Activation without Selection: Parallel Right Hemisphere Roles in Language and Intentional Movement?

57, 151–178 (1997) BL971837

BRAIN AND LANGUAGE ARTICLE NO.

Activation without Selection: Parallel Right Hemisphere Roles in Language and Intentional Movement? LORIE RICHARDS University of Kansas Medical Center AND

CHRISTINE CHIARELLO Syracuse University Because language and praxis each require the activation and selection of knowledge structures in long-term memory (MacKay, 1985, 1987), it is reasonable to consider whether hemisphere asymmetries for such processes span both domains. Language and skilled movement are thought to be strongly lateralized to the left hemisphere in most individuals. Yet, although recent evidence suggests that the right hemisphere also contributes to language use in context, few similar arguments have been made for the right hemisphere’s involvement in motor planning. In this paper, we review some of the evidence for a right hemisphere role in language and action processing and propose that within each domain the right hemisphere activates a range of relevant knowledge structures without selection.  1997 Academic Press

Language and skilled movement are functions traditionally attributed to the left cerebral hemisphere, primarily because aphasia and apraxia are much more common after left, compared to right, hemisphere injury. However, within the past 20 years, lateralization research with clinical (Joanette, Goulet, & Hannequin, 1990; Zaidel, 1983) and normal populations (Chiarello, 1988a, 1991) has suggested that the right hemisphere (RH) also plays an important role in the full expression of linguistic competence. The RH has been implicated in prosody (Ross, 1981), figurative language (Myers & Linebaugh, 1981), pragmatics and discourse processing (Gardner, Brownell, Wapner, & Michelow, 1983; Hough, 1990), and some aspects of lexical seThis paper was supported by a Career Development Award from the Center on Aging at the University of Kansas Medical Center to the first author and NIMH Grant MH43868 to the second author. Address reprint requests to Lorie Richards, Ph.D., Department of Occupational Therapy Education, 3033 Robinson, University of Kansas Medical Center, 3901 Rainbow Boulevard, Kansas City, KS 66160-7602. E-Mail: [email protected]. 151 0093-934X/97 $25.00 Copyright  1997 by Academic Press All rights of reproduction in any form reserved.

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mantic processing (Chiarello, 1988b). Thus, left hemisphere (LH) linguistic predominance does not imply RH linguistic incompetence. Rather, the view that emerges from this research is that the RH plays an important, if not absolutely crucial, role in many subtle aspects of language processing.1 In contrast, much less attention has been given to a potential RH role in skilled action and motor planning. This may be because neuropsychological research focuses on apraxia as a disorder of motor expression. However, praxis, no less than language, depends on a system of related concepts that are organized in memory and activated in the course of both perception and production (MacKay, 1985, 1987). In this paper, we will suggest that a RH contribution to skilled movement might be discernable if one could hone in on the interactions within long-term memory that initiate and organize movement planning in context. The assumption that similar processes operate in language and action comprehension and production is not new. Several models of action processing have been formulated that propose either parallel structure and processing in the language and action systems (MacKay, 1985; Rothi, Ochipa, & Heilman, 1991), or, at least, a shared conceptual system (Roy & Square, 1985; Roy & Hall, 1992). According to these models, comprehending and producing contextually appropriate intentional language and actions requires activating several different types of knowledge represented in long-term memory (MacKay, 1985; Rothi et al., 1991; Roy & Hall, 1992). Language and willful motor acts originate from shared intentions (MacKay, 1985, 1987). These intentions determine the goal of the act, but do not specify how the goal is to be realized. For example, the goal of having the door closed could be met by asking someone to close the door, requiring further activation of language representations. On the other hand, the same goal could be realized by shutting the door oneself, requiring further activation of motoric representations. Rothi et al. (1991) suggest a similar model; activated semantic information activates representations in either action output or language output lexicons. These output lexicons contain representations of motor programs for the output of either speech (phonology), writing (orthography), sign, or actions. Parallels have also been drawn between action and language comprehension. Rothi et al. (1991) suggest that both require access to a common semantic system. Both actions and language are thought to activate structural representations in modality-specific input lexicons. These representations in turn activate information in semantic memory in order to assign meaning to the structural descriptions (Riddoch & Humphreys, 1987). While it is possible to imitate both language and actions without accessing semantic memory, 1 In this paper we adopt a broad definition of language processing to include a range of verbal communicative functions including concept activation, text coherence inferencing, discourse processing, etc. We prefer the term grammar to refer to strict rule-governed linguistic processes such as phonology, syntax, and formal linguistic semantics.

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comprehension of these acts requires semantic activation (Rothi et al., 1991). Because it has been suggested that the comprehension and production of both language and action have similar bases, it is not unlikely that they utilize comparable processes in activating knowledge structures in long-term memory. In addition, comprehension and production of action and language depend on the ability to select stable patterns of activation and to suppress others within representational domains in long-term memory (MacKay, 1985; Roy & Hall, 1992). As reviewed below, selection appears to be predominantly a function of the LH in semantic comprehension. Therefore, it is reasonable to ask whether selection in action processing may also be carried out primarily by the LH. We begin by reviewing evidence relevant to the activation and selection of meanings within conceptual memory. Semantic priming studies investigating lateralization of comprehension processes in neurologically normal persons provide the most relevant data base, since hemisphere differences in activation/selection processes involved in language production have rarely been explored. We next summarize evidence for a RH role in language and communicative function based on studies of RH-injured patients, highlighting those findings potentially relevant to activation/selection processes. Evidence for RH involvement in action planning and comprehension is then discussed and potential parallels to the language findings are noted. While critical experiments are still needed, we argue that current data implicate comparable lateralization for activation and selection processes used in language and intentional movement. I. EVIDENCE FOR RH ROLE IN LANGUAGE PROCESSING

1. Hemisphere Asymmetries in the Activation and Selection of Word Meanings Faster and more accurate responses are made to target words when preceded by semantically related, as compared to unrelated, prime words. Such semantic priming is usually not larger for words presented to the LH, although right visual field/left hemisphere (RVF/LH) advantages are uniformly obtained in word recognition tasks (Chiarello, 1988a). Since greater priming is sometimes obtained for words presented to the RH, such priming can reveal unique RH language processes in the context of overall LH linguistic predominance (Chiarello, 1991). Semantic priming represents a class of effects which index a variety of meaning interactions that can occur when two words are processed in close temporal proximity (Neely, 1991). On the one hand, priming can reflect the passive spread of activation among related meanings in long-term conceptual memory (Collins & Loftus, 1975). Recognizing a word involves activating its corresponding meaning representation, making it more available for sub-

