An introduction to Experiment Three:

Based on the promising results of Experiment Two we wished to investigate the

phenomenon further by providing a second control condition to our experiment. More

specifically we wanted to include a condition in which the ccPAS protocol was mimicked

superficially yet with parameters manipulated so that no changes were expected. This was

something that ideally would have been completed as part of Experiment Two, but the time

constraints of an already lengthy experiment meant that this was difficult, if not impossible

to perform. As such despite the promising results that we uncovered in the previous

experiment we could not exclude alternative explanations for the change in beta frequency

and the subsequent change in TWI size. In this case, instead of the specific temporal

parameters of the ccPAS pulse timing (i.e. utilising a reduced beta frequency to base the

timings on) inducing this change in processing speed, it could alternatively be a marker of a

general sensory processing slow-down. This could be as a result of simply inserting

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specific properties of our stimulation resulted in this change or if by blindly stimulating this

network, using any random pulse timing we would have induced identical changes

regardless of the specific properties.

As we did not as yet have compelling evidence to determine this one way or

another, in Experiment Three we aimed to provide somewhat similar research, yet we

manipulated the parameters of the stimulation, so that we would instead have expected no

change between pre- and post-ccPAS measurements of both the IBF and the TWI. Our main

aim during this study was to once again use the ccPAS method to stimulate the functional

connection between the somatosensory and the visual cortices. However in the previous

study we attempted to modulate this connection by basing the pulse timing directly on a

reduced version of the participant’s IBF. This was with a view to fine-tuning the network and

subsequently reducing the speed of the oscillations subtending it, ultimately resulting in us

observing a reduced occipital IBF value post-ccPAS. As well as a subsequent increase in the

relative size of the tactile-induced TWI.

In this current investigation however, we based the timing of the paired TMS coil

pulses exclusively on the exact IBF value (in ms). According to our theory, this value

corresponds exactly to the time that it takes for the influence of the somatosensory cortex

to reverberate into the visual cortex. As we were stimulating at this exact value, and not

reducing it, as per the previous investigation, we theorised that this would not have resulted

in any reduction in the IBF value. If we were indeed to find this to be the case then we

would also have expected to see no change between pre-stimulation and post-stimulation

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This study had two overall aims, initially we looked to uncover further evidence as to

the underlying mechanisms governing multisensory information. Our second major aim here

was to provide further testing to a relatively new method of neuro-stimulation. As we have

utilised this method somewhat differently to other researchers we wished to ensure that

our methods are carefully thought out and leave as little room for ambiguity as possible.

Firstly we looked to stimulate the functional connectivity over far longer-range networks

than had previously been demonstrated; as such we aimed to stimulate using much larger

pulse delays. Secondly we also hoped to provide evidence for the first time to suggest that

this method can also be used reliably to inform our understanding of the oscillatory activity

subtending the functional connections that it is stimulating. As such, providing this control

condition we hoped to further develop on the compelling evidence that already exists from

the previous investigation and to confirm the efficacy of the methods that we used.

If indeed our postulations were correct and the communication between cortices are

dependent on the wave frequency travelling between them, stimulating at, or around the

beta frequency would allow us to demonstrate an ability to modulate these functional

connections simply by modulating the specific wave frequencies associated with them.

Our reasoning behind this investigation is that by stimulating using the exact

communication time of this network (and not a reduced time) we would have expected to

see no change in beta speed post-ccPAS. Due to the tight link between these beta processes

and the tactile-TWI values presented in the previous experiments, we also expected to find

no changes in the temporal window of this illusion post-ccPAS.

After participants first completed a tactile-induced DFI task (left hand only) we then

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induced TWI and the IBF values to post-ccPAS measures. Stimulation once again occurred

between electrode C4 (right somatosensory cortex) to Oz (visual cortex), as was the case in

the previous study. We then directly compared the change in IBF and TWI seen in

Experiment Three with the results seen in Experiment Two. Where we expected to see no

change in post-ccPAS IBF or TWI, compared to pre-ccPAS measures.

If our intended findings were confirmed, then we would take this as promising evidence

for a frequency specific mechanism subtending the tactile-induced DFI, specifically

governing the somatosensory visual functional network giving rise to the studied illusion.

Finally, we would also provide evidence that the ccPAS method could reliably be used to

experimentally modulate the functional connectivity between longer-range networks

(utilising longer pulse delays than what has previously been reported). This would also

suggest to us that the method can reliably be used to modulate the speed of neuro-

oscillatory process and that this method is both network (left vs. right somatosensory to

visual) and frequency specific.

In this experiment we were indeed able to uncover some further promising results.

Firstly we were able to provide evidence of the expected correlations. Crucially, we also

found no significant change in IBF value after the TMS protocol took place. We took this as

compelling evidence that in the previous experiment we were indeed able to explicitly

manipulate the properties of the functional connectivity between the somatosensory and

the visual cortices using our method. We take this as promising support to our initial

postulations, and only by stimulating this network using the specific parameters that we

used (i.e. a reduced IBF value) could we replicate the results. Furthermore, pre- and post-

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magnitude of this value took place. We take this as further evidence for the role of

functional connectivity between cortices in influencing the properties of multisensory

integration. This investigation also provides evidence for the efficacy of the ccPAS method in

stimulating longer-range functional connections. It also demonstrates for the first time that

the technique can also be used to inform our understanding of neuro-oscillatory processes

and how these relate to the communication between two functionally connected areas of

the brain.

The follow chapter will now look to assess the ccPAS method in more detail. It will also

discuss another technique we are using, namely encephalography as well as discussing some

of the wider background of the TMS machine that the ccPAS protocol relies upon.

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