High-definition
transcranial direct current stimulation (HD-
tDCS) has recently been developed as a noninvasive brain stimulation approach that increases the accuracy of current delivery to the brain by using arrays of smaller "high-definition"
electrodes, instead of the larger pad-
electrodes of conventional
tDCS. Targeting is achieved by energizing
electrodes placed in predetermined configurations. One of these is the 4x1-ring configuration. In this approach, a center ring
electrode (
anode or cathode) overlying the target cortical region is surrounded by four return
electrodes, which help circumscribe the area of stimulation. Delivery of 4x1-ring HD-
tDCS is capable of inducing significant neurophysiological and clinical effects in both healthy subjects and patients. Furthermore, its tolerability is supported by studies using intensities as high as 2.0 milliamperes for up to twenty minutes. Even though 4x1 HD-
tDCS is simple to perform, correct
electrode positioning is important in order to accurately stimulate target cortical regions and exert its neuromodulatory effects. The use of
electrodes and hardware that have specifically been tested for HD-
tDCS is critical for safety and tolerability. Given that most published studies on 4x1 HD-
tDCS have targeted the primary motor cortex (M1), particularly for
pain-related outcomes, the purpose of this article is to systematically describe its use for M1 stimulation, as well as the considerations to be taken for safe and effective stimulation. However, the methods outlined here can be adapted for other HD-
tDCS configurations and cortical targets.