Sensory Processing Notes (Psychology 100)

Vision

• Sensations involve different forms of energy; visible light is just one modality beneath the broad umbrella of sensations.
• Visible light range: $400\,\text{nm} \le \lambda \le 700\,\text{nm}$ (short waves violet to long waves red).
• Light can be reflected by objects; white objects reflect the entire range of visible wavelengths, while black objects reflect none.
• Photons are the smallest units of visible light.
• Spectrum context: visible light sits between invisible long waves (infrared, beyond red) and invisible short waves (ultraviolet, beyond violet).
• Hue, saturation, and brightness are core dimensions of color perception; brightness corresponds to the amplitude of light waves.

The Eye: From Photon to Neural Signal

• Cornea focuses light and flips the image; the shape of the cornea affects focus and can contribute to refractive errors when misshapen.
• Aqueous humor fills the front chamber; vitreous humor fills the back chamber.
• Iris controls the size of the pupil; pupil size regulates the amount of light entering the eye.
• Lens provides accommodation to focus near vs. far objects; ciliary muscle alters lens shape for accommodation.
• Other key components: pupil, lacrimal structures not detailed here, and the retina at the back of the eye.

Retina and Photoreceptors

• Retinal cells include photoreceptors and several interneuron types that process signals before sending them to ganglion cells.
• Photoreceptors basics:
  • Photopigment consists of retinol (a type of vitamin A derivative) bound to opsin; interaction with light triggers a chemical reaction that initiates neural signaling.
  • Two main photoreceptor types:
  • Rods: optimized for low light and motion detection; important for scotopic (night) vision.
  • Cones: optimized for color and fine detail; important for photopic (daylight) vision.
• Photoreceptors sit in the retina in a layered structure with photoreceptor cells feeding into bipolar cells, which then feed into retinal ganglion cells that form the optic nerve.

Retina Architecture (Key Cells and Layers)

• Photoreceptors (rods and cones) at the back of the eye detect light.
• Bipolar cells convey signals from photoreceptors to ganglion cells.
• Horizontal cells and amacrine cells provide lateral connections that shape receptive fields.
• Retinal ganglion cells transmit signals to the brain via the optic nerve.
• The fovea is the central region specialized for high-acuity vision; it has a high density of cones and no rods.
• The blind spot corresponds to the location where the optic nerve exits the retina (no photoreceptors there).

Distribution of Rods and Cones; Dark Adaptation

• Rods are distributed more densely in the peripheral retina and are highly sensitive in low light; cones are concentrated in the fovea and support color and detail.
• Dark adaptation: the process by which eyes increase sensitivity in darkness; rods adapt more slowly than cones but offer higher sensitivity in dim light.
• Rod-dominated adaptation is slower than cone-dominated adaptation; cones adapt quickly but saturate at lower light levels.
• Rods are critical for night vision; cones support color vision and high-resolution detail.

Spectral Sensitivity and Color Vision

• Spectral sensitivity curves show how photoreceptors respond to different wavelengths:
  • Blue (short wavelengths), Green (middle wavelengths), Red (long wavelengths) cone classes contribute to color perception.
  • Rods also contribute to luminance perception, particularly in low light.
• Spectral sensitivity details (typical): blue peaks around near 450 nm, green around ~550 nm, red around ~570–600 nm; rods peak around ~498 nm under standard lighting but are highly sensitive across a broad range in dim light.
• Trichromatic theory (Young-Helmholtz): color perception arises from the differential firing rates of the three cone types (blue, green, red).
  • This theory predicts how mixtures of cone responses produce colors; examples include color experiences like yellow (involves simultaneous activation of green and red cones) and white (summing across all three types).
• Opponent-process theory: color perception is organized in opponent pairs (red-green, blue-yellow) with neural processes that encode differences between pairs; this explains afterimages and certain color contrasts.
• Combined view: both trichromatic coding at the photoreceptor level and opponent-process processing in subsequent neural stages contribute to color vision.

