Language Acquisition Devices vs. Domain-General Mechanisms

Fluency in a language clearly relies on specialized knowledge about that language’s phonology, syntax, and semantics. Human infants are remarkably skilled at acquiring this knowledge, in stark contrast to non-human neonates and, to a lesser extent, human adults. While it may seem reasonable to suggest that a dedicated, language-specific mechanism underlies these skills, there is surprisingly little evidence that such a mechanism is required. Instead, interactions among general-purpose cognitive mechanisms seem sufficient for explaining language acquisition.

At first glance, it can seem obvious that a specialized “language acquisition device” is at work in language acquisition. For example, infants younger than six months of age are able to discriminate a wide variety of phonemes, but lose the ability to discriminate the phonemes that are not a part of (what will become) their native tongue across the next 6 months. Instead, they develop “categorical perception,” in which discrimination between category boundaries is near perfect, but discrimination within category boundaries is near chance. This phenomenon was initially heralded as a result of “speech mode” processing (Diehl, Lotto & Holt, 2004) and was considered a uniquely human adaptation enabling language. Subsequently, categorical perception was demonstrated among macaques, chinchillas, and even quail – none of which have language in the human sense – suggesting instead that this is merely a characteristic of general-purpose auditory processing (Hauser, Chomsky & Fitch, 2002).

One might also suspect a specialized mechanism for language acquisition in phonetic context effects. Consider the stimulus length effect – where the distinction between stops and glides (e.g., /b/ vs. /w/) is signaled reliably only by the duration of the following vowel. Closely related is the “compensation for coarticulation” effect, in which perception of a syllable can be affected by a syllable preceeding it by up to 50ms. Although previously used as evidence that linguistic perception is uniquely tuned to the peculiars of human vocalization, more recent evidence suggests these effects also arise from perceptual contrast effects in auditory processing common to non-human animals (Diehl, et al., 2004). Thus, the apparent role of language-specific mechanisms in phonology can instead be explained by more general purpose mechanisms.

Evidence from grammar learning is often considered less equivocal. For example, although non-human animals (and even some artificial neural networks) have demonstrated human-like phonological processing (Hutzler et al., 2004), only humans seem capable of syntactical constructions as rich as those in language. Yet there are several reasons to suspect that grammar actually arises from domain-general processes. For example, grammaticality judgments can be primed, such that a given construction is considered more grammatical if it has been recently encountered (Luka & Barsalou, 2005) – and priming has been demonstrated in domains as diverse as visual identification and familiarity judgments. This indicates that the mechanisms underlying grammar are fundamentally similar to those used in these other domains.

Grammar’s recursivity and hierarchical rule structures are also frequently argued to support the existence of language-specific mechanisms (e.g., Premack, 2005), but this too can be a result of domain-general processing. For example, the sensitivity of human infants to non-adjacent dependencies in language-like stimuli can result from general-purpose statistical learning (Gomez, 2002). Other evidence suggests that the representation of hierarchical rules may be an organizing principle of prefrontal cortex (Bunge & Zelazo, 2006) and not specific to language per se. There even appears to be disagreement over whether non-human animals are capable of recursivity – for example, Cullicover & Jackendoff (2003) refer to overwhelming evidence “that the behavior of many animals must be governed by combinatorial computation."

In summary, there is no solid evidence to suggest that language-specific mechanisms are at work in language acquisition. Tomorrow's post will cover language disorders, and the extent to which they implicate a language-specific mechanism.


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Anonymous Anonymous said...

you dont address any of pinkers work on regular and irregular verbs? I read some of this stuff a while back and dont really know where the consensus is right now....?

10/17/2006 07:15:00 AM  
Blogger Chris Chatham said...

Yeah, that's true I didn't touch the overregularization debate here. But since neural nets have been able to show the same developmental patterns both in normal and in disordered populations, I think it's uncontroversial that you don't need a specialized language device to explain past tense errors.

10/17/2006 07:46:00 AM  

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