Memory Systems_How does the brain store memories ?

Introduction

The brain contains numerous systems for performing functions related to sensation, action, and emotion. Each system includes billions of neurons with an enormous number of interconnections. Previously, we explored the mechanisms that guide the construction of these systems during brain development. Although prenatal development is impressive, the difference between a newborn baby and a Nobel Prize winner largely comes down to what has been learned and remembered. From the moment we take our first breath, sensory stimuli modify our brain and influence our behavior. We learn a wide range of things, from simple facts like "snow is cold" to more abstract concepts like "an isosceles triangle has two sides of equal length." Some of what we learn involves ingrained motor patterns, such as driving or playing soccer. Brain lesions can differentially affect different types of remembered information, suggesting the existence of multiple memory systems.

Experience-Dependent Brain Development and Learning

There is a close relationship between what we called experience-dependent brain development and what we call learning. Visual experience during infancy is essential for the normal development of the visual cortex and helps us recognize images, such as our mother’s face. Visual development and learning likely involve similar mechanisms but occur at different times and in different cortical areas. Learning and memory are lifelong adaptations of brain circuitry to the environment, enabling us to respond appropriately to situations we have experienced before.

Anatomy of Memory

This chapter discusses the anatomy of memory and the different parts of the brain involved in storing particular types of information. Chapter 25 will focus on the elementary molecular mechanisms that can store information in the brain.

Types of Memory and Amnesia

Learning is the acquisition of new knowledge or skills, while memory is the retention of learned information. We learn and remember various things, and these different types of information might not be processed and stored by the same neural hardware. No single brain structure or cellular mechanism accounts for all learning, and the way in which a particular type of information is stored may change over time.

Declarative and Nondeclarative Memory

Psychologists have extensively studied learning and memory, distinguishing between declarative memory and nondeclarative memory. During our lives, we learn many facts and store memories of events. Memory of facts and events is called declarative memory. A distinction within declarative memory is between episodic memory, which concerns autobiographical life experiences, and semantic memory, which concerns facts. Declarative memory is what people usually mean by "memory," but we also have nondeclarative memories.

Nondeclarative memory includes procedural memory, which involves skills, habits, and behaviors. Procedural memory is stored in the brain and is accessed for tasks we learn and for reflexes and emotional associations. This type of memory operates smoothly without conscious recollection. Nondeclarative memory is often called implicit memory because it results from direct experience, while declarative memory is called explicit memory because it involves more conscious effort.

Declarative memories are often easy to form and easily forgotten, while nondeclarative memories usually require repetition and practice over a longer period but are less likely to be forgotten. There is great diversity in the ease and speed with which new information is acquired, and studies of humans with abnormally good memories suggest a remarkably high limit on the storage of declarative information.

Types of Procedural Memory

Procedural memory involves learning a motor response (procedure) in reaction to a sensory input. Procedural memories form through two categories of learning: nonassociative learning and associative learning.

Nonassociative Learning

Nonassociative learning involves a change in behavioral response over time in response to a single type of stimulus. There are two types: habituation and sensitization. Habituation is learning to ignore a stimulus that lacks meaning, while sensitization intensifies a response to stimuli.

Associative Learning

In associative learning, behavior is altered by forming associations between events. There are two types: classical conditioning and instrumental conditioning. Classical conditioning involves associating a stimulus that evokes a measurable response with a second stimulus that normally does not evoke this response. Instrumental conditioning involves associating a motor act with a meaningful stimulus, typically a reward.

Types of Declarative Memory

Long-term memories are those that can be recalled days, months, or years after they were originally stored, while short-term memories are held by the brain only temporarily. Short-term memory can be erased by head trauma or electroconvulsive therapy (ECT) used to treat psychiatric illness, while long-term memories are not affected. Working memory, lasting on the order of seconds, is distinguished from short-term memory by its very limited capacity, the need for repetition, and the short duration. Different digit spans in different modalities suggest multiple temporary storage areas in the brain.

Amnesia

Forgetting happens as often as learning in daily life. Certain diseases and injuries to the brain can cause amnesia, a serious loss of memory and the ability to learn. Amnesia can result from trauma and may involve retrograde amnesia, which is memory loss for events before the trauma, or anterograde amnesia, which is the inability to form new memories following brain trauma. Transient global amnesia is a sudden onset of anterograde amnesia that lasts for a short period and is often accompanied by retrograde amnesia for recent events preceding the attack. The cause of transient global amnesia is not clearly established but may involve brief cerebral ischemia or other factors that affect cerebral blood flow.

