doi:10.1093/brain/awh349
Brain (2005), 128, 615–627
Abstract and concrete concepts have structurally
different representational frameworks
Sebastian J. Crutch1,2 and Elizabeth K. Warrington1
1
Dementia Research Group, Department of Clinical
Neurology, Institute of Neurology, University College,
and 2Division of Neuroscience and Psychological
Medicine, Imperial College, London, UK
Correspondence to: Sebastian Crutch, Dementia Research
Group, The National Hospital for Neurology and
Neurosurgery, 8-11 Queen Square, London WC1N 3BG, UK
Email: [email protected]
Summary
The architecture supporting our conceptual knowledge of
abstract words has remained almost entirely unexplored.
By contrast, a vast neuropsychological, neurolinguistic
and neuroimaging literature has addressed questions
relating to the structure of the semantic system underpinning our knowledge of concrete items (e.g. artefacts and
animals). In the context of semantic refractory access dysphasia, a series of experiments exploring and comparing
abstract and concrete word comprehesion are described.
We demonstrate that semantically associated abstract
words reliably interfere with one another significantly
more than semantically synonymous abstract words,
while concrete words show the reverse pattern. We report
the first evidence that abstract and concrete word meanings are based in representational systems that have qualitatively different properties. More specifically, we show
that abstract concepts, but not concrete concepts, are
represented in an associative neural network. Furthermore, our patient was found to have significantly greater
difficulty in identifying high frequency than low frequency
abstract words. This observation constitutes the first
evidence of an inverse word frequency effect. Our results
challenge the generality of many existing models
of human conceptual knowledge, which derive their
structure from experimental findings in the concrete
domain alone.
Keywords: semantic refractory access dysphasia; conceptual knowledge; abstract concepts; concrete concepts
Abbreviations: RSI = response-stimulus interval
Received April 7, 2004. Revised July 7, 2004. Accepted October 18, 2004. Advance Access publication November 17, 2004
Introduction
In almost 30 years since the first publication detailing a selective impairment of semantic memory, research has revealed
an ever more fine-grain categorical organization of human
knowledge for concrete concepts (for a review, see Capitani
et al., 2003). Over the same period, there has been a great
paucity of comparable investigations of abstract conceptual
knowledge. This is despite the fact that, in the original report
of semantic memory deficits, the performance of two patients
tested using the same stimuli revealed a double dissociation between concrete and abstract word comprehension
(Warrington, 1975). The more unusual pattern of a selective
impairment of concrete word knowledge with relatively preserved abstract word knowledge was subsequently replicated
(e.g. Warrington and Shallice, 1984; Sirigu et al., 1991;
Breedin et al., 1994). The paucity of experimental work on
the abstract portion of our vocabulary has not gone unnoticed
though, with Breedin et al. (1994) noting that:
‘the present results also point to a gap in the psychological
literature on semantic representation, which has little to say
about the manner in which abstract concepts are represented’ (p. 649).
The importance of the distinction between concrete and
abstract concepts has long been recognized in a number of
cognitive domains. For example, the role of item concreteness
has been stressed in the context of normal adults’ recall,
recognition and lexicial decision abilities (Paivio, 1971;
James, 1975), neurologically damaged patients’ reading skills
(e.g. deep dyslexics; Marshall and Newcombe, 1973), and
children’s vocabulary development (Brown, 1957; Gentner,
1982). Typically, subjects show a processing advantage for
concrete words (e.g. leaf, house, sword) over abstract words
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S. J. Crutch and E. K. Warrington
(e.g. pity, victory, deceit). This superiority has been attributed
variously to: (i) abstract words lacking the direct sensory
referents of concrete words (Paivio, 1986); (ii) greater
availability of contextual information in the knowledge
base for concrete words (Schwanenflugel and Shoben,
1983); and (iii) concrete words being supported by more
semantic features than abstract words (Plaut and Shallice,
1991, 1993). One common theme in these theories is that
a quantitative distinction is drawn between concrete and
abstract concepts, with concrete items more strongly represented. By contrast, we argue here that the fundamental
distinction is rooted in qualitatively different principles of
organization.
Theories of the representational framework underpinning
conceptual knowledge have largely been restricted to
the domain of concrete items, frequently being stimulated
by examinations of the phenomenon of category-specific
semantic deficits. Indeed, it was the observation of the
selective inability of some patients to retrieve semantic information concerning certain classes of items which led
Warrington (1981) to posit that semantic space had a finegrain categorical organization. It should be noted, however,
that in advocating a role for categorical information, it was
specifically predicted that other types of information such as
contextual association would be shown not to be powerful
organizational principles:
‘I have argued that the verbal semantic systems are
categorical in organization . . . In putting such an argument
are we rejecting any alternative model of semantic
organization? Yes, I think so. Such a formulation is almost
certainly incompatible with notions that consider verbal
semantic organization necessarily to be in the form of a
network of words related by associations. To be more
concrete: lion/danger or horseshoe/luck would be close
in an associative network organization but distant in a
categorical organization’ (Warrington, 1981; p. 422).
Nonetheless, other researchers have proposed that concepts
are underpinned by an associative network in the sense that
category-specific effects emerge after regional damage to an
associated set of semantic features (e.g. Allport, 1985; Farah
and McClelland, 1991; Hodges et al., 1994, 1995; McClelland
and Rogers, 2003). Although we address the debate surrounding concrete concept organization by explicitly testing
Warrington’s (1981) prediction, the focus of the current
experimental investigations is upon the less frequently studied
knowledge of abstract concepts. In particular, we compare
whether abstract and concrete word meanings are based
in representational systems that have the same or different
properties.
One approach to studying the properties of the semantic
system underlying concrete concepts has been to examine
the comprehension performance of patients with semantic
refractory access dysphasia. Refractoriness is an observable,
behavioural phenomenon, which can be defined by the
influence of temporal factors upon performance of a
cognitive task. More specifically, the performance of individuals with semantic refractory access disorders is facilitated
if the delay between each response and the presentation of
the subsequent stimulus is increased (response-stimulus
interval; RSI).
The problem underlying refractoriness has been
described as:
the reduction in the ability to utilize the system for a period
of time following activation
Warrington and McCarthy, 1983; p. 874.
