Frequency-specific insight into short-term memory capacity

Abstract

The digit span is one of the most widely used memory tests in clinical and experimental neuropsychology for reliably measuring short-term memory capacity. In the forward version, sequences of digits of increasing length have to be reproduced in the order in which they are presented, whereas in the backward version items must be reproduced in the reversed order. Here, we assessed whether transcranial alternating current stimulation (tACS) increases the memory span for digits of young and midlife adults. Imperceptibly weak electrical currents in the alpha (10 Hz), beta (20 Hz), theta (5 Hz), and gamma (40 Hz) range, as well as a sham stimulation, were delivered over the left posterior parietal cortex, a cortical region thought to sustain maintenance processes in short-term memory through oscillatory brain activity in the beta range. We showed a frequency-specific effect of beta-tACS that robustly increased the forward memory span of young, but not middle-aged, healthy individuals. The effect correlated with age: the younger the subjects, the greater the benefit arising from parietal beta stimulation. Our results provide evidence of a short-term memory capacity improvement in young adults by online frequency-specific tACS application.

Keywords: beta frequency; digit span; posterior parietal lobe; short-term memory; transcranial alternating current stimulation.

Fig. 1.

Experimental design, with five transcranial alternating current stimulation (tACS) conditions (5, 10, 20, and 40 Hz and sham) run during the digit span (DS) forward or backward memory task. The red (target) electrode is placed on the left posterior parietal cortex while the blue (reference) electrode is placed on the ipsilateral shoulder.

Fig. 2.

A: changes of Digit Span Forward score during online tACS. The 20-Hz stimulation increased memory performance with respect to the sham condition and to the middle-aged group. Error bars represent SE. *P < 0.05, **P < 0.01, significant effects. B: correlation between age and normalized DS scores (obtained by the percentage of 20 Hz changes with respect to sham condition).