With the help of Eyetracker (Eyelink in this case), we can get the realtime gaze position and therefore we can monitor the gaze position to make sure it falls into a certain area before the participants start the trial.

% make sure participants are fixating on the central
while isEyeLink  % if Eyelink was connected
status = Eyelink('newfloatsampleavailable');  % check to see if everything is fine in Eyelink
if status /=0; % if all is fine
evt = Eyelink('newestfloatsample'); % get the newest float sample from Eyelink
realtime.x = evt.gx(1);  % get the x axis of gaze
realtime.y = evt.gy(1);  % get the y axis of gaze
end;

if abs(realtime.x-512)\<=50&abs(realtime.y-384)\<=50  % if the gaze falls into  a 50\*50 pixel square around the cross
break; % jump out the loop and start the trial
end
end