Abstract
N40 is a well‐known component of evoked potentials with respect to the auditory and somatosensory modality but not much recognized with regard to the visual modality. To be detected with event‐related potentials (ERPs), it requires an optimal signal‐to‐noise ratio. To investigate the nature of visual N40, we recorded EEG/ERP signals from 20 participants. Each of them was presented with 1800 spatial frequency gratings of 0.75, 1.5, 3 and 6 c/deg. Data were collected from 128 sites while participants were engaged in both passive viewing and attention conditions. N40 (30–55 ms) was modulated by alertness and selective attention; in fact, it was larger to targets than irrelevant and passively viewed spatial frequency gratings. Its strongest intracranial sources were the bilateral thalamic nuclei of pulvinar, according to swLORETA. The active network included precuneus, insula and inferior parietal lobule. An N80 component (60–90 ms) was also identified, which was larger to targets than irrelevant/passive stimuli and more negative to high than low spatial frequencies. In contrast, N40 was not sensitive to spatial frequency per se, nor did it show a polarity inversion as a function of spatial frequency. Attention, alertness and spatial frequency effects were also found for the later components P1, N2 and P300. The attentional effects increased in magnitude over time. The data showed that ERPs can pick up the earliest synchronized activity, deriving in part from thalamic nuclei, before the visual information has actually reached the occipital cortex.
Highlights
The C1 component of visual evoked potentials (VEPs) has been known for some time and has been extensively studied by electrophysiologists for its sensory and attentional characteristics (Bodis-Wollner et al, 1992; Capilla et al, 2016; Clark et al, 1995; Jeffreys & Axford, 1972; Proverbio et al, 2010; Proverbio, Del Zotto, & Zani, 2007; Regan, 1989; Zani & Proverbio, 2009, 2018, 2020; Zhang et al, 2015), existence itself of an N40 thalamic visual potential recorded by the scalp is still a matter of debate
Post hoc comparisons showed that N2 was significantly (P = 0.009) larger to target than close non-targets only for to 3 c/deg target gratings, whereas for all spatial frequencies, N2 elicited by targets differed (P < 0.0005) from those elicited by far non-targets
We identified a negative deflection of about 40 ms in latency, quantified between 30 and 55 ms over the occipital areas (O1, O2, PPO1, PPO2), which was modulated by attention, being more negative for the target than for all other stimuli
Summary
The C1 (or P/N80) component of visual evoked potentials (VEPs) has been known for some time and has been extensively studied by electrophysiologists for its sensory and attentional characteristics (Bodis-Wollner et al, 1992; Capilla et al, 2016; Clark et al, 1995; Jeffreys & Axford, 1972; Proverbio et al, 2010; Proverbio, Del Zotto, & Zani, 2007; Regan, 1989; Zani & Proverbio, 2009, 2018, 2020; Zhang et al, 2015), existence itself of an N40 thalamic visual potential recorded by the scalp is still a matter of debate. Choi et al, 1977) It was first identified, using deep recording techniques, in response to flash stimulation in primates (Kraut et al, 1985, 1990). Neurophysiological recordings using multichannel electrodes have shown that lamina 4C in the macaque contributes to the surface flash-VEP N40 (Kraut et al, 1985; Schroeder et al, 1992) and to the pattern VEP N40 (Schroeder et al, 1991). Tenke et al (1993) reported that the main contribution to the initial scalp-recorded N40 of the VEP in monkey is generated by a combination of presynaptic activation of the axon terminals of the thalamo-cortical afferents and of excitatory presynaptic potentials on the stellate cells within lamina 4C
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