In this video, we describe a simple time and cost efficient protocol for using transcranial direct current stimulation for psycho and neurolinguistic experiments aimed at studying in a naturalistic, yet fully controlled way the role of the cortical areas of the human brain in word learning, and a comprehensive set of behavioral procedures for assessing the outcomes. Language is a highly important, yet poorly understood, function of the human brain. While studies of brain activation patterns during language comprehension are abundant what is often critically missing is causal evidence of brain areas'involvement in a particular function, not least due to the unique human nature of this ability, and a shortage of neurophysiological tools to study causal relationships in the brain non-invasively.
Recent years have seen a rapid rise in the use of transcranial direct current stimulation, or tDCS, which is an easy, inexpensive, and safe non-invasive technique that can modulate the state of the stimulated brain area which it does putatively by shifting the excitation and inhibition thresholds, and thus enables the study of its particular contribution to specific functions. Here, we will describe the use of tDCS in a psycholinguistic word learning experiment. Here, we will consider application of tDCS to language areas in a word learning experiment.
In general, the case of word learning is taken here as one example of a neurolinguistic experiment, and the tDCS part of the procedure should not change substantially for other types of language experiments which target the same areas. Yet, we use this opportunity to also highlight major methodological considerations in the word acquisition experiment per se, which is the second main aim of the current report. It is important to obtain all legal and ethical permissions for studies on human subjects that are required by your local legislation.
In our study, the permission was granted by the University of St.Petersburg Research Ethics Committee. Conduct all measurements in a soundproof, or at least sound attenuated, camera. Sound insulation is very important, since any extraneous sound, noise, human speech, and so on can significantly affect the performance, and thus influence the data.
To avoid interference by unnecessary subject experimenter contact, only the screen, headphones, or speakers, and any input devices, such as keyboard or button boxes are to be located inside the camera. Interaction with the experimenter is to be held over intercom, unless personal contact is required. The optimal parameters for text presentation are black text on gray background.
We use font face Arial size 27. To reduce delays and jitter in visual presentation, we recommend using a video card and a monitor with a refresh rate of 100 hertz and higher. To measure reaction times, use research grade response pads which have been ergonomics and more precise timing in comparison with conventional keyboards.
We have chosen words of Russian language which were controlled for their length and overall structure to avoid any basic effect of surface properties on higher level processing. To create multiple lists, we divided the word into sets which didn't differ statistically on their and syllabic frequency. This was established using T-tests.
Novel word forms were created by transposing the alternate syllables within each set of words. So, thus the novel words resembled existing words in terms of orthographic and phonological structure. As a result, we created a list of word forms to be learned and a list of similar unlearned pseudo words from one set of existing words.
All three stimulus types that shared one set would therefore potentially enter into lexical competition as neighbors after the learning stage. Further control list of words and pseudo words that didn't share this similarity were also prepared. The sets were counterbalanced around the subject groups in the way that they played different experimental roles to minimize any effects of surface form on newly acquired semantics.
We also created novel meanings associated with the new words in the process of learning. They were made of obsolete or real concepts not present in the participants'language. For contextual learning of novel semantics, we created five sentences for each of the new concept describing situations through which one could understand the meaning of the novel words gradually from general to more specific context.
Novel words were represented in their base form. The length of the sentences and the number of words were controlled and balance between conditions. Each sentence consisted of eight words according to the minimal length of sentences in Russian, which allow you to understand the context.
As you are reading these sentences, you can understand that circunal is a kind of negative feeling which appears when you have to remember too many passwords from your accounts. To assess acquisition and comprehension of word forms and semantics we use several tasks, free recall, cue recognition, lexical decisions, semantic definition, and semantic judgment. In the free recall task, participants had to reproduce as many new word forms as they could remember by typing them into the prepared spreadsheet.
The recognition and lexical decisions tasks were used the same stimuli and speed of presentation and differ only in instructions. The items were presented sequentially and randomly. In the recognition task participants had to press button one if they encountered the word during the experiment or press two if they didn't.
In the lexical decision, they had to press button one if presented what made sense or press two if it didn't. Semantic tasks were used to estimate the acquisition of normal meanings and the correspondence between that meaning and the surface form. In the semantic definition task, participants are given a list of the items, those presented previously in the learning phase.
