Functional perimetry combined with topographical vep analysis

Abstract

The processing of visual input depends on the position of the visual stimuli in the visual field. Based on the anatomical structure of the retina and the cortex, the function and perception vary with the location in the visual field. Due to the low signal-to-noise ratio, electrophysiological recordings in human subjects commonly have to use large stimuli and, therefore, yield poor spatial resolution. The combination of the method of quasi-simultaneous stimulation of many small (1.5° × 1.5° squares) visual field elements by binary m-sequences and topographical recordings allowed us to reconstruct the potential maps elicited at each of 54 visual field locations independently. Twenty-two normal subjects participated in the experiments and observed monocularly a stimulation field of 13.5° × 9° filled with the 54 squares. Mean luminance was 6.5 cd/m² and contrast was 95%. The EEG was recorded in 30 channels with a dense array of electrodes over the occipital brain areas. Individual noise levels of the subjects were estimated and significant signals were analyzed quantitatively. We determined three components between 90 ms and 220 ms latency. Both global field power (GFP) and topography of the components were affected by retinal stimulus location, showing a significant decline of GFP with retinal eccentricity. Our data demonstrate that even small retinal targets may evoke brain activity which can be recorded simultaneously. Scalp field topography depends critically on the exact stimulus location within the foveal and parafoveal retinal areas while response strength mainly depends on eccentricity.

Key Words:

• Visual evoked potential (VEP)
• brain electrical topography
• m-sequence
• local luminance
• perimetry.