Naming without knowing and appearance without associations: Evidence for constructive processes in semantic memory?

Abstract

This study describes a patient (SE) with temporal lobe injury resulting from Herpes Simplex Encephalitis, who displayed a previously unreported impairment in which his knowledge of associative and functional attributes of animals was disproportionately impaired by comparison with his knowledge of their sensory attributes (including their visual properties and characteristic sounds). His knowledge of man-made objects was preserved. A striking aspect of the present case was that the patient remained able to name many animals from their pictures, despite making gross errors in generating associative information about these same animals. This suggests that a semantic representation incorporating stored sensory knowledge may be sufficient for naming (at least for biological categories) and associative information may be unnecessary. Semantic knowledge may normally incorporate more information than is necessary for identification. SE's errors were found to be confabulatory and reconstructive in nature and it is argued that this aspect of his performance challenges passive conceptions of semantic memory couched in terms of a catalogue of stored representations. It is proposed that the patient's disorder affects a dynamic, constructive, and inferential component of his knowledge base, and that this component is sensitive to semantic category.     

Charting the progression in semantic dementia: Implications for the organisation of semantic memory

Abstract

A patient, JL, with the syndrome of semantic dementia was assessed longitudinally over a two-year period. The data presented here address the controversy concerning the hierarchical organisation of semantic memory. On a range of category fluency tests, when first tested JL was just within the normal range on the broadest categories of animals and household items, but was virtually unable to produce any instances of specific categories such as breeds of dog or musical instruments. Longitudinal fluency data for the animal category demonstrate that while JL continued to produce the most prototypical responses (cat, dog, horse), other animal labels dropped out early from his vocabulary. On the picture-sorting tests from our semantic memory test battery, JL's discrimination between living things and man-made objects was preserved for a substantial time in conjunction with a marked decline in his sorting ability for more specific categories, particularly features or attributes (e.g. size, foreign-ness, or ferocity of animals).

An analysis of naming responses to the 260 Snodgrass and Vanderwart pictures on four occasions suggests a progressive loss of the features of semantic representations that enable discrimination between specific category instances. There was a progressive decline in circumlocutory and category co-ordinate responses with a rise in broad superordinate and cross-category errors. The latter are of particular theoretical interest; on session I, all cross-category errors respected the living/man-made distinction, but by session IV almost half of such errors failed to respect this distinction. The emergence of category prototypes was another notable feature, particularly in the living domain: at one stage, land (or four-legged) animals were all named either cat, dog, or horse. By contrast, within the man-made domain, items were frequently described in terms of their broad use or function, until eventually no defining features were produced. These findings are discussed in the context of competing theories of semantic organisation.     

Experimental Studies in Ocular Behavior: I. The Dodge Mirror-Recorder for Photographing Eye-Movements

Two experiments are reported concerning the effects of semantic encoding operations on the subsequent usefulness of interitem semantic relations in recall. These experiments employ the Mathews paradigm in which Ss first make judgments about the semantic relatedness of words presented in triplets and subsequently have their memory for the words tested. These experiments examine the effects of variations in interitem associative strength (Experiment 1), and task instructions and triplet order (Experiment 2). The results are discussed in terms of the general effects of the semantic judgment task on the encoding of task-relevant and task-irrelevant semantic attributes of words.     

Looking for meaning: Eye movements are sensitive to overlapping semantic features, not association

Theories of semantic memory differ in the extent to which relationships among concepts are captured via associative or via semantic relatedness. We examined the contributions of these two factors, using a visual world paradigm in which participants selected the named object from a four-picture display. We controlled for semantic relatedness while manipulating associative strength by using the visual world paradigm’s analogue to presenting asymmetrically associated pairs in either their forward or backward associative direction (e.g., ham—eggs vs. eggs—ham). Semantically related objects were preferentially fixated regardless of the direction of presentation (and the effect size was unchanged by presentation direction). However, when pairs were associated but not semantically related (e.g., iceberg—lettuce), associated objects were not preferentially fixated in either direction. These findings lend support to theories in which semantic memory is organized according to semantic relatedness (e.g., distributed models) and suggest that association by itself has little effect on this organization.     

