Time-resolved spectral analysis
Time-frequency representations (TFR) of power were computed time-locked to sentence and to
picture onset. Time-resolved power was calculated at frequencies ranging between 2 and 40 Hz,
using a sliding time window of three cycles that was advanced in steps of 25 ms and of 2 Hz.
Prior to taking the Fourier transform, the data in each time window was multiplied with a
Hanning taper. For the comparison between constraining and nonconstraining contexts, the TFRs
were averaged over trials for each context type. Relative power change was calculated as the
difference between the power in the two sentential constraint conditions divided by their average.
Figure 1 shows the relative power changes as a function of sentential constraint timelocked to the sentence onset (left panels) and time-locked to picture onset (right panels). The
black boxes below each panel indicate the stimulus being present at each time point. The values
for the x-axis are presented at the bottom. The sentence in the example is “She keeps her lipstick
and mascara in her purse”. Note that not all words in the sentence are presented in the figure. At
the beginning of the sentence, virtually no differences are observed as a function of constraint.
Around the presentation of the two words preceding the picture, however, 15-20% power
decreases are observed in a broad frequency range over left posterior sensors, extending more in
time for the naming task. Over left frontal sensors in the naming task, power decreases are more
restricted to the pre-picture interval. Thus, these results do show that the differences in beta
desynchronisation as a function of sentential constraint evolve over time and are mostly
prominent around the last word of the sentence, but absent for the first words of the sentence.
Figure 1. Time-frequency representations of the sentential constraint effect (shown as relative
power changes) for the naming task (top) and judgement task (bottom) averaged over the sensors
highlighted in each row. The black boxes below each panel indicate the stimulus being present at
each time point. The values for the x-axis are presented at the bottom. The sentence in the
example is “She keeps her lipstick and mascara in her purse”. Only the first three words and the
last two words are shown in the figure.
Source-level interaction effect
Figure 2 shows the results of the source-level interaction analysis. The top row shows the relative
power changes masked by statistical significance such that only significant source locations are
shown in the figure. The middle row shows the relative power changes masked by their
magnitude, that is, only locations showing absolute power changes of at least 5% are shown. The
bottom row shows the t-values of the cluster-based permutation test thresholded at a t-value of ±
2. Negative values indicate locations where desynchronisation was larger in the naming task than
in the judgement task. Positive values indicate locations where desynchronisation was larger in
the judgement task than in the naming task.
Figure 2. Source-level interaction effects, masked by the statistically significant clusters (top
row), masked by the magnitude of power changes thresholded at 5% (middle row), and masked
by the t-values of the cluster-based permutation test thresholded at ± 2. Negative values indicate
locations where desynchronisation was larger in the naming task than in the judgement task.
Positive values indicate locations where desynchronisation was larger in the judgement task than
in the naming task.