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sequent processing. Related word meanings will also be indirectly activated due to the passive spread of activation through the semantic network. Hence, responses to words can be facilitated when preceded by a semantically related word due to passive meaning activation processes. Priming attributable to meaning activation is more likely under the following conditions: primes are pattern masked or very briefly presented, short prime–target stimulus onset asynchrony (SOA), low proportion of related trials, and/or word pronunciation responses. On the other hand, semantic priming can also measure subsequent processes that (1) amplify or dampen this initial activation (i.e., meaning selection) and (2) compare or integrate the meanings of prime and target words. Thus, priming can be amplified (Neely, Keefe, & Ross, 1989) or obscured (Balota & Lorch, 1986) when the task favors the development of semantic expectancies or prime–target meaning comparisons (Neely et al., 1989) or when sufficient time has elapsed to allow meaning selection to occur (Simpson & Burgess, 1985). Such processes will produce facilitation for strongly related and/or contextually appropriate words, and may also produce slowing for unrelated, indirectly related, and/or contextually inappropriate target words (Balota & Lorch, 1986; Burgess & Simpson, 1988b; Neely, 1977). The following conditions will promote priming sensitive to processes beyond meaning activation: long duration/clearly visible primes, lengthy SOA, high proportion of related trials, lexical decision responses. Under conditions conducive to the measurement of passive meaning activation, most single word priming studies report equivalent priming across the visual fields (Chiarello, Burgess, Richards, & Pollock, 1990; Chiarello, Senehi, & Nuding, 1987; Eglin, 1987; Marcel & Patterson, 1978; Richards & Chiarello, 1995; Walker & Ceci, 1985—but see also Abernethy & Coney, 1990). Stimulus pairs in such studies consisted of strongly associated words, some of which were also members of the same semantic category (e.g., CAT–DOG). Thus, there is little evidence for hemisphere differences in the activation of strongly related meanings (see also Burgess & Simpson, 1988a; Nakagawa, 1991). In contrast, when stimuli are nonassociated category members (e.g., GOAT–DOG), there is evidence for meaning activation only within the LVF/RH (Chiarello, 1985; Chiarello et al., 1990; Chiarello, & Richards, 1992; Michimata, 1987). The latter effects are independent of whether the prime is a highly typical or atypical member of the category (Chiarello & Richards, 1992). Since strongly related words are activated to the same extent in each hemisphere, but nonassociated category members are activated only within the RH, this implies that words activate a broader set of related meanings within the RH. Thus, priming studies conducive to the measurement of passive meaning activation obtain bilateral effects or, when semantic overlap is not great, RH predominance. In contrast, many priming processes that may occur subsequent to initial meaning activation appear to depend on the LH. For example,

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when subjects make use of prime–target relations to generate expectancies or to make meaning comparisons, they will perform more poorly following unrelated than neutral primes (Neely, 1977, 1991). This reaction time (RT) cost, or inhibition, is usually obtained only for words presented to the RVF/ LH (Chiarello et al., 1987; Michimata, 1987; Nakagawa, 1991; but see also Chiarello, Richards, & Pollock, 1992). This suggests that the failure to detect a semantic relation between two successive words is more disruptive to LH, than to RH, processing. Additionally, Nakagawa (1991) reported that RT costs were also obtained for ‘‘remotely’’ associated words (e.g., SHOES– TIGHT) at a long (750 msec) SOA, but only for RVF/LH trials. Thus, the LH may be biased toward detecting close semantic relations (see also Rodel, Cook, Regard, & Landis, 1992), thereby treating weakly related words as if they were unrelated. Such processes would be advantageous for the ability to rapidly integrate meanings across successive words. Two recent studies suggest that such lexical integration processes may be available preferentially to the LH (Chiarello et al., 1992; Faust, Kravetz, & Babkoff, 1993). Because meaning integration is enhanced to the extent that successive words are related to each other (Colombo & Williams, 1990), Chiarello et al. (1992) reasoned that greater postactivation priming should be obtained when prime and target words were related via two semantic relations than when they were related via only one. They found such an effect under high proportion priming conditions, but only within the RVF/LH. In other words, the LH obtained greater benefit from words strongly related to the prior context (i.e., the prime), while the RH obtained equal priming regardless of the number of semantic relations. Similarly, Faust et al. (1993) compared priming obtained from one-, three-, and six-word primes, which, when followed by a related target, constituted meaningful Hebrew sentences. RVF/LH targets obtained increasing amounts of priming as prime length increased, while, for LVF/RH targets, priming was no greater from three- or six-word primes than from one-word primes. Since the target predictability increased with prime length, this result likewise suggests that the LH, but not the RH, is sensitive to the degree of semantic/syntactic constraint provided by prior context. A rather different result reported by Beeman, Friedman, Grafman, Perez, Diamond, and Lindsay (1994) provides an interesting complement to these data. LVF and RVF target words (e.g., WEDDING) were preceded by threeword primes that were either related (white–ceremony–tuxedo) or unrelated (soap–tunnel–mouse) to the target. Related primes were selected such that each word was, at best, only weakly associated to the target. Under low proportion related conditions, equivalent VF priming was obtained. However, with a higher proportion of related primes, much greater priming was obtained for LVF/RH trials. Beeman et al. (1994) suggested that when ‘‘controlled’’ processing of primes was enhanced, the LH narrows its attentional focus to encompass only highly related concepts, thereby reducing the benefit

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to be obtained from weakly related meanings. If this interpretation is correct, it suggests that, even when the hemispheres do not differ in the extent of information initially activated, the RH may function to maintain activation for weakly related information that is irrelevant for subsequent LH processes. The latter point is highlighted in a study of ambiguous word primes (Burgess & Simpson, 1988a). Most research suggests that all meanings of ambiguous words are initially activated, but that eventually only the dominant or contextually appropriate sense remains available (e.g., Swinney, 1979). Burgess and Simpson (1988a) obtained this result for words presented to the RVF/LH: at a brief (35 msec) SOA all meanings were primed. At a longer (750 msec) SOA the dominant meaning was primed, but the subordinate meaning was inhibited (i.e., related subordinate targets had slower responses than unrelated targets). However, there was no evidence for meaning selection for LVF/RH targets: at the 750-msec SOA both dominant and subordinate targets remained activated. Thus, it appears once again that by failing to utilize a postactivation process available to the LH, a wide range of meanings remains activated within the RH. The semantic lateralization data obtained thus far permits some provisional conclusions that can serve to guide investigation into a potential RH role in intentional movement. First, both hemispheres appear to activate closely related conceptual information to a similar extent during comprehension. Second, peripherally related semantic information may be initially activated only within the RH. Third, the left, but not the right, hemisphere supplements meaning activation with additional selection and integration processes. These later operations serve to modulate and restrict the set of available meanings to those that are closely related to the current context. Because the RH only minimally employs such processes, it functions to maintain a broader range of related meanings, including those that may have been discarded by more focal LH semantic processes. Finally, the evidence suggests not just that meaning activation is broader within the RH, but also that it less discriminant. That is, there is little distinction among these various meanings in the degree of their activation. For example, the LVF/RH obtains equal priming from high and low dominant category primes (Chiarello & Richards, 1992), from a single strongly related associate as from three weak associates (Beeman et al., 1994), from words related via one or two semantic relations (Chiarello et al., 1992), and from single and multiple word sentence primes (Faust et al., 1993). These data imply that the RH may subserve widespread activation of knowledge structures without subsequent selection, in contrast to the selection/amplification/integration processes which the LH uses to narrow the range of activated representations. 2. Disruptions in Language Use Following RH Injury While more diffuse RH activation may be ill-suited to rapid meaning selection and integration processes needed for sentence comprehension, it may

