What can be inferred from a double dissociation?

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How is behaviour measured?
In terms of performance on wide range of tasks that are presumed to involve different combinations of mental functions
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What is cognitive neuropsychology concerned with?
Impact of brain damage on task performance
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What pattern does cognitive neuropsychology use?
Impairment and sparing in patients following brain injury in order to constrain theories of normal cognitive structures and processes
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Patterson and Plaut (2009)
The gold standard was always a double dissociation
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What is a double dissociation?
Patient is impaired in one task but performs normally in a different task.
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What was Teuber's definition of double dissociation?
One brain-injured patient (A) shows unimpaired performance on Task I (e.g. tactile discrimination) but impaired performance on Task II (e.g. visual discrimination) while a second patient (B), with a different lesion site, shows the reverse pattern
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Does frontal lobe lesions impair memory for designs or memory for words?
Right frontal lesion and left frontal lesion and a healthy control
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What do right frontal lesions have?
Right frontal lesion have a poor memory for designs at 66% and a good memory for words 90% performance
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What about left frontal lesions?
Good memory for designs (93%) performance and a poor memory for words (60%)
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What does the opposite performance of left vs right frontal patients suggest?
non overlapping component operations
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What did healthy controls perform at?
An average rate of 95% therefore there isn't a ceiling effect
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What did double dissociations help support?
The early argument proposed by Ungerlieder and Mishkin's (1982) early work with monkeys that suggested there were separate visual processing streams
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What did Milner and Goodale (1992) propose?
They proposed the reinterpretation of the dual pathway model
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What is the first ventral stream?
the visual perception and processing of object form and object recognition, to transform visual information into a perceptual representation of the world.
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What is the dorsal stream involved in?
Controlling actions interacting with the goal object
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The original study was quickly followed by what?
A stream of research which investigated the visuomotor capabilities of DF (Milner et al, 1991)
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What could patient DF?
scale her grip and orientate her wrist correctly, similarly to controls (Milner & Goodale, 1995)
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What tasks did DF fail?
Matching orientation tasks
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What is a matching orientation task?
appearing to choose orientations at random, yet when asked to reach towards a slot and ‘post’ an item she performed at a similar level to controls (Goodale et al., 1991).
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What was the suggestion?
D.F.’s visuomotor abilities suggested that the undamaged dorsal stream was controlling the visuomotor abilities, without the input of the damaged ventral stream.
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What did it emphasise?
The functional dissociation within the visual system
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What is visual agnosia?
Inability to recognise line drawings, geometric shapes
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What was vision?
Intact and there is no damage to the eye itself but patients can't relate what they see and what they know
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Where is the visual stream?
begins in the retina, parvocellular retinal ganglion cells project to the parvocellular layers of the LGN, which transmit to the V1 then to the V4 in the occipito-temporal region.
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What is optic ataxia?
Difficulty in pointing or grasping movement
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What do optic ataxia patients have problems with?
Reaching to visually guided goals in peripheral vision and the deficit leaves voluntary eye movement largely unaffected
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What is the cortical neurologic deficit?
A result of a lesion to the superior parietal lobule and areas around the intraparietal sulcus
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There are no primary what?
Sensory or motor deficits involved in lesions to this region and thus the problem is at a more integrative sensorimotor level
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What can it affect?
One limb in one or both visual hemifields (Not purely visual or spatial disorder)
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What was the tasks performed?
Grasp object (visually guided movement): grasp lines connects points where index finger and thumb made contact with object.
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What was the other tasks?
Discriminate shapes visually (Same/different)
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How did visual agnostics perform?
Poorly on the discriminate shapes tasks (Failed to compare objects based on outline shape) Performed well on the grasp object tasks
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How did optic ataxia perform?
Performed well on discriminate shapes task - Performed poorly on the grasp object task: Unable to use visual information about object shape to control movement
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Crawford et al (2003) argument?
- It could be argued that a dissociation does not show genuinely different mechanisms or functional subsystems but simply a difference in how two tasks are processed.
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What are the differences in performance due to?
Task difficulty rather than a specific impairment in a circumscribed function
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What is a good example of double dissociation?
May be seen in speech and language comprehension which are similar mental processes happening in different areas of the brain independent from each other
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What is Wernicke's aphasia?
Fluent aphasia, normal production, speech sounds and fluent nonsense
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What is often used?
Neologism (New word):word without obvious relation to a recognizable word. e.g., “glester” (as opposed to approximate words in speech apraxia)
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What are the damaged areas in Wernicke's Aphasia?
Postero-superior temporal, opercular supra marginal angular and posterior insular gyri: Damage to these areas leads to wernicke's aphasia
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What is Broca's aphasia?
