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Learning, Memory, & the Brain



The medical dictionary defines memory as the ability or process of reproducing or recalling what has been learned and retained especially through associative mechanisms (Merriam-Webster). However, extensive research studies that span through many decades reveal that memory is far more complex, resulting in networks of memory systems that serve various purposes. Due to the complexity, researchers have classified memory into two primary forms of memories, which are long term memory and short-term memory. Within the two broad forms of memory are further subclassifications that further detail the different types of memories within long term and short-term memory. For instance, one form of long-term memory is subdivided into declarative (explicit memory), as episodic (episodic or autobiographical), and as semantic memory (which pertains to facts and general knowledge). Nondeclarative memory, also known as implicit memory, is a different form of long-term memory and pertains to a type of memory that reflects the acquisition of skills, habits, priming, or conditioning. Emotional memory is a third form of long-term memory and is involved more to attraction, avoidance, or fear. While the study of memory is extensive and involves various brain and memory systems, or processes such as the encoding, consolidation, and retrieval of memory, the present paper gives primary focus to declarative and nondeclarative memory and its neural substrates. The taxonomy of these memory systems, along with its associated neural substrates are illustrated in the following graph.


 

Declarative Memory

Declarative memory is a conscious form of memory and is composed of semantic memories, which are fact-based types of memory and of episodic memories, which refers to a conscious recollection of personal and autobiographical memories.


Martinelli, et al., 2012 defined semantic memory as “all of the acquired knowledge about the world and is the basis for nearly all human activity”. Along with semantic memory, episodic memory is also a form of declarative or conscious type of memory system (Tulving, 2002). Episodic memory is form of memory that allows for individuals to recall past experiences whether they be personal or autobiographical. They are a form of conscious recollection of personal experiences and contains information under events that have occurred in the past. That is, while semantic memory records information about life, episodic memory allows for the person to recall personal experiences about such events (Tulving, 2002). It is therefore logical to state that in a hierarchical sense, semantic memory may be viewed as a form of platform for memories in which episodic memory is dependent upon (Tulving, 2002). It has further been found that semantic memory appears first during the development when compared to episodic memory (Tulving, 2002).  

         

Neural Substrates of Declarative Memory

Various research studies have identified general brain regions such as the frontal and temporal lobes which are implicated with declarative memory and these brain regions receive information from the neocortex and acetylcholine, serotonin, and noradrenaline motor and sensory ascending systems (Kolbe & Whishaw, 2009).


Within the temporal lobes lies the hippocampus, a brain region that is part of the limbic system and is heavily implicated with declarative memory and is known to have a major role in learning and memory (Dhikav, & Anand, 2012). The tubelike brain structure extends from the lateral neocortex of the medial temporal lobe. It is deep-seated within the temporal lobe and can be vulnerable to neurological and psychiatric disorders. However, the hippocampus may be more well known for its memory functions. The hippocampus is the long-term memory storage of sensory input and works in conjunction with the neocortex and other regions to organize and store information. From an anatomical standpoint, the hippocampus consists of two gyri which are the Ammon’s horn, which contain pyramidal cells and the Dentate gyrus which contain granule cells. These cells are found to be motor and sensory cells, respectively. Its connection to the rest of the brain consists of two major pathways, the perforant and the fimbria-fornix pathways. Its connections to other brain regions involve the posterior neocortex, thalamus, frontal cortex, basal ganglia, and hypothalamus (Dhikav, & Anand, 2012).


          The role that the hippocampus has with memory has been well established through extensive research (Brown & Aggleton, 2001). However, due to the associated structures that are also implicated with memory retention, there has been debate as to what degree the hippocampus is implicated with declarative memory. There is evidence that the hippocampus is responsible for the retention of memory in learning though for a shorter time span, leaving other associated brain structures to be implicated with the storage of longer-term memory functions (Kolb, & Whishaw, 2009). The researchers also suggest that earliest memories can be assessed in the neocortex. In contrast, other researchers have found that the removal of the hippocampus, without compromising adjacent structures, all earlier memories were no longer accessible, resulting in retrograde amnesia (Kolbe, & Whishaw, 2009). Not only was retrograde amnesia present, but so was anterograde amnesia.   


          The rirhinal cortices, particularly, the perirhinal and entorhinal cortex are areas of the brain that have been found to be more difficult to research though are found to have direct implications on declarative memory. Regarding the perirhinal cortex, this area is associated with recognition memory. To illustrate on the relationships, Brown and Aggleton (2001) used the example in which normal memory functioning will allow an individual to recognize various pieces of information when he or she sees another person that he or she has previously met. Such information may include, name, the location where the individual was first met, and other details. The perirhinal cortex allows for this information to become accessible and is a form of recognition memory. In contrast, an individual may see a person that he or she has previously met, and this person may be familiar. However, no other information can be recollected. The role of the perirhinal cortex therefore plays a different role than that of the hippocampus as this brain structure (hippocampus) may not even recall ever meeting the individual.


