Writing's Shadow: Corticospinal Activation during Letter Observation

Abstract

We can recognize handwritten letters despite the variability among writers. One possible strategy is exploiting the motor memory of orthography. By using TMS, we clarified the excitatory and inhibitory neural circuits of the motor corticospinal pathway that might be activated during the observation of handwritten letters. During experiments, participants looked at the handwritten or printed single letter that appeared in a random order. The excitability of the left and right primary motor cortex (M1) was evaluated by motor-evoked potentials elicited by single-pulse TMS. Short interval intracortical inhibition (SICI) of the left M1 was evaluated using paired-pulse TMS. F waves were measured for the right ulnar nerve. We found significant reduction of corticospinal excitability only for the right hand at 300-400 msec after each letter presentation without significant changes in SICI. This suppression is likely to be of supraspinal origin, because of no significant alteration in F-wave amplitudes. These findings suggest that the recognition of handwritten letters may include the implicit knowledge of "writing" in M1. The M1 activation associated with that process, which has been shown in previous neuroimaging studies, is likely to reflect the active suppression of the corticospinal excitability.

Brain responses to handwritten and printed letters differentially depend on the activation state of the primary motor cortex

“…Nonetheless, neural populations involved in biological motion perception might also play a role, since handwritten letters are traces of biological motion and biological motion analyses are carried out at similar latencies (between 260 and 360 ms) in the right posterior region (Jokisch et al, 2005;Krakowski et al, 2011). The timing of this effect nicely fits recent data by Nakatsuka et al (2012) who used single pulse TMS to show that at 300-400 ms after visual stimulus onset, the cortico-spinal excitability decreased more when participants viewed handwritten than printed letters.…”

Contribution of writing to reading: Dissociation between cognitive and motor process in the left dorsal premotor cortex

“…Therefore, only single-pulse stimulation, which allows us to minimize artifacts on electric muscle responses, was applied. If the handwritten character used here was able to elicit the motor representations of writing gestures as we assumed, this type of character but not the printed one should induce changes in MEPs in digit muscles as previously reported in the literature [Nakatsuka et al, 2012].…”

Brain activation patterns resulting from learning letter forms through active self-production and passive observation in young children

“…Second, our ROI analysis showed that the left precentral gyrus was only recruited above baseline for the actively learned cursive letters, and not for the passively learned or unlearned cursive letters. The left precentral gyrus has been associated with letter perception in several studies and is thought to activate stored motor programs associated with letter production (Longcamp et al, 2005a, 2006b, 2011; James and Gauthier, 2006; Nakatsuka et al, 2012). This idea is supported by the James and Atwood (2009) results, which found that, in adults, motor experience (writing) with pseudoletters led to recruitment of this region, but when the pseudoletters were learned with only visual practice, the precentral gyrus was not recruited any more so than before learning took place.…”

“…More recent studies also found recruitment of motor regions during perception of cursive letters. One study using Transcranial Magnetic Stimulation (TMS), found that recognition of handwritten cursive letters involved the motor cortex, as evidenced by reduced corticospinal excitability for the right hand (Nakatsuka et al, 2012). Another study found that Exner's area in the left dorsal premotor cortex was sensitive to whether dynamic cursive letters were presented forward or backward (Nakamura et al, 2012).…”

“…The recently uncovered neural networks for cursive letters in the adult brain implicate the motor cortex as a major part of the distinction seen in processing printed and cursive letters (Longcamp et al, 2006b, 2011; Nakatsuka et al, 2012; Nakamura et al, 2012). Although letter perception recruits the left motor cortex, perceiving cursive letters appears to enhance this activation in adults.…”