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Temporal order of strokes primes letter recognition Jim Parkinson and Beena Khurana University of Sussex, Falmer, East Sussex, UK

Does the perception of objects that are the result of human actions reflect the underlying temporal structure of the actions that gave rise to them? We tested whether the temporal order of letter strokes influences letter recognition. In three experiments, participants were asked to identify letters that temporally unfolded as an additive sequence of letter strokes, either consistent or inconsistent with common writing action. Participants were significantly faster to identify letters from consistent temporal sequences, indicating that the initial part of the sequence contained sufficient information to prime letter recognition. We suggest that letter perception reflects the temporal structure of letter production; in other words, Simon sees as Simon does.

Individuals learn and generate many actions whilst interacting with the environment, and in doing so they perceive the consequences. Among the most pervasive of learnt actions that give rise to contingent perceptual products are drawing in general and script writing in particular. Could the tandem coupling of the acts of writing with the perception of resultant figures consequently link perceptual outcomes to their microgenesis? Writing is an inherently serial process unfolding over time such that sequences have consistent and particular temporal signatures. It follows that the perceptual products of writing will also temporally unfold. Thus, the act of writing a single letter character entails the sequential and additive occurrence of perceptual products in a given temporal order. Does the perception of the endproduct, the complete letter in this case, reflect the sequential processes by which it is created?

Temporality is normally associated with events rather than objects of perception. Events, in general, have a beginning, middle, and end (Zacks & Tversky, 2001). However, here we extend this theoretical framework to the percepts of objects that are the result of human actions. Consider products of actions that temporally unfold as having the structure of an event. Thus, the perception of an action-based object will also have a beginning, middle, and end. The concept of a temporally extended formation of a final percept has been expressed as the microgenesis of experience. This idea stresses the forward-oriented nature of experience; that is, perceptual experience is a dynamic process with a developmental history that unfolds over time. Whilst each subsequent stage of this developing percept may supersede the previous stages in terms of overall informational content, the developmental history is not

Correspondence should be addressed to Jim Parkinson or Beena Khurana, Department of Psychology, University of Sussex, Falmer, East Sussex, BN1 9QH, UK. E-mail: [email protected] or [email protected] The authors acknowledge and appreciate the constructive comments of members of the PAC Lab at the University of Sussex on issues of design and presentation: Romi Nijhawan, Gerrit Maus, Martin Thirkettle, Ruth Habibi, and Mike Beaton. # 2006 The Experimental Psychology Society http://www.psypress.com/qjep

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DOI:10.1080/10.1080/17470210600937460

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eclipsed (Rosenthal, 2004). This temporally extended nature of perception can therefore lead to the anticipation of future percepts based upon the (short-term) developmental history of the perception. Sanocki (2001) demonstrated that the visual system is sensitive to the temporal order of appearance of parts of a figure. Figure recognition is better when large-scale fragments are presented prior to small-scale fragments than vice versa. However, these results may reflect the visual system’s processing hierarchy (large to small) rather than temporal order per se. We suggest that if the percept is the result of a welllearnt action with a particular temporal order, then the microgenesis of the percept will not only reflect the order of the action, but will lead to the anticipation of the final percept. That is, the perception of an action-based object is “active” in that the registration of the resultant end-state is inextricably accompanied by the temporal process—the reflection of the action—that gave rise to it (Freyd, 1987; Shepard, 1984). Precedence exists for the influence of character production on perception. Character formation can be reliably extracted from static handwritten examples of a character, aiding recognition (Freyd, 1983). Character production can influence online perception as well. For example, Chinese character strokes have a particular order, position, and direction. Individuals fluent in writing Chinese perceive the apparent motion of the direction of a stroke in a Chinese character to be congruent to the way of writing that stroke, opposing the direction predicted by basic mechanisms of apparent motion (Tse & Cavanagh, 2000). Non-Chinese writers do not experience this action-congruent direction of apparent motion, indicating a top-down influence of writing action on the perception of direction of motion. This top-down effect in Chinese readers occurs only when a character is presented with writing cues as to the character’s production, such as a script that appears handwritten (chirographic) as opposed to a script composed of only horizontal and vertical lines (Li & Yeh, 2003). We propose that there may be a similar form of perceptual sensitivity to the temporal order of

