• Sensation – the stimulus-detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain
  • Perception – active process of organizing the stimulus input and giving it meaning

Sensory Processes

  • Stimulus detection – absolute threshold designated as the lowest intensity at which a stimulus can be detected 50% of the time
  • Signal detection theory – concerned with the factors that influence sensory judgments
    • Decision criterion – standard of how certain a person must be that a stimulus is present before they will say they detect it
    • Increased rewards for noticing stimuli often results in lower detection thresholds
    • Increased danger/punishment for noticing stimuli often raises detection threshold
  • Difference threshold – smallest difference between two stimuli that can be perceived 50% of the time (just noticeable difference – jnd)
    • Weber’s Law – to perceive a difference between two stimuli, one must differ by a constant ratio
      • Value for weights = 1/50, therefore if 50 lbs. is lifted, increased weight will only be detected at 51 lbs.
      • Smaller fraction = higher sensitivity
      • Doesn’t apply to extremely high or low stimulation intensities
  • Sensory adaptation – the diminishing sensitivity to an unchanging stimulus
    • Perception of stimuli will decrease if constantly present

The Sensory Systems


  • The Human Eye
    • Light enters eye through cornea (transparent protective structure)
    • Pupil – adjustable opening that dilates or constricts to control amount of light entering
    • Iris – controls the pupil
    • Lens – elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects
      • Image flipped and reversed onto retina
      • Ability to see clearly depends on lens’ ability to focus image onto retina
        • Myopia (nearsightedness) – lens focuses image in front of retina
        • Hyperopia (farsightedness) – lens focuses image behind retina
    • Retina – multi-layered tissue at rear of eyeball
  • Photoreceptors: Rods and Cones
    • Retina covered in light-sensitive receptor cells
    • Rods – black and white receptors
      • Function best in dim light
    • Cones – color receptors
      • Function best in bright light
    • In humans, rods are everywhere except fovea (direct center of retina)
      • Cones decrease in concentration distant from the fovea
    • Rods and cones send message to brain via two additional layers of cells
      • Bipolar cells have synaptic connections with rods and cones
      • Bipolar cells synapse with ganglion cells, whose axons form into optic nerve
    • Cones in the fovea each have private line to a single bipolar cell (unlike others, which have many rods/cones for each bipolar cell)
      • Visual acuity (ability to see fine detail) increases with image directly on fovea
    • Blind spot exists at point where ganglion cells exit to form optic nerve
  • Transduction – process where characteristics of a stimulus are converted into nerve impulses
    • Rods and cones accomplish transduction through photopigments
    • Absorption of light be photopigments increases release of neurotransmitters
  • Brightness Vision and Dark Adaptation
    • Dark adaptation – the progressive improvement in brightness sensitivity that occurs over time in low illumination
      • Cones adapt completely in 10 minutes
      • Rods continue adapting for 30 minutes, allowing extreme sensitivity to light
  • Color vision
    • Trichromatic theory – three types of color receptors in retina (blue, green, red)
      • All colors produced by combination of wavelengths between these three colors
      • Flaws in theory:
        • Yellow produced by red and green, yet people with red-green color blindness can see yellow
        • Color afterimage (image in different color appears after stimulus shown for a while then withdrawn)
    • Opponent-process theory – three color receptors, each responding to two different wavelengths (red-green, blue-yellow, black-white)
      • Explains color afterimage issue
    • Dual processes in color transduction
      • Modern dual-process theory combines both theories to account for color transduction process
      • Cones contain one of three different photopigments that are sensitive to blue, green, and red
        • Different combinations of intensities will produce different colors
      • Opponent processes occur, but not in cones
        • Ganglion cells respond in opponent-process by altering firing rate
    • Color-deficient vision
      • Dichromat – color blind to only one system (red-green or yellow-blue)
      • Monochromat – completely colorblind (only sees black-white)
  • Analysis and Reconstruction of Visual Scenes
    • Feature detectors
      • Optic nerve sends nerve impulses to brain (thalamus, then primary visual cortex)
      • Groups of neurons in the cortex are organized to receive and integrate sensory nerve impulses from specific regions of retina
      • Feature detector cells fire selectively to stimuli that have specific characteristics
        • Certain cells fire when horizontal line present, others when other angles present
        • Parallel processing – different cells analyze stimuli and construct unified image of its properties
    • Visual association processes
      • Information analyzed and reconstructed in primary visual cortex is routed to other regions known as visual association cortex


