Cortex and Mind Chapter 5. Memory is stored knowledge about the internal and external environments; it includes perceptual and motor knowledge.

Transcription

1 Cortex and Mind Chapter 5 Memory is stored knowledge about the internal and external environments; it includes perceptual and motor knowledge.

2 Review of classical classification of learning and memory 1. Learning a. Associative learning Involves relations among different stimulus types. i. Classical conditioning: association of initially neutral stimulus with a physiological response. Also called Pavlovian conditioning from Pavlov's famous experiment pairing light and sound stimuli with salivation response. Before conditioning: US produces UR; CS produces no effect. Conditioning involves paired presentation of CS with US. After conditioning: CS produces CR (same as UR). ii. Operant conditioning: association of behavior and its consequences (how to "operate" something). Animal has to perform some task to achieve a goal. Each response is followed by a reinforcement or punishment.

3 b. Non-associative learning Involves experience with only a single stimulus type. i. Habituation: decrement in magnitude of a response to repeated stimulation ii. Sensitization: progressive amplification of a response to repeated stimulation (e.g. repeated scratching of skin becomes painful) iii. Imprinting: formation of fixed behaviors during critical period, usually in early life

4 2. Memory a. Explicit vs implicit i. Explicit or Declarative memory Cognitive memory involving recognition of sensory patterns, attachment of emotional value & in humans the ability to describe details about time, place & circumstance. It is accessible to conscious recollection and available to multiple response systems. It is relatively fast & may occur with only 1 presentation. It often involves association of simultaneous stimuli. ii. Implicit or Procedural memory Behavioral memory resulting from learning a skill or operation. Includes several kinds of abilities, all of which are unconscious and expressed through performance. It is slow & accumulates through repetition over many trials.

5 b. Temporal scale These are approximate phenomenological categories -- not necessarily different mechanisms. i. Iconic memory: lasts on a time scale of a few seconds -- related to continued activity in sensory system. Sometimes referred to as storage in a "sensory buffer". ii. Short-term memory: lasts on a time scale of seconds to minutes -- e.g. number sequences iii. Intermediate-term memory: lasts on a time scale of hours to days -- e.g. environmental positional relations iv. Long-term memory: lasts on a time scale of weeks to years -- relatively permanent

6 c. Stages of processing i. Encoding: transfer of information from sensory buffers to STM, and then to LTM. ii. Consolidation: gradual strengthening and stabilization of LTM. iii. Retrieval: recall of information from STM & LTM to perform a task. The stage model is non-physiological. From a neurophysiological perspective, memory is a result of cortical network formation by Hebbian principles: a) encoding occurs by the activation of cortical networks b) consolidation occurs by the modification of synaptic connections in those networks, forming new cognits or altering old ones c) retrieval occurs by the activation of existing cognits

7 Encoding & Consolidation: Formation of memory Long-Term Memory (LTM) is stored in cortical networks (cognits). Sensory information is encoded by perceptual categorization: (i) sensory input activates existing cognits (STM); (ii) novel input leads to creation of new cognits (new activity, STM, leading to synaptic strengthening, LTM). 1) Encoding involves the activation of pre-existing cognits and the strengthening of their connections, or the formation of new cognits if the sensory information is sufficiently novel. Pre-existing cognits are strengthened, or new cognits are created if pre-existing cognits are insufficient to explain the sensory information. 2) Encoding is followed by consolidation: cognits are expanded and strengthened by modification of synaptic connections between columnar assemblies in the cortex. 3) Consolidation occurs under the influence of a variety of factors, including: a. attention b. repetition (rehearsal & practice) c. emotion (affect) d. motivation (goals)

8 4) Consolidation is controlled by the hippocampal system, with emotional evaluation provided by the amygdala. Consolidation occurs by Hebbian principles of association: a. At the cellular level, memory formation occurs by synaptic modification that associates the activities of different cells. Glutamate is the major neurotransmitter at cortical excitatory synapses: the NMDA glutamate receptor is activated by synaptic activity in a Hebbian manner. The NMDA glutamate receptor is thought to be the basis for long-term potentiation (LTP) in the cortex. b. At the network level, this synaptic modification leads to the functional association of cognits within and across modalities. c. Memory that is unique to an individual person involves cognits of association cortex.

