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Functional Communication Within a Perceptual Network Processing Letters and Pseudoletters

Herdman, Anthony T.

Journal of Clinical Neurophysiology: October 2011 - Volume 28 - Issue 5 - p 441–449
doi: 10.1097/WNP.0b013e318230da5f
Original Article

Summary Many studies have identified regions within human ventral visual stream to be important for object identification and categorization; however, knowledge of how perceptual information is communicated within the visual network is still limited. Current theories posit that if a high correspondence between incoming sensory information and internal representations exists, then the object is rapidly identified, and if there is not, then the object requires extra detailed processing. Event-related responses from the present magnetoencephalography study showed two main effects. The N1m peak latencies were approximately 15 milliseconds earlier to familiar letters than to unfamiliar pseudoletters, and the N2m was more negative to pseudoletters than to letters. Event-related beamforming analyses identified these effects to be within bilateral visual cortices with a right lateralization for the N2m effect. Furthermore, functional connectivity analyses revealed that gamma-band (50–80 Hz) oscillatory phase synchronizations among occipital regions were greater to letters than to pseudoletters (around 85 milliseconds). However, during a later time interval between 245 and 375 milliseconds, pseudoletters elicited greater gamma-band phase synchronizations among a more distributed occipital network than did letters. These findings indicate that familiar object processing begins by at least 85 milliseconds, which could represent an initial match to an internal template. In addition, unfamiliar object processing persisted longer than that for familiar objects, which could reflect greater attention to inexperienced objects to determine their identity and/or to consolidate a new template to aid in future identification.

School of Audiology and Speech Sciences, Faculty of Medicine, University of British Columbia, Vancouver, British Columbia, Canada.

Address correspondence and reprint requests to Anthony T. Herdman, PhD, School of Audiology and Speech Sciences, Faculty of Medicine, University of British Columbia, 2177 Wesbrook Mall, Vancouver, BC, Canada V6T 1Z3; e-mail:

Grants from the Michael Smith Foundation for Health Research, Natural Sciences and Engineering Research Council of Canada, and the Rotman Research Institute funded this project.

Partial findings from this study were presented at the Cognitive Neuroscience Society Meeting in Montreal from April 17 to 20, 2010, and the BIOMAG 2010 meeting in Dubrovnik, Croatia, from March 28 to April 1, 2010.

Copyright © 2011 American Clinical Neurophysiology Society