Update variable value with slider in MATLAB

I have a struct file:     x.head = imread ('brain.jpg');         whos x

I have a slider:

 G3.slide = uicontrol('style','slide',...
                 'unit','pix',...
                 'position',[400 280 120 30],...
                 'min',-10,'max',10,'val',1,...
                 'sliderstep',[1/20 1/10],...
                 'callback',{@sl_call3,G3});

calling the function:

function [] = sl_call(varargin)

% Callback for the slider.
[k,x] = varargin{[1,3]};
set(x.head,x.head*get(k,'value'))

But when I squeeze, I get the error:

  

Reference to non-existent field 'head'.   Error in Project / sl_call (line 87) set (x.head, x.head * get (k, 'value')) Error while evaluating   UIControl Callback