Working Memory

Our brains acquire all kinds of information through our sensory systems, but we pay attention to only a fraction of it. To serve immediate behavioral needs, some of this sensory information is held in mind by working memory, such as a phone number we must remember in order to call. Unlike long-term memory, working memory has a very small capacity, as shown by the digit span described earlier.

Quantifying Working Memory Capacity

There are subtleties to the quantification of working memory capacity. For example, more words can be held in memory if they are short common words. Also, more words and numbers can be held in working memory if they can be chunked into meaningful groups (e.g., a 12-digit number is easily held when chunked into three years, such as 1945, 1969, 2001). Working memory can be thought of as a limited resource that can be used in a variety of ways. There are tradeoffs in the amount and precision of stored information that are influenced by the behavioral significance of the information.

Conversion to Long-Term Memory

Information held in working memory might be converted into long-term memories, but most of it is discarded when no longer needed. Research in both animals and humans suggests that working memory is a capability of the neocortex found in numerous locations in the brain.

The Prefrontal Cortex and Working Memory

One of the most obvious anatomical differences between primates (especially humans) and other mammals is that primates have a large frontal lobe. The rostral end of the frontal lobe, the prefrontal cortex, is particularly highly developed. Compared to the sensory and motor cortical areas, the function of the prefrontal cortex is relatively poorly understood. But because it is so well developed in humans, the prefrontal cortex is often assumed to be involved in characteristics that distinguish us from other animals, such as self-awareness and the capacity for complex planning and problem-solving.

Delayed-Response Task and Prefrontal Lesions

Some of the first evidence suggesting that the frontal lobe is important for learning and memory came from experiments performed in the 1930s using a delayed-response task. A monkey was first shown food being placed in a well below one of two identical covers on a table. A delay period followed, during which the animal could not see the table. Finally, the animal was allowed to see the table again and received the food as a reward if it chose the correct well. Large prefrontal lesions seriously degraded performance in this delayed-response task, as well as other tasks including a delay period. The monkeys performed increasingly poorly as the delay period was lengthened, implying that the prefrontal cortex may be involved in retaining information in working memory.

Working Memory for Problem Solving and Planning

Experiments conducted more recently suggest that the prefrontal cortex is involved with working memory for problem-solving and the planning of behavior. One piece of evidence comes from the behavior of humans with lesions in the prefrontal cortex. Recall the case of Phineas Gage, who had difficulty maintaining a course of behavior after severe frontal lobe damage. He could carry out behaviors appropriate for different situations, but had difficulty planning and organizing these behaviors.

Wisconsin Card-Sorting Test

The Wisconsin card-sorting test can demonstrate problems associated with prefrontal cortical damage. A person is asked to sort a deck of cards with a variable number of colored geometric shapes. The cards can be sorted by color, shape, or number of symbols, but at the beginning of the test, the subject isn’t told which category to use. The subject begins putting cards into stacks and is informed when errors occur, by which the subject learns what sorting category is to be used. Then, after ten correct card placements have been made, the sorting category is changed, and the subject starts over again. People with prefrontal lesions have great difficulty on this task when the sorting category is changed; they continue to sort according to a rule that no longer applies. This suggests they have a working memory deficit that limits their ability to use recent information to change their behavior.

Other Deficits in Tasks

The same kind of deficit is seen in other tasks. For example, a person with a prefrontal lesion might be asked to trace a path through a maze drawn on a piece of paper. Although the patient understands the task, he or she will repeatedly make the same mistakes, returning to blind alleys. These patients do not learn from their recent experience in the same way as a normal person, suggesting a working memory deficit.

Neuronal Response Patterns in Prefrontal Cortex

The neurons in the prefrontal cortex have a variety of response types, some of which may reflect a role in working memory. Two response patterns were observed while a monkey performed a delayed-response task. One neuron responded when the animal first saw the food wells, was unresponsive during the delay interval, and responded again when the animal saw the food wells again. This response pattern simply correlates with visual stimulation. More interesting is the response pattern of another neuron, which fired only during the delay interval. This cell was not directly activated by the stimuli in the first or second interval in which the monkey saw the food wells. The increased activity during the delay period may be related to the retention of information needed to make the correct choice after the delay, suggesting a role in working memory.

