EPPP

Sensation refers to the process by which sensory receptors detect physical energy from the environment and convert it into neural signals. This process, called transduction, occurs when specialized receptor cells transform stimuli into electrochemical signals that travel along sensory neurons to the brain.

The absolute threshold represents the minimum amount of stimulus energy required to detect a stimulus 50% of the time. The difference threshold, or just noticeable difference (JND), is the smallest detectable difference between two stimuli. Weber's Law states that the JND is proportional to the magnitude of the original stimulus. For example, detecting a weight difference requires a larger absolute change when lifting heavier objects.

Signal detection theory explains how we make decisions about the presence or absence of stimuli under conditions of uncertainty. This theory accounts for both sensory capabilities and decision-making factors including motivation, expectations, and consequences of hits, misses, false alarms, and correct rejections.

Light enters the eye through the cornea, passes through the pupil (regulated by the iris), and is focused by the lens onto the retina. The retina contains two types of photoreceptors: rods and cones. Rods, numbering approximately 120 million, are highly sensitive to light and enable scotopic (dim light) vision but do not process color. Cones, numbering approximately 6 million, require more light and are responsible for photopic vision, color perception, and visual acuity.

The fovea, located at the center of the retina, contains the highest concentration of cones and provides the sharpest visual acuity. The optic nerve carries visual information from the retina to the brain. The point where the optic nerve exits the eye creates the blind spot, which lacks photoreceptors.

Trichromatic theory, proposed by Young and Helmholtz, explains color vision through three types of cones sensitive to short (blue), medium (green), and long (red) wavelengths. Opponent-process theory, developed by Hering, proposes that color perception involves three opponent channels: red-green, blue-yellow, and black-white. Both theories are correct and operate at different levels of visual processing.

Visual information travels from the retina through the optic nerve to the lateral geniculate nucleus (LGN) of the thalamus, then to the primary visual cortex (V1) in the occipital lobe. Feature detectors in V1 respond to specific visual features such as edges, angles, and movement. The ventral stream (what pathway) processes object recognition, while the dorsal stream (where/how pathway) processes spatial location and movement.

Sound waves are characterized by frequency (perceived as pitch, measured in Hertz), amplitude (perceived as loudness, measured in decibels), and complexity (perceived as timbre). The human ear detects frequencies between approximately 20 Hz and 20,000 Hz.

The outer ear (pinna and auditory canal) collects and funnels sound waves to the tympanic membrane (eardrum). The middle ear contains three ossicles (malleus, incus, stapes) that amplify sound vibrations and transmit them to the oval window of the cochlea. The inner ear contains the cochlea, a fluid-filled spiral structure lined with the basilar membrane and hair cells that transduce mechanical vibrations into neural signals.

Place theory explains high-frequency sound perception through the location of maximal basilar membrane displacement. Frequency theory explains low-frequency perception through the firing rate of auditory neurons matching the sound wave frequency. Volley principle extends frequency theory by proposing that groups of neurons fire in alternating volleys to encode frequencies above individual neuron firing limits.

Auditory information travels via the auditory nerve to the cochlear nucleus, superior olivary complex, inferior colliculus, medial geniculate nucleus of the thalamus, and finally to the primary auditory cortex in the temporal lobe.

The somatosensory system processes touch, pressure, temperature, and pain through mechanoreceptors, thermoreceptors, and nociceptors distributed throughout the skin and body. Information travels through the spinal cord to the somatosensory cortex in the parietal lobe, organized somatotopically as the sensory homunculus.

Pain perception involves two pathways: fast pain (sharp, localized) transmitted by myelinated A-delta fibers, and slow pain (dull, diffuse) transmitted by unmyelinated C fibers. Gate control theory proposes that spinal cord mechanisms can modulate pain signals before they reach the brain. Endogenous opioids (endorphins, enkephalins) provide natural pain relief by binding to opioid receptors.

The chemical senses include gustation (taste) and olfaction (smell). Taste receptors on the tongue detect five basic tastes: sweet, sour, salty, bitter, and umami. Olfactory receptors in the nasal cavity can distinguish thousands of odors. Olfaction is unique in that signals travel directly to the olfactory bulb and then to the limbic system without thalamic relay, explaining the strong connection between smell and emotional memory.

The vestibular system, located in the inner ear, detects head position and movement through the semicircular canals (rotational movement) and otolith organs (linear acceleration and gravity). This system is essential for balance, spatial orientation, and coordinating eye movements with head movements.

Gestalt principles describe how the brain organizes sensory information into meaningful patterns. Figure-ground organization distinguishes objects (figures) from their backgrounds. Proximity groups nearby elements together. Similarity groups elements with shared features. Continuity perceives smooth, continuous patterns. Closure fills in gaps to perceive complete forms. Common fate groups elements moving in the same direction.

Depth perception relies on monocular cues (usable with one eye) and binocular cues (requiring both eyes). Monocular cues include relative size, interposition (overlap), linear perspective, texture gradient, motion parallax, and atmospheric perspective. Binocular cues include retinal disparity (different images on each retina) and convergence (inward eye movement for near objects).

Perceptual constancies allow us to perceive objects as stable despite changing sensory input. Size constancy maintains perceived object size despite distance changes. Shape constancy maintains perceived shape despite viewing angle changes. Color constancy maintains perceived color despite lighting changes. Lightness constancy maintains perceived brightness despite illumination changes.

Top-down processing uses knowledge, expectations, and context to interpret sensory information. Bottom-up processing builds perception from raw sensory data without prior expectations. Perception typically involves interaction between both processes.