Preservation of categorical knowledge in Alzheimer's disease: A computational account

Abstract

The distinction between knowledge of specific exemplars and knowledge of their general categories is central to much theorising on the nature of semantic memory. The dissociation between exemplar and category knowledge observed in Alzheimer's Disease (AD) would appear to support this distinction, and to suggest that different neural systems are involved in the representation of exemplar and category knowledge. We review the evidence for preserved category knowledge in the semantic memory impairment of AD, and propose an alternative interpretation, according to which category and exemplar knowledge are both represented in the same distributed neural substrate. The relative preservation of category knowledge is a consequence of the greater frequency, and hence greater robustness, of the representation of attributes shared by all or most members of a category, compared to exemplar-unique attributes. We test and confirm the computational adequacy of this hypothesis in two computer simulations.     

Semantic memory disorders

Semantic memory encompasses knowledge of objects, facts and words. A number of brain regions are probably involved, but the left infero-lateral temporal lobe appears to play a key role. The separability of semantic memory from episodic (or autobiographical) memory is a focus of current debate. Impaired semantic memory is a common feature of Alzheimer's disease but is invariably overshadowed by a profound deficit in episodic memory. In semantic dementia, a rarer disorder associated with focal temporal-lobe atrophy, there is selective loss of semantic memory, characterized by preservation of superordinate knowledge of words, and objects, but loss of finer-grained information. This pattern can be interpreted as a degradation of features from a distributed network of semantic representations. Following Herpes simplex encephalitis, patients sometimes show disproportionate loss of knowledge for natural kinds (e.g. animals) with relative preservation of knowledge about artefacts, this may reflect differential damage to neural systems critical for perceptual as opposed to functional features, perceptual properties being more salient in knowledge about natural kinds.     

The selective impairment of semantic memory

The selective impairment of semantic memory is described in three patients with diffuse cerebrallesions. These patients, selected on the basis of a failure to recognize or identify common objects (agnosia for objects), were investigated in detail. In particular, their perceptual, language and memory functions were assessed, and the limits and properties of their recognition difficulties explored.

It was found that knowledge of pictorial representations of objects, and of words, was impaired or impoverished, and in both instances knowledge of subordinate categories was more vulnerable than superordinate categories. Evidence is presented that this impairment of semantic memory cannot be accounted for by intellectual impairment, sensory or perceptual deficits, or expressive language disorder. The implications of damage to the semantic memory system for the operation of other cognitive systems, in particular short and long-term memory functions, are considered. Some tentative evidence for the structural basis for a hierarchically organized modality-specific semantic memory system is discussed.     

Associative recognition processes are modulated by the semantic unitizability of memoranda

Although memory of episodic associations is generally considered to be recollective in nature, it has been suggested that when stimuli are experienced as a unit, familiarity processes might contribute to their subsequent associative recognition. To investigate the effect of semantic relatedness during episodic encoding on the processes of retrieval of associative information, we had participants interactively encode pairs of object pictures, vertically arranged so as to suggest a functional or configural relationship between them. Half the pairs were independently judged to be of related objects (e.g., a lamp over a table) and half of unrelated objects (e.g., a key-ring over an apple). At test, participants discriminated between intact, recombined, and new pairs while event related potentials (ERPs) were recorded. In an early ERP marker of retrieval success generally associated with familiarity processes, differences related to associative memory only emerged for related pairs, while differences associated with item memory emerged for both related and unrelated pairs. In contrast, in a later ERP effect associated with recollection, differences related to associative memory emerged for both related and unrelated pairs. These findings may indicate that retrieval of episodic associations formed between two semantically related visual stimuli can be supported by familiarity-related processes.     

Latent structure in measures of associative,semantic, and thematic knowledge

There has been much debate about the relation between knowledge for meaning (semantic memory) and knowledge for words in context (associative memory). Many measures of that knowledge exist, but do they all measure the same thing? In this study, scaling, clustering, and factor-analytic techniques were used to reveal the structure underlying 13 variables. Semantic similarity determined from lexicographic measures is shown to be separable from the associative strength determined from word association norms, and these semantic and associative measures are in turn separable from abstract representations derived from computational analyses of large bodies of text. The three-factor structure is at odds with traditional views of word knowledge. The expression of long-term knowledge about words and the concepts they represent may be better viewed in terms of associative, semantic, and thematic information.     

