fMRI evidence for a phonological excitation account of associative facilitation in picture naming Greig de Zubicaray1, Kori Johnson1, Katie McMahon1, Niels Schiller2

1University of Queensland, Australia. 2University of Leiden, Netherlands.

MOG

IFG (Ptri)

IFG (Ptri)

pMTG

/STGIFG (Ptri)

IFG (Porb)

d

c

b

a

pMTG

/STG

AG

MOG

MOG

Associate > Unrelated

Associate > Neutral

Categorical > Neutral

Unrelated > Neutral

Studies of semantic context effects in speech production typically distinguish between two types of relationships between experimental stimuli; those involving shared semantic features and those involving association. In the former, stimuli are members of the same category (e.g., animal) and are usually coordinates (e.g., mouse and fox) that share visual or structural features (such as fur and four legs), while in the latter the conceptual relationships are more heterogeneous. They may be reflected in the co-occurrence of items in language use (e.g., mouse and cheese), or in normative output of free-association to lexical cues.

Introduction Associative and categorical relationships produce different context effects in the picture-word interference (PWI) paradigm.

Compared to distractor words unrelated to the target picture name, categorically related distractors slow naming responses when presented at short stimulus onset asynchronies (SOAs), an effect termed semantic interference (SI). By contrast, associate distractors facilitate naming responses at predominantly early SOAs. Recent accounts of associative facilitation in the PWI paradigm have attributed the effect to semantic priming (e.g., Mahon et al., 2007). However, an earlier phonological account attributed the effect to activation spreading between excitatory links at the word form level (Cutting & Ferreira, 1999).

The aim of the present fMRI study was to determine the locus of the associative facilitation effect in PWI, contrasting predictions from the rival accounts. Candidate regions were determined from a meta-analysis that indicated roles for the left mid-portion of the middle temporal gyrus (MTG) and left posterior MTG/superior temporal gyrus (pSTG) in lexical-semantic and word form processing, respectively (Indefrey & Levelt, 2004). A semantic priming account of associative facilitation thus predicts reduced activity in left mid-MTG, the opposite polarity to that predicted for SI. By contrast, if associative facilitation is due to the excitation of connected word forms, one would expect this to be reflected in activity primarily in left pMTG/STG.

Methods Participants. 17 healthy participants (9 female) with a mean age of 20 years. Stimuli. 25 B/W line drawings of common objects served as targets. For each picture, a categorically related distractor word was selected that was not an associate, along with a word that was associatively related and not a member of the same superordinate category according to free association norms. The target pictures and distractor words were alternately paired in related and unrelated lists to build conditions. Distractor words were a subset of those used by Sailor et al. (2009). Target pictures were also presented without distractor words in a neutral condition. Procedure. Two experimental blocks comprising 63 and 62 trials, respectively (25 pictures presented in 5 conditions: neutral/no distractor, associatively related, categorically related, associative control/unrelated, categorical control/unrelated). A fixation point (+) was shown for 50 ms, followed by the presentation of the distractor word at an SOA of -150 ms and the picture. Distractor and picture remained on the screen for 375 ms and 750 ms respectively. Naming responses were recorded with an MR-compatible microphone and RTs determined with voice-key software. Image aquisition. Images were acquired with a 4T Bruker Medspec system. 125 GE EPI volumes of 36 axial 3.5 mm slices (0.1 mm gap) were acquired (true TR, 3 s; effective TR, 15 s; TE, 30 ms; flip angle, 90o). Naming trials were interleaved with image acquisition using sparse temporal sampling to capture the peak BOLD signal response to task-related neural activity and allow stimuli to be presented and responses made without interfering gradient noise. Image analysis. Data were realigned, normalised to atlas space, smoothed with a 9 mm FWHM kernel and analysed in SPM8 using a repeated measures ANOVA on the participants’ contrast images from each of the 5 experimental conditions, followed by planned t-contrasts. A priori regions of interest (ROIs) were defined for small volume corrections (SVCs). SPMs were height thresholded at p < .001 and cluster thresholded at p < .05 corrected across the whole brain and for ROIs.

Results Error trials (5.6%) were excluded from analysis. Mean naming RTs as a function of distractor condition are given in Table 1. There was a significant main

effect of condition, F(4, 64) = 12.7, MSE = 2055.33, p < .001. Planned contrasts (one tailed paired t-tests) revealed faster RTs for associates compared to their unrelated control distractors, t(16) = -2.08, p < .05. Pictures in both associate and control conditions were named more slowly compared to the neutral, naming alone condition, t(16) = 3.64, p < .005 and t(16) = 4.06, p < .005, respectively. However, RTs for categorically related distractors showed only a marginally significant trend toward being named more slowly than their unrelated controls, t(16) = 1.68, p = .056. Pictures in both categorically related distractor and control distractor conditions were named more slowly compared to the neutral, naming alone condition, t(16) = 6.30, p < .001 and t(16) = 4.94, p < .001, respectively.

