Conclusion:

To conclude, in this study we set out to provide a complex investigation of the

properties of both the auditory-induced and the tactile-induced Double Flash Illusions,

mainly focusing on the latter of these two effects. The main aim here was predominantly to

focus on providing specific details on the underlying mechanisms subtending the two

illusory effects. This was with a view of informing our overall understanding as to the way in

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We first wished to re-confirm previous findings that had suggested a tight functional

relationship between occipital alpha frequencies and the temporal profile for the auditory-

induced DFI (Cecere et al., 2015). In this instance, we were indeed able to replicate these

previous findings, finding a significant correlation that was also shown to survive Robust

Skipped Correlations (Pernet et al., 2013), we take this as evidence of the robustness of this effect.

We were also able to provide key details on the tactile-induced DFI, providing for the

first time a measurement of its temporal profile (100 ms). We also provided evidence of a

relationship between this TWI and the corresponding TWI of the auditory-induced effect.

Incidentally these values were also found to not differ significantly from one another.

What we also found was evidence of visual beta frequencies playing a role in this

tactile-visual processing, providing evidence of a tight relationship between peak beta

frequency in the visual cortex and the TWI for this tactile-induced effect. This was found to

be the case for when tactile stimuli were presented to both the left hand and to the right.

Robust Skipped Correlations once again supported the robustness of these effects.

Instead of both illusory effects being determined by local visual processes we believe

this may mean that there is a differential effect for both tasks. It could be that the functional

connectivity between the two interconnected cortices set the fate of the illusory effects. We

theorise that the processing speed of information subtending these connection could be set

by the pre-synaptic region. Thus the time it takes for information from the auditory or

somatosensory cortex to reverberate into the visual cortex could correspond to the time it

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the case of the auditory-visual effect this would thus implicate alpha processes and beta

processes would subsequently be implicated for the tactile-visual counterpart.

In Experiment Two we wished to stimulate the connection between the

somatosensory cortex and the visual cortex to test for the specific hypothesis that the

oscillatory frequency coding for the TWI is determined by the specific timing of network

communication. In this research we were able to implement a modified version of a neuro-

modulatory protocol known as ccPAS (originally introduced with the specific aim of

modulating functional connectivity) in order to experimentally modify the connectivity

between the target areas. This was done by specifically reducing the timing of

communication between the two nodes of the target network which we thought was

indexed by the speed of occipital beta frequencies. The manipulation of the ccPAS timing to

reflect a slower pace of beta oscillations resulted in a corresponding reduction in beta

speed, as measured at the visual cortex, which in turn was shown to subsequently correlate

with an increase in the temporal size of the TWI for the tactile illusion. This once again

provides evidence that it appears to be properties of the connection subtending the two

interconnected regions that sets the fate of the illusory response, rather than local

properties as was originally assumed.

Finally, in Experiment Three we also provided a control condition to this ccPAS

protocol. In this case, instead of stimulating at a time slightly slower than normal processing

speed (i.e. the cycle duration of one beta wave), we stimulated at a time directly

corresponding to this value. As we were only stimulating at the normal processing speed of

the network we theorised that we should expect no change in beta processing speed, nor

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we wished to mimic the conditions of the previous stimulation, using a parameter that

should result in no beta modulation. This was with a view of finding a differentiation of

results and hence eliminating the possibility that previous findings only occurred as a result

of a general slowing down of neuro-oscillatory processes associated with inserting noise into

the system.

In this condition, in line with our expectations, no change in beta processing speed

was found post-ccPAS. In addition, no change in the size of the TWI was found either. This

suggests that the stimulation that we used in the previous investigation was strictly

frequency specific and the changes that we observed we not simply due to a general

slowing down of processing as a result of inserting noise into the system. This also provides

further evidence of the role of functional connectivity in multisensory processing.

In this investigation we concluded that the specific mechanism subtending the

specific parameters of the illusory effects may be comparable across sensory modalities but

simultaneously they also reflect the peculiarity of each sensory modality that is being

utilised, including temporal resolution. In other words, we theorise that auditory and tactile

crossmodal induced visual illusions might have been caused by the specific oscillatory

properties of each sensory signal’s pairing. The different oscillatory processes linked to

these effects could be explained as the specific computational speed needed by the cross-

sensory network to efficiently integrate information, thus representing the optimal

quantum for temporal binding between a given cross-sensory pair when impacting visual

processing specifically.

In addition to providing information on these illusions and in turn general

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the ccPAS method as a way of modulating long-range multisynaptic connections. Previous

research has only demonstrated the effectiveness of the method in terms of shorter-range

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

Here however we have provided evidence to suggest that the method can also be used to

stimulate at a longer-ranges between more remote, but still functionally interconnected

areas of the brain (in this case, from the somatosensory cortex to the visual cortex). In

providing this research we also demonstrate the specificity of the method. Finally we also

provide evidence for the first time that this method can be used as a way of investigating

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