When space merges into language

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Abstract

We present data from right brain-damaged patients, with and without spatial heminattention, which show the influence of hemispatial deficits on spoken language processing. We explored the findings of a previous study, which used an emphatic stress detection task and suggested spatial transcoding of a spoken active sentence in a ‘language line’. This transcoding was impaired in its initial portion (the subject-word) when the neglect syndrome was present. By expanding the original methodology, the present study provides a deeper understanding of the level of spoken language processing involved in the heminattentional bias. To ascertain the role played by syntactic structure, active and passive sentences were compared. Sentences comprised of musical notes and of a sequence of unrelated nouns were also compared to determine whether the bias was manifest with any sequence of events (not only linguistic ones) deployed over time, and with a sequence of linguistic events not embedded in a structured syntactic frame. Results showed that heminattention exerted an influence only when a syntactically structured linguistic input (=sentence with agent of action, action and recipient of action) was processed, and that it did not interfere when a sequence of non-linguistic sounds or unrelated words was presented. Furthermore, when passing from active to passive sentences, the heminattentional bias was inverted, suggesting that heminattention primarily involves the logical subject of the sentence, which has an inverted position in passive sentences. These results strongly suggest that heminattention acts on the spatial transcoding of the deep structure of spoken language.

Introduction

The spatial dimension has been so relevant in the evolution of the nervous system that sensory and motor processing cannot take place unless their spatial co-ordinates are coded. Coslett (1999) claimed that the influence of space also holds for higher-order more abstract functions (including language), which are less obviously related to space.

Coslett's (Coslett, Schwartz, Goldberg, Haas, & Perkins, 1993) considerations originated from clinical observations carried out on an aphasic patient, J.F., whose anomia became more or less severe depending on the hemifield stimuli were presented to. According to Coslett (1999), the importance of spatial information for every action we perform is so strong that each stimulus we perceive is automatically marked with reference to its co-ordinates in the egocentric space, even if spatial information does not seem relevant for the task at hand (“Spatial Registration Hypothesis”). Following this reasoning, any stimulus presented in the contralesional field (for patient J.F., the right hemispace) should activate the left hemisphere attentional system for the impaired right hemispace. This explains J.F.'s worse performance with stimuli in the contralesional field, while his performance in the left hemispace was supported by his intact right hemisphere (Coslett et al., 1993).

This interpretation stresses the importance of the extra-personal, egocentric space linguistic stimuli arise from (Coslett, 1999). Other authors hypothesize that any event expressed through language, whether spoken or written, automatically activates an “internal” spatial representation.

The latter explanation is postulated by Chatterjee, Maher, Gonzalez Rothi, and Heilman (1995b). They described an agrammatic patient, W.H., who assigned the thematic roles of agent and recipient of a given action on the basis of a spatial or temporal strategy. For example, in describing a picture, this patient was asked to report who did something to whom (agent and recipient of action). He tended to consider the figure located on the left as the agent of action (spatial strategy), or to match the first noun he heard with the figure located on the left (temporal strategy) (Chatterjee et al., 1995b).

According to Chatterjee et al. (1995b), conceptual knowledge of thematic roles is actually independent of language, and is matched with linguistic content only subsequently, by means of the grammatical and syntactic algorithms specific to one's own language. Due to a brain lesion, these linguistic algorithms were impaired in their agrammatic patient. Therefore, he based his thematic role detection exclusively on the automatic spatial representation of events, which proceeds naturally from left (agent of action) to right (recipient of action), with the verb being the vector defining the thematic roles and supporting the whole event frame.

To further support this idea, Chatterjee et al., 1995a, Chatterjee et al., 1999 asked non brain-damaged subjects to draw an assigned thematic role. Their reasoning was that if there is a spatial representation underlying a spoken sentence, and an “implicit propensity” to conceive events as traversing space from left to right, with the agent of the action on the left of this representational space, the subjects should draw a given action proceeding from left to right. They showed that the majority of subjects located the agent closer to the left margin of the page, and the recipient of the action closer to the right margin of the page (Chatterjee et al., 1995a, Chatterjee et al., 1999).

