The minimum example I can give you is:
sld = uicontrol('Style', 'slider',...
'Min',1,'Max',50,'Value',41,...
'Position', [5 5 120 20],...
'Callback', @(obj, evt) viewer(obj, evt));
Of course you have to change you function viewer.
function viewer(obj, evt)
.....
Additionally, I would suggest to load view_data.mat outside the Callback function and provide (an) additional argument(s) to the CallBack with your x_layer1, y_layer1 , z_layer1 variables.
Finally, remove the clf command from function viewer because it will clear you current figure window and eventually erase the slider.
