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Semantic amnesia is a type of amnesia that affects semantic memory and is primarily manifested through difficulties with language use and acquisition, recall of facts and general knowledge. A patient with semantic amnesia would have damage to the temporal lobe.
Memory has two classifications—short-term memory and long-term memory. Long-term memory can store information for a long duration and is subdivided into non-declarative (implicit) memory for learned skills and habits, and declarative (explicit) memory for knowledge of facts and events. [1] Declarative memory consists of semantic memory and episodic memory.
Semantic memory refers to acquired facts and general knowledge about the world. Examples include the name and physical attributes of objects and events, origins and history of objects, causes and effects of events or objects, associations between concepts, categories, opinions, beliefs, knowledge of historical events, etc. [2] The context in which semantic information is learned does not need to be remembered, such as who was involved, what was around the object, when the event occurred, where the object was seen, or how the event took place. One way of testing an individual's semantic memory is through the person's naming ability. People could be asked to name a series of objects ranging from low difficulty levels to high difficulty levels. For example, one could be asked to name a flower (easy) and a pipette (difficult). In addition to naming tests, vocabulary tests are also used to assess people's ability to define words and to appropriately select synonyms of words [3]
Semantic amnesia occurs as a result of impairments to semantic memory. Patients with semantic amnesia experience difficulty in learning new semantic information while some are unable to retain and retrieve it. Essentially, the ability to learn new facts and general knowledge would be compromised. However, research has suggested that some patients may find it easier to learn new semantic information when they can relate to it on a personal level and make connections to facts and knowledge they already know. [3]
The medial temporal lobe is essential for declarative memory, and consists of different regions that contribute towards its function. [4] It is located in the inner part of the temporal lobe, which is found near the division of the left and right hemisphere. The limbic system is situated deep inside the medial temporal lobe, and consists of the hippocampus, the amygdala, the cingulate gyrus, the thalamus, the hypothalamus, and other organs. [5]
Episodic and semantic memory are both dependent on the hippocampal region.[ medical citation needed ]
Lesions to the medial temporal lobe affects both episodic and semantic memory. [3] Immediate or short-term memory remains intact. [4]
Damage to the hippocampus can have long-term implications on semantic memory as well as difficulties in remembering names, dates, and events. Recalling verbal information is affected when the damage is on the left hippocampus.[ medical citation needed ]
Figure 1. Table showing the memory type, neuroanatomy, symptoms associated with each memory dysfunction, and cognitive testing deficit [adapted from Matthews, B. (2015). Memory Dysfunction. Behavioral Neurology and Neuropsychiatry, 613-626]. [6]
| Memory Type | Neuroanatomy | Symptoms | Cognitive Testing Deficit |
|---|---|---|---|
| Episodic | Medial temporal lobe | Unable to remember most recent experience (e.g. vacation); Unable to recall locations of places (e.g. where a particular store is located) | Recall of oral narrative and word list recall; Recall of figure location in space |
| Semantic | Anterior and inferior temporal lobe | Unable to recall the number of weeks in a year; Identifying most items as "things"; Unable to distinguish between objects or understand their meaning | General knowledge, picture naming, category fluency |
Semantic amnesia progressively evolves into dementia. Semantic dementia, a degenerative disorder, causes a progressive loss of semantic and conceptual knowledge. The region of the brain associated with semantic dementia is the left anterior temporal lobe [7] Patients experience difficulties in verbal identification of stimuli and have poor word knowledge and association abilities. Other symptoms associate with semantic dementia include anomia (a form of aphasia), category fluency, comprehension, and language problems. [7] However, visuospatial, attentional, and executive functions remain normal. [7] Retention of numerical knowledge and music is unaffected.
Anomia is the inability to name objects, and is a common early symptom of semantic dementia. [8] As a result, it becomes difficult to correctly name objects. For example, a pen may be referred to as a pencil, a car as a truck, or a glass may be considered to be a cup.
In later stages, the ability to categorize information deteriorates and no distinction remains between objects of different nature, such as hot versus cold, plant versus animal, etc. In fact, the perception towards objects also changes, such that everyday objects may appear strange or frightening. [8]
Studies have shown that as semantic impairment deteriorates, autobiographical impairment also becomes worse. [9] The patient is unable to remember events that occurred throughout their life, such as achievements, milestones, major setbacks and successes, to name a few. This finding suggests that episodic memory may also be affected.
