Posts Tagged ‘context memory’

Brain Regions Associated with Long-Term Memory

September 11, 2019

This post is based on an important book by Scott D. Slotnick titled “Cognitive Neuroscience of Memory.” Remember to consult the website
to see the anatomical information referred to in this post.

Dr. Slotnick writes, The term episodic memory can refer to many other related forms of memory including context memory, source memory, “remembering,” recollection, and autobiographical memory, which refers to a specific type of episodic memory for detailed personal events. As the names imply context memory and source memory refer to the context in which something occurred and source memory refers to where the event occurred.

Episodic memories are related to activity in both control regions and sensory regions of the brain. Sensory cortical activity reflects the contents of memory. The control regions that mediate episodic memory include the medial temporal lobe, the dorsolateral prefrontal cortex, and the parietal cortex. There are many regions associated with episodic memory but the primary regions are the medial temporal lobe, the dorsolateral prefrontal cortex, and the parietal cortex. The parahippocampal cortex processes the context of previously presented information such as the location or the color.

The hippocampus binds item information and context information to create a detailed episodic memory. Dr. Slotnick provides the following example. “If an individual went on a vacation to Newport Beach in California and later recalled meeting a friend on the beach, that individual’s perirhinal cortex would process item information (the friend), the parahippocampal cortex would process context information (the area of the beach on which they were standing), and the hippocampus would bind this information and context information into unified memory.”

Semantic memory refers to knowledge of facts that are learned through repeated exposure over a long period of time. These facts are processed and organized in semantic memory, which provides the basis for much thought. Subjectively, semantic memory is associated with “knowing.” Semantic memory includes definitions and conceptual knowledge, and this cognitive process is linked to the field of language.

Semantic memory has been associated with the left dorsolateral prefrontal cortex (in a different region associated with episodic memory), the anterior temporal lobes, and sensory cortical regions. The left dorsolateral prefrontal cortex may reflect the processing of selecting a semantic memory that is stored in other cortical memories. For example, naming animals activates more lateral inferior occipital-temporal cortex that has been associated with the perception of living things, while naming tools activates more medial inferior occipital-cortex that has been associated with perception of nonliving things.

In a study of Alzheimer’s patients, the impairment in an object naming task, which depends on intact semantic memory, was more highly correlated with cortical thinning in the left anterior temporal lobe. This finding suggests that the left anterior temporal lobe is necessary for semantic memory.

During long-term memory the hippocampus binds information between different cortical regions. But long-term memory may only depend on the hippocampus for a limited time. In the standard model of memory consolidation, a long-term memory representation changes from being based on hippocampal-cortical interactions to being based on cortical-cortical interaction, which takes a period of somewhere between 1 to 10 years. A person with hippocampal damage due to a temporary lack of oxygen might have impaired long-term memory for approximately 1 year before the time of damage from retrograde amnesia and have intact long-term memories for earlier events. This suggests that the hippocampus is involved in long-term memory retrieval for approximately 1year as more remote long-term memories no longer demand on the hippocampus so they are not disrupted.

The activity in the hippocampus did not drop to zero for older semantic memories but was well above baseline for events that were 30 years old. This indicates that the hippocampus was involved in memory retrieval for this entire period. If the hippocampus was no longer involved, the magnitude of activity in this regions would have dropped to zero for remote memories.

There is a growing body of evidence that the hippocampus is involved in long-term memories throughout the lifetime. As such, the process of consolidation does not appear to result in the complete transfer from hippocampus-cortical memory representation to cortical-cortical memory representations.