The memory deficits in alzheimer-type dementia: A review

This review is an account of recent experimental studies of memory deficits at the early stages of Alzheimer-type dementia, evaluating these studies in relation to current theories of memory functioning in humans. Whilst memory deficits are found to be widespread, some aspects are more resilient to impairment than others. For example, the processes associated with articulatory rehearsal in working memory are unimpaired despite a reduction in performance on most tests of primary memory. The “implicit” aspects of secondary memory appear to remain unimpaired, in contrast to a marked decline in “explicit” or “episodic” memory. In addition, there is evidence that the rate of forgetting from secondary memory is normal. Some aspects of episodic and semantic memory are found to be impaired as a consequence of a decline in the efficient organisation and processing of verbal material at encoding or retrieval. It is concluded that the deficits share particular features found in organic amnesia, but with additional deficits which relate to impairments in other domains of functioning.     

The perceptual-mnemonic/feature conjunction model of perirhinal cortex function

The perirhinal cortex was once thought to be “silent cortex”, virtually ignored by researchers interested in the neurobiology of learning and memory. Following studies of brain damage associated with cases of amnesia, perirhinal cortex is now widely regarded as part of a “medial temporal lobe (MTL) memory system”. This system is thought to be more or less functionally homogeneous, having a special role in declarative memory, and making little or no contribution to other functions such as perception. In the present article, we summarize an alternative view. First, we propose that components of the putative MTL system such as the hippocampus and perirhinal cortex have distinct and dissociable functions. Second, we provide evidence that the perirhinal cortex has a role in visual discrimination. In addition, we propose a specific role for perirhinal cortex in visual discrimination: the contribution of complex conjunctive representations to the solution of visual discrimination problems with a high degree of “feature ambiguity”. These proposals constitute a new view of perirhinal cortex function, one that does not assume strict modularity of function in the occipito-temporal visual stream, but replaces this idea with the notion of a hierarchical representational continuum.     

Cue location does not account for all selective associations in food aversion learning

Studies of food aversion learning have been widely interpreted as yielding evidence for selectivity in the associability of cues and consequences. On the one hand, taste cues were easily conditioned to interoceptive consequences (e.g. “illness”), but only with difficulty to exteroceptive consequences (e.g. shock). On the other hand, audiovisual stimuli were easily conditioned to shock, but only with difficulty to illness (Revusky and Garcia, 1970). Sullivan (1984) has emphasized that cue location and temporality have been important uncontrolled variables in many food-aversion studies and has argued that when these factors are taken into account, the supposed selectivity related to sensory modality is lost. Thus, Sullivan (1984) argues that shocks, like “illness”, are readily associated with tastes provided that the tastes are intrinsic attributes of the substances ingested.

Sullivan's important article may have demonstrated significant weaknesses in the evidence for specific associations of exteroceptive cues with shock rather than with “illness”, and this point will not be considered further. However, there are two aspects of Sullivan's case that cannot be substantiated. Firstly, the attempt to demonstrate that taste cues are as readily associated with shock as they are with “illness” is unconvincing. Secondly, unjustified assumptions are made about the nature of the unconditioned stimulus in conditioning with interoceptive consequences, which is presumed to be “illness” despite extensive evidence to the contrary.     

Remembering, Familiarity, and Source Monitoring

Two experiments investigated recollective experience in a source monitoring task. Subjects saw an array of objects and performed, watched, or imagined actions involving pairs of objects. In a subsequent recognition test, subjects indicated whether their recognition judgements were made on the basis of conscious recollective experience (“remember” responses), or on some other basis such as familiarity (“know” responses). The proportions of correct “remember” responses for both objects and actions decreased from performed, through watched, to imagined actions, whereas the proportions of correct “know” responses were uninfluenced by the source of the memories. In addition, the relationship between recollective experience and accuracy of source judgement varied across sources. Source accuracy for performed actions was obtained only in “remember” responses, whereas source accuracy for imagined actions was obtained only in “know” responses. Source accuracy for watched actions was obtained in both “remember” and “know” responses. The findings suggest that the types of memory attributes available at retrieval determine the quality of subsequent memory experience, and it is proposed that memories with strongly self-referential attributes (arising from performed actions) powerfully cue recollective experience during retrieval.     

The Critical Role of Cholinergic Basal Forebrain Neurons in Morphological Change and Memory Encoding: A Hypothesis

It has been known for a long time that cholinergic basal forebrain neurons which project to the cerebral cortex play a role in learning and memory. Behavioral studies following lesions, for example, repeatedly have suggested multiple learning-related roles for these neurons. Apart from behavioral studies, cholinergic neurons have been shown to possess extraordinarily plastic axons. This plasticity has not been related comprehensively to mnemonic devises, even though morphological changes in the CNS are prime candidates for the neural engram. In this paper, I propose a hypothesis that relates these two characteristics of cholinergic neurons. This hypothesis is that plastic cholinergic axon terminals induce structural reorganization in their targets during memory storage. Possible intracellular mechanisms are examined, whereby acetylcholine release in the cerebral cortex could cause postsynaptic structural changes. Finally, the characteristics of the overall cholinergic–cholinoceptive cell “engram” are elaborated with particular attention paid to the encoding of the stimulus properties along with the context and meaning of the stimulus.     

