Fig. 1. Set up and experimental tasks. The experiment included two variants (conditions) of the delayed choice reaction time tasks. The sequence of events for the condition of STM was as follows: (i) a cross at the center of the monitor as a visual warning stimulus lasting 1 s, (ii) a couple of vertical bars as a visual cue stimulus lasting 2 s, (iii) a blank screen as a delay period lasting 3.5–5.5 s, (iv) a circle at the center of the monitor as a go stimulus lasting 1 s, and (v) a finger movement as a motor response. Subjects had to click either left button of a home-made device if the taller bar was at the left monitor side or right button if the taller bar was at the right monitor side. This device had two large buttons distant about 10 cm each other, which were electronically connected to the mouse of the computer giving the visual stimuli. Compared to the STM task, the condition with no memory load (NSTM) had visual cue stimuli lasting up to the go stimulus.

Fig. 2. Model of a typical frequency spectrum computed from EEG (or magnetoencephalographic, MEG) data. Frequency bands of interest were theta, alpha 1, alpha 2 and alpha 3, which were computed according to a well-known procedure based on the detection of IAF peak (see Section 2 for further explanations).

Fig. 3. The grand average waveforms of the theta ERD during the NSTM and STM conditions in the young (N young) and elderly (N old) subjects. These waveforms refer to representative frontal, central and parietal electrode sites.

Fig. 4. The grand average waveforms of the alpha 1, alpha 2 and alpha 3 ERD/ERS during the NSTM and STM conditions. These waveforms refer to representative frontal, central and parietal electrode sites in the young (N young) and elderly (N old) subjects.

Fig. 5. Three-dimensional color maps of the theta ERD peak during the delay period of the NSTM and the STM conditions in the young (N young) and elderly (N old) subjects. The data refer to those illustrated in Fig. 3. Color scale: maximum ERD and ERS are coded in white and violet, respectively. The maximal (%) value of the ERD/ERS is reported.

Fig. 6. Three-dimensional color maps of group alpha 1, alpha 2 and alpha 3 ERD peak (T1) and ERS peak (T2) during the delay period during the NSTM and STM conditions in the young (N young) and elderly (N old) subjects. The data refer to those illustrated in Fig. 4 and the color scale is as in Fig. 5. The maximal ERD/ERS values were ±36% for the alpha 1, ±45% for the alpha 2, and ±42% for the alpha 3.

Fig. 7. Means across subjects (±S.E.) of the ERD/ERS amplitude as provided by the ANOVA design. In particular, these means refer to a statistical interaction among the factors Group (N young, N old), Region of interest (left frontal, right frontal, left central, right central, left parietal, right parietal), and Time period (ERD peak or T1, “ERS” peak or T2). The results of Duncan post hoc testing are indicated by asterisks.

Table 1. List including 46 electrodes (augmented 10–20 system) used for the present experiments

The position of the labels in the list roughly represents the position of the corresponding electrodes over the scalp. Of note, hyphen between two electrode labels indicates that the electrode is located halfway the two labeled electrodes.

Table 2. Mean latency (±S.E.) of theta, alpha 1, alpha 2, and alpha 3 event-ERD/ERS peaks

These peaks refer to both STM and NSTM conditions for the young (N young) and elderly (N old) subjects. Zerotime indicates the onset of the visual cue stimulus (i.e. two vertical bars).