           The entorhinal cortex can be best described as a primary structure that regulates input and output of information of the hippocampal formation. Outer layers of this brain structure are an area in which cortical information is received and subsequently relayed to the structures in the hippocampus. However, information is projected back to the cortex via this structure.  However, Canto et al (2008) has proposed that the entorhinal cortex is not simply a structure that relays the input and output of information between the hippocampus and outer brain regions. Rather, in addition to relaying information, the entorhinal cortex has the responsibility of organizing information into specific sets of inputs and outputs. Such flow of information has been found to be significantly disrupted in Alzheimer’s disease in the latter stages of the disease, further suggesting an alteration of function and structure of the entorhinal cortex (Chrobak, et al., 2000).


The Frontal Cortex is also heavily implicated in the encoding and retrieval of declarative memory. This particular brain region has been found to become activated in specific brain regions and even tends to exhibit brain lateralization with certain memory functions. For instance, the left prefrontal cortex is found to be more involved in the encoding of semantic information than retrieving it. A similar pattern is found with episodic information. This brain region is more committed to encoding information and retrieving the information. However, the right prefrontal cortex has been found to be more involved in the retrieval of episodic memory, which contrasts with the left prefrontal cortex of the brain (Kolb & Whishaw, 2009; Buckner & Koutsaal, 1998). Buckner and Koutsaal further elaborated in stating that the left prefrontal cortex was seen to be even more specialized in the encoding of the information when distractor tasks are employed; tasks that are more engaging and that lead to long term storage.

Specific prefrontal brain regions identified as being implicated with explicit memory also involve the left ventrolateral frontal cortex which has been found to be activated with memory encoding of single or groups of words though retrieval of information has not been localized to this particular area (Buckner & Koutsaal, 1998). However, the right dorsolateral frontal cortex and the posterior parietal cortex for both hemispheres are involved with memory retrieval.

Interestingly, similar brain regions, though on the opposite hemisphere become activated in individuals that are right hemisphere dominant for language (Gabrieli, et al., 1998).   


Nondeclarative Memory (Implicit)

Whereas declarative or implicit memory is characterized by the recall of previously learned information, and requires a conscious effort to retrieve, nondeclarative implicit memory does not require a conscious effort to recall and does not tend to fade with time. That is, there is no need for the explicit recollection of memories of information or events. Implicit memory is also a form of procedural memory, and this relates to learning that is skilled and may include motor and cognitive skills. An example of implicit learning is acquiring the skills of how to ride a bicycle. Once an individual has learned the skill, no effort in retrieving information on how to ride a bicycle is longer required. However, studies have found that it is possible to become impaired with nondeclarative memory while having declarative memory sparred (Kolbe & Whishaw, 2009). For instance, an individual may become impaired in not being able to learn a procedure such as learning how to operate a fax machine though may have no difficulties in recalling information of events, which would be declarative memories.

  

Neural Substrates of Nondeclarative Memory 

Certain brain regions have been found to be implicated with nondeclarative. The regions include the basal ganglia, the motor cortex, and the cerebellum.


          The basal ganglia is a group of subcortical nuclei that is responsible for motor control, motor learning, executive functions and behaviors, and emotions Lanciego et al., 2012). The basal ganglia can be categorized as input nuclei, output nuclei, and intrinsic nuclei. In terms of anatomical structures, the basal ganglia is composed of the dorsal striatum, ventral striatum, and globus pallidus and associated regions include the substantia nigra, ventral tegmental area, and the subthalamic nucleus. Lesion studies and studies of patients with Parkinson’s and Research with Huntington’s Disease (heavily implicated with the basal ganglia problems) have revealed that contextual implicit learning is compromised and not only includes implicit motor learning but also visual implicit learning and habit formation (van Asselen et al., 2009). According to (Knowlton, 2020), the basal ganglia mediates learning and memory in which a stimulus -response associations (or habits) are acquired. This process observed from the basal ganglia is independent from other memory brain structures such as the hippocampus. While implicit learning is associated to the basal ganglia, the dorsal striatum has been found to be the most implicated with learning and memory as it pertains to nondeclarative memory and learning, and this process is seen to occur as habit learning (Knowlton, 2020). However, the basal ganglia can also be seen as a brain structure that assists with memory organization (Knowlton, 2020). The basal ganglia is known to receive information or signals from all areas of the cerebral cortex through corticostriatal pathways that are well organized. However, certain corticoganglial loops exists and these may be implicated with meaning and memory functions. According to Packard and Knowlton (2020), a hypothesis about learning and memory in relation to the basal ganglia describe fronto-cortical-striatal loops that are used by the basal ganglia to train the cortex to produce learned motor responses when presented with a pattern of sensory information. It is then concluded from this hypothesis and from lesion studies that the basal ganglia plays an important mnemonic role. Up to now, it is appropriate to state that the mnemonic role that the basal ganglia plays in learning and memory is essentially a form of habit learning. However, habit learning is not one dimensional, as proposed by Seger and Spiering (2011). According to these researchers, there are at least five definitional features of habit learning that should be considered. Specifically, the features include of habit learning that have been proposed include one that is inflexible, slow, or incremental, unconscious, automatic, and insensitive to reinforcer devaluation. About ‘inflexible’ as a feature of habit learning and characteristic of nondeclarative memory functions, more context dependent forms of learning as opposed to inflexible, would be more indicative of memory functions that are characteristic of the hippocampus. That is, novelty in a task may require for continual adjustments to be made in interpreting the stimuli, and thus, not allowing for conditioning, skills, or habits to be reinforced.


