Cognitive Neuroscience Lecture 18: Memory 2

*L18: Memory: Neural*
Mechanisms

Learning/memory is changes in relative strength of
connections/transmissions.

E.g.:

" (A) synapses may increase in number "

(B) Synapses may develop separate zones "

(C) A single Synaptic spine may divide in two "

 (D) New dendritic
spines may form

Working Memory

o
Requires executive functions:

§
Initiation, inhibition, shifting, monitoring

§
Involves prefrontal cortex

o
Delay-period activity

§
Monkey pfc: orientation of 135 deg.

§
Delay period activity to targets in this
orientation but NOT during encoding or response

·
**Implies**
that prefrontal cortex active during delay

§
Delay-period activity in human fMRI

·
Memory trials:

o
face: study, delay, face: recognition

·
Non-memory trials:

o
Scrambled face, blank, scrambled face

·
Delay period activity in prefrontal regions

o
As you go to higher regions, it becomes more
face selective and has more memory delay

o
Spatial WM: location of faces

o
Face WM: remember identity of faces

o
Different prefrontal regions are active for
different types of WM

Neural substrates WM: evidence from defecits

o
Prefrontal – executive control

§
Spatial and face use diff parts of prefrontal

o
Verbal WM: supramarginal & IPL

§
Individuals with strikingly limited verbal WM
capacity (low digit span) have lesions in the left IPL (supramarginal gyrus),
adjacent to language areas

§
**Suggests**
that phonological loop (verbal) of WM involves supramarginal gyrus and inferior
parietal lobe

§
KF & others had **left parietal/temporal lesions and STM deficits** due to stroke,
tumor

o
Visual WM:

§
Thing ur not paying attention toàv1

§
Thing ur paying attention toàVSTM

o
Capacity of WM

§
**Parietal**
cortex (bilateral intraparietal sulcus) tracks number of objects in WM

§
Response in bilateral intraparietal sulcus
tracks number of objects in visual WM

§
Plateaus at 4

§

Medial Temporal Lobe: limbic system

o
HM: removed all of entorhinal cortex and
perirhinal cortex (plus amygdala)

§
Removed most of hippocampus but structural MRI
showed some left, but not functioning

§
HM: bilateral lesions affecting amygdala &
hippocampus, leaving posterior hippocampus intact

o
Role of hippocampus in memory:

§
Rapid consolidation—initial storage of
declarative memories

·
New memories stored in a hippocampal-dependent
way

·
LTP

§
Slower consolidation

·
Interacts with neocortex over days, months
years, stable memory trace in neocortex that no longer depends on hippocampus

·
Older memories are hippocampal-independent

·
Evidence from retrograde amnesia

§
Ribot’s law

·
In cases of amnesia produced by brain damage,
recent more affected than remote

§
ECT graded retrograde amnesia implies that
memory changes over time after initial learningà
hippocampus first, then neocortex

§
Train an animal on a task, perform hippocampal
lesion x days after training, test animal, results: graded retrograde

·
**Implies**
medial temporal lobe structures (including hippocampus) critical for
consolidation

§
Types of retrograde:

·
Graded

·
**Focal:**
remote memories more vulnerable

o
Lessons from MTL amnesias

§
Cognition and memory are distinct

·
Can have normal IQ and poor learning/memory

§
Acquisition of new memories and consolidation of
recent ones is distinct function. It is located in MTL (eg hippocampus)

§
MTL not required for

·
Immediate working memory

·
Procedural nondeclarative memory

·
Storage site for long term memory

§
**Ultimate**
LTM Storage: neocortical, largely nonfrontal

·
Neocortical

·
Semantic memory loss in specific domains

o
Language, object recognition, faces, reading,
navigation, number processing deficits

o
Forms of “memory” loss

§
Loss of info learned in these domains

·
Perception: viewing pictures –occipital/visual

·
Memory: retrieval of same pictures or sounds
from memory

·
Arrows indicate regions of overlap between
memory and perceptual activations

·
**Areas**
active during memory retrieval (without stim) include areas active in
processing stim

**Non-declarative**
memory

o
Study phase: read a list of words

o
**Priming**
effect (VISUAL):

§
Implicit test: resented for increasing time
until they can be identified correctly

·
time to ID is shorter if you’ve already seen the
word before

§
Explicit test: was this word on the list you
read the previous day?

