locating memories Flashcards
wayfinding
is the set of cognitive processes required to get from here to there
how do we navigate?
egocentric
object positions are framed in relation to the self (“I”)
spatial reference frames
allocentric
spatial reference frame
object (including self) positions are framed in relation to external objects
absolute: unchanging
intrinsic: depends on orientation of the reference object ie/ right/left
categorical wayfinding
technique
qualitative or nominal relationships (general terms)
ie/ above/below, in/on
affect valence biases object spatial memory (good-up; bad-down)
sense of division of things
coordinate wayfinding
technique
quantitative or metric relationships (specific)
ie/ 4.67m NW, 3 times this far
categorical wayfinding brain area
spatial perception is lateralized to the left cerebral hemisphere
left parietal lesion patients make more categorical errors
coordinate wayfinding brain area
spatial perception is lateralized to the right cerebral hemisphere
posterior parietal cortex
right parietal lesion patients make more coordinate errors
actual distances between objects
place cells
receive input from many grid cells and code for specific places
located in dentate gyrus
grid cells
receive multimodal input and respond to distinct spatial frequencies
located in entorhinal cortex
time cells
are sensitive to intervals between key events
located in both dentate gyrus and entorhinal cortex
Parts of the hippocampal index theory
Formation
Recognition
Recall
Memory Failure
formation
experience is represented in many cortical regions and stored in hippocampus by LTP as an index n
not a memory but a pointer
recognition
similar stimuli activate the index, which triggers cortical and subcortical associations of an engram
recall
a sufficient subset of stimuli activate enough of the index to then activate the whole index and engram
memory failure
insufficient subset of information to reactivate an index
episodic memory formation
hippocampus is blind to modality; does not itself discriminate visual, auditory, somatic, olfactory, or other sensory-specific information
- intrinsic theta oscillations (~8Hz), with gamma bursts in each cycle
- the sequence with each gamma burst involves place cells for recently past, current, and upcoming places
one theory proposes that the hippocampus is a general sequence generator for memory
- the “index” is a sequence, not a mere reference to a cell assembly
dorsal stream contribution
the dorsal stream supports spatial awareness with three output pathways:
- Prefrontal Pathway
- spatial working memory - Premotor pathway
- visually guided action
- dissociable from purely perceptional or cognitive estimates of distance - medial temporal cortex
- spatial navigation, wayfinding
inferior parietal lobule (IPL)
region of association cortex in the right hemisphere specialized for tracking distances
what is the rIPL specialized for
spatial distances
temporal distances (intervals)
social distances
parts of rIPL do not discriminate between distance type
what are the three egocentric layers of space
personal
peripersonal
extrapersonal
cultural and individual variation exist in average comfortable social distances
personal
within ones body
peripersonal
within reach
can be adjusted, ue/ when driving
extrapersonal
beyond reach
where are memories located
episodic memories must encode a location, but the memory itself must also be stored somewhere
engrams are not stored as discrete packets of info in a specific location in the brain
rather, memories are stored as distributed networks of neurons whose connections have been mutually strengthened by encoding and consolidation (everything that contributes to the experience)
memory storage: structure of engrams
when an axon of cell A is near enough to excite a cell B and repeatedly or persistently take part in firing it, some growth processes or metabolic change rakes place in one or both cells such that A’s efficiency, as one of the cells firing B, is increased
cells that fire together wire together
techniques for studying engram location have evolved
A. Large cortical ablation
B. small cortical ablation
C. stereotaxic surgery (BLA)
- basolateral amygdala
D. stereotaxic surgery
how to identify engram neurons?
cell must be active during learning
cell must be active during test
when activated they can generate the engram behaviour
when inhibited they prevent (or reduce) the engram behaviour
immediate early genes
genes whose protein products are present after cell activation
tagged (co-expressed) with fluorescent protein to measure it
problem: it degrades in minutes
TetTag mouse
engineered so FOS promotes the expression of Tetracycline-transactivator (ttTA) system
tTA promotes reporter gene LAC
Detecting Cell Activity
Temporal control over the expression of LAC offers longer tracking of cell activity
A. DOX (doxycycline) - inhibits LAC production is an antibiotic that blocks LAC expression when consumed
- Remove DOX, LAC is expressed
B. Fear conditioning with no DOX causes expression of LAC
- add DOX during retrieval
- detect coincidence of LAC and ZIF
C. LAC and ZIF found more often in cells active in animals who froze more after fear conditioning
- sign of both encoding and retrieval
Optogenetics
DOX prevents expression of ChR2 (light sensitive receptor)
After being fear condition in Context B, mice froze to blue light applied to dentate gyrus in Context A, even on DOX
How are engram cells chosen
Experiment:
1. There is a difference between fear retention based on training-test interval
A. short intervals (10s) are remembered much less than longer intertrial intervals (8m)
B. CREB infusion “restores” fear memory trace for 10s intervals
- engram cells targeted to die cause the the erasure of the conditioned memory
A. CREB can be overexpressed by injecting it into neurons
B. CREB overexpression leads to likely selection as engram cell
C. Neurotoxin (diptheria) targets and kills cells that overexpress CREB
D. Animals that overexpress CREB and treated with diptheria fail to remember
How does CREB help a neuron
does it up-regulate genes that are involved in consolidation?
- It does but thats not why
CREB changes the intrinsic excitability
- lowers firing thresholds, making it more likely to join hebbian cell assembly