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sequences since various segments of an action and therefore the percepts associated with each action segment occur in a specific temporal order. It is intuitive that the process of viewing an action is more informative than viewing only the end state of an action. If the perception of the action reflects the temporal sequence of performing the action, perceiving the first part of the perceptual sequence in the correct temporal order should lead to an expectation of subsequent parts of the perceptual sequence. In other words, presentation of action-consistent sequences of perceptual precursors should prime perceptual endstates, whereas presentation of action-inconsistent perceptual sequences should not. The action of writing a letter is a good example of the temporally sequential nature of human actions. In general, individuals have a well-learnt, largely invariant method of writing letters that is shared among those with a common educational history. One aspect of letter writing relevant to the present study is the largely consistent temporal order in which each stroke of a letter is produced. Pre-exposing Chinese readers to the “early” as opposed to “late” strokes of a character facilitates recognition of the character, suggesting that character production can impact character accessibility (Flores d’Arcais, 1994). We begin with the premise that both the action of writing a letter and the consequent perceptual products will share the same sequential temporal signature with regard to stroke order. Now consider an animation of a letter as an additive sequence of strokes that unfolds over a short period of time. Our initial experiment tested whether writing consistent temporal order of strokes primes the perception of the final product—that is, the complete letter. On the other hand, does temporal stroke order information that is inconsistent with writing action delay the recognition of the complete letter?

EXPERIMENT 1 In Experiment 1 participants were presented capital letters H, N, and their experimentally

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defined nonletter counterparts for purposes of discrimination (Figure 1). Nonletter stimuli were created from letters by moving one stroke to alter their overall shape. Participants were required to categorize letters versus nonletters by means of a two-alternative forced-choice response, while reaction times and error rates were measured. Critically, two dynamic presentation modes provided temporal stroke order information in the form of short three-frame stroke-by-stroke

animations. In the consistent presentation mode (see Figure 1a), the stroke order presented was consistent with that used by the majority of right-handed individuals (Simner, 1981). In the inconsistent mode (see Figure 1b), the stroke order was inconsistent with that used by the majority. In contrast, the immediate mode (Figure 1c) did not include any temporal information; the target frame immediately followed fixation.

Figure 1. The time course and contents of the frames used in the various stimulus sequences. In Experiments 1 and 3, stimuli were based on capital letters N and H. In the three dynamic modes, consistent (1a), inconsistent (1b) and dynamic neutral (1f; Experiment 3 only), a fixation point was presented for 1,000 ms, followed by each frame at 100 ms intervals. In the immediate mode (1c; Experiment 1 only), the target frame immediately followed the fixation frame. While only the immediate and dynamic neutral modes for letter N-based stimuli are shown, equivalent stimuli for letter H-based trials were also used in Experiments 1 and 3. Figures 1d and 1e show the stimulus sequences based on the letter Z that were used in Experiment 2. Note that regardless of whether a trial involved the consistent or inconsistent presentation modes, the experimental task to decide whether a particular stimulus was a letter or a nonletter is only resolved in the final, target frame. In all experiments, the target frame remained visible for only 100 ms. THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 0000, 00 (0)

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Method

Results

A total of 12 volunteers, naı¨ve regarding the hypotheses with normal or corrected-to-normal vision, participated. Stimuli were presented using SuperLab Pro 1.75 software on a monitor set to a refresh rate of 100 Hz. The visual stimuli of each frame were black on a white background. At a viewing distance of 1 m, both letters and nonletters subtended a horizontal visual angle of 2.248. Letters subtended a vertical visual angle of 2.248, and nonletters 4.488. The experiment consisted of randomized trials in a 3 (presentation mode)  2 (letter: H or N)  2 (letter validity: letter or nonletter) design. Trials began with a centrally presented small circular fixation point for 1,000 ms. Each frame in the dynamic presentation modes was 100 ms in duration and was immediately replaced by the next, following the relevant sequence for the mode. The target frame was removed from the screen 100 ms after onset, and reaction times were measured from target frame onset. The experiments were conducted in a cubicle lined with black-out curtains with low-level ambient light provided from above. Responses using the index finger of the right hand to indicate “letter” and that of the left to indicate “nonletter” were made on a computer keyboard. Each experimental session consisted of 24 practice followed by 240 experimental trials (2 and 20 trials per condition, respectively).

