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. 2012;7(12):e51791.
doi: 10.1371/journal.pone.0051791. Epub 2012 Dec 12.

VEP correlates of feedback in human cortex

Yury Petrov  1 , Jeffrey Nador, Jiehui Qian
Affiliations

VEP correlates of feedback in human cortex

Yury Petrov et al. PLoS One. 2012.

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.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Top: small receptive fields.
A larger stimulus engages more neurons; VEPs for individual quadrants sum up linearly to the full stimulus VEP. Bottom: large receptive fields. A larger stimulus engages the same neurons; neuronal responses saturate, VEPs for individual quadrants sum up nonlinearly to the full stimulus VEP. The neuronal response input–output nonlinearity is illustrated on the right.
Figure 2
Figure 2. Subject-averaged scalp VEPs at 100 msec after stimulus onset.
The back of the head is shown. Green and red dots indicate the nasion and tragi points respectively. Top: responses to individual quadrant stimuli. Bottom: sum of the four quadrant responses compared to the full-disk response.
Figure 3
Figure 3. Scalp VEPs at an occipital electrode (top panel) and a frontal electrode (bottom panel).
Individual subject’s VEPs are shown by thin traces, the averaged VEPs – by bold traces. Black curves show the sum of four VEPs, one for each quarter-disk stimulus, red curves show full-disk VEPs.
Figure 4
Figure 4. Maximal amplitudes of scalp VEPs (top) and skull-scalp currents (bottom) averaged across subjects.
The maximum activity distributions are shown on flattened scalp surface. Dark dots on the scalp surface indicate electrode locations. The top (North) of each surface map corresponds to nasion, the bottom (South) – to the occipital pole area. Colormaps are not on the same scale across the panels, instead each panel’s scale was set to its full range of amplitude variation. The ranges were (for panels from left to right) 9.8, 5.9, 3.7 formula image for scalp potentials, and 10.9, 7.8, 6.1 formula image for skull-scalp current densities.
Figure 5
Figure 5. Averaged response epochs at the activation hotspots (see the inset).
Quadrant, half-disk, and full-disk VEPs are shown in black, blue, and red respectively. Error bars showing the standard error of the mean (SEM) represent average response variation across the raw epochs. Peaks chosen for further analysis are indicated by the black arrow (the earliest peak) and gray, blue, and green arrows (later peaks).
Figure 6
Figure 6. Amplitudes of the skull-scalp current peaks (normalized values) as a function of the relative stimulus area.
Data for peaks of different latencies are shown with different colors, the corresponding peaks are shown in Figure 5 by arrows of the same color. Solid curves show quadratic polynomial fits to the data. The response nonlinearity measure, formula image, is plotted in the bottom right panel for all peaks.
Figure 7
Figure 7. Left: Response nonlinearity shown in the last panel in Figure 6 plotted as a function of the time from the stimulus onset for the corresponding peaks.
The solid line shows a linear fit to the data compounded over all ROIs except TP. Middle: amplitudes of the three LO peaks and the TP peak compared between contralateral (solid bars) and ipsilateral (hashed bars) stimulation. The bars are color coded by the same colors as the respective peaks in Figure 5. Right: the laterality index for the three LO peaks plotted as a function of the peak latency. The solid line shows a linear fit to the data.
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No external funding was used for this study.
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