For Chatterjee and colleagues, the presence of a spatial representation underlying spoken language reflects the primitive way the mind conceptualizes and represents events, a way that has been successively obscured—but not deleted—during the course of development by the evolution of language. Analogously, passing from phylogenesis to ontogenesis, Mandler (1996) observed that in the young child emerging linguistic concepts are mapped on a pre-existing conceptual system, which has space as a fundamental constituent dimension.

If space and language interact at some still unidentified level, forming the spatio-linguistic representation outlined above, it is theoretically possible to observe representational abnormalities as a consequence of (1) a brain lesion modifying spatial processing and (2) a brain lesion modifying linguistic processing in two opposite, but converging, lines of action.

Some evidence for the latter is provided by the two aphasic patients described above, i.e., by Coslett et al. (1993) and by Chatterjee et al. (1995b). Instead, some evidence for the former is provided by two patients with hemispatial neglect, one described by Baxter and Warrington (1983) and the other by Caramazza and Hillis (1990).

Patient O.R.F., described by Baxter and Warrington (1983), underwent two consecutive vascular accidents within a short period of time. The first parietal lesion caused conduction aphasia (the patient was left-handed), phonological dyslexia, and mild neglect dyslexia, without other signs of spatial neglect; the second vascular event, again striking the right parietal region, produced an additional impairment of word-spelling abilities that was not present before. The word–spelling deficit was observed only for letters at the beginning of words; for example, if the examiner spelled the word “b-e-g-g-a-r” aurally letter by letter, the patient reported “vinegar” (Baxter, & Warrington, 1983).

Interestingly, this bias was manifested when spelling proceeded either from the beginning to the end of the word, or from the end to the beginning of the word. Baxter and Warrington (1983) hypothesized that the patient was representing the stimulus on an inner screen, and the symptoms resulted from an interaction between representational neglect and neglect dyslexia. The authors affirmed that the patient reported “reading” the aurally spelled words on an “inner screen”.

Instead, patient N.J., described by Caramazza and Hillis (1990), exhibited severe neglect affecting the right half of space after a lesion to the left hemisphere; he had no language deficits. N.J. was submitted to several reading and spelling tests: horizontal reading, vertical reading, mirror-reverted reading, delayed copy, aural spelling, recognition of aural spelling, written spelling and backwards written spelling. When reading, the patient made errors exclusively at the end of words, irrespective of the form of the input (vertical, horizontal or mirror reversed). Also, on the spelling tasks errors appeared only at the right end of a word (N.J. reported “exceed” after the examiner spelled “e-x-c-e-s-s“), irrespective—also for spelling—of the form or order of the input (written and aural spelling, forward and backward spelling). According to Caramazza and Hillis (1990), reading and spelling both involve a common pre-lexical level of representation, which they call “grapheme description”. At this level of representation, the abstract identity of the single letters compounding the word is computed, regardless of the form/orientation of the input; therefore, damage to this format-independent level would impair both reading and spelling in a similar way and in conformity with the heminattentional deficit.

Since the spelling tasks were performed aurally, the behavior of both patients strongly suggests a spatial transcoding of spoken language which, given its spatial format, undergoes the same bias observed in neglected patients for visuo-spatial stimuli in the contralesional hemispace.

The clinical reports of these two patients document the influence of hemineglect on spoken language processing, but with reference to a very peculiar type of spoken language: spelling. Although spelling is performed aurally, in many respects it is actually very close to written language. This is probably why Baxter and Warrington defined the deficit described in their patient as a rare type of “dysgraphic syndrome”, highlighting with the term “dysgraphic” the affinity of spelling and written language.

With regard to the effects of hemineglect on genuine spoken language, a neglect patient -J.L.- was reported by Barbut and Gazzaniga (1987). As a consequence of a right parietal lesion, in addition to errors affecting the most leftward letters of words in reading, writing and spelling, J.L. also produced errors in spontaneous speech. For example, he often omitted the beginning of a word, saying “bulance” instead of “ambulance”, or “portant” instead of “important”. Most of his errors were omissions, but the examiner also observed that the patient often made substitutions, forming semantically unrelated words. According to Barbut and Gazzaniga (1987), the pattern of errors exhibited by their patient was consistent with the hypothesis that some aspects of language processing involve some spatial mechanisms responsible for the representation of what they call “conceptual space”.