Imaging Working Memory in the Human Brain

Human brain imaging experiments suggest that numerous brain areas in the prefrontal cortex are involved in working memory. In one study by Courtney et al., brain activity was recorded by positron emission tomography (PET) while subjects performed two working memory tasks. In the identity task, three face photographs were briefly shown in succession; each image was at a different location and the subject looked at each face to memorize it. The subject indicated whether the face was the same as one of the memorized faces. In the location task, a similar paradigm was used, but the subject’s task was to memorize the locations of the three faces presented before the delay, with the identities of the faces being irrelevant. Both experiments looked for brain activity during the delay interval between the memorization and test phases.

Brain Areas Involved in Working Memory

The brain areas that demonstrated significant working memory activity in these experiments included six areas in the frontal lobe that showed sustained activity during the delay period, suggesting a role in working memory. Three areas exhibited stronger sustained activity for facial identity than spatial location, one area was more responsive to spatial memory, and two areas were active equally in facial and spatial memory tasks. An interesting unanswered question is whether working memory for other types of information is held in the same or different brain areas.

Area LIP and Working Memory

Cortical areas outside the frontal lobe have also been found to contain neurons that appear to retain working memory information. An example is provided by the lateral intraparietal cortex (area LIP), buried in the intraparietal sulcus. Area LIP is thought to be involved in guiding eye movements because electrical stimulation here elicits saccades in monkeys. The responses of many neurons in area LIP of monkeys suggest that they are also involved in a type of working memory. This pattern is evident in a delayed-saccade task, in which the animal fixates on a point on a computer screen and a target is briefly flashed at a peripheral location. After the target goes off, there is a variable length delay. At the end of the delay period, the fixation point disappears, and the animal’s eyes make a saccadic movement to the remembered location of the target. The neuron begins firing shortly after the peripheral target is presented, and the cell keeps firing throughout the delay period. The neuron stops firing only after the saccadic eye movement begins.

Modality-Specific Working Memory Responses

Other areas in the parietal and temporal cortex have been shown to have analogous working memory responses. These areas seem to be modality-specific, just as the responses in area LIP are specific to vision. This is consistent with the clinical observation that there are distinct auditory and visual working memory deficits in humans produced by cortical lesions.

Declarative Memory

We've seen that sensory information can be temporarily held in mind by working memory, but how does the brain retain information for a longer time? Even before humans evolved to the point that we could cram for neuroscience exams by drawing cartoons of the brain, we needed to remember many things—the location of the river to drink from, where to find food, which cave to call home. To understand the neural basis of declarative memory storage, we first need to examine where in the brain it is stored. In other words, we must explore the location of a memory, known as an engram or memory trace. For example, when you learn the meaning of a word in a foreign language, where in your brain is this information stored; where is the engram?

The Neocortex and Declarative Memory

In the 1920s, American psychologist Karl Lashley conducted experiments to study the effects of brain lesions on learning in rats. Well aware of the cytoarchitecture of the neocortex, Lashley set out to determine whether engrams resided in particular association areas of the cortex, as was widely believed at the time. In a typical experiment, he trained a rat to run through a maze to get a food reward. On the first trial, the rat was slow getting to the food because it would enter blind alleys and have to turn around. After running through the same maze repeatedly, the rat learned to avoid blind alleys and go straight to the food. Lashley was investigating how performance on this task was affected by different lesions in the rat’s cortex. He found that a rat given a brain lesion after it had learned to run the maze then made mistakes and went down blind alleys it had previously learned to avoid. Apparently, the lesion damaged or destroyed the memory for how to reach the food.

Impact of Lesion Size and Location

How did the size and location of lesions affect learning and memory? Interestingly, Lashley found that the severity of the deficits caused by the lesions (both learning and remembering) correlated with the size of the lesions but was apparently unrelated to the location of the lesion within the cortex. Based on these findings, he speculated that all cortical areas contribute equally (are equipotential) to learning and memory; it was simply a matter of performance on the maze task becoming poorer as the lesion became bigger and the ability to remember the maze worsened. If true, this would be a very important finding because it implied that engrams are based on neural changes spread throughout the cortex rather than being localized to one area. The problem with this interpretation was that Lashley’s lesions were large, each damaging multiple brain areas possibly involved in learning or remembering the maze task. Another problem was that the rats may have solved the maze in several different ways—by sight, feel, and smell—and the loss of one memory might have been compensated for by another.