Semantic distance in memory structure: The retrieval of conceptual relationships

Four experiments are described which aim to distinguish the relative contributions of measures of semantic distance based on formal and normative criteria. Experiment I replicates a previous finding by Collins and Quillian (1969) that sentence confirmation RTs support a hierarchically-organised memory model. Experiments II and III minimised the role of syntactic processes and examined the times taken to “see the relationship” between pairs of concepts. The results also supported a hierarchical model but cast doubt upon the formal distinction between superset and property relationships. Experiment IV showed similar results using only property relationships of the “has” form. Multiple regression analyses of the data indicate that “number of intervening links” is a more consistent predictor of RT than “associative” measures of semantic relatendness and confirm strong linearity effects consistent with a hierarchical model of storage.     

Palatability shift of a salt-associated incentive during sodium depletion

Previous studies of natural palatability-sensitive reactions, elicited from rats by tastes, have indicated that the taste of concentrated NaCl becomes more palatable during states of body sodium depletion. A training procedure based upon “sensory preconditioning” or “irrelevant incentive” designs was used here to establish gustatory conditioned labels (quinine or citric acid) for either NaCl or fructose while rats were in a normal physiological state. Experiment 1 replicated demonstrations by others that gustatory conditioned labels for salt can attract and act as independent incentives during the salt appetite induced by sodium depletion. Experiment 2 used the taste reactivity measure to show that the enhanced palatability of the taste of NaCl transfers to produce enhanced palatability of the taste of the isolated conditioned label for salt in a state-dependent fashion. These results offer further support for the proposition that conditioned incentives not only predict hedonic events to follow, but themselves become attributed with the hedonic properties of their reinforcers.     

Motivationally neutral stimulation of the nucleus basalis induces specific behavioral memory

The cholinergic system has been implicated in learning and memory. The nucleus basalis (NB) provides acetylcholine (ACh) to the cerebral cortex. Pairing a tone with NB stimulation (NBstm) to alter cortical state induces both associative specific tuning plasticity in the primary auditory cortex (A1) and associative specific auditory behavioral memory. NB-induced memory has major features of natural memory that is induced by pairing a tone with motivational reinforcers, e.g., food or shock, suggesting that the cholinergic system may be a “final common pathway” whose activation promotes memory storage. Alternatively, NB stimulation might itself be motivationally significant, either rewarding or punishing. To investigate these alternatives, adult male rats (n = 7) first formed a specific NB-induced memory (CS = 8.0 kHz, 2.0 s paired with NBstm, ISI = 1.8 s, 200 trials), validated by post-training (24 h) frequency generalization gradients (1–15 kHz) of respiration interruption that were specific to the CS frequency. Thereafter, they received the same level of NBstm that had induced memory, while confined to one quadrant of an arena, and later tested for place-preference, i.e., avoidance or seeking of the quadrant of NBstm. This NBstm group exhibited neither preference for nor against the stimulated quadrant, compared to sham-operated subjects (n = 7). The findings indicate that specific associative memory can be induced by direct activation of the NB without detectable motivational effects of NB stimulation. These results are concordant with a memory-promoting role for the nucleus basalis that places it “downstream” of motivational systems, which activate it to initiate the storage of the current state of its cholinergic targets.     

Does it all go together when it goes? The nineteenth Bartlett memorial lecture

As groups of people age, the differences in the cognitive abilities of the most and least able become more extreme. This increase in between-individual variance is accompanied by an increase in within-individual variance: the difference between individuals' levels of performance on their best and least well retained skills. The implications of increasing between-individual variance are discussed in terms of the range of different factors that may affect cognitive ageing. Increases in within-individual variance are discussed in terms of differences betweeen “fluid” and “crystallized” abilities. The usefulness of this distinction and its functional implications are questioned. The hypothesis that age-related declines in “fluid” abilities are best modelled in terms of declines in a single factor is evaluated. Evidence is presented of disparate rates of decline, even of “fluid” cognitive abilities, such as performance on IQ tests, ability on information-processing tasks, and efficiency on memory tasks. Data from large-scale cross-sectional studies suggests that cognitive skills do not “all go together when they go”, but that there may, rather, be characteristic patterns, or syndromes, of cognitive ageing.     

