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Visual motion processing and TMS: increase of neural noise or reduction of signal strength?
No full textLa stimolazione magnetica transcranica (TMS) è una metodica utilizzata nelle neuroscienze cognitive, che permette di superare l’approccio correlazione fra cervello e funzioni cognitive associate, stabilendo invece una relazione causale fra area cerebrale e processo cognitivo in atto. Nonostante la TMS sia ampiamente impiegata a scopi di ricerca sia clinica che di base, i meccanismi di azione attraverso cui opera sono poco chiari. L’ipotesi di “lesione virtuale” è l’unico inquadramento teorico disponibile al fine di interpretare i dati proveniente da studi TMS. In analogia con studi di lesioni su animali o in pazienti umani, l’ipotesi di “lesione virtuale” stabilisce che la TMS agisca inducendo un blocco transitorio e reversibile della funzionalità della popolazione neurale coinvolta dalla stimolazione. In questo modo è stato possibile studiare il ruolo funzionale dell’area stimolata all’interno della rete di connessioni cerebrali ingaggiate dal compito cognitivo in atto, senza l’interferenza di effetti confondenti tipici di studi di lesione, ad esempio fenomeni di riorganizzazione cerebrale o di plasticità. Nonostante l’”ipotesi di lesione virtuale” abbia permesso un notevole incremento dell’impiego della TMS nelle neuroscienze cognitive, essa non propone nessuna indicazione precisa circa i meccanismi di azione attraverso cui la TMS agisca. In particolare, appare che la TMS possa agire sia riducendo l’intensità del segnale neurale, sia introducendo rumore neurale nell’area stimolata. Entrambi questi meccanismi inducono in ultima analisi ad un impoverimento della prestazione comportamentale del soggetto.
Scopo della presente trattazione è di indagare in che modo l’attività elettrica indotta dalla TMS nel tessuto cerebrale interagisce con l’attività neurale già presente nell’area stimolata.
Nel primo esperimento è stato applicato un protocollo di stimolazione ripetitiva (rTMS) durate un compito di discriminazione della direzione di punti in movimento. Attraverso analisi psicofisiche, è emerso che la rTMS agisca inducendo rumore neurale in V5/MT, un’area visiva extra-striata, responsabile dell’elaborazione di stimoli visivi in movimento.
Al contrario, l’applicazione di un singolo impulso TMS (spTMS) alla stessa area corticale e durante l’esecuzione dello stesso compito cognitivo, ha rivelato che la spTMS agisca sopprimendo l’intensità del segnale visivo. Questi ultimi dati sono stati confermati anche quando lo stato fisiologico di V5/MT è stato variato attraverso l’orientamento dell’attenzione ad una particolare caratteristica dello stimolo visivo. Nel terzo esperimento infatti, è stato trovato che spTMS riduca il guadagno che un indizio attenzionale opera durante un compito di discriminazione di direzione di movimento sull’intensità della forza del segnale, mentre spTMS non interferisce con la capacità di uno stesso indizio attentivo di ridurre il rumore neurale all’interno dell’area.
In conclusione, differenti paradigmi di TMS inducono differenti risultati circa l’interazione tra l’attività neurale in atto e l’attività indotta dalla stimolazione. Un singolo impulso magnetico agisce sopprimendo la forza del segnale nell’area stimolata, mentre impulsi ripetuti introducono rumore neurale. L’importanza del settaggio dei parametri di stimolazione (ad esempio intensità e frequenza) e dello stato fisiologico e delle proprietà morfologiche dell’area stimolata, così come tenere in considerazione in che modo l’area target opera sono discussi come elementi fondamentali negli studi TMS.
Nel primo capitolo (Introduzione) verrà introdotto l’argomento cardine, concentrandosi su alcuni principi basilari di funzionamento della TMS. La trattazione sarà poi rivolta alla principale ipotesi teorica utilizzata per spiegare gli effetti indotti dalla TMS (ipotesi di “lesione virtuale”), considerandone vantaggi e limiti. Saranno inoltre introdotte alcune evidenze sperimentali sugli effetti della TMS sull’attività neurale provenienti da studi su animali. Infine, verranno prese in considerazione le proprietà morfologiche e funzionali di V5/MT e alcuni dati presenti in letteratura circa l’applicazione della TMS su questa area cerebrale durante un compito di discriminazione di direzione di movimento.
