UBC Theses and Dissertations
An electrocortical investigation of word recognition in a backward masking paradigm Brandeis, Daniel Ulrich
Three aspects of stimulus content, i.e. meaningfulness, familiarity and task relevance, were manipulated without the subjects awareness. A number of subliminal (backward-masked) stimuli were presented to the subject whose task it was to estimate an interval of 1 sec (starting with the presentation-flash) by pressing a button. Supraliminal words were randomly interspersed among these, subliminal stimuli, appearing above or below the masked field. Whenever the subject detected a previously assigned target among the supraliminal stimuli, he/she was required to press the button as fast as possible. The meaningfulness of the subliminal material was manipulated using words, nonwords and blanks. Three groups of words were used: the targets, the nontargets and other, 'new' words (which were never presented supraliminally). Task relevance (targets vs. nontargets) and familiarity ('new' words vs. other words) were thus manipulated. Unexpectedly, detection performance was better with words than with nonwords. This suggests that detection is a late process drawing on lexical information. Several components of the event related potential (ERP) differentiated as early as 140 msec poststimulus between sub-and supraliminal conditions. More importantly, differences within the subliminal conditions were observed: familiarity was discriminated after 260 msec and simple presence of a string after 300 msec. These results are consistent with the conclusions drawn from detection performance, and they support the notion that backward masking does not disrupt processing.
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