The impact of brain stimulation on planning abilities in healthy adults

transcranial direct current stimulation (tDCS) will be used in the current study. tDCS is safe, non-invasive form of brain-modulation. Using a common 9 volt battery, tDCS sends a weak DC current through two sponge electrodes placed upon the scalp and held in place using an elastic strap. tDCS modifies spontaneous neuronal excitability via a tonic hypo- or hyperpolarization of neuronal resting membrane potential. Depending upon the polarity of the current flow, regional excitability can either be increased (anodal tDCS) or decreased (cathodal tDCS). The tDCS device itself is rather simple. A single 9 volt battery powers a constant current generator that delivers current between two electrodes: a cathode (negative charge) and an anode (positive charge). When delivering tDCS, these electrodes (which can vary in size, shape, and basic material) are placed directly on the scalp. The constant current generated by the tDCS device flows from the cathode (-) to the anode (+) thereby creating a circuit directly through the brain. Several parameters determine the effects and efficacy of tDCS. The first, and perhaps most important, of these parameters is electrode position. As the neural effect of each electrode differs, it is essential the target being stimulated and the polarity of stimulation be determined and understood prior to application. Typically, the desired electrode is placed directly over the intended target whilst the second electrode is placed in either a neutral (vertex, orbital bone, &c.) or related (contralateral neural region) reference location. The next parameter is electrode size. As can be assumed, electrodes come in many sizes (typically from 1 cm2 to 35 cm2). Although larger electrodes affect a larger area of underlying cortex, an increase in the electrode surface area necessarily decreases the amount of current generated at each point. Accordingly, proper electrode size is essential to elicit desired effects. Another parameter is current density. Determined by dividing the current strength (typically between 0.01 and 2.0 mA) by the electrode size, the current density determines the induced field strength in the brain and, accordingly, tDCS effects. It has been shown in humans that larger current densities generate stronger and longer lasting effects. The final parameter essential to tDCS is stimulation duration. The duration of stimulation typically determines the duration of tDCS after-effects. For this study, 60 healthy participants will undergo one session of tDCS. During this session, 2 mA of tDCS will be applied for 20 minutes over the dorsal lateral prefrontal cortex (DLPFC). Each sponge electrode will be 9 cm2. As tDCS creates a small electric current between two poles, there are two unique electrodes : a positive anode and a negative cathode. Typically, the neural region under the positive anode becomes hypo-polarized thereby making the underlying neurons more likely to fire. Conversely, the neural region under the negative cathode becomes hyperpolarized thereby making the underlying neurons less likely to fire. Due to these dual electrode effects, it is important to have two stimulatory conditions: each condition will have the electrodes placed in the same areas, however the anode and cathode will be switched. The position of the electrodes will be defined as: Primary Electrode: We will place the primary electrode over the right DLPFC in accord with the standard 10/20 system. Tweny the participants will have the positive anode placed in this location, twenty will have the negative cathode placed here, and twenty participants will receive no stimulation (control group). Participants who receive no stimulation will still have a primary electrode applied to the scalp, however, in this condition polarity will not matter as stimulation will only last for 30-seconds: just long enough to generate a slight tingle and blind the subject to his/her condition but not long enough to generate any effect. Secondary Electrode: We will place the secondary electrode over the left dorsal lateral prefrontal cortex in accord with the standard 10/20 system.1 The secondary electrode will be the opposite polarity of the primary electrode: accordingly, those with a positive anode as the primary electrode will have the negative cathode as the secondary, and vice versa. The tDCS device to be used in the current study is the Iontophoresis Device (trade name Chattanooga Ionto)