An impairment of word finding or naming abilities (“anomia”) is the most common symptom of aphasia following brain injury and can be caused by a breakdown at any of the processing components involved in word production . Importantly, naming performance can be improved in individuals with aphasia [2–4]. In fact, although naming treatments often utilize multiple exposures to repeated stimuli, a single application of a language task (“facilitation”) can have an effect upon subsequent naming performance [2–5]. This is also the case for unimpaired speakers, with previous behavioral research showing that the simple act of naming a picture once can speed subsequent naming of the same picture . However, the mechanisms underpinning both the facilitation of unimpaired naming and treatment-induced improvement in individuals with aphasia remain uncertain. The present study, therefore, aimed to examine the effects associated with a specific semantic facilitation technique on subsequent picture naming performance in both individuals with aphasia and age-matched controls using fMRI.
In unimpaired speakers, improved naming performance following facilitation may reflect a form of repetition priming. These priming effects are thought to modify access to stored cognitive representations during first presentation, so that recognition and retrieval processes are enhanced on subsequent presentations [7, 8]. It has been argued that the successful treatment of naming deficits in individuals with aphasia may rely on the same mechanisms that underlie priming in unimpaired naming [2, 9]. Successful naming is a complex linguistic operation involving multiple sub-processes. Most theoretical models of spoken word production include at least two major levels of processing involved in picture naming: a lexical-semantic stage, where the meaning of a concept or picture is connected to its corresponding abstract lexical representation, and a phonological stage, where the relevant lexical-semantic unit is mapped onto the phonological word form [10–13]. Although underlying deficits and resulting symptom profiles differ markedly amongst individuals with aphasia, facilitatory techniques have traditionally targeted either the semantic or phonological level of processing [14, 15]. This is due to the commonly held belief that semantic and phonological techniques target distinct components of the impaired word production system [16, 17]. More recently, it has been proposed that these different types of tasks may also engage different neural regions [18–21]. It has been suggested that semantically based tasks are most effective for individuals whose primary area of deficit involves the semantic level of processing and, similarly, phonological tasks more effective for those with a phonological impairment [16, 22]. However, this is not always the case and it is difficult to determine with any certainty the processing level at which facilitation is taking place . Indeed both semantic and phonological processing are likely to occur during most language-related tasks, indicating that the differences between such techniques may be overstated [23, 24].
It also appears that the beneficial effects resulting from phonologically- and semantically-focused tasks differ with respect to their longevity. Behavioral studies in both healthy controls and individuals with aphasia provide evidence for short-lived improvements (up to periods of several minutes) following a phonological task (but see ) and longer lasting benefits from a semantic task [4, 8, 25]. However, in the majority of studies utilizing a semantically focused task the phonological word form is provided in either the spoken or written modality, and it is thought that these longer-term effects may in fact be acting by strengthening the mapping operation between the processing levels . This idea is supported by treatment and facilitation studies in aphasia that find limited benefits from semantic tasks that do not involve the relevant word form and which conclude that the phonological form is the critical element of such techniques [26, 27]. While there is clearly a need to more comprehensively investigate the longevity of priming effects on naming, these findings support the proposal that tasks directly targeting either the lexical-semantic or phonological levels alone result in only short-term benefits, whereas longer lasting effects rely on activation of both semantics and phonology, by strengthening the mapping between them [24, 25].
The use of functional neuroimaging in aphasia is beginning to provide more evidence regarding the neurocognitive substrates of treatment-induced improvement . For example, studies utilizing a variety of linguistic tasks have shown that reorganization of function and recovery is dependent upon modulation of neural activity in spared left hemisphere language-related regions [29, 30]. The role of the right hemisphere is less clear, with some research pointing towards a maladaptive functional reorganization to right hemisphere language homologues [31, 32] and other studies proposing that both right and left hemisphere mechanisms contribute significantly to language recovery . Various explanations have been put forward to account for inconsistencies across the literature, including the observation that the nature of relateralization may vary depending upon the size of the lesion [34, 35]. Additionally, areas not traditionally associated with linguistic processing have been shown to support recovery from anomia in some individuals . There is a growing body of literature attempting to detail the various neural mechanisms underlying improvement of language function, including research exploring the role of specific tasks used in treatment and the exact nature of their effects.
By way of example, several studies relevant to the current investigation have utilized fMRI to quantify neural changes prior to, and following, some form of intensive semantically focused treatment. A single case pilot study by Davis, Harrington and Baynes  used a decision-based semantic intervention and found improved covert verb generation associated with increased activation in several of their defined regions of interest following treatment, including the left inferior frontal gyrus and right posterior inferior temporal region. The authors suggest that increased activity in the left inferior frontal area may be due to their treatment task requiring selection of semantic information from competing alternatives. Increased activity in the right inferior temporal region, an area contralateral to the participant’s lesion, was taken to reflect a compensation of function by the right hemisphere homologue, or a change in strategy. Although one of the aims of their study was to determine whether word retrieval could be improved without explicit naming, silent production of responses during scanning can be problematic. It is difficult to ensure task compliance, and previous research has identified differences in brain activation patterns for covert and overt verbal responses  which may limit the findings. Additionally, the auditory cue used to prompt verb generation included the word form of the pictured item (e.g. for the picture ‘BALL’, the corresponding question was “What do you do with a ball?”). While the task was verb generation, participants heard the word form of the pictured item and may have attempted to covertly name the picture, making it difficult to isolate any effects to semantic processing.
