Memory, Lecture 11 Flashcards
The Function-Structure Relationship
Function and structure do not always match exactly.
A structure can participate in multiple functions.
A function may rely on multiple structures.
Definitions of Episodic Memory (EM)
EM refers to the ability to recall specific events and experiences.
Tulving’s definition emphasises “mental time travel” and re-experiencing past events.
Other criteria for EM include the ability to create links between unrelated bits of information and to place past experiences within a particular time and place.
EM is a result of associative learning, where the context of an episode is bound together for later retrieval.
Information Flow within the Medial Temporal Lobe (MTL)
The MTL is responsible for memory processing and consolidation.
Information is initially collected through the perirhinal and parahippocampal cortices.
Information then passes to the entorhinal cortex, before ultimately reaching the hippocampus.
The MTL contains a large network of connections within and among its subregions, allowing for extensive information processing.
Hierarchical Organisation of the MTL
The MTL is hierarchically organised, with different regions responsible for different aspects of memory processing.
The perirhinal and parahippocampal cortices are responsible for object and scene recognition, respectively.
The entorhinal cortex is responsible for spatial processing and acts as a gateway to the hippocampus.
The hippocampus is responsible for binding and consolidating information into long-term memory.
Introduction to Memory Systems and Amnesia
Amnesia can be caused by neurosurgery, strokes, head injury, certain viruses, or long-term alcoholism
Retrograde amnesia is the inability to remember events before the onset of amnesia
Anterograde amnesia is the inability to form new long-term memories after the onset of amnesia
Patient HM underwent bilateral medial temporal lobectomy, resulting in profound anterograde amnesia
The Good and Bad of HM’s Surgery
HM’s surgery resulted in reduced severity and frequency of convulsions
HM’s IQ improved from 104 to 118 after the surgery
HM experienced minor retrograde amnesia for events within the 2 years preceding the surgery
HM had profound anterograde amnesia and could not form new long-term memories after the surgery
HM’s Anterograde Amnesia and Digit Span Test
HM had preserved memory of the past and good short-term/working memory
HM could not form new, long-term memories
HM could not repeat more than 7 digits after 25 trials of the digit span + 1 test
Normal subjects can repeat up to 18 digits
HM’s Implicit Memory and Mirror Drawing Task
HM’s implicit memory was intact, and he could retain certain types of tasks by improved performance
Implicit memory is non-declarative, while explicit memory is declarative
HM’s MTL was critical for making new memories and retrieving old memories
The Subsequent Memory Paradigm
The Subsequent Memory Paradigm aims to evaluate encoding-phase activity leading to successful (versus unsuccessful) memory
Wagner et al. (1998) investigated whether brain activity at encoding predicted which stimuli would be remembered or forgotten
Activity in the left ventrolateral prefrontal cortex and left MTL was predictive of later remembered versus forgotten stimuli
Familiarity is the sense of memory that a stimulus has been encountered before, while recollection is memory for the context or other associative information about a previous encounter with a stimulus.
Hippocampus as binder of relational memories
Eichenbaum et al. (2007) propose a model where:
The perirhinal cortex processes item representations (important for familiarity)
The parahippocampal cortex processes “context” (including scene perception)
The hippocampus binds items in context (important for recollection)
Ranganath et al. (2004) found that the hippocampus supports recollection-based recognition (but not familiarity) independently of the type of stimulus, while familiarity memory is supported by the rest of the MTL.
Episodic Memory & Consolidation
Consolidation is the process that stabilizes a memory over time after it is first acquired.
Synaptic consolidation is the structural changes in the synaptic connections between neurons that may take hours to days to complete.
System consolidation is the gradual shift of memory from the hippocampus to the cortex.
There are two theories of system consolidation:
A) Standard consolidation theory -> Temporary role of hippocampus
B) Multiple trace theory -> Permanent role of hippocampus
Ribot’s Law states that memory loss following brain damage has a temporal gradient, where more recent memories are more likely to be lost than remote memories, which have undergone system consolidation.
Temporal gradient in amnesia: Patient PZ
Patient PZ had Korsakoff’s syndrome at the age of 65.
Standard consolidation model suggests that initially, the hippocampus plays an active role in “binding” the activity of disparate cortical “modules,” but over time, the hippocampus plays less of a role.
PZ was unable to learn new paired associates and remembered some famous people from the 1930-1940s but not later.
Memory Reactivation and System Consolidation
Memory reactivation is the core mechanism of consolidation.
Reactivation leads to the reinstatement of patterns of neural activity in the cortex.
Such reactivation subsequently results in the stabilization and refinement of cortical traces.
This iterative process leads to storage and recall becoming completely dependent on the cortex, and independent of the hippocampus.
Retrieval of nonconsolidated and consolidated memory
An event with audio (A), spatial (S), and visual (V) information is encoded.
The hippocampus contains a unified representation of the event.
When a retrieval cue containing only spatial and visual information of the event is encountered before consolidation, the hippocampus plays a critical role.
After the memory is fully consolidated, the connections with the hippocampus become unnecessary.
The retrieval cue accesses the memory directly from the cortical network of connections that form the unified representation of the memory.
Temporal Gradient in Amnesia and Semantic Dementia
SD: Semantic dementia - damage to anterior temporal lobes (cortex)
AD: Alzheimer’s disease – damage to hippocampus and related structures
SD patients can remember recent but not old events because memories not yet completely dependent on cortex – yet to be transferred out of hippocampus
Evidence for consolidation from amnesia: To explain retrograde memory loss, we assume that old memories are not fully consolidated at the time of injury.