Tactile short-term memory

Subjects tried to recall the location of a tactile stimulus on the underside of the forearm after delays of 0, 3, 5, 10, 15, 30, 45 and 60 sec. When “rehearsal” was prevented by requiring subjects to count backwards during the delay, accuracy of recall decreased systematically reaching an asymptote after 45 sec. When subjects were left free to “rehearse,” this did not affect the decline in accuracy over the first 10 sec. Between 10 and 15 sec. there was a significant increase in accuracy followed by a slow decline which had not reached asymptote by 60 sec. It is suggested that tactile STM (short-term memory) depends on two processes, a fading sensory trace which is unaffected by distraction and a less labile system which does not appear to be verbal but which depends on “rehearsal.”     

The effect of an irrelevant dimension on “same-different” judgements of multi-dimensional stimuli

Subjects made “same”-“different” judgements of simultaneously presented pairs of visual stimuli which could vary either in shape and colour independently, or in shape alone. In both conditions only shape was relevant to the “same”-“different” judgement. In the former condition “same” and “different” reaction times (RTs) were shorter when the states (“same” or “different”) of the relevant and the irrelevant dimension, colour, were the same. This result is interpreted as support for either a perceptual or a response interference hypothesis. The presence of an irrelevant dimension did not appear to affect differentially “same” and “different” judgements. The need for a re-evaluation of the results from other studies of multi-dimensional stimulus discrimination is discussed.     

Short-term memory for repeated and non-repeated items

Digit sequences containing repeated items are retained differently in short-term memory from sequences containing no repeated items. The repeated items are remembered better or worse than items in the corresponding positions of “all-different” sequences depending on the number of times the item is repeated, the number of items repeated, the number of items intervening between the occurences of a repeated item, and the position of the repeated items in relation to the beginning and end of the sequence. In every type of repetition studied, except one, memory for the non-repeated items in sequences with repeated items is better than for the corresponding items of all-different sequences. This is true in some cases despite significant specific interference between the (non-repeated) items following the separated occurrences of repeated items. The negative effects in memory for repeated items and the positive effects in memory for non-repeated items are greater when the items are presented at the rate of five per sec. than at one per sec., contrary to the hypothesis that differential rehearsal is responsible for these effects. The results are interpreted as supporting an “associative,” as opposed to a “non-associative,” theory of short-term memory, as this distinction is defined in the paper.     

Reversing the word-length effect: A comment on Caplan, Rochon, and Waters

Caplan, Rochon, and Waters (1992) report a failure to observe the poorer immediate serial recall for words of longer spoken duration obtained by Baddeley, Thomson, and Buchanan (1975) and subsequently replicated by others. Indeed, they find a significant reversal of this effect. We present evidence that the material used by Caplan et al. differs only minimally in spoken duration under speeded articulation conditions (Exp. 1 = 1.9%, Exp 2 = 2.31%), in contrast to a clear difference in the case of the original Baddeley et al. material (24.5%). It is further suggested that the reversal of the word-length effect may result from differences in acoustic similarity between the “long” and “short” word sets used by Caplan et al. We conclude that the evidence continues to indicate that longer spoken duration is associated with reduced memory span.     

Acoustic similarity and interference in short-term recognition memory: Some results of a decision-theory analysis

The following three studies of single-probe recognition memory set out to show the effect on the signal-detectability measures of d' and β (Tanner and Swets, 1954) of variations in the acoustic similarity of interfering material, which may either precede or follow the item to be remembered (proactive or retroactive interference --PI or RI). The first experiment studies a situation employed by Wickelgren (1966a), who reported that acoustically similar RI substantially reduced d'. It is shown that this effect could have been due to biases in Wickelgren's original designs, and that when a bias-free design is used, the fall in d' is only of borderline significance.

To investigate this problem further, a design was evolved in which two items were presented for memorizing, which varied in acoustic similarity to each other, and (after a distracting task) a probe was presented with one of three questions: Was this the first item of the pair? Was it the second? or, Did it occur in either position? In the first case, recognition-memory with RI of varying acoustic similarity was being studied, and as in the first experiment, it was found that similarity slightly reduced d'. With the second question, PI effects were being studied, and here negligible differences were found. With the third type of question, a “location-free” test, no effects of similarity were found. The last result rules out Posner's (1967) “acid-bath” explanation of similarity effects in interference: an explanation in terms of “differentiation” (or “filtering”) was also invalidated by the results of a third experiment, in which the same effects were found even though similarity varied only between stimulus items and interference, and not between these and the probe. Wickelgren's (1966b) associative model appears to have least difficulty in accommodating these results, though even this needs certain ad hoc assumptions to be able to do so.     

