Neuroimaging Experiments

In order to test the feasibility of using the LED fibre-optic system for stimulus presentation in the MRI environment, three fMRI experiments were conducted.

3.2.4.1 Experiment 1

The first experiment tested whether the visual stimulus produced by the fibre-optic system can elicit activation in the primary visual cortex. In this experiment, the participants were asked to fixate on a central fixation dot while passively viewing an alternating visual stimulus presented in either the left or right visual field. Four fibre-optic prisms were mounted on a flat surface near the top of each participant‟s lap, with two prisms each on the left and right visual fields, respectively, with a central fixation dot in between. Visual stimuli were positioned in the peripheral visual field, 8o horizontally from the central fixation point and 1o above the fixation, stimulus dimensions were 1o (W) × 0.8o (H). Stimuli were presented in white color at 25% maximum brightness level. Participants viewed the visual stimuli through the mirrors mounted on top of the head coil, directly above their eyes. They were instructed to keep their eyes open and to maintain central fixation throughout the experiment. Stimuli were presented in a block design with 15s block duration of alternating „stimulus‟ and „rest‟ periods (see Figure 3.5).

With the aim of distinguishing lateralized visual cortex activation, „stimulus‟ periods were further divided into two distinct „left stimulus‟ and „right stimulus‟ conditions. In the „left stimulus‟ condition, a 4Hz contrast-reversing checkerboard-like stimulus was presented on the left side of the visual field. Similarly, in the „right stimulus‟ condition, the same type of stimulus was presented on the right hand side. Flickering checkerboard-like stimuli were chosen because it has been shown in many visual retinotopic mapping studies that this kind of stimulus elicits a strong and reliable haemodynamic response in the occipital cortex (Engel et al., 1997). Three right-handed participants (two males, one female, aged 29-32, with normal or corrected-to-normal vision) were recruited for the first experiment and gave written

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informed consent according to the guidelines approved by Cardiff University‟s School of Psychology Ethics Committee. Each participant was scanned in one 25-minutes session, consisting of five individual runs, which lasted for 300s each. Overall, 25 blocks were obtained for each condition.

Figure 3.5. Diagram of the experimental procedure of Experiment 1. For each stimulus block, odd and

even frames were continuously alternated at 4Hz for a period of 15s. A 15s rest period followed, during which no stimulus was presented. Following the rest period, another alternating pattern was displayed for 15s, this time in the opposite visual field.

3.2.4.1 Experiment 2

The second fMRI experiment was conducted to test the feasibility of the fibre-optic system for presenting coloured visual stimuli. Several human neuroimaging studies have demonstrated that ventral occipital regions (V2/3 and V4) respond strongly to chromatic stimuli in both passive viewing (Beauchamp et al., 1999; Wade et al., 2002) and colour discrimination tasks (Barrett et al., 2001; Ting Siok et al., 2009). In this experiment, the participant was asked to perform a simple colour discrimination task by judging whether the colour of the stimulus presented on the left hand side was the same or different to the stimulus colour presented on the right hand side. Visual stimuli were presented using two fibre-optic

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prisms, with each prism in the left and right visual field surrounding a central fixation dot. Stimuli were positioned in the peripheral visual field, 6.5o horizontally from the central fixation dot and 1o vertically above fixation. The dimension of each stimulus was 0.5o (W) x 0.8o (H). Consistent with the first experiment, the participant was instructed to view the stimulus through the mirror and maintain central fixation throughout the experiment.

A simple block design was used in the fMRI task with 15s block duration alternating between „stimulus‟ and „rest‟ (see Figure 3.6). Each block consisted of 5 individual trials. For each trial, 2Hz flickering checkerboard-like stimulus was presented in both left and right visual fields for 1s, followed by response collection for another 1s, and a 1s ISI. The „stimulus‟ blocks were further divided into blocks containing either coloured-stimuli or achromatic gray- stimuli. In the coloured stimuli condition, the stimulus on the left hand side was displayed with either the same or different hue value as that of the stimulus on the right hand side. During the response period, the participant was instructed to press one out of two response buttons that signified whether the colours in each visual field were the same or different, using a LumiTouchTM MRI optical keypad (Photon Control, Inc.). Three different hue values were used for the stimuli, including pure red, pure green and pure blue. Prior to the experiment, the brightness level of each colour was matched subjectively by the participant. Equal numbers of „same‟ and „different‟ coloured stimuli were presented, with the order of stimuli randomized. Gray-stimuli blocks were introduced as the control condition. In this condition, instead of comparing the colour of the stimuli, the participant was asked to compare the brightness level between the left and the right stimuli. All stimuli in this block were presented in white colour with three different levels of brightness (10%, 15%, and 25%). One right-handed male participant (aged 28, with normal vision) was recruited for the second experiment and gave written informed consent according to the guidelines approved by Cardiff University‟s School of Psychology Ethics Committee. The participant was scanned in a short 24-minutes session, consisting of three individual runs, each lasting 8 minutes. Overall, 24 blocks were obtained for each condition.

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Figure 3.6. Diagram of the experimental procedure for Experiment 2. In the coloured-stimuli blocks,

the participant performed a simple colour discrimination task by comparing the hue of the left and right stimulus (same or different), whereas in the grey stimuli block, the participant compared the brightness of the left and right stimulus.

3.2.4.3 Experiment 3

In the third and final experiment, a simple multisensory fMRI experiment was conducted to establish the feasibility of combining the fibre-optic system with the QuaerosysTM tactile stimulator. The aim of the experiment was to visualize brain regions involved in bimodal visuo-tactile processing. Several neuroimaging studies of multisensory interactions in the human brain have shown that multimodal stimuli strongly activated cortical multisensory regions including the lateral occipital region, MT, the superior temporal region, the ventral intraparietal region, premotor cortex, and others (Macaluso and Driver, 2005; Beauchamp, 2005).

A simple blocked fMRI design (Figure 3.7) was employed, which allowed us to compute the contrast of activations correlated with bimodal visio-tactile processing against activations correlated with unisensory processing (visual or tactile). Each block consisted of 5 individual trials. On each trial, a low-frequency (6Hz) or high-frequency (12Hz) stimulus was presented for 500ms, and the participant performed a frequency discrimination task by pressing the

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appropriate response button. Visual stimuli were presented using one fibre-optic prism positioned 6.5o to the left of the fixation dot. The subject rested the tip of their left index finger on top of the tactile stimulator positioned directly below the prism, inside the black Perspex box (Figure 3.1f). Similar checkerboard-like patterns were used for both visual and tactile stimuli. The colour of the visual stimuli was white at 25% maximum brightness level and the intensity of the tactile stimulation was at 30% maximum level. In the multisensory condition, both visual and tactile stimuli were delivered simultaneously at the same frequency. The participant who was recruited for the second experiment also participated in the third experiment and was scanned again in a 30-minute session, consisting of four individual runs, each lasting 7.5 minutes. Overall, 20 blocks were obtained for each condition.

Figure 3.7. Diagram of the experimental procedure of Experiment 3. In all conditions, the participant

performed a simple frequency discrimination task by deciding whether the frequency of the visual flicker and/or tactile vibration was high (12Hz) or low (6Hz). The stimulus was presented for 500ms and the response was collected up to 1s after the stimulus offset.

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