Sensing the environment

Sensory Processing

  • Sensation
    • Threshold: the smallest signal that can be detected.
    • Weber's Law: The threshold for detecting a stimulus-change is a constant ratio. Eg. You are holding a dumbbell, and someone puts a stone on that dumbbell, can you detect it with your eyes closed? For humans, the stone must be at least 2% of the dumbbell for you to detect that change.
    • Signal detection theory: used to predict when and how a signal will be recognized amidst other sensory information. There are four possible outcomes:
      1. Hit - signal present and detected
      2. Miss - signal present, but not detected
      3. False Positive - signal absent, but detected (eg. hallucinations)
      4. Accurate Rejection - signal absent and not detected
    • Sensory adaptation: Our sensory neurons adapt to (become less sensitive/ignores) stimuli that are constantly there. For example, our inner ear uses this process to make recurring loud noises sound softer.
    • Psychophysics: the psychological study of the relationship between a physical stimuli and mental response. For example, a series of different pitches played for a subject to test which he/she can detect (humans can detect pitches ranging from 20 to 20,000 Hz).
  • Sensory receptors
    • Sensory pathways: the pathways followed by a nerve impulse from a sensory organ (eg. eye, tongue, ear, skin) to the brain or spinal cord.
    • Types of sensory receptor
      • By stimulus:
        • Thermoreceptor- detects temperature (heat)
        • Chemoreceptor- detects chemicals
        • Photoreceptor / Electromagnetic - detects light/electromagnetic waves (ie. rods and cones in the retina)
        • Mechanoreceptor- detects touch and sound (mechanical stimuli such as vibrations, as well as pressure applied to skin)
        • Baroreceptor- detects pressure (ie. blood pressure within vessels)
        • Nocireceptor- detects pain (specialized chemoreceptor)
      • By location:
        • Exteroceptor- located near/at the surface of the skin, detects stimuli on the surface of the body.
        • Interoreceptor- (aka Viceroreceptors) located near visceral organs/blood vessels and is linked with the autonomic nervous system. For example, we know we are hungry when nerve endings in our digestive mucous membranes detect an increase of fatty acids in our stomach.
        • Proprioceptor- located near tendons, ligaments, joints, and skeletal muscles. A specialized mechanoreceptor, detects position of the body.


  • Structure and function of the eye
    • eye structure
      • Light first travels through the cornea
      • Then through the pupil (hole in the iris muscle)
      • Lens = focuses light on retina.
      • Ciliary muscles: changes the shape of the lens to focus light
      • Vitreous humor = fluid.
      • Retina = screen on the back of the eye = contains photoreceptors.
      • Photoreceptors
        • Photoreceptor cells located on the back of the retina.
        • Rods = senses light and dark (no color), more sensitive.
        • Cones = senses color, less sensitive.
        • Rhodopsin = chemical responsible for light reception = Retinal (chemical) + Opsin (transmembrane protein)
        • Light converts cis-retinal → trans-retinal.
        • trans-retinal then causes hyperpolarization of photoreceptor cell, which prompts the chain of events that sends signal to the brain.
        • Sends signal to brain via a bundle of nerves on the back of the retina (where the blind spot is)
    • Optic nerve: transmits signal from the photoreceptors to the brain
    • function: take in light, focus it, form an image on the retina, which is trasmitted to the brain via the optic nerve
  • Visual processing
    • Visual pathways in the brain
      • Nerve impulses travel from each eye along the optic nerves and meet at the optic chiasm
      • Here, half of the nerves from each side cross and resume to the back of the brain
      • The left side of the brain receives half of the left optic nerve and half of the right optic nerve
      • The same goes for the right side
      • The binocular field of vision is where left and right fields of vision overlap.
    • Parallel processing: parallel processing is what allows our brains to quickly process visual information such as color, depth, motion, and size, simultaneously, instead of one by one.
    • Feature detection: the Feature Detection Theory describes why a particular part of our brain is triggered when we look at something (ie. looking at animals trigger one part of the brain, and looking at words trigger a different part.)


