Signal Detection Theory and the Anatomy of Human Vision

Introduction to Signal Detection Theory

• Definition and Purpose: Dr. Jardin defines Signal Detection Theory (SDT) as the study of how decisions are made when only two possible outcomes exist. It aims to understand the decision-making process under conditions of uncertainty.

• The Decision Matrix (Email/Spam Example):

  • In an email system, a computer algorithm must decide if an incoming message is "good email" or "spam."

  • Yes (Decision): The algorithm classifies the email as good.

  • No (Decision): The algorithm classifies the email as spam.

• Four Possible Outcomes:

  • Hit: Correctly identifying a signal that should be accepted. Example: A message from a friend is correctly identified as good and sent to the inbox.

  • Miss: Failing to identify a signal correctly, resulting in an improper label. Example: A good message is misidentified as spam and sent to the spam folder.

  • Correct Rejection: Properly identifying and rejecting a non-signal. Example: The algorithm recognizes spam correctly and prevents it from entering the inbox.

  • False Alarm: Identifying a non-signal as a signal. Example: A spam message is incorrectly identified as good and makes its way into the inbox.

• TSA Airport Screening Example:

  • Hit: Identifying a person carrying a dangerous item.

  • Miss: Failing to identify an actual threat, allowing them to pass through security.

  • Correct Rejection: Correcting identifying a person who is not a threat and has nothing wrong with them.

  • False Alarm: Misidentifying a safe individual as a threat when they have nothing dangerous.

• Attentional Factors: SDT becomes more challenging over extended periods due to fatigue. Making "yes/no" decisions for long durations taxes the attentional system and increases the difficulty of accurate detection.

Sensation, Transduction, and the Nature of Light

• Sensation and Vision: Vision is considered the best-understood sense and serves as the primary framework for discussing sensory processing.

• Transduction: This is the process of converting external energy from the environment (e.g., light or sound waves) into internal electrochemical signals, specifically neural impulses or action potentials.

  • Sensory Receptors: These are specialized cells that make the process of transduction possible.

  • Neural Code: Once transduction is complete, the brain uses a pattern of neural firing to translate energy into meaningful experiences of sound, touch, or taste.

• The Stimulus: Light:

  • Light is a form of electromagnetic radiation that travels in waves (though it can also take the form of particles).

  • Wavelength: The distance from one peak of a wave to the next. This determines the psychological experience of color (or hue).

  • Amplitude: The height of the wave. This determines the psychological experience of brightness.

• Visible Spectrum: The human eye can only discern a very narrow band of electromagnetic wavelengths.

  • Human Range: Approximately $400$ to $750\,nm$ (nanometers).

  • Species Differences: Bees can perceive wavelengths between $300$ and $700\,nm$, allowing them to see ultraviolet (UV) light. This reveals hidden patterns and colors on flowers that lead the bee to pollen and nectar, which are invisible to humans.

• Sensory Limitation: The limitations of sensory receptors define the limitations of conscious experience; humans cannot experience what their senses cannot register.

Anatomy of the Eye

• Initial Path of Light:

  • Cornea: A tough, transparent tissue covering and protecting the front of the eyeball.

  • Aqueous Humour: A small area of clear fluid behind the cornea.

  • Pupil: An opening in the center of the iris through which light passes.

  • Iris: A muscle that controls the size of the pupil.

• The Pupillary Response:

  • Dilate: The pupil expands in the dark to increase light entry.

  • Constrict: The pupil shrinks in bright light to protect the retina. Constriction typically happens faster than dilation.

  • Psychological Factors: Pupils also fluctuate in size based on emotions, motivation, concentration, sexual attraction, and arousal states (e.g., a cat's pupils dilating before it pounces).

• Focus and Translation:

  • Lens: Focuses light onto the back of the eye.

  • Retina: Located at the very back of the eye, containing receptors necessary to transduce light into neural impulses.

Neural Processing in the Retina

• Retinal Structure: The retina is approximately the thickness of a single sheet of paper but consists of several complex cell layers.

