Psycholinguistics and Neurolinguistics

Speech error types (spoonerisms, metathesis)

Speech errors are unintended slips in production, and their regularities show how utterances are planned before they are spoken. Most errors exchange, anticipate, perseverate, shift, delete or substitute units that belong to the same category and occupy corresponding positions, such as onset with onset, vowel with vowel, stem with stem and affix with affix. Errors that violate the syllable structure of the intended utterance are not expected, so a sound that is moved into a slot it could not occupy in the target form is a sign that the error is not a natural one. A spoonerism is an exchange of the initial sounds of two words, so that the onsets swap places and the words are heard in a new combination, as in 'read the list' becoming 'lead the rist'. Metathesis is the transposition of sounds within a single word, as in the historical or dialectal alternation between 'cavalry' and 'calvary'. Shift errors move an affix to the wrong stem, anticipation errors bring a later segment forward, and deletion errors drop a segment altogether. The term slip of the tongue is reserved for production errors, in which the speaker plans one unit and articulates another. A slip of the ear is a perception error, in which the listener analyses the acoustic signal as a different word or phrase. Exchanging sounds within a phrase during speech is a slip of the tongue, whereas mishearing an intact utterance is a slip of the ear. Malapropisms, which are substitutions of a word by a similar-sounding one, are usually discussed separately as lexical selection errors.

Aphasia types

Aphasia is an acquired disorder of language production or comprehension that results from damage to the language areas of the brain. Causes include open or closed head trauma and neurodegenerative conditions that become more common in advanced age. Broca's aphasia is non-fluent: speech is effortful and telegraphic, grammatical morphology is often omitted (agrammatism), and comprehension of complex sentences such as passives is impaired, while the production of speech is clearly the most affected function. Wernicke's aphasia is fluent but meaningless: speech flows easily yet contains jargon and nonsense words, and comprehension is markedly impaired. Anomic aphasia is characterised by word-finding difficulty, in which the speaker knows what an object is but cannot retrieve its name, so circumlocution is used in place of simple words. Conduction aphasia leaves fluent speech and comprehension relatively intact, but the ability to repeat an utterance is disrupted. Dysphonia, a disorder of the voice, is not an aphasia type, and attention deficit or lack of motivation are not causes of aphasia.

Language-cognition dissociation

A dissociation occurs when one ability is impaired while another is relatively spared, so that language and cognition can be separated in clinical populations. Williams syndrome combines intellectual disability with relatively fluent and well-developed language, which is a classic dissociation of language from general cognition. Dyslexia is a specific reading disorder that appears with typical general intelligence, and specific language impairment (SLI) is a language deficit that occurs with normal nonverbal cognition. Autism spectrum disorder is not a clear dissociation case in this sense, because language and social-cognitive impairments occur together. The same logic applies to motor control: a language function can be lost while general motor skills remain intact, as seen in a signer who loses sign production with Broca's aphasia but can still draw and manipulate objects. Conversely, a person who has lost all voluntary movement can still speak fluently, showing that vocal language output does not depend on the general motor system. These cases indicate that the motor control required for language is separate from general motor skill, and that Broca's aphasia reduces fluency in a signed language as well as a spoken one.

Experimental sampling and descriptive statistics

In sampling, probability (random) methods give every member of the population a known, non-zero chance of selection, and they include simple random, systematic, stratified and cluster sampling. In stratified sampling the population is divided into strata and random samples are drawn within each stratum, while in cluster sampling whole clusters are selected at random. Snowball sampling, which recruits participants through referrals, and convenience sampling, which uses the most accessible people, are non-probability methods because selection is not random. Descriptive statistics summarise the spread of data around the mean, and the standard deviation is the standard measure of that spread. For a sample it is computed as s=\sqrt{\frac{\sum_{i=1}^{n}(x_i-\bar{x})^2}{n-1}}, where the denominator n-1 corrects for estimating the mean from the sample. A data set whose values lie tightly around the mean has a small standard deviation, whereas one with values far from the mean has a large one, so the spread can often be judged by inspection before any calculation is made.

Statistical learning in infants

Statistical learning is the infant's ability to detect distributional regularities in the input, such as the frequency with which one syllable follows another. In speech, the transitional probability between adjacent syllables tends to be high within words and low across word boundaries, so infants can use the drop in probability to segment a continuous stream into word-sized units before they know the lexicon. Statistical learning is therefore a mechanism of language acquisition, distinct from numerical cognition, from the data that researchers collect, and from the learning of phonemic categories.

Dyslexia types (phonological vs surface)

Dyslexia is a reading disorder, and its subtypes are described by which of the two routes to pronunciation is impaired. The sublexical (phonological) route converts graphemes into phonemes by rule, and the lexical route retrieves the pronunciation of familiar words from memory. In phonological dyslexia, the grapheme-to-phoneme conversion route is impaired, so the patient cannot reliably map letters to sounds and cannot read aloud pseudowords or other made-up strings that have no stored entry. In surface dyslexia, the lexical route is damaged and the patient depends on rule-based conversion, so irregular words are regularised and mispronounced. Difficulty in distinguishing homographs and in recognising semantic mismatches reflects impaired meaning processing and belongs to deep or semantic dyslexia rather than to the phonological subtype.