9 Summary: 1) memory encoding involves the activation of pre-existing cognits and the strengthening of their connections if the sensory input is recognized; or it involves the formation of new cognits if the sensory input is not recognized. 2) memory consolidation involves the expansion & modification of cognits. 3) synaptic modification of cognits occurs according to Hebbian principles in a self-organizing manner, i.e. as a result of concurrent activity without outside control. 4) synaptic modification occurs as a result of concurrent activity: (a) at different hierarchical levels within unimodal cognits; (b) between unimodal cognits of different modalities; and (c) between unimodal cognits and symbolic cognits of association cortex.

10 Retrieval from Short-term memory Recall: Evidence for a two-stage model of memory Ebbinghaus (1885) began the modern history of human memory studies when he tested his ability to recall lists of nonsense syllables.

11 1) Graphs of retention in such studies appear to suggest that memory consists of two stages: short-term and long-term.

12 The Atkinson-Shiffrin model of human memory was based on the view that sensory information first passes into a short-term memory store (first stage) and then into a long-term memory store (second stage). It was thought that the rapid decline in retention in the first 3 days represented a short-term memory storage (with limited capacity & rapid decay), and that the much slower decline thereafter represented a long-term storage (with unlimited capacity & little or no decay). For information to be retained permanently, it had to be transferred from short-term to long-term storage. Two effects are well known in free recall studies: primacy effect: better recall of the first words in a list than subsequent words recency effect: better recall of the last words in a list than preceding words

13 2) The two-stage model of memory was apparently supported by evidence that presentation of distractors between word presentation and recall interferes with the recency effect, but not the primacy effect. This result would be predicted if the first words in the list pass quickly into long-term storage and are thus not as vulnerable to interference. 3) The model also appeared to have support from the anterograde amnesia suffered by patients with bilateral hippocampal damage: they could recall recent events but could not form new long-term memories. This result was interpreted as meaning that the hippocampi are required for the transfer of short-term memory items into long-term memory storage. 4) There were also reports that some patients with anterograde amnesia had lost the primacy effect while retaining the recency effect. This result would be predicted if the ability to transfer items from STM to LTM were lost. That is, if the first items in the list were not transferred to LTM, their recall would not be better than latter items, and they would have lost the primacy effect. The recency effect would remain because it would only depend on the newness in STM of the last items in the list.

14 Evidence against a two-stage model of memory 1) Warrington reported cases of amnesic patients who had lost the recency effect while retaining the primacy effect. 2) When plotted on log-log axes, the retention graphs do not suggest two stages. Rather, they indicate that the rate at which information is lost (i.e., the forgetting rate) slows down over time. 3) Evidence from electrical currents applied to the head in humans and animals suggested that the extent of the loss of recent memories that was produced was a simple function of time after learning. That is, these results did not indicate two stages of memory. Conclusion: a) Short-term memory does undergo a process of consolidation in which information becomes relatively permanent. b) However, consolidation appears to operate on information as soon as it enters short-term memory and it does so continuously over time. The cognitive networks of the cortex appear to be the only neural substrate for memory storage. Consolidation appears to occur by strengthening of synapses in cortical networks.

15 Short-term memory does not require a separate storage site in the brain, or a separate system to maintain it. However, it does represent active and operant memory -- working memory -- memory for time-limited information that is used to accomplish goals of behavior.

16 Perceptual memory The content of memory is almost always heterogeneous, i.e. it consists of many different kinds, e.g. sensory, spatial, motor, semantic, emotional, conceptual. This fact suggests that perceptual memory is represented by a heterarchical network, i.e. one that spans widely dispersed cognits. Any perceptual memory is an associative conglomerate of sensory and semantic features at many levels of the cognitive hierarchy of perceptual knowledge. Fuster, p Perceptual memory is also hierarchical it grows into unimodal and polymodal associative cortex from an innate (phyletic) sensory memory in primary sensory cortices.

17 From Mesulam, 1998: 1) The figure on the left illustrates how a vast associative cognit develops in transmodal areas, associating unimodal networks in different patterns of association. The highest levels of integration correspond to conceptual memory. 2) The figure on the right illustrates a hypothesized transmodal network node that associates concurrent activity in different transmodal cognits.