The response patterns for letter N trials (all conditions) did not differ from those for letter H trials1 and were therefore combined. A 3  2 repeated measures analysis of variance (ANOVA) indicated a main effect of mode of presentation, F(2, 22) ¼ 56.6, p , .001, no effect of letter validity, F(1, 11) ¼ 2.45, p ¼ .146, and a significant interaction between the variables of presentation mode and letter validity, F(1.3, 13.8) ¼ 8.87, p , .01 (see Figure 2).2 Post hoc t tests revealed that while letters were recognized quicker in the consistent than in the inconsistent mode, t(11) ¼ 4.80, p ¼ .001, mean difference ¼ 40.5 ms, Pearson r ¼ .82, nonletters in the consistent mode produced longer reaction times than those in the inconsistent mode, t(11) ¼ 4.70, p , .01, mean difference ¼ 15.9 ms, r ¼ .82.3 Consistent mode letters were recognized an average of 76 ms faster than those in the immediate mode, t(11) ¼ 7.70, p , .001, r ¼ .92. Both letters and nonletters in the immediate mode were recognized more slowly than those in the inconsistent mode, t(11) ¼ 2.34, p , .05, mean difference ¼ 35.5 ms, r ¼ .58, and t(11) ¼ 9.92, p , .001, mean difference ¼ 61.7 ms, r ¼ .95, respectively. All mean error rates were below 4% (see inset, Figure 2), and no statistically significant effects were found in the error rates. Upon completion participants were asked questions about the experimental stimuli. Although the obvious difference between animated and nonanimated trials was commented upon, no participant stated the differ-

1 Initially, the data were analysed with a 3  2  2 ANOVA (presentation mode by letter by letter validity). The letter (H or N) variable did not interact to any significant level with any other variable, indicating that the response patterns were consistent across the two letter types. 2 The Greenhouse–Geisser correction is quoted for the interaction as sphericity could not be assumed: Mauchley’s test, x2(2) ¼ 8.92, p , .05. 3 Prior to this experiment, we ran a version that was identical (N ¼ 12), except that the target frame was not removed from screen until a response was made. The pattern of results was the same as that in Experiment 1, except that there was no significant difference in reaction times between consistent and inconsistent dynamic presentation of nonletter stimuli, t(11) ¼ 0.380, p ¼ .711, r ¼ .11. Leaving the target frame onscreen until a response is made may have enabled participants to delay their responses as a function of increased exposure to the target frame, reducing the salience of the prior stroke order information and somewhat diluting the nonletter effects.

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Figure 2. Reaction times to determining whether the target was a letter or nonletter using both N and H stimuli in Experiment 1. Error bars are SE. Pair-wise t-tests were performed between conditions. Two-tailed results as indicated:   p , .01,  p , .05. Inset shows percentage error rates in Experiment 1 (error bars are SE).

ence between the dynamic conditions. Thus, the pattern of responses is not based on the conscious awareness of the congruence between the consistent mode and writing action.

Discussion Experiment 1 provides initial evidence for temporal stroke order priming of letter recognition without an attendant effect on identification accuracy. Intriguingly, people are largely unaware of well-learned and well-executed movements. In concurrence, when we asked people to describe how they write the capital letter N (say), no one immediately produced declarative knowledge. Instead, everyone took to “air-writing”: performing the action of writing the character with a finger in order to provide verbal descriptions. Based on introspective reports of participants in Experiment 1, the action-consistent facilitation appears to be independent of reportable knowledge of its basis. The influence of temporal stroke order on letter perception manifested itself in Experiment 1 in two separate ways. First, action-consistent temporal stroke order facilitated the identification

