Research reportNaming facilitation induced by transcranial direct current stimulation
Introduction
A large number of neuroimaging studies highlight that the ability to name actions or objects is achieved by a wide and complex cerebral network. This system involves, among other areas, the left prefrontal and temporal areas [22], [35], as demonstrated by neuroimaging [35], [41] and brain lesion [10], [23] studies. The crucial role of the left dorsolateral prefrontal cortex (DLPFC) in action naming has also been confirmed by repetitive transcranial magnetic stimulation (rTMS) studies [6], [7], [8]. Cappa et al. [6] reported that high frequency rTMS of left DLPFC significantly reduced the vocal reaction times for naming of action pictures. This interesting result has been subsequently confirmed in Alzheimer disease patients [7], extending the effect to object naming [8] and to a patient affected by primary progressive aphasia [11].
Recently, a lot of interest has been captured by the rediscovery of a cerebral stimulation technique that acts through the application of a very low direct current [27], [37]. Transcranial direct current stimulation (tDCS) seems to act by modulating the resting membrane potential, in an opposite direction depending on the polarity (anodal vs. cathodal) of the electrode placed on the chosen area. A very interesting characteristic is the duration of these neuromodulatory effects. The first studies on the human motor cortex [29], [31] have shown that 13 min of anodal stimulation induce 90 min of increased cortical excitability (enhanced resting motor evoked potentials – MEPs amplitude) or that, in a similar way, but with opposite results, cathodal stimulation causes 60 min of diminished cortical excitability (reduced resting MEPs amplitude).
The mechanisms underlying these effects have been first studied in animals, in the 1960s [4], [9], [15], [16], [39]. In humans, Liebetanz et al. [21] demonstrated that short-term tDCS effects are related to membrane depolarisation (anodal stimulation) or hyperpolarisation (cathodal stimulation), while long-term tDCS effects involve the participation of glutamatergic NMDA receptors. We now know that synaptic plasticity, i.e., modulation of the strength of synaptic connections on the basis of experience, is dependent on NMDA receptors, as we know that plasticity is the basis for learning and memory [26]. If we consider that a brain injury, such as stroke or a neurodegenerative disease, can damage this system, the opportunity to non-invasively modulate the functioning of these mechanisms can open new prospects for the neurorehabilitation of brain-damaged patients [24], [40].
In the past years, several studies have sought to durably modify cortical excitability. Behavioural facilitatory effects have been highlighted with regard to implicit motor learning [33], associative learning [12], working memory [13], [34], pitch memory [44], perception [2] and language [17], [25], [43]. This facilitatory function may be very important, not only in establishing the role of the stimulated area, but also because it can be used to enhance reduced function in cognitive neurorehabilitation.
Our work aims to explore the effects of tDCS on picture naming, making use of a task that was previously studied with rTMS in normal [6] and Alzheimer's patients [7], [8]. We hypothesize that anodal stimulation of the DLPFC can generate a facilitatory effect, namely a decrease of the vocal reaction times in action and/or object naming.
Section snippets
Methods
Two picture naming experiments were conducted with normal young subjects after the end of a tDCS period over the DLPFC.
Subjects
Twelve healthy subjects (4 males, mean age 24.1 years, standard deviation 3.7, range 19–32) took part in the experiment 1 and 12 other healthy subjects participated in the experiment 2 (6 males, mean age 21.8 years, standard deviation 1.0, range 20–23). Each subject participated in only one of the two experiments.
Subjects were native Italian speakers, right-handed and with normal or corrected-to-normal vision. We did not include subjects with a history of seizures, implanted metal objects,
Picture naming task
Stimuli for the picture naming task were presented on a personal computer screen using the software Presentation v. 12.0 (http://www.neurobs.com). All of the stimuli were black and white two-dimensional line drawings taken from the corpus of the CRL-IPNP (Center for Research in Language – International Picture Naming Project; http://crl.ucsd.edu/∼aszekely/ipnp), a broad set of 795 action and object pictures. These items have been tested in healthy and patient populations across seven different
Transcranial direct current stimulation
The stimulation was delivered by a battery-driven, constant current stimulator (neuroConn GmbH, Ilmenau, Germany) through a pair of saline-soaked sponge electrodes (7 cm × 5 cm). A constant current of 2 mA was applied for 8 (experiment 1) or 10 (experiment 2) min, with a ramping period of 10 s both at the beginning and at the end of the stimulation. The current density (0.057 mA/cm2) was maintained below the safety limits [36]. The electrodes were kept firm by elastic bands and an electroconductive gel
Procedure
In the two experiments, subjects were seated in front of a computer screen, in a quiet room in semi-darkness. In the experiment 1, they performed the picture naming task immediately after anodal, cathodal and sham stimulation. The three sessions and, therefore, three experimental blocks were separated by a 1-h pause (i.e., washing-out) period. Their order of execution was accurately balanced. The procedure is shown in Fig. 2a.
In experiment 2, the subjects performed the picture naming task
Data analysis
The subjects’ performances were recorded with a microphone placed in front of the participant. Vocal responses were digitised with the GoldWave v. 5.15 (GoldWave, Newfoundland, Canada) software, with a sampling rate of 11,025 Hz.
We measured accuracy, giving 1 point to each error (no response, semantic error, visual error) and then calculating the mean for each subject in each condition.
The latency of the verbal response (vocal reaction time – vRT) was measured manually on the screen, marking the
Experiment 1
All of the subjects tolerated the stimulation well; no subjects reported adverse effects or asked to interrupt the experiment.
Discussion
In this study, we show that anodal stimulation of the left DLPFC exerts a facilitation effect on picture naming in healthy subjects. The absence of a significant differential stimulation effect on the attentive task or on the perception of sensations questionnaire ruled out the possibility of facilitation due to non-specific effects, such as enhancement of arousal or attention. Moreover, the observation of a facilitatory effect in both experiments indicates robustness of the result.
Our
Acknowledgement
We thank Umberto Boldi for some help with data collection
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