Please try the following:
% Window duration (in seconds):
dur = 0.5;
% Spectrogram settings (in samples):
winSize = round(fs*dur);
overlap = round(winSize/2);
fftSize = winSize;
% Plot the spectrogram:
spectrogram(signal,winSize,overlap,fftSize,fs,'yaxis');
Then try experimenting with the value of dur until the spectrogram provides a good balance between time resolution and frequency resolution.
