Poster E24, Saturday, August 18, 3:00 – 4:45 pm, Room 2000AB

Electrophysiological evidence for a left hemisphere bias towards letters as early as 100ms

Kurt Winsler1,2, Stephanie Osmond1, Katherine Midgley1, Phillip Holcomb1, Jonathan Grainger3;1San Diego State University, 2University of California, Davis, 3Aix-Marseille Université

ERP studies of letter and word recognition have found that the brain dissociates between these frequently encountered stimuli and other similarly complex visual stimuli such as symbols or false fonts as early as the N1. This typically left-lateralized effect, often termed an N170 effect, has been taken as evidence of the lower bound on the time course of orthographic processing. The left lateralized nature of these effects agree with other research which has shown that visual linguistic information is faster and more accurately processed when presented to the left hemisphere than the right hemisphere. The current study further examines such hemispheric differences as well as early visual ERP effects by presenting letters and symbols to either the right or left visual field. Additionally, font size was manipulated to probe the lateralization of lower level sensory-perceptual processing. Thirty participants completed a 2AFC character detection task, in which they viewed 216 arrays of three symbols or letters, briefly presented at two degrees to the right or left of fixation. Of these trials, half were presented in a smaller font and half were in larger font, but the center-to-center distance between the characters was preserved. Thus, the design was 2 (presentation side) x 2 (character type) x 2 (character size), with 27 trials per cell. ERP results were analyzed according to electrodes contralateral to presentation side, that is, hemispheric results are comparing right and left electrode sites following presentation to the left or right visual fields respectively. In both hemispheres, larger stimuli elicited more negative-going N170s. Letters generated larger N170s than symbols in both hemispheres, although this effect was considerably greater in the left hemisphere. These findings replicate previous research demonstrating enhanced N170s to letters compared to less familiar stimuli, particularly in the left hemisphere. In keeping with previous research, there were more positive-going P1 waves to the larger stimuli, regardless of which hemisphere it was presented to. However, for stimuli presented to the left hemisphere (i.e. RVF), there were more positive P1s to symbols than to letters. This is evidence that the left hemisphere can show differential activation between letters and other stimuli as early as 100ms. This finding may have important implications for understanding the process of reading and how this skill interacts with hemispheric specialization.

Topic Area: Perception: Orthographic and Other Visual Processes