18 The solidity of long-term memory refers to its resistance to loss from injury. Factors influencing memory solidity: 1) hierarchical rank: higher rank is usually associated with greater redundancy of content & with greater access to more association pathways; greater hierarchical rank greater resistance to injury 2) strength of connections: memories with associations that are more specific (e.g. dates, names) are less solid (more vulnerable to loss) than memories with associations based on general concepts. The latter are related to more memories and thus more reinforced by repeated activation. Memory is also subject to hemispheric specialization. Each hemisphere can acquire memories and motor skills apart from the other. Nonetheless, some memory networks extend across both hemispheres through associative fibers in the corpus callosum.

19 Executive memory The history of research on the frontal lobe has emphasized its role in action rather than memory, except as a store of short-term or working memory. However, the study of patients with frontal lobe injury provides ample evidence for the frontal cortex as a store of executive memory. Just as in the non-frontal cortices the perceptual hierarchies are based on low-level sensory areas, the executive hierarchy in the frontal lobe is based on low-level motor areas. Deficits caused by frontal lobe lesions: 1) inability to recount serial actions (retrospective memory) 2) planning deficit: inability to formulate new plans of behavior (prospective memory) The planning deficit may be seen as a disorder in the representation of executive sequences (i.e. temporal cognits or schemas of action). Networks in the lateral prefrontal cortex in particular are responsible for new schemas, plans, and programs of action.

20 The lateral prefrontal networks receive efferent copies of movements from motor cortex and subcortical motor structures, as well as the resulting kinesthetic inputs. The efferent copy is thought to be a form of corollary discharge, whereby a motor area sends a secondary signal to some other area at the time that it sends a primary signal to the muscles, so that the other area can adjust for the consequences of the movement.

21 The theory of corollary discharge originated with Hermann von Helmholtz, who proposed that when we decide to move our eyes, a signal is sent to the neurons controlling eye movements, and at the same time a corollary discharge is sent to the visual system to cancel out any motion signals that result from corresponding motions of the retinal image. He saw this as necessary so that we do not perceive any external movement whenever we move our eyes across a stationary scene or stimulus. He made 2 predictions: 1) whenever a corollary discharge and visual movement signals from the retina reach the visual areas of the brain together, there will be NO perception of apparent object motion. This is the basis for the perceptual property called space constancy. 2) whenever either a corollary discharge or movement signals from the retina reach the visual areas alone, there WILL be perception of apparent object motion.

22 von Helmholtz found proof for his theory in patients with paralysis of the eye muscles. (This can also be tested experimentally by injecting curare into the eye muscles). These patients perceived that whenever they tried to move their eyes, the world would seem to jump in the same direction as the attempted eye movement. He explained that in these patients, the motor areas of the brain still send a command to move the eyes, but the eyes fail to move due to the paralysis. The retinal image does not actually move because the paralyzed eyes cannot move. Yet, the motor system still sends a corollary discharge signal to the visual system. The corollary discharge would normally cancel out movement of the retinal image. The retinal image would normally move in the opposite direction to the attempted eye movement. (Push your eyeball in 1 direction; which way does the image move?) The corollary discharge signal would normally cancel it by acting as a retinal image moving in the same direction as the attempted eye movement. But the retinal image does not move in these patients, and the visual perceptual system interprets the corollary discharge signal as retinal image movement in the same direction as the attempted eye movement. Corollary discharge and sensory feedback allow learning of motor tasks by modifying movements so that the actual consequences of an action match the intended consequences. The matching process may involve the same basic neuronal mechanisms in the frontal hierarchy as are used in the perceptual hierarchies for recognition matching.

23 A prediction about executive memory comes from microelectrode studies in monkeys: prefrontal neurons integrate sensory information over time for the execution of behavioral acts that are contingent on that information. In a task requiring the association of tones with colors over time [Color Plate 2], Fuster et al (2000) recorded prefrontal neurons that had joint preferences for paired sensory items, e.g. low-pitch/green or high-pitch/red. This is a delayed paired-associate task in which the monkey responds according to previously learned associations. The recorded neurons represented samples from prefrontal networks that stored the associations of tones and colors. Successful selection of the correct matching color depends on both working memory (holding the sample information) and long-term memory networks (storing the pairedstimulus associations).

24 Memory consolidation is thought to operate in the same way for executive memory as for perceptual memory: 1) Concrete, stereotypical, and automatic movements are consolidated in motor cortex. 2) More general and abstract components of action are consolidated in prefrontal cortex. These include the rules and contingencies of a task. 3) With practice, the common elements of many action sequences generate overarching prefrontal networks that abstract the commonalities of those elements into action schemas. Symbolic memory: 1. The action schema is a symbolic representation that is equivalent to the perceptual symbolic representations of the posterior cortex. 2. The high-level action schema of the frontal lobe may be the form in which social regulations are instantiated in the brain.

