Chapter 7 - Brain substrates
Semantic memory
Stored in cortex
Episodic memory
Hippocampus
Sensory cortex
Cortical areas that specialize in one sensory modality •Specific types of semantic knowledge are stored in cortical areas devoted to processing that kind of information
Association cortex
Involved in associating information within and across modalities •For ex., association cortex helps us link the word ‘dog’ with visual image of dog and semantic information about dogs
Studies suggest that specific categories of semantic information are encoded by specific groups of neurons.
Recordings from human cortex show neurons tuned to specific semantic categories
a neuron from the Medial temporal lobe of an awake human fires to pictures of Steve Carell but not to photos of other celebrities.
In humans, the MTL includes •Hippocampus •Amygdala •Surrounding cortex
HM
Anterograde amnesia
Loss of ability to form new episodic and semantic memories
Damage to the hippocampus = impairment in episodic memory
The hippocampus is critical for forming new episodic memory
Rats with hippocampal damage make many errors when trained on working memory version of radial arm maze (proactive interference).
Birds with hippocampal damage lose the ability to locate their caches (they still store food normally but forget where they put it)
There is evidence to suggest that selective hippocampal damage spares semantic memory
HM could learn new sematic information after laborious study
Patients with damage that extends further into MTL are unable to learn new semantic information
New semantic information depends on cortical regions in the MTL (parahippocampal and perirhinal cortices), rather than hippocampus itself
Subsequent memory paradigm
Participants were scanned while viewing a list of words (asked to categorize the words as abstract or concrete; incidental encoding) •Then, given surprise recognition test •Go back and compare brain activity for words later remembered vs. words later forgotten •Reveals brain activity required to properly encode and consolidate semantic memories
•Hippocampal area of the left temporal lobe •Left frontal cortex
False memory
In the related word paradigm, the hippocampus is “fooled” by the lure word (the word that is related but not presented).
A small area of the parahippocampal cortex is only activated by the words seen, not the lure word, suggesting a possible physical way of distinguishing true from false memories!
Retrograde amnesia
loss of memories for events that occurred before the injury
Ribot gradient
memory loss is worse for events that occurred shortly before injury than for more distant events
People with bilateral MTL damage also show some retrograde amnesia in addition to anterograde amnesia
Standard consolidation theory
Multiple trace theory
An episodic memory consists of many components (sights, sounds, etc.) that are stored in sensory and association cortex •Initially, the hippocampal region helps link these components into a single episode •Over time, the components become linked to one another directly and no longer require the hippocampus •So, the MTL is required for initial storage and retrieval but its contribution diminishes over time •Explains why brain disruption usually damages recent memories (still undergoing consolidation) but not older memories (fully consolidated)
•Some patients have severe retrograde amnesia that is not temporally graded. •To account for such extensive retrograde amnesia, Moscovitch and Nadel developed the Multiple Trace Theory •When an event occurs, it can be stored as an episodic by an ensemble of neurons in the hippocampus and neocortex•Each time that memory is retrieved, the retrieval itself becomes a new episodic memory (similar to reconsolidation)
Retrograde amnesia (RA) is a loss of memory-access to events that occurred, or information that was learned, before an injury or the onset of a disease
Thus, an old and often-recalled event can have multiple ‘memory traces’ in the brain •Over time, the general content or gist of the memory can become a semantic memory which is stored in cortex, independent of the hippocampus •But the original episodic memory, with specific details of time and place, is never fully independent of the hippocampus. •Suggests that spared memories after MTL damage are actually semantic rather than true episodic
Frontal Cortex
regions of cortex in frontal lobes,may help determine what information we store (and remember) and what information we don’t store (and forget).
Recall that left frontal lobe (in addition to MTL) was active during incidental encoding of well remembered items
Other areas of prefrontal cortex may suppress hippocampal activity, inhibiting storage and retrieval of “unwanted” memories