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be essential for the comprehension of figurative language, discourse, and garden path recovery (Beeman, in press; Chiarello, 1991; Peterson & Burgess, 1993). We will now review these and other areas in which RHinjured patients show alterations in language tasks. While an exhaustive review of this literature is beyond the scope of this article (see Joanette et al., 1990), we cite representative studies and highlight findings which may be relevant to hemisphere asymmetries in activation and selection processes. 2.1. Prosody and verbal communication of emotion. Aprosodias (disorders in the comprehension and/or production of speech prosody) are more common after right- than left-hemisphere injury (Ross, 1981). Experimental investigations of unilateral brain injury suggest that there may be more than one RH process that contributes to prosodic comprehension. On the one hand, the ability to extract meaning from temporally extended speech contours is disrupted after RH injury, as indicated by studies using acoustically filtered stimuli for the interpretation of linguistic or emotional prosody (Heilman, Bowers, Speedie, & Coslett, 1984; Tompkins & Flowers, 1985).2 Dichotic listening studies with normal subjects likewise find RH advantages for judgments of both affective and linguistic prosody (Ley & Bryden, 1982; Shipley-Brown, Dingwall, Berlin, Yeni-Komshian, & Gordon-Salant, 1988). However, there are also well-documented deficits in the interpretation of verbally communicated emotion after RH injury, even when prosodic cues are eliminated. For example, relative to LH-injured and control subjects, RH patients are disproportionally impaired in the identification/interpretation of written emotional words and sentences (Borod, Andelman, Obler, Tweedy, & Welkowitz, 1992) and of verbal descriptions of emotional expressions (Blonder, Bowers, & Heilman, 1991). Thus, one cannot entirely eliminate affective factors as a contributor to prosodic comprehension deficits associated with RH injury. Prosody production deficits have also been found when RH-injured patients produce speech in context (Behrens, 1988; Shapiro & Danly, 1985), but not in a repetition task (Ryallis, Joanette, & Feldman, 1987, as cited by Joanette et al., 1990). This could imply a conceptual contribution to such deficits, in that prosody must be recruited to express the patient’s thought or attitude in the former, but not in the latter, situations. The association of both production and perception deficits with RH injury is likewise consistent with a central conceptual locus. In fact, the kinds of meanings that underlie the expression and interpretation of intonation and affective communication (e.g., feelings, attitudes, statement modality) are much broader than those needed to convey more precise propositional meanings. The more broadly 2 While RH-injured patients are impaired relative to controls in a variety of prosody comprehension tasks (Borod, 1992), their performance, while qualitatively different, is not always worse than that of LH-injured patients (Van Lancker & Sidtis, 1992).

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based semantic activation that we have argued characterizes the RH may be particularly well suited to this function. 2.2. Lexical semantic processing. Most of the research relevant to a RH role in lexical semantic processing has been obtained from lateralization studies of split-brain (Zaidel, 1983) and neurologically normal (Chiarello, 1991; see above) persons. Indeed, on standard tests of word meaning comprehension, RH-injured patients appear normal. Semantic priming is also apparently normal after RH injury, at least when strongly related prime–target pairs are used (Henik, Dronkers, Knight, & Osimani, 1993).3 However, other studies implicate a RH lexical semantic contribution under some conditions. As described above, RH-injured patients are impaired, relative to LH-injured and normal control subjects, in the identification and discrimination of emotion when conveyed via printed words or sentences (Borod et al., 1992). For example, the RH patients were impaired at identifying the emotion shared by three words or conveyed in a simple sentence and at determining whether word pairs represented the same or different emotions. Interestingly, the RH patients were also subnormal in three ‘‘control’’ tasks in which nonemotion words/sentences were judged based on ‘‘characteristics of people’’ (e.g., ‘‘do CLEVER and SMART represent the same or a different characteristic?’’). However, because the RH patients were disproportionally impaired on the emotion tasks, the data provided evidence for two separate RH deficits, one involving the identification of emotion and the other judgments of subtle lexical meanings. Semantic similarity judgments for more concrete words are sometimes (Chiarello & Church, 1986), but not always (Joanette et al., 1990), impaired after RH injury. It is notable that the words used in the former study were the same weakly related category items that had previously shown greater LVF/RH meaning activation in a study of normal college students (Chiarello, 1985). Judgment tasks also reveal RH-injured patients to be insensitive to connotative (Brownell, Potter, Michelow, & Gardner, 1984) and metaphoric (Brownell, Simpson, Bihrle, Potter, & Gardner, 1990) word meanings (see Section 2.4, below). Collectively, such findings suggest a RH role in lexical discriminations requiring the appreciation of emotional content, word meaning nuances, or category membership. With respect to lexical production, RH-injured patients may be subtly impaired in word choice when expressing feelings (Bloom, Borod, Obler, & Koff, 1990). Verbal fluency tasks have also been reported to be sensitive to RH injury. When asked to name as many words as possible from a given semantic category, RH-injured patients produce fewer items than normal, after the first 30 sec of production (Joanette, Goulet, & LeDorze, 1988). Since the most highly accessible category members would likely be produced first, this may represent a deficit in the ability to access and/or report more 3 As priming for strongly related word pairs is usually obtained bilaterally (see above), we would not expect to see a loss of such priming after RH injury.

