6.3 Maintenance of information the dorsal visual stream (Experiment 3). 96
6.3.2 Time-frequency activity patterns in the dorsal visual stream. 97
Next, I investigated whether the alpha frequency range (9-13Hz) activity patterns in the dorsal visual stream depend on working memory condition. This a priori hy-pothesis was derived from the previous MEG study byJokisch and Jensen [2007].
The authors observed that the alpha activity over the parieto-occipital MEG sites was decreased during the maintenance of orientation as compared to identity and control. I used surrogate-based cluster corrected for multiple comparisons t-test (Maris and Oostenveld, 2007) to identify significant changes in the alpha power during memory maintenance across three conditions. Power was decreased during maintenance of orientation as compared to identity and control (pcorrected < 0.05;
see Figure 6.11). Importantly, no significant difference was observed between the alpha power during maintenance of identity and control. Although the effects were sustained through the entire maintenance interval, the clusters of significant change were most pronounced during an early part of the retention interval (around 500-900ms and 500-800ms after the stimulus offset for the orientation-control and orientation-identity, respectively). This replicates previous results of Jokisch and Jensen[2007] associating maintenance of orientation in WM with decreased alpha activity in the dorsal visual stream. As described above abundant evidence has shown that the alpha activity reflects functional [Bonnefond and Jensen, 2012, Jensen et al., 2012, 2014, Jokisch and Jensen, 2007, Klimesch, 2012, Thut et al., 2006] and neural [Haegens et al., 2011, Spaak et al., 2012] inhibition. Conse-quently, the decrease of power in the alpha frequency range relates to the release from neural inhibition (i.e. disinhibition). The current results add to evidence that activity in the alpha band reflects allocation of resources by inhibiting and disinhibiting activity in the dorsal visual stream (see Figure 6.11).
To quantify differences in the gamma frequency range I used 3x3 repeated mea-sures ANOVA with memory condition (orientation, identity and control) and time interval (1-900ms, 901-1800ms, 1801-2700ms) as factors. Neither main effects (all F < 0.54, all p > 0.54) nor interaction was significant (F(4,24) = 0.54, p=0.70).
6.3.3 Intra-regional CFC in the dorsal visual stream.
Roux and Uhlhaas[2014] extended the multiplexing model of WM suggesting that theta-gamma and alpha-gamma cross-frequency coupling might provide distinct mechanisms for maintenance of different information in WM. Such an extended model posits that the alpha-gamma phase-amplitude coupling may play a role in suppressing of redundant visual features. To test this I calculated the CFC be-tween the phase of alpha and the broadband gamma frequency (31-150Hz) both recorded from the same sites in the dorsal visual stream. To test if the observed CFC is stronger than chance I generated a reference distribution of the surrogate CFC data by independent shuffling labels of the low- and high-frequency trials.
0 1 2 3 4
Figure 6.11: Time-frequency activation patterns in the dorsal visual stream.
Group average of time-frequency activation patterns normalized to the time in-terval prior to stimulus presentation. The color maps reflect percentage change of baseline separately for each condition (OR, ID, CO reflect maintenance of orientation, identity and control, respectively). The grey bars above the spec-trograms indicate trial sequence with the memory stimulus presentation (S), maintenance and the probe (P). The vertical dashed lines indicate time points of these events. The horizontal dashed lines mark the alpha power spectrum between 9 and 13Hz. The bar plot (lower right) presents baseline normalized alpha power averaged during the maintenance period (2.7s). Error bars show
SEM.
This procedure keeps intact the spectral properties of the data as well as the actual analytic amplitude and phase time series at each time point while randomizing the trials structure between the two signals. This procedure was performed 100 times.
If the observed modulation index is caused by spurious coupling, the empirical value should not be improbable given the reference distribution of surrogates ob-tained in permutation test. In other words, empirical value might be attributed a rank-based p-value given the reference distribution of surrogate data under the null hypothesis that there is no coupling. This procedure was performed sepa-rately for each subject leading to separate significance test for each participant.
The empirical modulation index exceeded the 95thpercentile of the surrogates (cor-responding to the threshold of p < 0.05) for all seven subjects who had electrodes in the dorsal visual stream (see Figure 6.12). The mean modulated frequency was at 90 ± 28.6Hz (mean ± standard deviation). This result shows that the gamma power is indeed modulated by the phase of alpha in the dorsal visual stream. This corroborates the extended multiplexing model of WM as well as the alpha inhibi-tion model of WM. How does the CFC relate to changes in the alpha power will be further investigated. In particular one might expect that the alpha duty cycle increases for task relevant condition (i.e. lower alpha power relates to increased duty cycle in the dorsal stream during the maintenance of orientation).
7
Figure 6.12: Intra-regional CFC in the dorsal visual stream. The alpha-gamma coupling in the dorsal visual stream for each of the 7 patients having electrodes in the dorsal visual stream. All seven patients (each represented by single dot) show increased empirical CFC (x-axis) compared to the 95th surrogate (y-axis; left). The alpha-gamma coupling in the dorsal visual stream is reliably stronger than chance which is indicated by all dots being below the diagonal (left) and by all blue bars being above the red bars (middle). Single subject’s example of the alpha-gamma phase-amplitude coupling (right panel).
Normalized gamma (upper part of the right panel) is non-uniformly distributed across phase of the alpha oscillation (lower part). Time on x-axis is relative to the alpha peak. Color indicates percentage change of baseline. The gamma power is attenuated particularly strong during the peak of alpha oscillations.