How can I make my scatter plot markers opaque, or semi transparent?

I am only 2 weeks old in computer programming so please bare with me my background is neuroscience!

So I have come far enough in a script to create a scatter plot and overlay it onto an image. I have managed to have the colour as a function of z-scores (Z) and size as a function of variable durS - which is awesome. All I need to know now is how to make the markers semi transparent so that I can see the overlap of markers and the image underneath.

Here is my code so far:

X = alldatamat(:,2); %xcoor Y = alldatamat(:,3); %ycoor Z = (pupilz); %colour as a function of z scores durS = alldatamat(:,4); %size of marker as a function of duration

figure(1); imshow(imselect); % brings up selected image hold on; Z = remap(Z,0,255); %% this is a made up function to retain colours when superimposed on image ps = scatter(X,Y,durS,Z,'filled');

please help me make markers semi transparent (I tried alpha and played around with a few other things), or maybe suggest another way I can do this. In your answer please assume I know the least! Thanks!