Neural mechanisms of working memory - week 8 (Chris) Flashcards
(14 cards)
Fuster (1974)
Monkey neurophysiology studies suggested a role for the prefrontal cortex (PFC) in working memory.
In this paradigm, monkeys see a piece of food in a tray, a shutter comes down and the tray is closed. Then when the shutter opens the monkey has to remember where the food was located.
Single neuron recordings from 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 PFC hold a representation of the to-be-remembered stimulus (e.g. location of the food).
Neurons in prefrontal cortex (PFC) show sustained, elevated responses during the delay period of a working memory task
Goldman-Rakic (1987): Standard model of working memory
Sustained activation in PFC during the delay period of a WM task reflects a neuronal WM ‘template’, a temporary representation of to-be-remembered information
Evidence from monkey neurophysiology
Oculomotor delayed response task
In this task monkeys saw a cue on the left or right of fixation and had to maintain their eye gaze at the centre for 3 seconds and then make an eye movement (a saccade) in the direction of the cue. So they had to hold the direction of the cue in memory for 3 seconds.
They found that single neurons in PFC showed direction-specific firing during the delay period of this task (between the cue and response).
They interpreted this as showing a direct neurophysiological correlate of a WM template, a temporary representation of the spatial location indicated by the cue.
Supporting human neuropsychological evidence for a role of the PFC in working memory
Petrides and Milner (1982)
Self-ordered WM task given to patients with frontal and temporal cortex lesions
Patients instructed to touch one picture per sheet of paper and not to touch the same picture twice
Patients with frontal lesions disproportionately impaired on this task
Evidence from monkey neurophysiology for what/where dissociation in working memory
Spatial delayed response task
Pattern delayed response task
B shows the response of a neuron in the inferior convexity – the lower, ventral part of the PFC
C shows the response of a neuron in the dorsolateral (upper, dorsal) part of the PFC.
As you can see, the neuron in the inferior ventral PFC shows higher activation to the patterns and lower activation to spatial cues
Whereas the neuron in the upper dorsal PFC shows higher activation to spatial cues and lower activation to patterns.
What vs where in working memory - supporting evidence from human neuroimaging (PET)
Object WM task – remember the identities of 3 faces
- Activation in ventral PFC
Spatial WM task – remember the locations of 3 faces
Activation in dorsal PFC
Multivoxel pattern analysis (MVPA)
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
Algorithm is then able to ‘decode’ what the subject is looking at simply by viewing their pattern of brain activity
Linden et al. (2012)
Subjects performed task requiring them to hold several objects in WM
On each trial, required to decide whether a single object was part of the memory set
4 categories of objects – faces, bodies, flowers, scenes
Trained pattern classifier to learn patterns of activation for each category
Tested ability of pattern classifier to predict which category subject was holding in WM on each trial
The regions holding category related information are exclusively in the posterior part of the brain.
The implication of this is that the same regions that enable us to process an object when we see it with our eyes are also involved in storing temporary representations of those objects in WM.
Evidence against ventral/dorsal what/where distinction in PFC
Task required monkey to remember an object and first make an eye movement to the correct object, then make an eye movement to the correct location
Over half of neurons showed both object and location selectivity
This study found that single neurons in PFC can encode both the location of an object and also its identity.
Explaining PFC activation during WM: Adaptive coding model of PFC neurons (Duncan)
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 information
Explaining the neuropsychological evidence for a role of the PFC in working memory: The role of other processes
What processes does this task require?
Storage of previously touched item
Suppression of previously touched item
Selective attention to a novel item
Planning/strategy use
Sustained attention
Can we decode information about items held in WM from patterns of activation in prefrontal/parietal cortex?
Riggall and Postle also looked into the question of where in the brain different types of information are represented during working memory.
They scanned people performing a WM task where they had to memorise a moving dot array, and during the delay period were cued to either remember the direction or the speed of the dots.
Then they were shown a probe that was either a match or mismatch.
The key thing here is that when the cue appeared subjects had to follow a rule (attend to the speed or the direction of the dots in WM) and also hold in WM a specific stimulus (the particular speed at which the dots were moving, or the direction in which they were moving).
They found that they could decode which direction and how fast the dots were moving but only from visual cortex and temporal cortex. PFC provided no information about this.
In contrast, task instructions could be decoded from PFC and parietal regions.
fMRI evidence for the internal attention hypothesis of WM
Subjects maintained 2 objects in WM
Subsequently cued either to maintain both objects (non-selective attention condition) or a single object (selective attention condition) in WM
Finally asked to decide whether any of the objects in an array matched the object(s) they were holding in WM
Activation in PFC was greater for the selective condition than the non-selective condition
Connectivity analysis demonstrated that activation in PFC modulated activation in different occipitotemporal regions depending on which stimulus the subject maintained in WM
Combined TMS/fMRI showed that disruption of PFC activity caused distal effects on occipitotemporal activation, thus establishing direction of causation
PFC seems to send an attentional bias signal to sensory-specific regions to enhance processing of the task-relevant object during WM
Distributed neuronal architecture of WM
This leads to a model of WM in which lower level visual regions maintain the temporary representations of items held in WM (the templates) and PFC/parietal regions hold a representation of the task rules for manipulating this information (the central executive!)
