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Bursts of high frequency random noise stimulation (tRNS) increase arousal in a discriminative reaction time task.
No full textArousal reflects a state of generalized physiological activation, and different neuronal nuclei are responsible for its regulation through widespread projections to the cortex. Reaction times (RTs) have been used as a behavioral measure of arousal, and in a condition of increased arousal it has been showed a reduction of RTs, denoting a performance improvement. Recent modeling studies on transcranial electrical stimulation (tES) suggest a wide diffusion of the current across different regions of the cortex. On this basis it has been suggested that, sometimes, the effect of the tES might not be specific for the stimulated site but it might induces a general arousal. In the light of these results, the aim of the present study was to increase arousal using tES during a discriminative RTs task, in order to improve the performance. 16 healthy volunteers (8 F, mean age 25) participated to the experiment. The task was a continuative quasi-random visual presentation, for 100 ms, of digits from 1 to 9, with a variable inter-trial interval of 800-2000 ms. Subjects had to press response buttons for target digits (8-9) which were presented after a warning digit (1). We evaluated RTs using a paradigm that has been showed to be sensitive to the modulation of the arousal. Skin conductance response was recorded during the experiment to have also a physiological index of arousal. The experiment was divided in 6 blocks. During blocks 2, 3 and 4, bursts of high frequency random noise stimulation (tRNS) were administered to the subjects concurrently to the presentation of the warning digit, with the aim to increase the endogenous activation related to the preparation of the response. Each burst (2 mA) lasted for 900 ms. We used a pair of circular electrodes (22.8 cm2) placed with a fronto–occipital montage (FPz–Oz). Real and sham stimulation was balanced in a within subjects design. Results showed a significant reduction of RTs during the second block of stimulation (interaction between condition and block, p = <.001) denoting a performance improvement. Concerning skin conductance, analyses showed a main effect of condition (p = <.001) with a higher phasic activation during the real stimulation session, which was maintained over time. During the sham session, instead, the phasic activation decreased in a faster way, according to a physiological habituation to the repetitive stimuli. These results support the effectiveness of bursts of tRNS to increase arousal over time, measured both with a physiological (skin conductance) and a behavioral (RTs) index. Modulations over the behavioral performance, however, are weaker in comparison to the physiological ones, probably due to the less sensitivity of the behavioral measure used in the present research. The study of the tES effects on other behavioral tasks through the use of additional physiological measures of arousal (e.g., pupil dilation, EEG) are the future steps needed to support this promising research line
CORRELATI ELETTROFISIOLOGICI DELLA PERCEZIONE DEL CONTRASTO
No full textIntroduzione:
La percezione del contrasto è uno degli aspetti primari della percezione visiva e, nell’uomo, è stata solitamente studiata con un approccio di tipo psicofisico. Nel presente lavoro abbiamo registrato l’attività elettroencefalografica in soggetti sani durante un compito di detezione e discriminazione in cui veniva presentato uno stimolo gabor a diversi livelli di contrasto.
Metodo:
In questo studio hanno partecipato 6 soggetti. Lo stimolo presentato variava, con incremento esponenziale, tra 6 diversi livelli di contrasto, il più basso dei quali corrispondeva al livello di soglia psicofisica, calcolata individualmente per ogni soggetto. Le analisi si sono focalizzate sia sulle risposte comportamentali (accuratezza e tempi di reazione), che sugli indici elettrofisiologici evocati dallo stimolo, come i potenziali evento relati (P100, N2pc) e l’attività oscillatoria cerebrale.
Risultati:
I risultati hanno messo in evidenza come sia gli indici comportamentali che gli indici elettrofisiologici incrementavano seguendo un trend non lineare rispetto alla modulazione del contrasto, che aumentava in maniera esponenziale tra le diverse condizioni sperimentali. Mentre l’ampiezza della componente visiva precoce P100 si modulava in base alle caratteristiche fisiche dello stimolo, la componente più tardiva N2pc variava con un andamento simile alla performance del soggetto, apparendo predittiva del comportamento. L’analisi delle frequenze ha inoltre evidenziato un incremento in potenza, in relazione alla variazione del contrasto, in specifiche bande di frequenza come theta (4-7Hz) e alpha (8-14Hz), mentre non è stata osservata alcuna modulazione delle frequenze alte.