Fluency also affected by speech apraxia (Co-occuring with Broca's aphasia), disordered programming of articulatory muscles, manifest in approximate words, language comprehensions: essentially intact
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What is looking at?
Double dissociation.
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For example?
A patient diagnosed with Broca’s Aphasia would have difficulty in producing fluent speech however would still understand speech. Whereas Wernicke's is the opposite.
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Who is Patient HM?
Removes parts of the medial temporal lobe on both sides of his brain
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Could not do what?
commit new events to long term memory, although his short term memory was intact- for example, his scores on the digit span subset of the Weschler Adult Intelligence Scale were six forward and five backward
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What is an explanatory hypothesis for patient's HM pattern of impairment can appeal to what?
Subtraction of the more that long term memory requires, while the remainder of the memory system continues as before
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What is the hierarchical model?
This was a dominant model in the 1960s which predicts that if short term memory is impaired, then long term memory will be impaired as well
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What did Patient KF show?
Short term memory with spared long term memory
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How did KF perform?
very poorly on digit span and when presented with two digits could recall only the first: his performance was also poor when letters or words were used instead of digits
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What was normal?
His ability to commit new material to long term memory - assessed for example, by the paired associate learning subtest of the Wechsler Memory scale was normal
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Patient HM showed what?
intact short-term memory as assessed by Digit Span but impaired ability to commit new information to long-term memory while patient KF showed the reverse pattern.
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What was a heterarchical model of normal memory?
there is a route into long-term memory that is relatively independent of the system for auditory-verbal short-term memory can account for this double dissociation.
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In the case of patient HM's pattern of impairment, What was the explanatory hypothesis?
damage to cognitive components that are implicated in long-term memory but not in auditory-verbal short-term memory
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What was the balance of support?
heterarchical or two-route model of normal memory because it allows a better explanation of the double dissociation of impairments than the previously dominant hierarchical model does.
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Who was patient Mu?
- Bilateral lesions of occipital-parietal cortex, shows severe spatial disorientation with relatively well preserved semantic knowledge - Brain damage following an overdose of dextromoramide (Palfium).
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What did he have a digit span of?
7 forwards and 4 backwards
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What was semantic memory?
- Semantic memory was good as shown by performance on the verbal subtests of the WAIS-R and by his score of 50/60 on the Spot-the-word test (Baddeley et al, 1992)
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What could he not complete?
Other areas of difficulty in visa spatial tasks included spatial imagery, picture scanning, matching and visual short term memory
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How did he perform on the tests? Benton et al, 1983
Chance level on the mental rotation of a manikin, picture scanning: 2/9 on this subset, picture matching was also poor with the score of 5/16 on the forced choice version of the benton visual retention test
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What about visuomotor problems?
MU’s performance on Corsi blocks was strikingly poor, to the extent that he was unable to touch a single block indicated by the tester (span 0). - However, when asked to touch parts of his body, MU could do this without error
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What was wrong with JBR?
- Bilateral temporal lobe damage and shows severe semantic problems and no impairment on visuo-spatial tasks - Sustained brain damage as a result of herpes simplex virus encephalitis
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What was he bad at?
Recognised none of the items in the graded naming test and was poor at recognising the same items from their names - His semantic memory problems also led him to score very poorly on an oral version of Spot the word test (Baddeley et al, 1992).
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What was asked?
When asked to give descriptions of 50 famous people, JBR stated that he had not heard of most of them
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What was the semantic task?
- The battery contains 48 stimulus items, representing three categories of living items (land animals, sea creatures, and birds) and three categories of manufactured items (household items, vehicles, and musical instruments),
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What did Mu mostly perform at?
- MU mostly performed at the level of controls, with the exception of the ‘sea creatures’ category (MU 8 items, range for young controls 10-20). JBR again fell consistently well below the range of control subjects’
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In except what?
All categories except vehicles where he was near the lower limit of the normal range
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What was these findings support?
observations of normal or near-normal semantic memory for MU, and very impaired semantic memory for JBR
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What was the visual object and space perception task?
Dot counting, number location, cube analysis and control
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What was involved in each of these tasks?
subjects are asked to count groups of 5 to 9 randomly positioned dots, find a X against a noisy background (Control), counting the cubes,
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What is the problem?
Normal controls performed at ceiling level on these tasks, a problem which often affects studies of semantic memory. Therefore, we cannot conclude that MU’s semantic memory is entirely normal;
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What did JBR show?
JBR showed a discrepancy in performance between living and non living items, scoring somewhat better (Although impaired overall) on non living items
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For example?
JBR’s scores were best for the ‘vehicles’ category, though this was a topic in which he had been particularly interested prior to his illness
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Patterson and Plaut (2009)

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