Preliminary evidence for performance enhancement following parietal lobe stimulation in Developmental Dyscalculia

Teresa Iuculano, Roi Cohen Kadosh, Teresa Iuculano, Roi Cohen Kadosh

Abstract

Nearly 7% of the population exhibit difficulties in dealing with numbers and performing arithmetic, a condition named Developmental Dyscalculia (DD), which significantly affects the educational and professional outcomes of these individuals, as it often persists into adulthood. Research has mainly focused on behavioral rehabilitation, while little is known about performance changes and neuroplasticity induced by the concurrent application of brain-behavioral approaches. It has been shown that numerical proficiency can be enhanced by applying a small-yet constant-current through the brain, a non-invasive technique named transcranial electrical stimulation (tES). Here we combined a numerical learning paradigm with transcranial direct current stimulation (tDCS) in two adults with DD to assess the potential benefits of this methodology to remediate their numerical difficulties. Subjects learned to associate artificial symbols to numerical quantities within the context of a trial and error paradigm, while tDCS was applied to the posterior parietal cortex (PPC). The first subject (DD1) received anodal stimulation to the right PPC and cathodal stimulation to the left PPC, which has been associated with numerical performance's improvements in healthy subjects. The second subject (DD2) received anodal stimulation to the left PPC and cathodal stimulation to the right PPC, which has been shown to impair numerical performance in healthy subjects. We examined two indices of numerical proficiency: (i) automaticity of number processing; and (ii) mapping of numbers onto space. Our results are opposite to previous findings with non-dyscalculic subjects. Only anodal stimulation to the left PPC improved both indices of numerical proficiency. These initial results represent an important step to inform the rehabilitation of developmental learning disabilities, and have relevant applications for basic and applied research in cognitive neuroscience, rehabilitation, and education.

Keywords: Developmental Dyscalculia; learning; neural compensation; rehabilitation; transcranial electrical stimulation.

Figures

Figure 1
Figure 1
Artificial digits. Symbols used as stimuli during the learning phase and the numerical Stroop task and their equivalent as everyday digits—adapted from Tzelgov et al. (2000). Reprinted from Cohen Kadosh et al. (2010), with permission from Elsevier.
Figure 2
Figure 2
Schematic outline of the experimental design in a typical daily session. (A) tDCS was delivered for 20 min from the start of the training. In this example, anodal stimulation is applied to the right parietal lobe (red arrow), whereas cathodal stimulation is delivered to the left parietal lobe (blue arrow). (B) The training continued after the termination of the stimulation. (C) Once the training ended, the subjects performed the numerical Stroop task and (D) the number line task. The time next to each image reflects the elapsed time from the beginning of the daily session until its termination in a cumulative fashion. Please note that on Day 1 only, the session ended after the learning phase—thereby it did not include the experimental tasks (i.e., Stroop-task and number line task). Reprinted from Cohen Kadosh et al. (2010), with permission from Elsevier.
Figure 3
Figure 3
Number Line task. Subjects were asked to map the given symbol, which appeared randomly at the left upper corner—as in the current example—or at the right upper corner, on the physical line. Subjects were instructed to place each symbol on the line according to its magnitude. Reprinted from Cohen Kadosh et al. (2010), with permission from Elsevier.
Figure 4
Figure 4
Learning functions for the two DD individuals. DD1 received Right Anodal—Left Cathodal (RA-LC) stimulation to the PPC (dotted red line); DD2 received Left Anodal—Right Cathodal (RC-LA) stimulation to the PPC (solid blue line). The improvement in the learning task over blocks (x-axis) was modeled using a power law function. Non-linear regression showed an equivalent fit for both participants (RA-LC, R = 0.9; RC-LA, R = 0.96).
Figure 5
Figure 5
Numerical Stroop task. Congruency effect (measured in terms of accuracy) for the two DD individuals. DD1 did not exhibit the canonical Congruency effect (Congruent > Neutral > Incongruent), while DD2 showed a clear Congruency pattern in the predicted direction. For DD1: Neutral > Congruent (p < 0.05); Congruent vs. Incongruent (p = 0.13); Incongruent vs. Neutral (p = 0.08). For DD2: Congruent > Incongruent (p < 0.05); Congruent vs. Neutral (p = 0.07); Incongruent vs. Neutral (p = 0.16). Data are mean ± standard error (SE) of the mean. *p < 0.05.
Figure 6
Figure 6
Numerical distance by congruency effects. Effects measured in terms of RTs for each DD individual. DD1 did not exhibit the canonical Congruency effect (Incongruent > Neutral > Congruent), while DD2 showed a Congruency pattern related to the numerical distance between stimuli. For DD1 both Congruent as well as Incongruent trials were slower than Neutral trials (p < 0.001) and no effect of numerical distance was evident [F < 1]. In DD2, the canonical pattern typical of the Congruency effect (Incongruent > Congruent) was only present for small distances (e.g., 2–4) (p < 0.05); while the reverse pattern (Congruent > Incongruent) characterized DD2's performance with large numerical distances (e.g., 2–7) (p < 0.05). Main effects are shown in black (DD1's profile). Interaction is shown in shades of green (DD2's profile). Data are mean ± standard error (SE) of the mean. *p < 0.05; ***p < 0.001.
Figure 7
Figure 7
Average location of subjective responses on the number line task plotted for each type of stimulation. Linear regression lines and equations are indicated for each type of stimulation (Red line—Right Anodal-Left Cathodal stimulation received by DD1; Blue line—Left Anodal-Right Cathodal stimulation received by DD2).

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