CASE STUDY: Categorical in American Sign

When we listen to a speaker of our native language we tend not to hear a continuous flow of sound (which it mostly is) – rather, we perceive the utterance as distinct units. Anyone trying to learn a second language is likely to have found that when spoken by a native of the language it sounds like a continuous flow of sounds.

The perception of as distinct units rather than as a continuous stream is known as categorical perception and has mostly been studied using speech. However, Emmorey et al. (2003) studied categorical perception in users of American (ASL). The experiments compared deaf signers and hearing non-signers on ASL.

There are three important aspects to ASL: hand configuration, place of articulation and movement. The first refers to the configuration of the hands and fingers, the second refers to the location of the hands about the body and the third refers to the movement of the hands and arms. However, not all hand configurations and places of articulation are distinctive in ASL. The author hypothesised that deaf signers but not hearing non-signers would show categorical perception for distinctive hand configurations and places of articulation.

A computer program was used to generate a continuous movement of the hands from one sign to another; for example, from PLEASE, signed by showing the back of the hand with thumb and four fingers extended, to SORRY, which is signed in a similar way but with only the thumb extended (see figure below). Two tasks were used: a discrimination task and a categorical task.

For the discrimination task, eleven images or frames were created for each word pair (e.g., the signs for PLEASE and for SORRY, and nine equally spaced intermediate images). Participants were presented with two images that were two frames apart, and then a third image that was either identical to the first or second image. The task was to identify whether the third image was the same as the first image or the second image.

Example of two signs used in the study.

For the categorisation task, subjects were shown the two signs (the two end points on the continuum) and then one further sign, which was a randomly chosen image from the eleven. Participants had to say (in a forced-choice paradigm) whether the image represented one or other of the two signs (end points).

The results showed that deaf signers showed a categorical perception effect with the discrimination task, but the hearing non-signers did not. Neither group showed categorical perception with the categorical task. This was determined by comparing responses at the category boundary, where deaf signers showed a significantly higher accuracy in the task than hearing non-signers.

This study shows that categorical perception is not unique to speech processing but can occur in ASL. Thus, the categorical perception may “arise naturally as a part of language processing, whether that language is signed or spoken” (Emmorey et al., 2003, p. 39).

Reference Emmorey, K., McCullough, S. & Brentari, D. (2003). Categorical perception in American Sign Language. Language and Cognitive Processes, 18(1): 21–45 [doi: 10.1080/01690960143000416].