May I know how to find Time frequency power spectrogram of a signal?
Short-time Fourier transform

Traditional Fourier analysis provides no information about evolution of frequency components over time. However, Short-time Fourier transform (STFT), a method which applies a short-time window to the signal and performs a series of Fourier transforms within this window as it slides across all the data, can overcome this limitation, providing a time–frequency representation of the signal. 

In MATLAB, try

doc spectrogram


Continuous Wavelet Transform

Alternatively, continuous wavelet transforms (CWT) provide a useful approach in investigating non-stationary signals, this is often regarded as an “optimal” solution with regard to time and frequency resolution. Reportedly, CWT perform better at detecting abrupt changes in non-stationary signals compared to STFT.

In MATLAB, try

doc cwt


Generally, EEG signals have a low signal to noise ratio (SNR), therefore it is preferred to use ensemble averaging over trials for your time-frequency analysis (TFA), should the study design allow it.