VIS Lecture 1 - Basics of vision I: The eye

Why Study Visual Perception?

• Allows for the design of a device that can locate, describe, and identify objects in an environment, including their spatial relationships.
• Enables a device to travel, evaluate and reflect on its processed information, have conscious experiences, categorize information, and store details for future use.
• Provides the knowledge necessary to pass psychology exams and progress in the field.
• Facilitates answering common questions about visual perception, like "Why do we experience visual illusions?".
• Enables answering questions from other experts in various fields, such as why building a perception machine is so difficult.
• Provides insight into one of psychology's oldest scientific disciplines.
• It is essential for understanding the human mind and behavior.
• Visual perception is necessary for perception, and thus determines behavior.
• Visual perception involves the transformation of an environmental stimulus.
• Receptor processing, neural processing, and light transformation all contribute to perception.
• Perception relies on knowledge, recognition, and action.

Intro to Visual Perception - Overview

• Week 1 focuses on the basics of vision, the eye, and brain physiology.
• Week 2 will cover the psychology of visual perception, including models, theories, and tests, as well as what happens when perception goes wrong.
• Weeks 3 & 4 will cover color, depth/size, motion, object perception, and recognition features and objects.
• Week 5 includes an interim revision and formative class test and discussion.

Learning Tools

• Textbook: "Sensation and Perception" by E.B. Goldstein (2014), 8th-10th edition.
• Lectures and lecture slides, including optional "blue boxes".
• Self-test slides with one learning outcome and three multiple-choice questions.
• Knowledge trees.

Questions

• Post any administrative or subject-related questions on Blackboard in the Visual Perception folder.

Basics of Vision I: The Eye

• The light-sensitive surface covers ~75% of the inner eye.
• The diameter of the eye is 2.5 cm.
• Retina is the surface on the back of the eye that is responsible for sensing light and color.
• Macula is an oval-shaped pigmented area near the center of the retina of the human eye.
• Fovea is the reference point for central vision, highest acuity and most detailed vision.
• Sclera is the outer layer of the eye made of a tough, fibrous coat.
• The lens is the part of the eye that focuses light onto the retina.
• Optic disc is a blind spot, there are no photoreceptors at this location
• Iris regulates the amount of light entering the eye.
• Pupil admits light into the eye.
• Cornea is the transparent front part of the eye that refracts light.
• The ciliary muscle is crucial for focusing the eye.

Macular Degeneration

• Macular degeneration involves the deterioration of the retina in the macula/fovea area.
• The condition results in irreversible blindness and loss of central vision.

Visual Process - Light

• Light is electromagnetic energy, measured in wavelengths, that enables the perception of objects.

Visual Process - Transformations

• The first transformation involves the conversion of light waves into a retinal object representation.
• The second transformation involves converting the retinal object representation into an electrical signal.

Visual Process - 1st Transformation

• The transformation of light reflects from an object into a retinal image (retinal object representation).
• The level of light refraction is dependent on the thickness of the lens.
• Accommodation refers to focusing on near/far objects.
• Ciliary Muscles relax/contract to change the thickness of the lens, which bends light to fall onto the fovea.
• A problem with accommodation is the fixed distance between the lens and the retina.
• With accommodation to a far object, relaxed ciliary muscles, results in a slim lens, and light is bent at a small angle.
• With accommodation for a near object, contracted ciliary muscles, results in a thick lens, and light is greatly bent.
• Accommodation helps the eye to focus on an object as its distance varies.
• A far point is the maximum distance of objects from the eye for which a clear image can be seen.
• Near point is the minimum distance of object from the eye for which a clear image can be seen.

Common Vision Problems

• Myopia (nearsightedness) involves far objects appearing out of focus, can result from a lens that's too thick or an eyeball that's too long, it requires concave correction lenses that diverge the light.
• Hyperopia (farsightedness) involves near objects appearing out of focus, usually because the eyeball is too short, it requires convex correction lenses that converge light.

Visual Process - 2nd Transformation - Transduction

• Is when the retinal image is transformed into electrical energy.

Layers of the Retina

• They contain light waves, nerve fibers, photoreceptors, horizontal and vertical connectors, and senders to the brain.
• Light-sensitive chemicals in the receptors outer segments change when they are hit by light.
• The chemical reaction sparks an action potential, which travels to the ganglion cells via the bipolar cells.
• The optic disk is a blind spot where there are no photoreceptors.

Photo Receptors - Rods and Cones

Rods and cones differ in:

• Absolute number.
• Density and distribution in the retina.
• Ability to dark adaptation.
• Absolute sensitivity.
• Acuity.
• Spectral sensitivity.
• Colour vision.
• 120 million rods per human eye.
• 6 million cones per human eye.
• Rods are more sensitive to shorter wavelengths with a maximum at 500 nm.
• Cones are more sensitive to longer wavelengths, with a maximum at 560 nm.

Dark Adaptation

• The eye's sensitivity to light increases in dark adaptation.

Absolute Light Sensitivity

• Rods have a higher amount of light sensitivity than cones.

Visual Acuity

• Cones have better visual acuity than rods, especially in light-adapted eyes.

Neural Convergence

• Multiplies neurons synapse onto fewer neurons.
• On average, 1 ganglion receives input from 120 rods and 6 cones.
• More convergence happens for rods than cones.
• Increased light sensitivity occurs in rods, this is known as the ability to detect light.
• Increased acuity in cones allows for detail vision.
• Separate ganglion cells receive input from separate cones, this is detailed information about different light sources is available.
• A same ganglion cells receives inputs from multiple rods, this is known as information about different light sources is combined.

Purkinje Shift

• Increased sensitivity to short wave lengths in dark adapted eye.

Colour Vision

• Rod Vision = Dark-adapted operators are at low luminance with no color sensation.
• Cone vision = Light-adapted operators are at high luminance allowing the following:
  • S cones respond to short wave lengths (Blue).
  • M cones respond to medium wave lengths (Green).
  • L cones respond to long wave lengths (Red).

Two Types of Vision

• Photopic vision: Cone-dominated, foveal and peripheral vision, light-adapted vision, insensitive in dim light, operates at high luminance, has a high visual acuity, most sensitive to long wavelengths and is the basis of colour vision.
• Scotopic vision: Rod-dominated, peripheral vision, dark-adapted vision, sensitive in dim light, operates at low luminance, has a low visual acuity, is sensitive to short wavelengths and has no colour sensation.
• Mesopic vision is a combination of both rod and cone vision together.