Temporal frequency dependence of the polarity inversion between upper and lower visual field in the pattern-onset steady-state visual evoked potential.
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Abstract
Purpose: According to the cruciform model, the upper and lower halves of the visual field representation in the primary visual cortex are located mainly on the opposite sides of the calcarine sulcus. Such a shape would have consequences for the surface-recorded visual evoked potential (VEP), as V1 responses to stimulation of the upper and lower hemifield manifest with opposite polarity (i.e., polarity inversion). However, the steady-state VEP results from a complex superposition of response components from different cortical sources, which can obscure the inversion of polarity. The present study assesses the issue for different stimulation frequencies which result in different patterns of superposition in the steady-state response.
Methods: Sequences of brief pattern-onset stimuli were presented at different stimulation rates ranging from 2 Hz (transient VEP) to 13 Hz (steady-state VEP). The upper and lower hemifields were tested separately and simultaneously. The data were assessed both in the time domain and in the frequency domain.
Results: Comparing the responses to the stimulation of upper and lower hemifield, polarity inversion was present within a limited time interval following individual stimulus onsets. With increasing frequency, this resulted in an approximate inversion of the full steady-state response and consequently in a phase shift of approximately 180° in the time-domain response. Polarity inversion was more prominent at electrode Pz, also for transient responses. Our data also demonstrated that the sum of the hemifield responses is a good approximation of the full-field response.
Conclusion: While the basic phenomenon of polarity inversion occurs irrespective of the stimulus frequency, its relative impact on the steady-state response as a whole is the largest for high stimulation rates. We propose that this is because longer-lasting response components from other visual areas are not well represented in the steady-state VEP at higher frequencies.
期刊介绍：
Documenta Ophthalmologica is an official publication of the International Society for Clinical Electrophysiology of Vision. The purpose of the journal is to promote the understanding and application of clinical electrophysiology of vision. Documenta Ophthalmologica will publish reviews, research articles, technical notes, brief reports and case studies which inform the readers about basic and clinical sciences related to visual electrodiagnosis and means to improve diagnosis and clinical management of patients using visual electrophysiology. Studies may involve animals or humans. In either case appropriate care must be taken to follow the Declaration of Helsinki for human subject or appropriate humane standards of animal care (e.g., the ARVO standards on Animal Care and Use).
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