They should try to define each of them and type their definitions into the spreadsheet. In the semantic judgment task, participants were presented with a word and three definitions. They should choose one correct definition for each word by pressing the corresponding button.
Only one of the definitions were correct. In addition, we suggest including none of this or not sure option. Transcranial direct current electrical stimulation, tDCS, generates neuro modulations and spontaneous and neuronal activity.
Dependent on the orientation of the cells with respect to current, the membrane potential can be shifted slightly towards hyperpolarization. For instance, for motor system it happens during anodal stimulation, or depolarization for cathodal stimulation. This change in your neural excitability will lead to various temporary alterations in the brain function, including the motor sensory and high level cognitive functions.
Here, we will consider application of tDCS to language areas such as Wernicke's and Broca's areas of human in a word learning experiment. It's important to remember the main difference between tDCS and other non-invasive brain stimulation methods, such as transcranial magnetic stimulation, first, since there is not a simpler way to determine sensitivity to tDCS by threshold assessment, a single protocol is applied for all subjects. Second, it's very difficult to accurately estimate the stimulation area.
One can only speak about the hypothetical area. Third, it's also difficult to estimate the duration of offline stimulation effects. Presumably, the main effect of stimulation are observed up to one hour after the termination of stimulation.
However, the effect of stimulation can sometimes be detected even one day after stimulation. There are three types of stimulation, cathodal, with a cathode electrode over target area, anodal, with the anode over target area, and placebo, or sham stimulation. Use sponge electrodes measuring five by five centimeters.
The electrodes should be soaked in physiological saline solution for five minutes before application. In order to minimize the effect of stimulation on other areas of the brain put the anodal electrode at the base of the neck on the right side. We use spongy electrodes measuring two by five centimeters.
Particular attention should be paid to prevent spreading of the solution beyond the boundaries of the electrode application zone. Special care should be taken to keep the surrounding electrode area dry. The optimal protocol for cathode stimulation is the stimulation by one milliamp current for 10 minutes.
First, the current rises to one milliamp over the initial 20 seconds, and at the end of the stimulation it decreases to zero in the last 20 seconds of the stimulation. The sets with contextual sentences containing the novel words are presented in a random order. Each sentence starts with a word by word presentation.
After this, the entire sentence appears on the screen to ensure its full understanding. Participants have to press an indicated response button with their index finger of the left hand after reading the whole sentence. The left hand is used to reduce interference with the language errors of the left hemisphere.
Duration of sentence presentation is five seconds. Then a crosshair appears at the center of the screen for 500 milliseconds and the next sentence begins. The sets of the sentences are separated from each other by appearance of three crosshairs for two seconds.
Each new concept presentation starts with another crosshair present for 500 milliseconds before the sentence words are flashed. Each word is presented for 500 milliseconds, the empty screen and the background color between the words within one sentence for 300 milliseconds. We used a battery of different tests since tDCS effects on language processing are not well known.
To assess learning effects both immediately and following the overnight consolidation stage we recommend breaking the stimulus set into two subset, equally distributed across stimulus conditions and counterbalanced across the subject group. We are running the assessment tasks immediately after the learning protocol and after a 24 hour delay. This protocol allows data analysis using different tests comparing two sets of samples coming from continuous distributions with equal medians, like Wilcox and Rank Sum test, or Mann-Whitney U test, or medians to sample T-test if the distribution is normal.
Within each group, there were notable differences in accuracy scores and reaction times between the two assessment sessions. In the sham group novel word recognition was significantly better on the first day than on the second one. In the cathodal group reaction time latency in the recognition task was significantly faster for novel words than for competitor pseudo words on the first day, but not on the second one.
The results of lexical decision task showed that after cathodal stimulation both on the first and on the second day there was significantly better performance for novel words than for pseudo word competitors. In the sham group, however, this effect was significant on the second day only. tDCS of language cortices exemplified here by Wernicke's area produces a complex pattern of behavioral outcomes.
Contextual presentation of new words after tDCS significantly expands the possibilities of simultaneous study of acquisition of word form per se and of its semantics. To disengage the various effects a battery of different tests is needed, which could test the processes at different levels of short and long term memory, lexical access, semantic processing, and so on. The cognitive effects achieved during the transient stimulation phase are maintained over a longer period and may therefore be possibly used for modulating word acquisition and processing in practical settings.
