Extralinguistic Deficits Account for the Early Lexical Impairments of Dementia of the Alzheimer's Type (DAT)

Lexical impairments in DAT have been studied for decades (e.g., Aronoff et al., Reference Aronoff, Gonnerman, Almor, Arunachalam, Kempler and Andersen2006; Irigaray, Reference Irigaray1967; Kempler, Reference Kempler1988). Patients with DAT have trouble recalling names and other words, often substituting pro-forms (e.g., “he,” “it”), using conceptually related words (e.g., “dog” for “horse”), or pausing when they cannot generate a target word in conversation or in structured tasks. Anomia, at least in spontaneous speech and simple picture-naming tasks, could be due to extralinguistic deficits or a deterioration of the underlying semantic/conceptual system. Extralinguistic deficits can include inattention to the task, forgetting the target word, or being distracted by related competitor responses. A semantic/conceptual impairment, defined as a loss of underlying semantic memories, would be manifested in lexical production, as well as any other task that relies on that meaning, including comprehension, knowledge of category relationships, attributes, and the like.

Data from several sources can help shed light on this issue. Studies have shown that word-picture matching is relatively intact in DAT, even when naming is not, suggesting that the semantic system is sufficiently preserved to support distinguishing semantically related words from one another (e.g., Astell & Harley, Reference Astell and Harley1998; Rogers, Ivanoiu, Patterson, & Hodges, Reference Rogers, Ivanoiu, Patterson and Hodges2006). Several other tasks have been used to probe the semantic system in DAT more deeply, including word definition (Astell & Harley, Reference Astell and Harley2002) and similarity judgments in which participants sort or arrange words so that similar items are grouped together (e.g., Aronoff et al., Reference Aronoff, Gonnerman, Almor, Arunachalam, Kempler and Andersen2006; Bonilla & Johnson, Reference Bonilla and Johnson1995; Ober & Shenaut, Reference Ober and Shenaut1999). In both definition and similarity judgment tasks, patients with DAT have performed similarly to healthy controls, giving correct (although impoverished) definitions and grouping categorically similar words together.

Semantic priming is another method to examine underlying semantic knowledge. Priming results that resemble those obtained from healthy individuals suggest that automatic semantic activation is intact in individuals with DAT (e.g., Ober, Reference Ober2002; Ober, Jagust, & Koss, Reference Ober, Jagust and Koss1991; Shenaut & Ober, Reference Shenaut and Ober1996). However, it should be noted that priming studies of individuals with DAT have yielded contradictory results (i.e., lack of semantic priming, hyperpriming, or priming effects comparable to control participants). It has been noted that even partially degraded semantic information might result in intact semantic priming, making it difficult to ascertain the degree of semantic/conceptual loss (Giffard, Desgranges, & Eustache, Reference Giffard, Desgranges and Eustache2005). These priming data and those documenting qualitatively intact performance on tasks of word comprehension, word definition, and similarity judgments imply that the well-documented and pervasive symptom of anomia in DAT can be attributed to extralinguistic cognitive deficits. That said, it is undeniable that as the disease progresses, data from a range of lexical semantic tasks, including those mentioned earlier, show deterioration of the semantic/conceptual system (e.g., Aronoff et al., Reference Aronoff, Gonnerman, Almor, Arunachalam, Kempler and Andersen2006).

The overall picture that emerges from the literature at this point is one of a slow stepwise deterioration of a semantic system. Early on in the process, the loss of semantic features is minimal and does not interfere with performance on a wide range of semantic tasks (e.g., word comprehension and definition, similarity judgment, priming). At this point, memory and attention are often moderately impaired and do interfere with task performance. However, as the semantic deterioration progresses, naming errors change in quality (early on, contrast coordinates: “apple” for “pear”; later, superordinates: “fruit” for “apple”; still later: “I don't know”), comprehension deteriorates and deficits in other lexical tasks will become more apparent as well (see Aronoff et al., Reference Aronoff, Gonnerman, Almor, Arunachalam, Kempler and Andersen2006, and Gonnerman, Andersen, Devlin, Kempler, & Seidenberg, Reference Gonnerman, Andersen, Devlin, Kempler and Seidenberg1997, for discussion of how this progression can be conceptualized and map onto specific task performance). At these later points in the dementing process, the semantic/conceptual system is no longer intact, and errors in naming and other lexical tasks can be attributed to a combination of extralinguistic and semantic/conceptual deficits.

