The Science of Sight: Psychophysics, Eye Anatomy, and Neural Vision

Posted on Sep 4, 2025 in Psychology

Foundations of Sensation and Perception

Psychophysics: Measuring Perception

Weber’s Law and Just Noticeable Difference

Ernst Weber discovered that the smallest detectable change in a stimulus is a constant proportion of the stimulus level.

Weber’s Law: ΔI/I = k (where ΔI is the change in intensity we can detect, I is the intensity of the stimulus, and k is a constant).

The Just Noticeable Difference (JND) is the minimum amount of difference between two stimuli required to reliably notice a change.

Fechner’s Contributions to Psychophysics

Gustav Fechner sought to understand the relationship between physical stimuli and psychological experience. He developed three methods and Fechner’s Law, which attempts to demonstrate the relationship between environmental energy and internal perception.

Key Psychophysical Concepts

• Absolute Threshold: The minimum amount of energy required in the environment to detect a stimulus.
• Magnitude Estimation: Describes the mathematical relationship between stimulus intensity and perceived magnitude.
• Steven’s Power Law: Sensation (S) is related to stimulus intensity (I) by an exponent (b). S = aI^b (where a is a constant).
• Signal Detection Theory: A method for determining an individual’s sensitivity to a stimulus. The Receiver Operating Characteristic (ROC) curve plots hits versus false alarms, and misses versus correct rejections (CR).
• ‘d” represents your sensitivity to a stimulus, while ‘criterion’ is the amount of evidence you require to make a decision.

Neural Basis of Sensation

Pioneering Discoveries in Neuroscience

Helmholtz theorized about the time it takes for a person to feel a stimulus. Santiago Ramón y Cajal discovered that neurons do not physically touch each other, establishing the concept of the synapse.

Transduction and Neural Firing

Transduction: The process by which energy from the environment is converted into an electrical signal by sensory receptors.

The more excitation a cell receives, the sooner it can overcome its refractory period. The rate at which a cell fires indicates the strength of the signal.

Doctrine of Specific Nerve Energies (DoSNE)

The Doctrine of Specific Nerve Energies (DoSNE) states that the quality of the sensation elicited by a stimulus depends on the specific nerve excited, rather than the stimulus itself.

Brain Imaging for Sensory Research

Electroencephalography (EEG)

Electroencephalography (EEG) uses electrodes placed on the scalp to measure electrical activity from neurons.

Event-Related Potentials (ERP)

Event-Related Potentials (ERP) are measures of brain response to specific stimuli, requiring the averaging of many EEG recordings.

Functional Magnetic Resonance Imaging (fMRI)

Functional Magnetic Resonance Imaging (fMRI) measures local brain activity patterns based on Blood Oxygen Level Dependent (BOLD) signals. BOLD refers to the ratio of oxygenated to deoxygenated hemoglobin.

Fundamental Sensory Processes

Sensory Adaptation

Adaptation: If a stimulus remains constant, our senses adapt, and we cease to perceive the stimulation.

Cross-Modality Matching

Cross-Modality Matching: The ability to match the intensities of sensations originating from different sensory modalities. This allows us to observe sensory differences.

Sensation vs. Perception

Sensation: The process of detecting energy in the environment.

Perception: The process of interpreting sensations into a meaningful experience.

Light Analysis Pathway

The pathway of light analysis involves physical energies being converted into neural codes, which then lead to a psychological experience.

The Visual System: Structure and Function

Understanding Light and Vision

The Electromagnetic Spectrum

Visible light constitutes only a small portion of the electromagnetic spectrum.

Advantages of Binocular Vision

Having two eyes provides several advantages:

• Enhanced depth perception
• Wider peripheral vision
• Redundancy (vision if one eye is compromised)
• Ability to cover the blind spot
• Increased sensitivity to weak signals

Anatomy of the Eye

Key Structures and Their Roles

• Cornea: The clear tissue at the front of the eye, serving as the primary refractive surface. It provides approximately 80% of the eye’s focusing power and is fixed.
• Aqueous Humor: A watery solution located between the cornea and the lens.
• Iris and Pupil: Control the amount of light entering the eye. The iris is responsible for eye color.
• Lens: Allows for adjustments in the eye’s focus, contributing about 20% of the eye’s focusing power.
• Vitreous Humor: A clear gel situated between the lens and the retina, helping to maintain the eye’s shape.
• Retina: A layer of neurons at the back of the eye that contains photoreceptors.