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distantly related category members. Subsequent analyses (Joanette & Goulet, 1988) revealed that the RH-injured patients produced both fewer than normal subclasses of items (e.g., farm animals, African animals), as well as fewer items within the subclasses they did produce. Collectively, these findings implicate the RH in the exhaustive retrieval of semantic category members, particularly those that are not highly accessible. A related finding has been reported in verbal memory tasks. Recall of word lists is typically enhanced when the list items include members of the same semantic category. Further, such items are recalled in category clusters, implying that the words are semantically encoded by category despite being randomly ordered in the stimulus list. Although patients with RH injury are usually not impaired in verbal learning tasks, their recall is not enhanced for category lists, nor do they demonstrate category clustering in recall (Villardita, 1987; Villardita, Grioli, & Quattropani, 1988; Welte, 1993). While this insensitivity to semantic category membership was demonstrated in a memory, rather than a language processing, task, it could reflect a failure to activate passively categorical information in memory. It is important to note that lexical semantic deficits subsequent to RH injury have been documented in tasks requiring judgments or evaluation of meaning rather than meaning comprehension per se. Similarly, semantic retrieval impairments are evidenced as alterations in normal output strategies rather than frank anomias. Finally, many of the deficits involve semantic categorizations of one sort or another. This suggests that RH meaning activation processes may primarily support (1) comprehension processes beyond the lexical denotative level (see Section 2.3, below) and (2) use of noncentral semantic features for category discriminations and/or retrieval. 2.3. Discourse processing and inferencing. Abnormalities in the comprehension and production of narrative speech and texts have been found after RH injury in a variety of experimental settings. Narrative speech in such patients is described as less coherent and informative than normal (Joanette & Goulet, 1990; Joanette, Goulet, Ska, & Nespoulous, 1986), and these patients have difficulty arranging sentences into a coherent story (Delis, Wapner, Moses, & Gardner, 1983; Scheiderman, Murasugi, & Saddy, 1992). RH-injured patients may ‘‘miss the point’’ of connected discourse and have difficulty extracting the theme of a narrative (Hough, 1990). The ability to draw some types of text inferences can also be impaired (Brownell, Potter, Bihrle, & Gardner, 1986; Beeman, 1993), especially when these depend on disparate sources of information (Brownell, Carroll, Rehak, & Wingfield, 1992). It has been suggested that widespread meaning activation within the RH could function to support discourse level processes that are disrupted after RH injury (Beeman, in press; Chiarello, 1991). A recent study provides the first empirical evidence to support such a connection (Beeman, 1993). Using an on-line semantic priming task, Beeman showed that a deficit in drawing

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coherence inferences from text among RH-injured patients was attributable to failure to activate inference-relevant information during text comprehension. He suggested that widespread, diffuse semantic activation is needed to support coherence inferences and that an intact RH is needed to maintain this type of information during discourse comprehension. This is consistent with the lateralization data reviewed earlier: either hemisphere can support activation of strongly related meanings, but the RH additionally maintains activation for more peripherally related information. RH-injured patients have difficulty revising their initial interpretations of texts in light of subsequent contradictory or surprising information (Brownell et al., 1986; Rehak, Kaplan, Weylman, Kelly, Brownell, & Gardner, 1992) or when they must reassign previously given syntactic roles (Schneiderman & Saddy, 1988). Their impairment in the appreciation of verbal humor can also be attributed to an inability to reinterpret previously given information in order to appreciate a punchline (Bihrle, Brownell, Powelson, & Gardner, 1986; Brownell, Michel, Powelson, & Gardner, 1983). It is important to point out that the RH patients in these studies were able to detect the presence of textual incongruities, but yet were unable to construct a viable alternate interpretation. Previously reviewed lateralization data suggested that the LH rapidly selects and integrates word meanings that are consistent with prior context and may actually suppress alternate senses. In contrast, the RH, lacking selection mechanisms, appears to maintain activation for multiple meanings. An intact RH, then, could serve as a substrate for retrieval of alternate meanings that were suppressed within the LH (Burgess & Simpson, 1988b; Chiarello, 1991). With RH injury this substrate would no longer be available, thereby hindering reinterpretations of various sorts. The discourse data imply a role for the RH in comprehending and producing discourse themes and in maintaining information needed for text inferencing and reinterpretations. While additional processes are no doubt involved, each of these abilities may depend on the ability to maintain broad semantic information without selection. 2.4. Figurative language use. RH-injured patients tend to prefer literal interpretations of phrasal metaphors and idioms (Winner & Gardner, 1977; Van Lancker & Kempler, 1987), even when the context biases the figurative meaning (Myers & Linebaugh, 1981). Similarly, RH patients have difficulty appreciating the intent of statements that are not literally true, thereby failing to detect sarcasm or lies (Kaplan, Brownell, Jacobs, & Gardner, 1990; Tompkins & Mateer, 1985). Injury to the RH also impairs the production and interpretation of indirect requests (Stemmer, Giroux, & Joanette, 1994). For example, indirect requests may be inappropriately interpreted as literal questions (Foldi, 1987) and RH patients may be unable to use linguistic context to determine whether literal or indirect interpretations are appropriate (Weylman, Brownell, Roman, & Gardner, 1989). Stemmer et al. (1994) attributed such deficits to the need to construct and monitor more than one mental

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model of a text or discourse, as RH interpretive deficits were not found when the text supported the construction of a single mental model. Thus, an intact RH may be a prerequisite for properly interpreting a range of figurative constructions that involve ‘‘double meanings.’’ Metaphors, idioms, sarcasm, indirect requests, as well as word connotations, all possess multiple, and sometimes contradictory, levels of meaning. Indeed, a full appreciation of figurative expressions appears to require consideration of several simultaneously conveyed meanings which may not be literally consistent with each other (Chiarello, 1991; Glucksberg, 1993; Stemmer et al., 1994). If, as we argue, the RH normally functions to maintain multiple interpretations, then RH injury would be expected to impair the ability to appreciate figurative meanings due to overreliance on more selective LH interpretative processes. 2.5. Summary. As reviewed here, hemisphere differences in activation/ selection mechanisms, which were posited based on lateralization research in the normal brain, can be extended to account for some of the subtle linguistic alterations that accompany RH injury. However, this requires us to assume that activation and selection processes used to process single words are also applicable to sentence and discourse level meanings. This assumption is not unreasonable in light of research that demonstrates bridges between word meaning activation and higher order processes within the RH (Beeman, 1993). We can now consider whether corresponding processes might underlie a RH role in intentional movement. While there are many fewer studies in this area, current findings do imply some RH contribution to action processing. II. BASIS FOR LANGUAGE AND ACTION SIMILARITIES

The belief that action processing occurs in the LH (Geschwind, 1965; Liepmann, 1980) stems from data obtained with RH-injured patients which shows little evidence of impairment in action production, in contrast to severe impairment among LH-injured patients (Lehmkuhl, Poeck, & Willmes, 1983; Duffy & Duffy, 1990; Roy, Square-Storer, Hogg, & Adams, 1991). A similar situation exists when comparing the incidence of aphasia for RHand LH-injured patients. However, as described above, recent research has delineated a role, albeit a subordinate one, for the RH in language processing. Under the assumption that there may be similarities in language and action processing, a similar role for the RH in intentional movement comprehension and production can be proposed. Action planning involves both conceptual and production systems (Roy & Square, 1985). The comprehension and production of contextually appropriate actions in the real world requires the activation of several different types of conceptual knowledge represented in long-term memory (Roy & Hall, 1992). This includes knowledge of objects and tools and their contex-

