The user can make a selection by blinking. Because not every eye blink should be made a selection, the minimum and maximum blink time must be set by experimenting.
Choose Tools > Eye tracking > Selection.
Select the Blink option.
From the Timing options group, click Minimum blink time.
Do one of the following:
Click on to decrease the time, click on to increase the time.
Drag the slider to the left to decrease the time, drag it to the right to increase the time.
From the Timing options group, click Maximum blink time.
Do one of the following:
Click on to decrease the time, click on to increase the time.
Drag the slider to the left to decrease the time, drag it to the right to increase the time.
to decrease the time, click on 
to increase the time.