Sensation and Perception: An Overview

Sensation and Perception

• Sensation refers to the acquisition of raw information by the sense organs
• It also refers to the subjective feeling produced by stimulation of a sensory nerve
• It can be objectively viewed as an inference regarding a person's inner state based on observable behavior
• Perception refers to the processing of sensory information from receptors
• Perception is the fusion of information from multiple sensory channels
• Perception involves the conscious mental registration of a sensory stimulus

Psychophysics, Sensory Organs and Receptive Fields

• Psychophysics quantifies the strength of a sensation
• A sensory ending is a peripheral branch of a sensory nerve
• A sensory organ is a sensory ending in contact or association with receptors
• A sensory unit is an entire sensory nerve, from its peripheral beginning to its central termination
• The receptive field is the spatial area where a stimulus of sufficient intensity and quality evokes a discharge in the afferent sensory nerve fiber

Receptor Classification and Sensory Modality

• Receptors are specialized non-neuronal cells capable of transduction
• Mechanoreceptors are sensitive to changes in pressure, like the Pacinian corpuscle
• Chemoreceptors are sensitive to changes in blood pH, blood gases, and chemicals
• Thermoreceptors are sensitive to changes in skin and core temperature
• Photoreceptors are sensitive to variations in the wavelength of light
• Nociceptors are sensitive to potentially harmful stimuli

Receptor Location and Receptor Activity

• Exteroceptors are located on the body's surface and mediate temperature, pain, touch, pressure, vision, and audition
• Interoceptors are located inside the body and include pressoreceptors, visceral receptors, blood vessel receptors, and CNS receptors
• Proprioceptors are involved in position sense, including muscle spindles and Golgi tendon organs
• Generator potential develops when a stimulus exceeds threshold value and matches the receptor type
• Transduction involves opening cation channels or cyclic nucleotide-gated channels, leading to depolarization
• Receptor potential is a graded potential that can undergo summation
• After summation, an action potential is produced and encoded
• The impulse is then conducted to the appropriate center for processing

Basic Principles of Sensation

• Peripheral branches of a sensory unit intertwine with adjacent units, creating partially shifted overlap on the sensory surface
• Different sensations arise from different sets of nerve fibers because of their central connections based on Mueller’s Doctrine
• Within a given set of nerve fibers, stimulation elicits an identical sensation
• Receptors have a low threshold for a specific energy form, conferring specificity based on the Law of Adequate/Appropriate Stimulus
• The adequate stimulus for vision depends on the wavelength of light entering the retina
• Specific receptors do not exist according to Pattern Theory
• Different sensations stem from the same receptors responding in various ways to different stimuli

Adaptation

• Receptors respond differently to continuing stimuli
• Rapidly-adapting (phasic, dynamic) receptors discharge a few impulses at stimulus onset and removal, exemplified by touch, olfactory receptors, and Pacinian corpuscles
• Slowly adapting (tonic, static) receptors respond with high-frequency discharge that increases with stimulus persistence; examples include muscle spindles, pressoreceptors, A and C fibers

Contrast and Spatial Processing

• Opposing receptors mediate contrast, or opposite sensations
• Simultaneous contrast: stimulation of a receptor type diminishes sensitivity in neighboring areas for the same stimulus and augments it for the opposite sensation
• Successive contrast after a stimulus ceases, the excitation threshold for that receptor type increases, while the opposite receptor type's threshold decreases
• After image: receptors continue firing long after stimulus removal, exhibited by tonic receptors, absent in phasic receptors
• Accuracy of localization relies on the sensory unit size since the smaller the unit, the more precise the localization
• Greater number of overlapping units yields more accurate localization due to diminished overlapping area size (lateral inhibition)
• The greater the number of activated units, the lesser the degree of localization

Weber-Fechner & Stevens’ Laws

• Stimulation of a sensory pathway at any point central to the sense organ results in sensation projected to the periphery based on Projection
• Weber-Fechner Law: the felt sensation = the logarithm of stimulus intensity, appropriate for a limited intensity range
• Stevens’ Power Law asserts that internal sensations increase as a power function
• Stevens' law equation: Mean magnitude estimate = aXb

Ways of Measuring sensation

• Qualities of sensation include
• Quality (modality): qualitative continuum.
• Intensity: affected by generator potential magnitude, lateral inhibition, and the number of activated receptors.
• Affect: the pleasantness or unpleasantness of a sensation.

Somesthesis Sensations

• Crude somatosensory mechanical sensations by unmyelinated fibers.
• Vibration is encoded by the Pacinian corpuscle.
• Pressure is encoded by the Ruffini ending.
• Touch is encoded by peritricial nerve endings, Meissner corpuscles, and Merkel discs.
• Aδ fibers for cold and C fibers for warm sensations

Touch Sensation and Perception

• Appropriate touch stimulus is uneven bending of hair or skin
• Touch sensitivity varies across the body being greater in exposed and mobile parts
• Localization fairly accurate, depending on skin region, those with low thresholds (greater sensitivity) being more accurate
• Two-point discrimination (smallest separation of two points producing two distinct touch impressions) also greater int he more mobile parts of the body

Thermoreceptors

• Cold receptors stimulated at 15°C, peak at 25°C, and end at 44°C
• Warm receptors stimulated at 32°C, peak at 45°C, and end at 47°C.
• Skin temperatures of 31°C–36°C create a neutral zone, or physiological zero, where skin feels neither warm nor cold
• As skin temperatures occur between 45°C and 49°C, they stimulate cold and pain fibers

Pain Sensation, Modulation and Pathways

• Defined as unpleasant sensory/emotional experience associated with tissue damage or described in terms of such damage
• Composed of sensation plus individual’s reaction consisting of emotional overtones, widespread somatic/autonomic activity, and volitional avoidance/escape efforts
• Factors influencing reaction include age, sex, race, nature/duration/intensity, personality and significance
• Segmental mechanisms of pain modulation consist of Gate Control and Windup phenomenon
• Suprasegmental mechanisms include release of natural pain modulators, which can include endorphins, enkephalins, dynorphins, and serotonin
• Pain receptors are naked terminals of Aδ and C fibers
• Descending inhibitory pathways: Periaqueductal grey, Raphe nucleus, Locus coeruleus

Olfactory Receptors & Location

• Located within the olfactory epithelium along the walls of the olfactory cleft
• The cells themselves are distributed among the epithelial cells
• A yellow pigment also saturates it
• Stimuli travel via eddy currents made possible by sniffing

Odorants & Classification

• Odorants, or odorivectors, evoke the olfactory sense and possess two physical properties
  • They are volatile; air-borne molecules are released and carried by sniffing to the olfactory mucosa
  • They are fat-soluble
• No chemical element is odorous in an atomic form
• The odors of organic compounds have been predicted sometimes from the structure of their molecules
  • Ethers are almost always fragrant
  • Esters almost always smell of fruit.
• Human olfactory sensations cannot be well-described in English, so have traditionally been designated as smelling "like something”
• Classified using the Zwaardemaker, Amoore, Henning and Crocker-Henderson designations

Gustatory System

• Taste receptors consist of modified epithelial cells located on the dorsal surface of the tongue
• Taste buds are absent in the center of the tongue
• One such small papilla contains numerous receptors numbering between 50 and 150
• Five primary taste qualities can be recognized: Salt, Sour, Sweet, Bitter and Umami
• Gustatory sense is acute, but insensitive to relative concentrations, instead easily experiencing fatigue
• Olfactory and cutaneous sensations are closely related to the taste system