Refractoriness has a number of behavioural consequences.
First, refractoriness causes response inconsistency; with repetitive testing of a set of stimuli, identification of items may be
achieved once but become unreliable thereafter. Secondly, in
patients with refractory access disorders, the strong and reliable effects of word frequency are often reduced or absent.
Finally, refractoriness leads to a sensitivity to the semantic
relatedness of test stimuli. Refractoriness can be observed
to affect not only individual concepts, but also previously
unprobed concepts which are represented close in neural
space. It is the last of these three consequences that has
been exploited to reveal a number of very fine-grain organizational principles in the domain of concrete vocabulary (e.g.
types of man-made artefacts: Warrington and Crutch, 2004)
and in the domain of proper names (e.g. cities: Crutch and
Warrington, 2003a).
At the physiological level, the refractory access phenomena
may be caused by damage to the neuromodulatory systems
which mediate neural activation levels compared with a relative sparing of the neurons which directly encode semantic
information (Gotts and Plaut, 2002). This neuromodulatory
damage results in excessive neuronal depression and a refractory period during which subsequent neural firing is blocked
or reduced.
Refractory access disorders have also been observed
to affect non-semantic components of the language system
(e.g. refractory anomia: McCarthy and Kartsounis, 2000;
refractory dyslexia: Crutch and Warrington, 2001). Consequently, in studying comprehension abilities in patients
with refractory access conditions it is essential to verify that
the refractory processes held to affect behaviour are operating at a semantic and not pre- or post-semantic level. In the
case of the patient reported in this paper, A.Z., this was
achieved by examining her ability to match spoken words
to pictures grouped in arrays by semantic relationship (e.g.
goat, deer, bear) compared with phonological relationship
(i.e. rhymes such as goat, boat, coat). A.Z. made significantly more errors in the semantic than phonological condition, indicating that at least a major component of her
comprehension deficit results from an impairment at the
level of semantic processing (Warrington and Crutch, 2004).
A semantic locus for A.Z.’s refractory access deficit is
further corroborated by the presence of semantic distance
effects on word–picture matching tasks when the identity of
a set of items is probed under both semantically close (i.e. all
Abstract and concrete concepts
potential responses drawn from the same category) and
semantically distant array conditions (i.e. all potential
responses drawn from different categories). Most importantly in relation to the experiments reported in this
paper, equivalent semantic distance effects are also observed
on tasks conducted entirely within the verbal domain,
namely spoken word–written word matching. These effects
provide strong evidence that the matching procedure is
mediated at least in part at a semantic level (e.g. Crutch
and Warrington, 2003a, 2004a).
It is this demonstration of semantic mediation on verbal
matching tasks that opens the way for exploration of the
domain of abstract conceptual knowledge. Indeed one
reason for the dearth of abstract word experimentation is
that, by their nature, low concreteness concepts have low
imageability and hence cannot easily be submitted to investigation using traditional neuropsychological techniques such
as picture naming or picture identification. Furthermore, the
use of spoken word to written word matching tasks is unsuitable for the majority of patients with a central semantic
deficit (e.g. patients with semantic dementia) because the task
might still be performed without recourse to semantic mediation via a process of print–sound correspondence. As noted
though, these techniques can and have been used to explore
fine-grain categories within a wide-ranging vocabulary in
patients with a refractory access disorder of semantic processing. Here the technique is applied for the first time to
abstract concepts.
From a more practical perspective, semantic distance
effects have typically been measured in the concrete domain
by comparing patients’ abilities to identify target items from
among distractor items drawn from the same semantic category (e.g. celery, radish, leek, onion) as opposed to distractors
drawn from different semantic categories (e.g. celery, rhino,
flask, helicopter). In the case of patients with a semantic
refractory access deficit, refractoriness is observed to build
up much more quickly in the former than in the latter condition (e.g. Forde and Humphreys, 1995; Warrington and
Cipolotti, 1996; Warrington and Crutch, 2004). However,
most abstract words are not as easily classified as concrete
words. For example, the word ‘dog’ can be classed as an
animal (superordinate) and has various exemplars such as
‘labrador’ and ‘alsatian’ (subordinates), but the word ‘luck’
cannot easily be categorized in the same manner. Many
abstract words do, however, have synonyms, i.e. other
abstract words which have essentially the same meaning
(e.g. reluctant, unwilling, averse). Indeed, in the studies
reported here we use synonymy as a marker of semantic
similarity among abstract words.
In the context of a semantic refractory access disorder, this
series of experiments attempts to measure the relative decrement in comprehension accuracy as influenced by similarity
and association of meaning. Thereby, we attempt to compare
directly semantic similarity and semantic association as competing principles of organization of both abstract and concrete
word semantics. In doing so, we document the effects of
617
abstractness and frequency upon the comprehension abilities
of our patient.
Case report
A.Z. is a 73-year-old right handed lady who became globally
aphasic following a left middle cerebral artery haemorrhage in
January 2002. A CT scan demonstrated extensive damage to
the left middle cerebral territory with left parietal, temporal
and posterior frontal involvement. A.Z.’s spontaneous speech
was fluent yet jargonistic with frequent semantic and phonemic errors. She also demonstrated a gross and pervasive
impairment of her reading, writing and naming skills. In particular, she was unable to read high frequency monosyllabic
words, could not write her own name and named only two
items from the Boston Naming Test.
With regard to her comprehension skills, A.Z. exhibited a
mild static semantic impairment. On the short form of the
British Picture Vocabulary Scale (Dunn et al., 1982), she
scored 22 out of 32 on oral presentation and 23 out of 32
with written stimuli. Furthermore, on the Pyramid and Palm
Trees Test (Howard and Patterson, 1992), A.Z. gained the
weak score of 38 out of 52 on both the word–word matching
and picture–picture matching versions of the task. Comprehension performance on a high frequency word-picture
matching task revealed no evidence of a category-specific
semantic deficit (McKenna and Warrington, 1978).
