Analysis of the visual spatiotemporal properties of American Sign Language

Abstract

Careful measurements of the temporal dynamics of speech have provided important insights into phonetic properties of spoken languages, which are important for understanding auditory perception. By contrast, analytic quantification of the visual properties of signed languages is still largely uncharted. Exposure to sign language is a unique experience that could shape and modify low-level visual processing for those who use it regularly (i.e., what we refer to as the Enhanced Exposure Hypothesis). The purpose of the current study was to characterize the visual spatiotemporal properties of American Sign Language (ASL) so that future studies can test the enhanced exposure hypothesis in signers, with the prediction that altered vision should be observed within, more so than outside, the range of properties found in ASL. Using an ultrasonic motion tracking system, we recorded the hand position in 3-dimensional space over time during sign language production of signs, sentences, and narratives. From these data, we calculated several metrics: hand position and eccentricity in space and hand motion speed. For individual signs, we also measured total distance travelled by the dominant hand and total duration of each sign. These metrics were found to fall within a selective range, suggesting that exposure to signs is a specific and unique visual experience, which might alter visual perceptual abilities in signers for visual information within the experienced range, even for non-language stimuli.

Keywords: Eccentricity; Image statistics; Motion; Sign language; Speed.

Figure 1.. Example 2-D motion trajectory.

Position (x, y) of the right dominant hand for the ASL phrase, sign know easy (English translation: “To sign the word ‘know’ is easy”) is plotted. In this example, the target sign is know, represented by the solid line, while the carrier phrase is represented by the dashed line, with larger dashes used for sign and smaller dashes for easy. (The z dimension, not shown here, was also recorded.)

Figure 2.. Scatterplot of hand position over time.

Position coordinates are shown for a) signs, b) sentences and c) narratives. All samples from each stimulus type are presented, separately for the three signers, in each figure. These position coordinates are presented both in terms of centimeters and, for x and y planes only, in degrees from the origin (between the signer’s eyes, defined as 0,0,0). Values are plotted for the frontoparallel plane, i.e., height (y) and width (x), assuming one is facing the signer. On the bottom and left axes, the metric is in centimeters. On the top and right axes, the metric is in visual degrees, assuming a 1.52 meter viewing distance. If one is facing the signer from twice the distance (e.g., 3 meters), the x and y degrees labels would simply be halved, are plotted for the frontoparallel plane, i.e., height (y) and width (x), i.e., assuming one is facing the signer. For each figure, a larger circle depicts the average position for each signer.

Figure 4.. Speed vs. Distance Plots.

3D Speed (in centimeters per second) and distance (centimeters) values across all signs are plotted in separate figures for the three signers, RB, DH, and VM. For each signer, each dot represents the average speed value of a single sign as a function of the sign’s cumulative distance traveled by the hand. The dashed line is the model of constant speed, the thin line is the model of constant duration (see text). The bold line is a logarithmic fit, and the correlation coefficient is presented for this fit.