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tual environments, knowledge of action orderings, and abstract action frames. Several models of action planning have suggested that conceptual representations may be shared by language and intentional movement systems (Roy & Square, 1985; MacKay, 1985; Rothi et al., 1991). The activation of conceptual information assigns meaning to structural representations, whether they be actions or lexical representations (orthographic or phonologic). Activation of conceptual information may also underlie the generation of abstract goals and intentions, regardless of the means (linguistic, motoric) by which the goal is to be satisfied. According to Roy and Square (1985), there are two levels of production for intentional movements. During lower level production, the actual muscular parameters needed to perform movements are determined (MacKay; 1985; Roy & Square, 1985). This is primarily at the execution end of motor planning and is not the focus of this review.4 Rather, we concentrate our discussion on higher level production, where abstract action programs are assembled or accessed from memory (Lashley, 1917; Heilman, 1979; Keele, Cohen, & Ivry, 1990; Roy & Square, 1985; but see Liepmann, 1980; Alexander, DeLong, & Crutcher, 1992). These action programs contain spatial and temporal information related to classes of actions (e.g., hammering, waving) (Liepmann, 1980; Roy & Square, 1985; MacKay, 1985). As with conceptual information, action programs or their components may be stored in interconnected networks of related actions. Several action programs may be activated in any given situation. Producing the contextually appropriate intentional movement requires the selection of one of the activated action programs (MacKay, 1985; Roy & Hall, 1992). Selected action plans may then run off automatically, requiring attentional processing only at key points. Failure to attend at these key points may cause an inappropriate or unintended action program to run which results in action errors (Reason, 1979; Roy, 1982; Roy & Square, 1985). We will now discuss the evidence for activation and selection of actionrelated information in the RH. The strongest evidence for RH involvement in the planning of intentional movements would be the observation of action impairments or action comprehension deficits following RH damage. Inferring RH processing for action processing in individuals with LH damage is problematic since residual functioning could be subserved by the damaged LH. For this reason, we focus our review on studies that have compared action production and comprehension in individuals with RH damage to neurologically normal controls. Because we wish to propose a RH contribution for high level action functions that are typically attributed to the LH, our 4 We focus here on those programming processes that occur prior to actual motor innervation. As primary motor neurons from the RH innervate muscles on the left side of the body, it is clear that at this level the RH is heavily involved. What is more in question is the role of the RH in more central processes leading up to specific motor command signals.

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review omits those areas of praxis for which there is already extensive evidence for RH control (e.g., dressing and constructional apraxia, see Arena & Gainotti, 1978; DeRenzi & Faglioni, 1976). III. EVIDENCE FOR RH ROLE IN ACTION PROCESSING

As the RH has been found to be important for some, but not all language processes, the RH may be involved in some, but not all, action planning processes. Only by differentiating the processes involved in intentional movement planning will we discover which processes are available in the RH. However, many studies are limited in their ability to discriminate processes because of the measures used to assess action production. In many investigations, a gross rating scale is used to score the produced actions. These rating scales range from simple correct–incorrect ratings (Roy et al., 1991) to 5-point scales which allow for basically correct actions that are simply distorted (Edwards, Deuel, & Baum, 1991). Often these rating scales are combined with error classification analyses (Haaland & Flaherty, 1984). As these latter authors point out, the error analyses typically emphasize motor rather than conceptual aspects of actions. Because of this, it is not easy to differentiate production from conceptual errors or programming from execution errors. Recently, there have been attempts to go beyond measuring accuracy. The time from the onset of the movement cue to the completion of the movement has been divided into two components. RT has been used as a measure of motor programming, while movement time (the time from the onset of the movement until the completion of the movement) has been used as a measure of execution. Only aiming movements and imitation of meaningless movements have been studied using such measures. RH-injured individuals have demonstrated subtle impairments in both the comprehension of meaningful gestures (Duffy, Duffy, & Pearson, 1975; Gainotti & Lemmo, 1976) and in producing aiming, nonsense, and meaningful movements. Tasks involving the production of aiming movements require moving a limb from a starting base to one or more targets (e.g., Haaland & Harrington, 1989a, 1989b). In the second type of task, subjects imitate meaningless movements, either in isolation or within a sequence (e.g., Kimura, 1977; Roy & Square, 1992). The third type of task involves pantomime of meaningful movements, either to verbal command or in imitation (e.g., Roy & Black, 1993; Haaland & Flaherty, 1984). While the first two tasks measure the integrity of action production systems, pantomime may require conceptual as well as production systems. After reviewing the literature in each of these areas, we offer an interpretation based on putative hemisphere differences in activation and selection processes. 1. Gesture Comprehension There are few investigations of gesture recognition. Most have examined the ability of individuals with LH damage to recognize gestures (e.g., Borod,

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Fitzpatrick, Helm-Estabrooks, & Goodglass, 1989; Duffy & Watkins, 1984). Fewer have examined gesture recognition in individuals with RH damage. Gainotti and Lemma (1976) found evidence that RH damage resulted in gesture recognition impairments. They performed a pantomime and the subject selected the corresponding picture from among three foils. While most subjects with RH damage recognized the gestures without error, there were six subjects who performed less than perfectly. In contrast, only one of the neurologically intact control subjects made any errors on the gesture recognition test. Duffy et al. (1975) also found evidence of pantomime recognition impairment following RH damage. Individuals with RH damage performed less accurately than normal controls. However, in a later study, Duffy and Duffy (1981) failed to replicate these results using a modified (four items omitted) version of the gesture recognition test. Thus, there is only weak evidence, at best, that the RH is involved in the comprehension of actions. However, it must be kept in mind that the gesture recognition tests used in the above studies involved very simple gestures, perhaps akin in complexity to single words. As reviewed earlier, simple word recognition and comprehension of denotative word meanings is not impaired following RH damage. Instead, language comprehension impairments following RH damage have been observed only with more complex stimuli or tasks. It is more likely that a RH role in action comprehension would be observed under more complex task conditions, such as comprehension of mimed narratives or gestural humor. 2. Production of Aiming Movements Whether the RH is involved in the programming and execution of simple aiming tasks is controversial. Haaland and Harrington (1989a) had subjects with RH and LH injury move a stylus to target circles displayed to the right or left of the starting point. Subjects always moved from the starting position into the hemispace ipsilateral to the hand used. All movements consist of an initial ballistic or open loop movement, which is thought to be preprogrammed and is executed without adjustment for incoming sensory feedback. These ballistic movements are followed by corrective or closed loop movements to reach the desired target. Closed loop movements, while still requiring some preprogramming, are modified as sensory information regarding the movement is interpreted in route. (Schmidt, 1988). Haaland and Harrington (1989a) examined each phase separately. Individuals with RH injury performed similarly to neurologically intact subjects in reaction time, movement time, the distance traveled per stroke, and the constant and variability of error. There was no evidence for RH control of either the planning of the initial open loop movement nor in the corrective, closed loop movements. In another study (Haaland & Harrington, 1994), subjects performed alternate taps between two targets as rapidly and accurately as possible, and there