Therefore, despite having no viable expressive language
skills, systematic investigation of A.Z.’s comprehension abilities remained possible with the use of simple written word or
picture pointing tasks. These techniques have previously been
applied to three aspects of her conceptual knowledge, namely
place names (Crutch and Warrington, 2003a), person names
(Crutch and Warrington, 2004) and modality-specificity
(Warrington and Crutch, 2004). In this paper, however, we
tackle a much larger and arguably more important component
of conceptual knowledge, namely the realm of abstract words.
The investigations reported here were conducted between
June 2003 and January 2004, and were designed to document
and compare A.Z.’s comprehension of abstract and concrete
vocabularies.
Experiment 1: Frequency and abstractness
effects in abstract word identification
Words are often classified as ‘abstract’ or ‘concrete’, particularly in the context of neuropsychological experimentation.
However, abstractness/concreteness may in fact constitute a
continuum ranging between highly abstract concepts (e.g.
tendency) and highly concrete concepts (e.g. spoon). If this
were the case, then a number of reports of concreteness effects
in patients may reflect a greater expression or an exacerbation
of a pre-existing abstractness gradient. In this experiment, we
explored the influence of the degree of abstractness and word
frequency upon A.Z.’s word identification skills. More specifically, abstractness was manipulated without resorting to
stimuli drawn from the concrete extreme of any potential
618
S. J. Crutch and E. K. Warrington
continuum; only words traditionally classified as abstract
were included in the experiment.
Methods
The stimuli consisted of two sets of 144 abstract written words with
frequency ratings and abstractness ratings drawn from the MRC
Psycholinguistic Database (Coltheart, 1981; see Appendix 1). All
words were between five and eight letters in length. The abstract
words in both sets were divided equally into a matrix of three levels
of frequency (low, middle and high frequency) 3 three levels of
abstractness (highly abstract, moderately abstract and mildly
abstract). For the frequency ratings, there was no overlap between
the matrix cells, with each frequency band defined by a strict range of
Thorndike–Lorge ratings (low: 1–15; middle: 100–200; high: >300).
Similarly for the abstractness ratings, there was no overlap between
the ratings that defined each band (very low: 200–300; low: 300–400;
mild: 400–500). The 16 items in each matrix cell of each set were
arranged into four written word arrays of four items. The experiment
was conducted as a spoken word–written word matching task, so that
for each group of four arrays, one item was probed from each array in
sequence until every item had been probed once. A.Z. was required to
point to the target written word following each spoken word stimulus.
The experiment was conducted at a natural presentation rate with
a stimulus-response interval of 1 s. For the first set of words, low,
middle and high frequency arrays were tested in order for the very
low abstractness words, and latterly for the low and mildly low
words. For the second set of words, the identical procedure was
used except that arrays corresponding to each cell of the matrix
were presented in the reverse order. More specifically, the high
frequency/mild abstractness cell was tested first and the low
frequency/very low abstractness cell was tested last.
Results
The percentage correct responses for each cell of the word
matrix (both stimulus sets combined) together with the
percentage total correct responses for each frequency and
abstractness band are shown in Table 1. A x2 test for trend
of odds revealed that A.Z. showed significant trends to better
performance not only with less abstract words [x2(1) = 8.95,
P < 0.01] but also with lower frequency words [x2(1) = 4.56,
P < 0.05].
Comment
These results demonstrate that A.Z. shows an increase in
impairment in written word identification the greater the
abstractness of the target word. However, the pattern of
Table 1 Percentage correct responses on the spoken
word–written word matching, frequency by concreteness
matrix task (Experiment 1)
Word frequency
Total
Low Medium High
Abstractness Very low (N = 32)
Low (N = 32)
Mildly low (N = 32)
Total
59%
66%
88%
71%
50%
75%
78%
68%
50%
47%
66%
54%
53%
63%
77%
64%
performance is atypical in the sense that the results reported
here document for the first time the reversal of the typical
word frequency effect. Patients with semantic refractory
access disorders have previously been shown to be relatively
insensitive to the effects of word frequency (i.e. the absence of
a frequency effect), but have never before exhibited an actual
reversal of the tendency. The superiority of low frequency
word performance is particularly striking, as an advantage for
high frequency words is almost ubiquitous in clinical and
experimental studies of all components of the language
processing system.
Experiment 2: The effect of temporal factors
upon abstract word comprehension
In a number of previous word comprehension studies, we have
demonstrated that A.Z. exhibits the core features of a refractory access deficit. In particular, A.Z. shows a sensitivity to
temporal factors, with performance being facilitated by
increasing the length of the interval between making each
response and the presentation of the subsequent stimulus
(RSI). However, these demonstrations have been conducted
in the domains of concrete concepts and proper nouns. Therefore, in order to study semantic distance effects in A.Z.’s
comprehension of abstract words, it was considered necessary
to establish whether the influence of temporal factors was also
apparent in this domain of conceptual knowledge.
Methods
The stimuli consisted of 20 written abstract words, arranged into
five arrays each containing four items (e.g. wait, task, ache, stiff; see
Appendix 2). For each array, the name of every written word was
spoken by the examiner four times in a pseudo-random order. Following
the presentation of each spoken word target, A.Z. was requested to point
to the corresponding written word in the array. This procedure was
conducted twice for each array, once under a fast presentation rate (1 s
RSI) and once under a slow presentation rate (10 s RSI). The two
temporal conditions were alternated in an ABABA BABAB design.
Results
The results for each condition are shown in Fig. 1. A
Wilcoxon matched-pairs signed-ranks test showed that A.Z.
made significantly more errors in the fast condition than in the
slow condition (z = 2.50, P < 0.02; Fig. 1A). Having probed
each item four times, an item-specific analysis was conducted
to examine on which probe each error occurred. Serial position curves showing the percentage error rate on each probe
are given in Fig. 1B. In the fast condition, a Cochran Q test
revealed a significant difference in the number of errors
made on each probe [Q(3) = 16.80, P < 0.001]. This difference
reflects a gradual increase in error rate with successive probes
following initially accurate performance. In the slow condition however, A.Z.’s performance was almost at ceiling and
there was no significant difference in the number of errors
on each probe [Q(3) = 3.62, P > 0.3]. Thus, A.Z.’s ability to
comprehend the meaning of abstract words is characterized by
Abstract and concrete concepts
619
Fig. 1 Results from the abstract word refractoriness experiment (Experiment 2). (A) Percentage correct responses in the fast (1s RSI)
and slow (10s RSI) conditions. (B) Percentage error rate made by A.Z. in the fast (filled squares) and slow (open squares) conditions.
at least two of the core features of a refractory access disorder,
namely sensitivity to temporal factors and inconsistency.
