Early Visual Pathway Origins of SSVEP Intermodulation Components Induced by Luminance Flicker

Dual-frequency modulation strategies in SSVEP-BCIs not only elicit fundamental responses but also induce robust intermodulation components (IMCs) through nonlinear neural integration, significantly expanding the available coding space. While IMCs can arise from various levels of the visual hierarchy depending on the stimulus complexity, the fundamental physiological origins of IMCs induced by pure luminance flicker-specifically whether they arise from retinal nonlinearity or cortical integration-remain debated. To resolve this, we employed a head-mounted display to physically isolate visual fields, comparing dichoptic hemifields (cortical) and monocular hemifield (retinal) conditions across varying spatial densities. Our results provide evidence that IMCs in scalp EEG during luminance modulation originate predominantly from retinal nonlinearities. Furthermore, while the geometric shape of the stimulus exerts a significant influence, the total length of the spatial boundaries separating the two frequencies acts as the dominant determinant of nonlinear output intensity. To elucidate why high-frequency IMCs often appear attenuated despite this robust boundary-driven generation, we constructed a spectral synthesis model ( $\text {A} = \beta \cdot \text {ITC} \cdot \text {Freq}^{-\alpha }$ ). This analysis demonstrated that the scalp-recorded amplitude is directly proportional to Inter-Trial Coherence (ITC) but is severely dampened by the brain's intrinsic ${1}/\text {f}$ capacitive filtering, confirming that the apparent weakness of high-frequency components is an artifact of physical transmission.