Depending on the context, aphasia is typically defined as an acquired language disorder resulting from brain injury that affects expressive and receptive language. McNeil & Pratt (2001), however, pointed out the inadequacy of this definition of aphasia, and there continues to be international efforts to clearly define aphasia for research and clinical purposes (Berg et al., 2022). Some debated components for inclusion in the operationalization of aphasia include the dimensionality of aphasia (e.g., which dimensions, how many; Halai et al., 2022), the role of cognitive processes in aphasia (e.g., Minkina et al., 2017; Villard & Kiran, 2017), the inclusion of consequences of aphasia (e.g., activity, participation, and other psychosocial impacts of language impairment; Berg et al., 2022; Martin et al., 2007), and the etiology of aphasia (e.g., focal versus diffuse lesions; Berg et al., 2022).

Critically, how aphasia is operationally defined has direct influence on the design and implementation of aphasia diagnostic tests (Spreen & Risser, 2003) and on how rehabilitation for aphasia may be approached. As Schuell (1964) stated, “what you do about aphasia depends on what you think aphasia is” (p. 138). Furthermore, an operational definition has direct implication also for what is not to be studied (Petheram & Parr, 1998); aphasia operationalization is not immune from this aspect. Therefore, it is critical that we continuously examine and update the definition of aphasia because “a lack of conceptual clarity for a construct is the first step toward measurement heterogeneity, measurement flexibility, and the profusion of untested measures” (Flake & Fried, 2020; p. 460). Clearly, the relationship between operationally defining a construct and assessing that construct is symbiotic (Scheel et al., 2021). If there is a lack of clarity in what aphasia diagnostic tests capture, our understanding of aphasia will suffer and, in turn, will negatively affect aphasia rehabilitation. Therefore, the purpose of this paper is to examine the content of aphasia diagnostic batteries as one window into how the field, historically and currently, defines aphasia.

There is agreement among validity theorists that test validation requires a detailed, substantive explanation of the mechanisms that produce observed response outcomes, either in terms of a causal relationship between the underlying attribute and the response (Borsboom et al., 2004) or the processes that determine responses to individual items (Zumbo, 2009). As Zumbo (2009, p. 71) writes, “validity, per se, is not established until one has an explanatory model of the variation in item responses and/or scale scores and the variables mediating, moderating, or otherwise affecting the response outcome.” Therefore, validity of tests goes beyond convergent correlations with other tests but considers how underlying theories may shape those tests (Zumbo, 2009). We limit the focus of our paper to language impairment since dominant diagnostic tests of aphasia have been closely linked to how aphasia has been conceptualized (historically) as a language disorder.

To understand how the field thinks about aphasia, one must consider the long history of diverse epistemological influence and debate (Petheram & Parr, 1998). We cannot provide a full account of the history of clinical aphasiology in this paper; we refer interested readers to Tesak & Code (2008). Instead, we briefly outline three major epistemological streams in the field (Figure 1) and how they have shaped our understanding of aphasia today.

The first epistemological stream is the localizationist approach to aphasia, which focuses on the brain-behavior relationships underlying language impairment. The prominence of brain-behavior relationships and localizationism in clinical aphasiology is not surprising since most definitions of aphasia include the qualities of acquired and brain injury in their definitions. This epistemological perspective has led to the development of the classical aphasia sub-types (e.g., Broca’s, Wernicke’s, Conduction), defined in the late 1800s by Wernicke and Lichtheim (Lichtheim, 1885), that are still in common use over 125 years later (Graves, 1997).

The second epistemological stream is holism, which emphasizes that language is distributed in the brain, a direct response against the localizationism movement in aphasiology. In particular, proponents of a holism perspective argue that the use of aphasia sub-types based on localization is not useful for understanding a specific patient’s functioning and behaviors (Friedrich, 2006; Joswig & Hildebrandt, 2017; Tremblay & Dick, 2016). Furthermore, what causes aphasia disrupts not just language, but the whole person, including psychosocial impacts (Goldstein, 1927), planting the seeds for the more modern life participation approach to aphasia (Chapey et al., 2000).

The third epistemological stream is the field of psycholinguistics, which emerged from the psychology of language. Psycholinguistics began to be applied to aphasiology in the late 1800s (Steinthal, 1871), also in response to the localizationism movement. Proponents of the psycholinguistic perspective of aphasia emphasized the need to understand precisely what is being localized in the brain. “Language” is too general of a term; thus, a focus was on understanding which linguistic domains were impacted by aphasia, and how so (Spreen, 1968).

The differing perspectives on aphasia, along with the seminal work of Weisenberg & McBride (1935) to systematically assess language and cognition in persons with and without aphasia, culminated in the release of several aphasia diagnostic tests beginning in the 1960s and 70s, some of which are still popular in clinical aphasiology today. Many of the initial aphasia diagnostic batteries had a very large, primary focus on assessing language impairment given the current (at that time) definition of aphasia. In general, the strong influence of the localizationist epistemology in aphasiology has led to diagnostic tests being grouped together by whether they align with the localizationist epistemology or whether they were developed in (opposing) response. We selected a series of aphasia diagnostic tests to examine in the present paper based on this coarse categorization: We selected the Boston Diagnostic Aphasia Examination (BDAE; Goodglass & Kaplan, 1972), the Western Aphasia Battery (WAB; Kertesz, 1979), and the Quick Aphasia Battery1 (QAB; Wilson et al., 2018) as historically and currently dominant aphasia tests that embody (at least in part) the localizationist epistemology (or, at minimum, the importance of brain-behavior relationships in assessment), while we selected the Minnesota Test for the Differential Diagnosis of Aphasia (MTDDA; Schuell, 1964), the Porch Index of Communicative Ability (PICA; Porch, 1967), and the Comprehensive Aphasia Test (CAT; Swinburn et al., 2004) as historically and currently dominant aphasia tests developed in (opposing) response to localizationist epistemology. We provide a brief summary of each of these diagnostic tests in Table 1.

Continued research on defining the linguistic and cognitive domains and neural processes associated with aphasia (e.g., Halai, et al., 2022; Lacey et al., 2017), as well as calls for clearer translation of diagnostic tests for clinical practice (Byng et al, 1990; Nickels, 2005) suggest that a clear and specific operational definition of aphasia continues to elude us. Even the progress of Berg and colleagues (2022), in coordination with the Collaboration of Aphasia Trialists, to produce a modern definition of aphasia, highlights that there are points of an aphasia definition that are not unanimously agreed upon. Given that the role of any diagnostic test is to 1) identify whether a condition is present or not and 2) provide detailed information on the nature of the condition if present, the content of aphasia diagnostic tests may be critically informative to understanding how aphasia may be defined in the field.

Although current views of test validity emphasize the primacy of construct validity (e.g., Zumbo, 2009), validity is currently generally considered a unitary property that may be supported by different kinds of evidence, among which is evidence based on test content (AERA, APA, NCME, 1999; Messick, 1995; Sireci, 1998). We used methodology described by Fried (2017) in statistically investigating content overlap among depression scales to examine and quantify the content of the six aphasia diagnostic tests indicated above. While we would anticipate that tests may vary to some degree in content, based most likely on differing epistemological influences (otherwise there would be a single diagnostic test for aphasia), we also anticipate that there should be a set of core content that is consistent across tests since they all presumably assess the same construct – aphasia (or at minimum language impairment, given the primary aims of the selected tests). It is this set of core content that may implicitly point to a possible operational definition of aphasia that would be accepted by the field, which may or may not need to be updated or refined based on current research and perspectives of aphasia.