Chapter 24: Memory Systems Neuroscience: Exploring the Brain, 4e Chapter 24: Memory Systems

Introduction Learning and memory: lifelong brain adaptation to environment Several similarities between experience-dependent brain development and learning Similar mechanisms at different times and in different cortical areas Memories range from stated facts to ingrained motor patterns. Anatomy: several memory systems Evident from effects of brain lesions

Types of Memory and Amnesia Learning: acquisition of new information Memory: retention of learned information Declarative memory (explicit) Facts and events Nondeclarative memory (implicit) Procedural memory—motor skills, habits

Types of Declarative and Nondeclarative Memory

Types of Procedural Memory A type of nondeclarative memory Involves learning a motor response In reaction to sensory input Occurs in two categories of learning Nonassociative learning Associative learning

Types of Nonassociative Learning (a) Habituation (b) Sensitization

Associative Learning Behavior altered by formation of associations between events In contrast to changed response to a single stimulus Classical conditioning (Pavlov) Pairing of unconditional stimulus with conditional stimulus Instrumental conditioning (Thorndike) Associate a response with a meaningful stimulus

Classical Conditioning

Types of Declarative Memory Working memory Temporary storage, lasting seconds Short-term memories—vulnerable to disruption Facts and events stored in short-term memory Subset are converted to long-term memories. Long-term memories Recalled months or years later Memory consolidation: process of converting short- to long-term memories

Memory Consolidation

Amnesia Amnesia: serious loss of memory and/or ability to learn Causes: concussion, chronic alcoholism, encephalitis, brain tumor, stroke Limited amnesia (common)—caused by trauma Dissociated amnesia: no other cognitive deficits (rare) Retrograde amnesia: memory loss for things prior to brain trauma Anterograde amnesia: inability to form new memories after brain trauma

Amnesia Produced by Trauma to the Brain

Amnesia—(cont.) Transient global amnesia Sudden onset of anterograde amnesia Lasts a shorter period, from temporary ischemia (e.g., severe blow to head) Symptoms: disoriented, ask same questions repeatedly; attacks subside in couple of hours; permanent memory gap

Working Memory We pay attention to small fraction of sensory information. Some sensory information held briefly in working memory. Small capacity—limited resource Mostly discarded, some may be converted to long-term memory A capability of neocortex found in numerous brain locations

Prefrontal Cortex and Working Memory Primates have a large frontal lobe. Functions of prefrontal cortex: self-awareness, capacity for planning and problem solving

Wisconsin Card-Sorting Test To demonstrate problems associated with prefrontal cortical damage

Working Memory Activity in Monkey Prefrontal Cortex

Imaging Working Memory in the Human Brain Numerous brain areas in prefrontal cortex involved in working memory. PET study: Six frontal lobe areas show sustained activity correlated with working memory. Identity task Location task Unknown whether working memory for other types of information is held in same or different brain areas

Human Brain Activity in Two Working Memory Tasks

Area LIP and Working Memory Cortical areas outside frontal lobe also involved in working memory. Lateral intraparietal cortex (area LIP) Involved in guiding eye movements Stimulation causes saccades. Demonstrated in delayed-saccade task in monkeys Other modality—specific areas of parietal and temporal cortex have analogous working memory responses

LIP Neuron Response in Delayed-Saccade Task

The Neocortex and Declarative Memory Lashley’s rat experiments Cortical lesions produce memory deficits. Speculated all cortical areas contribute equally (equipotential) Equipotential capacity later disproved But memory engrams can be widely distributed in the brain

Hebb and the Cell Assembly External events are represented in cortical cells. Cells reciprocally interconnected  reverberation Simultaneously active neurons—cell assembly Consolidation by “growth process” “Fire together, wire together” Hebb on the engram Widely distributed among linked cells in the assembly Could involve neurons involved in sensation and perception

Hebb’s Cell Assembly and Memory Storage

The Medial Temporal Lobes Important for consolidation and storage of declarative memories Demonstrated by: Electrical stimulation in the temporal lobe Neural recordings from the temporal lobe

Medial Temporal Lobes and Declarative Memory

Information Flow through Medial Temporal Lobe

Electrical Stimulation of the Human Temporal Lobes Temporal lobe stimulation Effects different from stimulation of other areas of neocortex Penfield’s experiments Stimulation  sensations like hallucinations or recalling past experiences Temporal lobe: apparent role in memory storage Caveat: complex sensations reported by minority of patients, all with abnormal brains (epilepsy)

Human Neural Recordings from the Medial Temporal Lobe Neurons found that preferentially respond to categories Faces, household objects, outdoor scenes Invariant neurons—respond to variety of images are structurally or conceptually related Individual neurons respond selectively to one person’s face. Many questions remain.