Lashley's Conclusions and Subsequent Research

Subsequent research has proven Lashley’s conclusions to be incorrect. All cortical areas do not contribute equally to all memories. Nonetheless, his conclusions that all of the cortex participates in memory storage, and that an engram can be widely distributed in the brain, are correct and important. Lashley had a major impact on the study of learning and memory because he led other scientists to consider ways in which memories might be distributed among the vast number of neurons of the cerebral cortex.

Hebb and the Cell Assembly

Lashley’s most famous student was Donald Hebb. Hebb reasoned that it was crucial to understand how external events are represented in the activity of the brain before one can hope to understand how and where these representations are stored. In his remarkable book The Organization of Behavior, published in 1949, Hebb proposed that the internal representation of an object consists of all the cortical cells that are activated by the external stimulus. Hebb called this group of simultaneously active neurons a cell assembly. Hebb imagined that all these cells were reciprocally interconnected. The internal representation of the object was held in working memory as long as activity reverberated through the connections of the cell assembly. Hebb further hypothesized that if activation of the cell assembly persisted long enough, consolidation would occur by a “growth process” that made these reciprocal connections more effective; neurons that fired together would wire together. Subsequently, if only a fraction of the cells of the assembly were activated by a later stimulus, the now-powerful reciprocal connections would cause the whole assembly to become active again, thus recalling the entire internal representation of the external stimulus—in this case, the circle.

Hebb's Twofold Message

Hebb’s important message about the engram was two fold:

(1) It could be widely distributed among the connections that link the cells of the assembly.

(2) It could involve the same neurons that are involved in sensation and perception. Destruction of only a fraction of the cells of the assembly would not be expected to eliminate the memory, possibly explaining Lashley’s results. Hebb’s ideas stimulated the development of neural network computer models. Although his original assumptions had to be modified slightly, these models have successfully reproduced many features of human memory.

The Location of Engrams

Where is the engram for a foreign language? Look to the regions of the brain in the temporal and parietal lobes that normally process language. A lesion here can disrupt your memory of a foreign word but leave intact the memory of your foreign-born grandmother’s face. However, although declarative memories may finally reside in many areas of the neocortex, decades of research indicate that to get there, they must pass through structures in the medial temporal lobes.

Studies Implicating the Medial Temporal Lobes

A variety of experiments indicate that structures in the medial temporal lobe are particularly important for the consolidation and storage of declarative memories.

Anatomy of the Medial Temporal Lobe

The temporal lobe is located under the temporal bone, so named because the hair of the temples is often the first to go gray with the passage of time (tempus is Latin for “time”). The association of the temporal lobe with time was fortuitous as this region of the brain is important for recording past events. The medial portion of the temporal lobe contains the temporal neocortex, which may be a site of long-term memory storage, and a group of structures interconnected with the neocortex that are critical for the formation of declarative memories.

Key Structures of the Medial Temporal Lobe

The key structures are the hippocampus, the nearby cortical areas, and the pathways that connect these structures with other parts of the brain. The hippocampus is a folded structure situated medial to the lateral ventricle. The name means “seahorse,” a resemblance you can see in the hippocampus. Ventral to the hippocampus are three important cortical regions that surround the rhinal sulcus: the entorhinal cortex, which occupies the medial bank of the rhinal sulcus; the perirhinal cortex, which occupies the lateral bank; and the parahippocampal cortex, which lies lateral to the rhinal sulcus. Inputs to the medial temporal lobe come from the association areas of the cerebral cortex, containing highly processed information from all sensory modalities. For instance, inferotemporal visual cortex projects to the medial temporal lobe, but low-order visual areas such as striate cortex do not. This means that the input contains complex representations, perhaps of behaviorally important sensory information, rather than responses to simple features such as light–dark borders. Input first reaches the rhinal and parahippocampal cortex before being passed to the hippocampus. A major output pathway from the hippocampus is the fornix, which loops around the thalamus before terminating in the hypothalamus.

