Friday, August 3, 2012

A lexicon without semantics?

I spend a lot of time thinking about words. The reason I am so focused on words is that they sit right at that fascinating boundary between “perception” and “cognition”. Recognizing a spoken word is essentially a (rather difficult) pattern recognition problem: there is a complex and variable perceptual signal that needs to be mapped to a particular word object. But what is that word object? Is it just an entry in some mental list of known words? Are perceptual properties preserved or is it completely abstracted from the surface form? Does the word object include the meaning of the word, like a dictionary entry? The entire contents of all possible meanings or just some context-specific subset?

At least going back to Morton’s (1961) “logogen” model, and including the work of Patterson & Shewell (1987) and Coltheart and colleagues (e.g., Coltheart et al., 2001), researchers have argued that the lexicon (or lexicons) must represent words in a way that is abstracted from the surface form and independent of meaning. In part, this argument was based on evidence that some individuals with substantial semantic impairments could nevertheless reasonably accurately distinguish real words from fake words (the “lexical decision” task).

An alternative approach, based on parallel distributed processing and emphasizing emergent representations (e.g., McClelland, 2010), argues that the “lexicon” is really just the intermediate representation between perceptual and semantic levels, so it will necessarily have some properties of both. Michael Ramscar conducted a very elegant set of experiments showing how semantic information infiltrates past-tense formation (Ramscar, 2002): given a novel verb like “sprink”, if participants were led to believe that it meant something like “drink”, they tended to say that the past tense should be “sprank”, but if they were led to believe that it meant something like “wink” or “blink”, then they tended to say that the past tense should be “sprinked”. In other words, past-tense formation is influenced both by meaning and surface similarity. Tim Rogers and colleagues (Rogers et al., 2004) showed that the apparent ability of semantically-impaired individuals to perform lexical decision was really based on visual familiarity: these individuals consistently chose the spelling that was more typical of English, regardless of whether it was correct or not for this particular word (for example, “grist” over “gryst”, but also “trist” over “tryst”; “cheese” over “cheize”, but also “seese” over “seize”).

Data like these have been enough to convince me that the PDP view is right, but there are a few counter-examples that I am not sure how to explain. Among them is a recent short case report of a patient with a severe semantic deficit (semantic dementia), but remarkably good ability to solve anagrams (Teichmann et al., 2012). She was able to solve 18 out of 20 anagrams (“H-E-T-A-N-L-E-P” --> “ELEPHANT”) without knowing what any of the 20 words meant. Neurologically intact age-matched controls solved essentially the same number of anagrams (17.4 ± 1.5) in the same amount of time. Related cases of “hyperlexia” (good word reading with impaired comprehension) have also been reported (e.g., Castles et al., 2010). I can imagine how a PDP account of these data might look, but to my knowledge, it has not been developed.

Castles, A., Crichton, A., & Prior, M. (2010). Developmental dissociations between lexical reading and comprehension: Evidence from two cases of hyperlexia. Cortex, 46(10), 1238-47. doi:10.1016/j.cortex.2010.06.016 Coltheart, M., Rastle, K., Perry, C., Langdon, R., & Ziegler, J. (2001). DRC: A dual route cascaded model of visual word recognition and reading aloud. Psychological Review, 108(1), 204-256. 
McClelland, J. L. (2010). Emergence in Cognitive Science. Topics in Cognitive Science, 2(4), 751-770. doi:10.1111/j.1756-8765.2010.01116.x. 
Morton, J. (1961). Reading, context and the perception of words. Unpublished PhD thesis, University of Reading, Reading, England.
Patterson, K., & Shewell, C. (1987). Speak and spell: Dissociations and word-class effects. In M. Coltheart, G. Sartori, & R. Job (Eds.), The cognitive neuropsychology of language (pp. 273-294). London: Erlbaum.
Ramscar, M. (2002). The role of meaning in inflection: why the past tense does not require a rule. Cognitive Psychology, 45(1), 45-94.
Rogers, T. T., Lambon Ralph, M. A., Hodges, J. R., & Patterson, K. E. (2004). Natural selection: The impact of semantic impairment on lexical and object decision. Cognitive Neuropsychology, 21(2-4), 331-352.
Teichmann et al. (2012). A mental lexicon without semantics. Neurology. DOI: 10.1212/WNL.0b013e3182635749

1 comment:

  1. Hi Dan,

    Stumbled on this page searching for an article of Herr Ramscar's and thought to leave a quick comment. We recently published an update to the 2002 paper in LCP using a reading time measure. I thought you might also find these papers on semantics interesting:

    Baayen, R. H. and Moscoso del Prado Martin, F. (2005) Semantic density and past-tense formation in three Germanic languages. Language 81, 666-698.

    Ramscar, M & Dye, M. (2011) Learning language from the input: Why innate constraints can’t explain noun compounding. Cognitive Psychology, 62(1), 1-40.

    And check out Baayen et al.'s (2011) reading model based on principles of naive discriminative learning :)

    Cheers, Melody