Time Frequency Resolution In BESA
|Modules||BESA Research Complete (or Basic+Coherence)|
|Version||5.2 or higher|
The time-frequency resolution of the complex demodulation used in BESA depends on the user-settings for “Frequency and Time Sampling” in the “Coherence” tab of the ERP dialog box. The following table lists the 50% power and amplitude drops, respectively, as a function of the time-frequency sampling setting. Frequencies are given relative to the center frequency of a sharp frequency oscillation, latencies are given to the latency of a sharp time event. For example, with a time-frequency setting of 1.0 Hz, 50 ms, a sine wave with sharp frequency f will be smoothed to a time-frequency signal that has a full power width at half maximum of 2×1.42 Hz. In other words, at frequencies f ± 1.42 Hz, the displayed power in the time-frequency plot is half of the displayed power at frequency f. A sharp time-event, (e.g. a sharp spike in the time-domain) will be smoothed to a time-frequency signal of 2×78.8 ms full power width at half maximum.
|Time-Frequency Sampling||50% Power Drop||50 % Amplitude Drop|
|0.20 Hz / 250 ms||±0.283 Hz, ±394 ms||±0.397 Hz, ±554 ms|
|0.25 Hz / 200 ms||±0.354 Hz, ±315 ms||±0.496 Hz, ±443 ms|
|0.5 Hz / 100 ms||±0.708 Hz, ±158 ms||±0.993 Hz, ±222 ms|
|1.0 Hz / 50 ms||±1.42 Hz, ±78.8 ms||±1.99 Hz, ±111 ms|
|2.0 Hz / 25 ms||±2.83 Hz, ±39.4 ms||±3.97 Hz, ±55.4 ms|
|2.5 Hz / 20 ms||±3.54 Hz, ±31.5 ms||±4.96 Hz, ±44.3 ms|
|5.0 Hz / 10 ms||±7.08 Hz, ±15.8 ms||±9.93 Hz, ±22.2 ms|
|10.0 Hz / 5 ms||±14.2 Hz, ±7.88 ms||±19.9 Hz, ±11.1 ms|
The following two plots show the frequency blurring (left) and the temporal blurring (right) for a time-frequency sampling of 2.0 Hz / 25 ms. When applying a denser frequency sampling, the frequency resolution will increase, and the time resolution will decrease proportionally (i.e. the x-axis scaling will change proportionally).