Sensory Systems: Vision and Auditory Mechanisms

Comprehensive vocabulary flashcards covering the optical, retinal, and cortical visual systems, as well as acoustic physics, cochlear anatomy, auditory transduction, and spatial localization.

Cornea

The clear front surface of the eye that refracts (bends) most incoming light, providing primary focusing power.

Refraction

The bending of light rays so they can focus properly onto the retina.

Pupil

The opening that allows light to enter the interior of the eye.

Iris

The colored, muscular portion of the eye that controls pupil size to regulate how much light enters.

Lens

The internal optical structure that changes shape to adjust focus and direct where light focuses for different viewing distances.

Accommodation

The process by which the eye's lens changes its shape to focus light from objects at different distances.

Retina

The light-sensitive neural tissue located at the back of the eye containing photoreceptors and retinal circuitry.

Fovea

The specialized central region of the retina packed with cones that provides the sharpest visual acuity.

Optic Disc

The site where axons of retinal ganglion cells exit the eye to form the optic nerve; it contains no rods or cones.

Blind Spot

The area in the visual field corresponding to the optic disc, where no photoreceptors are present to detect light.

Perceptual Completion

The neural process in which the brain fills in missing visual information at the blind spot using surrounding patterns.

Accommodation for Near vs. Far Objects

Focusing on near objects causes the lens to become rounder (thicker) and the pupil to constrict; focusing on far objects causes the lens to become flatter (thinner).

ON-Center Pathway

A visual pathway where an increase in light in the receptive field center leads to ON bipolar cell depolarization and increased firing in ON-center ganglion cells.

OFF-Center Pathway

A visual pathway where a decrease in light (darkness) in the receptive field center depolarizes OFF bipolar cells and increases firing in OFF-center ganglion cells.

Myopia

A refractive error where light focuses in front of the retina, causing distant objects to appear blurry; corrected using a diverging (concave) lens.

Hyperopia

A refractive condition where light focuses behind the retina when accommodation is relaxed due to insufficient focusing power.

Presbyopia

An age-related loss of lens flexibility that impairs the ability to accommodate and focus on nearby objects.

Astigmatism

A visual defect characterized by blurry or distorted vision caused by an uneven curvature of the cornea or lens.

Rods

Photoreceptors specialized for dim-light vision that exhibit high sensitivity, high convergence, lack of color discrimination, and low detail resolution.

Cones

Photoreceptors specialized for bright-light vision that mediate color detection and high visual acuity with low convergence.

Scotopic Vision

Dim-light vision mediated predominantly by rods.

Photopic Vision

Daylight vision mediated by cones.

Photopigment

A light-sensitive molecule embedded in photoreceptor outer segments consisting of a protein (opsin) and a chromophore (retinal) that absorbs photons.

Photopigment Bleaching

The structural change that occurs when light hits a photopigment, rendering it temporarily incapable of absorbing another photon until regenerated.

Convergence (Retinal)

The neural arrangement where multiple photoreceptors funnel synaptic signals onto shared downstream bipolar and ganglion neurons.

Dark Current

The continuous inward flow of positive ions into photoreceptors in darkness, maintained by $cGMP$-gated cation channels remaining open.

Phototransduction

The biological cascade by which photoreceptors convert light absorption into an electrical signal: light decreases $cGMP$, closes cation channels, reduces the dark current, hyperpolarizes the cell, and reduces glutamate release.

Horizontal Cells

Retinal interneurons that modulate communication between photoreceptors and bipolar cells, providing lateral inhibition that generates center-surround receptive fields.

Amacrine Cells

Retinal interneurons that modify synaptic signaling between bipolar cells and ganglion cells.

Lateral Inhibition

A neural wiring mechanism in which active neighboring neurons suppress the activity of adjacent pathways to enhance borders and contrast.

Mach Bands

A perceptual illusion where visual edges appear exaggeratedly lighter or darker than their physical luminance, produced by retinal lateral inhibition.

Nasal Retina

The inner half of the retina near the nose whose nerve fibers cross to the contralateral side of the brain at the optic chiasm.

Temporal Retina

The outer half of the retina near the temples whose nerve fibers remain uncrossed (ipsilateral), projecting to the same side of the brain.

Optic Chiasm

The X-shaped anatomical site where retinal ganglion cell axons originating from the nasal retinas cross to the opposite hemisphere.

Lateral Geniculate Nucleus (LGN)

The visual thalamic relay station that receives input from the optic tract and projects to the primary visual cortex via optic radiations.

Magnocellular Pathway (M Pathway)

The visual processing stream specialized for detecting motion, rapid changes, high temporal resolution, and luminance contrast, with lower spatial resolution.