·
Good performance in explicit, but no visual
priming in the implicit for **right**
occipital l damage patient

o
**Priming**
effect (SEMANTIC)

§
Faster at reading/making decisions for words
preceded by related words

§
Neural substrates sensitive to difference: **left anterior temporal lobe**

o
**Skill**
learning (motor):

§
fMRI study of motor learning

§
participants learn precisely timed mvts

§
after 5 days of practice, less use of primary
motor cortex, s1, premotor, and cerebellum

§
reduced activity àskill
is learned (less planning needed)

·
**repetition**
attenuation

McClelland, McNaughton, & o’Reilly:

o
Complementary learning systems hypothesis: 2
learning systems, neocortical & hippocampal

o
Neocortex:

§
Slow learning

§
Small adjustments to connections

§
Integration into complex knowledge structures

§
Priming

§
Implicit

§
Procedural

o
Hippocampus:

§
Learns Quickly

§
Supports rapid one trial learning

§
Teacher to neocortex until neocortical
connection changes are robust

§
Declarative knowledge

§
Explicit learning and retrieval

Encoding and retrieval

o
Recognition memory test: was this previously
there?

o
**Encoding:**
hits vs misses

o
**Retrieval:**
hits vs correct rejection  (discriminate
between old and new)

o
**False**
alarms (confabulations): failure from encoding or retrieval

**Frontal**
lobe and encoding : Posterior LIFG Wagner et al

o
Encoding period scanned—sort fMRI data according
to whether or not the word was correctly or incorrectly remembered later (later
tested outside scanner)

o
Compare BOLD response during encoding for
remembered vs. forgotten word to ID areas for successful encoding

o
Posterior LIFG—higher for remembered words

o
Frontal areas: differences in activation at time
of encoding, predict accuracy at time of test

o
**For**
verbal tasks: left hemisphere

o
**Visual**
tasks: bilateral

**Encoding**
and retrieval: posterior parietal lobes

o
Encoding: hits vs misses (how well did u
remember it)

o
Retrieval: hits vs correct (how well can u
discriminate old and new)

o
**Both**
associated with parietal lobes, possibly due to attention in encoding and
retrieval

**Recollection**
and familiarity: MTL

o
Episodic: higher or lower prob of recall

o
But “butcher on bus” means it’s not always black
and white

o
Recollection: specific

o
Familiarity: feeling sthing is “old” but no
details

o
Recognition confidence higher when more things
are old

o
MTL Recollection regions should show greater
activity at time of encoding for items where source is remembered than for
items where source is forgotten

§
Successful recollection used region Xàregion X used for
successful recollection

§
**Source**
memory is an index of recollection

o
MTL for familiarity should show more activity
for trials that later had higher confidence ratings

§
**More**
confidence indicates stronger familiarity

o
Recollection: accurate source memory

§
**MTL:**
Hippocampus and parahippocampal

o
Familiarity: correlation with confidence rating

§
**A**
different part of MTL: entorhinal/rhinal cortex

o
Conclusion: recollection and familiarity depends
on different structures within MTL

False memories/confabulations

o
high levels of false alarms to theme word,
accompanied by high confidence

DRM associate lists

o
Hippocampus: semantically associated false memory
= true memory

o
Parahippocampal gyrus: true memories >
false/new memories

o
**Suggests**
hippocampus = semantic retrieval, and parahippocampal is sensory retrieval

Functions of a constructive memory (not rote)

o
**Economy:** don’t
need every detail; gist is better

o
**Constructive**
episodic simulation hypothesis: **episodic: ability to recollect past helps**
with future

§
Use episodic to imagine possible future
scenarios and this constructive activity requires flexible access to past
experience