Fridriksson et al.  also utilized a semantic treatment. Their study examined the neural correlates of improved naming by comparing both a semantic and phonological intensive treatment in three individuals with aphasia, two of whom suffered from non-fluent type aphasia. Their semantic treatment took the form of a detailed auditory cueing hierarchy that did not include the phonological form until the final cue, which for both treatment hierarchies involved auditory repetition of the target item. All three participants benefitted behaviorally to varying degrees from both techniques and generally showed increased activity within neural regions not traditionally linked to language processing. The authors proposed that their findings of activation in regions not normally associated with language processing (e.g., right entorhinal cortex and bilateral precuneus) may represent a form of compensatory cortical adaptation, as opposed to any specific repair of the normal language network [14, 38].
Researchers examining brain activity before and after treatment of aphasia commonly face the difficulties inherent in interpretation of imaging data when comparing patterns of activity during poor performance prior to treatment, to good performance following treatment. A complementary way to advance understanding of the mechanisms underlying treatment is to look more directly at the brain activity occurring during performance of a particular treatment task. Perani et al.  compared activation during both a phonological and semantic covert verbal fluency task in five participants with aphasia. For patients with good recovery, the principal finding regarding the semantic task was activation in the left inferior frontal gyrus, possibly reflecting the use of an effective and effortful lexical retrieval strategy. However, in the case of those patients with impaired performance, there was extensive bilateral prefrontal activation, which may have indicated increased “mental effort” required for retrieval attempt rather than retrieval success .
A more recent study compared a semantic judgment task to a control task  in eight participants with aphasia who differed in terms of site of lesion. The results of the experiment indicated that all individuals without damage to the left inferior frontal region activated the left inferior frontal gyrus, similar to their control group of eight individuals. They concluded that successful completion of a complex semantic judgment task required the inhibition of competing items. In contrast, those participants who did have some inferior frontal lesion involvement activated contralateral regions, in addition to perilesional left frontal regions. It should be noted that the semantic judgment task used in their study involved the presentation of written words and therefore included the orthographic form of stimuli. The presence of the word form means that orthographic, phonological and lexical-semantic processes were likely to be engaged during performance of the task.
Although no definitive picture has emerged regarding the neural correlates associated with performance of particular language tasks used in treatment, or the mechanisms involved in the recovery of language following treatment, together these studies demonstrate that semantic interventions can result in positive effects for some individuals. Facilitation-induced improvement in performance, however, has not been as extensively investigated. Studies exploring the facilitatory effects of specific treatment tasks upon subsequent naming are principally limited to behavioral studies [2–5]. The examination of naming facilitation in conjunction with neuroimaging may increase our knowledge regarding the relationship between facilitation and treatment and may enable us to determine how specific treatments are having their effects at a neurocognitive level. Moreover, the ability to measure the bases of successful naming facilitation may provide a method for predicting positive treatment outcomes in individuals with anomia .
Therefore, in the present study we aimed to determine the behavioral and neurocognitive effects of picture naming facilitation by a prior semantic task in both unimpaired speakers and individuals with aphasia using fMRI. Unlike other studies, we did not utilize fMRI before and after a semantic task, or simply have participants perform the task within a scanner. Instead, participants performed a semantic facilitation task and then fMRI was used to investigate the differences in neural activation between facilitated and unfacilitated conditions during subsequent picture naming. To minimize the involvement of any phonological processing, the semantic questions used during the facilitation task did not include the phonological word form and participants were not required to produce the word form in response. We also sought to determine the longevity of any effects by manipulating the timing of the prior semantic task. Consequently, the long-term condition was facilitated several days prior to subsequent naming, and the delay between facilitation and subsequent naming for the short-term condition was a period of several minutes. Importantly, the three main picture naming conditions of interest (long-term facilitated, short-term facilitated and unfacilitated) were presented within a single scanning session. To our knowledge, no other neuroimaging study has utilized this specific design to test the underlying cognitive mechanisms involved in successful picture naming following facilitation with a targeted semantic task in both unimpaired speakers and individuals with aphasia.
It was hypothesized that participants with aphasia would engage similar regions to age-matched controls or their right hemisphere homologues during successful naming of previously facilitated items, with associated modulation of activity primarily in regions linked to semantic processing. This would be evident by minimal involvement of areas linked to phonological processing and significant changes in activation within a combination of spared left hemisphere semantic regions (or areas close to damaged semantic regions), and/or in the right hemisphere homologues of those regions. Further, we tested the hypothesis that our targeted facilitation task would result in relatively short-term effects upon subsequent picture naming, based on previous behavioural observations [8, 25, 40]. Findings in line with this hypothesis would indicate that temporary facilitation was occurring selectively at the lexical-semantic level, as opposed to strengthening the links between the lexical-semantic and phonological levels of processing. It was also hypothesized that any longer lasting effects would involve areas linked to both lexical-semantic and phonological processing, indicating that more durable facilitation from a semantic task is dependent upon a strengthening of connections between the two levels of processing .