| dc.description | In the acquired reading disorder known as letter-by-letter reading, words can only be pronounced after each letter is identified individually. The objective of this research was to investigate the cognitive mechanisms that underlie this disorder. These mechanisms were studied in a brain-damaged patient (GM) who demonstrated letter-by-letter reading, as well as an additional impairment in reading irregular words. Comparisons were made to a control group of six brain-damaged patients without reading deficits. The first set of investigations (Experiments 1--5) examined the effects of different psycholinguistic variables in pronunciation and lexical decision tasks to confirm the diagnosis of GM's dyslexia. The second set of investigations (Experiments 6--9) evaluated several prominent hypotheses concerning the deficit that underlies letter-by-letter reading. The hypothesis that letter-by-letter reading results from difficulty processing "abstract letter identities" was tested using a letter matching task with a phonetic confusability manipulation. Comparison with individual control subjects suggested that GM was able to process letters abstractly and that previous positive results may have been due to scaling artifact. The hypothesis that letter-by-letter reading results from difficulty in rapid processing of multiple letters was tested using a rapid serial visual presentation (RSVP) paradigm. GM was found to require an extended period of time after he had processed one letter before he was able to reliably identify a second letter. The final hypothesis that letter-by-letter reading results from an impairment that is not specific to letters was tested using an RSVP task in which processing of letters was compared to that of numbers. GM's deficit was also found to extend to numbers, and this was confirmed in a matching task with letters and numbers intermixed. Implications of these findings for theories of letter-by-letter reading, as well as normal reading and brain organization, are discussed. | en_US |
