Dynamic information about letter production influences visual letter perception:
Evidence from an acquired letter recognition deficit

• ¹ Johns Hopkins University, Cognitive Science, United States
• ² Macquarie University, Cognitive Science, Australia

Letter recognition is traditionally assumed to involve matching a perceptual representation of a stimulus letter with a stored representation of a static letter shape. However, people learn not only to recognize letters, but also to write them, and several researchers have suggested that knowledge concerning the dynamics of letter production (e.g., knowledge about the direction and sequencing of writing strokes) may play a role in letter recognition (James & Atwood, 2009; Longcamp, Lagarrigue, & Velay, 2010; Parkinson & Khurana, 2007). We report results from NGN, a 77-year-old man who suffered a large left ventro-medial lesion as a consequence of stroke. NGN is severely impaired in visual letter identification. His perception of letter shapes is intact, but his ability to map these shapes onto abstract letter representations is severely disrupted. We investigated whether dynamic presentation of letters might improve NGN’s letter identification. Upper-case letters were presented individually in dynamic or static conditions (see Figure 1). In the Dynamic-Forward condition a moving dot drew a path from the beginning to the end of the letter as it is typically written, until the entire letter was displayed. In the Static condition the whole letter was displayed for the full duration required to “write” the letter in the dynamic condition. NGN’s letter identification accuracy was significantly higher in the Dynamic-Forward condition (90%) than in the Static condition (73%), p < .001. Two additional dynamic conditions established that the advantage of the Dynamic-Forward condition was not due to the mere presence of motion in this condition (a possibility suggested by the findings of Rauschecker et al., 2011), or to the fact that the stimulus shape evolved over time. In the Dynamic-Backward condition a moving dot “wrote” the letter from end to beginning; and in the Dynamic-Random condition the dots making up the letter shape appeared in random order at the same rate as in the other dynamic conditions, until the entire letter was displayed. In both of these conditions, NGN’s performance was significantly worse than in the Dynamic-Forward condition (ps < .01), and no better than his performance in the Static condition: Accuracy was 80% in the Dynamic-Backward condition and 74% in the Dynamic-Random condition. These results demonstrate that mere motion, or mere change over time, are not sufficient to facilitate NGN’s letter identification; only the typical writing motion is helpful. Our findings are consistent with hypotheses holding that knowledge of writing motions contributes to letter identification, and also suggest that dynamic letter displays may be worth exploring for purposes of rehabilitation. Our results also raise an interesting point about rehabilitation strategies in which individuals with letter identification deficits are asked to trace the shapes of visually-presented letters on their hands, in the air, or on a surface. Facilitatory effects of the tracing strategies on letter identification performance have typically been attributed to the tactile or kinesthetic representations activated by tracing (e.g., Starrfelt, Olafsdóttir, & Arendt, 2013). Our results suggest that the dynamic visual information provided by the tracing motions may also play an important role.