Improving Awareness for Spatial Neglect With tDCS

The aim of this study is to improve awareness in right-brain-damaged patients with USN following stroke using transcranial direct current stimulation (tDCS). tDCS is a neuromodulatory technique that delivers a low-intensity direct current (e.g., 1-2 mA) to cortical areas facilitating (anodal tDCS) or inhibiting (cathodal tDCS) spontaneous neuronal activity. This stimulation is delivered by a stimulation device with two (or, rarely, more) electrodes placed on the scalp. The electrode assembly most commonly used for tDCS comprises (1) a conductive rubber electrode, (2) an electrode sponge, and (3) an electrolyte-based contact medium (e.g., saline, gel, or conductive cream) to facilitate delivery of current to the scalp, as well as (4) any materials used to shape these components or otherwise direct current flow (plastic casing, rivets). The tDCS mechanism of action is a subthreshold modulation of neuronal membrane potentials, which alters cortical excitability and activity dependent on the current flow direction through the target neurons. tDCS does not induce activity in resting neuronal networks, but modulates spontaneous neuronal activity: consequently, the amount and direction of effects critically depend on the previous physiological state of the target neural structures. In this sense, tDCS represents a neuromodulatory technique which do not induce massive synchronized discharge of action potentials as the Transcranial Magnetic Stimulation.

The introduction of tDCS in the clinical context took place about 15 years ago. Seminal studies demonstrated that weak, direct electric currents could be delivered effectively transcranially: specifically, anodal direct current stimulation was shown to increase cortical excitability, whereas cathodal stimulation decreased it. tDCS has been tested in thousands of subjects world-wide with no evidence of important side effects. For instance, it has been used in healthy individuals to change the efficacy of various motor and cognitive processes, with only mild and transient effects such as tingling and itching sensation under the stimulation electrode, moderate fatigue and headache. Moreover, since it is well-tolerated also in patients, a growing body of evidence is focused on the use of tDCS in clinical context for rehabilitative purposes: after brain damage, the induction of enhancement of the damaged hemisphere through anodal tDCS is effective in reducing the cognitive deficits caused by the lesion (e.g., stimulating perilesional areas).

We will select USN patients, showing anosognosia for neglect symptoms, from a previous study. If only a few patients will accept to participate in this protocol, new right-brain-damaged patients will be recruited at HUG.

A protocol using anodal tDCS (i.e., excitatory stimulation) will be administered. We will target brain areas that could be involved in anosognosia for USN and, if these sites are damaged by the lesion, perilesional unimpaired regions will be stimulated, as the rationale is that excitatory stimulation of these sites can restore the defective error monitoring, and temporarily ameliorate anosognosia.

Before and immediately after each stimulation, the three measures of anosognosia (i.e., explicit self-rating, SCR and EEG error-related potentials) will be recorded: this procedure has been chosen in order to compare the tDCS effects on the different features of anosognosia. To control for the size of the parietal and frontal damage, which could influence the beneficial effect of tDCS, we will estimate the number of voxels damaged in these regions through the lesion maps of patients.

Each one of the three experimental sessions is expected to last no more than 2h30min. This includes: 15/20min to welcome the participant, explaining the experiment and make him sign the informed consent and the screening criteria for tDCS ; 30min of neuropsychological screening (only for patients who did not undergo Study 1 or only for the first session); 20min of preparation of the set-up (connecting the cables for SCR, placing the EEG cap and connecting all the electrodes, installing the tDCS electrodes); 60min for the experiment's recording (anosognosia + tDCS + anosognosia); 10/15min for cleaning up the set up and doing a short debriefing with the participant.