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were also no deficits associated with RH injury. It is interesting to note that in both studies, the RH subjects had mostly anterior damage. Other studies have found movement deficits following RH injury. Haaland and Harrington (1989b) again had subjects move a stylus to a target circle. As the location of the target circle differed on each trial, the direction of movement from trial to trial was now uncertain. In general, when movements were made with the ipsilateral arm, subjects with RH damage had few deficits compared to neurologically intact subjects. However, when movements were made with the contralateral arm (in nonhemiparetic subjects), subjects with RH damage had slower reaction times than normal controls, suggesting RH involvement in the planning of simple aiming movements. Haaland and Harrington (1989b) suggested that the use of the ipsilateral limb may not be sufficiently sensitive to detect hemisphere effects in movement control. It is interesting that, in contrast to the studies in which no RH performance deficits were found with simple aiming movements, the RH-lesioned subjects in this study had primarily posterior damage. It is not clear whether variations in lesion location, or in foreknowledge of the direction in which to move, can account for discrepancies between the previously discussed studies. In the Haaland and Harrington (1989a, 1994) studies, subjects had prior knowledge of movement direction, used the ipsilateral hand, and had primarily anterior damage. No RH performance deficits were found. However, in the study by Haaland and Harrington (1989b), subjects were never informed of movement direction until the target appeared for each trial, used the contralateral hand, and had primarily posterior damage. Here, RH performance deficits were found. We suggest that movement uncertainty may be the critical variable for observing a RH contribution to action planning. When performing routine motor tasks, the relevant motor programs are known in advance. The motor program for each component of each task is probably rapidly selected upon the completion of the previous component, a process requiring the engagement of attention at key points in the task (Reason, 1979; MacKay, 1985). The LH may be responsible for focusing attention on the relevant motor program in order to select it. However, under movement uncertainty, it might be beneficial to keep more than one action plan activated. With two possible movement choices, selection of one plan prior to target onset would result in errors approximately 50% of the time or in slowed reaction times due to the necessity of activating or reactivating the correct program. If, in contrast, activation were maintained for multiple action programs, the appropriate action program would have a greater chance of being available for selection after the target was presented. If the RH functioned to activate multiple action programs without selecting one, RH advantages or effects of RH damage would be expected on tasks with movement uncertainty. This is supported by a study in which neurologically normal subjects used their left and right hands to point to a target (Elliot, Roy, Goodman, Carson, Chua, & Moraj,

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1993). Continuously visible targets were situated either to the left or to the right of the starting position. On each trial, a tone indicated the direction for the next movement. Hence, movement direction was uncertain until the tone was presented. Left hand responses were initiated faster than right hand responses, especially when movements were made into the left hemispace, implying that the RH had an advantage in programming these movements. However, once initiated, movement times and accuracies demonstrated a right hand advantage, indicating LH superiority in controlling movement execution. Thus, the Elliot et al. (1993) study supports a RH role for programming aiming movements under directional uncertainty, possibly due to the benefits of simultaneous activation of more than one action program within the RH. 3. Imitation of Meaningless Movements Those studies examining how subjects with RH damage imitated meaningless movements have found little evidence for RH involvement in motor control for single movements (Lehmkuhl et al., 1983; Roy & Square, 1992) or for repetitions of the same movement (Harrington & Haaland, 1991). However, when the task was to imitate sequences of different movements, deficits were found with RH damage. RH-damaged subjects exhibited movement impairments for sequences of three or more meaningless movements in space (Kolb & Milner, 1981; Roy & Square, 1992), for manipulations of buttons, levers, and handles (Kimura, 1977; Harrington & Haaland, 1991), as well as for meaningful pantomimes (Roy et al., 1991; see also below). Movement errors consisted primarily of sequence (misorderings or omissions) and incomplete (movements produced with too little force or using less than the correct number of digits) errors. Harrington and Haaland (1991) found that reaction times of subjects with RH damage were similar to those of neurologically intact subjects, suggesting that both groups preprogrammed movements similarly. However, movement times were more variable in the RH-damaged subjects than in the control group, especially for the longer sequences. There was also a trend for disproportionate slowing of different, as compared to repetitious, sequences following RH damage, particularly for individuals with posterior damage. Harrington and Haaland (1991) suggested that, at least for subjects with right posterior damage, programming continued during movement and that RH damage led to difficulties in this programming. One potential explanation is that the LH may rapidly select action programs and inhibit others. There may be a limit on the number of action programs that can be selected at any given time (MacKay, 1987). Thus for long sequences of unique movements the LH would have to activate new action programs as the sequence progressed. This would slow sequence output from the LH. If the RH maintained activation for multiple action pro-

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grams, however, the appropriate action programs would be available even for sequences consisting of several elements. RH damage could interfere with the availability of these additional programs and hence produce slowing when the coordination of multiple action programs is required. Thus, there may be a RH role in action planning during high level production. The RH appears to be involved in the programming of sequences of unique movements and in situations in which movement direction is not known prior to the signal to move. Several authors have suggested that the RH is important for programming the spatial components of these tasks. However, as action programs contain spatial and temporal information (Leipman, 1980), we prefer the explanation that these are situations in which the continued activation of multiple action programs is beneficial. The RH may not employ selection processes, and, hence, may function to maintain multiple action programs during action processing. However, further empirical work will be needed to determine whether it is this multiple action program activation or the spatial requirements of the tasks that is the important variable in RH participation in action production. 4. Pantomime of Meaningful Movements The research discussed above has implicated a RH role in action planning during high level production. As the RH has been implicated more strongly in accessing conceptual information for language tasks, it is of interest to determine whether there is also a RH role in semantic access during action processing. It is unlikely that research using simple aiming tasks or the imitation of nonsense movements will provide information regarding hemisphere control of action-related conceptual information. Because these tasks are devoid of real world contextual meaning, they probably do not access the conceptual system (Rothi et al., 1991). Access of the conceptual system is likely only for the third type of movement task that has been used for investigating RH control of action processing: comprehending and producing to verbal command pantomimes of meaningful movements. The literature is divided on whether or not individuals with RH injury perform less accurately than neurologically intact subjects when producing meaningful movements. Lehmkuhl et al. (1983) and Roy et al. (1991) both found that subjects with RH injury were unimpaired compared to neurologically intact subjects on the performance of single meaningful gestures. In contrast, Roy and Black (1993) found that RH-injured individuals had more movement errors than did controls. In fact, individuals with RH injury were as impaired in producing movements as were the LH-injured subjects. There is no apparent reason for the discrepant findings as all three studies used similar stimuli, scoring systems, and samples of subjects with regard to lesion location. Haaland and Flaherty (1984) also found that subjects with RH injury made