Extralinguistic Deficits Explain Sentence and Discourse Comprehension and Production Impairments of DAT

Sentence production in DAT is characterized by intact morphosyntactic structure (i.e., subject-verb agreement, well-formed plural and tense markings) (Kavé, Reference Kavé2003; Kavé, Leonard, Cupit, & Rochon, Reference Kavé, Leonard, Cupit and Rochon2007; Kemper, LaBarge, Ferraro, Cheung, & Storandt, Reference Kemper, LaBarge, Ferraro, Cheung and Storandt1993; Kempler, Curtiss, & Jackson, Reference Kempler, Curtiss and Jackson1987). Although sentence production in DAT is not error-free, Altmann, Kempler, and Andersen (Reference Altmann, Kempler and Andersen2001) demonstrated that grammatical errors made by patients with DAT are similar in type and proportion to those made by healthy elderly, suggesting that similar processes are operating in the grammar of both groups.

Sentence comprehension can appear impaired in offline tasks that involve listening to and remembering instructions while selecting one of several choices in a response array or responding to information questions about the material presented (Kempler, Almor, Tyler, Andersen, & MacDonald, Reference Kempler, Almor, Tyler, Andersen and MacDonald1998; Rochon, Waters, & Caplan, Reference Rochon, Waters and Caplan2000). The fact that patients with DAT do not show effects of syntactic complexity and that their performance correlates with measures of working memory have led authors to conclude that sentence comprehension impairments can be attributed to extralinguistic deficits in executive function (e.g., working memory). If offline sentence comprehension deficits are due to memory impairment, performance on online comprehension tasks, which minimize extralinguistic task demands, should be intact. Indeed, Almor, MacDonald, Kempler, Andersen, and Tyler (Reference Almor, MacDonald, Kempler, Andersen and Tyler2001) demonstrated in an online cross-modal naming paradigm that patients with DAT performed similarly to healthy elderly in processing subject-verb agreement. Small, Andersen, and Kempler (Reference Small, Andersen and Kempler1997) showed that speech rate alteration can modulate sentence comprehension for patients with DAT, suggesting, again that extralinguistic factors can play a significant role in sentence processing for this population. Consistent with these data, Kavé and Levy (Reference Kavé and Levy2003) demonstrated in both online and offline tasks that participants with DAT, like healthy controls, were sensitive to violations of tense and person. Taken together, these data suggest that grammatical processing may be grossly intact in DAT, at least with respect to relatively simple and robust grammatical phenomena (e.g., subject-verb agreement).

However, the data with regard to grammatical comprehension are not altogether uniform. For instance, Price and Grossman (Reference Price and Grossman2005) collected data from patients with DAT using an online word detection paradigm. They found that although patients with DAT were sensitive to violations of verb transitivity, they were not sensitive to violations of thematic role assignment. Therefore, sensitive and specific tasks may reveal islands of impairment within the sphere of grammatical processing in DAT. Overall, the bulk of studies suggest that impairment in sentence comprehension is not the result of grammatical deficits per se, but rather can be attributed to extralinguistic factors, in particular, impairments in attention and working memory.