• Photoreceptors: Located in the innermost layer at the back of the retina.

  • Rods: Rod-shaped receptors. They are highly sensitive to light, allowing for vision in low-light conditions. They do not distinguish between frequencies (black and white vision only) and are mostly found in the periphery.

  • Cones: Pointed/cone-shaped receptors. They are much less sensitive than rods and require strong light. They are sensitive to different wavelengths, allowing for color vision and high visual acuity (sharpness).

• The Three Types of Cones:

  • Short Wavelength (Blue): Maximally sensitive to $420$ to $440\,nm$.

  • Middle Wavelength (Green): Maximally sensitive to $534$ to $545\,nm$.

  • Long Wavelength (Red): Maximally sensitive to $564$ to $580\,nm$.

  • Note: It is preferred to use "Short, Middle, Long" because individual cones do not provide "color" information directly; the brain interprets the activation later.

• Cellular Layers and Signaling:

  1. Photoreceptors (Rods and Cones) are activated by light.

  2. Bipolar Cells: The next layer up; these cells integrate signals from multiple photoreceptors through synapses.

  3. Ganglion Cells: These integrate information from multiple bipolar cells.

  4. Optic Nerve: Formed by the axons of ganglion cells, which carry the signal from the retina to the brain.

• The "Inside-Out" Architecture: Light must pass through the liquid of the eyeball, blood vessels, and the ganglion/bipolar layers before it actually reaches the photoreceptors at the very back of the retina.

Specialized Structures and Adaptations

• Fovea: A small central region of the retina densely packed with cones. It provides the clearest vision and is used for tasks like reading.

  • Size: Measures only about $1$ to $2$ degrees of visual angle.

  • Visual Angle Experiment: If you hold your thumb at arm's length, the area it covers on your retina is approximately $1$ degree of visual angle.

• Pigmented Epithelium: A dark-colored membrane behind the photoreceptors in humans and other diurnal (day-active) animals. Its purpose is to absorb stray photons to prevent blurring, ensuring clear, crisp vision.

• Reflecting Tapetum (Tapetum Lucidum): A reflective membrane found in nocturnal animals (like dogs and cats) instead of a pigmented epithelium.

  • Function: It bounces stray photons back through the photoreceptors for a second chance at absorption.

  • Trade-off: Increases sensitivity to low light (night vision) but causes a loss of visual acuity (crispness). This reflection is why animal eyes appear to "glow" in the dark.

Theories of Color Perception

• Young-Helmholtz (Trichromatic) Theory:

  • Developers: Thomas Young ($1802$) and Hermann von Helmholtz ($1850s$).

  • Concept: Based on behavioral evidence, they logicized that the eye must contain three types of receptors maximally sensitive to specific wavelengths.

  • Mechanism: Color perception results from mixing inputs from these three receptors, similar to mixing paint to create new colors.

  • Significance: Correctly predicted the existence of three cone types $100$ years before physical confirmation and explains color blindness (missing one or more cone types).

• Opponent Process Theory:

  • Developer: Ewald Hering.

  • Concept: Colors are derived from three antagonistic (opponent) systems:

    1. Black and White

    2. Blue and Yellow

    3. Red and Green

  • Afterimages Experiment: Staring at a red/yellow image and then looking at a white space produces a green/blue afterimage.

  • Mechanism: Staring at a color fatigues that specific part of the system; when removed, the opposite (antagonistic) color is briefly perceived because the system is unbalanced. The Trichromatic theory cannot explain this phenomenon.

• Synthesis: Both theories are correct but apply to different stages of processing:

  • Trichromatic Theory: Applies to the level of the retina (photoreceptors).

  • Opponent Process Theory: Applies to later stages of visual processing in the visual centers of the brain.

Questions & Discussion

• Afterimage Exercise: The speaker instructs the audience to stare at a dot in the center of a yellow and red image for $30$ seconds without blinking.

• Outcome: Upon removing the image, the audience should see an afterimage in blue and green. If not seen, the speaker suggests moving closer to the screen and trying again without moving the eyes.