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more errors than neurologically intact controls. In particular, RH-damaged subjects made many body-part-as-object (BPO)-2 errors. BPO errors are those in which the person’s body part (typically the hand) takes on the spatial characteristics of the objects to be pantomimed. The more primitive are the BPO-1 errors in which the body part is used as the object. For example, when asked to pantomime brushing teeth, the index finger is extended from a fisted hand and is rubbed back and forth across the teeth. BPO-1 errors are typical of 4- to 8-year-old children’s pantomimes. BPO-2 errors are less primitive, indicating rudimentary conceptual separation of the self and external objects. With BPO-2 errors, the subject grasps the imaginary object correctly, but in pantomiming its use, fails to take the spatial characteristics of the object into account. For example, the subject grasps an imaginary toothbrush with a fisted hand, but rubs his/her fist against the teeth. BPO-2 errors are typical of 8- to 12-year-old children. (Kaplan, 1968). If BPO errors are conceptually primitive pantomimes, deficits associated with RH damage could implicate the RH in the access of conceptual information during movement. However, BPO-2 errors can also be construed as spatial errors (Haaland & Flaherty, 1984). This latter interpretation would implicate a production system locus for such errors. Hence, the evidence for difficulties producing meaningful movements associated with RH damage is mixed and not clearly attributable to errors in conceptual access. Unfortunately, paradigms that can clearly discriminate production from conceptual errors have not been used when examining praxis in individuals with RH injury. We now turn to a discussion of one type of movement production difficulty experienced by subjects with LH damage. While it is more difficult to surmise RH contributions from such evidence, the impairments in making transitive (those in which an object is manipulated—brushing teeth), compared with intransitive (those that involve no objects—waving), movements that have been found following LH damage (Haaland & Flaherty, 1984; Gonzalez-Rothi, Heilman, Mack, Verfaille, & Brown 1988; Rapcsak Ochipa, Beeson, & Rubens, 1993) may offer some insight into RH control of conceptual action information. Not surprisingly, both Haaland and Flaherty (1984) and Roy et al. (1991) found that individuals with LH damage made more pantomime errors than intact individuals and sometimes made more errors than their RH-damaged counterparts. It was particularly with transitive movements that individuals with LH damage were the most impaired compared to those with RH damage. Haaland and Flaherty (1984) found that many of the errors made by individuals with LH damage were BPO-1 errors. That these impairments in producing transitive gestures may be due to RH control of action programming is suggested by a case study described by Rapcsak et al. (1993). GK was a strongly right-handed individual who had suffered a massive LH stroke 15 years previously. As an MRI verified that nearly the entire LH was destroyed, the authors concluded that GK’s

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functioning was almost totally dependent upon the RH. Despite this he was able to live alone and reported no difficulties with object use. During the study, GK performed a variety of movements, including transitive, intransitive, meaningless, and axial movements and sequences of nonsense movements. He also recognized and discriminated gestures. Consistent with previous studies, GK showed impairments with meaningless movements only when combined into sequences. The movement errors were primarily spatial–temporal, with some omissions and perseverations occurring. His performance of meaningful movements, on the other hand, demonstrated a large dissociation between intransitive and transitive movements. While he performed most of the intransitive movements correctly, he was severely impaired in his ability to pantomime transitive gestures. GK’s errors during transitive pantomimes were primarily spatial–temporal and BPO-1 errors. However, content errors were never observed; the correct intent of the action was typically apparent. Thus, while the intact RH could apparently construct the correct intention from accessed conceptual information, the conceptual information available could not support a completely correct movement production in the absence of objects. In contrast, his production of these same transitive gestures dramatically improved when he was able to actually manipulate the objects. One explanation that has been offered for the difference in production ability for transitive and intransitive movements following LH damage is that transitive gestures are more dynamic in nature than intransitive gestures (Roy, Square, Adams, & Friesen, 1985), suggesting that the LH has a unique role in programming and executing movement transitions (Kimura, 1977). However, the transitive and intransitive gestures used by Haaland and Flaherty (1984) cannot be divided along such a dimension as both required similar movement transitions. Yet only the transitive movements proved exceptionally difficult for the subjects with LH damage compared to those with RH damage. Haaland and Flaherty (1984) and Rapscak et al. (1993) suggested, rather, that the production of transitive pantomimes occurs outside of the context in which the pantomimed actions typically occur. Producing transitive pantomimes in the absence of supporting context requires the integration of intrapersonal (the relationship of one’s body parts to each other) and extrapersonal (the spatial relationships among objects and object parts) space and that the representations of these two spatial components be distinct from each other. The selection of a stable pattern of conceptual activation is required for the construction of well-separated representations of the self and external objects. In contrast, intransitive gestures require only the internal organization of intrapersonal space. Because the production of intransitive gestures requires no external objects, only one’s body, there is always a more typical context available under pantomiming conditions (Rapscak et al., 1993). Thus, intransitive actions may require the activation of less conceptual information and fewer manipulations of this information. Both Haa-

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land and Flaherty (1984) and Rapscak et al. (1993) suggest that individuals with LH damage are unable to activate well-separated representations of the self and the external objects involved in transitive pantomimes. The inability to construct a unique representation of intended object–action interactions could be the consequence of a RH system that activates a large amount of action information without being able to select one pattern of activation. Individuals with LH damage are often able to perform transitive movements if they are actually manipulating objects (thereby eliminating the need to maintain an internal representation of the object) (Rapscak et al., 1993; Rothi et al., 1991; but see Poizner, Soechting, Bracewell, Rothi, & Heilman, 1989). They are also often relatively able to perform intransitive gestures (Haaland & Flaherty, 1984; Kertesz & Hooper, 1982) (for which there is already relatively rich contextual support). These observations suggest that potent sensory/contextual cues (of which manipulating the object is the strongest) may help selection occur. This implies that the RH may not be devoid of selection mechanisms in action processing, but that these selection mechanisms may require very powerful external cues to operate. Thus, the RH may be involved in the access of action-relevant, conceptual information. There is some, albeit weak, evidence for gesture impairments following RH damage. However, the definitive studies that have the potential to carve out the RH’s role in gesture recognition have not been done. There is also evidence that the RH may not be able to support the construction of the independent conceptual representations of self and environment that are required for the pantomime of transitive gestures. This latter deficit could be the result of diffuse activation of action information. To summarize, there is some evidence to suggest that the RH is involved in the production and conceptualization of actions. During high level production programming, it appears that the RH, particularly the posterior RH, is involved in the programming of sequences of more than two different movements. The RH also appears to be involved in programming movements when there is movement uncertainty. The current evidence relates to directional uncertainty, although other types of uncertainty (force required, type of movement required, etc.) have not been explored. While it seems that the RH is able to program transitive movements during the manipulation of objects, transitive pantomimes may not be supported by the RH. It has been suggested that this is due to an inability to construct adequate conceptual representations of intra- and extrapersonal interactions to support pantomimes of transitive movements (Haaland & Flaherty, 1984; Rapscak et al., 1993). Some individuals with RH damage are poorer at recognizing gestures than neurologically intact individuals. All of these findings can potentially be explained by a RH system which activates a broad set of action-related information without the ability to select one pattern of activation in the absence of powerful external cues.