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of letters. Second, action-consistent temporal stroke order inhibited the identification of nonletters relative to the inconsistent mode. For both letters and nonletters in the consistent mode, the first two frames contain temporal stroke order information consistent with writing letters, thus priming letter recognition even in the case of the target frame resulting in a nonletter, thereby impeding the response to a nonletter. Essentially, it is as if temporally consistent information leads the system “down the garden path”, to use the well-known concept from psycholinguistics. We have posited a priming account that invokes the influence of letter production. However, the allocation of attention based on either left-to-right reading strategies or unidirectional movement could also account for the better performance in the consistent condition.4 For consistent mode letters (Figure 1a) the three strokes unfold in a left-to-right manner whereas in the inconsistent mode (Figure 1b) there is a left –right – left unfolding. Thus, in the inconsistent mode, attention has to backtrack in order to follow the appearance of strokes. In other words, the consistent mode permits the smooth, unidirectional movement of attention in the normal direction of English reading, whereas the inconsistent mode requires a nonsmooth, backtracking of attention in a direction opposite to that of reading. In order to test this competing attention movement account, we employed a letter that calls for unidirectional attention shifts regardless of presentation mode. The stimuli were based on a computer-generated capital letter Z. The consistent presentation mode used a stroke order reflecting writing action (Figure 1d), whereas the inconsistent (Figure 1e) did not. However in both modes attention has to shift vertically in one direction only, either top-to-bottom or bottom-to-top, without having to backtrack. Therefore these stimuli permit a more equitable test of the attention movement account versus

With thanks to Axel Buchner and an anonymous reviewer for highlighting this alternative account. THE QUARTERLY JOURNAL OF EXPERIMENTAL PSYCHOLOGY, 0000, 00 (0)

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temporal stroke production.

order

priming

via

letter

significant effect was found in the error rate data (inset Figure 3).

EXPERIMENT 2

Discussion

Experiment 2 was identical to Experiment 1, except that only two presentation modes were used: consistent and inconsistent (Figures 1d and 1e). A total of 12 new volunteers naı¨ve to the hypothesis participated.

The results of Experiment 2 mirror those of Experiment 1 with stimuli that are not biased in terms of the allocation of attention. The findings with the letter Z do not easily lend themselves to an attention movement account based on either reading patterns or unidirectional shifts. One further concern regarding Experiment 1 is the appropriateness of the immediate mode as a baseline condition against which temporal stroke order priming is being assessed. The immediate mode is biased unfavourably in that no temporal information about the upcoming target frame is provided. In the dynamic modes participants may have been in a better position to anticipate the onset of the target frame because the first two frames act as temporal cues for the onset of the target frame. In the immediate mode without any temporal cues, the onset of the target frame may not have permitted adequate time for response preparation. This in turn may have caused the relatively poor performance compared to the dynamic modes. The above considerations do not impact upon the differences between the consistent and inconsistent modes; however, they do bring into question whether the speeded identification of letters in the consistent mode is entirely due to stroke order priming. It is integral to the main hypothesis to determine whether providing temporal cues to the onset of the target frame in the immediate mode will produce performance similar to that in the consistent mode. Were this to be the case, then performance benefits in the consistent mode compared to the immediate mode would be principally due to temporally cueing of the target frame. Experiment 3 addressed this issue by providing participants with a two-frame temporal cue before the target frame in all modes, whilst retaining a distinction between consistent and inconsistent temporal stroke order information and the absence of stroke order information in

Results A 2  2 repeated measures ANOVA revealed significant effects of presentation mode, F(1, 11) ¼ 97.6, p , .001, and letter validity, F(1, 11) ¼ 5.87, p , .05, and a significant interaction between presentation mode and letter validity, F(1, 11) ¼ 11.2, p , .01 (Figure 3). Post hoc t tests showed that consistent letters were recognized faster than inconsistent letters, t(11) ¼ 6.55, p , .001, mean difference ¼ 52 ms, r ¼ .89. Consistent nonletters were also recognized faster than inconsistent nonletters, t(11) ¼ 3.66, p , .004, mean difference ¼ 15.9 ms, r ¼ .74; however, the difference was significantly less than the difference between letters, as revealed by a significant interaction. No

Figure 3. Reaction times to determining whether the target was a letter or nonletter using Z stimuli in Experiment 2. Error bars are SE. Pair-wise t-tests were performed between conditions. Twotailed results as indicated:   p , .01. Inset shows percentage error rates in Experiment 2 (error bars are SE).

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the new baseline condition (dynamic neutral mode).

EXPERIMENT 3 Experiment 3 used the same stimuli and procedure as those in Experiment 1, with the following exception: The immediate mode was replaced with a dynamic neutral mode consisting of two initial neutral frames of 100 ms each. The first frame contained a dot twice the size of the fixation point, and the second frame a dot four times its size, both localized at the fixation point (Figure 1f). Thus, in Experiment 3 all three presentation modes were equivalent in terms of temporal precue information. A total of 20 participants5 naı¨ve to the hypothesis volunteered to take part. All had normal or corrected-tonormal vision.

