Detecting anticipatory events in handwriting movements À

We investigated how visual processes exploit specific anticipatory movements observed in handwriting gestures. Previous research has shown that the kinematic information contained in the downstroke of an l is exploited to predict the identity of the forthcoming letter. Here, we determined the moment at which prediction takes place. Two between-letter effects were examined: changes in size (ll vs le) and changes in rotation direction (le vs ln). Results show that with only 75% of the l downstroke trajectory (or 60% of the downstroke time) subjects are already capable of predicting the identity of the letter following the l , that is well before the end of the downstroke. Analysis also reveals that identification takes place after the presentation of the movement acceleration phase. The visual perception of motor anticipation seems to involve the detection of motor events. DOI:10.1068/p2864 ÀA pre-test for this research has been reported in Kandel, Boe« , Orliaguet, and Zesiger, 1995 `̀ Visual recovery of motor anticipation in handwriting: Analysis of threshold detection'', in Proceedings of the 7th International Conference on Handwriting and Drawing, 6 ^ 10 August (London, Ontario: International Graphonomics Society) pp 56 ^ 58. In other words, the geometric figure (circle) is interpreted in terms of the movement that produced it (elliptic), eliciting a perceptual illusion. Kandel et al (1994) and Orliaguet et al (1997) suggested that motor information could be used during visual processes to anticipate forthcoming motor sequences. In everyday life we are permanently predicting other people's movements. For instance, when a person puts his/her hand forward to give us an object, we predict the final position of the movement so we place our hand at the right place for grasping it. In other words, the visual system starts a spatiotemporal `reading' process of the motor sequence to predict and adapt the organism to the following one. So, the ability to predict movements has a very important adaptative function. Kandel et al (1994) showed that anticipatory information contained in a handwriting gesture can be exploited by perceptual processes to predict the forthcoming motor sequences. For example, by viewing the production of a letter l, subjects can predict the identity of the following letter (eg ll , le, ln) well before its spatial information becomes available. Complementary experiments showed that the visual perception of motor anticipation is observed with pairs of letters (eg ll , le, lm) as well as with letters within words (eg fille, filet, filme) (Kandel et al 1993). This ability is also found with small and large letters and for a large range of movement times (Kandel et al 1995). Prediction scores are higher in conditions where the stimulus provides kinematic information (dynamic presentation) than in conditions where only spatial information is available (static presentation) (Orliaguet et al 1997). The subject does perceive the shape differences, but these differences do not provide enough information to predict the identity of the forthcoming letter. In addition, this prediction ability is not learned during the experiment. It appears from the first trials and subjects are not aware of the quality of their performance. It is noteworthy that similar results were observed in other kinds of perceptual tasks. In audiovisual speech, visual information can become available well before its acoustic output (Benguërel and Cowan 1974). Cathiard and Lallouache (1992) showed that a French [ y ] can be correctly identified 160 ms before the voiced output. They observed a correspondence between the identification functions and the acceleration peak of the upper lip as well as the interolabial area. Furthermore, the visual perception of anticipatory grasping movements enables subjects to know whether a glass will be grasped for drinking, throwing, or displacing (Orliaguet et al 1996). The aim of the present study was to explore when visual processes detect anticipatory information during the presentation of dynamic handwriting movements. More precisely, the goal was to determine the moment at which the subject can predict the following letter. In other words, how much information about the downstroke of an l is required to predict whether it will be followed by another l , an e, or an n? Two types of contextual effects were explored: changes in size (ll vs le) and changes in rotation direction (le vs ln). The methodological principle was to provide different amounts of kinematic information about the l's downstroke and assess subjects' predictions based on that information. 2 Experiment 1 Cursive handwriting implies the production of continuous sequences of letters varying in shape and size. Several studies have shown that the shape and kinematics of a letter fluctuate as a function of the production of surrounding letters (Thomassen and Schomaker 1986; van Galen et al 1986). That is to say, when reproducing the letter l , the movement time of its downstroke is a function of the spatial constraints of the following letter (Orliaguet and Boe« 1990; Boe« et al 1991). Changes in size (ll vs le) as well as in size and rotation direction (le vs ln) entail temporal differences in the downstroke of the l . Movement time of the l downstroke is shorter when it is followed by another l than when it is followed by an e and, in turn, is shorter still when it is followed by an n. This indicates that the motor system anticipates the following letter while 954 S Kandel, J-P Orliaguet, L-J Boe«