Figure

Auditory word identification for previously presented words in amnesics and controls. (a) All words originally presented in the same voice; data from Schacter and Church (1995). (b) Words originally presented in six different voices; data from Schacter et al. (1995).

The greatest problem with the theory is that amnesic patients sometimes fail to show intact performance on tests of implicit memory. For example, consider a study on perceptual priming by Schacter, Church, and Bolton (1995). It resembled the study by Schacter and Church (1995), in that perceptual priming based on auditory word identification was investigated. However, it differed in that the words were initially presented in six different voices. On the word-identification test, half the words were presented in the same voice and half were spoken by one of the other voices (re-paired condition). The normal controls showed more priming for words presented in the same voice, but the amnesic patients did not, as shown in Figure 7.9 (b).

How can we explain these findings? In both the same voice and re-paired voice conditions, the participants were exposed to words and voices they had heard before. The only advantage in the same voice condition was the fact that the pairing of word and voice was the same as before. However, only those participants who had linked or associated words and voices at the original presentation would benefit from that fact. As Curran and Schacter (1997, p. 41) concluded, "Amnesics may lack the necessary ability to bind voices with specific studied words." This view of the major deficit in amnesia is discussed more fully later.

Data-driven and conceptually driven processes

Roediger (1990) emphasised the distinction between data-driven and conceptually driven processes. He claimed that implicit memory tasks usually depend on data-driven processes, whereas explicit memory tasks generally depend on conceptually driven processes. From this perspective, it could be argued that the reason why amnesic patients typically perform well on implicit memory tasks but poorly on explicit memory tasks is because they have fairly intact data-driven processes but impaired conceptually driven processes.

A key prediction from Roediger's approach is that the memory performance of amnesics depends more on whether data-driven or conceptually driven processes are used than on whether explicit or implicit memory is involved. Amnesics should perform especially well relative to normals when data-driven processes are required at learning and at test, but should perform particularly poorly when conceptually driven processes are needed at learning and test.

Recent evidence has mostly favoured the view that amnesics have impaired explicit memory over Roediger's assumption that the key problem is in conceptually driven processes. For example, Vaidya et al. (1995) made use of four retrieval conditions:

1. Perceptual cues (word fragments); explicit test.

2. Perceptual cues (word fragments); implicit test.

3. Conceptual cues (word associates); explicit test.

4. Conceptual cues (word associates); implicit test.

According to Roediger's theory, the amnesic patients should have been impaired mainly on the two conceptual tests (3 and 4). In fact, the amnesic patients showed impaired performance on the two explicit tests (1 and 3), and intact performance on the two implicit tests (2 and 4). Thus, amnesics' performance was predicted much better by the distinction between explicit and implicit memory than by that between data-driven or perceptual processing and conceptual processing. Similar findings were reported by Cermak, Verfaellie, and Chase (1995). As a result, it seems that Roediger's theory does not provide an adequate account of long-term memory in amnesic patients.

Evaluation

Roediger's approach has served the valuable function of focusing on some of the key processes involved in learning and memory. Some evidence (e.g., Blaxton, 1992) supports the application of transfer appropriate processing theory to amnesia, and amnesic patients generally have impaired conceptual rather than perceptual processing. However, there is increasing evidence that impaired long-term memory functioning in amnesics depends more on explicit memory than on conceptual processing.

Declarative versus procedural knowledge

One of the most influential theoretical approaches to amnesia is based on the notion that there are two or more long-term memory systems. According to Masson and Graf (1993, p. 6), "a memory system is a collection of correlated functions that are served by anatomically distinct brain structures." Cohen and Squire (1980) proposed a memory systems account based on the distinction between declarative knowledge and procedural knowledge. This distinction is closely related to that made by Ryle (1949) between knowing that and knowing how. Declarative knowledge corresponds to knowing that, and covers both episodic and semantic memory. Thus, for example, we know that we had porridge for breakfast this morning, and we know that Paris is the capital of France. Procedural knowledge corresponds to knowing how, and refers to the ability to perform skilled actions (e.g., how to ride a bicycle; how to play the piano) without the involvement of conscious recollection. Thus, declarative memory corresponds fairly closely to explicit memory and procedural memory to implicit memory.

Cohen (1984, p. 96) provided more formal definitions of declarative and procedural knowledge. Procedural knowledge is involved when "experience serves to influence the organisation of processes that guide performance without access to the knowledge that underlies the performance." Declarative knowledge is represented "in a system.in which information is.first processed or encoded, then stored in some explicitly accessible form for later use, and then ultimately retrieved upon demand."

Damage to memory systems

According to Cohen (1984), amnesics have severe impairment of the memory system involved in declarative memory, but they have a relatively intact procedural learning system. In support of this position, we have seen that amnesics cannot readily form new episodic or semantic memories, and declarative knowledge consists (by definition) of episodic and semantic memories. Amnesics acquire many motor skills as rapidly as normals, which is in line with the contention that their procedural learning skills are unimpaired.

Theorists focusing on the distinction between declarative and procedural knowledge have tried to identify the brain structures involved. As we saw earlier in the chapter, amnesia can be produced by damage to various brain structures. Chronic alcoholics who develop Korsakoff's syndrome have damage to the diencephalon, and often also to the frontal lobes. The two principal structures of the diencephalon are the hypothalamus and the thalamus, and the dorsomedial thalamic nucleus seems to be of particular importance in amnesia (see Figure 7.5).

Amnesia following herpes simplex encephalitis or surgery to reduce the incidence of epileptic seizures is caused by damage to the medial temporal lobes, and within them the hippocampus has been especially implicated in memory function (Parkin & Leng, 1993). It should be noted that the diencephalon and the medial temporal lobes are nearby structures within the limbic system.

