Depends on exactly what you mean by local min or max, and I don't see any black or red circles in the picture you posted. You could try movmin()/imerode() or movmax()/imdilate() to get the min or min in a moving window. The local min in the window may not necessarily be a valley and the local max may not necessarily be a peak. To get peaks and valleys you'd need islocalmin/islocalmax (very simple determination) or findpeaks for a more comprehensive classification of what you call a peak or value. To get valleys you need to invert your signal
[peakValues, indexesOfPeaks] = findpeaks(signal);
[valleyValues, indexesOfValleys] = findpeaks(-signal);
valleyValues = -valleyValues; % Invert back to get the original, non-negated values.
There are a bunch of complicated options in findpeaks() to fine tune what you consder a peak or valley.
