Loss of memory is referred to as Amnesia. People suffering with amnesia, also called amnestic syndrome usually remember information themselves but have trouble learning new information and forming new memories. Amnesia can be caused due to damage in the area of the brain responsible for memory process and storage. Unlike temporary memory loss, amnesia can be permanent.
Features of amnesia
There are two main features of amnesia. They are, Anterograde amnesia which means following the onset of amnesia, people face difficulty in learning new information.
Retrograde amnesia means individuals face difficulty in remembering past events and familiar information.
There is another rare type of amnesia that is dissociative amnesia (psychogenic amnesia) which usually arises from emotional shock or trauma. In this case, the individual may lose personal memories and autobiographical information but only briefly.
Anterograde and retrograde amnesia as detailed in the image below
Figure 1: Features of amnesia.
Neuroanatomy
The process of memory are of three kinds. They are
Episodic memory
It refers to the learning and recollection of autobiographical information in one’s life.
Episodic memory is dependent on hippocampal formation and its connection.
Semantic memory
It is the acquired knowledge of meaning of verbal and perceptual concepts.
Semantic memory is dependent on the functioning of the middle of inferior temporal gyri of the neocortex.
Implicit memory
It refers to learning of sensory-motor skills.
Implicit memory is dependent on cortical, and subcortical motor systems.
“Work of Karl Lashley and Wilder Penfield in the year the 1950s and 1960s has made clear that memories are not stored in just one part of the brain but distributed throughout the cortex” Any disease or injury to the brain can affect memory.
Amnesia can occur from damage to brain structures that form limbic systems, that control emotions and memories. The amnesia that is caused due to brain injury or damage is called neurological amnesia. The causes of neurological amnesia are stroke, inflammation of brain tissue (encephalitis), tumors, seizures, and degenerative diseases such as Alzheimer’s disease.
The parts of the brain involved with memory are amygdala, hippocampus, cerebellum, and prefrontal cortex is depicted in the diagram below.
Figure 2: Parts of the brain involved with memory
Diagnosis
Several tests can be used to diagnose amnesia, they are
Cognitive tests to determine extent of memory loss from memory evaluation tests.
Imaging tests such as CT, and MRI to look for abnormalities or brain damage.
Blood tests to check for infections and nutritional deficiencies.
EEG to check for seizures.
Brain imaging is found to be an important diagnostic tool in linking descriptions of brain and cognition. Brain imaging investigation has led to a comprehensive understanding of the structure and function.
A special type of MRI called functional MRI produces image of blood flow to the areas of the brain, it images the brain’s anatomy and determines which part of the brain are handling critical functions.
In the field of psychology and neurology, the contribution of brain imaging techniques plays a vital role from localizing the damage or lesion to witnessing the recovery after treatment.
Memory-related hippocampal activation in sleeping disorder using fMRI
Over the years, one of the interesting questions every one of us have is “how young children gain the capacity to remember their past events?”
In the study conducted by Janani Prabhakar et al (2018 pp 6500-6505) said that the early hippocampus process have been involved in this ability of children to remember their past, but due to lack of methods, it has inhibited the assessments in early developments. In this study, Janani Prabhakar et al employed the fMRI paradigm that captured memory-related hippocampal function during natural sleep in toddlers at night.
With MRI, scientists faced difficulty in assessing the population that cannot complete demands of behavioral tasks therefore, Janani Prabhakar, etc developed an fMRI paradigm that overcomes these challenges and allowed them to examine memory-related hippocampal function in toddlers. Assessment of hippocampal function and its contribution to early episodic memory in infants have been difficult without fMRI.
An fMRI Investigation on three multiple kinds of episodic memory
Episodic memory is different from other forms of memory systems which allows humans to remember past experiences. Hung-Yu Chen et al (2017) conducted a study using fMRI as the tool, where the participants studied set of scenes and two types of memory tests were such as, picture memory test and life memory test were performed while undergoing fMRI.
