Methods Experimental Setup And Scanning

The aims of the current experiment were to determine the optimal stimulation rates that would produce either the clearest delineation of somatopy or the most consistently statistically significant change in BOLD signal in SI.

5.3.1 Subject and Scanning Details

Nine right-handed males (mean age 25, age range 21-34) served as subjects. The local ethics committee approved the experimental procedure and all subjects signed a consent form before being scanned. The data were acquired on a Siemens MAGNETOM Vision (Siemens, Erlangen, Germany) at 2T. Each BOLD-EPI volume scan consisted of 48 transverse slices (inplane matrix 64x64; voxel size 3x3x3mm; TE=40ms; TR=4.11s).

1120 volume scans in total were collected from each subject (5 sessions x 224 scans per session). A T1-weighted high-resolution MRI of each subject (1 X 1 X 1.5 mm resolution) was acquired to facilitate anatomical localisation of the functional data. Auditory stimuli were delivered by a custom-built sound delivery system, with headphones designed to attenuate scanner noise (Palmer etal., 1998).

5.3.2 Experimental Setup

The five gutters were positioned on the appropriate fingers of the right hand of each subject, and adjusted so that the distal nozzle was positioned over the centre of the digit pad (in a similar fashion to Figure 5.3). The five distal nozzles were connected to individual airlines so that each of the digit pads could be stimulated independently. Examples of stimuli at each frequency were delivered before scanning to ensure that subjects could perceive the stimuli, and that similar intensities were perceived on each finger. Most subjects (7/9) reported that the lOHz stimulation felt slightly weaker than the IHz stimulus.

During scanning, subjects lay supine on the scanner bed with their right hand resting on their torso. They were instructed to close their eyes and concentrate on the pattern of stimulation. Subjects received 24 epochs of airpuff stimulation (6 epochs x 4 different rates - 1,2,5 and lOHz) per session (224 scans in length), delivered to a single digit. Each subject received five sessions of stimulation, one per digit. Each epoch of stimulation was 5 scans in length (5x4.11 TR = 20.55s) and was alternated with periods of no stimulation lasting

4 scans (4x4.11 = 16.44s). Epoch order was pseudo-randomised (i.e. blocks of epochs of 1,2,5 and lOHz stimulation were randomised) within-sessions and across subjects to account for possible expectation and order effects. Session order (i.e. digit stimulation order) was randomised across subjects.

Amplitude-modulated white noise stimuli were played periodically to prevent subjects hearing the airpuffs during scanning (repetition rate IHz; duration 500ms, attack 50ms decay 450ms; programming by I. Johnsrude). All subjects reported being unable to hear the stimuli.

5.3.3 Image Preprocessing and D ata Analysis 5.3.3.1 Spatial Preprocessing

Data preprocessing was carried out using SPM99 (Wellcome Dept, of Cognitive Neurology, London, UK; http:/www.fil.ion.ucl.ac.uk/spm) implemented in Matlab5 software. All preprocessing was carried out in a similar fashion to previous experiments (as described in section 3.2.3.3). The initial four scans were removed to allow for T % saturation effects (as opposed to two in section 3.2.3.3), and subjects’ scans were realigned across all five sessions. Two subjects’ data sets contained significant amounts of task- correlated motion (as detected by inspection of movement parameters) and were therefore excluded from subsequent analysis. All functional volumes were smoothed using a 8mm FWHM isotropic Gaussian kernel.

5.3.3.2 Data Analysis

Data analysis used SPM99 as before. Functional volumes from all sessions were treated as a timeseries, and experimental effects estimated using a multi­ session design matrix that included separate session mean terms. Each subject’s data were analysed separately in a single-subject, five-session design matrix. Individual session partitions of the design matrix consisted of the timecourse of the four experimental covariates (IHz, 2Hz, 5Hz and lOHz stimulation) modelled as box-car functions convolved with the expected HRE, and six covariates representing the estimated movement parameters for each scan (obtained from the realignment parameters). To remove low-frequency noise the data were high-pass filtered using a set of discrete cosine basis functions with a minimum cut-off period of 370s. Temporal autocorrelation was dealt with using the method of Worsley and Friston (1995), by temporally smoothing the session time series with a Gaussian kernel of 6s FWHM.

Linear contrasts of the covariates were evaluated for the effects of stimulation at each rate/digit (4x5=20 contrasts) and the average effect of stimulation at all rates/digit (1x5=5 contrasts). Results were displayed as a voxelwise statistical parametric map of t values. As I was most interested in the contralateral SI and bilateral superior lateral sulcal areas, voxels in these areas were reported as significantly active with a p value of /7<0.00001 (corrected for the number of voxels using the areas’ estimated volumes; Kennedy et al., 1998). Voxels in other brain areas were reported as significant if they survived a correction for multiple comparisons over the entire volume 0?<0.05 corrected). I did not use a cluster threshold for the current analysis to ensure maximal sensitivity to potentially small effects.

5.4 Results

5.4.1 Single Rate/Digit Contrasts

The main effects of each single digit/frequency contrast (5digitsx4 frequencies = 20 contrasts) were examined for each subject. The results of each contrast are summarised in Table 5.1 below. I wanted to address two independent issues: which rate of stimulation produced the ’best’ pattern of digit somatopy within SI (i.e. that which agreed with the Penfieldian pattern); and which rate of stimulation was best at eliciting SI activation consistently across digits, whether the spatial pattern was somatopical or non-somatopical. The assessment of both of these hypotheses was made difficult due to the lack of activations in almost half the subjects. Three subjects showed little activation in any o f the single rate/digit contrasts tested (subject 4, subject 6 and subject 7). The subsequent analyses therefore focused on the remaining four subjects.