How We Perceive Light

Perception of Light

How Light Enters the Eye

• Light is bent through the lens as it enters the eye.
• The amount of light entering the eye is controlled by the iris.
• Light is focused upside down onto the retina after passing through the lens.
• Light rays from the top of an object are focused on the bottom of the retina, and vice versa.
• The image on the retina is inverted (upside down).

Image Processing in the Brain

• The visual input from the retina is integrated in the occipital lobes.
• The brain turns the image right side up, so we perceive the world correctly.

Iris Function

• The iris adjusts the amount of light entering the eye.
• The iris is a two-part muscle composed of:
  • Circular muscle fibers (pupillary constrictors)
  • Radial muscle fibers (pupillary dilators)

Pupillary Constrictors

• They make the pupils smaller.
• Controlled by the parasympathetic nervous system.

Pupillary Dilators

• They radiate out like spokes and open up the pupil size.
• Controlled by the sympathetic division.
• When the sympathetic division is activated (e.g., when scared), pupils dilate to maximize sensory inputs.
• Pupillary dilators are triggered by emotions, including physical attraction.
  • Pupil dilation can indicate attraction.

Eye Function: Distance vs. Nearby

Looking at Far Away Objects

• Light wavelengths are fairly parallel.
• The lens doesn't need to do much to focus light onto the retina.

Looking at Close Up Objects

• Light is more scattered.
• The eye makes adjustments to focus on the close object.
• These adjustments are known as the near response.

Near Response

• Involves:
  • Convergence of the eyes
  • Pupil constriction
  • Accommodation of the lens

Convergence of the Eyes

• Eyes move towards the close object.
• The eyes cross slightly.

Pupil Constriction

• Reduces distracting stimuli from the periphery.
• Allows only light from the object being viewed to enter.
• Controlled by the pupillary constrictor.

Accommodation of the Lens

• The shape of the lens changes due to the actions of the ciliary body on suspensory ligaments.
• This change in shape bends light into one focused spot on the retina.

Emmetropia: Harmonious Vision

• Looking at an object far away (over 20 feet).
• Light comes straight into the eye.
• The lens focuses that light on the back of the eye on the retina without much effort.

Vision Conditions

Hyperopia: Farsightedness

• The lens has a shape such that light is focused behind the retina.
• The image on the retina is scattered.
• Close-up objects are blurry.

Myopia: Nearsightedness

• The lens shape focuses light in the middle of the vitreous humor.
• Light is scattered by the time it hits the retina.
• Far away objects look blurry.
• Close-up objects are seen without issues.