Neuro-functional pathways and Connection with Diseases
Types of Memory
Traditionally, there are three types of memory defined: short-term memory (STM), long-term memory (LTM) and working memory (WM). Hebb defines the short-term as the mechanism of continuous reverberation of stimuli and information in cellar systems and as a result neurons excite one another for some time after the original stimulus (Nadel and Hardt, 2010, 253). In other words, STM is an ability to hold information about the stimuli for the brief period. If this reverberatory activity leads to restructuring of synaptic connections, the long-term memory appears. LTM is an ability of human brain to storage the information for a continuous period. In humans, the STM lasts until the rehearsal; otherwise the duration of capturing the information won’t exceed 18-20 seconds. Working memory, which is very similar to STM, is emphasizing not the duration of capturing the information, but the state of using it during some actions. Thus working memory is a short-term memory used for the purposes of specific activity which induces activation in “prefrontal cortex, parietal cortex, thalamus, medial–temporal regions, and cerebellum” (Nadel and Hardt, 2010, 253).
Cellular and neuropsychological mechanisms of memory
Tolman discovered that all the information about the external world undergoes internalization by the use of special mental maps. Nadel has found that these mental maps are in the hippocampus, but recent studies have shown that even more cortical regions and correspondent cellular formations participate in the process of storing information. Among these systems are “adjacent entorhinal cortex, pre- and parasubiculum, retrosplenial cortex, parietal cortex, frontal cortex and other areas” (Derdikman and Moser, 2010, 561).
Studies of rats conducted by Derdikman and Moser have shown that place cells in the hippocampus are responsible for creating cognitive maps of the environment – memorizing the space. Nevertheless, these studies have also shown that this cortical region is also responsible for encoding stimuli of different modalities if the task requires. Thus, hippocampus and place cells are responsible for episodic memory (Derdikman and Moser, 2010, 562). Grid cells of the entorhinal cortex are also responsible for space orientation and are working like a “map of rat’s position” (Derdikman and Moser, 2010, 563). The rapid change of different spatial maps helps the navigation in the space by the use of cellar memory and restructuring of it.
The Role of Long-term Potentiation and Depression in Memory Formation
The long-term potentiation is a flexibility of synaptic connections which undergoes positive changes in information transmission due to activities. The long-term depression is a complementary process which is the reduction of mnemonic activity due to deactivation of cellar transmission (Bliss & Cooke, 2011, 3). According to researchers, the processes similar to LTP and LTD have a crucial role in memory formation. The reason for this suggestion is that LTP is highly demonstrable in hippocampus activity and inhibitors of hippocampus activity also cause LTD. Moreover, the biochemical changes which occur after induction of LTP and similar to those which take place in the brain during the memory process. Nonetheless, the question of LTP and LTD role in memory formation is still controversial and requires further studies (Lynch, 2004, 87).
The Consolidation and Reconsolidation processes. Facilitation of memory during sleeping
According to the traditional understanding of memory process, it is a way from receiving information during some events, encoding it and consolidating it into fixed memory. The consolidation of memory is strengthening of recollecting of the information learned. According to Nadel et al., the process of memory is nonlinear. It means that there might be some changes in the consolidated memory which is fragile. New stimuli change the structure of fixed memory by making it labile and influencing it by novel information, protein injections, etc. Thus, reconsolidation is the change of the unstable memory connections and restructuring of them due to the new information of according to the reaction on certain stimuli.
It is a recent finding that during night relief the connections between information learned restructures in the way we can better recollect it. Nevertheless, there was no reasonable evidence for this conclusion. The rat studies conducted by Girardeau et al. (2006, 1122-1123) have shown a connection between the facilitation of sharp wave–ripples (SPW-R) and long-term memory storage. To underpin this fact, scientists suppressed SPW-Rs in the hippocampus during the post-learning sleep, which resulted in deterioration of memory consolidation. The reconsolidations of information learned mean that SPW-Rs are crucial for the process of consolidation as well. Nonetheless, the partial ability to recollect the information learned, even after suppressions, is the result of other cortical zones participation in the consolidation process.
Amnesia and Memory Recovering
Amnesia is a lost or disturbances of memory due to functional of psychogenic changes. The damages of storing, encoding and retrieval of information cause the inability to create new memories of to recover old ones (Nadel and Hardt, 2010, 252).
Retrograde amnesia is the loss of access to memories formed before events causing brain injury. There are two possible reasons for this process. The first is that memories which a person had during the traumatic experience are present in the brain but are not accessible. According to the second position, these memories were not consolidated simultaneously with the action and that is why they are lost forever. The anterograde amnesia is an inability to produce new memories after the traumatic event which caused amnesia. According to the studies, the reason for the development of both types of amnesia is damage of the medial temporal lobe and hippocampal formation (Nadel and Hardt, 2010, 252).
In addition, the episodic recent memories are more vulnerable to amnesic process that earlier information. Nonetheless, during retrograde amnesia caused by the hippocampal damage, “even remote event memories cannot be recalled in their entirety” (Nadel and Hardt, 2010, 256). Nevertheless, the studies have shown that the exposure to the amnesic agent which caused retrograde amnesia helps to recover and recollect the precious information (Miller & Matzel, 2006, 493).
References
Bliss, T., & Cooke, S. (2011). Long-term potentiation and long-term depression: A clinical perspective. Clinics, 66(1), 3-17.
Derdikman, D., & Moser, E. (2010). A Manifold of Spatial Maps in the Brain. Space, Time and Number in the Brain, 14(12), 41-57.
Girardeau, G., Benchenane, K., Wiener, S., Buzsáki, G., & Zugaro, M. (2009). Selective suppression of hippocampal ripples impairs spatial memory. Nature Neuroscience Nat Neurosci, 12(10), 1222-1223.
Lynch, M. (2004). Long-Term Potentiation and Memory. Physiological Reviews, 84(1), 87-136.
Miller, R., & Matzel, L. (2006). Retrieval failure versus memory loss in experimental amnesia: Definitions and processes. Learning & Memory, 13, 491-497.
