The following text is for questions 21 to 25.
 

    The eye is the most important sensory organ of the human body. Four fifths of all information from the outside world reaches the brain through the eye. But how does this wonderful organ works?

    Once light reaches the retina, the light-sensing cells (photoreceptors) convert light into electrical impulses. Photoreceptor cells are either rods or cones. Rods are concentrated along the outer perimeter of the retina; they help us to see images in our peripheral vision and also to see in dark and dimly lit environments. Cones are concentrated in the macula, the centre of the retina, allowing us to see fine visual detail in the centre of our vision; they also allow us to perceive colour. Together, the photoreceptors convert light into electrical impulses that are passed through the rest of the retina, through the optic nerve to the brain. The visual centres in the brain interpret the information received from the retina, enabling us to see.

(Adapted from http://www.health.gov.au)

"... they help us to see images in our peripheral vision ...." (Paragraph 2)

What does the underlined word refer to?