Experiment 3: The influence of semantic
similarity upon abstract adjective and
verb comprehension
divine, sacred, religious, holy). The same words were also arranged
into four semantically unrelated arrays, each containing one word
from each the semantically related arrays (e.g. divine, sparse, furious,
sudden). In an identical manner, the items within the verb set were
also arranged into both semantically related (e.g. kneel, sit, stand,
lie) and semantically unrelated written word arrays (e.g. kneel, sing,
bake, drive). The tasks were conducted as spoken word–written word
matching tests. Each item in an array was probed four times in a
pseudo-random order and a 1 s RSI was adopted. Comprehension
of each set of words was tested separately and, within each set, the
arrays were presented in an ABBA order.
The exploration of semantic organization in the context of
refractory access dysphasia has typically depended upon the
manipulation of semantic distance effects. In particular, it
has been shown that, despite accurate performance initially,
refractoriness builds up more quickly between semantically
related concepts than semantically unrelated concepts leading
to a higher overall error rate. Semantic similarity has been
demonstrated to affect concrete word comprehension performance not only in patient A.Z., but also in other semantic
refractory access patients (e.g. Forde and Humphreys, 1995;
Warrington and Cipolotti, 1996). However, equivalent effects
of semantic similarity in the abstract word domain have not
previously been investigated. In this experiment, comprehension of two classes of abstract words was investigated, namely
adjectives and verbs. Unlike in the concrete domain in which
‘semantic relationship’ is most frequently defined by membership of a common semantic category, few abstract concepts can
be classified in the same manner. Consequently, we examined
semantic distance among adjectives using synonyms derived
from a thesaurus to form semantically similar and semantically
dissimilar groups. By contrast, the verbs selected in this task
were not synonymous but were chosen because they formed
coherent, semantically related clusters of concepts.
The percentage correct responses for adjectives (both word
sets combined) and verbs under the conditions of semantic
relatedness and unrelatedness are given in Fig. 2A. Despite
the semantic similarity of words in the related arrays,
Wilcoxon matched-pairs ranked-sums tests revealed no difference between A.Z.’s ability to identify adjectives or verbs
under the two conditions (adjectives: z = 1.16, P > 0.2;
verbs: z = 0.93, P > 0.3). Serial position curves demonstrating
the percentage error rate for each probe of adjective and verb
items are shown in Fig. 2B. Cochran Q tests revealed that A.Z.
showed a significant difference in the number of errors made
on successive probes with both the adjectives [related: Q(3) =
29.40, P < 0.001; unrelated: Q(3) = 17.89, P < 0.001] and the
verbs [related: Q(3) = 8.48, P < 0.05; unrelated: Q(3) = 10.00,
P < 0.02]. Examination of Fig. 2B indicates that these differences reflect increasing error rates with repeated probes
following an almost ceiling level of performance on probe 1.
Methods
Comment
The stimuli consisted of two sets of 16 adjectives and one set of 16
verbs (see Appendix 3). The items within each adjective set were
arranged into four semantically related written word arrays (e.g.
Overall, the data reported in this experiment suggest that
refractoriness does not build up more quickly among semantically related abstract words more than semantically unrelated
Results
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S. J. Crutch and E. K. Warrington
Fig. 2 Performance on the adjective and verb spoken word–written word matching tasks (Experiment 3). (A) Percentage total correct
responses in the synonym and non-synonym conditions. (B) Percentage error rate on each successive probe of the test stimuli.
words. The equivalence of performance on related and unrelated arrays of both adjectives and verbs was unexpected, and
very different to the pattern of performance predicted for
concrete nouns.
Experiment 4: The influence of semantic
similarity upon abstract and concrete
word comprehension
In the context of a semantic refractory access dysphasia,
semantic distance effects are attributed to the abnormal, deleterious effects which activating a concept has upon other
concepts that partly share neural space. Thus, the failure to
observe semantic distance effects among abstract words in
Experiment 3 was highly surprising, not only because such
effects are almost ubiquitous within the concrete domain, but
because the observation implies that abstract words with similar meanings do not necessarily share neural space. In this
experiment, the influence of semantic similarity upon A.Z.’s
comprehension abilities was further investigated by comparing and contrasting abstract and concrete words. Given
that there are very few adjectives with high concreteness
values, the abstract words chosen for this experiment were
mainly nouns and verbs.
Methods
The stimuli consisted of two sets of abstract words (see Appendix 4)
and two sets of concrete words (see Appendix 5). Within each set
of abstract words, the items were arranged into semantically synonymous written word arrays each containing four abstract synonyms
(e.g. deceit, trick, steal, cheat). The same words were also arranged in
semantically non-synonymous arrays consisting of one word from
each of four synonymous arrays (e.g. deceit, strike, mush, screen). In
an identical fashion, the items within each set of concrete words were
arranged into four written word arrays in which each item was drawn
from the same semantic category (e.g. goose, pigeon, crow, sparrow).
The same concrete words were also arranged into four different
semantic category arrays containing one item from each of the
same category arrays (e.g. goose, melon, pullover, biscuit). All
other aspects of the experimental design were identical to the procedure employed in Experiment 3.
Results
The combined percentage correct responses for the synonymous and non-synonymous abstract word conditions are shown
in Fig. 3A. A Wilcoxon matched-pairs signed-ranks test
revealed no difference in response accuracy between the
synonymous and non-synonymous array conditions (z =
0.22, P > 0.8). Serial position curves demonstrating the percentage error rate for each probe of the abstract words are also
shown in Fig. 3A. Cochran Q tests revealed that A.Z. showed
a significant difference in the number of errors made on
successive probes [synonyms: Q(3) = 17.48, P < 0.001;
non-synonyms: Q(3) = 12.00, P < 0.01]. Inspection of the
serial position curves suggests that these differences reflect
an increase in the percentage error rate across consecutive
probes of the test stimuli for both synonymous and nonsynonymous conditions alike.