Squire, Knowlton, and Musen (1993) argued that the major brain structures underlying declarative or explicit memory are located in the hippo-campus and anatomically related structures in the medial temporal lobes and the diencephalon, with the neocortex being the final repository of declarative memory. McKee and Squire (1992) found that amnesics with medial temporal lobe lesions showed similar forgetting rates to amnesics with diencephalic lesions at retention intervals of between 10 minutes and one day. These findings led Squire et al. (1993) to argue that the diencephalon and medial lobe structures are of comparable importance to declarative or explicit memory.

Some researchers have used PET scans to study the brain structures involved in declarative or explicit memory. Squire et al. (1992) found that blood flow in the right hippocampus was much higher when participants were performing a declarative memory task (cued recall) than a procedural memory task (word-stem completion). This supports the view that the hippocampus plays an important role in declarative memory. Similar findings were reported by Schacter et al. (1996).

The frontal lobes are also generally damaged in Korsakoff patients, and so it is important to consider their role in declarative memory. Episodic memory seems to depend on the frontal lobes as well as on the diencephalon (Wheeler et al., 1997). An important aspect of episodic memory is temporal discrimination, i.e., remembering when events or episodes occurred. Shimamura, Janowsky, and Squire (1990) found that frontal lobe patients were poor at reconstructing the order in which words in a list had been presented, in spite of having normal recognition memory for those words. However, many patients without frontal lobe damage show poor temporal discrimination.

It is harder to identify the brain structures underlying procedural or implicit memory, because implicit memory consists of several unrelated skills and processes. However, as was discussed earlier, much progress has been made. Sensori-motor skill learning seems to depend on the basal ganglia and the cerebellum, and perceptual skill learning involves the right parietal cortex and the left inferior occipito-temporal cortex. The parts of the brain involved in perceptual priming probably depend on the sense modality involved (e.g., visual; auditory). With visual perceptual priming tasks, bilateral occipito-temporal areas seem to be involved. In contrast, conceptual priming involves left frontal neocortex.

Why are humans equipped with separate brain systems underlying declarative or explicit memory and procedural or implicit memory? Squire et al. (1993, pp. 485-486) argued that each major brain system has its own particular function:

One system involves limbic/diencephalic structures, which in concert with neocortex provides the basis for conscious recollections. This system is fast, phylogenetically recent, and specialised for one-trial learning... The system is fallible in the sense that it is sensitive to interference and prone to retrieval failure. It is also precious.giving rise to the capacity for personal autobiography and the possibility of cultural evolution.

Other kinds of memory have also been identified. Such memories can be acquired, stored, and retrieved without the participation of the limbic/diencephalon brain system. These forms of memory are phylogenetically early, they are reliable and consistent, and they provide for myriad, non-conscious ways of responding to the world .they create much of the mystery of human experience.

Synthesis

One of the major developments in theories of amnesia in recent years has been a growing consensus on the key features of amnesia. Similar views have been expressed by Baddeley (1997), Curran and Schacter (1997), and Cohen, Poldrack, and Eichenbaum (1997), as is apparent from the following quotations:

• "What appears to be lost [in amnesia] is. the record of new links formed in the process of episodic learning.. .all conscious links between new experiences are hard to form for amnesics" (Baddeley, 1997, p. 306).

• "The medial temporal lobe [often damaged in amnesia] is critically involved with binding or integrating information that may be stored in separate critical modules" (Curran & Schacter, 1997, p. 42).

• "The functional deficit in amnesia is the selective disruption of declarative memory, i.e., of a fundamentally relational representation supporting memory for the relationships among perceptually distinct objects that constitute the outcomes of processing of events" (Cohen et al., 1997).

What is common to these positions is the notion that amnesics find it hard to store integrated or linked information in long-term memory. As Cohen et al. (1997) have the most developed theory (representing a modified form of the theory put forward by Cohen and Squire, 1980), we will focus on it in detail. They argued that declarative memory is impaired in amnesia, whereas procedural memory is not. Declarative memory was defined earlier, and procedural memory "accomplishes experience-based tuning and modification of individual processors, and involves fundamentally inflexible, individual (i.e., nonrelational) representations (Cohen et al., 1997, p. 138).

Evidence that this theoretical approach may be superior to the one based on the explicit/implicit memory distinction was reported by Whitlow, Althoff, and Cohen (1995). They presented amnesic patients and normal controls with real-world scenes, and asked them to respond as rapidly as possible to questions (e.g., "Is there a chair behind the oranges?"). After that, the participants answered questions when presented with three kinds of scenes:

1. Repeated old scenes.

2. New scenes.

3. Manipulated old scenes, in which the positions of some of the objects had been altered.

The participants' eye movements were recorded as they viewed the scenes.

What did Whitlow et al. (1995) find? Both groups answered faster to old scenes (whether repeated or manipulated) than to new scenes. This could be explained on the basis that the task relies on implicit

(al Controls

(b) Amnesic patients

(al Controls

(b) Amnesic patients

Repeated Manipulated New old scenes old scenes scenes

Repeated Manipulated New old scenes old scenes scenes

Repeated Manipulated New old scenes old scenes scenes

Stop Anxiety Attacks

Stop Anxiety Attacks

Here's How You Could End Anxiety and Panic Attacks For Good Prevent Anxiety in Your Golden Years Without Harmful Prescription Drugs. If You Give Me 15 minutes, I Will Show You a Breakthrough That Will Change The Way You Think About Anxiety and Panic Attacks Forever! If you are still suffering because your doctor can't help you, here's some great news...!

Get My Free Ebook


Post a comment