Extralinguistic processing deficits have also been used to explain discourse impairments in DAT. Patients with DAT are known to have difficulty constructing an informative and coherent narrative. Their narratives are often repetitive with topic changes, unclear references (e.g., “he,” “there”), and lack of coherence and informativeness (Chapman, Highley, & Thompson, Reference Chapman, Highley and Thompson1998; Laine, Laakso, Vuorinen, & Rinne, Reference Laine, Laakso, Vuorinen and Rinne1998; Ripich & Terrell, Reference Ripich and Terrell1988). The association between discourse and working memory in DAT was made explicit by Almor and colleagues. Almor, Kempler, MacDonald, Andersen, and Tyler (Reference Almor, Kempler, MacDonald, Andersen and Tyler1999) demonstrated that the speech of patients with DAT contained more pronouns (compared to full nouns) than the speech of healthy participants. Furthermore, the researchers demonstrated in an online cross-modal naming paradigm that patients with DAT were less sensitive than healthy participants to the appropriateness of pronominal references in short discourses. Sensitivity to pronoun appropriateness positively correlated to working memory scores, providing further evidence for the contribution of extralinguistic deficits to the discourse impairment observed in DAT.

Semantic/Conceptual Deficits Explain the Early Lexical Impairments of Semantic Dementia (SD)

Anomia and word comprehension deficits are the hallmark of SD. Unlike DAT, in which initial anomia occurs in the context of relatively intact semantic/conceptual knowledge, in SD, anomia is generally accompanied by impairments in other semantic/conceptual tasks from the earliest point. This has prompted several scholars to propose an underlying modality-independent semantic/conceptual deficit in SD.

In SD, word retrieval deficits are usually accompanied by word comprehension impairments, although the naming deficit is generally more severe than the comprehension deficit (e.g., Rogers et al., Reference Rogers, Ivanoiu, Patterson and Hodges2006). Additionally, patients with SD present deficits in various tasks that rely on underlying semantic/conceptual representations, including defining words (Warrington, Reference Warrington1975), sorting objects (Hodges & Patterson, Reference Hodges and Patterson1996), and demonstrating the use of objects (Hodges, Bozeat, Lambon, Patterson, & Spatt, Reference Hodges, Bozeat, Lambon, Patterson and Spatt2000). Rogers et al. recently reported results of verbal and nonverbal semantic/conceptual tasks from 42 patients with SD. The patients with SD, in addition to being predictably impaired on a naming task, were also impaired on comprehension tasks using words (e.g., word-picture matching and the word version of the Palm Trees and Pyramids Test) and a semantic conceptual task with pictures (the picture version of the Palm Trees and Pyramids Test). In this and other studies, the naming deficit in SD appears to emerge early in the course of the disease and correlate significantly with performance on other semantic/conceptual tasks (Grossman & Ash, Reference Grossman and Ash2004).

Data suggest that the underlying semantic/conceptual deficit in SD, like DAT, progresses. Early in the course of SD, fine distinctions are first lost between items that share many perceptual and conceptual features (e.g., “apple” and “pear”), leading to naming errors at coordinate level. Distinctions between dissimilar objects remain clearer, preserving the ability to select, for instance, the odd-man out from an array, as in the Palm Trees and Pyramids Test. At this early stage in the dementia, sufficient semantic information remains in the system to allow accurate performance on tasks of attribute and category structure (e.g., “Is pear a fruit?”) and many comprehension tasks. As damage to the system progresses, more general distinctions are lost (animate vs. inanimate, abstract vs. concrete, category knowledge), interfering with an increasing number of semantically based tasks. Verfaellie and Giovanello (Reference Verfaellie and Giovanello2006) recently documented in a 4-year longitudinal study of a patient with SD this pattern of (1) an early and persistent semantic impairment in word generation and (2) a progression of impairment on a task of implicit attribute (concrete/abstract) identification.

Taken as a whole, the data suggest that the lexical deficit in SD seems to progress from one predominated by anomia to one in which an underlying semantic deficit is undeniable (see Grossman & Ash Reference Grossman and Ash2004, for review). Unlike DAT, the deterioration of an underlying semantic/conceptual system appears earlier and across a variety semantic/conceptual tasks.