The percentage correct responses on the concrete word
stimulus sets are shown in Fig. 3B. The results for both
word sets were considered together. A Wilcoxon matchedpairs signed-ranks test showed that A.Z. was significantly less
accurate at pointing to the target word in same semantic
category arrays than different semantic category arrays
(z = 3.62, P < 0.001). Serial position curves showing the
percentage error rate for successive probes of the test items
under both same category and different category conditions
are also given in Fig. 3B. Cochran Q tests again demonstrated a significant difference in response accuracy between
probes under the same category condition [Q(3) = 14.76,
P < 0.01] and a trend towards a similar pattern in the
different category condition [Q(3) = 7.43, P < 0.1]. Both
these results describe a gradual increase in the error rate
across successive probes.
Abstract and concrete concepts
621
100
100
90
90
80
80
70
70
Error rate (%)
Correct responses (%)
A. Abstract words (semantic similarity)
60
50
40
60
Non-synonyms
40
30
30
20
20
10
10
0
Synonyms
50
0
Probe 1 Probe 2 Probe 3 Probe 4
Synonyms/Non-synonyms
B. Concrete words (semantic similarity)
100
100
90
90
80
***
70
70
Error rate (%)
Correct responses (%)
80
60
50
40
60
Different
40
30
30
20
20
10
10
0
Same
50
0
Probe 1 Probe 2 Probe 3 Probe 4
Samecategory/Differentcategory
C. Abstract words (semantic association)
100
100
90
90
80
70
***
70
Error rate (%)
Correct responses (%)
80
60
50
40
60
Non-associated
40
30
30
20
20
10
10
0
0
Probe 1 Probe 2 Probe 3 Probe 4
Associated/Non-associated
D. Concrete words (semantic association)
100
100
90
90
80
80
70
70
Error rate (%)
Correct responses (%)
Associated
50
60
50
40
60
Non-associated
40
30
30
20
20
10
10
0
Associated
50
0
Associated/Non-associated
Probe 1 Probe 2 Probe 3 Probe 4
Fig. 3 Performance on the abstract and concrete spoken word–written word matching experiments. Percentage total correct responses and
the serial position curves showing the percentage error rate on successive probes of the test stimuli in close (filled squares) and distant (open
squares) arrays are shown. (A) Abstract word synonyms experiments (Experiment 4). (B) Concrete categories experiments (Experiment 4).
(C) Abstract word associates experiments (Experiment 5). (D) Concrete word associates experiments (Experiment 5).
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S. J. Crutch and E. K. Warrington
Comment
The evidence presented in this experiment strongly suggests
that refractoriness does not build up more quickly among
abstract words which have similar meanings than among
those which do not. By contrast, significant effects of semantic
relatedness were observed in the comprehension of more concrete words. The surprising absence of the semantic relatedness effect among abstract words cannot be attributed to the
grammatical class of the abstract word stimuli in question as
equivalent null results were obtained using adjectival stimuli
as when using noun and verb stimuli.
Experiment 5: The influence of semantic
association upon abstract and concrete
word comprehension
Having established that abstract word representations are
unaffected by semantic similarity (i.e. category) relative to
concrete words, we evaluated an alternative potential principle of organization: semantic association. Associated words
are those whose meanings are not synonymous but which are
often bound together in real world or sentential contexts
(e.g. salute, army, general, respect). The significance of associative links between concepts was examined using the same
spoken word to written word matching technique described
in Experiments 3 and 4.
Methods
The stimuli consisted of two sets of 16 abstract words (see
Appendix 6) and two sets of 16 concrete words (see Appendix 7).
For all stimulus sets, the items were arranged into arrays each
containing four associated but non-synonymous words (e.g. abstract:
exercise, healthy, fitness, jogging; concrete: farm, cow, tractor, barn).
The same words were also arranged into four non-associated arrays
with one word drawn from each associated array (e.g. abstract:
exercise, gamble, punch, future; concrete: farm, sailor, shelf, oven).
As before, the tasks were conducted as spoken word–written word
matching tests. Each item in an array was probed four times in a
randomized sequence, using a 1 s RSI. Comprehension of each set
of words was tested separately and, within each set, the arrays were
administered in an ABBA design.
Results
The percentage correct responses (sets 1 and 2 combined)
for the abstract words under both associated and nonassociated array conditions are shown in Fig. 3C. A Wilcoxon
matched-pairs signed-ranks test showed that A.Z. showed a
difference between conditions with significantly worse performance on the associated word arrays than non-associated
word arrays (z = 4.23, P < 0.001). Serial position curves
showing the percentage error rate across the four probes of
each test item are also given in Fig. 3C. Cochran Q tests
demonstrated a strong trend towards different accuracy levels
between probes in the associated array condition [Q(3) = 6.48,
P < 0.1] and a significant difference between probes in the
non-associated condition [Q(3) = 15.67, P < 0.01]. Inspection
of the serial position curves suggests that both these effects
correspond to an increase in the percentage error rate across
consecutive probes of the test stimuli.
The percentage correct responses on the concrete word
stimulus sets are shown in Fig. 3D. The results for both word
sets were considered together. A Wilcoxon matched-pairs
signed-ranks test revealed no significant difference between
the associated and non-associated conditions (z = 0.23, P >
0.8). As with the abstract word stimuli, serial position curves
showing the percentage error rate for successive probes of the
test items are also given in Fig. 3D. Cochran Q tests also
demonstrated significant differences in response accuracy
between probes under both the associated array condition
[Q(3) = 15.42, P < 0.01] and the non-associated array
condition [Q(3) = 22.83, P < 0.001]. Both these results
describe a gradual increase in the error rate across successive probes.
Comment
The significant effect of semantic association upon the comprehension of abstract words makes a striking contrast to the
absence of semantic similarity effects in the abstract synonyms tasks. It appears that refractoriness builds up much more
easily among associated than synonymous abstract words,
implying that the representations underpinning associated
abstract words are much closer neural neighbours than the
representations which support words which have very similar
meanings. In the concrete domain, evidence of the reverse
pattern has been documented. The absence of an effect of
semantic association upon the comprehension of concrete
words stands in contrast to the strong effect of semantic
similarity. Thus these results give further strength to the
claim that, unlike abstract concepts, concrete concept representations are organized by semantic category and not by
semantic association.
