Categorical perception of the index finger as an American Sign Language phonological construct

In American Sign Language, specific handshapes found in the manual alphabet and/or numbering system (such as the pointed index finger-the number "1"-and its bent counterpart-the letter "x") are assumed to be phonologically distinct under the rationale that bentness of the finger(s) constitutes a phonemic distinctive feature. This study investigates this notion, building on previous behavioral studies of American Sign Language categorical perception of bentness of the index finger. We examined varying levels of bentness between 180° and 90° using a scale ranging from the signs BLACK and SUMMER in the standard dialect. Experiment 1 found that when presented with all levels of bentness, signing participants identified a categorical boundary around 130°. Experiment 2 used EEG to investigate vMMN and P300 responses to within-category and between-category oddballs. This is the first study to our knowledge to use EEG to investigates categorical perception in ASL. The results of Experiment 2 indicated that signing participants did not consistently agree on the 130° boundary, citing a "lack of context" to identify the sign. However, those signing participants who consistently categorized signs according to the boundary determined in Experiment 1 showed stronger P300 responses to between-category differences than to within-category differences. However, even these participants showed no CP effects in the vMMN. This, combined with individual variation in where the category boundary lies, suggests that CP bentness may be a weaker phonological feature in ASL than spoken phonemic features, or may instead be a phonetic, not phonological, feature.