Twelve adults (6F, 6M) aged 21 to 33 participated in the experiment. All were right handed, native English speakers with no personal or family history of epilepsy or any other neurological condition. Each gave written informed consent after the experimental methodology was explained. The experiments were approved by the Local Research Ethics Committee.
There were 50 trials of each task. Half of the words referred to man-made items (e.g. “axe”, “canoe”, “pillow”) while half were natural items (e.g. “apple”, “cat”, “rain”). All of the stimuli were highly familiar, concrete nouns matched across category for rated familiarity (mean for man-made vs. natural: 530 vs. 530, t45
<1.0), written frequency (18 vs. 25, t41
<1.0), number of letters (5.3 vs. 5.5, t48
<1.0), and number of syllables (1.6 vs. 1.7, t48
<1.0) (Coltheart, 1981
). There was a trend for man-made items to be rated as less concrete than natural items (592 vs. 606, t36
=2.0, p=0.06). The same stimuli were used in both tasks so that any differences between tasks could be unambiguously associated with the tasks rather than the stimuli. To avoid biasing the result with repetition priming effects, stimulus repetitions were counterbalanced across tasks. On average, word repetitions were separated by 105s.
In the scanner, a mixed block and event-related design was used. Tasks were blocked in an ACBC fashion with semantic (A) and perceptual (B) decisions separated by 13 seconds of rest (C). Stimuli were presented for 500ms with 2700ms of blank screen between trials for an inter-trial interval of 3.2s. Stimuli were presented out of phase with data acquisition to ensure an unbiased sampling of the haemodynamic response (Price, Veltman, Ashburner, Josephs, & Friston, 1999
). Within each of the task blocks, 10 trials were presented and the items were pseudorandomly ordered. During the session 150 T2*-weighted images were collected. An additional 4 dummy volumes were collected at the start of the session to allow for T1 equilibrium before the test trials began. In total, the scanning session lasted 7.5 minutes. The data from this experiment form a subset of a study of task differences reported previously (Devlin, Matthews, & Rushworth, 2003
Scanning was carried out using the Varian-Siemens 3 Tesla MRI scanner at the Functional Magnetic Resonance Imaging of the Brain (FMRIB) Centre in Oxford. A Magnex head-dedicated gradient insert coil was used in conjunction with a birdcage head radio-frequency coil tuned to 127.4 MHz. A gradient-echo EPI sequence was used for image collection (TR 3s, TE 30ms, 64 × 64 resolution, 256mm × 256mm FOV). 25 slices were employed to cover the brain with 5mm slice thickness and in-plane resolution of 4mm. An automated shimming algorithm was used to reduce magnetic field inhomogeneities (Wilson et al., 2002
) and a TE of 30ms jointly optimized BOLD contrast-to-noise and image signal-to-noise while minimizing intra-voxel de-phasing.
Functional images were realigned (Friston et al., 1995
) to correct for small head movements using the Statistical Parametric Mapping software (SPM99, Wellcome Institute of Cognitive Neurology, www.fil.ion.ucl.ac.uk/spm99
). Translation and rotation corrections did not exceed 2.3mm and 2.5°, respectively for any of the participants. The mean image created by the realignment procedure was used to determine the parameters for transforming the images onto the MNI mean brain. The normalization parameters were then applied to the functional images (Ashburner & Friston, 1997
; Ashburner, Neelin, Collins, Evans, & Friston, 1997
). Finally, each image was smoothed with a 5mm at full-width half-maximum (FWHM) Gaussian filter. The SPM software was used to compute a random-effects analysis using the general linear model. Trials were modelled as events and the stimulus train was convolved with a “canonical” HRF (Friston et al. 1994; Glover 1999) which was used in all subsequent linear contrasts. Temporal derivatives were included to better fit regional deviations in timing and thus reduce unmodelled variance; they were not used in any contrasts. The interaction between Task and Category was computed by contrasting Natural > Man-made for the semantic task with Man-made > Natural in the perceptual task. Because the interaction is two-tailed, the regional BOLD response in each condition was plotted to determine the direction of the interaction. All analyses were limited to the posterior fusiform gyri bilaterally using four a priori
regions-of-interest (ROIs) based on previously published findings. Chao et al. (2002)
reported activation for Animals > Tools at −38 −58 −10 and +36 −58 −10 while the reverse contrast revealed activation at −25 −57 −7 and +22 −57 −5. Each of these coordinates served as the centre of a spherical volume with a 10mm radius which was used to restrict the search space. Given the specificity of the search space, the statistical threshold was set to p<0.05, uncorrected.