W8.1 (C): The neural basis of working memory Flashcards
(15 cards)
What is the strongest evidence for two functions being dissociable?
Double dissociation
What do studies into neurophysiology of monkeys show about working memory?
Suggested a role for the PFC in working memory
Monkeys saw a piece of food in a tray- a shutter came down and the tray is closed. When the shutter opens, the monkey has to remember where the food was located. Single neuron recordings from the PFC showed elevated neuronal firing during the delay period i.e. when the shutter is down. This was interpreted as showing that neurons in the PC hold a representation of the to be remembered stimulus e.g. location of the food
What does sustained activation in the PFC during the delay period of a WM task reflect?
A neuronal working memory template which is a temporary representation of to be remembered info
What further evidence for this model into sustained activation and working memory (monkeys)?
Single monkey neuron recordings where monkeys completed an oculomotor delayed response task (maintained eye gaze at the centre for 3 seconds and then made saccades in the direction of the cue)
Direction specific firing of PFC neurons- interpreted as showing a direct neurophysiological correlate of a WM template
Neurons in pFC seem to encode actual location/target for which the monkey has to make an eye movement
What supporting neuropsychological evidence is there for a role of the PFC in working memory?
Petrides and Miller- administered a self ordered task to patients with frontal lobe lesions where they saw sheets of paper- patients had to touch one of the images and then give the next sheet where they would touch a different image, and so on. They found patients with frontal lobe lesions were much more impaired on this task, indicating a deficit in working memory- this was interpreted as evidence that the PFC holds a representation of info over the short periods of time of a WM task
Consistent with the idea that PFC is important for storing short term info over a period time and the storage buffer for temporary info
Separate storage buffers for different types of info
How did Wilson et al test what vs where systems in working memory?
Two types of visuospatial WM- where there was one system for objects and another for spatial locations- they extended what we already know about visual object recognition into the pFC- the idea there are two visual streams
Monkeys were trained to perform a delayed response task- they saw a cue that instructed them to make eye movement in a particular direction and then had to remember that information before making the response
The key change to the task was that in addition to the standard spatial cues there were also pattern cues, in which a pattern appeared in the centre of the screen and instructed monkeys to make an eye movement in a particular direction- in spatial trials the monkey had to remember the spatial location of cue and in pattern trials they had to maintain the identity of the pattern
What does the standard model of working memory suggest?
There are two types of visuospatial WM- one system for objects and one for spatial locations- extends what we already know about visual object recognition into the PFC and the idea that two visual streams extends into the PFC and enables remembering this info over a short period of time
What evidence is there for what vs where in working memory from human neuroimaging (PET) scans?
Courtney et al (1996)- Subjects were required to either remember the identity or location of three faces and to say whether the test stimulus matched any of the identities or locations- activation for the object WM task seen in ventral PFC and activation task seen in dorsal PFC
Either a spatial or object task so they were asked to ignore one piece of data- same idea for ventral and predorsal cortex
How can multi voxel pattern analysis be used in patterns of working memory and the PFC?
Takes advantage of fine grained patterns of activation in the brain, uses machine learning techniques to teach an algorithm about the pattern of neural activation associated with a particular stimulus- the algortithm is then able to decode what the subject is looking at, simply by viewing their pattern of brain activity
How did Linden et al (2012) map brain activation and information during category specific visual working memory?
Subjects performed a task requiring them to hold several objects in WM- on each trial, they were required to decide whether a single object was part of the memory set (4 categories of objects) Trained pattern classifier to learn patterns of activations for each category and tested the ability of pattern classifier to predict which category subject was holding in WM on each trial
Fed this data into a pattern classifier- brain regions in which the pattern classifier was able to correctly predict which category the subjects were holding in the WM
Action occurring in the back of the brain rather than front- predictive activity from posterior brain regions- broadly, occipto temporal cortex
Conclusions is that: PFC does not seem to hold temporary representations of info in WM; these temporary representations appear to be stored in brain regions involved in sensory processing, in this case, the visual cortex
Implication is that these same regions enable us to process objects when we see them are also involved in storing temporary representations of those objects in WM
What evidence is there AGAINST what/where distinction in PFC?
Task required monkeys to remember an object and first make an eye movement to the correct object and then to correct location- over half of neurons showed both object AND location specificity- they seem to show flexible response properties that are not fixed
How can the adaptive coding model of PFC neurons explain PFC activation during WM?
PFC neurons show flexibility in their response ‘tuning’
Unlike neurons in other brain regions, response properties of PFC neurons are not fixed but adapt to encode currently task relevant info
Clearly something different about the way PFC neurons behave
How can activation in dLPFC during spatial WM be explained?
The concept of chunking- showed that when ppts had to learn a specific sequence of squares, they divided spatial WM tasks into structured and unstructured conditions
Generally, activation increases when things are harder, but in this case, the easier task activated pfc. Conclusion is that this may be due to chunking where ppts try to make it easier for themselves by taking the info stores in working memory and putting it into a manageable chunk
What role can other processes play in explaining the neuropsychological evidence for a role of the PFC in working memory?
Whilst v specific to damage, they are not very specific in terms of actual processes being measured
Tasks don’t just measure WM but instead a variety of other processes such as storage of previously touched items, selective attention to novel items and engaging in planning etc
So just because patients w lesions perform badly doesn’t mean the PFC is mediating memory and nothing else
What are they key points?
Human and monkey studies converge to suggest the pFC is NOT organised according to the type of stimulus held in WM (what vs where)
PFC may not store info in the WM at all, so what is the role of the PFC?
We have seen the ability to follow complex task rules is a key component of fluid intelligence- does the PFC play a role in this?