Reasoning abilities remain relatively unaffected. [9]
Transient semantic amnesia may result from a temporary dysfunction of the inferolateral temporal lobes. [10] As the name suggests, this type of amnesia occurs temporarily and in most cases the patient's memory resumes back to normal.
An example of a case study of transient semantic amnesia involves a patient who was admitted into hospital with an acute loss of memory for common words and their meanings. For example, he was unable to understand and differentiate between a "car" and an "engine". His speech was fluent, he could perform quick mental maths, and when reviewed by the doctor, his orientation and verbal anterograde memory was normal. [10] However he had been suffering from migraine for a long time. [10] When reviewed after a month, the patient was back to normal based on a cognitive assessment as well as CT scans. He was also able to remember the sequence of events that led to his admission into the hospital.
This case suggests a transient loss of semantic memory, with preservation of episodic memory. [10] Semantic dementia has been associated to this syndrome, with the following being core symptoms: [10]
The first case of selective semantic memory deficit was characterized by a loss of verbal vocabulary and impaired knowledge of animals and objects. [11]
Treatment for amnesia really depends on the cause. Options may include psychiatric treatment, medications, sedation, dietary advise, abstinence, and nursing homes [12]
While there is no single medication that can cure semantic amnesia and dementia instantly, there are treatment options available to slow down the progression of the symptoms, especially anomia. Furthermore, amnesic patience may require repetition of information to learn, as perhaps they are unable to learn semantic information from reading it in just one trial, which normal healthy individuals would be able to do. [3]
Studies on dissociations between episodic and semantic memory have shown that while one form of memory can be impaired, the other can remain relatively intact. [9] However, there is scope for research on exploring associations between the two, and whether their impairments can occur simultaneously or have an effect on each other. [9]
Long-term memory (LTM) is the stage of the Atkinson–Shiffrin memory model in which informative knowledge is held indefinitely. It is defined in contrast to short-term and working memory, which persist for only about 18 to 30 seconds. LTM is commonly labelled as "explicit memory", as well as "episodic memory," "semantic memory," "autobiographical memory," and "implicit memory".
Source amnesia is the inability to remember where, when or how previously learned information has been acquired, while retaining the factual knowledge. This branch of amnesia is associated with the malfunctioning of one's explicit memory. It is likely that the disconnect between having the knowledge and remembering the context in which the knowledge was acquired is due to a dissociation between semantic and episodic memory – an individual retains the semantic knowledge, but lacks the episodic knowledge to indicate the context in which the knowledge was gained.
Henry Gustav Molaison, known widely as H.M., was an American who had a bilateral medial temporal lobectomy to surgically resect the anterior two thirds of his hippocampi, parahippocampal cortices, entorhinal cortices, piriform cortices, and amygdalae in an attempt to cure his epilepsy. Although the surgery was partially successful in controlling his epilepsy, a severe side effect was that he became unable to form new memories.
The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain.
In neurology, anterograde amnesia is the inability to create new memories after an event that caused amnesia, leading to a partial or complete inability to recall the recent past, while long-term memories from before the event remain intact. This is in contrast to retrograde amnesia, where memories created prior to the event are lost while new memories can still be created. Both can occur together in the same patient. To a large degree, anterograde amnesia remains a mysterious ailment because the precise mechanism of storing memories is not yet well understood, although it is known that the regions of the brain involved are certain sites in the temporal cortex, especially in the hippocampus and nearby subcortical regions.
Semantic memory refers to general world knowledge that humans have accumulated throughout their lives. This general knowledge is intertwined in experience and dependent on culture. New concepts are learned by applying knowledge learned from things in the past.
Episodic memory is the memory of everyday events that can be explicitly stated or conjured. It is the collection of past personal experiences that occurred at particular times and places; for example, the party on one's 7th birthday. Along with semantic memory, it comprises the category of explicit memory, one of the two major divisions of long-term memory.
In neurology, retrograde amnesia (RA) is the inability to access memories or information from before an injury or disease occurred. RA differs from a similar condition called anterograde amnesia (AA), which is the inability to form new memories following injury or disease onset. Although an individual can have both RA and AA at the same time, RA can also occur on its own; this 'pure' form of RA can be further divided into three types: focal, isolated, and pure RA. RA negatively affects an individual's episodic, autobiographical, and declarative memory, but they can still form new memories because RA leaves procedural memory intact. Depending on its severity, RA can result in either temporally graded or more permanent memory loss. However, memory loss usually follows Ribot's law, which states that individuals are more likely to lose recent memories than older memories. Diagnosing RA generally requires using an Autobiographical Memory Interview (AMI) and observing brain structure through magnetic resonance imaging (MRI), a computed tomography scan (CT), or electroencephalography (EEG).