Electrical Stimulation of the Human Temporal Lobes

One of the most intriguing and controversial studies implicating the neocortex of the temporal lobe in the storage of declarative memory traces involved electrical stimulation of the human brain. Wilder Penfield’s work involved patients whose brains were electrically stimulated at numerous locations prior to the ablation of the seizure-prone region as part of surgical treatment for severe epilepsy. Stimulation of somatic sensory cortex caused the patient to experience tingling sensations in regions of skin, whereas stimulation of motor cortex caused a certain muscle to twitch. Electrical stimulation of the temporal lobe occasionally produced more complex sensations than stimulation in other brain areas. In a number of cases, Penfield’s patients described sensations that sounded like hallucinations or recollections of past experiences. This is consistent with reports that epileptic seizures of the temporal lobes can evoke complex sensations, behaviors, and memories.

Interpretation of Temporal Lobe Stimulation

Are these people reexperiencing events from earlier in their life because memories are evoked by the electrical stimulation? Does this mean that memories are stored in the neocortex of the temporal lobe? One interpretation is that the sensations are recollections of past experiences. The fact that such elaborate sensations resulted only when the temporal lobe was stimulated suggests that the temporal lobe may play a special role in memory storage. However, other aspects of the findings do not clearly support the hypothesis that engrams are being electrically activated. For instance, some brain-stimulated patients said they saw themselves, something that we cannot normally experience.

Also, it is important to appreciate that complex sensations were reported only by a minority of the patients, and all of these patients had an abnormal cortex associated with their epilepsy. There is no way to prove whether the complex sensations evoked by temporal lobe stimulation are recalled memories. However, it is clear that the consequences of temporal lobe stimulation and temporal lobe seizures can be qualitatively different from stimulation of other areas of the neocortex.

Neurons and Memory Formation

Some neurons are selective for recognizing specific people or landmarks, like the Eiffel Tower. These neurons lie between visual and memory coding. However, they may not be essential for recognition since even individuals with hippocampal damage, like H.M., could recognize familiar people and objects. These neurons likely play a role in forming new memories.

Temporal Lobe Amnesia

The removal of temporal lobes has significant effects on learning and memory.

The famous case of H.M. highlights this.

The Case of H.M.: Temporal Lobectomy and Amnesia

Henry Molaison (H.M.) had an operation in 1953 to treat severe epilepsy, involving the removal of an 8 cm portion of his medial temporal lobe, including parts of the hippocampus. Although the surgery reduced his seizures, it resulted in profound anterograde amnesia. He could not form new memories and had some retrograde amnesia, losing memories from several years before his surgery.

Memory Retention and Learning Post-Surgery

Despite his memory loss, H.M.'s working memory remained largely intact. He could recall information temporarily with constant rehearsal, and he could learn new tasks, though he had no memory of learning them. This indicates that the brain structures for procedural memory were still functional.

Significance of H.M.'s Case

H.M.'s case helped establish that the medial temporal lobe is critical for memory consolidation but not for memory retrieval. His ability to retain some memories from before the surgery suggests that not all memories are stored in the medial temporal lobe.

Animal Models of Human Amnesia

Studies in macaque monkeys have shown that damage to the medial temporal lobe results in anterograde amnesia, similar to H.M. The delayed non-match to sample (DNMS) task demonstrated that monkeys with such lesions have difficulty remembering objects after a delay.

Memory Deficits from Temporal Lobe Lesions

Monkeys with medial temporal lesions performed poorly on DNMS tasks with longer delays, indicating a memory deficit. This deficit was not specific to visual memory, as it also occurred when monkeys could touch but not see the objects.

Hippocampus and Memory

The hippocampus plays a vital role in memory consolidation. In rats, hippocampal lesions impair spatial memory, as seen in the radial arm maze experiment. However, rats can still learn procedural tasks, suggesting a distinction between different types of memory.

Place Cells in the Hippocampus

Place cells in the hippocampus respond to specific locations, helping animals navigate their environment. These cells adapt to changes in the environment, such as stretching the space or altering visual cues. This adaptability underscores the hippocampus's role in spatial memory.

Learning and Adaptation of Place Cells

In experiments with rats, researchers initially observe place cells within a small box and determine the fields of several cells. When a hole is cut in the side of the box, allowing the rat to explore a larger area, some cells develop place fields outside the smaller box after the rat has explored the new environment. These cells seem to learn by altering their receptive fields to suit the new environment, indicating a possible role in spatial memory.

Role of Hippocampus in Humans

While the existence of place cells in the human brain is uncertain, PET imaging studies show hippocampal activation during virtual or imagined navigation. In one experiment, subjects navigated a virtual town, showing increased activation of the right hippocampus and left tail of the caudate when navigating, especially compared to a control condition with directional arrows guiding them.

