Visual evoked cortical potential (VECP) elicited by sinusoidal gratings controlled by pseudo-random stimulation

Abstract

The contributions of contrast detection mechanisms to the visual cortical evoked potential (VECP) have been investigated studying the contrast-response and spatial frequency-response functions. Previously, the use of m-sequences for stimulus control has been almost restricted to multifocal electrophysiology stimulation and, in some aspects, it substantially differs from conventional VECPs. Single stimulation with spatial contrast temporally controlled by m-sequences has not been extensively tested or compared to multifocal techniques. Our purpose was to evaluate the influence of spatial frequency and contrast of sinusoidal gratings on the VECP elicited by pseudo-random stimulation. Nine normal subjects were stimulated by achromatic sinusoidal gratings driven by pseudo random binary m-sequence at seven spatial frequencies (0.4-10 cpd) and three stimulus sizes (4°, 8°, and 16° of visual angle). At 8° subtence, six contrast levels were used (3.12-99%). The first order kernel (K1) did not provide a consistent measurable signal across spatial frequencies and contrasts that were tested-signal was very small or absent-while the second order kernel first (K2.1) and second (K2.2) slices exhibited reliable responses for the stimulus range. The main differences between results obtained with the K2.1 and K2.2 were in the contrast gain as measured in the amplitude versus contrast and amplitude versus spatial frequency functions. The results indicated that K2.1 was dominated by M-pathway, but for some stimulus condition some P-pathway contribution could be found, while the second slice reflected the P-pathway contribution. The present work extended previous findings of the visual pathways contribution to VECP elicited by pseudorandom stimulation for a wider range of spatial frequencies.

Figure 1. Mean VECP kernel waveforms obtained from 9 subjects at three spatial frequencies.

Left column: first order kernels (K1). Center column: second order kernel first slices (K2.1). Right column: second order kernel second slices (K2.2). Due to their amplitude versus spatial frequency (Fig. 2) and amplitude versus contrast (Figure 3) functions we have suggested that K2.1 is mainly dominated by the M-pathway response or a mixture of M- and P-pathway influence, whereas K2.2 is mainly dominated by the P-pathway response (see the text for more details).

Figure 2. Mean RMS amplitude of VECP kernels across the spatial frequency domain at high contrast stimulation.

First order kernels showed very small or no signal at all stimulus conditions (K1, left). The highest response of the second order kernel first slice occurred at low spatial frequencies (K2.1, center). The highest response of the second order kernel second slice occurred at intermediate spatial frequencies (K2.2, right). Error bars are standard errors of the means (SEM).

Figure 3. Mean RMS amplitude of VECP kernels for different contrast at three spatial frequencies (0.4, 2, and 10 cpd).

First order kernels showed small or no signal at all stimulus conditions (K1, left). Second order kernel first slice (K2.1, center) and second slice (K2.2, right) amplitude saturated at high contrast at all spatial frequencies, the effect being more robust at low and intermediate spatial frequencies. We have fitted the contrast response mean values with power functions (black curves) and estimated their saturation index (z). K2.1 functions have larger z than K2.2 functions, especially at low spatial frequency. To exploit further this issue, we fitted the data point with Michaelis-Menten functions (red curves). This allowed us to compare contrast gain (g) of amplitude versus contrast functions for K2.1 and K2.2. The difference in contrast gain was aligned with the hypothesis that K2.1 is dominated by M-pathway contribution while K2.2 is dominated by P-pathway contribution. However, the difference in saturation indicated that they are similar or even contrary to the above hypothesis. One possibility is that K2.1 has a mixed contribution of the M- and P-pathways. Error bars are SEM.

Figure 4. VECP components of the second order kernel first slice (K2.1), N1, P1 and P2 (right).

Mean amplitude of the VECP components of the second order kernel first slice along the spatial frequency domain at high contrast stimulation (left). Circles and squares represent P1 and P2 components, respectively. They showed opposite spatial frequency tuning: P1 amplitude was high-pass tuned while P2 amplitude was low-pass tuned. Error bars in the left panel are SEM.

Figure 5. VECP components of the second order kernel first slice, P1 and P2, elicited by stimulus at different contrast levels.

(A–C) VECP waveforms at three contrast levels. (D–F) Component amplitudes as a function of stimulus contrast. At low spatial frequency, P2 component dominated at all contrast levels. At intermediate spatial frequency, P1 and P2 components co-existed only at high contrast. When contrast was lowered, P1 component became very small while P2 component remained large. At high spatial frequency, both components were present at 25–99% contrast levels and, in addition, P1 is larger than P2 at the highest contrast level. Error bars in the lower panels are SEM.

Figure 6. Influence of the stimulus size in the VECP waveforms of the second order kernel first slice (K2.1).

At low spatial frequencies, the waveforms were similar for all stimulus sizes and the P2 component dominated the waveforms (left). At intermediate spatial frequencies, the P1 component was small or absent for small stimuli (4°) but was present for large stimuli (8° and 16°), while P2 amplitude was similar across all stimulus sizes (center). At high spatial frequencies, the two components largely overlap for small stimuli, but remained separated for large stimuli (left).

Figure 7. Mean VECP kernel waveforms obtained from 9 subjects at three spatial frequencies and 99% contrast.

Left column: second order kernel first and second slices (K2.1 and K2.2, respectively). Center column: first principal component waveforms extracted from K2.1 and K2.2. Right column: second principal component waveforms extracted from K2.1 and K2.2.

Figure 8. Contrast response functions for K2.1 first (filled circles) and second (empty circles) principal components at three spatial frequencies (0.4 cpd, 2 cpd, and 10 cpd) (A–C), and for K2.2 first (filled circles) and second (empty circles) principal components at the same spatial frequencies (D–F).

K2.1 first principal component is more sensitive to contrast than K2.1 second principal component and K2.2 first and second principal components. We fitted power functions to the mean values (not shown for clarity) and observed that the largest difference between K2.1 first and second principal components were seen at 0.4 cpd (z = 0.84 and 0.37, respectively) and 2 cpd (z = 0.57 and 0.38, respectively) and between K2.2. first and second principal components were seen at 2 cpd (z = 0.57 and 0.37, respectively). We fitted Michaelis-Menten functions to the mean values (not shown for clarity) and observed that the largest difference between K2.1 first and second principal components were seen at 0.4 cpd (g = 1.06 and 0.31, respectively). The difference in contrast gain are suggestive that K2.1 first principal component is dominated by M-pathway response, whereas the other components are dominated by the response of a less contrast sensitive pathway such as the P-pathway. Error bars are SEM.

Figure 9. Spatial frequency response functions for K2.1 first and second principal components at three contrast levels (99%, 50%, and 25%) (A–C), and for K2.2 first and second principal components at the same contrast levels (D–F).

K2.1 first principal component amplitude generally decreases as spatial frequency increases whereas K2.2 second principal component amplitude and amplitudes of K2.2 first and second components generally increases when spatial frequency is increased. These results also suggest that the K2.1 first principal component is dominated by M-pathway response whereas the other components are dominated by P-pathway response. Error bars are SEM.