Neuroscience/Objectives/Lecture 38
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Objectives: 27 - 28 - 29 - 30 - 31 - 32 - 33 - 34 - 35 - 36 - 37 - 38 - 39 - 40 - 41-42 - 43 - 44-45 - 46 - 47 - 48 - 49 - 50 - 51 - 52 - 53 - 54
Learning and memory
Describe the major temporal categories of memory.
- Very short-term memory
- Also called sensory memory, this type of memory lasts from 0.5 to 3 seconds depending on the modality. It holds both iconic (0.5 s) and echoic (2-3 s) memories, which are subserved by low-level visual and auditory circuits, respectively.
- Short-term memory
- Also called working memory, this maintains memories for seconds to minutes.
- Long-term memory
- LTM maintains memories for minutes to years to possibly an entire lifetime. There are several LTM systems, including declarative, non-declarative, explicit, and implicit systems, all of which share some overlap.
Describe the architecture proposed by Baddeley for working memory. Describe the neural structures that working memory is thought to depend upon.
Baddeley's model of working memory employs a central executive, a sort of spotlight that can focus in on particular pieces of information. The central executive then triggers verbal (phonological) and spatial subsystems to rehearse and store this information in order to maintain it in memory.
The central executive is believed to reside in the dorsolateral prefrontal cortex (BA 46). The verbal rehearsal system is in the left premotor cortex and Broca's area, while the verbal storage system is in the left inferior parietal lobule (BA 39,40). Similarly, the spatial rehearsal center is the right premotor cortex, and the spatial storage center is the right parietal cortex.
Clinical damage to the frontal or parietal lobes yields working memory deficits. This can be caused by trauma, stroke, etc. Also some diseases specifically target frontal lobe function. Patients with phenylketonuria are unable to convert phenylalanine to tyrosine and ultimately dopamine, thereby disrupting the dopaminergic projections to the prefrontal cortex. Dopaminergic projections to the prefrontal cortex are selectively damaged because they turn over very rapidly.
Explain declarative memory. List the brain structures declarative memory is thought to depend upon.
Declarative memory maintains explicit (ie, consciously accessible) memories of words, facts, and events. Two types of declarative memory—episodic and semantic—are known. Episodic (or autobiographical) memory stores memories of personal events with a time and place context. It is this system that one uses when "remembering". Semantic memory stores general knowledge about facts, concepts, and words not linked to a particular event. It is this system that one uses when "knowing".
Learning new declarative memories depends upon medial temporal lobe structures, including the hippocampus, parahippocampal gyrus, and diencephalon. As these memories become consolidated into long-term memory, medial temporal lobe structures are less involved in their storage. Instead, storage of these memories becomes increasingly dependent on neocortical temporal lobe regions and sensory and motor regions involved in particular concepts. For example, the concept for hammer lies in the precentral gyrus, presumably because knowledge of a hammer involves the neural circuitry required to actually use the hammer.
Medial temporal lobe lesions result with anterograde amnesia. Neocortical lesions lead to long-term memory deficits as well. Damage to the lateral temporal lobe (eg, caused by Pick's disease) leads to general semantic deficits, including semantic dementia. Left motor lesions (eg, caused by Parkinson's disease) leads to tool-naming and tool-knowledge problems.
Explain non-declarative memory. List and describe the different types of non-declarative memory systems that have been posited.
Non-declarative memory is implicit (ie, not consciously accessible) memory. There are several types, including procedural memory, priming, basic associative learning, and non-associative learning.
| Type | Notes | Neural substrates | Clinical correlates |
|---|---|---|---|
| Procedural memory | Memory for skills and habits (eg, riding a bike) | Striatum, motor cortex, cerebellum | Parkinson's disease, Huntington's disease |
| Priming (perceptual representation) | Identification of words and objects on the basis of form and structure (visual, auditory, and structural description subsystems) | Posterior neocortical regions | |
| Basic associative learning: musculoskeletal responses | Eg, eye-blink response in mammals | Cerebellum | Cerebellar lesions |
| Basic associative learning: emotional responses | Eg, classical fear conditioning | Amygdala | Amygdala lesions |
| Non-associative learning | Eg, habituation, dishabituation to a stimulus | Reflex pathways |
Explain the significance of patient HM in the study of memory.
Following bilateral medial temporal lobe resection in 1953, HM had impaired declarative memory as evidenced by his anterograde amnesia and temporally grade retrograde amnesia. Despite this, HM's working memory, reasoning abilities, and abstract thinking abilities were spared, as were his non-declarative memories. This suggested that the brain has multiple memory storage systems.