Recently, a decade after the literature reported above, a paper by Rinaldi, Marangolo, and Pizzamiglio (2003) added another piece of evidence to this field of investigation. The authors described a different type of hemineglect effect on the processing of spoken language than that previously observed. When asked to compare two spoken sentences (subject/verb/object) with regard to their emphatic stress, patients with right hemisphere lesions and spatial neglect made significantly more errors when the stress was placed at the beginning of the sentence (on the subject-word). The bias exhibited by neglected patients towards the initial word of the sentence suggests a transcoding of spoken language into a spatial format. The auditory trace left by the spoken sentence would be automatically transformed into an analog with an intrinsic spatial component. This analog would then be affected by the presence of hemineglect in the same way this deficit affects spatial information processing, which is on the contralesional side. In other terms, it appears that the word at the beginning of the sentence falls on the “left” side of the spatial transcoding of the sentence and is, therefore, neglected similar to the way left visuo-spatial information is neglected by these patients in the external space.

The present investigation aims at expanding the original methodology and qualifying the nature of the interaction between the spatial and linguistic mechanisms underlying the observed effect.

As in the previous experiment, the first question concerns the use of a unique format, i.e., active sentence structure. The replication or the change in the results using active and passive sentences will contribute to understanding whether the effect is related to the position of the stress in the auditory string of words or, instead, to its syntactic structure.

The second question is whether the comparison of the stress location in the two sentences produces a bias in hemineglect patients only when linguistic strings are used, or whether, instead, it takes place also when two non-linguistic strings of acoustic events are compared. In other words, if the bias in hemineglect patients can also be observed when they compare the stress embedded in two musical sentences, the effect can be attributed exclusively to the need to locate a target stimulus in a sequence of events spaced over time.

Third, do the stimuli need to be meaningful sentences or can the same effect of neglect be observed with a sequence of non-related words? Does neglect affect the representation of the event expressed in the spoken sentence, where a verb supports the event frame and defines the thematic roles, or does it affect any sequence of three nouns, independently of the presence of a verb?

Section snippets

Subjects

This study was carried out on 26 right brain-damaged subjects admitted to the Santa Lucia Foundation I.R.C.C.S. in Rome for rehabilitation (mean age 61.7 years, S.D. 11.37; education 9.7 years, S.D. 4.19) (see Table 1 for individual patients’ data).

For each patient, unilaterality of the brain lesion was documented by a CT or an MR scan. Each patient admitted to the hospital underwent a standard neuropsychological examination, which included a battery for the diagnosis of hemispatial neglect for

Active–passive linguistic condition

An ANOVA 2 × 2 × 2 was computed to compare the performance of the two groups of patients on the two sentence types, active and passive; factors used were “Group” (N−/N+ patients), “Sentence Type” (Type I/Type II) and “Condition” (active/passive). “Group” and “Condition” showed no main effects (p = 0.20 and p = 0.53); however, “Sentence Type” was significant (Type I: mean 19.16, S.D. 3.54; Type II: mean 18.34, S.D. 3.54) (F1,24 = 10.52, p = 0.003). The “Group” × “Sentence Type” interaction was not significant

Active versus passive linguistic condition

First, the present results provide additional support for the results obtained in our previous study (Rinaldi et al., 2003), confirming that neglect patients have a specific impairment in processing the initial part of an active sentence (positional bias), namely, the subject-word. In the present study, a noteworthy additional result emerged from a comparison of the active versus the newly introduced passive condition, which in neglect patients is the inversion of the positional bias. This

General discussion

Our data show that in brain-damaged patients an interaction exists between the presence of neglect and the type of task administered. The task consisted of comparing the stress positions in two heard sentences, a comparison, which directly involved the acoustic traces left by the spoken sentences in the working memory. When they were given two sequences of words to compare, patients with unilateral neglect made more errors when the cue (the stress) appeared in the initial part than in the

Acknowledgement

This research has been supported by grant of Ministero Italiano della Sanità.

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