What is the function of horizontal cells?

• A. Transmit signals to the brain.
• B. Relay photoreceptor signals.
• C. Regulate intraocular pressure.
• D. Lateral inhibition and contrast enhancement.

Answer: (D)

Horizontal cells act as inhibitory interneurons in the retina, spreading signals laterally across photoreceptors. They integrate input from neighboring photoreceptors and send inhibitory feedback to them (and to adjacent horizontal cells), creating a center-surround receptive-field arrangement. This lateral inhibition sharpens contrast and helps detect edges by suppressing activity in the surround when the center is stimulated, enhancing the difference between light and dark borders. They are not the path that carries signals to the brain—that role belongs to ganglion cells—and they do not regulate intraocular pressure.