  • Frequency – number of sound waves or cycles per second (Hz = one cycle per second)
  • Amplitude – vertical size of the sound waves (decibels – db)
  • Transduction system of ear is made up of bones, membranes, and tubes
    • Sound waves vibrate eardrum, which vibrates three bones (hammer, anvil, and stirrup)
      • Amplify sound waves more than thirty times
    • Cochlea – coiled, snail shaped tube that contain basilar membrane (sheet of tissue)
    • Organ of Corti rests on the basilar membrane
      • Has thousands of tiny hair cells that are actual sound receptors
      • Sound waves cause waves in liquid, which bend hairs, causing release of neurotransmitters
  • Sound localization – ability to notice location of sound due to timing difference in sound wave reception in ear
  • Two types of hearing loss:
    • Conduction deafness – problems involving the mechanical system that transmits sound waves to the cochlea
    • Nerve deafness – caused by damaged receptors within the inner ear, or damage to the auditory nerve

Taste and Smell: The Chemical Senses

  • Taste (gustation) and smell (olfaction) are sensitive to chemical molecules rather than energy
  • Taste buds – chemical receptors along edges and back surface of tongue
    • Consists of bitter, sour, salty, and sweet receptors
  • Receptors of  smell are long cells that project through the lining of the upper part of the nasal cavity and the mucous membrane
  • Pheromes – chemical signals found in natural body scents
    • Menstrual synchrony – tendency of women who live together to have similar menstrual cycles

The Skin and Body Senses

  • Receptors in skin and internal organs sense pressure, pain, warmth, and cold
    • Mixtures form other sensations, such as itch
  • Kinesthesis – provides us with feedback about our muscles’ and joints’ positions and movements
    • Cooperates with the vestibular sense (sense of body orientation or equilibrium)

Perception: The Creation of Experience

  • Two different types of processing functions:
    • Bottom up processing – takes individual elements of the stimulus and combines into a unified perception
    • Top down processing – sensory information is interpreted in the light of existing knowledge, concepts, ideas, and expectations
      • Accounts for psychological influences on perception

Role of Attention in Perception

  • Attention involves processes of (1) focusing on certain stimuli, and (2) filtering out other incoming information
    • Studied through technique called shadowing
      • Participants hear two messages simultaneously through earphones, and must repeat one of the messages word for word
      • Most can complete this, but cannot repeat second message
  • Attention strongly influenced by nature and personal factors
    • Internal factors (motives and interests) influence which stimuli are noticed

Perceptions Have Organization and Structure

  • Synthesia – stimuli in one sensory modality give rise to perceptions in other modalities
  • Gestalt theorists believe strongly in top-down processing
    • Wholes perceived are often more than the sum of their parts
    • Figure ground relations – perceptual organization in which a focal stimulus is perceived as a figure against a background of other stimuli
    • Gestalt laws of perceptual organization – four ways in which people group and interpret stimuli
      • Similarity – when parts of a perception are perceived as similar, they will be perceived as belonging together
      • Proximity – elements that are near one another are likely to be perceived as part of the same configuration
      • Closure – people tend to close the open edges of a figure or fill in gaps of an incomplete figure, so that their identification of the form is more complete
      • Continuity – people link individual elements together so that they form a continuous line or pattern that makes sense

Perception Involve Hypothesis Testing

  • Perceptual schema – a mental representation or image of a stimulus to compare it with
  • Perception is an attempt to make sense of stimulus input, finding the best interpretation of sensory information that can be arrived at based on our knowledge and experience

Perception is Influenced by Expectations

  • Perceptual set – a readiness to perceive a stimulus in a particular way based on expectations, motives, emotions, or beliefs

Stimuli are Recognizable Under Changing Conditions

  • Perceptual constancies – ability to recognize stimulus characteristics under varying conditions
    • Example – ability to recognize both an open door and closed door as still being a door
    • Shape constancy allows the recognition of people and objects from many different angles
    • Brightness constancy causes the relative brightness of objects remains the same under different conditions of illumination

Perception of Depth, Distance, and Movement

  • Brain translates information from retina (only in two dimensions – length and width) into three-dimensional perceptions using two cues:
    • Monocular depth cues – require only one eye
      • Use of light and shadow to create 3D image
      • Linear perception allows depth cues (two lines converging into the difference)
      • Interposition, height, clarity, and relative size also contribute
    • Binocular disparity – require both eyes
      • Perceptions from both eyes are combined into one image (example – 3D glasses)
      • Convergence – produced by feedback from muscles that turn eyes inward to view a near objects

Perception of Movement

  • Primary cue for perceiving motion is movement of stimulus across the retina
  • Relative movement of an object against a structured background is a movement cue
  • Stroboscopic movement – illusory movement produced when a light is briefly flashed in darkness, and then, a few milliseconds later, another is flashed nearby
    • Light appears to move, though it is simply quick flashing of light in a movement pattern


  • Compelling but incorrect perceptions that can be understood as erroneous perceptual hypotheses about the nature of the stimulus
  • Size constancy may be distorted to create an illusion of distance
author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0

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