Summary of Experiment Two:

The main aim of Experiment Two was to provide further information regarding the

relationship between neuro-oscillatory processes and the tactile-induced DFI. The ultimate

goal here was to expand our understanding of the underlying mechanism behind the DFI

tasks. With the ultimate view to provide us with further details of the mechanism underlying

general multisensory processing.

We first wished to provide some evidence of a possibility to experimentally

manipulate beta frequencies using the ccPAS method of neuro-modulation. As the results of

Experiment One had suggested to us that instead of local oscillatory activity influencing the

properties of the illusory effects, it was indeed properties of the functional connection

between cortices that set the fate of the illusion. Thus, we wished to stimulate this

connection with a view to reducing the speed of the beta processes that, according to our

theory, subtends it. Whilst evidence exists to suggest a capability to use tACS to modulate

local visual processes in order to modulate the auditory-induced DFI (Cecere et al., 2015),

we wished to modulate properties of a functional connection, thus for our research a

different methodology was required. Evidence exists to suggest that the ccPAS method can

be used to modulate functional connectivity between two regions of the brain (Buch et al.,

2011; Chiappini et al., 2018; Rizzo et al., 2011; Rizzo et al., 2009; Romei et al., 2016).

However, this was the first time that the stimulation had occurred over such a long-range

network. Furthermore, the method has not in the past been used in the investigation of

neuro-oscillatory processes. Hence whilst we used an established (whilst still relatively new)

method, we wanted to assess the capabilities of this technique beyond what had already

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As stimulation took place exclusively on the right hemisphere we should only expect

to see an effect when the tactile stimuli are presented to the participant’s left hand, but not

their right hand. During this investigation we were measured occipital beta whilst each task

was taking place (either the left-hand or the right-hand task). As a result two different

connections are at play here, one subtending the right somatosensory area and the visual

cortex (as in the left-hand condition) and one subtending the left somatosensory area and

the visual cortex (as in the right-hand condition). This should have led to a differentiation of

occipital beta tuning depending on which task was being completed. As we were using the

ccPAS method to exclusively target the network in use for the left-hand condition (i.e. right

somatosensory to visual), post-ccPAS measures of the IBF speed and the TWI size should

have been the same as pre-ccPAS measures when tactile stimuli were presented to the right

hand. Specifically we expected to see a modulation of the tactile-induced DFI only when the

network involved corresponded to the one targeted by the ccPAS, showing the state-

dependent network specificity of the effect.

Crucially, our intention here was to reduce the speed of the beta processes

subtending the connection. In this case we first measured each person’s visual IBF while

they performed the tactile-induced DFI task either with tactile stimuli being presented to

either their left or their right hand. This IBF value (in ms) is what we hypothesised to

correspond to the time it takes information from somatosensory regions to reach the visual

regions. We then used IBF value that was uncovered when tactile stimuli were presented to

the left hand and calculated, for each individual, a value that was 3 Hz slower than the

original IBF value. We then used the duration of one cycle of this new modulated frequency

value (in ms) to set the spacing of the paired TMS pulses. These were delivered exclusively

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tune the communication of the network at a rather slower rate corresponding to a value

slower than that of the IBF. In other words, the manipulation was aimed at reducing the

speed of the beta processes subtending the connection between the somatosensory and

the visual cortices. We theorised that this would manifest as a reduced beta frequency in

visual areas post-ccPAS.

In line with our hypothesis, our results showed a clear decrease in beta frequency

post-ccPAS, as measured in the visual cortex. This suggests that our method was successful

in reducing the speed of the beta processes subtending the connection. Crucially, this effect

of stimulation appears strictly state-dependent as it was only found for those blocks in

which the tactile stimuli were delivered over the left hand. Instead, no difference was found

when they were delivered to the right hand. This finding would be consistent with a

network specific effect, as the modulation of the beta peak in expected directions can only

be appreciated for the right (but not left) somatosensory to visual cortex network

manipulation, the same network targeted by our ccPAS procedure.

Given previous findings of the correlation between beta processes and the tactile-

induced DFI, we assumed that by modulating the speed of these beta processes we may

also have been able to modulate certain aspects of the perceived illusion which we have

shown to be related to the speed of beta oscillations, namely the size of TWI. If a change

was found here then this provide support to out theory that the properties of the tactile-

induced DFI are causally set by the properties of the functional connection subtending the

two cortices. As we found that the length of IBF appears to be directly related to the size of

the TWI, we also expected that a decrease in IBF speed following our ccPAS protocol should

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expected to once again be state-dependent. This was as no change in beta speed was found

in blocks where the tactile stimuli post ccPAS were delivered to the right hand, and hence

this should lead to no change in TWI.

Again, what we found was in line with our hypothesis, we found that post-ccPAS

measurements of the tactile-induced TWI appeared to be significantly wider than pre-ccPAS

measurements. This was exclusively the case for when the tactile stimuli were presented to

the left hand, but not the right, once again demonstrating the network specificity of this

effect. In this instance we believe this may represent promising evidence that highlights the

potential relevance of neural communication at the network level through frequency

specific oscillatory activity.

Despite these promising results, at the conclusion of Experiment Two we did not

have any particularly compelling evidence to discount one other alternative explanation for

the pattern of these results. That is we could interpret the reduction in beta speed (and in

turn the widening of the TWI) as a result of inserting noise into the network. This in turn

would be independent of the precise timing that we used in this experiment. If this was to

be the case then these findings tell us very little about the specific network and instead

would suggest that we have simply induced a degradation of the TWI as a result of noise

insertion. As a result we felt that it was imperative to conduct a further experiment, this

time making use of a control condition to the ccPAS procedure. In other words a condition

mimicking the stimulation whereby no changes are expected. The option that we decided to

utilise was a method where we based the timing of the paired TMS pulses on the IBF value

exactly, that is the normal timing that we believe the system to operate at (instead of a

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mimic entirely the procedure that participants experienced during Experiment Two, yet as

we were stimulating at normal processing speed we should expect to see no modulation of

IBF or TWI post-ccPAS. Of course this would not be the case if the changes observed in

Experiment Two were as a result of this possible noise insertion, in which case we should

expect to see results similar to those found in the current investigation. Using this method

to tease apart these two hypotheses, Experiment Three looked to investigate this further.