Il secondo, terzo e quarto capitolo rappresentano il fulcro sperimentale della tesi, in cui tre esperimenti verranno dettagliatamente introdotti, descritti e discussi separatamente.
Nel secondo capitolo (rTMS e elaborazione di movimento) è stato applicato un paradigma di stimolazione ripetitiva durante un compito di discriminazione di direzione di movimento.
Nel terzo capitolo (spTMS e elaborazione di movimento), è stato utilizzato lo stesso compito utilizzato nell’esperimento precedente sotto l’effetto di un spTMS.
Nel quarto capitolo (spTMS, attenzione e elaborazione di movimento), lo stato fisiologico di V5/MT è stato modificato attraverso l’attenzione rivolta ad una particolare caratteristica dello stimolo visivo (fornendo un indizio attentivo rivolto alla direzione di movimento). E’ stato applicato un paradigma di spTMS a V5/MT allo scopo di studiare l’interazione fra l’effetto indotto dalla modulazione attenzionale e l’effetto indotto dalla TMS sulla percezione visiva.
Nell’ultimo capitolo (Capitolo 5 -Discussione generale) le principali scoperte dell’intero lavoro saranno riassunte e discusse.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
Reliability of TMS phosphene threshold estimation: toward a standardized protocol
No full textPhosphenes induced by transcranial magnetic stimulation (TMS) are a subjectively described visual phenomenon employed in basic and clinical research as index of the excitability of retinotopically organized areas in the brain. Objective: Phosphene threshold estimation is a preliminary step in many TMS experiments in visual cognition for setting the appropriate level of TMS doses; however, the lack of a direct comparison of the available methods for phosphene threshold estimation leaves unsolved the reliability of those methods in setting TMS doses. The present work aims at fulfilling this gap. Methods: We compared the most common methods for phosphene threshold calculation, namely the Method of Constant Stimuli (MOCS), the Modified Binary Search (MOBS) and the Rapid Estimation of Phosphene Threshold (REPT). In two experiments we tested the reliability of PT estimation under each of the three methods, considering the day of administration, participants' expertise in phosphene perception and the sensitivity of each method to the initial values used for the threshold calculation. Results: We found that MOCS and REPT have comparable reliability when estimating phosphene thresholds, while MOBS estimations appear less stable. Conclusions: Based on our results, researchers and clinicians can estimate phosphene threshold according to MOCS or REPT equally reliably, depending on their specific investigation goals. We suggest several important factors for consideration when calculating phosphene thresholds and describe strategies to adopt in experimental procedures.M.R. was supported by Juan de la Cierva postdoctoral fellowship (JCI-2012-12335) from the Spanish Government
Effects of right parietal transcranial magnetic stimulation on object identification and orientation judgments.
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The neural basis of the Enigma illusion: a transcranial magnetic stimulation study.
No full textThe 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 400ms 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
The neural mechanisms of the effects of transcranial magnetic stimulation on perception.
No full textTranscranial magnetic stimulation (TMS) is a technique used to study perceptual, motor, and cognitive functions in the human brain. Its effects have been likened to a "virtual brain lesion," but a direct test of this assumption is lacking. To verify this hypothesis, we measured psychophysically the interaction between the neural activity induced by a visual motion-direction discrimination task and that induced by TMS. The visual stimulus featured two elements: a visual signal (dots that moved coherently in one direction) and visual noise (dots that moved randomly in many directions). Three hypotheses were tested to explain the impairment in performance as a result of TMS: 1) a decrease in signal strength; 2) an induction of randomly distributed neural noise with an accompanying decrement in system sensitivity; and 3) a suppression of relevant information processing and addition of neural noise. We provide evidence in favor of the second hypothesis by showing that TMS basically acts by adding neural noise to the perceptual process
Is transcranial alternating current stimulation effective in modulating brain oscillations?
Full text linkTranscranial 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 tas
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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