It should be mentioned that there are data that suggest the semantic/conceptual deficit in SD may not be as pervasive or as general as just stated. Several authors have proposed that visual feature information is disproportionately affected in SD. This would explain patients’ inability to make judgments with regard to categories that crucially rely on perceptual information (e.g., fruits and vegetables) and their relatively preserved abilities to do so with categories that rely less on perceptual distinctions (e.g., numbers and other abstract concepts) (e.g., Crutch & Warrington, Reference Crutch and Warrington2006; Halpern et al., Reference Halpern, Glosser, Clark, Gee, Moore and Dennis2004; Vesely, Bonner, Reilly, & Grossman, Reference Vesely, Bonner, Reilly and Grossman2007).

Sentence and Discourse Abilities Appear Preserved in SD

In contrast to the progressive deterioration of the semantic/conceptual system underlying lexical processing, syntactic and morphological aspects of connected speech remain essentially unaffected by SD. Most reports of patients with SD describe speech output as fluent and paraphasic, but with little or no mention of grammatical construction problems. It can be argued that SD leads to a dissociation between impaired conceptual knowledge and intact language structure in production and comprehension. For example, a longitudinal case study by Kavé and colleagues (Reference Kavé, Leonard, Cupit and Rochon2007; Rochon, Kavé, Cupit, Jokel, & Winocur, Reference Rochon, Kavé, Cupit, Jokel and Winocur2004) found that, despite clear semantic deficits that affected message content, the patient demonstrated intact morphological and syntactic structure in language performance. Grossman, Rhee, and Moore (Reference Grossman, Rhee and Moore2005) studied three patients with SD using an online measure of sentence comprehension (i.e., sensitivity to grammatical violations) and found that patients with SD performed similarly to healthy control participants. Preservation of language structure appears to extend to discourse production (Ash et al., Reference Ash, Moore, Antani, McCawlet, Work and Grossman2006), but there is little relevant discourse data available to date.

Lexical Deficits of Progressive Nonfluent Aphasia (PNFA) Are Attributed to a Phonological Access Impairment in Word Production

Anomia is also a hallmark of PNFA. Anomia in PNFA is apparent in conversational speech, confrontation naming, word reading, and word repetition (Clark, Charuvastra, Miller, Shapira, & Mendez, Reference Clark, Charuvastra, Miller, Shapira and Mendez2005; Zakzanis, Reference Zakzanis1999). In contrast to errors found in SD, the speech errors of individuals with PNFA are more likely to be phonological than semantic paraphasias (Clark et al., 1999; George & Mathuranath, Reference George and Mathuranath2005). Furthermore, the impairment is typically seen in tasks that involve word production, whereas relatively good performance characterizes tasks that do not require language production (e.g., single-word comprehension, verbal and visual versions of the Palm Trees and Pyramids Test) (Rogers et al., Reference Rogers, Ivanoiu, Patterson and Hodges2006).

In addition to preserved word comprehension abilities, certain nonlanguage cognitive abilities also remain relatively preserved, at least in mild and moderate stages of PNFA. In one recent study, individuals with PNFA were impaired on naming, verbal fluency tasks, and memory for words (word list recall), but not on visual recall and figure drawing (Zahn et al., Reference Zahn, Buechert, Overmans, Talazko, Specht and Ko2005). Relatively preserved or stable performance has also been documented on a range of nonverbal tasks, including a modified Visual Verbal Test, a visual reproduction test, face recognition, and Raven's Progressive Matrices (Weintraub, Rubin, & Mesulam, Reference Weintraub, Rubin and Mesulam1990; Wicklund, Johnson, & Weintraub, Reference Wicklund, Johnson and Weintraub2004).

These data suggest the lexical deficit in PNFA is confined to lexical production processes and is likely due to impaired access to phonological representations and/or motor programming more than a semantic impairment (Clark et al., Reference Clark, Charuvastra, Miller, Shapira and Mendez2005; George & Mathuranath, Reference George and Mathuranath2005, case 2; Hodges & Patterson, Reference Hodges and Patterson1996; Rogers et al., Reference Rogers, Ivanoiu, Patterson and Hodges2006; Weintraub et al., Reference Weintraub, Rubin and Mesulam1990).