In picture memory test, participants were asked to report for each scene if it is recollected from prior study episode.
In life memory test, participants were asked to report each scene if it reminded them of a specific event from their pre-experimental lifetime.
In behavioral assessment during picture memory test and life memory test, the performance in picture memory test was accurate.
On considering the fMRI result, it showed contrast of old scenes leading to successful retrieval in two types of tests revealed differential activity in numerous regions in the brain.
fMRI result acquired from life memory test and picture memory test is detailed in the image below.
Figure 3: Differential activation for successful recollection in life memory test and picture memory test.
The BOLD signal obtained by successful retrieval of previously studied scenes in picture memory test was contrasted with successful retrieval by previously studied scenes in a life memory test.
From f-MRI, it was known that the regions active during autographical retrieval include bilateral hippocampus, left amygdala, bilateral superior frontal gyrus, angular gyrus, medial prefrontal cortex, and bilateral retrosplenial complex. The regions that were more activated during the recognition process include right middle frontal gyrus, bilateral insula, right inferior parietal cortex, precuneus and midcingulate cortex.
By detecting changes in the blood flow, fMRI measures brain activity. In this study, t not only showed contrast in episodic memory but also examined the brain’s functional anatomy helped us understand about different areas of the brain that were activated during particular process of memory.
Finding the self: an event-related fMRI study
On considering memory function, knowledge about one's self is remembered better than other types of semantic information. In this study W.M.Kelley et al used event-related functional magnetic resonance imaging indexed by BOLD contrast to investigate potential neural substrates of self-referential processing.
In this study, participants were imaged while making judgments about trait adjectives under three experimental conditions. They are,
Self-relevance.
Other relevance.
Case judgment.
From functional resonance imaging data, relevance judgments and case judgments were found to have activation of left inferior frontal cortex and anterior cingulate whereas, the self-referential process was found to have activation of medial prefrontal cortex. From this observation, author describes that “self-referential processing is functionally dissociable from other forms of semantic processing within the human brain.”
Commonly activated regions across all the three trial types were occipital lobes, parietal lobes, motor cortex, thalamus, and cerebellum.
Additionally, f-MRI results showed that regions of striate and extrastriate visual cortex, parietal cortex, dorsal frontal cortex, motor cortex, and cerebellum were commonly activated, other than these regions, activations were also observed in the medial anterior cingulate gyrus, left thalamus and left caudate nucleus.
On observing activations in brain regions during each trial, for certain processes, some regions of the brain exhibited increased activation while other regions exhibited significantly decreased activation.
In this study, functional magnetic resonance imaging not only shows activations in the brain areas during particular process but also helps us to compare the results of the areas that weren’t activated for particular process whereas to the same regions that showed higher activations during other processes. It also helps in identifying increase and decrease of activation in the regions of the brain.
Memory in frontal lobe epilepsy: an fMRI study
Focal epilepsies are associated with structural and functional changes and usually spread to the areas beyond the seizure onset. In this study, Maria Centeno et al (2012) investigated functional anatomy of memory in patients with frontal lobe epilepsy (FLE) using the fMRI memory encoding paradigm.
fMRI results show that patients with frontal lobe epilepsy with normal memory showed increased activation in middle and bilateral inferior frontal gyrus compared to control group and frontal lobe epilepsy patients with impaired memory. Patients with impaired performance had decreased activation in amygdala and hippocampal activation compared to controls and patients with frontal lobe epilepsy with normal recognition scores.
fMRI did not show any differences in frontal activations between controls and patients with memory impairment. f-MRI data provides evidence of activation of both frontal and medial temporal lobe areas in impairment of memory function in patients with frontal lobe epilepsy.
In normal recognition memory, there was increased recruitment of frontal areas, contralateral to epileptic foci. Poor performance was associated with decreased activation in mesial temporal areas.
Functional correlates of different performances is detailed in the image below.
