Default vs. frequent; Other kinds of evidence

Mark McConville
Henry S. Thompson
9 March 2012
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1. Regularity: Majority or defaultness?

There are two other important ways in which English plural morphology is similar to English past tense morphology.

Firstly, there are many more nouns that take the regular plural suffix than form irregular plural forms.

Secondly, the regular plural suffix is the default way of forming the plural, used when an irregular plural form cannot be found, for whatever reason:

2. Default vs. frequent, cont'd

So, there are two distinct ways of understanding morphological regularity:

These two different interpretations coincide in English, with both past tense and plural inflection

This "coincidence" is why it is possible to train R&M-style pattern associators to mimic English inflectional morphology

What evidence can we find from other languages?

Are there languages where the psycholinguistically default plural is not in the majority?

3. German past tense inflection

German past tense morphology is similar to English

But the relative proportions of regular versus irregular verbs differ.

German has more irregular verbs than English

So, based on one counting method at least, German regular verbs appear to be in a minority.

However, the regular past tense suffix is clearly the psycholinguistic default:

4. German plural morphology

German has eight different ways of forming plural nouns

The first seven classes are reckoned by German linguists to be completely irregular.

The eighth class (-s) is the least frequent

But -s appears to be the default plural ending, just like -s in English:

German linguists have called -s the "Notpluralendung" - the emergency plural.

Thus, the German -s plural is a textbook example of the distinction between the two senses of morphological regularity

Since the -s plural is so infrequent, pattern associators cannot learn to mimic its default behaviour.

Pinker takes this as evidence that regularity cannot be reduced to a pattern stamped on children's minds by repeated exposure to words.

5. Correlation or causation?

Connectionist explanations of regular morphology may well have got things back-to-front.

The connectionist argument runs as follows:

But the evidence from German suggest the opposite direction of causation:

The reason that German has fewer regular verbs (and regular plurals) than English is simply that Germany was never conquered by a French speaking ruling aristocracy.

6. The causal arrow, cont'd

Similarly, the following argument is a common misconception:

But we have already seen many examples of regular verbs becoming irregular:

So, languages don't consistently evolve in either direction

Same is true more widely about language change

7. Data from other languages

Dutch (another West-Germanic language):

French (a Romance language - still Indo-European):

8. More languages

Hungarian (non Indo-European):

Arabic (Semitic - non-European):

Hebrew (also Semitic):

9. Still more languages; Conclusion

Chinese (Sino-Tibetan - isolating i.e. no inflection):

Arapesh (New Guinean):

Moral: all languages appear to have the characteristic signs of rules - the existence of default words or morphemes to fall back on when memory lets speakers down.

So the dual mechanism account is strongly supported

10. Cognitive neuroscience

Psychology has traditionally been the study of the human mind as a black box

New techniques allow neuropsychologists to study the living brain without invading it

These techniques allow us to study living brains in action, including injured brains.

DNA testing allows us to pinpoint genes responsible for inherited psychological conditions.

Together, these advances have led to a new field of study: cognitive neuroscience.

Can we isolate words and rules in different parts of the brain?

Can we learn anything from cases of direct neurological damage?

11. Brain as computational system

Can we identify a bit of brain devoted to words and a bit devoted to rules?

Unfortunately, the brain is not the kind of organ in which every function needs to be carried out by a chunk of tissue with a recognisable shape.

The brain is a computational system

Also: we cannot force experimental subjects to just relax and think pure past tense thoughts

So to prove that regular and irregular verbs rely on different parts of the brain

12. Human brain anatomy

The human brain consists of billions of neurons, connected by trillions of synapses.

These are all organised into dozens of lumps, sheets and strands in a convuluted 3D structure.

The brain has two hemispheres

Each hemisphere is divided into parts by the central sulcus and the Sylvian fissure

[no description, sorry]

The major language areas are on the banks of the Sylvian fissure

So, at a very rough level we can postulate:

13. Double dissociation

The supersized human brain is a vulnerable organ, which can be damaged by a variety of causes - tumours, infection, malnutrition, blocked/burst arteries, falls, bullets, car accidents.

Brain-damaged patients are a crucial source of evidence for cognitive scientists.

Imagine that a patient with an injury to brain part X can no longer do feat Y but can still do feat Z

But this is too simple

Neuropsychologist Hans-Lukas Teuber pointed out that links between mind and brain should be based on a double dissociation, involving two kinds of patient and two kinds of task

This would suggest that X and Y differ in the kind of work that they do, and that the kind of work done by X involves task Z whereas the kind of work done by Y involves task W.

14. Double dissociation and spelling rules

Spelling rules give us a nice example of double dissociation involving language.

Consider the following two conditions:

Some brain-damage patients have surface dyslexia but not phonological dyslexia.

Other brain-damage patients have phonological dyslexia but not surface dyslexia.

So we can conclude that the brain contains two distinct routes from printed text to sound:

Each of these routes can be damaged independently of the other.

15. Double dissociation and connectionist models

Connectionist models such as R&M's try to capture regular and irregular forms in a single pattern associator memory.

This approach makes it impossible to simulate double dissociation.

Connectionist modellers generally simulate damage by eliminating/weakening connections at random.

This leads to a single dissociation involving inability to handle irregular words

Claims that connectionist models have been able to exhibit double dissociation are only valid for artificially small toy models

16. Agrammatism

Aphasia is an impairment of language following an injury to the brain

Agrammatism is a symptom of some forms of aphasia, where patients have difficulty

Agrammatic aphasics have less trouble retrieving individual words and idioms.

Examples of agrammatic speech:

Agrammatism is often caused by damage to Broca's area, and surrounding regions of the frontal lobe.

17. Agrammatism and inflection

Agrammatics have particular problems with inflectional suffixes either leaving them out or using the wrong one

But studies have shown that such patients make fewer such errors with irregular past tense and plural forms than they do with regular ones

18. Agrammatism cont'd

Similar studies have discounted alternative explanations, e.g.

Studies involving language production among agrammatics have shown similar results

19. Anomia

The other half of the double dissociation involves anomia

Anomic patients generally have words stuck on the tip of their tongue, and resort to circumlocution, pronouns, and generic words like "something" or "stuff".

For example, here is an anomic patient trying to talk about his elbow:

And a clock:

Anomia is often caused by damage to posterior parts of the brain, in and around Wernicke's area.

20. Jargon aphasia

Some anomics exhibit jargon aphasia

Interestingly, jargon aphasics often inflect their jargon with regular suffixes ("a self-administered wug test"!) -

Again, this suggests that regular inflection is computed in a different part of the brain from the parts in which words are stored and retrieved.

Studies have shown that anomic patients:

So, anomic patients appear to be the mirror image of agrammatic patients.

21. Priming and double dissociation

Recall the priming effect in lexical decision tasks with regular and irregular past tense forms

William Marslen-Wilson and Lorraine Tyler investigated priming effects in agrammatic and anomic patients.

In two agrammatic patients

And in one anomic patient

Again we see a classic double dissociation, suggesting that regular and irregular inflection are handled in different parts of the brain.