Morphosyntactic and Extralinguistic Impairments Both Contribute to Sentence Processing Deficits of PNFA

Patients with PNFA exhibit grammatical deficits in both production and comprehension. The nonfluent language production of patients with PNFA is characterized by short sentences with phonological and grammatical errors in spontaneous speech. Difficulties are also present in structured language production tasks, such as sentence completion and picture description (Grossman & Ash, Reference Grossman and Ash2004; Grossman et al., Reference Grossman, D'Esposito, Hughes, Onishi, Biassou and White-Devine1996; Hodges & Patterson, Reference Hodges and Patterson1996). Discourse production is impaired, often characterized by limited output and omission of information (Ash et al., Reference Ash, Moore, Antani, McCawlet, Work and Grossman2006).

Sentence comprehension impairments appear in explicit, offline tasks, such as sentence-picture matching with grammatically complex sentences and part V of the Token Test (e.g., Grossman et al., Reference Grossman, D'Esposito, Hughes, Onishi, Biassou and White-Devine1996; Thompson, Ballard, Trait, Weintraub, & Mesulam, Reference Thompson, Ballard, Trait, Weintraub and Mesulam1997). Such deficits can be attributed to morphosyntatic impairment or to extralinguistic task-performance factors, including attention and memory. To address the role of attention and memory in sentence comprehension, Grossman and colleagues (Reference Grossman, Koenig, DeVita, Glosser, Moore and Gee2003, Reference Grossman, Rhee and Moore2005) directly compared the relationship between sentence comprehension and extralinguistic cognitive abilities in PNFA. They reported that poor performance on offline sentence comprehension tasks correlated with impairments in auditory short-term memory. These results along with slowed processing in online tasks, have led them to reasonably conclude that slowed processing time allows sentence-level material to degrade in short-term memory during the comprehension process. Thus, the combination of slowed processing and impaired short-term memory results in impaired sentence comprehension.

Recent evidence provided by Grossman and his colleagues, however, suggests that at least some individuals with PNFA experience syntactic impairment per se (Grossman & Ash, Reference Grossman and Ash2004; Grossman & Moore, Reference Grossman and Moore2005; Peelle, Cooke, Moore, Vesely, & Grossman, Reference Peelle, Cooke, Moore, Vesely and Grossman2007). For example, Peelle et al. demonstrated that individuals with PNFA, as compared to healthy older participants, were insensitive to grammatical errors despite their reasonable sensitivity to thematic role violations in a word-monitoring task. In an online monitoring task, extralinguistic factors are not expected to affect performance and therefore cannot account for the impairment observed in individuals with PNFA.

Individuals with Premorbid High Levels of Education, Literacy, and Mental Activity Show a Delayed Onset of Dementia Symptoms

Cognitive reserve refers to particularly rich neuronal connections in the brains of individuals who have achieved high education levels and engaged in complex mental activity across the life span. It is theorized that cognitive reserve offers protection against the cognitive changes associated with the pathology of dementia (Stern, Reference Stern2002). That is, despite neural dysfunction, individuals with higher levels of cognitive reserve may not show behavioral symptoms of dementia as early or as significantly as those with less cognitive reserve. The protection afforded by cognitive reserve manifests as lower incidence rates and delayed onset of the dementia symptoms (e.g., Manly, Schupf, Tang, & Stern, Reference Manly, Schupf, Tang and Stern2005; Perneczky, Diehl-Schmid, Pohl, Drzezga, & Kurz, Reference Perneczky, Diehl-Schmid, Pohl, Drzezga and Kurz2007; Valenzuela & Sachdev, Reference Valenzuela and Sachdev2006). For example, Valenzuela and Sachdev conducted a meta-analysis of 22 longitudinal studies of cognitive reserve and incidence of dementia and found that premorbid higher education, higher IQ, and high levels of leisure and mental activities all were associated with lower (up to 47% lower) incidence of dementia.