Explicit memory is one of the two main types of long-term human memory, the other of which is implicit memory. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and concepts. This type of memory is dependent upon three processes: acquisition, consolidation, and retrieval.
Semantic dementia (SD), also known as semantic variant primary progressive aphasia (svPPA), is a progressive neurodegenerative disorder characterized by loss of semantic memory in both the verbal and non-verbal domains. However, the most common presenting symptoms are in the verbal domain. Semantic dementia is a disorder of semantic memory that causes patients to lose the ability to match words or images to their meanings. However, it is fairly rare for patients with semantic dementia to develop category specific impairments, though there have been documented cases of it occurring. Typically, a more generalized semantic impairment results from dimmed semantic representations in the brain.
The Papez circuit, or medial limbic circuit, is a neural circuit for the control of emotional expression. In 1937, James Papez proposed that the circuit connecting the hypothalamus to the limbic lobe was the basis for emotional experiences. Paul D. MacLean reconceptualized Papez's proposal and coined the term limbic system. MacLean redefined the circuit as the "visceral brain" which consisted of the limbic lobe and its major connections in the forebrain – hypothalamus, amygdala, and septum. Over time, the concept of a forebrain circuit for the control of emotional expression has been modified to include the prefrontal cortex.
Transient global amnesia (TGA) is a neurological disorder whose key defining characteristic is a temporary but almost total disruption of short-term memory with a range of problems accessing older memories. A person in a state of TGA exhibits no other signs of impaired cognitive functioning but recalls only the last few moments of consciousness, as well as possibly a few deeply encoded facts of the individual's past, such as their childhood, family, or home perhaps.
Memory disorders are the result of damage to neuroanatomical structures that hinders the storage, retention and recollection of memories. Memory disorders can be progressive, including Alzheimer's disease, or they can be immediate including disorders resulting from head injury.
Ribot's law of retrograde amnesia was hypothesized in 1881 by Théodule Ribot. It states that there is a time gradient in retrograde amnesia, so that recent memories are more likely to be lost than the more remote memories. Not all patients with retrograde amnesia report the symptoms of Ribot's law.
Kent Cochrane, also known as Patient K.C., was a widely studied Canadian memory disorder patient who has been used as a case study in over 20 neuropsychology papers over the span of 25 years. In 1981, Cochrane was involved in a motorcycle accident that left him with severe anterograde amnesia, as well as temporally graded retrograde amnesia. Like other amnesic patients, Cochrane had his semantic memory intact, but lacked episodic memory with respect to his entire past. As a case study, Cochrane has been linked to the breakdown of the single-memory single-locus hypothesis regarding amnesia, which states that an individual memory is localized to a single location in the brain.
Transient epileptic amnesia (TEA) is a rare but probably underdiagnosed neurological condition which manifests as relatively brief and generally recurring episodes of amnesia caused by underlying temporal lobe epilepsy. Though descriptions of the condition are based on fewer than 100 cases published in the medical literature, and the largest single study to date included 50 people with TEA, TEA offers considerable theoretical significance as competing theories of human memory attempt to reconcile its implications.
Retrospective memory is the memory of people, words, and events encountered or experienced in the past. It includes all other types of memory including episodic, semantic and procedural. It can be either implicit or explicit. In contrast, prospective memory involves remembering something or remembering to do something after a delay, such as buying groceries on the way home from work. However, it is very closely linked to retrospective memory, since certain aspects of retrospective memory are required for prospective memory.
The neuroanatomy of memory encompasses a wide variety of anatomical structures in the brain.
Amnesia is a deficit in memory caused by brain damage or brain diseases, but it can also be temporarily caused by the use of various sedatives and hypnotic drugs. The memory can be either wholly or partially lost due to the extent of damage that was caused.
In psychology, confabulation is a memory error consisting of the production of fabricated, distorted, or misinterpreted memories about oneself or the world. It is generally associated with certain types of brain damage or a specific subset of dementias. While still an area of ongoing research, the basal forebrain is implicated in the phenomenon of confabulation. People who confabulate present with incorrect memories ranging from subtle inaccuracies to surreal fabrications, and may include confusion or distortion in the temporal framing of memories. In general, they are very confident about their recollections, even when challenged with contradictory evidence.
