Distinct Patterns of P1 and C2 VEP Potentiation and Attenuation in Visual Snow: A Case Report

Visual snow syndrome, characterized by persistent flickering dots throughout the visual field, has been hypothesized to arise from abnormal neuronal responsiveness in visual processing regions. Previous research has reported a lack of typical VEP habituation to repeated stimulus presentation in patients with visual snow. Yet these studies generally used pattern-reversal paradigms, which are suboptimal for measuring cortical responses to the onset of foveal stimulation. Instead, these responses are better indexed by the C2, a pattern-onset VEP peaking 100–120 ms after stimulus onset. In this case study, we analyzed the C2 and its adaptation profile in data previously collected from a single patient with visual snow using a “double-pulse” presentation paradigm. In controls, shorter intervals between stimulus pairs were associated with greater attenuation of the C2 VEP, with recovery from adaptation at longer stimulus onset asynchronies (SOAs). However, the visual snow patient showed the opposite pattern, with reduced C2 amplitude at longer SOAs despite distinct C2 peaks at the shortest SOAs. These results stand in contrast not only to the pattern of C2 VEP attenuation in controls, but also to a lack of adaptation previously reported for the pattern-onset P1 VEP in this patient. Exploratory source localization using equivalent current dipole fitting further suggested that P1 and C2 VEPs in the visual snow patient arose from distinct sources in extrastriate visual cortex. While preliminary, these results support differential patterns of VEP attenuation and potentiation within the same individual, potentially pointing toward multiple mechanisms of abnormal neuronal responsiveness in visual snow syndrome.

Topographical Analysis of the Onset VEP in the Detection of Paracentral Visual Field Defects

Transient visual evoked potentials (VEP) were recorded simultaneously from 16 electrodes evenly placed over posterior scalp locations covering the occipital, posterior parietal and temporal areas. Interhemispheric amplitude difference of the N70 deflection was established across 6 homologous lateral electrode pairs in 15 normal controls and 32 patients with chiasmatic or retrochiasmatic cerebral lesions. Twenty-three of these had known homonymous or bitemporal field defects while 9 had normal fields on routine perimetry. Significant interhemispheric asymmetry of any single electrode pair occurred in 55% of the 32 patients with known pathology, while the cumulative yield of all electrode pairs was over 80 percent. The diagnostic yield of individual electrode pairs was significantly different: the electrode pair placed over the temporo-parietal junction detected the highest number of abnormalities. The cumulative abnormality taken over all lateral electrode pairs could be described with a curve well fitted with a probability summation function. It is inferred that the contribution of several independent generator sources is reflected in the N70 of the pattern onset VEP. The results suggest that multichannel recording of the interhemispheric amplitude distribution of the N70 of the onset VEP is useful for the evaluation of paracentral visual field defects.

Topography and homogeneity of monkey VI studied through subdurally recorded pattern-evoked potentials

Using small checkerboard stimulus fields, we have recorded visually evoked potentials (VEPs) in an alert rhesus monkey from an array of 35 electrodes chronically implanted between dura and arachnoid to study mass neuronal activity in striate and peristriate visual cortex. Although the principal purpose of this work was to study in detail cortical mapping in this particular animal for future intracortical recordings, we report here the usefulness of our approach for the non-invasive study of cortical processing, in particular of cortical magnification and receptive-field properties over the central 6° of the visual field.The striate and extrastriate components in the pattern onset VEP both have a double negative-going waveform, with N–P–N peak latencies of 75–100–135 ms and 90–115–160 ms, respectively, for small element sizes and moderate contrasts; latencies may be 5 ms shorter for large element sizes and high contrast. We found little activity at electrode locations over visual areas beyond V2. The waveforms and timing permit some careful speculation concerning intracortical processing and VEP generation.The complex logarithmic form of the retinotopical projection provides a satisfactory model for our data, if a value of 1–1.2° is used for the offset parameter a. Our data suggest that the most abundant receptive-field size in foveal striate cortex has a center diameter of 12′. This size remains constant up to 2° eccentricity, and increases only slowly up to 4°. The smallest receptive-field sizes seem to be independent of eccentricity, throughout the central 4° of Vl, with a value of 4–8′, in agreement with single-cell data reported by Dow et al. (1981) and Van Essen et al. (1984).