Process dissociation procedure reveals age differences in unconscious influences on memory for possible and impossible objects

Abstract

Forty-eight younger and 48 older adults performed inclusion and exclusion tasks for line drawings of possible and impossible objects that were encoded semantically or globally. Participants' performance was transformed into estimates of conscious and unconscious influences on memory via the Process Dissociation Procedure (PDP). Five major findings were obtained. First, developmental differences were observed in the relative strength of conscious and unconscious influences on memory such that conscious influences were stronger for younger than older adults, whereas unconscious influences were stronger for older than younger adults. Second, unconscious influences on memory were demonstrated for possible and impossible objects. Third, unconscious influences on memory were obtained for objects that were encoded in both a global and a semantic fashion. Fourth, age-related differences in conscious and unconscious influences on memory were unaffected by object type. Fifth, estimates of conscious and unconscious influences on memory were unrelated to scores on psychometric measures of visual-spatial ability: Primary Mental Ability-Space (PMA-Space) and Benton Facial Recognition Task (BFRT). Collectively, these findings have implications for our understanding of the relative strength of conscious and unconscious memory processes in younger and older adults as well as the different types of unconscious memory processes that are recruited by the PDP in comparison to the traditional priming methodology.     

Visual object agnosia without prosopagnosia or alexia: Evidence for hierarchical theories of visual recognition

Abstract

A single case study is presented of a patient, Mr. W, with a selective deficit in recognizing pictures and real objects, linked to impaired stored visual knowledge about objects. Despite this, Mr. W maintained a preserved ability both to read aloud printed words and to recognize famous faces, when compared with age-matched control subjects. In addition, his access to semantic information from words was superior to that from pictures. The data provide evidence that visual agnosia can occur without alexia or prosopagnosia, contrary to recent proposals (Farah 1990, 1991). This finding is consistent with a hierarchical model of visual object recognition in which agnosia can reflect impaired stored knowledge of objects without accompanying perceptual deficits. The selective recognition deficit for objects further indicates that stored knowledge concerning different classes of visual stimuli (common objects, faces, and words) is separately represented in the brain.     

Visual Imagery Processing and Knowledge of Famous Names in Alzheimer's Disease and MCI

ABSTRACT

The study of memory for famous people and visual imagery retrieval was investigated in patients in the early stages of Alzheimer's disease (AD) and in the prodromal stage of AD, so-called Mild Cognitive Impairment (MCI). Fifteen patients with AD (MMSE ≥23), 15 patients with amnestic MCI (a-MCI) and 15 normal controls (NC) performed a famous names test designed to evaluate the semantic and distinctive physical features knowledge of famous persons. Results indicated that patients with AD and a-MCI generated significantly less physical features and semantic biographical knowledge about famous persons than did normal control participants. Additionally, significant differences were observed between a-MCI and AD patients in all tasks. The present findings confirm recent studies reporting semantic memory impairment in MCI. Moreover, the current findings show that mental imagery is lowered in a-MCI and AD and is likely related to the early semantic impairment.     

Structural Comparison of Cognitive Associative Networks in Two Populations1

The cognitive associative structure of 2 populations was studied using network analysis of free‐word associations. Structural differences in the associative networks were compared using measures of network centralization, size, density, clustering, and path length. These measures are closely aligned with cognitive theories describing the organization of knowledge and retrieval of concepts from memory. Size and centralization of semantic structures were larger for college students than for 7^th graders, while density, clustering, and mean path length were similar. Findings presented reveal that subpopulations might have very different cognitive associative networks. This study suggests that graph theory and network analysis methods are useful in mapping differences in associative structures across groups.     

Discriminating Semantic from Episodic Relatedness in Young and Older Adults

ABSTRACT

The ability of young (aged 18–30) and older (aged 60–80) adults to discriminate pre-experimental (semantic) from experimental (episodic) associations was examined. Participants studied a list containing semantically related and unrelated word pairs and then made either associative recognition (Experiments 1a and b) or semantic relatedness (Experiment 2) judgments at various response deadlines. For associative recognition judgments, both young and older adults benefited from semantic relatedness, leading to more hits for related than unrelated pairs, and at the long response deadline, older adults' performance on those pairs matched that of young participants. Also, both young and older adults demonstrated superior discrimination for unrelated lures whose members had originally been studied in related pairs – evidence for recall-to-reject processing in both age groups. In making semantic relatedness judgments, both young and older adults showed an episodic priming effect. When older adults can rely on long-standing associations, their performance resembles that of young adults – both in associative recognition and in episodic priming.     