Focusing Light: Refraction and Accommodation

The Lens System

The lens system works through refraction (the bending of a wave as it passes at an angle).

Advantages: Creates bright images.

Disadvantages: Images can be out of focus.

Accommodation and Refractive Errors

Accommodation: The eye’s lens changes shape to focus near or far objects onto the retina.

Our ability to accommodate declines with age, a condition known as Presbyopia.

• Hyperopia (Farsightedness): The eyeball is too short, causing the focal point to be behind the retina.
• Myopia (Nearsightedness): The eyeball is too long, causing the focal point to be in front of the retina.
• Emmetropia: Normal refraction, resulting in no eyesight issues.
• Astigmatism: Caused by an unequal curving of one or more of the eye’s refractive surfaces, typically the cornea.

The Blind Spot

The Blind Spot is the area where ganglion cells leave the back of the retina, lacking photoreceptors.

Retinal Processing and Photoreceptors

Neurons of the Retina

The retina contains five main types of neurons:

• Photoreceptors
• Bipolar cells
• Ganglion cells
• Amacrine cells
• Horizontal cells

Types of Vision

• Photopic Vision: Daytime and color vision, primarily activating cones.
• Scotopic Vision: Low-light vision, primarily activating rods.

Rods vs. Cones: Key Differences

Rods and cones are distributed across the retina, though not uniformly.

Stimuli are measured by the size of the image they project onto the retina.

The Fovea and Retinal Pathways

The Fovea is the center of the retina, consisting primarily of cones, responsible for high-acuity vision within approximately 2 degrees of the visual field.

Retinal Pathways

• Vertical Pathway: Involves photoreceptors, bipolar cells, and ganglion cells.
• Lateral Pathway: Regions of the retina interact through lateral inhibition, involving horizontal and amacrine cells.

Advanced Retinal Processing

Interactions within the Retina

• Horizontal Cells: Responsible for lateral inhibition, creating the center-surround receptive field structure.
• Amacrine Cells: Interact with both bipolar cells and ganglion cells.
• Lateral Inhibition: A pattern of interaction among neurons in the visual system where activity in one neuron inhibits the response of an adjacent neuron.

Bipolar and Ganglion Cell Types

• Midget Bipolar Cells: Receive input from a single cone and pass it to a single ganglion cell.
• M Ganglion Cells: Receive excitatory input from diffuse bipolar cells and project to the magnocellular layer of the Lateral Geniculate Nucleus (LGN).
• P Ganglion Cells: Receive excitatory input from midget bipolar cells and project to the parvocellular layer of the LGN.

Receptive Fields and Visual Phenomena

Receptive Field: The region of the retina where stimuli can influence the firing rate of a neuron.

• ON-Center Receptive Field: Responds best to light in the center of the field.
• OFF-Center Receptive Field: Responds best to darkness in the center of the field.

Ganglion cells act as filters, allowing some information to pass while suppressing others.

Mach Bands: Illusory light and dark bands perceived near a light-dark border.

Contrast: The difference in luminance or brightness between adjacent areas.

Dark Adaptation Curve: A plot illustrating how visual sensitivity changes in the dark over time.

Essential Terminology in Vision Science

Glossary of Key Concepts

• Photoreceptor: A specialized receptor cell that responds to light.
• Diffuse Bipolar Cell: A bipolar cell whose processes spread out to receive input from multiple photoreceptors (often rods or multiple cones).
• Duplex: Refers to the retina having two distinct systems for vision (rods for scotopic, cones for photopic).
• Graded Potential: A shift in electrical charge in a small, localized area of a neuron, varying in magnitude.
• Hyperpolarization: The movement of a cell’s membrane potential away from the resting potential in a more negative direction.
• Photoactivation: The reaction that occurs when light strikes photoreceptors, initiating a neural signal.
• Photon: A quantum of light; a particle of light.
• Retinitis Pigmentosa: A genetic degenerative disease that affects the retina, leading to progressive vision loss.
• Rhodopsin: The photopigment found in rod cells that is responsible for light sensitivity in low-light conditions.
• Visual Acuity: The sharpness of vision; the ability to see fine detail.
• Depolarization: A loss of the difference in electrical charge between the inside and outside of a cell membrane, typically due to changes in ion permeability.
• K+ Channels: When potassium (K+) channels open, K+ ions leave the cell, leading to hyperpolarization and making the cell less likely to fire an action potential.