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