The Effects of Temporal Lobectomy (H.M.)

Temporal Lobectomy and Amnesia (H.M.) Removal of temporal lobes had no effect on perception, intelligence, personality. Anterograde amnesia so profound he could not perform basic human activities (and partial retrograde amnesia) He could not recognize the doctor who studied him for nearly 50 years. Impaired declarative memory, but spared procedural memory (mirror drawing)

An Animal Model of Human Amnesia Studies of macaque medial temporal lobe using experimental ablation Delayed match-to-sample and delayed non-match to sample (DNMS) tests Recognition memory tasks Amygdala and hippocampus not significantly involved in recognition memory Much still unknown about specific brain areas Collectively, medial temporal structures critical for consolidation of memory

Delayed Non-Match to Sample (DNMS) Task Medial temporal lobe structures shown important for memory consolidation

Effect of Medial Temporal Lobe Lesions on DNMS Performance

The Diencephalon and Memory Processing

The Diencephalon and Memory Processing: The Case of N.A. Radar technician accidentally stabbed through left dorsomedial thalamus Less severe amnesia, but like H.M.: anterograde and some retrograde amnesia Korsakoff’s syndrome: chronic alcoholism—thiamin deficiency Symptoms: confusion, confabulations, severe memory impairment, apathy Can lead to lesions in dorsomedial thalamus and mammillary bodies Suggests mechanisms involved in consolidation distinct from processes that recall memories

Memory Functions of the Hippocampal System Memory formation, retention, retrieval involve system of interconnected brain areas Hippocampus involved in various memory functions Binds sensory information for memory consolidation Supports spatial memory of location of objects of behavioral importance Involved in storage of memories for some length of time

Effects of Hippocampal Lesions in Rats (a) Normal rats go down each maze arm for food only once - but not with hippocampal lesions (b) Normal and lesioned rats learn which arms are baited and avoid the rest

Spatial Memory and Place Cells Learning Morris water maze requires hippocampus. Place cells fire when animal is in a specific place. Place fields dynamic

Place Cells in Humans PET imaging in human brain related to spatial navigation of a virtual town

Grid Cells Identified in rodent neural recordings Unlike place cells Respond when animal is at multiple locations that form hexagonal grid Likely also grid cells in human entorhinal cortex Place cells, grid cells, and hippocampal neurons showing sensitivity for head direction  brain region highly specialized for spatial navigation

A Rat Place Cell and a Grid Cell

Hippocampal Functions Beyond Spatial Memory O’Keefe and Nadel: hippocampus specialized for creating spatial map of environment Apparent important role in spatial memory Other hippocampal function theories Important for working memory Integrates or associates sensory input Odor discrimination Hippocampus links different experiences together.

Odor Discrimination Experiment

Consolidating Memories and Retaining Engrams Declarative memory formation involves system of interconnected brain structures: Take in sensory information Make associations between related information Consolidate learned information Store engrams for later recall Components include hippocampus, cortical areas around hippocampus, diencephalon, neocortex, and more.

Two Models of Memory Consolidation Standard model of memory consolidation Information from neocortex areas associated with sensory systems sent to medial temporal lobe for processing Synaptic consolidation, systems consolidation Multiple trace model of consolidation Engrams involve neocortex, but even old memories also involve hippocampus. Multiple memory traces

Reconsolidation Rat experiments Reactivating a memory makes it sensitive to change as when first formed (before consolidation) Reconsolidation: the reactivation effect Human reconsolidation experiments Recalling a memory makes it susceptible to change Hippocampal activity Profound implications for treatment of stress associated with unpleasant memories

Procedural Memory Different memory types involve different brain structures. The striatum involved in habit learning and procedural memory Caudate nucleus + putamen = striatum Key location in the motor loop Input from frontal and parietal cortex Output to thalamic nuclei and cortical areas involved in movement

The Striatum and Procedural Memory in Rats Lesions in striatum disrupt procedural memory (habit learning) —but not declarative memory Standard radial arm maze performance (declarative memory) depends on hippocampus. Modified radial arm maze performance (performance memory) depends on striatum. Damaged hippocampal system: degraded performance on standard maze task Damaged striatum: impaired performance of the modified task

Habit Learning in Humans and Nonhuman Primates Effects of selective brain lesions on memory  comparable in rodents and primates. In monkeys Striatum damage—effect on performance memory but not declarative memory Effects of human diseases Consistent with striatum’s role in procedural memory Studies of patients with amnesia and Parkinson’s disease

Performance of Patients with Amnesia and Parkinson’s Disease

Concluding Remarks Learning and memory Involve changes widely across the brain Memories classified based on: Duration Kind of information stored Brain structures involved Physiological basis for memory storage? Initially held in fragile form Long-term memories more robust: structural brain changes?