Although the protection value of cognitive reserve has been consistently associated with lower incidence of dementia, controversy remains regarding the relation between cognitive reserve and rate of cognitive decline once dementia is recognized (e.g., Scarmeas, Albert, Manly, & Stern, Reference Scarmeas, Albert, Manly and Stern2006). Higher cognitive reserve has been associated with slower decline in some studies (e.g., Manly et al., Reference Manly, Schupf, Tang and Stern2005; Wilson et al., Reference Wilson, Fleischman, Myers, Bennet, Bienias and Gilley2004) but with faster decline in others (e.g., Scarmeas et al.; Stern, Albert, Tang, & Tsai, Reference Stern, Albert, Tang and Tsai1999). Those who found faster decline with higher reserve suggest that the protective effect of cognitive reserve delays the appearance of the dementia symptoms; therefore, by the time the symptoms do appear, the progression of the underlying neuropathology is more advanced and is manifested relatively abruptly.

The variables that compose cognitive reserve include level of education and occupation, social engagement, stimulation in the environment, and levels of intelligence and literacy. Manly and her colleagues (e.g., Manly, Touradji, Tang, & Stern, Reference Manly, Touradji, Tang and Stern2003, Reference Manly, Schupf, Tang and Stern2005) have argued that literacy level is a better measure of reserve than education level, especially for an ethnically varied population. In an epidemiological study of over 3,000 randomly sampled individuals older than 65 years, Manly et al. (Reference Manly, Schupf, Tang and Stern2005) found that literacy level (measured as reading levels in English, using subtests of the Wide Range Achievement Test-Version 3) predicted decline in language, memory, and executive function better than did years of formal education. Specifically, low literacy was associated with faster decline in memory and language scores.

A relation between premorbid literacy skills and cognitive decline was reported by Kemper and her colleagues (e.g., Kemper, Greiner, Marquiz, Prenovost, & Mitzner, Reference Kemper, Greiner, Marquiz, Prenovost and Mitzner2001). In their longitudinal study of a large sample of nuns, Kemper and her colleagues found that higher linguistic skills in early adulthood, as measured by the proportions of complex sentences (in terms of linguistic structure as well as density of ideas) in writing samples, were associated with lower incidence rates of dementia. They did not find that the number years of education of the studied individuals was related to the rate of decline in their linguistic skills over time. Taken together, these data suggest premorbid language or literacy abilities predict the onset of cognitive decline in dementia, highlighting the interrelation of language and other cognitive resources in dementia.

An additional protective variable, proposed by Bialystok, Craik, and Freedman (Reference Bialystok, Craik and Freedman2007), pertains to premorbid proficiency and use of more than one language. Their study suggests that the special cognitive and language behaviors associated with bilingualism can mitigate the onset of the dementia symptoms. We briefly review data from studies of dementia in bilingual speakers. The first group of data reviewed pertains to decline associated with dementia, whereas the second pertains to preserved abilities.

Deficient Executive and Switching Skills Explain Patterns of Language Impairments in Bilinguals with Dementia

The languages of bilingual and multilingual speakers who are proficient in their languages are assumed to be processed by largely overlapping neural networks (e.g., Abutalebi & Green, Reference Abutalebi and Green2007). Consequently, dementia-related language impairment would be expected to affect all languages, preserving premorbid levels of relative proficiency. Support for this can be found in several studies of bilingual dementia (e.g., Filley et al., Reference Filley, Ramsberger, Menn, Wu, Reid and Reid2006; Hyltenstam & Stroud, Reference Hyltenstam, Stroud, Hyltenstam and Obler1989; Salvatierra, Rosselli, Acevedo, & Duara, Reference Salvatierra, Rosselli, Acevedo and Duara2007). For example, Filley et al. reported the language difficulties of an English-Chinese bilingual woman with progressive aphasia. Her language deficits included paraphasic errors and impaired repetition abilities equally in both languages. Similarly, Friedland and Miller (Reference Friedland and Miller1999) reported impairments reflecting levels of pre-onset proficiency in four English-Afrikaans speakers with mild to moderate dementia, diagnosed with DAT.