25 The case of Phineas Gage The role of the brain in social cognition was first documented by John Harlow, a physician who attended Phineas Gage, a railroad construction supervisor who was injured by a tamping rod. The rod entered the head through his lower left jaw, traveled through the eye socket, and damaged parts of orbitofrontal and ventromedial prefrontal cortex before exiting through the top of the skull.

26 Gage recovered from the accident, but underwent pronounced changes in his behavior and personality. He had previously been friendly and reliable. After the accident, he became irritable, profane, short-tempered, and inconsiderate of others. He also was incapable of making plans. He was not able to keep his job as supervisor and became a wandering drifter. For a time, he was a side-show attraction with a traveling carnival. His skull and the rod are on display at a medical museum at Harvard University.

27 Adaptation to the social environment Since the time of Harlow, evidence as accumulated that the prefrontal cortex plays essential roles supporting social interactions. Adaptation to the social environment is critical for humans. From the time of birth, humans require social interaction for survival. In order to function in a social environment, many demands are placed on the brain s information processing systems. Functions are required that promote self-interest, self-enhancement, and self-protection. fmri studies show that cooperation among humans activates the same neural systems as do appetitive stimuli and symbolic rewards. Thus, cooperative activity in a social group can have significant reward value.

28 The orbitofrontal and ventromedial parts of prefrontal cortex are part of the reward system in the brain. Together with the amygdala, the orbitofrontal cortex has been associated with the abstract representation of reward. As part of this role, orbitofrontal cortex has been proposed to be involved in determining the socio-emotional relevance of social information. This means that it determines the social propriety of action schema, i.e. whether a particular action schema is socially appropriate or not.

29 Memory solidity: Local frontal lesions at lower hierarchical levels are more harmful to low-ranking memory than they are at higher levels to high-ranking memory. This is because of the higher degree of interconnectivity of network nodes at higher levels due to divergence and convergence of connections in ascending the hierarchy. Executive-perceptual memory: Memory of past sequences of behavior in autobiographical context, i.e. with respect to specific time and space references, involves networks that incorporate both executive and perceptual items. These networks probably depend on the heavy interconnectivity of prefrontal and posterior parietal cortex. Future memory: 1. Evidence that the lateral prefrontal cortex is essential for the representation of plans suggests that this area can form within itself networks that represent future action. 2. Once such networks are formed, goal-directed behavior requires that certain ones be selectively activated in choosing one course of action over another. This is a form of memory retrieval for the future. Thus, prefrontal cortex may be said to store memory for the future.

30 3. Since the ability to store multiple future actions has great survival value, it is probably related to the high degree of phylogenetic expansion of prefrontal cortex in humans. This ability may also play an important role in creative thought and general intelligence.

31 Retrieval of memory Memory retrieval entails activation of networks that were previously created by encoding and strengthened by consolidation. Stage Process Perception Patterned Activation Encoding Patterned Activation Patterned Synapse Formation Consolidation Patterned Synapse Strengthening Retrieval Patterned Re-activation In consolidation, synaptic modification occurs between columnar assemblies according to Hebbian principles as a result of their concurrent activity in perception. Thus, consolidation preserves the patterns of association of perceptual activity in the structure of cognits. Retrieval involves reactivation of the concurrent activity in the same patterns that produced the cognits. In the convergence zone hypothesis of Damasio, reactivation of a high-level cognit also re-activates the lower-level cognits from which it was formed.

32 Factors that alter memory retrieval: 1) loss of cognit solidity (synaptic strength) due to aging or disease: some amnesias (loss of retrieval ability) result from lowered solidity (fewer and/or weaker synapses), e.g. in Alzheimer s Disease. 2) physical insult: some amnesias result from a mechanism failure. After blunt force trauma, memory for events before and after the trauma is impaired. Over time, retrieval ability improves, with the period of amnesia both before and after trauma shrinking. Eventually, the bulk of the amnesia disappears, but some period of time remains irretrievable due to lack of consolidation. 3) psychogenic block: dynamic defenses against anxiety may make emotion-laden memories irretrievable by normal recall. 4) over-retrieval: obsessive-compulsive disorder leads to uncontrollable retrieval of perceptual memory, motor memory, or both. Not all contents of a memory are equally retrievable. Factors affecting degree of access to memory content: a) degree of consolidation at different hierarchical levels (e.g. general facts recalled better than specific facts) b) practice c) rehearsal d) attention e) type of retrieval (e.g. recognition retrieval easier than recall retrieval)

