Caitlin Hof amp Steven Larson Spike Timing Measurements Measures the firing rate of neurons Is assumed to be a correlate to working memory Is done with single neuron electrodes electrophysiology and EEG ID: 600085
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Slide1
Synaptic Theory of Working Memory
Caitlin Hof & Steven LarsonSlide2
Spike Timing Measurements
Measures the firing rate of neurons
Is assumed to be a correlate to working memory
Is done with single neuron electrodes (electrophysiology) and EEG
Has allowed for the theoretical measurement and identification of working memory length and locationSlide3
Synaptic Theory
Memories are preserved in the short term by constant activation of neuronsMeasuring activity spikes shows what is thought to be memory
Memories in working memory can be reactivated by a general stimulus
Adjusting chemical balance within the synapses affects length of working memorySlide4
Synaptic Theory (Continued)
Short term plasticity leads to the creation of Polychronous
Neuronal Groups
PNGs form and dissipate to hold temporary memories
These are energy inefficient but hold memory in its purest formSlide5
Visual STM
Where it occurs are Prefrontal Cortex and Inferior Temporal RegionsAblations to area TE affect STM while the
Perirhinal
cortex affects LTM
Activity in the PFC takes place in fewer neurons but continues during distractors
Suggests that a circuit like series of neurons stores the stimulus for later comparisonSlide6Slide7
Synaptic Theory of Working Memory
Gianluigi
Mongillo
,
Omri
Barak, &
Misha
Tsodyks
Science Volume 319 14 March 2008Slide8
Working Memory
Holds information temporarilyProcessing
Crucial to cognitive tasksSlide9
Delayed response paradigm
Stimulus kept until task execution
Stimulus specific spiking activity – neuronal correlate of working memory
Stimulus
Time Delay
ResponseSlide10
Theoretical Construct
Delay activity – intrinsic cell properties
OR
Persistent reverberations in selective neural populationsSlide11
Theoretical Construct Issues
Both theories suggest working memory might not be entirely in spiking memory
Energetically expensive to hold information in spiking form
High metabolic cost of action potentialsSlide12
Alternative Theory
Presented alternative accountBased on properties of excitatory synaptic transmission in prefrontal cortexSlide13
Prefrontal Cortex
Higher order functions
Cognitive behavior
Personality
Decisions
Planning
Thoughts and actions for internal goals
Executive function
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Alternative Theory
Prefrontal cortex implicated in working memory
Excitatory synaptic transmission is
facilitatory
Sensory -> depressionSlide15
Alternative Theory
Proposed: An item is maintained in the working memory state by short-term synaptic facilitation mediated by increased residual calcium levels at the presynaptic terminal of the neurons that code for this itemSlide16
A Little Biology
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Alternative Theory
Removal of residual calcium from presynaptic terminals is relatively slow
Memory can be transiently held for about 1 s without enhanced spike activatedSlide18
Methods
Recurrent network of integrate and fire neuronsNetwork encodes a set of memories (items)
Composed of randomly selective populations of excitatory neurons
Connections between the neurons coding for the same memory stronger than connections between different populationsSlide19
Figure 1BSlide20
Figure 1ASlide21
Figure 1 ASlide22
Memory Maintenance with Synaptic Facilitation (Figure 2A)
Stimulation – loading one item into working memory by transient external excitation to the corresponding neuronal population Slide23
Memory Maintenance with Synaptic Facilitation (Figure 2B)Slide24
Memory Maintenance with Synaptic Facilitation (Figure 2C)Slide25
Memory Maintenance with Synaptic Facilitation (Figure 2)
The use of residual calcium at synaptic terminals as a memory buffer requires low
emmision
rates
Buffer content is not substantially affected by the neural activity of the rest of the cortexSlide26
Conclusions
Working memory can be maintained by short-term synaptic facilitationAccumulation of residual calcium in the presynaptic terminals could carry the information about the recalled memory in a working form, reducing the need for metabolically costly action potentials
Memories -> spiking memory
A result of global reactivating input to the network or by virtue of the intrinsic network dynamicsSlide27
Conclusions
Not all stimuli enter working memoryDecision to allow items into working memory is mediated by attention – represented by the global excitatory output
Performance of human observers on memory tasks positively correlates with the level of neural activity during the presentation of the items
Model predicts that residual calcium at the presynaptic terminals should be enhanced during working memory even if there is no increase in firing rateSlide28
Conclusions
Suppressing the spiking activity for several hundred milliseconds should still allow for the memory to reactivate after the suppressing input is withdrawn
Groups of neurons could exhibit brief epochs of coherent firing
Model provides a possible target for a pharmacological interference with working memory
Manipulations that modify the facilitation/depression balance in the memory related cortical areas are predicted to have a strong effect on the stability and duration of memory