Results Once again, with no difference in the results for N and H trials,6 data from these two conditions were combined and were analysed by a 3  2 repeated measures ANOVA. Significant main effects of presentation mode, F(2, 38) ¼ 86.2, p , .001, and letter validity, F(1, 19) ¼ 35.0, p , .001, and a significant interaction between presentation mode and letter validity, F(2, 38) ¼ 13.6, p , .001, were observed (Figure 4). Analysis with post hoc t tests confirmed that consistent mode letters produced faster reaction times than did inconsistent mode letters, t(19) ¼ 5.81, p , .001, mean difference ¼ 33.3 ms, r ¼ .80, and dynamic neutral mode letters, t(19) ¼ 8.49, p , .001, mean difference ¼ 46.2 ms, r ¼ .89. Nonletters produced slightly but nonsignificantly quicker reaction times in the inconsistent than in the consistent mode, t(19) ¼ 1.81, p ¼ .087, mean difference ¼ 8.8 ms, r ¼ .38.

Figure 4. Reaction times to determining whether the target was a letter or nonletter using both N and H stimuli in Experiment 3. Error bars are SE. Pair-wise t-tests were performed between conditions. Two-tailed results as indicated:   p , .01,  p , .05, † p , .06, †† p , .09. Inset shows percentage error rates in Experiment 3 (error bars are SE).

Since letter recognition latencies in the consistent mode were much faster than those in the inconsistent and dynamic neutral modes, the presence of temporal order stroke priming occurs even with equivalent target onset precueing. On the other hand, even though letters in the inconsistent mode were recognized faster than those in the dynamic neutral mode, t(19) ¼ 2.05, p ¼ .055, r ¼ .42, this mean difference of 12.9 ms was less than half that found between letters in the inconsistent and immediate modes in Experiment 1. Thus target frame precueing did aid response preparation in the dynamic neutral mode, as compared to the immediate mode (Experiment 1). Error rates showed a similar pattern to that in Experiment 1, and all error rates were below 5% (inset Figure 4). In order to fully gauge the magnitude of the temporal stroke order priming effect, data from consistent and inconsistent trials in Experiments 1 and 3 were combined. A 2  2 repeated measures ANOVA showed significant effects of presentation mode, F(1, 31) ¼ 25.4, p , .001, and letter validity, F(1, 31) ¼ 11.2, p ¼ .002, and the interaction between presentation mode and letter validity, F(1, 31) ¼ 52.2, p , .001. Again,

5 With thanks to Axel Buchner for the suggestion to increase the number of participants in Experiment 3, thereby increasing the power of the experiment. 6 There was no significant interaction effect of letter type (H or N) with any other variable based on a 3  2  2 ANOVA.

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consistent letters produced quicker reaction times than did inconsistent letters, t(31) ¼ 7.60, p , .001, mean difference ¼ 36 ms, r ¼ .81, whilst consistent nonletters produced slower reaction times than did inconsistent nonletters, t(31) ¼ 3.46, p ¼ .002, mean difference 11.5 ms, r ¼ .53. These merged data attest the strength and persistence of the temporal stroke order priming effect and demonstrate a significant, but weaker “garden path” effect.

Discussion All presentation modes in Experiment 3 provided temporal cues to the target frame, and therefore the differential response latencies can be attributed to differences in temporal stroke order. If prior to the presentation of the complete letter (target frame), the strokes that make up a letter are visually presented in the order used in writing the letter, recognition of the letter is facilitated. This priming effect improves performance relative to a dynamic neutral mode, showing that the performance enhancement is not solely the result of improved anticipation of target onset. The inconsistent mode produced slightly better performance than the dynamic neutral mode. It appears that strokes presented in the inconsistent order do improve recognition beyond that achieved by providing participants temporal cues to target onset. However, part priming from letter strokes is rendered quite weak in the absence of their temporal order being consistent with writing action. In other words, the “what” in vision is not independent of the “when” in vision. Finally, if the target is a nonletter but the first two frames present strokes in an order consistent with writing, recognition of the nonletter is impaired. This too indicates that consistent stroke order primes letter recognition, such that when the target frame is a nonletter, responses are delayed.