The fMRI analyses (Figure 1) revealed significantly increased activity for associates compared to their unrelated controls solely in the left pMTG/STG and angular gyrus (Z = 4.78, peak -54, -60, 24; ROI Z = 4.11, peak -51, -57, 18). No significant activity was detected for the categorically related distractors compared to their unrelated controls at the whole brain level or in any ROI. Compared to the neutral condition, associates showed increased activity in the pars triangularis and pars orbitalis of the left IFG (Z = 6.09, peak = -54, 21, 18), left pMTG/STG and angular gyrus (Z = 5.0, peak -60, -48, 6; ROI Z = 5.0, peak -60, -48, 6), and right MOG (Z = 4.29, peak 42, -69, 6). Categorically related distractors also showed increased activity compared to the neutral condition in the pars triangularis of the left IFG (Z = 3.62, peak = -48, 18, 21), and right MOG (Z = 4.39, peak 39, -81, -3). Unrelated distractors showed increased activity compared to the neutral condition in the left IFG (pars triangularis; Z = 4.48, peak = -54, 21, 18) and right pMTG/MOG (Z = 5.0, peak 42, -69, 0).

References Cutting, J. C., & Ferreira, V. S. (1999), 'Semantic and phonological information flow in the production lexicon'. Journal of Experimental Psychology: Learning, Memory, and Cognition, vol. 25, pp. 318-344.

Indefrey, P., & Levelt, W. J. M. (2004), 'The spatial and temporal signatures of word production components'. Cognition, vol. 92, pp. 101-144.

Mahon, B. Z.et al. (2007). Lexical selection is not by competition: A reinterpretation of semantic interference and facilitation effects in the picture word interference paradigm. Journal of Experimental Psychology: Learning, Memory and Cognition, vol. 33, pp. 503-535.

Sailor, K., et al. (2009), ‘Exploring the time course of semantic interference and associative priming in the picture-word interference task’. Quarterly Journal of Experimental Psychology, vol. 62, pp. 789-801.

Figure 1. Cerebral regions showing significant activity in the planned contrasts (t-tests) within regions showing a significant main effect of distractor condition, superimposed on the surface of an individual brain (height thresholded at p < .001 and cluster thresholded at p < .05). (a) Associative > Unrelated, left lateral view (b) Associative > Neutral (no distractor), left and right lateral views; (c) Categorical > Neutral; (d) Unrelated > Neutral, left and right lateral views. IFG - inferior frontal gyrus; Porb - pars orbitalis; Ptri - pars triangularis; pMTG/STG - posterior middle and superior temporal gyri; AG - angular gyrus; MOG - middle occipital gyrus.

Funded by : ARC DP1092619 GZ supported by FT0991634 NS supported by the Netherlands Institute for Advanced Study (NIAS)

Table 1. Mean naming latencies (RTs, in milliseconds) as a function of distractor condition and effects Distractor Type RT Effect Associate 1237 (97)

-22

Associate control 1259 (116)

Categorical 1287 (108) 17

Categorical control 1270 (110)

Neutral (none) 1206 (91) Standard deviations in parentheses !

Discussion In the present study, associative distractor-target relationships resulted in significant facilitation of picture naming latencies compared to an unrelated condition.

This facilitation effect occurred in conjunction with a significant increase in fMRI activity solely in left posterior temporal cortex. By contrast, categorical distractor-target relationships had only a marginal effect on picture naming latencies at the same SOA, and no significant activity increases or decreases were observed in the fMRI data compared to an unrelated distractor condition. Overall, pairing distractor words with target pictures during naming elicited fMRI activity in a predominantly left hemisphere cortical network compared to naming alone.

The finding of increased fMRI activity solely in the left pMTG/STG for the significant associative facilitation effect is in line with a phonological excitation explanation, if one assumes the activation of multiple word forms, at least temporarily (Cutting & Ferreira, 1999). The SI effect was marginally significant in the naming RT data in the present study, hence the results can be considered broadly consistent with those reported by Sailor et al. (2009). However, the RT results of the current sample were considerably slower than those reported in comparable behavioural studies, and the increased variability might explain the failure to detect significant fMRI activity corresponding to SI. More research is needed to delineate the extent to which phonological and other accounts can explain associative facilitation effects for different types of semantic relation.

www.fmrilab.net greig.dezubicaray@uq.edu.au