Discussion
The observation and manipulation of semantic relatedness
effects in patients with semantic refractory access disorders
enable the investigation of principles governing the organization of semantic memory. It is held that refractoriness may be
observed between related concepts because the activation
of one representation leads to the partial activation of other
representations that share neural space. In previous reports
of our patient A.Z., we have described a number of such
effects in the conceptual domains of concrete items and proper
nouns (Crutch and Warrington, 2003a, 2004, 2005;
Warrington and Crutch, 2004). In this paper, we have
extended this approach to the rarely studied domain of
abstract words. Sets of synonymous abstract words were
selected to be analogous to the semantically related categorical groupings of concrete items employed in earlier studies.
However, to our great surprise, we were entirely unable to
detect semantic relatedness effects among synonymous
abstract word stimuli. This unexpected absence of a relatedness effect was documented for several sets of stimuli. It was
Abstract and concrete concepts
demonstrated that adjectives (Experiment 3), verbs (Experiment 3) and also mixed noun and verb parts of speech arrays
(Experiment 4) were all equally immune to the effect. By
contrast, using an identical spoken word–written word matching procedure, strong semantic distance effects continued to
be observed among concrete words (Experiment 4). The
unpredicted null results led us to consider alternative forms
of relationship between abstract words and, in particular, contextual associative relationships. On a task comparing A.Z.’s
ability to identify associated and non-associated abstract
words, A.Z. showed significantly worse performance in the
associated condition (Experiment 5). However, remarkably
no effect of semantic association was detected on an equivalent task involving concrete words (Experiment 5). Thus, we
have documented evidence of a double dissociation between
abstract words which show an effect of semantic contextual
association but not semantic similarity, and concrete words
which show an effect of semantic similarity but not semantic
association. On the basis of these contrasting patterns of
results, we would claim that abstract concepts are represented
in an associative neural network whereas concrete concepts
have a categorical organization. Indeed, while some researchers have claimed that concrete concepts are imbedded in a
hierarchy of superordinate and subordinate representations,
this principled organization appears not to apply to a more
abstract vocabulary. We would predict that these contrasting
organizations must be reflected in the neural substrate of
conceptual knowledge.
In this discussion, we first consider existing views in the
cognitive neuroscience literature upon the difference between
abstract and concrete concepts before considering the implications of our own results. In both of these considerations,
we make a clear distinction between theories of how the
abstract–concrete difference is acquired and theories of
how that difference is represented.
It has been claimed that the abstract–concrete difference
reflects the manner in which these concepts are acquired
and the roles they play in language (Breedin et al., 1994).
Experience of the perceptual features of objects via our
five sensory channels appears to play a key role in the acquisition of concrete concepts (e.g. Landau et al., 1992,
1998). Abstract concepts, however, may be acquired in
the context of language without any direct perceptual
input. As a consequence, Breedin et al. (1994) suggested
that: ‘exposure to multiple sentence contexts plays an important role in the development of representations for abstract
concepts’ (p. 650).
Consistent with this viewpoint, abstract words have been
claimed to have richer associations, and indeed more
meanings, than concrete words (Paivio et al., 1968; Schwanenflugel and Shoben, 1983). In particular, the meaning of
concrete words (e.g. horse) tends to be constant across most
contexts, whereas abstract words (e.g. appeal) often have
related but distinct meanings depending on the context in
which they are being used. Experimental support from this
notion comes from Schwanenflugel and colleagues who in a
623
series of timed comprehension, naming and meaningfulness
judgement tasks showed that the provision of sentence or
paragraph contexts eliminated the traditional bias toward
faster responses with concrete items (Schwanenflugel and
Shoben, 1983; Schwanenflugel et al., 1988, Schwanenflugel
and Stowe, 1989).
The question of whether abstract words are represented in
a qualitatively or quantitatively different way from concrete
words has been considered as an explanation of some abstractness effects (e.g. abstract word anomia; Franklin et al., 1995).
However, theoretical accounts of conceptual organization have
tended to either concentrate upon quantitative differences
between the two types of words or to only model concrete
concepts to the exclusion of abstract knowledge. For example,
Plaut and Shallice (1993) hypothesized that the difference was
a reflection of concrete words being supported by a greater
number of semantic features. To examine this notion, they
developed a connectionist model of oral reading via semantics
in which lesions at any locus produced the typical concreteness effect (i.e. more errors for abstract words). However, this
result would appear to be unsurprising given that an a priori
decision was taken to train the model on a greater number of
semantic features for concrete than abstract terms. By contrast, the model of Allport (1985) only considers meaning for
imageable words, positing the existence of attribute domains
which correspond to different sensory properties. For those
concrete concepts for which the model does offer a system of
organization, the underlying representational framework is
held to be associative in nature, with connections between
distributed features according to their co-occurrence.
To our knowledge, only one previous report has claimed, as
we do, that the representational difference between abstract
and concrete words might be qualitative rather than quantitative (Breedin et al., 1994). However, these researchers characterized the difference in terms of discrepancy in the number
of perceptual and functional features required to distinguish
abstract and concrete concepts. Our results suggest that irrespective of the type of information which constitutes abstract
and concrete concepts, a much more fundamental difference
exists in the actual architecture of those representations.
The evidence that the spread of refractoriness between concrete
concepts is exacerbated by semantic similarity but not semantic
association confirms Warrington’s prediction that in the
domain of concrete concepts, the primary organizational principle is categorical and not associative (Warrington, 1981). By
contrast, it appears that the representations of abstract concepts are predominantly linked to other associated rather than
semantically similar items. This conclusion is compatible
with a wide range of previous observations about the effects
of word concreteness, particularly those of Schwanenflugel
and colleagues detailed above. We would regard the facilitatory effect of sentential contexts as a further example of the
importance of the associative links between abstract concepts.