33 Role of hippocampus in memory retrieval Lesion analysis: patients with bilateral hippocampal damage (e.g. H.M.) show memory retrieval problems: a) It was originally believed that H.M. s impairment was primarily anterograde, i.e. for information following surgery (reflecting an inability to store new long-term memories). b) However, subsequent testing revealed that he also had extensive retrograde amnesia (reflecting an inability to retrieve previously stored memories). His retrograde amnesia covered most events 3-4 days prior to surgery. Neuroimaging: PET studies show hippocampal activation in memory retrieval. Anatomical connectivity: signals pass from sensory areas to hippocampal system & hippocampal system sends feedback to sensory neocortex In conclusion: current thinking is that the hippocampal system is necessary for: (1) retrieval of memories for a short time in the past (perhaps those memories not completely consolidated); as well as (2) the consolidation of new memories

34 Categories of neural input that initiate memory retrieval: 1. sensory input 2. inputs from other activated cognits 3. inputs from the internal milieu: a. influences from visceral systems b. influences from limbic system We next consider each of these three types of initiation in more detail.

35 1) Memory retrieval by sensory input a) Microelectrode studies in inferior temporal cortex of monkeys trained on paired associates show that neurons there respond similarly to either of two associated visual stimuli. This suggests that representations of both stimuli have been incorporated into an IT cognit, and that it can be activated by either stimulus. b) Microelectrode studies in somatosensory cortex of monkeys show that neurons there respond similarly to the touch of an object and to a visual stimulus that has been behaviorally associated with it. c) Organization of memory by category facilitates the recall of memory items. This may be due to the associative structure of memory networks across hierarchical levels. Sensory inputs arriving at one level may activate associated memory contents at the same level, and at higher and lower levels. Thus, cueing of an item s category may facilitate its recall through cross-level associations.

36 2) Memory retrieval by other memory networks One memory network may be activated as part of a sequence of associative retrievals. The activated network may subsequently retrieve a second network through associative connections. In principle, any memory retrieval operation externally or internally initiated may act as the source of further retrievals.

37 3) Memory retrieval by the internal milieu (a. visceral systems) Subcortical brain structures integrate visceral functions (temperature control; fluid control and thirst; eating, digestion, and hunger; reproduction; sleep; stress control; fear). The amygdala-hypothalamus-septum system sends signals to the orbitofrontal cortex, which may then activate memory networks. For example, the feeling of hunger may activate memories related to food.

38 3) Memory retrieval by the internal milieu (b. limbic system) External stimuli may feed into the amygdala at the same time as, or before, they reach cortex. The confluence of internal and external signals in the cortex may underlie such phenomena as state-dependent learning and fear conditioning, where internal signals about the state of the organism affect the learning process. Cortical networks that associate visceral and emotional inputs with perceptual items from the external senses may be considered to represent emotional memory. Emotional coloring of memory can sometimes distort retrieval. In false memory, the recollection of long-term memory may be inaccurate or incorrect due to its emotional content.

39 The terms explicit and implicit memory have traditionally referred to semantic and motor types of memory. Fuster, however, considers them to relate to different degrees of consolidation and memory strength: any memory may be explicit when it is newly consolidated, but may gradually become more and more implicit as it consolidates as a result of continued usage and/or practice. Explicit memory may require greater conscious effort for its retrieval than implicit memory. The phenomenon of priming is the facilitation of memory retrieval of an item as a result of previous exposure to a stimulus that is related to that item: a) Priming may be understood as the partial pre-activation of cortical memory networks that are in some way associated with the priming stimulus. b) The facilitating effect of priming may be seen by the enhanced probability, speed, or accuracy of the retrieval. c) The first stimulus need not be recognized for priming to occur, nor must it even reach conscious awareness.

40 Summary on memory retrieval: 1) the cerebral cortex may be considered as a vast web that associates all kinds of long-term memories, i.e. that contains cognits for all kinds of knowledge. 2) the retrieval of memory from this web can be very selective, i.e. retrieval may activate very specific, localized cognits. 3) the retrieval of memory may spread from one cognit to another in a sequence of recollections ( a train of thought ).