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That the RH may process movement information as it does language information, via activation without selection, remains an interesting, but unproven conjecture. We conclude by briefly outlining some research strategies that could provide empirical support for our view of a possible RH role in action planning. All involve situations in which the selection of one pattern of action information would be costly and the maintenance of multiple patterns beneficial. Based on evidence from language tasks and our reinterpretation of RH action deficits, it is precisely in such situations that evidence for a RH contribution to action planning should be found. 5. Suggested Research Paradigms The first paradigm focuses on the principle of movement uncertainty. When the required movement parameters are not known prior to the necessity of movement, it may be beneficial to maintain activation for multiple action possibilities. Evidence for such a result was discussed above (Elliot et al., 1993; Haaland & Harrington, 1989b). In these studies, the direction of movement on any given trial was uncertain until the trial began. However, neither of these studies was designed to investigate the movement direction uncertainty question and thus did not directly compare a condition in which the direction of movement was known ahead of time to a condition in which it was not. If multiple action programs are available in the RH, but one action program is selected and the other inhibited in the LH, individuals with RH injury should have elevated RTs compared to neurologically intact subjects due to the necessity of activating or reactivating the correct action program. Subjects without neurological impairments should demonstrate a left hand advantage in RT. In contrast, there should be no benefit to having more than the required action program available when the direction of movement is known prior the movement. In this condition, individuals with RH damage should perform similarly to those without brain damage. A role for the RH in planning movements under conditions of directional uncertainty is logical given the implication that the RH is involved in the processing of spatial information (Springer & Deutsch, 1989) related to movements. It will be interesting to discover whether it is the spatial element of directional uncertainty that is important for delineating a RH role in movement production or whether it is the uncertainty dimension that is relevant. It would also be useful to explore other types of uncertainty (force uncertainty, and timing uncertainty). The latter investigation would be particularly interesting given that the LH is usually implicated in the timing of intentional movements (Tzeng & Wang, 1984; Kitterle, Christman, & Hellige, 1990). A second fruitful paradigm would involve a form of semantic priming of action information. Klatzky and her colleagues (Klatzky, Pellegrino, McCloskey, & Doherty, 1989; McCloskey, Klatzky, & Pellegrino, 1992) had

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subjects learn associations of symbols and hand shapes (e.g., the . sign was associated with clenching the fist). They then presented the symbols as primes, and subjects made sensibility judgements to target phrases describing actions (CLIMB THE LADDER). When the subjects had manually made the hand shapes during training (but not when they verbalized the association), symbols primed sensibility judgements to related targets. Klatzky and colleagues (Klatzky et al., 1989; McCloskey et al., 1992) argued that actual motor information was activated and that this aided the comprehension of the linguistic descriptions of actions. In order to address the amount of action information that becomes activated in the RH, concrete nouns whose referents afford at least two actions (e.g., COIN affords the action PINCH when picking it up, but affords the action POKE when sliding it across the table) could be presented as primes and symbols representing actions presented laterally as targets. Target recognition could be measured several ways, one of which is to measure the RT for the production of the movements indicated by the target symbols. Varying the SOA between the onset of the prime and the onset of the target (from at least 35 to 750 msec) would allow delineation of the time course of action information. This is important to the understanding of RH and LH activation patterns of action information as differing temporal activation patterns have been found in the hemispheres for multiple meanings of ambiguous words (Burgess and Simpson, 1988a). If the RH activates a broad set of action information and that information remains in an activated state, priming would be expected in the LVF for all related targets even at long SOAs. If the LH selects a small set of action information and inhibits other information, priming only for the most typical action with an object may be found at long SOAs. Finally, as Rapscak et al. (1993) have suggested that the production of transitive pantomimes cannot be supported by the RH due to the lack of contextual support, it would be interesting to vary the amount of context available in the primes to determine the amount of contextual support needed for RH action selection. One way to do this would be to present two primes: one a noun affording multiple actions, the second a verb indicating one of the possible actions (COIN, SLIDE). When targets representing the action suggested by the prime verb (COIN, SLIDE–POKE) were presented, priming would be expected in both visual fields. When the target represented an action afforded by the noun but different than the one indicated by the verb (COIN, SLIDE–PINCH), a different pattern of priming might be expected. In the RVF/LH no priming would be expected. There may even be suppression (related RTs slower than in the unrelated RTs) if the LH selects action information based on the prime and inhibits other action information. If there is enough context to support RH selection, the same absence of priming would be expected in the LVF/RH. However if the context is not sufficient to support RH selection, priming would be expected in the LVF/RH.

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CONCLUSIONS

The intent of this review was to identify plausible analogies between language and action, in order to stimulate a reconsideration of the role of the RH in action planning. Because language and skilled movement both depend upon systems of interrelated information in long-term memory, we have focused on activation and selection processes that may span both domains. The lateralization literature on meaning activation during language processing indicates that both hemispheres have access to a rich semantic system, but that each hemisphere may activate and maintain a somewhat different set of meanings. Selection mechanisms, in contrast, appear to depend more exclusively on the LH. We suggested that the RH may also maintain activation for a broad range of action information without selection, and that this may enhance the comprehension and production of complex intentional movements. However, we are not claiming that all hemisphere asymmetries for language or intentional movement are traceable to activation/selection processes. Rather we argue that some asymmetries may be common to these domains and that these are likely to depend on comparable activation and selection processes in long-term memory. We believe that this is potentially a fertile research area, capable of yielding insights into both the processing of action information and the hemispheric distribution of function. REFERENCES Abernethy, M., & Coney, J. 1990. Semantic and phonemic priming in the cerebral hemispheres. Neuropsychologia, 28, 933–946. Alexander, G., DeLong, M., & Crutcher, M. 1992. Do cortical and basal ganglionic motor areas use ‘‘motor programs’’ to control movement? Behavioral and Brain Sciences, 15, 656–665. Arena, R., & Gainotti, G. 1978. Constructional apraxia and visuospatial disabilities in relation to laterality of cerebral lesions. Cortex, 14, 463–473. Balota, D., & Lorch, R. 1986. Depth of automatic spreading activation: Mediated priming effects in pronunciation but not lexical decision. Journal of Experimental Psychology: Learning, Memory, and Cognition, 12, 336–345. Beeman, M. in press. Coarse semantic coding and discourse comprehension. In M. Beeman & C. Chiarello (Eds.). Right hemisphere language comprehension: Perspectives from cognitive neuroscience. Mahweh, N.J.: Erlbaum. Beeman, M. 1993. Semantic processing in the right hemisphere may contribute to drawing inferences from discourse. Brain and Language, 44, 80–120. Beeman, M., Friedman, R., Grafman, J., Perez, E., Diamond, S., & Lindsay, M. 1994. Summation priming and coarse semantic coding in the right hemisphere. Journal of Cognitive Neuroscience, 6, 26–45. Behrens, S. J. 1988. The role of the right hemisphere in the production of linguistic stress. Brain and Language, 33, 104–127. Bihrle, A. M., Brownell, H. H., Powelson, J., & Gardner, H. 1986. Comprehension of humorous and non-humorous materials by left and right brain-damaged patients. Brain and Cognition, 5, 399–412. Blonder, L. X., Bowers, D., & Heilman, K. M. 1991. The role of the right hemisphere in emotional communication. Brain, 114, 1115–1127.

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