Parvocellular Pathway (P Pathway)

The visual processing stream specialized for detecting fine details and red-green color differences, displaying high spatial resolution and slower temporal responses.

Primary Visual Cortex (V1)

The first major cortical recipient of visual input, situated in the occipital lobe and organized retinotopically.

Retinotopy

The spatial principle where adjacent points in the visual field are mapped onto adjacent anatomical locations in the visual cortex.

Cortical Magnification

The anatomical allocation where central foveal vision occupies a disproportionately large area of the visual cortex relative to its retinal size.

Scotoma

A blind or visually deficient spot within an otherwise intact visual field, often resulting from damage to visual pathways or $V1$.

Sound Frequency

The rate at which a sound wave repeats per second, measured in Hertz ($Hz$), which primarily determines perceived pitch.

Sound Period

The duration required to complete one full cycle of a sound wave, calculated as $\text{Period} = \frac{1}{\text{frequency}}$.

Sound Amplitude

The magnitude of pressure variations in a sound wave, which corresponds subjectively to perceived loudness.

Timbre

The qualitative acoustic dimension that allows a listener to distinguish two sounds that share identical pitch and loudness.

Fourier Analysis

A mathematical technique that deconstructs a complex acoustic waveform into its component sine-wave frequencies and amplitudes.

Harmonics

Frequencies in a complex tone that represent integer (whole-number) multiples of the lowest fundamental frequency.

Tympanic Membrane

The eardrum, a membranous boundary between the ear canal and middle ear that vibrates when struck by incoming sound waves.

Ossicles

The chain of three middle ear bones (malleus, incus, and stapes) that mechanically convey and amplify acoustic vibrations to the oval window.

Oval Window

The membrane-covered opening at the entrance of the cochlea that transmits mechanical vibrations from the stapes footplate into cochlear fluid.

Round Window

A flexible cochlear opening that bulges outward to relieve fluid pressure displaced by oval window movements.

Organ of Corti

The sensory receptor organ of hearing resting on the basilar membrane that contains inner and outer hair cells and stereocilia.

Inner Hair Cells

The primary sensory mechanoreceptors in the organ of Corti that transduce mechanical vibrations into electrical signals sent to the auditory nerve.

Outer Hair Cells

Receptor cells in the organ of Corti whose stereocilia contact the tectorial membrane and which change length to amplify cochlear vibrations and improve frequency selectivity.

Endolymph

The potassium-rich ($K^+$) fluid that fills the scala media (cochlear canal).

Helicotrema

The narrow aperture at the cochlear apex that connects the scala vestibuli (vestibular canal) with the scala tympani (tympanic canal).

Basilar Membrane: Base vs. Apex

The base is stiff, located near the beginning of the cochlea, and responds to high-frequency (high-pitch) sounds; the apex is flexible, located near the tip of the cochlea, and responds to low-frequency (low-pitch) sounds.

Hair Cell Transduction Mechanism

Bending stereocilia toward the tallest stereocilium opens mechanically gated channels, allowing $K^+$ influx, depolarization, $Ca^{2+}$ entry, and neurotransmitter release; bending away closes channels and hyperpolarizes the cell.

Conductive Hearing Loss

Hearing impairment caused by an obstruction or mechanical issue in the outer or middle ear that prevents sound vibrations from reaching the cochlea.

Sensorineural Hearing Loss

Hearing impairment resulting from damage to sensory hair cells inside the cochlea or along auditory nerve fibers.

Medial Superior Olive (MSO)

An auditory brainstem structure that measures interaural time differences ($\text{ITDs}$) to localize low-frequency sounds.

Lateral Superior Olive (LSO)

An auditory brainstem structure that detects interaural level differences ($\text{ILDs}$) to localize high-frequency sounds.

Place Coding

The acoustic frequency representation based on the specific physical location along the basilar membrane where hair cells are maximally displaced.

Phase Locking

The synchronized firing of auditory neurons at a specific, identical phase point during a sound wave's cyclical period.

Volley Principle

The mechanism where multiple auditory neurons coordinate their firing in staggered temporal volleys to track frequencies higher than single-neuron refractory limits.

Missing Fundamental

The perceptual phenomenon where the brain reconstructs and perceives the fundamental pitch of a harmonic sound even when the fundamental frequency itself is absent.

Interaural Time Difference (ITD)

The slight arrival time disparity between the two ears used to localize lower-frequency sounds in the horizontal plane.

Interaural Level Difference (ILD)

The difference in sound loudness between the two ears, caused by the acoustic head shadow, used primarily to localize high-frequency sounds.

Cochlear Nuclei

The first major brainstem auditory relay station that receives input directly from the auditory nerve.