Visual motion processing and TMS: increase of neural noise or reduction of signal strength?

Transcranial magnetic stimulation (TMS) is a tool which allows cognitive neuroscientists to come through the correlative approach between brain and cognition and to state a causal relationship between a cerebral area and a cognitive function. Although TMS is widely engaged in basic and clinic research, its precise mechanisms of action are poorly understood. The “virtual lesion” hypothesis is the only theoretical framework available in order to interpret TMS data. In analogy with the lesion studies on animals or humans, it states that TMS acts inducing a temporary and reversible lesion in the brain, allowing the experimenter to study the functional role of the stimulated area into the cognitive network without confounds such as cerebral reorganization or plasticity phenomena. Nevertheless, the “virtual lesion” hypothesis does not provide any clear suggestions about the mechanisms of action of TMS. It states that TMS can act either suppressing the strength of the signal or introducing random neural noise in the stimulated area, both leading to an impairment of subject’s performance. The aim of the present dissertation is to address how the electrical activity introduced by TMS in the neural system interacts with the on-going neural activity already present into the brain. In the first experiment repetitive TMS (rTMS) has been applied during a motion direction discrimination task. By means of psychophysics, the results showed that rTMS acts introducing neural noise into the V5/MT area, an extra-striate visual area responsible for motion perception. On the contrary, single pulse TMS (spTMS) applied on the same area and during the execution of the same task, led to different results: single pulse TMS suppressed the strength of the signal. These data were confirmed when the physiological state of V5/MT was changed by means of feature-based attention. In the third experiment spTMS reduced the gain of an attentional cue directed to a particular feature of the stimulus during a motion direction discrimination task, but it did not affect the tuning mechanism of feature-based attention. In conclusion, different TMS parameters lead to different results regarding the interaction between the on-going neural activity and the activity induced by TMS. Single pulse TMS acts by a suppression of the strength of the signal, while rTMS introduces neural noise in the stimulated area. The importance of TMS parameters, the importance of the physiological state of the stimulated area and its morphological properties, as well as how the target area works are discussed as constrains in TMS studies. In the first chapter (Introduction) the topic will be introduced, briefly focusing on some basic principle of TMS. It will be addressed the main theoretical framework within TMS data are interpreted (i.e., “virtual lesion” hypothesis), considering its advantages and limitations. Furthermore, some experimental evidences about the effects of TMS on the neural activity coming from animals’ studies will be introduced. Finally, I will refer to some basic knowledge on the morphological and functional properties of V5/MT, and to some TMS data present in literature about TMS application in a motion direction discrimination task. The second, third and fourth chapters represent the experimental corpus (main corpus) of the dissertation, in which three experiments will be detailed introduced, described and discussed, separately. In the chapter 2 (rTMS and motion processing) rTMS has been applied on V5/MT during a motion direction discrimination task. In the chapter 3 (spTMS and motion processing) the same task as in previous experiment, has been used with spTMS In the chapter 4 (spTMS, attention and motion processing), the physiological state of V5/MT has been changed by means of feature-based attention (by an attentional cue directed to the direction of motion). A spTMS has been delivered over V5/MT in order to study the interaction between attention and TMS modulation on visual perception. In the last chapter (chapter 5 –General discussion) the main findings of the entire work will be summarized and discussed.