Conclusioni:
I dati di questo studio dimostrano come l’attività corticale registrata dalle aree visive possa considerarsi maggiormente legata agli aspetti percettivi del soggetto piuttosto che alle caratteristiche fisiche dello stimolo
Arousing auditory stimulus modulates performance during a discriminative reaction time task
No full textThe aim of the present study was to investigate the effect of an arousing auditory stimulus without emotional connotation (brief bursts of white noise presented at a volume of 90 decibel) on normal subject performance, during a discriminative reaction time (RT) task. Two groups of subjects, with different arousal states, were recruited. The first group included subjects familiar with the experimental setting, while the second group included students participating to the experiment for a credit reward for a university course. The subjects of the latter group had a higher anxiety level as evaluated with a dedicated questionnaire. The task was a continuative quasi-random visual presentation of digits. Subjects had to press response buttons for target digits which were presented after a warning digit. During half the blocks of the task, the arousing auditory stimulus was presented to the subjects through headphones concurrently to the presentation of the warning digit. The first group of subjects showed a significant steep reduction of RTs during the presentation of the arousing stimulus, consistently with an increased arousal level, that was maintained over time. On the contrary, the second group showed slower RTs during the presentation of the arousing stimulus, due to the achievement of a non-optimal arousal level (too high). These results showed that it is possible to modulate the arousal, during a RT task, using an auditory stimulus without emotional connotation. The different performance of the subjects, related to the arousal state, also support the inverted U-shaped curve between arousal and performance
Nonlinear properties of visual perception: electrophysiological evidence in humans
No full textVisual perception in humans has been investigated mainly through a psychophysical approach, which allows investigating perceptual experience as a function of physical features variations. However, relatively little is known about the neuronal correlates of stimulus contrast perception in the human brain. In the current work we recorded the electroencephalographic (EEG) activity in healthy participants during a detection and discrimination task where a visual stimulus (i.e., gabor) was presented under six contrast levels. We analyzed how the contrast levels modulate behavioral and physiological responses. Six healthy volunteers (age 24-34 years; 4 female) participated in the study. The stimuli could have six different contrast levels, presented in a random order. Only the first value was adjusted at the individual threshold level, the others were an exponential increase of the first (~3%, 6%, 12%, 25%, 50% and 100%; Michelson contrast index). In each trial, the participants had to perform two tasks, first a gabor detection task (yes/no) and then a orientation discrimination task (leftward/rightward). EEG was recorded continuously during all the experiment from 32 electrodes mounted on an elastic cap according to the 10-20 international system. Analyses were focused both on the behavioral responses (i.e., accuracy and reaction times) and physiological responses, such as evoked oscillatory activity in the 4-80Hz frequency range and event related potentials (P100, N2pc). Results showed that both behavioural and electrophysiological indexes increased in a non-linear trend in relation to contrast modulation, which increased exponentially through the conditions. The frequency analysis revealed a modulation of specific frequency bands such as theta (4–7Hz) and alpha (8–14Hz) which showed an increase of power as the contrast increased. No modulation of the high frequencies was observed. Concerning event related potentials, changes in the early visual P100 amplitude were more affected by exogenous stimulation while the late N2pc component varied consistently with the performance, appearing to be predictive of the behaviour. The correlation analysis applied to the estimated spline functions of the behavioral and electrophysiological indexes revealed that theta frequency and N2pc component were highly correlated to response accuracy, more than alpha band and P100. These data suggest that activity in visual areas corresponds to subjects’ percept and both can be similarly modulated by physical variations
Combining transcranial electrical stimulation with electroencephalography: a multimodal approach
No full textAlthough numerous studies have been performed using transcranial electrical stimulation (tES), our understanding of tES-induced effects on neural activity remains limited, especially regarding the effects on neural networks. The use of an approach, such as electroencephalography (EEG) in combination with tES, could allow for a more detailed understanding of the neural mechanisms involved in these observed changes. Co-registration of tES and EEG might provide high temporal resolution information regarding tES-induced modifications/modulations to cortical activity that corresponds to different stages of processing. This article aims at presenting new knowledge about this recent and innovative approach that can possibly provide information about the dynamics of human brain functions beyond what is possible by the use of either method alone
Transcranial direct current stimulation effects on spontaneous and evoked cortical activity: a combined TMS/EEG study
No full textExcitability modulation of the motor system induced by transcranial direct current stimulation: A multimodal approach
No full textAnodal and cathodal transcranial direct current stimulations (tDCS) are both established techniques to induce
cortical excitability changes. Typically, in the human motor system, such cortical modulations are inferred
through changes in the amplitude of the motor evoked potentials (MEPs). However, it is now possible to directly
evaluate tDCS-induced changes at the cortical level by recording the transcranial magnetic stimulation
evoked potentials (TEPs) using electroencephalography (EEG).
The present study investigated the modulation induced by the tDCS on the motor system. The study evaluates
changes in the MEPs, in the amplitude and distribution of the TEPs, in resting state oscillatory brain activity
and in behavioral performance in a simple manual response task. Both the short- and long-term tDCS effects
were investigated by evaluating their time course at ~0 and 30 min after tDCS.
Anodal tDCS over the left primary motor cortex (M1) induced an enhancement of corticospinal excitability,
whereas cathodal stimulation produced a reduction. These changes in excitability were indexed by changes
in MEP amplitude. More interestingly, tDCS modulated the cortical reactivity, which is the neuronal activity
evoked by TMS, in a polarity-dependent and site-specific manner. Cortical reactivity increased after anodal
stimulation over the left M1, whereas it decreased with cathodal stimulation. These effects were partially
present also at long term evaluation.
No polarity-specific effect was found either on behavioral measures or on oscillatory brain activity. The latter
showed a general increase in the power density of low frequency oscillations (theta and alpha) at both stimulation
polarities.
Our results suggest that tDCS is able to modulate motor cortical reactivity in a polarity-specific manner, inducing
a complex pattern of direct and indirect cortical activations or inhibitions of the motor system-related network,
which might be related to changes in synaptic efficacy of the motor cortex
High frequency rTMS induces α and β increase: limitations of classical high versus low frequency opposition
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