Visual Processing and Pathways

• Receptive fields of retinal ganglion cells often have center-surround structures: on-center, off-surround vs off-center, on-surround.
• Early visual processing includes edge detection and contrast enhancement via simple cells and other cortical neurons.
• Visual pathway to the brain:
  • Retina → optic nerve → optic chiasm → optic tract → lateral geniculate nucleus (LGN) of the thalamus → primary visual cortex (V1, calcarine sulcus).
  • Some signals also project to the superior colliculus via the tectopulvinar pathway for orienting responses.
• Blindsight: residual visual capability in individuals with lesions to primary visual cortex, suggesting alternative pathways for some visual information.
• Color vision deficiencies (color blindness) reflect variations or deficiencies in cone types or post-receptoral processing.
• Receptive field maps and the proposed neural circuit: receptors feed into bipolar cells, which feed into ganglion cells; the organization underlies center-surround receptive fields that drive edge detection and spatial perception.
• Visual field maps: nasal and temporal retina inputs split at the optic chiasm; left visual field projects to the right hemisphere and vice versa.
• Meyer's loop and dorsal/ventral pathways:
  • Dorsal pathway (where/how): processes spatial location and motion; projects to parietal areas.
  • Ventral pathway (what): processes object identity and color; projects to temporal areas.

Visual Fields, Blind Spot, and Visual Cortex Layout

• Left visual field information projects to the right hemisphere; right visual field to the left hemisphere; due to partial crossing at the optic chiasm.
• Blind spot corresponds to the optic disc where the optic nerve exits the retina; no photoreceptors present.
• Primary visual cortex located in the occipital lobe; V1 processes basic visual features (edges, orientations, basic shapes).
• Simple cells in V1 have receptive fields that respond to oriented edges and bars of light; their activity reflects the organization of the visual scene.

Color Vision Tests and Color Blindness

• Color blindness tests assess the ability to distinguish color differences; results can indicate deficiencies in one or more cone types.
• Common color vision deficiencies involve reduced sensitivity to certain wavelengths rather than a complete lack of color vision.

Hearing: Basics and the Ear

• Sensory stimulus for hearing involves changes in air pressure (compression and rarefaction) transmitted as sound waves.
• Hertz (Hz) measure frequency (cycles per second) of sound; decibels (dB) measure amplitude/intensity of sound pressure.
• Key wave properties: wavelength corresponds to the distance between successive peaks; duration and periodicity influence pitch perception.

The Ear: Structures and Function

• Outer ear components: pinna (auricle) and external auditory canal; collect and funnel sound toward the eardrum.
• Tympanic membrane (ear drum) vibrates in response to sound waves.
• Middle ear ossicles: malleus (hammer), incus (anvil), stapes (stirrup); amplify and transmit vibrations from the eardrum to the oval window.
• Inner ear structures: cochlea contains the organ of Corti with basilar membrane, hair cells with stereocilia embedded in the basilar membrane, and the tectorial membrane.
• Oval window transmits mechanical energy into the cochlear fluids; round window accommodates fluid movement.
• Signal transduction: bending of hair cells’ stereocilia triggers neural impulses in auditory nerve fibers.
• The cochlea can be conceptualized as a snail-shaped organ where mechanical frequency mapping occurs along the basilar membrane (tonotopy).

The Cochlea and Hair Cells

• Hair cells: convert mechanical energy into neural signals; stereocilia movement drives ion channels and neurotransmitter release.
• Organ of Corti contains inner and outer hair cells that contribute to auditory transduction and amplification.
• Basilar membrane is structurally graded: stiffer at the base (high frequencies) and more flexible at the apex (low frequencies).
• Mechanisms of hair cell transduction involve bending of stereocilia, opening of ion channels, and release of neurotransmitters to auditory nerve fibers.

Pitch Detection: Theories

• Frequency Theory: neural firing rate matches the frequency of the sound; effective for low frequencies but limited by neural firing rates.
• Place Theory: different frequencies maximize vibration at specific locations along the basilar membrane; high frequencies peak near the base, low frequencies near the apex.
• Most models combine place coding for high-frequency components with rate coding for lower frequencies to explain a broad pitch range.

Auditory Processing and Neural Coding

• Auditory nerve fibers show frequency tuning; thresholds vary with frequency, illustrating the sensitivity profile of auditory processing.
• Auditory scene analysis: the brain segregates and organizes sounds from a complex environment into perceptual streams; multiple cues (timing, intensity, frequency) contribute to perceptual grouping.