However, differential impairment of the two languages of bilinguals with dementia was reported in the two cases described in Mendez, Saghafi, and Clark (Reference Mendez, Saghafi and Clark2004). One case was an English-Spanish-German speaker who experienced difficulty with naming and word comprehension. His deficits were greater in Spanish and German than in English (presumably his L1). The second case was a Spanish-English-Polish trilingual who had experienced progressive difficulty with word retrieval and word comprehension. His performance on language testing was worse in Polish, better in English (L2), and best, although also impaired, in Spanish (L1). The authors concluded that these data suggest differential impairment of the languages of multilingual speakers and differential processing of these languages in the multilingual brain. But, as has been proposed for bilingual aphasia (e.g., Green, Reference Green1998), what appears to be differential language impairment could reflect executive dysfunction, which might prevent appropriate language or code switching by impairing the ability to activate and inhibit the languages according to the communication situation.

Furthermore, evidence from bilingual dementia suggests that some (although not all) individuals with dementia tend to switch between their languages inappropriately, that is, even when their interlocutor does not speak or understand one of the languages (De Santi, Obler, Sabo-Abrahamson, & Goldberger, Reference De Santi, Obler, Sabo-Abrahamson, Goldberger, Joanette and Brownell1990; Friedland & Miller, Reference Friedland and Miller1999). This is in contrast to code switching typical of bilingual conversations. In most cases, bilingual speakers with dementia tend to switch to their first language (Friedland & Miller, Reference Friedland and Miller1999; Mendez, Perryman, Pontón, & Cummings, Reference Mendez, Perryman, Pontón and Cummings1999). Mendez et al. suggested that switching to the first language in dementia is explained by the fact that earlier learned information is more resistant to decline than later learned information. However, the inappropriate or involuntary switching to one language can also be explained by impaired cognitive function in executive control. According to this account, appropriate and inappropriate language choice in bilinguals is related to preserved or impaired executive skills.

Use of More Than One Language Can Enhance Cognitive Reserve and Delay Onset of Dementia

Bilingual and multilingual individuals, who have years of experience switching between languages and inhibiting or activating their languages in accordance to the communication situation, might have particularly enhanced executive function abilities (Bialystok, Craik, Klein, & Viswanathan, Reference Bialystok, Craik, Klein and Viswanathan2004). Support for this is found in a recent article by Bialystok and colleagues who demonstrated that enhanced executive function associated with bilingualism might be part of cognitive reserve discussed earlier. Bialystok et al. (Reference Bialystok, Craik and Freedman2007) reviewed the records of 184 patients who received a diagnosis of dementia and could be classified as either monolinguals (n = 91) or bilinguals (n = 93). The bilinguals varied in the age of second language learning and in the languages spoken (25 different first languages; most common languages other than English included Polish, Yiddish, German, Rumanian, and Hungarian). They found that the age of onset of dementia symptoms (on the basis of family report during the initial neurological assessment) of the bilingual speakers was significantly later (4.1 years) than that of the monolinguals. This was true despite the significantly fewer average number of years of education for the bilingual group as compared to the monolingual group. There was no difference between the two groups in rate of decline 4 years after the initial testing. The authors proposed that constant use of two (or more) languages could be taken as a type of complex mental activity and associated with delayed onset of dementia symptoms. The authors interpreted their findings of later onset of dementia symptoms in bilinguals as compared to monolinguals as the result of well-practiced attentional control that accompanies years of using two or more languages, attending to one and inhibiting another appropriately. The research leaves the question of whether and how the lifelong use of two languages provides a unique contribution to cognitive reserve unanswered. Clearly, these ideas are preliminary and need to be replicated.