Hippocampal Function in London Taxi Drivers

Studies on London taxi drivers, who must memorize a vast number of city streets, show that they have a larger posterior and smaller anterior hippocampus compared to a control group. The size of the posterior hippocampus correlates with the length of experience as a taxi driver, suggesting the human hippocampus plays a role in spatial navigation.

Hippocampal Lesions and Navigation Impairment

An interesting case is T.T., a London taxi driver who suffered bilateral hippocampal damage after nearly 40 years in his profession. While he could recognize city landmarks, his navigation was impaired, especially on smaller roads, suggesting the human hippocampus is crucial for detailed spatial memory.

Grid Cells in the Entorhinal Cortex

In addition to place cells, grid cells in the entorhinal cortex are spatially selective and respond when an animal is at multiple locations forming a hexagonal grid. Recent experiments suggest the presence of grid cells in the human entorhinal cortex, showing sinusoidal variation in fMRI signals during navigation, indicating a role in spatial orientation.

Standard Model of Memory Consolidation

The standard model of memory consolidation suggests that information processed in the hippocampal system is eventually stored in distributed areas of the neocortex. This process, known as systems consolidation, makes the hippocampus initially necessary for memory retrieval, but later the hippocampus is no longer needed as memories are stored in the neocortex.

Critiques of the Standard Model: Multiple Trace Model

The multiple trace model proposes that the hippocampus is always involved in memory storage, even for older memories. According to this theory, each time an episodic memory is retrieved, it forms a new memory trace involving both the hippocampus and neocortex. This model explains retrograde amnesia as dependent on the number of memory traces, suggesting that the hippocampus is crucial for retrieving episodic memories regardless of their age.

Reconsolidation of Memories

Research has shown that memories can be altered or erased even after they are consolidated. In experiments with rats, reactivated memories were wiped out by electroconvulsive shock, suggesting that reactivation makes memories sensitive to change, a process known as reconsolidation. This phenomenon has profound implications for understanding memory reliability and treating conditions like post-traumatic stress disorder.

Human Studies on Reconsolidation

In experiments with human subjects, reactivation of consolidated memories led to the incorporation of new sensory information into the old memories during reconsolidation. This finding suggests that recalling a memory can make it susceptible to change, highlighting the dynamic nature of memory and its implications for both everyday recollections and therapeutic interventions.

PROCEDURAL MEMORY

Thus far, we have focused on the brain systems involved in the formation and retention of declarative memories.

The neural basis of nondeclarative memory is complex because different types involve different brain structures.

The Striatum and Procedural Memory in Rodents

Various kinds of nondeclarative memory are thought to involve different parts of the brain. For example, evidence supports the involvement of the striatum in habit learning and procedural memory.

The basal ganglia, including the caudate nucleus and the putamen (striatum), are crucial for controlling voluntary movements. The striatum receives input from the frontal and parietal cortex and sends output to thalamic nuclei and cortical areas involved in movement. Studies suggest that the striatum is critical for procedural memory involved in forming behavioral habits.

The Striatum and Procedural Memory in Rodents

H.M.'s ability to learn new habits despite his inability to form new declarative memories suggests distinct circuitry for procedural memory. In rodents, lesions to the striatum disrupt procedural memory without affecting declarative memory. For instance, rats performing tasks with and without light cues show that lesions to the striatum impair performance on light-based tasks but not on standard tasks.

Recordings from the rat striatum show that neural responses change as animals learn a procedure associated with a reward, indicating the striatum codes a sequence of behaviors.

Habit Learning in Humans and Nonhuman Primates

Studies with monkeys show similar effects of brain lesions on memory in rodents and primates. Lesions to the medial temporal lobe impair declarative memory but not habit learning. Conversely, lesions to the striatum disrupt habit learning but not declarative memory.

Diseases like Huntington’s and Parkinson’s that affect the basal ganglia provide further evidence of the striatum’s role in habit learning. For instance, Parkinson’s patients struggle with tasks requiring stimulus-response habits but perform normally on declarative memory tasks.

CONCLUDING REMARKS

The human brain is constantly changing with experience. Learning and memory involve changes across the brain, with structures like the medial temporal lobe and diencephalon critical for memory consolidation. Research continues to explore how specific brain structures contribute to different types of memory.