPs. In addition, normative data are provided for a set of stimulus characteristics suitable for the clinical routine.

Changes in pattern electroretinograms to equiluminant red-green and blue-yellow gratings in patients with early Parkinson's disease

Summary In Parkinson’s disease (PD), the luminance pattern electroretinogram (PERG) is reported to be abnormal, indicating dysfunction of retinal ganglion cells (RGCs). To determine the vulnerability of different subpopulations of RGCs in PD patients, the authors recorded the PERG to stimuli of chromatic (red-green [R-G] and blue-yellow [B-Y]) and achromatic (yellow-black [Y-Bk]) contrast, known to emphasize the contribution of parvocellular, koniocellular, and magnocellular RGCs, respectively. Subjects were early PD patients (n = 12; mean age, 60.1 ± 8.3 years; range, 46 to 74 years) not undergoing treatment with levodopa and age-sex-matched controls (n = 12). Pattern electroretinograms were recorded monocularly in response to equiluminant R-G, B-Y, and Y-Bk horizontal gratings of 0.3 c/deg and 90% contrast, reversed at 1Hz, and presented at a viewing distance of 24 cm (59.2 × 59 degree field). In PD patients, the PERG amplitude was significantly reduced (by 40 to 50% on average) for both chromatic and luminance stimuli. Pattern electroretinogram latency was significantly delayed (by about 15 ms) for B-Y stimuli only. Data indicate that, in addition to achromatic PERGs, chromatic PERGs are altered in PD before levodopa therapy. Overall, chromatic PERGs to B-Y equiluminant stimuli exhibited the largest changes. Data are consistent with previous findings in PD, showing that visual evoked potentials (VEP) to B-Y chromatic stimuli are more delayed than VEPs to R-G and achromatic stimuli. The results suggest that the koniocellular subpopulation of RGCs may be particularly vulnerable in early stages of Parkinson’s disease.

Pigeon pattern electroretinogram: A response unaffected by chronic section of the optic nerve

SummaryRetinal evoked responses to sinusoidal gratings modulated in counterphase (pattern ERG) have been recorded from the pigeon eye. The pattern ERG amplitude depends upon the temporal frequency of the modulation, the contrast, the spatial frequency and the area of the stimulus. In 8 pigeons the pattern ERG has been recorded at different times after the unilateral section of the optic nerve. It has been found that the pattern ERG has a comparable amplitude in the two eyes as a function of the spatial frequency, 3 and 9 months after the section of the left optic nerve. At these times, histological evidence shows a drastic reduction in the density of the retinal ganglion cells on the operated side in comparison to the control one. These findings suggest that retinal sources other than the ganglion cells are responsible for the generation of the pigeon pattern ERG.

Binocularity in the little owl, Athene noctua. II. Properties of visually evoked potentials from the Wulst in response to monocular and binocular stimulation with sine wave gratings.

Visually evoked potentials (VEPs) have been recorded from the Wulst surface of the little owl, Athene noctua, in response to counterphase-reversal of sinusoidal gratings with different contrast, spatial frequency and mean luminance, presented either monocularly or binocularly. Monocular full-field stimuli presented to either eye evoked VEPs of similar amplitude, waveform and latency. Under binocular viewing, VEPs approximately doubled in amplitude without waveform changes. VEPs with similar characteristics could be obtained in response to stimulation of the contralateral, but not ipsilateral, hemifield. These results suggest that a 50% recrossing occurs in thalamic efferents and that different ipsilateral and contralateral regions converge onto the same Wulst sites. The VEP amplitude progressively decreased with increase of the spatial frequency beyond 2 cycles/degree, and the high spatial frequency cut-off (VEP acuity) was under binocular viewing (8 cycles/degree) higher than under monocular (5 cycles/degree) viewing (200 cd/m2, 45% contrast). The VEP acuity increased with increase in the contrast and decreased with reduction of the mean luminance. The binocular gain in both VEP amplitude and VEP acuity was largest at the lowest luminance levels. Binocular VEP summation occurred in the medium-high contrast range. With decreased contrast, both monocular and binocular VEPs progressively decreased in amplitude and tended to the same contrast threshold. The VEP contrast threshold depended on the spatial frequency (0.6-1.8% in the range 0.12-2 cycles/degree). Binocular VEPs often showed facilitatory interaction (binocular/monocular amplitude ratio greater than 2), but the binocular VEP amplitude did not change either by changing the stimulus orientation (horizontal vs. vertical gratings) or by inducing different retinal disparities.(ABSTRACT TRUNCATED AT 250 WORDS)

The electroretinogram of the little owl (Athene noctua)

Electroretinographic responses (ERGs) have been recorded from the cornea of the little owl (Athene noctua) in response to single light flashes and to alternating sinusoidal gratings (pattern) at different levels of light adaptation. Both flash- and pattern-evoked ERGs show scotopic as well as photopic components. The pattern evoked ERG is spatially tuned with tuning functions which shift towards lower frequencies by reducing the mean luminance. The retinal acuity is about 6 c/deg at 2.3 log cd/m2 and decreases progressively by reducing the mean luminance. No pattern ERG can be recorded beyond -6.7 log cd/m2 at any spatial frequency. The pattern ERG amplitude decreases progressively by reducing the contrast. The extrapolated contrast threshold is about 1%. Acuity and contrast sensitivity ERG values are in the range of those obtained by operant techniques in other species with duplex retinae such as owls and cats.

