Motor Cortex and Monkeys are Responsive to Statistical Regularities of Letter Strings
A cool new study in the Journal of Cognitive Neuroscience questions the notion that the premotor cortex response to action words is due to implicit motor simulation (de Zubicaray et al., 2013). Previously, the conceptual representation and/or simulation of action words in motor regions of the brain has been taken as evidence for embodied theories of language comprehension (Glenberg & Kaschak, 2002). These theories have been based on fMRI and EEG experiments showing that reading or listening to verbs that depict actions of the face, arm or leg activate somatotopically-specific regions of motor cortex (Hauk et al., 2004).
A review of the fMRI literature on the perception or production of nonwords (e.g., sonerge, rintoon) revealed peak activations in motor regions (shown above in Fig. 2 of de Zubicaray et al., 2013). Nonwords are devoid of semantic meaning and hence cannot elicit motor simulations. However, endings of words (e.g., –erge, –oon) can be highly predictive of grammatical category (verb and noun, respectively, for the examples above).
de Zubicaray and colleagues took advantage of these probabilistic sublexical cues when constructing stimuli for their own fMRI experiment. A set of nouns, verbs, noun-like nonwords, and verb-like nonwords were presented during a grammatical category judgment task (“identify whether a letter string on the screen seemed more likely to be a noun or a verb”). The noun stimuli were not related to body parts (e.g., cavern) and the verbs denoted manual actions (e.g., crumple). In brief, the results suggested that verbs and verb-like nonwords activated premotor cortex to a greater extent than nouns and noun-like nonwords.
The authors took this as evidence that the observed cortical responses to action verbs were due to ortho-phonological probabilistic cues to grammatical class and not to embodied motor representations. In other words, the spelling and pronunciation of word endings influenced activity in motor regions of the brain. Furthermore, the verb-like activations overlapped with the motor hand area, as defined by a localizer scan where participants viewed hand actions.
The results have challenging implications not only for theories of embodied language comprehension, but also for mirror neuron theories of action understanding. In monkeys, neurons in ventral premotor cortex (area F5) fire when they perform an action, and when they observe a similar action. fMRI studies in humans lack the spatial and temporal resolution of single unit recordings, but “mirror-like” activations in a parieto-frontal circuit have led to the hypothesis that the mirror system is responsible for comprehending the actions and intentions of others (Rizzolatti & Sinigaglia, 2010).1
If premotor cortex is sensitive to statistical regularities in nonwords, this presents a problem for grounding language in action areas of the brain.2
Speaking of statistical regularities in letter strings, an intriguing study in baboons suggested that they might be able to distinguish written English words from nonwords using an orthographic code and not just visual pattern matching (Ziegler et al., 2013). New evidence for this interpretation was provided by the transposed letter effect, where humans are able to read scrambled versions of words if the middle letters are reversed in a specific fashion (e.g., casino → caniso).3 This ability would trip you up when making lexical decisions, because caniso would be erroneously identified as a word.
Six trained ‘reading’ baboons actually showed the transposed letter effect, making more false positive errors for nonwords like caniso than for other types of nonwords. In the Discussion, Ziegler et al., (2013) put forth the bold interpretation that the monkeys were able to engage orthographic processes… which are not actually linguistic in nature:
In sum, word/nonword discrimination performance of monkeys is sensitive to a marker effect of orthographic processing—the transposed-letter effect—but not to the effects of visual similarity. This finding clearly suggests that monkeys use a truly orthographic code rather than a visual code.
. . .
Reading and writing are recent cultural inventions in humans. Although baboons do not have human-like language, they are sensitive to a classic marker of orthographic processing. These findings suggest that the front end of reading (Grainger & Dufau, 2012) is supported by neural mechanisms that are much older than the behavior itself and are not linguistic in nature (Platt & Adams, 2012).
Obviously, the monkeys are unable to translate orthography into phonology (i.e., read the words aloud) or perform grammatical judgments, so they wouldn’t show premotor activation for verb-like nonwords…
ADDENDUM (7/3/2013): I meant to link to Andrew D. Wilson and Sabrina Golonka’s Frontiers article, Embodied cognition is not what you think it is. They also have a nice new post on Grounded vs. embodied cognition: a (hopefully uncontentious) note on terminology.
2 One potential objection to the ‘end of embodiment’ argument is that verb-like nonwords primed semantic representations of actual verbs that are orthographically and phonologically similar.
3 But see If You Can Raed Tihs, You Msut Be Raelly Smrat for the limitations and boundary conditions of this effect.
de Zubicaray G, Arciuli J, McMahon K. (2013). Putting an “End” to the Motor Cortex Representations of Action Words. J Cogn Neurosci. Jun 27. [Epub ahead of print]
Glenberg AM, Kaschak MP. (2002). Grounding language in action. Psychonomic Bulletin & Review 9:558-565.
Hauk O, Johnsrude I, Pulvermüller F. (2004). Somatotopic representation of action words in human motor and premotor cortex. Neuron 41:301-7.
Rizzolatti G., Sinigaglia C. (2010). The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations. Nature Reviews Neuroscience 11:264-274.
Ziegler JC, Hannagan T, Dufau S, Montant M, Fagot J, Grainger J. (2013). Transposed-Letter Effects Reveal Orthographic Processing in Baboons. Psychol Sci. Jun 11. [Epub ahead of print]
Published on: July 3, 2013 @ 03:57