Concept Summary

Sensation vs. Perception

• Sensation is the conversion, or transduction, of physical, electromagnetic, auditory, and other information from the internal and external environment into electrical signals in the nervous system.
• Perception is the processing of sensory information to make sense of its significance.

• Sensory receptors are nerves that respond to stimuli and trigger electrical signals.

  • Sensory neurons are associated with sensory ganglia: collections of cell bodies outside the central nervous system.
  • Sensory stimuli are transmitted to projection areas in the brain, which further analyze the sensory input.
  • Common sensory receptors include photoreceptors, hair cells, nociceptors, thermoreceptors, osmoreceptors, olfactory receptors, and taste receptors.

• A threshold is the minimum stimulus that causes a change in signal transduction.

  • The absolute threshold is the minimum of stimulus energy that is needed to activate a sensory system.
  • The threshold of conscious perception is the minimum of stimulus energy that will create a signal large enough in size and long enough in duration to be brought into awareness.
  • The difference threshold or just-noticeable difference (jnd) is the minimum difference in magnitude between two stimuli before one can perceive this difference.
  • Weber’s law states that the jnd for a stimulus is proportional to the magnitude of the stimulus, and that this proportion is constant over most of the range of possible stimuli.

• Signal detection theory refers to the effects of nonsensory factors, such as experiences, motives, and expectations, on perception of stimuli.

  • Signal detection experiments allow us to look at response bias. In a signal detection experiment, a stimulus may or may not be given, and the subject is asked to state whether or not the stimulus was given. There are four possible outcomes: hits, misses, false alarms, or correct negatives.
• Adaptation refers to a decrease in response to a stimulus over time.

Vision

• The eye is an organ specialized to detect light in the form of photons.

  • The cornea gathers and filters incoming light.
  • The iris divides the front of the eye into the anterior and posterior chambers. It contains two muscles, the dilator and constrictor pupillae, which open and close the pupil.
  • The lens refracts incoming light to focus it on the retina and is held in place by suspensory ligaments connected to the ciliary muscle.
  • The ciliary body produces aqueous humor, which drains through the canal of Schlemm.
  • The retina contains rods and cones. Rods detect light and dark; cones come in three forms (short-, medium-, and long-wavelength) to detect colors.
  • The retina contains mostly cones in the macula, which corresponds to the central visual field. The center of the macula is the fovea, which contains only cones.
  • Rods and cones synapse on bipolar cells, which synapse on ganglion cells. Integration of the signals from ganglion cells and edge-sharpening is performed by horizontal and amacrine cells.
  • The bulk of the eye is supported by the vitreous on the inside and the sclera and choroid on the outside.

• The visual pathway starts from the eye, and travels through the optic nerves, optic chiasm, optic tracts, lateral geniculate nucleus (LGN) of the thalamus, and visual radiations to get to the visual cortex.

  • The optic chiasm contains fibers crossing from the nasal side of the retina (temporal visual fields) of both eyes.
  • The visual radiations run through the temporal and parietal lobes.
  • The visual cortex is in the occipital lobe.

• Vision, like all senses, is processed through parallel processing: the ability to simultaneously analyze and combine information regarding color, shape, and motion.

  • Color is detected by cones.
  • Shape is detected by parvocellular cells, with high spatial resolution and low temporal resolution.
  • Motion is detected by magnocellular cells, with low spatial resolution and high temporal resolution.

Hearing and Vestibular Sense

• The ear is divided into the outer, middle, and inner ear.

  • The outer ear consists of the pinna (auricle), external auditory canal, and tympanic membrane.
  • The middle ear consists of the ossicles: malleus (hammer), incus (anvil), and stapes (stirrup). The footplate of the stapes rests on the oval window of the cochlea. The middle ear is connected to the nasal cavity by the Eustachian tube.
  • The inner ear contains the bony labyrinth, within which is the membranous labyrinth. The bony labyrinth is filled with perilymph; the membranous labyrinth is filled with endolymph. The membranous labyrinth consists of the cochlea, which detects sound; utricle and saccule, which detect linear acceleration; and semicircular canals, which detect rotational acceleration.
• The auditory pathway starts from the cochlea and travels through the vestibulocochlear nerve and medial geniculate nucleus (MGN) of the thalamus to get to the auditory cortex in the temporal lobe.
• Sound information also projects to the superior olive, which localizes the sound, and the inferior colliculus, which is involved in the startle reflex.

Other Senses

• Smell is the detection of volatile or aerosolized chemicals by the olfactory chemoreceptors (olfactory nerves) in the olfactory epithelium.

  • The olfactory pathway starts from the olfactory nerves and travels through the olfactory bulb and olfactory tract to get to higher-order brain areas, such as the limbic system.
  • Pheromones are chemicals given off by animals that have an effect on social, foraging, and sexual behavior in other members of that species.
• Taste is the detection of dissolved compounds by taste buds in papillae. It comes in five modalities: sweet, sour, salty, bitter, and umami (savory).

• Somatosensation refers to the four touch modalities: pressure, vibration, pain, and temperature.

  • A two-point threshold is the minimum distance necessary between two points of stimulation on the skin such that the points will be felt as two distinct stimuli.
  • Physiological zero is the normal temperature of the skin to which objects are compared to determine if they feel “warm” or “cold.”
  • Nociceptors are responsible for pain perception. The gate theory of pain states that pain sensation is reduced when other somatosensory signals are present.
• Kinesthetic sense (proprioception) refers to the ability to tell where one’s body is in three-dimensional space.

Object Recognition

• Bottom-up (data-driven) processing refers to recognition of objects by parallel processing and feature detection. It is slower, but less prone to mistakes.
• Top-down (conceptually driven) processing refers to recognition of an object by memories and expectations, with little attention to detail. It is faster, but more prone to mistakes.
• Perceptual organization refers to our synthesis of stimuli to make sense of the world, including integration of depth, form, motion, and constancy.

• Gestalt principles are ways that the brain can infer missing parts of a picture when a picture is incomplete.

  • The law of proximity says that elements close to one another tend to be perceived as a unit.
  • The law of similarity says that objects that are similar appear to be grouped together.
  • The law of good continuation says that elements that appear to follow the same pathway tend to be grouped together.
  • Subjective contours refer to the perception of nonexistent edges in figures, based on surrounding visual cues.
  • The law of closure says that when a space is enclosed by a group of lines, it is perceived as a complete or closed line.
  • The law of prägnanz says that perceptual organization will always be as regular, simple, and symmetric as possible.