Aphasia What is aphasia? Types of aphasia Cognitive neuropsychology and aphasia Syndrome or symptoms? Computational cognitive neuropsychology
The forthcoming lectures • Take a look at several language processes that have interested neuropsychologists (e.g., listening, speaking, reading and writing). • Examine neuropsychological data which bear on theories about the nature and operation of these language processes. • Emphasis is on cognitive neuropsychology as a research discipline, but practical/clinical applications will also be illustrated. • Introduce computational neuropsychology.
What kind of language operations? • ModularInteractive mandatory, encapsulated non-mandatory domain specific non domain specific • Rule basedAssociative explicit grammatical rules pattern of connections operating on linguistic units operating among units symbolic units non symbolic units
What kind of representations? • Semantically transparentDistributed linguistic representations neuron like units with phonemes, syllables, words no immediate referent • DiscreteContinuous binary (all or none) activation level words as units words as patterns damage causes total loss partial loss of function
What is Aphasia? • Aphasia is a neurological disorder caused by damage to those areas of the brain that are responsible for language (frontal/temporal). • Primary signs of the disorder include difficulty in expressing oneself when speaking, trouble understanding speech and difficulty with reading and writing. • Aphasia is most commonly seen in adults who have suffered a stroke but aphasia can also result from a brain tumor, infection, head injury, or a dementia that damages the brain.
Types of aphasia • Aphasia can be divided into four categories: • (1) Expressive aphasia • conveying thoughts through speech or writing. • (2) Receptive aphasia • understanding spoken or written language. • (3) Anomic or amnesia aphasia • difficulty using the correct names for objects, people, places, or events. • (4) Global aphasia • loss of all comprehension and expression.
Differential diagnosis • There are two critical features of the language produced by aphasic patients: • Fluent/Non-fluent: does language production require great effort; are there many pauses and “ums” and “ers”; and is the number of words produced per unit of time very low? • Grammatical/Agrammatical: does language production lack grammatical structure; is it mostly just a string of nouns (note, though, that the nouns themselves can be the correct ones).
Broca’s aphasia • Speech is non-fluent, slow and laboured. • Grammatical structure is absent. • Comprehension of morphemes is normal.
Speech sample • Doctor: Why did you come to hospital? • Patient: “Ah…Monday…ah Dad and Paul…and Dad…hospital. Two…ah…doctors…and thirty minutes…and yes…ah…hospital. Wednesday…nine o’clock…doctors. Two doctors…and ah…teeth. Yeah…fine”.
Wernicke’s aphasia • Speech is fluent, but full of neologisms. • Grammatical structure is preserved. • Comprehension of morphemes is impaired.
Speech sample • Doctor: Why did you come to hospital? • Patient: “Never, now mista I wanna tell you this happened when happened when he rent. His - kell come down here and is - he got ren something. And he roden all these arranjen from the pedis on from iss pescid”.
What is the prognosis? • The outcome of aphasia is difficult to predict given the wide range of variability of the condition but some skills are easier to treat. • Generally, people who are younger or have less extensive brain damage fare better. • The location of the injury is also important and is another clue to prognosis. • In general, patients tend to recover skills in language comprehension more completely than those skills involving expression.
Cognitive neuropsychology • Cognitive psychologists study the behaviour of people with normally functioning mental systems to draw conclusions about the mind. • Cognitive neuropsychologists specialise in the study of behaviour of individuals whose language processes are not functioning normally (e.g. after a stroke or head injury). • They use data from studies of people with language impairments sustained after brain damage - aphasia - to test, extend or develop theories about normal language processing.
Six assumptions of cognitive neuropsychology • Brain injured patients show dissociations of language function that are informative. • We can make inferences about the functional architecture of normal language processing by studying these dissociations. • Double dissociations provide strong evidence for independent language processes. • The primacy of single case studies. • "Modularity of mind” (Fodor). • Associations of language impairments are not useful for cognitive neuropsychology.
Problems with associations • Traditional clinical neuropsychology is based on associations rather than dissociations. • Co-occurence of symptoms is used to group patients into syndromes e.g. Broca’s aphasia. • However, symptoms often co-occur for purely anatomical rather than functional reasons. • Even the study of relatively robust aphasic syndromes has not told us anything about the nature of language processing nor the role of specific brain regions for language tasks (though see Robinson et al 1999 for BA45).
Gerstmann’s syndrome • A pattern of co-occurring symptoms that result from a specific lesion in the angular gyrus (which is in the left parietal lobe). • finger agnosia • left-right confusion • agraphia • acalculia • Associations are OK for localisation of brain lesions but are not useful for understanding the organisation of language processes and are not necessary for lesion location (fMRI).
Aphasia and cognitive neuropsychology • The syndromes of aphasia tell us little about the nature of language processing beyond the fact that receptive and expressive language skills are dissociable and that grammar and meaning are processed independently in the human mind. • The study of language breakdown can be used to develop and to test theories of cognitive processing - this will be the focus of the lectures. • Theories can be used to design connectionist or computational models and brain imaging studies in order to understand brain-behaviour.
Computational psychology • This is an approach to cognitive psychology in which models of processing in some cognitive domain such as language (or memory, object recognition or perception) are translated into explicit computer programs. • These programs process input in exactly the same way that people do or so the modeler assumes (e.g. Plaut & Shallice, 1993). • Models that are expressed as computer programs are called computational models.
Why do computer modeling? • Not because we assume the brain or the mind is a computer but for two “scientific” reasons: • If you turn a theory into a computer program then you discover many parts of the theory need further specification – this forces the theorist to make the theory complete (Plaut et al., 1996). • Once there is a complete theory and a program that represents it you can test the theory more rigorously because the program can be used to explore how variables and experimental manipulations influence the model.
Computational cognitive neuropsychology • Computational models can be used to test theories like: data from normal subjects, brain imaging studies and brain damaged patients. • Computational models can be lesioned to test them against the performance of patients who are thought to have a specific cognitive deficit. • If the intact program behaves in the same way as human subjects do and the lesions of the program behave like patients then the theory that generated the program will look feasible.
Summary • Studies of aphasia have enabled us to develop theories of normal language processing and to identify the necessary cognitive processes. • Studies of aphasic patients have shown that the syndrome approach to neuropsychology is limited in what it can tell us about brain-behaviour relationships. • Computational cognitive neuropsychology aims to understand the behaviour of aphasic patients by lesioning connectionist models of language.
References • Plaut, D., & Shallice, T. (1993). Deep dyslexia: A case study of connectionist neuropsychology. Cognitive Neuropsychology, 10(5), 377-500. • Plaut, D.C., et al. (1996). Understanding normal and impaired word reading: Computational principles in quasi-regular domains. Psychological Review, 103(1), 56-115. • Kay, J. et al. (1993). The Psycholinguistic Assessment of Aphasia (PALPA). LEA. Hove. • Robinson, G. et al. (1998). Dynamic aphasia: an inability to select. Brain, 121, 77-89.
Glossary • Abstract words: refer to concepts (love peace). • Concrete words: refer to physical nouns (cat). • Content words: Nouns, verbs adjectives open (new words can be added). • Function words: articles, prepositions, conjunctions closed class. • Mental lexicon: store of information about words. • Morpheme: smallest unit of language that has meaning. • Orthography: visual characteristics of a word. • Phonemes: smallest unit of sound. • Pragmatics: context dependent meanings of words. • Prosody: changes in intonation giving meaning. • Semantics: the meaning of a word.