Use of visual evoked potentials (VEPs) to investigate mechanisms involved in the reading process

Abstract

Aim: In this study we have attempted to use VEPs directly to investigate visual mechanisms in the initial stages of the reading process. Method: The pattern onset mode of stimulation (using a 330 ms presentation time) approximately stimulates the intake of information during reading and accordingly we have adopted this technique to investigate possible responses at several cortical loci which are known to be involved in the reading process. To obtain the best results from localized areas of the cortex, a technique using a 3 cm radius triangular Laplacian derivation was adapted. Right and left hemifoveal responses were recorded with a Medelec Sapphire system, using pattern and letter stimuli. Signals arising from two putative generators, one in the striate cortex (5% up from inion, on the midline) and another possibly from the human homologue of primate area V4 (12.5% up from inion, 17% right), were studied using the 1–3° stimuli to determine contrast responses and spatial selectivity. Four averaged responses from 32 sweeps, filtered (1-50 Hz), were recorded for contrasts of 80% and 20% and spatial frequencies between 1–22.32 cpd. Results: The tuning function for checkerboard patterns at 80% contrast for the lateral generator peaked at 2.75 cpd and showed a rapid fall off. The midline response at 80% contrast peaked at 5 cpd and showed a much slower fall off. Responses to letter stimuli were attenuated to approximately 1 μV over V4 and 300 nanovolts from the angular/supramarginal gyri. No significant result was obtained from Wernicke's and Broca's areas. Conclusions: VEPs may be used to directly investigate the reading process. Further refinement of techniques is necessary to record responses from more specialized areas such as Wernicke's and Broca's areas.