The Pupil and Lens of the Eye

• Light enters the eye and reaches the retina, forming the basis for sight.
• Light can be conceptualized as photons or waves of electromagnetic energy with wavelengths between 380 and 760 nanometers.
• Two key properties of light are wavelength and intensity.

Pupil and Lens

• The irises regulate the amount of light reaching the retinas via the pupil.
• Pupil size adjustment balances sensitivity and acuity.
• High illumination constricts pupils, while low illumination dilates them.
• The lens, located behind the pupil, refracts for near vision and flattens for distant vision.
• Accommodation is the process of adjusting the lens to focus images on the retina.

Eye Position and Binocular Disparity

• Vertebrates possess two eyes due to their bilateral symmetry.
• Binocular disparity, the difference in image position on the two retinas, is greater for closer objects.
• The visual system uses binocular disparity to create a 3D perception from 2D retinal images.

Retina and Neural Signals

• The retina converts light into neural signals, conducts them, and participates in signal processing.
• Five types of neurons compose the retina: receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.
• Retinal neurons communicate chemically (synapses) and electrically (gap junctions).
• The retina's inside-out arrangement presents challenges, including light distortion and a gap in the receptor layer for ganglion cell axons.
• The fovea minimizes distortion, specializing in high-acuity vision.
• Completion creatively solves the blind spot issue by filling in gaps using surrounding receptor information.
• Surface interpolation is the process by which we perceive surfaces.

Cone and Rod Vision

• Two types of receptors exist: cone-shaped cones and rod-shaped rods.
• The duplexity theory of vision states that cones and rods mediate different kinds of vision.
• Spectral sensitivity refers to the relative brightness of lights at different wavelengths.
• Photopic spectral sensitivity is determined by judging brightness of wavelengths on the fovea.
• Scotopic spectral sensitivity is determined by judging brightness of wavelengths on the periphery of the retina at low intensity.
• Purkinje effect describes the shift in peak sensitivity towards blue-green in low light, making reds appear darker.
• Involuntary fixational eye movements include tremor, drifts, and saccades.
• Visual transduction is the conversion of light to neural signals by visual receptors.

Pathways and Organization

• Retina-geniculate-striate pathways conduct signals from the retina to the primary visual cortex via the lateral geniculate nuclei of the thalamus.
• The retina-geniculate-striate system exhibits retinotopic organization, mapping the retina at each level.
• Two parallel channels, parvocellular layers (P Layers) and magnocellular layers (M Layers), communicate through each lateral geniculate nucleus.
• Neuron reactions depend on whether there are on-center cells or off-center cells.
• The receptive fields of primary visual cortex neurons are classified as simple or complex.
• Simple cells have antagonistic "on" and "off" regions and are unresponsive to diffuse light.
• Complex cells have rectangular receptive fields, respond to specific orientations, and are unresponsive to diffuse light.
• Primary visual cortex is organized into functional vertical columns.
• Column location depends on retinal location, dominant eye, and preferred straight-line angle.
• Neuronal "preferences" become more complex from retina to thalamus to visual cortex.

Seeing Color

• Component theory (trichromatic theory) suggests three cone types with different spectral sensitivities encode color based on activity ratios.
• Opponent-process theory proposes two cell classes for encoding color and another for brightness.
• Color constancy is the perception of an object's color independent of reflected wavelengths.
• Retinex theory suggests the visual system calculates surface reflectance by comparing light from adjacent surfaces across wavelength bands.

Visual Cortex

• Primary visual cortex is the area that receives input from the visual relay nuclei of the thalamus.
• Secondary visual cortex receives input from the primary visual cortex.
• Visual association cortex receives input from secondary visual cortex and other sensory systems.
• The dorsal stream flows from the primary visual cortex to the dorsal prestriate cortex to the posterior parietal cortex.
• The ventral stream flows from the primary visual cortex to the ventral prestriate cortex to the inferotemporal cortex.
• Where versus what theory of vision suggests damage can selectively impair certain visual aspects.
• Akinetopsia is the inability to perceive motion smoothly.

Mechanisms of Perception

• Hierarchical organization is evident in sensory systems, with higher-level damage causing more specific deficits.
• Functional segregation and parallel processing are key features of sensory system organization.

Auditory System

• The auditory system's function is sound perception.

The Ear

• Sound waves vibrate the tympanic membrane, transferring vibrations to the ossicles (malleus, incus, stapes).
• Stapes vibrations trigger oval window vibrations, transferring vibrations to the cochlea's fluid.
• The organ of Corti, within the cochlea, is the auditory receptor organ.

Auditory Cortex

• Primate primary auditory cortex includes three areas: the core region, belt, and parabelt areas.
• Primary auditory cortex is organized in functional columns.
• Auditory cortex is tonotopically organized.
• Two large areas of association cortex are the prefrontal cortex and posterior parietal cortex.
• Damage to the auditory system can cause deafness.

Somatosensory System

• The somatosensory system relates to touch and pain and mediates bodily sensations: exteroceptive, proprioceptive, and interoceptive.
• Cutaneous receptors include free nerve endings, Pacinian corpuscles, Merkel's disks, and Ruffini endings.
• The dorsal column medial lemniscus system carries information about touch and proprioception.
• The anterolateral system carries information about pain and temperature.
• Somatosensory signals are conducted to association cortex in the prefrontal and posterior parietal cortex.
• Somatosensory agnosias include astereognosis (inability to recognize objects by touch) and asomatognosia (failure to recognize own body parts).

Perception of Pain

• Pain is paradoxical and is extremely important for survival.
• Pain has no obvious cortical representation.
• Cognitive and emotional factors can effectively suppress pain through descending pain control.

Chemical Senses

• Smell and taste are chemical senses.
• Molecules of food excite both smell and taste receptors and produce flavor.
• The chemical senses evaluate potential foods.

Olfactory/Gustatory System

• The olfactory system consists of the nose and nasal cavities, supporting the olfactory mucous membrane.
• The gustatory system is partially responsible for the perception of taste.
• Taste perception is stimulated when a substance in the mouth reacts chemically with taste receptor cells located on taste buds in the oral cavity, mostly on the tongue
• Broad tuning refers to each gustatory receptor and neuron responding to a wide range of tastes.
• Narrow tuning suggests that each gustatory receptor responds to only one taste, or at least to very few of them.
• Anosmia is the inability to smell.
• Ageusia is the inability to taste.

Selective Attention

• Attention can be focused endogenously (internal cognitive processes) or exogenously (external events).
• The cocktail party phenomenon suggests the brain blocks stimuli except for a particular kind while unconsciously monitoring blocked-out stimuli.
• Simultanagnosia is a difficulty in attending visually to more than one object at a time.