Equiluminant red-green and blue-yellow VEPs in multiple sclerosis

The primate visual system is composed by two color-opponent pathways—red–green (R-G) and blue–yellow (B-Y)—subserved by the so-called parvo- and koniocellular streams respectively. The authors’ aim was to compare the relative involvement of chromatic visual subsystems in multiple sclerosis (MS). In 30 MS patients with different forms of MS they recorded visual evoked potentials (VEPs) to onset (300 msec) and offset (700 msec) of equiluminant R-G and B-Y sinusoidal gratings of different contrast (90% and 25%). Equiluminance was established psychophysically by establishing the R-G and the B-Y color ratio at which chromatic gratings alternating at 15 and 10 Hz respectively had minimum visibility. The negative wave at stimulus onset with a peak latency of 120 to 160 msec was evaluated. Ordinary VEPs to luminance (LUM) contrast (black–white reversing checkerboards of 15’ check size and 50% contrast) were also recorded for comparison. Latencies of R-G VEPs were abnormal in 53.3% and 58.3% of patients at 90% and 25% contrast respectively, whereas abnormal B-Y VEPs were 56.6% and 48.3%. Latencies of LUM VEPs were abnormal in 45% of patients. Interocular latency asymmetries were abnormal in 59.2% and 33.3% of patients for R-G, and 51.8% and 62.9% for B-Y. Latency asymmetries for LUM VEP were abnormal in 46.4% of patients. The higher rate of VEP abnormalities found with equiluminant chromatic stimuli compared with achromatic stimuli confirms the general vulnerability of color-opponent visual pathways in MS, even if the number of patients with abnormal findings was not significantly different when both test conditions were compared. VEPs to R-G and B-Y equiluminant stimuli appear to be involved approximately to the same extent.

Retinal and tectal responses to alternating gratings are unaffected by monocular deprivation in pigeons

Retinal and tectal potentials to alternating gratings were recorded in pigeons raised monocularly deprived. Deprived eyes showed consistent myopia and anterior-posterior axis elongation. In addition, vitreal opacities were observed in 70% of the deprived eyes. In pigeons with vitreal opacities, a reduction of retinal and tectal response amplitude was consistently found in the high spatial frequency range. Pigeons with clear media of the deprived eyes showed normal retinal and tectal responses.

The pigeon pattern electroretinogram is not affected by massive loss of cell bodies in the ganglion layer induced by chronic section of the optic nerve.

In pigeons, electroretinographic responses to contrast reversal of sinusoidal gratings (pattern ERGs) were recorded before and after section of the left optic nerve. Ninety and 270 days following optic nerve section the ganglion cell layer of the side that underwent the surgical procedure showed an 80% reduction in the number of cell bodies as compared with the intact side. At these times the pattern ERGs showed comparable amplitudes in both eyes. There is a possibility that the inner nuclear layer of the pigeon retina plays a major role in the generation of the pattern ERG.

Interaction between photoreceptors and pigment epithelium in developing pigeon retina: an electrophysiological and ultrastructural study

In pigeons as in other vertebrates, the electroretinogram (ERG) first can be recorded simultaneously with the appearance of the photosensitive lamellae in the developing outer segments. At around 4–6 days after the bird has hatched a small negative-positive complex (a-b-wave) appears. Simultaneously, a c-wave can be recorded and the termination of the stimulus is followed by a positive d-wave. The ERG amplitude progressively increases during the first month as the lamellar structures complete their maturation. The c-wave amplitude shows a steady increase in the early post-hatching period which parallels the a-b complex maturation. In newly hatched pigeons the retinal pigment epithelial (RPE) cell consists of a small soma and long villi which extend toward the photoreceptors. During maturation a progressive increase in RPE thickness and number of cytoplasmic elements can be observed.

Recent advances in clinical neurophysiology of vision.

Publisher Summary Two principles in neuronal organization are pivotal to the understanding of visual cortical processing: multiple parallel pathways and anatomical segregation of functionally specialized areas. The functional specialization of separate anatomical areas is confirmed by the effect of selected lesions producing deficits limited to color, spatial perception, or movement. It must be acknowledged that anatomical segregation is far from absolute and some lesions limited to specific areas only produce a temporary effect with subsequent recovery. Anatomical studies in primates have demonstrated 305 interconnecting pathways among the various visual areas. Question remains on the reconciliation of the hypotheses of multiple parallel channels, functional specialization, and hierarchical organization with the unitary integrated phenomenology of visual experience. The hypothesis of a distributed network may bring these separate concepts together. The visual system is modular; modules are added as needed to increase information processing and each module operates via a distributed network with crucial nodal points (or information bottlenecks). The most crucial nodal point is VI where all incoming information is received and then distributed to the other areas of the network. Lesions at crucial nodal points are likely to produce specific visual deficits for example, homonymous hemianopia in VI lesions. Lesions in the network may be silent or transient. Other modules or network connections may compensate for the damaged area (plasticity).