Figure 4: (A, B) Patients with FLE with normal memory showed increased frontal activation when compared to controls and to patients with memory impairment. (C,D) Patients with FLE with impaired memory showed decreased amygdala and hippocampal activation when compared to controls and to patients with normal memory.
Thalamus abnormalities during working memory in schizophrenia: an fMRI study
Julie Bor et al (2011) aimed to identify and compare cerebral activation in schizophrenia patients and controls during working memory tasks. Owen et al (2005) stated that “neuroimaging studies in humans have consistently found robust activation of frontal, parietal and temporal regions during the working memory test.”
Bold fMRI responses obtained were as follows
For the control group, activation was found for verbal working memory and spatial working memory.
It encompassed bilateral posterior parietal cortex including precuneus and inferior parietal lobules, bilateral premotor cortex, bilateral prefrontal cortex, thalamus, and cerebellum.
Patients with verbal working memory showed a significant cluster of increased activation in the thalamus and basal ganglia (caudate nucleus, putamen nucleus, and globus pallidus) compared to controls.
For spatial working memory compared with verbal working memory reported same activation extended to cerebellum, other activations were found in right superior, middle frontal gyrus, and left middle frontal gyrus.
Figure 5: Increased BOLD activation in patients compared to controls for verbal working memory
Neural correlates working memory in children and adolescents with agenesis of the corpus callosum: an fMRI study
V.Siffredi et al aimed to investigate the functional organization of working memory in children with agenesis of corpus callosum using fMRI
They observed that during encoding process, there was activation in bilateral frontal areas (anterior cingulate, ventrolateral, and precentral areas)
During retrieval, compared to encoding activations were found in bilateral frontal and parietal-temporal regions.
An amazing fact is that research using functional magnetic resonance imaging suggests that verbs and nouns are stored in different ways in the brain.
References
Hung-Yu Chen; Adrian W. Gilmore; Steven M. Nelson; Kathleen B. McDermott (2017), Are There Multiple Kinds of Episodic Memory? An f-MRI Investigation Comparing Autobiographical and Recognition Memory Tasks. Neurosci 37(10), 2764-2775, doi:10.1523/JNEUROSCL1534-16.2017
Janani Prabhakar, Elliott G. Johnson, Christine Wu Nordahl, Simona Ghetti (2018), Memory-related hippocampal activation in the sleeping toddler. Psychological and Cognitive Sciences 115(25), 6500-6505, doi: 10.1073/pnas.1805572115
Kelley, W. M; Macrae, C. N; Wyland, C, L; Caglar, S; Inati, S; Heatherton, T. F (2002), Finding the self? An event-related f-MRI study. Journal of Cognitive Neuroscience 14(5), 785-794, doi: 10.1162/08989290260138672
Centeno, Maria; Vollmar, Christian; O’ Muirchearyaigh, Jonathan; Stretton, Jason; Symms, Mark R.; Barker, Gareth J.; Kumari, Veena; Thompson, Pamels J.; Duncan, John S.; Richardson, Marl P.; Koepp, Matthias J. (2012), Memory in frontal lobe epilepsy: An f-MRI study. Epilepsia 53(10), 1756-1764, doi: 10.1111/j.1528-1167.2012.03570.x
Dominique Sappey-Marinier; Danielle I barrola; Thierryd’ Amato; Marie-FrancoiseSuaud-Chagny; Mohamed Saoud (2011), Thalamus abnormalities during working memory in schizophrenia An f-MRI study. Schizophrenia research 124 (1), 49-53, doi: 10.1016/j.schres.2010.10.018
M.M.Spencer-Smith; P.Barrouillet; M.J.Vaessen; R.J.Leventer; V.Anderson; P.Vuilleumier (2017), Neural correlates of working memory in children and adolescents with agenesis of the corpus callosum: An f-MRI study, Neuropsychologia 107, 71-82, doi:10.1016/j.newuropsychologia.2017.09.008