Episodic and Semantic memory

Long-term memory is the last stage of the dual memory model proposed by Atkinson & Shiffrin. While short-term and working memory can hold information for about 0-30 seconds, information stored in the LTM can remain there indefinitely. Long-term memory is commonly labelled as explicit memory (declarative) as well as episodic memory, semantic memory, autobiographical memory, and implicit memory (procedural memory. 

Explicit LTM - conscious knowledge. Implicit LTM - knowledge that can influence thought and behvaiour without any necessary involvement of conscious awareness.

Episodic memory

• An LTM store that contains information about experiences, events, and occurrences that make up a person's life, and can be explicitly stated or conjured. For example, if one remembers the party they had on their 10th birthday, that's an episodic memory
• The essence of episodic memory is its capacity to represent a specific event and locate it in time and space
• Often tied to spatial or temporal events
• Episodic memories allow an individual to figuratively travel back in time

Episodic memory was coined by Endel Tulving in 1972. He was referring to the distinction between knowing and remembering. Knowing is more factual (semantic) whereas remembering is a feeling that is located in the past (episodic). Tulving defined 3 key properties of episodic memory recollection. These are a subjective sense of time, connection the self, and autonoetic consciousness. Autonoetic consciousness refers to a special kind of consciousness that accompanies the act of remembering which enables an individual to be aware of the self in a subjective time.

Semantic memory is one of the two types of declarative or explicit memory (memory of facts or events that is explicitly stored and retrieved). Semantic memory refers to general world knowledge that we've accumulated through our lives. This general knowledge is intertwined in experience and dependent on culture. Semantic memory is distinct from episodic memory, which is our memory of experiences and specific events that occur during our lives, from which we can recreate at any given time.

Network models: networks of various sorts play an integral part in many theories of semantic memory. Networks are composed of sets of nodes connected by links. The nodes may represent concepts, words, perceptual features, or nothing at all. The links may be weighted such that some are stronger than others.

Feature models: feature models view semantic categories as being composed of relatively unstructured sets of features. The semantic feature-comparison model proposed by Smith, Shoben, and Rips (1974) describes memory as being composed of feature lists for different concepts.

Hierarchical organisation: information organised into hierarchy of categories

Semantic dementia is a progressive neurodegenerative disorder that's characterised by the loss of semantic memory in both the verbal and non-verbal domains. The most common presenting symptoms are in the verbal domain however, and it is characterised as a primary progressive aphasia (neurological syndrome in which language capabilities slowly and progressively become impaired).

Semantic dementia patients show symptoms of surface dyslexia, a relatively selective impairment in reading low-frequency words with exceptional or atypical spelling-to-sound correspondences.

Semantic dementia is one of the three canonical clinical syndromes associated with frontotemporal lobar degenration. It is associated predominately with temporal lobe atrophy (let greater than right).

There is an agreement that semantic memory serves a different function and operates in a different way from episodic memory. However, there is a disagreement as to whether the two depend on fundamentally different learning and memory systems.

Information is registers in both systems the same way

• "There is no known method of readly encoding information into an adult's semantic memory without putting corresponding information in episodic memory or vice versa" - Wheeler et al, 1997

Baddeley (2002) speculates that semantic memories result from an accumulation of similar episodic memories that become knowledge when we're no longer able to retrieve individual learning episodes

Most patients with anterograde amnesia have problems with declarative memory as a whole (semantic and episodic memory)

Episodic / semantic distinction is intuitively plausible:

• They deal with different types of information
• There is something phenomenonologically different about them, e.g remembering last weekend vs. remembering the capital of France.

Tulving argues that episodic and semantic memory form separate memory systems - "Episodic memory shares many features with semantic memory out of which it grew.. but it also possesses features that semantic memory does not".

Amnesia patients

• Often have impaired episodic memory but intact semantic memory - Patient KC could slowly acquire new semantic memories but not episodic memories. KC couldn't remember the learning context and had no autonoetic consciousness
• Similarly, semantic dementia patients often have intact episodic memory but impaired semantic memory