Fig 1.
Schematic that illustrates the Sternberg task and an oscillatory model of multi-item WM.
(A) Sternberg task with 3 letters each presented for 700 ms at an interstimulus interval of 275–350 ms. This was followed by a maintenance period of 2 s, followed by a 700 ms probe. (B) The dynamics predicted by the LIJ model for the WM maintenance on a site selective to “Q” with respect to 3 different positions on the list. Each theta/alpha cycle may contain many (5–8) gamma cycles (slow gamma), but only 3 are shown. The gamma cycle (slow gamma) at which maximal high-frequency firing occurs (and thus theta phase) corresponds to the order of item presentation. LIJ, Lisman/Idiart/Jensen; WM, working memory.
Fig 2.
TFRs of power during encoding, maintenance, and probe.
(A) Average TFRs of power at 14 letter-selective sites. From left to right, the panels of this plot show the neural responses to baseline, tuned and untuned letters, response during maintenance, and responses to the probe, respectively. During encoding, gamma activity is elevated by tuned letters; theta/alpha decreases by either the tuned or untuned letter. During maintenance, theta/alpha is elevated, and gamma is low. (B) Average TFRs of power at 14 non-letter-selective sites, which show little change during encoding or maintenance. Underlying data available at http://orion.bme.columbia.edu/jacobs/data/. TFR, time-frequency representation
Fig 3.
The CFC was calculated by estimating the PAC for the data during the retention interval [27]. When combining all sensors, a coupling was observed between theta/alpha phase and gamma power. Note the coupling to slow gamma at 30–40 Hz and fast gamma activity at 75–120 Hz. Based on these results and the modulations in Fig 2, we focused the subsequent analysis on the theta/alpha band (7–13 Hz) and the gamma band (75–120 Hz). Underlying data available at http://orion.bme.columbia.edu/jacobs/data/. a.u., arbitrary unit; CFC, cross-frequency coupling; PAC, phase–amplitude coupling.
Fig 4.
Properties of PAC during maintenance.
(A) Average (14 sites) PAC is greater at letter-selective than at non-letter-selective sites. In right panel, differences were calculated for each subject; the average difference is large, showing that PAC is much larger at letter-selective sites (p < 0.012). (B) Average (14 sites) PAC is somewhat larger for tuned compared to untuned letters. In panel at right, difference for each subject was calculated before averaging. This difference was not statistically significant. (C) There is a significant difference (two-tailed t test; p = 0.012) of gamma power between letter-selective sites and non-letter-selective sites during the maintenance period. There is no significant difference between theta and gamma power over letter-selective sites when comparing the tuned letter and untuned letter during the maintenance period. Underlying data available at http://orion.bme.columbia.edu/jacobs/data/. a.u., arbitrary unit; PAC, phase–amplitude coupling
Fig 5.
(A,B) CFC in trials with slow or fast RT. (C) Average CFC (PAC) over the white box of A and B. There was no difference with respect to slow and fast RTs. (D) Theta/alpha and gamma power during maintenance. There was no difference with respect to fast and slow RTs. Error bars represent SEM. Underlying data available at http://orion.bme.columbia.edu/jacobs/data/. a.u., arbitrary unit; CFC, cross-frequency coupling; PAC, phase–amplitude coupling; RT, response time
Fig 6.
List position affects the theta/alpha phase that has maximal fast gamma power during maintenance.
(A) The deviation of fast gamma band power for each position from the average measured as a function of theta/alpha phase (Hilbert phase 7–13 Hz). Error bars represent SEM. (B) Average fast gamma power as a function of theta/alpha phase; the color indicates the item position (P1-P2-P3) yielding the maximum power at that phase bin. We consider this sequence statistically significant in terms of ordering (p = 0.0062; see Methods). Underlying data available at http://orion.bme.columbia.edu/jacobs/data/.