Abstract
Pattern Visual Evoked Potentials (VEPs) are electrical signals recorded from the visual cortex in response to pattern reversal stimuli. Although VEPs are typically evoked using a single field (monofield), horizontal and vertical meridian asymmetries arising from asymmetrical stimulations are well known. Typically these recordings need to be performed in separate trials. In this study, a new bideconvolution method employing two temporally jittered orthogonal sequences to stimulate two visual fields simultaneously is employed. Responses were recorded from three scalp postions (O1,Oz,O2) of normal subjects. Transient (2.5 rev/sec) and quasi-steady-state (QSS) (12.5 rev/sec) pattern VEPs were acquired under both monofield and bifield conditions. During recordings, subjects focused on one of five fixations along the bifield stimulation display. Each display field was driven by different orthogonal reversal sequences with two similar mean rates but slightly different temporal jitters. These biorthogonal sequences enabled us to extract two independent VEPs using continuous loop deconvolution algorithm (Ozdamar et al., 2014; Toft-Nielsen et al., 2014). VEP components (P60; N75; P100; N135) were analyzed using latency/amplitude measures. Responses were consistent and clearly distinguishable at both reversal rates. Under monofield conditions, all gaze fixations generated symmetric VEP responses. Under bifield recording conditions, two asymmetrical VEPs were generated by each active hemifield, with the highest amplitude occurring when fixation points corresponding to the active hemifield. All waveform components were clearly distinguishable against the residual noise levels. Bifield stimulation results are consistent with the exclusive representation of the visual hemifield in the contralateral visual cortex. Both P60 and P100 components show similar lateralization properties and may reflect different stages of the same process. Differences in bifield VEP morphology revealed by simultaneous recordings may have significance in exploring retinal, visual pathway and cortical topography and could greatly enhance the diagnostic utility of the pattern VEP in many visual disorders.