Information relevance, working memory, and the consideration of alternatives

People routinely focus on one hypothesis and avoid consideration of alternative hypotheses on problems requiring decisions between possible states of the world--for example, on the “pseudodiagnosticity” task (Doherty, Mynatt, Tweney, & Schiavo, 1979). In order to account for behaviour on such “inference” problems, it is proposed that people can hold in working memory, and operate upon, but one alternative at a time, and that they have a bias to test the hypothesis they think true. In addition to being an ex post facto explanation of data selection in inference tasks, this conceptualization predicts that there are situations in which people will consider alternatives. These are:

1. “action” problems, where the alternatives are possible courses of action;

2. “inference” problems, in which evidence favours an alternative hypothesis.

Experiment 1 tested the first prediction. Subjects were given action or inference problems, each with two alternatives and two items of data relevant to each alternative. They received probabilistic information about the relation between one datum and one alternative and picked one value from among the other three possible pairs of such relations. Two findings emerged; (1) a strong tendency to select information about only one alternative with inferences; and (2) a strong tendency, compared to inferences, to select information about both alternatives with actions.

Experiment 2 tested the second prediction. It was predicted that data suggesting that one alternative was incorrect would lead many subjects to consider, and select information about, the other alternative. For actions, it was predicted that this manipulation would have no effect. Again the data were as predicted.     

Acoustic factors in visual search

Previous work has shown that in searching for existing or absent “e.s” in printed prose, the presence or absence of silent “e.s” was less likely to be detected than that of pronounced “e.s.” It was suggested that the acoustic or kinaesthetic “image” was searched for evidence of an “e” in addition to the visual stimulus and that evidence from both sources was considered in making the appropriate response.

The present experiment employs mainly substitutive errors within words, which may or may not change their pronunciation. The results suggest that the form of the acoustic correlates has no bearing upon whether the words are detected as wrongly spelt, but that the presence or absence of an acoustic event corresponding in time to the spatial location of the error is important.     

Does the autistic child have a metarepresentational deficit?

This study examines the claim that autistic children lack a “theory of mind” because of an inability to metarepresent. We argue that if autistic children have a “metarepresentational” deficit in Leslie's (1987, 1988) sense of the term, then they should have difficulty not only with mental representations such as false beliefs, but also with external representations such as photographs. Autistic children's understanding of photographic representations was tested using Zaitchik's (1990) task. This task is modelled on the false belief task (Baron-Cohen, Leslie, & Frith, 1985: Wimmer & Perner, 1983) but involves “false” photographs where a photographic representation does not conform with the current state of the real world. Like Zaitchik (1990) we found that normal 3 and 4-year-olds found this task as difficult as the false belief task. In sharp contrast, however, the autistic children in our study passed the photograph task but failed the false belief task. As both tasks require the ability to decouple, this evidence challenges the view that autistic children lack “metarepresentational” ability in Leslie's sense. However, the results leave open the question of whether autistic children have a metarepresentational ability in the different sense of the term intended by Pylyshyn (1978), that is, representing the relationship between a representation and what it represents.     

Changes in behaviour associated with a nervous system poison (DDT.)

The main purpose of the investigation was to show that behaviour measures can be used to investigate the effects of those toxic drugs which produce “biochemical lesions” in the nervous system although the nature of the lesions still remains undetermined. The advantages of this approach are twofold. First, a psychological study may help to uncover the initial effects of the drug, and thus provide evidence which may lead to the ultimate understanding of the action of the drug. Second, and of value from the practical point of view, such a method may be used to detect toxicity.

In this study D.D.T. was used. Two experiments were performed on one control and four experimental groups of albino rats. Problem solving behaviour, speed and pattern of locomotion, and reaction to stress involving visual stimuli were observed.

Problem solving behaviour was found to be unaffected by the drug; no changes were found in speed of locomotion, but pattern of locomotion revealed that “ataxia” was one of the initial effects of D.D.T. poisioning. The experimental animals were found to be generally less reactive to “stress”; “hyper-irritability” reported in previous studies being explicable in terms of exaggerated motor responses.

The results obtained on “ataxia” showed that the procedure adopted here could be used to detect chronic D.D.T. toxicity in rats.     

Short report: Is spatial information encoded automatically in memory?

A dual-task paradigm was used to test Hasher and Zacks' (1979) hypothesis that spatial memory is automatic. Subjects saw two sets of 16 words each, the words being presented singly in random corners of a monitor screen. They were asked to remember the words and the corner in which each word was shown. In addition, subjects were given a concurrent task to perform. This task was either “easy” (counting aloud by ones) or “difficult” (counting aloud by sevens). Attention was focused either on the memory task or on the counting task. Word recognition was better when subjects carried out the easier competing counting task and when subjects concentrated mainly upon remembering the words and their positions. Contingent spatial memory was unaffected by either manipulation, supporting the hypothesis that spatial memory is automatic.