In an animal study done by Miskin et al., (1984) (Seger, & Spiering, 2011) animals that had a well intact hippocampus exhibited a capacity for one time learning and thus did not require for multiple trials to carried out for a skill or habit to be acquired. It is then concluded that memory and learning that is associated with the basal ganglia, is also characterized by a slow and incremental process. However, this process can be best conceptualized as habit learning and as gradual acquisition of learning and memory.

A clear distinction that can be made between declarative and nondeclarative memory rests of the idea that nondeclarative memory should be unconscious whereas declarative memory is more conscious or deliberate. While somewhat difficult to conceptualize, nondeclarative memory does have a form of awareness, in many instances, at one stage of developing a habit, skill, or becoming conditioned. It is at the reinforcement stage of developing this form of memory and learning that a conscious effort could be present. However, once memory and learning has been transferred to the basal ganglia, these processes can now be considered and assumed to be an unconscious effort. Pessiglione et al., (2008) has conducted research that indicates that the basal ganglia is involved in learning that is out of the realm of consciousness. However, his research also shows that in many instances, there is an awareness of what has been learned.


Implicit learning has also been found to be automatic as it does not require the processes involved to employ short term memory and selective attention to the degree in which implicit memory is able to be carried out even when other competing attentional stimuli are present. However, a discrepancy arises if it is believed that implicit memory should be automatic, even in the presence of competing stimuli. The reason for this discrepancy rests on the idea that a basic function of the basal ganglia is believed to be one in which it attends to executive functions but also, one in which a primary role of the basal ganglia is involved in task switching and selection of attention. To solve this dilemma, Forde et al., (2006) views the basal ganglia as having the capacity to engage in dual tasks and not necessarily be interrupted by independent tasks that could be viewed as competing against each other.


Other regions that are activated during an implicit learning task are found to also receive information through the thalamus from the cerebellum (Kolbe & Whishaw, 2009). Furthermore, while it is found that the cerebellum is implicated in motor responses as it pertains to implicit learning and memory, it is also found that conditioning, which is also a characteristic of implicit learning is significantly influenced by the cerebellum. To illustrate this, researchers conducted an experiment in which a puff of air was administered to the eyelid of a rabbit while being paired with a tone stimulus. Lesions to cerebellar pathways revealed that the rabbit continued to blink when the puff of air was administered though did no longer respond when the tone stimulus was presented (Thompson and Kim, 1996).


The cerebellum has been named “little brain” and is geographically located in the posterior portion of the brain and in proximity to the occipital and temporal lobes. Interestingly, while the cerebellum accounts for ten percent of the brain’s overall volume, this structure of the brain also accounts for over fifty percent of the total number of neurons found in the brain. In the past, the cerebellum’s primary function has been involved with motor processes. More specifically, it has been viewed as the brain region that is involved with the balance, coordination, and movements of moto responses. However, while the cerebellum has the responsibility of balance, posture, and coordinating voluntary movements, recent research has also found that the cerebellum plays an important role in motor learning such as fine tuning and adapting while also having other important roles with certain cognitive functions (Desmond and Fiez, 1998). It is therefore concluded that the cerebellum is implicated with initial learning of sequences, and detection and correction of errors in skill acquisition (Tiernan, n.d.). Furthermore, the cerebellum, as it relates to nondeclarative learning, is also involved in the coordination and timing of sequences thus allowing for the learned activity or event to be carried out without much analysis that might be observed with declarative learning.  


Conclusion

While other forms of memory and learning that may include emotional or short-term memory are not addressed in the present paper, it is evident that that declarative and nondeclarative memory functions employs many mechanisms and that its overall functions are complex. More recent research on the study of memory is intriguing as many of the findings of the last several years have revealed other functions, such as those of the cerebellum, that were not previously well understood.      


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