Episodic vs. Semantic Memory

Dissociation between Episodic and Semantic Memory in Amnesics

If episodic Opens in new window and semantic memory Opens in new window form separate memory systems, there should be several important differences between them.

Some of the strongest support for the separate-systems viewpoint comes from studies of brain-damaged patients (Greenberg & Verfaellie, 2010).

Much research has focused on the ability of amnesic patients to acquire episodic and semantic memories after the onset of amnesia Opens in new window. Thus, what was of interest was the extent of anterograde amnesia Opens in new window.

Spiers et al. (2001) reviewed 147 cases of amnesia involving damage to the hippocampus Opens in new window or fornix. Episodic memory was impaired in all cases, whereas many patients had significantly less severe problems with semantic memory.

The finding that the impact of hippocampal brain damage was much greater on episodic than on semantic memory suggests the two types of memory are distinctly different.

Note that the memory problems of amnesic patients are limited to long-term memory Opens in new window. According to Spiers et al. (2001, p. 359), “None of the cases was reported to have impaired short-term memory (typically tested using digit span — the immediate recall of verbally presented digits).”

We would have stronger evidence if we discovered amnesic patients with very poor episodic memory Opens in new window but essentially intact semantic memory Opens in new window.

Such evidence was apparently obtained by Vargha-Khadem et al. (1997). Two of the patients they studied had suffered bilateral hippocampal damage at an early age before developing semantic memory.

Beth suffered brain damage at birth and Jon at age 4. Both patients (Beth and Jon) had very poor episodic memory for the day’s activities, television programmes and telephone conversations.

In spite of this, Beth and Jon both attended ordinary schools. Their semantic memory based on levels of speech and language development Opens in new window, literary and factual knowledge (e.g., vocabulary) were within the normal range. However, healthy individuals with an intact hippocampus depend on it for semantic memory acquisition (Baddeley et al., 2015).

Subsequent research indicated Jon had some problems with semantic memory (Gardiner et al., 2008). When provided with various facts concerning geographical, historical and other kinds of knowledge, Jon’s rate of learning was significantly slower than that of healthy controls.

Why did Jon show a good ability to acquire semantic information in semantic memory in his everyday life?

Gardner et al. (2008) suggested this occurred because Jon devoted much time to repeated study of such information. Overall, Jon’s semantic memory was somewhat impaired but his episodic memory was grossly deficient. How can we explain this difference?

Vargha-Khadem et al. (1997) argued that episodic memory Opens in new window depends on the hippocampus Opens in new window whereas semantic memory Opens in new window depends on the underlying entorhinal, perirhinal and parahippocampal cortices. The brain damage suffered by Beth and Jon centered on the hippocampus.

Why do so many amnesics have great problems with both episodic and semantic memory?

The answer may be that they have damage to the hippocampus and the underlying cortices. This is very likely given the two areas are adjacent.

Some support for the above viewpoint was reported by Bindschaedler et al. (2011). They reported the case of VJ, a boy with severe atrophy of the hippocampus but relatively preserved surroundings areas such as perirhinal and entorhinal cortex.

VJ’s performance on semantic memory tasks (e.g., vocabulary) increased over time at the same rate as that of healthy controls even though he showed severe impairment on 82% of episodic memory tasks.

Retrograde Amnesia

So far we have focused on the ability of amnesic patients to acquire new episodic and semantic memories after the onset of amnesia.

What about amnesic patients’ retrograde amnesia (poor recall for memories formed prior to the onset of amnesia)?

Many amnesic patients show much greater retrograde amnesia for episodic than semantic memories. Consider KC. According to Tulving (2002, p. 13), “He cannot recollect any personally experienced events …, whereas his semantic knowledge acquired before the critical accident is still reasonably intact … his general knowledge of the world is not greatly different from others’ at his educational level.”

Retrograde amnesia for episodic memories in amnesic patients often spans several years. There is typically a temporal gradient with older memories showing less impairment than older ones (Bayley et al., 2006).

In contrast, retrograde amnesia for semantic memories is generally small except for knowledge acquired shortly before the onset of amnesia (Manns et al., 2003).

Several theories have been put forward to explain retrograde amnesia Opens in new window (Kopelman & Bright, 2012).

According to consolidation theory Opens in new window, there is a long-lasting physiological consolidation of episodic memories Opens in new window in the hippocampus Opens in new window.

After a period of several years, these memories are stored elsewhere, which protects them from the effects of hippocampal damage. This explains the temporal gradient, but the notion that consolidation lasts for several years is implausible.

An alternative theory is that episodic memories become more like semantic memories Opens in new window over time, thus protecting them from the effects of brain damage. According to this theory, remote semantic memories (e.g., for vocabulary) formed years before the onset of amnesia should be relatively intact but are often forgotten.

Kopelman and Bright (2012, p. 2969) concluded that, “The problem is that the confusing and contradictory empirical data do not really support any of the main theories.”

There is a final point. Episodic memories typically depend on a single learning experience, whereas most semantic memories (e.g., meaning of specific words) depend on several learning experiences.

This reduced learning opportunity for episodic memories compared to semantic ones may explain at least in part greater retrograde amnesia for the former.

Semantic Dementia

Are there brain-damaged patients having severe problems with semantic memory but with relatively intact episodic memory? The short answer is “Yes”.

Patients with semantic dementia Opens in new window have severe loss of concept knowledge from semantic memory even though their episodic memory and most cognitive functions are reasonably intact.

Note, however, that patients with semantic dementia differ in terms of their precise symptoms.

Semantic dementia always involves degeneration of the anterior temporal lobe, so that area is of great importance to semantic memory.

Areas such as perirhinal and entorhinal cortex are probably involved in the formation of semantic memories, whereas the anterior temporal lobes are where such memories are stored on a semi-permanent basis.

Patients with semantic dementia find it very difficult to access information about most concepts stored in semantic memory (Mayberry et al., 2011). However, their performance on several episodic memory tasks is good.

For example, Adlam et al. (2009) asked patients with semantic dementia to remember what tasks they had performed 24 hours earlier, where those tasks were performed and when during the session they occurred. The patients as a group performed comparably to healthy controls.

In sum, the evidence points to a double dissociation. Amnesic patients have very poor episodic memory but often have fairly intact semantic memory. In contrast, patients with semantic dementia have very poor semantic memory but their episodic memory is reasonably intact.