Manuela Ruzzoli

Modelling non-invasive brain stimulation in cognitive neuroscience

Non-invasive brain stimulation (NIBS) is a method for the study of cognitive function that is quickly gaining popularity. It bypasses the correlative approaches of other imaging techniques, making it possible to establish a causal relationship between cognitive processes and the functioning of specific brain areas. Like lesion studies, NIBS can provide information about where a particular process occurs. However, NIBS offers the opportunity to study brain mechanisms beyond process localisation, providing information about when activity in a given brain region is involved in a cognitive process, and even how it is involved. When using NIBS to explore cognitive processes, it is important to understand not only how NIBS functions but also the functioning of the neural structures themselves. We know that NIBS techniques have the potential to transiently influence behaviour by altering neuronal activity, which may have facilitatory or inhibitory behavioural effects, and these alterations can be used to understand how the brain works. Given that NIBS necessarily involves the relatively indiscriminate activation of large numbers of neurons, its impact on a neural system can be easily understood as modulation of neural activity that changes the relation between noise and signal. In this review, we describe the mutual interactions between NIBS and brain activity and provide an updated and precise perspective on the theoretical frameworks of NIBS and their impact on cognitive neuroscience. By transitioning our discussion from one aspect (NIBS) to the other (cognition), we aim to provide insights to guide future research.

The mechanism of transcranial magnetic stimulation in cognition

Carlo Miniussi*, Manuela Ruzzoli and Vincent Walsh Department of Biomedical Sciences and Biotechnology, National Institute of Neuroscience, University of Brescia, Italy Cognitive Neuroscience Section, IRCCS San Giovanni di Dio Fatebenefratelli, Brescia, Italy Department of Neurological and Vision Sciences, University of Verona, Italy Institute of Cognitive Neuroscience and Department of Psychology, University College London, UK

Is Transcranial Alternating Current Stimulation Effective in Modulating Brain Oscillations

Transcranial alternating current stimulation (tACS) is a promising tool for modulating brain oscillations, as well as a possible therapeutic intervention. However, the lack of conclusive evidence on whether tACS is able to effectively affect cortical activity continues to limit its application. The present study aims to address this issue by exploiting the well-known inhibitory alpha rhythm in the posterior parietal cortex during visual perception and attention orientation. Four groups of healthy volunteers were tested with a Gabor patch detection and discrimination task. All participants were tested at the baseline and selective frequencies of tACS, including Sham, 6 Hz, 10 Hz, and 25 Hz. Stimulation at 6 Hz and 10 Hz over the occipito-parietal area impaired performance in the detection task compared to the baseline. The lack of a retinotopically organised effect and marginal frequency-specificity modulation in the detection task force us to be cautious about the effectiveness of tACS in modulating brain oscillations. Therefore, the present study does not provide significant evidence for tACS reliably inducing direct modulations of brain oscillations that can influence performance in a visual task.

Effects of right parietal transcranial magnetic stimulation on object identification and orientation judgments

We investigated the role played by the right parietal lobe in object identification and the ability to interpret object orientation, using transcranial magnetic stimulation (TMS) to momentarily interfere with ongoing cortical activity. Short trains of TMS pulses (12 Hz) were applied to a site overlying the right intraparietal sulcus/inferior parietal lobe while subjects performed either object identification tasks (i.e., picture-word verification and categorizing objects as natural or manufactured) or object orientation judgment tasks (i.e., picture-arrow verification and deciding whether an object was rotated clockwise or counterclockwise). Across different tasks, right parietal TMS impaired orientation judgments, but facilitated object identification, compared to TMS applied to a brain vertex control site. These complementary findings demonstrate that the right parietal lobea region belonging to the dorsal visual streamis critical for processing the spatial attributes of objects, but not their identity. The observed improvement in object recognition, however, suggests an indirect role for the right parietal lobe in object recognition. We propose that this involves the creation of a spatial reference frame for the object, which allows interaction with the object and the individuation of specific viewing instances.

Transcranial stimulation and cognition

Noninvasive brain stimulation (NIBS) is a unique method for studying cognitive function. For the study of cognition, NIBS has gained popularity as a complementary method to functional neuroimaging. By bypassing the correlative approaches of standard imaging techniques, it is possible to establish a putative relationship between brain cognition. In fact, functional neuroimaging data cannot demonstrate the actual role of a particular cortical activation in a specific function because an activated area may simply be correlated with task performance, rather than being responsible for it. NIBS can induce a temporary modification of performance only if the stimulated area is causally engaged in the task. In analogy with lesion studies, NIBS can provide information about where and when a particular process occurs. Based on this assumption, NIBS has been used in many different cognitive domains. However, one of the most interesting questions in neuroscience may not be where and when, but how cognitive activity occurs. Beyond localization approaches, NIBS can be employed to study brain mechanisms. NIBS techniques have the potential to influence behavior transiently by altering neuronal activity, which may have facilitatory or inhibitory behavioral effects. NIBS techniques include transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (tES). TMS has been shown transiently to modulate neural excitability in a manner that is dependent mainly on the timing and frequency of stimulation (high versus low). The mechanism underlying tES is a change in neuronal membrane potentials that appears to be dependent mainly on the direction of current flow (anodal versus cathodal). Nevertheless, the final effects induced by TMS or tES depend on many technical parameters used during stimulation, such as the intensity of stimulation, coil orientation, site of the reference electrode, and time of application. Moreover, an important factor is the possible interactions between these factors and the physiological and cognitive state of the subject. To use NIBS in cognition, it is important to understand not only how NIBS functions but also the brain mechanisms being studied and the features of the area of interest. To describe better the advanced knowledge provided by NIBS in cognition, we will treat each NIBS technique separately and underline the related hypotheses beyond applications.