However, we should be careful to note that, given the probable
existence of a continuum between abstractness and concreteness, we suggest this associative/categorical dichotomy is
624
S. J. Crutch and E. K. Warrington
relative rather than absolute. More specifically, we predict
that middle concreteness items (e.g. nurse, chemistry) have
both associative and categorical connections, in more equal
proportions than concepts at either end of the spectrum. It is
difficult to envisage how existing models of conceptual
knowledge, which have mainly been derived from work on
category-specific deficits of concrete item knowledge, can be
extended to incorporate our findings on abstract words. Essentially, our findings suggest that attempting to model conceptual knowledge within a unitary system based on a single set
of network principles is over simplistic.
With regard to the development of different representational frameworks for concrete and abstract concepts, we
agree with Breedin et al. (1994) that multiple contexts may
play a major role. It is notable that in a dictionary, concrete items
typically have only a few fixed meanings (e.g. tomato occurs in
the context of gardening and eating). On the other hand, dictionaries tend to describe abstract words as having multiple meanings depending upon context (e.g. the word ‘set’ has >30
transitive and intransitive senses). Furthermore, the psycholinguistic literature makes a clear distinction between two types of
‘multiple meaning’, namely homonyms which have clearly
distinct, unrelated meanings (e.g. ‘clog’ = Dutch footwear, to
block up, etc.) and polysemous words which have multiple,
semantically related senses (e.g. ‘pain’ = punishment, grief,
to cause distress, etc.). Recent evidence from lexical decision
tasks in normal subjects has demonstrated the importance of
this latter distinction by showing a processing disadvantage
for words which have multiple unrelated meanings, but a processing advantage for words which have multiple related
senses (Rodd et al., 2002, 2004). However, it should be
noted that some researchers have suggested that all words
are polysemous to some extent, because their precise
meanings change in every unique context in which they appear
(Anderson and Nagy, 1991). We would argue that an associative network may have a greater capacity to represent the
multiple meanings of abstract words than a more formal, rigid
categorical network. Such a representational framework
also possesses the flexibility to capture the natural variability
in the semantics of both words with unrelated meanings and
words with related senses.
One further finding which is of particular note is that
our patient A.Z. not only exhibited a commonly observed
concreteness effect on written word identification tasks, but
also showed some evidence of an inverse frequency effect
(Experiment 1). An absence of an advantage for high
frequency words has been previously reported in several
studies of both refractory access patients (e.g. Warrington
and Cipolotti, 1996) and patients with static storage impairments of various components of the language system
(e.g. Hillis et al., 1990; Nickels and Howard, 1995). Indeed,
in another study of our patient A.Z., frequency effects
were noted to be absent on a spoken word–picture matching
task (Crutch and Warrington, 2004b). However, to the best
of our knowledge, A.Z. is the first patient on record to demonstrate significantly better performance with less commonly
occurring stimuli. One previous patient exhibiting a predisposition for low frequency word production has been
reported, but this was in the context of a classical jargon
aphasia in which the nature and information processing
locus of the impairment was unclear (Marshall et al.,
2001). The absence of frequency effects in refractory access
patients has been attributed to an impairment of the neuromodulatory system leading to higher than normal levels of
synaptic depression following the activation of a representation. Frequency effects are reduced or absent because synaptic
depression is greater when activity levels are higher, as in
the case of high frequency items (Gotts and Plaut, 2002).
We would claim that this explanation can be extended to
an actual inversion of the frequency effect; using very low
and very high frequency stimuli as in Experiment 1 may
generate a sufficiently large activity discrepancy as to
generate significantly more synaptic depression following
high frequency than low frequency word identification. It
should again be stressed that the inversion of the frequency
effect in A.Z. is not necessarily reliable and is very much
stimulus-dependent. On this account, an absence rather
than an inversion of the frequency effect was observed in
our earlier study of the same patient (Crutch and Warrington,
2004b) because the (pictureable) stimuli invoked a smaller
frequency range.
To summarize, we have reported a series of experiments
which indicate that abstract conceptual knowledge relies upon
qualitatively different representations than concrete conceptual knowledge. We would suggest that a deeper understanding of abstract word semantics and their relationship to other
aspects of the language system is long overdue, particularly
bearing in mind the importance of an abstract vocabulary to
our ability to communicate and produce fluent propositional
speech (Crutch and Warrington, 2003b). Overall, studies of
concrete word representations have yielded valuable insights
into the structure of human semantic memory, but it is important to bear in mind that they concentrate upon only a fraction
of our total vocabulary.
Acknowledgements
We wish to thank Dr D Cohen for allowing us to study a
patient under his care. We are also indebted to Katie Masters
for her kind assistance and for providing us with language
assessment information.