Age-related differences in associative memory depend on the types of associations that are formed

ABSTRACT

Building on well-established findings of age-related decline in associative memory, we examined whether the magnitude of age differences depends on the types of associations that are formed. Specifically, because of predominant age-related changes in the hippocampus, we expected to find larger age differences in recognition of between-domain than within-domain associations. Twenty younger and 20 older healthy adults were given two associative recognition tests, using face–name and word–word pairs, that were matched for difficulty level. As hypothesized, a three-way interaction indicated that, relative to item recognition, age differences in associative recognition were greater for between-domain face–name associations than for within-domain word–word associations. This dissociation is consistent with the idea that the hippocampus plays a prominent role in binding information received from distal neocortical regions. The discussion focuses on the roles of recollection and familiarity in supporting associative memory as well as implications for the remediation of age-related memory decline.     

Is a picture worth a thousand words? Evidence from concept definitions by patients with semantic dementia

Nine patients with semantic dementia (the temporal lobe variant of frontotemporal dementia) were asked to define concrete concepts either from presentation of a picture of the object or from its spoken name. As expected, the patients with the most severe semantic impairment produced the least detailed definitions, and the quality of the definitions overall was significantly related to concept familiarity. Further analyses of the definitions were designed to assess two key theoretical aspects of semantic organization. (i) Do objects and their corresponding names activate conceptual information in two neuroanatomically separable (modality-specific) semantic systems? If so, then-apart from any expected commonality in performance attributable to factors such as concept familiarity-one would not predict striking item-specific similarities in a patient's picture- and word-elicited definitions. (ii) Do sensory/perceptual features and more associative/functional attributes of conceptual knowledge form two neuroanatomically separable subsystems? If so, then one would predict significant dissociations in the prominence of these two types of information in the patients' definitions. The results lead us to favor a model of the semantic system that is divided by attribute type but not by modality.     

Development of memory structures for homographs using pathfinder network representations

Some studies with children have shown that there is no semantic priming at short stimulus onset asynchrony (SOA) in lexical decision and naming tasks for homographs. The predictions of spreading activation theories might explain this missing effect. There may be differences in children's and adults' memory structures. We have explored this hypothesis. The development of memory structure representations for homographs was measured by a Pathfinder algorithm. In Experiment 1, the three dependent variables were: the number of links in the network, closeness measures (C), and distances between nodes. Results revealed developmental differences in network structure representations in adults and children. In Experiment 2, results revealed that these differences were not due to the cohort effect. In Experiment 3, the relationship between associative strength, as measured by associative norms, and distances, as measured by Pathfinder algorithm, was explored. The results of these three experiments and empirical research from semantic priming experiments show that these differences in memory structure representations could be one of the sources of the missing semantic priming effect in children.     

Increased sensitivity to proactive interference in amnestic mild cognitive impairment is independent of associative and semantic impairment

Episodic memory deficit is the hallmark of amnestic mild cognitive impairment (aMCI). There is, however, an overlap in performance among patients with aMCI and elderly controls (EC). The memory deficit in aMCI therefore needs to be better characterized. Studies have shown that associative memory is selectively impaired in aMCI, and recent work suggested that aMCI may be hypersensitive to semantic proactive interference (PI). It is not known whether this increased PI is related to associative or semantic impairment. EC (n = 44) and patients with aMCI (n = 30) performed two tasks presenting a gradually increasing PI effect across four lists. One task used semantic cueing, the other phonological cueing. We controlled for associative memory by introducing it as a covariate and by matching our subjects for it. Patients with aMCI had a greater PI effect than EC matched for associative memory, regardless of the type of cueing. The increased PI effect in patients with aMCI is independent of their associative and semantic impairment.     

The verification of category and property statements

Two experiments compared the speed with which category and property statements of matched associative production frequency could be verified. The verification of category statements was consistently faster than that of property statements, for both true and false judgments. The category advantage did not interact with the level of production frequency for the true sentences. Implications for models of decision processes in semantic memory are discussed.     

Privileged access to action for objects relative to words

We compared action (pour or twist?) and contextual/semantic (found in kitchen?) decisions made to pictures of objects, nonobjects, and words. Although there was no advantage for objects over words in contextual/semantic decisions, there was an advantage for objects over words and nonobjects in action decisions. For objects, both action and contextual/semantic decisions were faster than naming; for words, the opposite occurred. These results extend the early results of Potter and Faulconer (1975) that there is privileged access to semantic memory for objects relative to that for words and privileged access to phonology for words. Our data suggest that, for objects, there is privileged access to action knowledge rather than to all forms of semantic knowledge and that this is contingent on learned associations between objects and actions.