GENERAL DISCUSSION In the past, theoretical accounts of visual perception have more often been constrained by the

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logic of sensory as opposed to motoric considerations. Recently, however, scepticism has been expressed regarding both the need for detailed sensory representations for perception and the independence between perception and action (O’Regan & No¨e, 2001). For example, the discovery of mirror neurons has shown that certain premotor areas are activated both when an action is performed and when a similar action performed by another is perceived (Buccino et al., 2004; Rizzolatti & Craighero, 2004). It is with respect to these forged links between perception and action that we hypothesize that temporal stroke order priming in letter recognition is based on motor patterns used in the production of letters. Past demonstrations of the influence of character production on perception have been limited to characters perceived to be handwritten (Flores d’Arcais, 1994; Freyd, 1983; Li & Yeh, 2003; Tse & Cavanagh, 2000). The distortions and irregularities in handwritten characters make feature analysis difficult, but they do provide cues to production that aid recognition. Feature analysis should be sufficient for the recognition of machine-produced letters such as those used in the present study, especially given that the target frame was identical. However, our findings show the influence of temporal order of strokes to be potent enough to impact the perception of machine-made letters that in their static state contain no dynamic information about their production. These findings are indicative of cross talk between representations used for perception and those for action. In other experimental scenarios it has been shown that the movement to grasp a bar of a given orientation is facilitated by prior exposure to a picture of a hand in the position congruent with the to-be-performed action (Vogt, Taylor, & Hopkins, 2003). That is, perception of an action can affect the production of the action. The opposite direction of interaction has also been observed, in that stored patterns of motor actions or knowledge about actions can constrain and affect the perception of movement (Viviani & Stucchi, 1992).

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Our findings are compatible with the idea that this interaction can be bidirectional. That is, action (motor codes) and perception (sensory systems) can engage in cross-talk, where the perception of an action sequence (i.e., two frames temporally consistent with writing action) engages the action codes (writing the letter), which then affects perception, leading to the expectation of the perceptual outcome of the action (priming the letter). One account for how this bidirectional link might be implemented is common coding theory (Hommel, Mu¨sseler, Aschersleben, & Prinz, 2001). According to this, early antecedents of actions and late products of perception share a common representational domain. Correspondingly, “the perception of an action should activate action representations to the degree that the perceived and the represented action are similar” (Jackson & Decety, 2004, p. 259). Perception of an action sequence activates the relevant motor codes as a form of action simulation (Knoblich & Flach, 2001), a view supported by neurophysiological evidence such as mirror neuron data in macaques (Gallese, Fadiga, Fogassi, & Rizzolatti, 1996) and in humans (Buccino et al., 2004; Rizzolatti & Craighero, 2004). Indeed, visual presentation of static letters activates specific areas of premotor cortex involved in the action of letter writing (Longcamp, Anton, Roth, & Velay, 2003). In turn, this activation of motor codes could lead to the prediction of future perceptual events consistent with the action. This feedback from motor codes to provide the perceptual system with a prediction is concordant with forward models that provide predictions of the consequences of motor commands and are implicated in the understanding of another’s intended actions (Wolpert, Ghahramani, & Jordan, 1995). The importance of temporal order in performing actions is well documented. Here we suggest that this temporal order is part and parcel of the perceptual representation of the process and result of the action. Our findings have shown that part order can prime the perception of the result of a well-learnt action. Thus, we advance the idea that perceiving a multipart action

performed in an action-consistent temporal order activates the motor code of the action. Based on the rest of the motor sequence, a prediction of the next stage in the action is forwarded to the perceptual system, essentially priming the perception of the next step, and ultimately the outcome, of the action. Whether it is the activation of the underlying motor code or the guiding of attention based on an activated motor code cannot be ascertained without brain activation correlates. However, we believe that this effect of temporal order priming for actions is an important aspect of action perception, as shown here for welllearnt actions such as handwriting. Indeed, this kind of common coding/forward model mechanism should act to facilitate cognitive processing between action and perception domains for all imitable actions. Original manuscript received 23 January 2006 Accepted revision received 20 July 2006 First published online day month year

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