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Appendix 1 Stimuli contained in both Set A
and Set B of the abstractness 3 frequency
matrix task (Experiment 1)
Very low abstractness words (concreteness rating 200–300)
Low frequency
Middle frequency
High frequency
Set A
Set B
Set A
Set B
Set A
Set B
avowal
excise
truism
feint
savant
levity
enigma
alacrity
analogy
overtone
inaction
heroism
alias
unbelief
impetus
bravery
dotage
impunity
throe
pacifism
infinity
clemency
sagacity
jeopardy
atrocity
reprisal
sanctity
protocol
craven
betrayal
heresy
aptitude
concert
motive
boast
tendency
loyalty
virtue
distress
wisdom
mercy
magic
upset
talent
dread
impulse
neglect
dignity
approval
misery
honesty
anxiety
devotion
economy
sorrow
patience
extreme
despair
estimate
nonsense
capacity
reality
extent
mystery
advice
emotion
blame
ideal
pride
quality
instance
memory
purpose
truth
effort
guess
feeling
chance
while
other
respect
delight
attitude
value
variety
trust
opinion
spite
success
spirit
manner
effect
cause
least
might
thought
626
S. J. Crutch and E. K. Warrington
Appendix 3 Adjective and verb stimulus
arrays used in Experiment 3
Appendix 1 Continued
Low abstractness words (concreteness rating 300–400)
Low frequency
Middle frequency
High frequency
Set A
Set B
Set A
Set B
Set A
Set B
emission
calculus
prefix
tenure
stoic
regency
sobriety
jargon
adultery
bequest
context
acidity
interim
foible
dogma
mutiny
stipend
pommel
pronoun
perjury
sequel
axiom
legality
truce
carnage
rigidity
velocity
edict
deity
mirage
equity
agility
culture
retreat
humanity
welfare
flutter
crisis
victory
portion
incline
bargain
proof
poverty
drama
budget
incident
genius
insult
skill
rescue
threat
session
amateur
transfer
menace
comedy
grief
angel
division
research
liberty
reaction
reward
profit
charm
fortune
strain
fashion
strength
relief
amount
beauty
result
number
problem
reason
answer
moment
right
claim
distance
century
occasion
protest
pleasure
peace
health
position
marriage
dream
month
sense
trouble
minute
business
Mild abstractness words (concreteness rating 400–500)
Low frequency
Middle frequency
High frequency
Set A
Set B
Set A
Set B
Set A
Set B
buffoon
zenith
vitriol
pestle
galaxy
abyss
dynasty
venom
cuisine
mileage
alphabet
segment
shale
quake
brawl
filth
oxide
curfew
molecule
abbess
mermaid
fissure
glutton
ignition
psalm
donor
citation
mosque
maize
rogue
microbe
sedative
shriek
comrade
fairy
embrace
saint
stalk
adult
pioneer
scent
device
signal
groan
angle
route
holiday
interior
footstep
craft
ceremony
grate
rebel
deposit
glitter
resort
autumn
disaster
lecture
exhaust
damage
bunch
shrug
payment
speech
patient
murder
income
shadow
breath
whisper
report
summer
notice
figure
party
matter
laugh
evening
night
leader
degree
object
accident
youth
design
meeting
travel
general
enemy
group
fight
story
country
voice
friend
Appendix 2 Stimulus arrays tested under
both fast and slow presentation conditions
(Experiment 2)
rest
job
firm
sore
ache
tall
chore
halt
tough
steep
wait
hurt
work
pain
high
stiff
great
task
hard
stop
Adjectives (Set 1)
Synonymous arrays
divine
scant
sacred
bare
religious
sparse
holy
deficient
Non-synonymous arrays
scant
furious
sacred
rash
annoyed
deficient
unexpected
divine
Adjectives (Set 2)
Synonymous arrays
hurried
silent
fast
quiet
quick
peaceful
speedy
still
Non-synonymous arrays
merry
silent
foolish
stupid
quick
glad
still
hurried
Verbs
Semantically related arrays
sing
kneel
shout
sit
laugh
stand
talk
lie
Semantically unrelated arrays
sing
shout
kneel
sit
bake
cook
drive
fly
enraged
furious
annoyed
angry
sudden
abrupt
rash
unexpected
sparse
holy
enraged
abrupt
angry
sudden
bare
religious
stupid
dense
thick
foolish
glad
happy
cheerful
merry
dense
quiet
happy
fast
speedy
thick
cheerful
peaceful
bake
cook
boil
fry
drive
fly
sail
ride
stand
fry
laugh
sail
ride
talk
boil
lie
Appendix 4 Stimulus arrays probed in
synonymous and non-synonymous conditions
of the abstract synonym task (Experiment 4)
Set 1
Synonymous arrays
boil
look
heat
peek
cook
glance
fry
see
Non-synonymous arrays
boil
heat
look
peek
gale
wind
clean
eat
Set 2
Synonymous arrays
beat
mask
knock
cloak
thump
hide
strike
screen
gale
wind
storm
breeze
clean
neat
pure
fresh
eat
chew
bite
taste
neat
chew
fry
glance
cook
storm
pure
bite
see
taste
fresh
breeze
cheat
trick
steal
deceit
soft
mush
pulp
smooth
Abstract and concrete concepts
Appendix 4 Continued
Non-synonymous
beat
cloak
cheat
soft
arrays
smooth
mask
trick
knock
Appendix 6 Continued
thump
hide
steal
pulp
screen
strike
deceit
mush
Appendix 5 Stimulus arrays probed in same
category and different category conditions of
the two word sets in the concrete categories task
(Experiment 4)
Set 1
Same category arrays
goose
cardigan
crow
jacket
sparrow
blouse
pigeon
pullover
Different category arrays
goose
banana
melon
cardigan
pullover
pizza
biscuit
sparrow
Set 2
Same category arrays
kettle
rabbit
toaster
hedgehog
oven
squirrel
fridge
badger
Different category arrays
squirrel
badger
onion
rose
toaster
parsnip
pansy
fridge
pizza
cake
bread
biscuit
melon
grapes
peach
banana
pigeon
bread
jacket
grapes
cake
blouse
crow
peach
leek
carrot
onion
parsnip
daffodil
tulip
rose
pansy
rabbit
kettle
daffodil
leek
tulip
oven
carrot
hedgehog
Appendix 6 Stimulus arrays probed in
associated and non-associated conditions of
the two word sets in the abstract associates
task (Experiment 5)
Set 1
Associated arrays
exercise
healthy
fitness
jogging
fight
punch
violent
struggle
627
gamble
casino
poker
chance
future
past
always
present
Non-associated arrays
gamble
casino
future
past
exercise
healthy
punch
fight
Set 2
Associated arrays
witch
gallery
spell
sculpture
magic
artist
curse
exhibition
Non-associated arrays
witch
gallery
exhibition
magic
funny
abroad
travel
laugh
poker
always
fitness
violent
chance
present
jogging
struggle
comedy
joke
laugh
funny
journey
travel
abroad
holiday
joke
sculpture
curse
journey
holiday
artist
comedy
spell
Appendix 7 Stimulus arrays probed in
associated and non-associated conditions of
the two word sets in the concrete associates
task (Experiment 5)
Set 1
Associated arrays
farm
oven
cow
apron
tractor
kitchen
barn
soup
Non-associated arrays
sailor
boat
shelf
wall
farm
cow
oven
apron
Set 2
Associated arrays
monkey
airport
cage
plane
banana
suitcase
zoo
ticket
Non-associated arrays
tent
zoo
monkey
plane
suitcase
canvas
brush
fire
sailor
boat
anchor
sea
shelf
wall
book
wood
anchor
book
tractor
kitchen
sea
wood
barn
soup
picture
brush
paint
canvas
tent
camp
woods
fire
paint
airport
woods
banana
picture
camp
ticket
cage