Abstract
It is known that neural responses become less dependent on the stimulus size and location along the visual pathway. This study aimed to use this property to find evidence of neural feedback in visually evoked potentials (VEP). High-density VEPs evoked by a contrast reversing checkerboard were collected from 15 normal observers using a 128-channel EEG system. Surface Laplacian method was used to calculate skull-scalp currents corresponding to the measured scalp potentials. This allowed us to identify several distinct foci of skull-scalp currents and to analyse their individual time-courses. Response nonlinearity as a function of the stimulus size increased markedly from the occipital to temporal loci. Similarly, the nonlinearity of reactivations (late evoked response peaks) over the occipital, lateral-occipital, and frontal scalp regions increased with the peak latency. Response laterality (contralateral vs. ipsilateral) was analysed in lateral-occipital and temporal loci. Early lateral-occipital responses were strongly contralateral but the response laterality decreased and then disappeared for later peaks. Responses in temporal loci did not differ significantly between contralateral and ipsilateral stimulation. Overall, the results suggest that feedback from higher-tier visual areas, e.g., those in temporal cortices, may significantly contribute to reactivations in early visual areas.
Highlights
The event-related potentials technique (ERP) has been actively used in brain research for the last half-century
Evoked potentials for individual quadrants are shown in the top row, their sum is shown in the bottom row where it can be compared with the visually evoked potentials (VEP) evoked by the full-disk stimulus
The occipital pole negativity represented by the cool colors changed only slightly between the sum of the quadrants and the full-disk stimuli unlike the occipito-temporal positivity represented by the hot colors, which was much weaker for the fulldisk stimulus
Summary
The event-related potentials technique (ERP) has been actively used in brain research for the last half-century. Because the ERP features usually reflect integrated activity of multiple cortical sites interpretation of their neurophysiological bases is problematic making the ERP technique very empirical [1,2,3,4]. Causality analysis can in principle be applied to such problems [6,7,8], but because ERPs normally constitute only a fraction of the on-going EEG activity it is likely that causality measures will be dominated by causal interactions within the endogenous EEG activity (brain rhythms, etc.) rather than by the exogenous event-related potentials. In this study we propose a different approach to this problem based on analyzing the amount of response saturation with stimulus area within the ERP waveform
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