The neural basis of the Enigma illusion: A transcranial magnetic stimulation study

The aim of this study was to test the role of the visual primary (V1) and the middle temporal area (V5/MT) in the illusory motion perception evoked by the Enigma figure. The Enigma figure induces a visual illusion that is characterized by apparent rotatory motion in the presence of a static figure. By means of repetitive transcranial magnetic stimulation (rTMS) we show that V5/MT is causally linked to the illusory perception of motion. When rTMS was applied bilaterally over V5/MT just prior to presentation of the Enigma figure, the perception of illusory motion was disrupted for approximately 400 ms resulting in a delayed illusion onset. In contrast, rTMS applied over V1 did not have any effect on the illusory perception of motion. These results show that V5/MT, a visual cortical area associated with real motion perception, is also important for the perception of illusory motion, while V1 appears not to be functionally involved in illusory motion perception.

Accurate and Rapid Estimation of Phosphene Thresholds (REPT)

To calibrate the intensity of transcranial magnetic stimulation (TMS) at the occipital pole, the phosphene threshold is used as a measure of cortical excitability. The phosphene threshold (PT) refers to the intensity of magnetic stimulation that induces illusory flashes of light (phosphenes) on a proportion of trials. The existing PT estimation procedures lack the accuracy and mathematical rigour of modern threshold estimation methods. We present an improved and automatic procedure for estimating the PT which is based on the well-established Ψ Bayesian adaptive staircase approach. To validate the new procedure, we compared it with another commonly used procedure for estimating the PT. We found that our procedure is more accurate, reliable, and rapid when compared with an existing PT measurement procedure. The new procedure is implemented in Matlab and works automatically with the Magstim Rapid2 stimulator using a convenient graphical user interface. The Matlab program is freely available for download.

The effect of TMS on visual motion sensitivity: an increase in neural noise or a decrease in signal strength?

The underlying mechanisms of action of transcranial magnetic stimulation (TMS) are still a matter of debate. TMS may impair a subjects performance by increasing neural noise, suppressing the neural signal, or both. Here, we delivered a single pulse of TMS (spTMS) to V5/MT during a motion direction discrimination task while concurrently manipulating the level of noise in the motion stimulus. Our results indicate that spTMS essentially acts by suppressing the strength of the relevant visual signal. We suggest that TMS may induce a pattern of neural activity that complements the ongoing activation elicited by the sensory signal in a manner that partially impoverishes that signal.

Confounders in the detection of minimal hepatic encephalopathy: a neuropsychological and quantified EEG study

Chronic alcohol misuse, HCV infection and cirrhosis may cause cognitive alterations. The aim of the present study was to assess the influence of alcohol misuse, HCV infection and cirrhosis per se on the neuropsychological and electroencephalogram (EEG) profile and to evaluate the role of alcohol misuse and HCV infections as potential confounding factors in the detection of minimal hepatic encephalopathy.

Modality-switching in the Simon task: The clash of reference frames

The representation of spatial information related to an event can influence behavior even when location is task-irrelevant, as in the case of Stimulus–Response (S-R) compatibility effects on the Simon task. However, unlike single-modality situations, which are often used to study the Simon effect, in real-life scenarios various sensory modalities provide spatial information coded in different coordinate systems. Here, we address the expression of S-R compatibility effects in mixed-modality contexts, where events can occur in 1 of various sensory modalities (i.e., vision, touch or audition). The results confirm that, in single-modality cases, Simon effects in vision are expressed in an external spatial frame of reference, while touch information is coded anatomically. Remarkably, when mixing visual and tactile trials in an unpredictable way, the Simon effect disappeared in vision whereas tactile Simon effects remained expressed in their own (anatomical) frame of reference. Mixing visual and auditory stimuli did not obliterate the visual Simon effect and S-R compatibility effects in an external reference frame were evident for both modalities. The extinction of visual Simon effects as a result of mixing visual and tactile modalities can be interpreted as a consequence of the dynamic reorganization of the weights associated to the different sources of spatial information at play.