Deafness and Hearing Impairment

• Conductive deafness: problems in the outer or middle ear that impede sound transmission to the inner ear.
• Sensorineural (nerve) deafness: damage to the inner ear (cochlea) or auditory nerve.
• Exposure to very high sound levels (dB) raises the risk of hearing loss over time; there's a danger level associated with prolonged exposure at certain decibel levels.
• Example risk levels range from everyday quiet to very loud sounds; protection is advised at high dB levels to prevent irreversible damage.

Cochlear Implants

• External sound processor captures sound and converts it into electrical signals.
• Internal implant delivers electrical stimulation to the cochlea via an array of electrodes, bypassing damaged hair cells to stimulate the auditory nerve directly.
• A cochlear implant can restore a sense of sound for individuals with severe to profound sensorineural hearing loss.

Smell and Taste: Chemical Senses

• Olfaction (smell): a chemical sense relying on airborne molecules; around 5 million olfactory receptors of many types contribute to odor perception.
• Olfactory receptors operate on a lock-and-key principle: specific odor molecules bind to specific receptor proteins, triggering neural signals.
• Olfactory information bypasses the thalamus (has a relatively direct route to olfactory cortex) and is associated with pheromones and signaling.
• Pheromones may play a signaling role in social and reproductive behaviors across species; humans also possess olfactory signaling mechanisms.

Olfactory System: Anatomy and Pathways

• Olfactory receptor neurons have cilia that detect odorants in the nasal mucosa.
• Afferent fibers of the olfactory nerve convey signals to the olfactory bulb, which processes smell information before routing to higher cortical areas.
• Pathway components include the cribriform plate of the ethmoid bone and olfactory receptor cells (basal, supporting, receptor cells).

Taste: Gustation

• Taste receptors exist for sweet, bitter, salty, sour, and umami; possibly a fifth basic taste (noted in some classifications).
• Detection mechanisms:
  • Sweet and bitter: primarily via chemical binding (“lock and key”) to receptor proteins.
  • Salty and sour: involve ion flow directly affecting receptor cells.
• Taste receptor cell turnover: receptors vary by individual and tend to decrease with age.

Taste Buds and Papillae

• Taste buds are clusters of receptor cells located within papillae on the tongue (circular, filamentous, and other papillae types).
• Taste buds have hairlike endings that extend into the taste pore and synapse with sensory nerves.

Skin Senses and Somatosensation

• Primary skin senses: light touch, pressure, pain, cold, and heat.
• Receptor density varies by body location, influencing the size of the somatosensory cortex representation (somatotopy).
• Pain is a common sensory experience and can be visceral in origin; dynamic touch involves movement and changes in pressure.

General Principles of Sensory Receptors

• Adaptation: sensory receptors reduce signaling in response to a constant, unchanging stimulus.
• Higher-level processing uses habituation, sensory gating, and selective attention to modulate perception and processing.
• These processes help manage the flood of sensory information and prioritize salient cues for behavior and decision-making.

Practical and Ethical Implications

• Subliminal perception: debates exist about the extent to which unnoticed stimuli influence behavior; interpretation depends on methodological rigor and definition of subliminal thresholds.
• Exposure to loud sounds: real-world implications for hearing health; guidelines emphasize limiting exposure duration and using protective equipment in loud environments.
• Color vision and accessibility: understanding color vision deficiencies informs design (e.g., signage, user interfaces) to enhance accessibility for all.
• Sensory augmentation and implants (e.g., cochlear implants) raise ethical considerations about device access, cost, and long-term impacts on perception.

Connections to Foundational Principles

• Weber’s law (just noticeable differences): the JND is proportional to stimulus magnitude; used to quantify sensation changes across modalities.
  • Expression: $\Delta I = k \cdot I$ where $\Delta I$ is the JND, $I$ is the stimulus intensity, and $k$ is a constant.
• Fechner’s law: subjective magnitude increases as a logarithm of stimulus intensity; links sensation to physical energy.
  • Expression: $S = k \log_{10}(I)$ where $S$ is perceived intensity.
• Energy conversion and transduction: sensory receptors convert physical energy (light, sound, chemical molecules) into neural signals via specialized transduction mechanisms (photopigments, hair cell mechanics, receptor binding).
• Topographic and functional specialization: distinct pathways (dorsal/ventral, on-center/off-surround, receptor-to-neuron chains) support specialized processing for spatial, spectral, and temporal information.
• Multimodal integration: perception arises from integrating diverse cues (visual-auditory-aolfactory-taste-somatosensory) to form coherent representations of the environment.