  • Structure and function of the ear
    • ear structure
      • Ear canal = auditory canal.
      • Tympanic membrane = eardrum.
      • Ear bones = malleus (hammer) → incus (anvil) → stapes (stirrup).
      • Vestibule = contacts the oval window (where stirrup vibrates), is continuous with semicircular canals and cochlea.
      • Cochlea = spiral = houses hair cells.
      • Semicircular canals = 3 of them perpendicular to one another = senses position and movement of the head, help you balance.
    • mechanism of hearing
      1. Sound enters ear.
      2. Hits ear drum (tympanic membrane)
      3. Malleus (hammer) → Incus (anvil) → Stapes (stirrup)
      4. Vibrates fluid in Cochlea.
      5. Transmits to fluid in Cochlea.
      6. Cochlear hair cells excited by vibrations, and sends signal to brain.
  • Auditory processing (e.g., auditory pathways in the brain)
    • Sound → ear drum → vibrations in cochlea → vestibulocochlear nerve (cranial nerve VIII) → brain
    • The auditory nerve then sends the message to an area of the brain called the brainstem
    • At the brainstem details of the signal such as frequency, location, duration, and intensity are deciphered.
    • Next, the signal passes through the thalamus and into the auditory cortex, a specific region found in the temporal lobe of the brain.
    • Here the signal is translated by neurons into a message that we can understand
  • Sensory reception by hair cells
    • The vibrating hair cells within the basilar membrane bend when they come in contact with the tectorial membrane.
    • This movement opens up ion channels within the hair cells themselves that release neurotransmitters.
    • These neurotransmitters stimulate dendrites from bipolar afferent neurons.

Other Senses

  • Somatosensation (e.g., pain perception)
    • pain, pressure, temperature, proprioception/position, and whether a muscle group of the body is being contracted.
    • perceived by skin, muscles, joints, and ligaments.
  • Taste (e.g., taste buds/chemoreceptors that detect specific chemicals)
    • Sweet (sugar, e.g. glucose): "T1R2" and "T1R3" receptors.
    • Salty (salt, e.g. NaCl): sodium channels
    • Bitter (basic, e.g. Quinine): T2R receptors
    • Sour (acidic, e.g. HCl): TRP (transient receptor potential) channel.
    • Umami (meaty, e.g. glutamate, amino acids and nucleotides): a combination of T1R1 and T1R3 receptors.
    • Taste bud
      • Spherical in shape, covered in taste hairs that are used to detect chemicals in our food, and are connected to taste receptor cells
      • A taste pore is located in the center of the taste bud
      • For food to be recognized by the taste bud, a chemical called a "tastant" must be present. This chemical is soluble to saliva, so it can be easily absorbed between and among the papillae (small fleshy projections covering the tongue) and into the taste pores.
      • Once taste hairs detect the tastant, they stimulate other taste receptor cells, which deliver the message to the gustatory area of the brain, where the signal can be translated into what we taste.
    • Pathway: tongue -> glossopharyngeal nerve (cranial nerve IX) -> brain
  • Smell
    • Olfactory cells/chemoreceptors that detect specific chemicals
      • Olfaction (smell) begins with olfactory cells found in the roof of the nasopharynx, (our nasal cavity)
      • The olfactory receptors within the nasopharynx detect chemicals in the air that dissolve the mucous-covered nasal membrane.
      • The nasal membrane contains olfactory nerves, which extend into the olfactory bulbs/nerves (cranial nerve I).
    • Pheromones: chemicals that behave as signals to the brain that cause a response common to members of the same species. They serve as important chemical messengers, and help animals to prepare for events such as danger, a discovered food source, and when it is time to mate.
    • Olfactory pathways in the brain
  • Kinesthetic sense: also known as "proprioception" senses position/movement of ligaments, muscles, and tendons.
  • Vestibular sense: controlled by hair cells in the inner ear. Responsible for our sense of balance (how we perceive gravity) and movement.


  • Bottom-up/Top-down processing
    • Bottom-up processing is a type of processing that begins with various sensory receptors, which pick up signals and send them to the brain for integration and processing. For example: prick your finger with a pin → sensory information sent to brain → feel pain.
    • Top-down processing begins in the brain, where information from previous experiences has already been brought up by one of the sensory systems. The brain draws on this information to interpret new sensory information. For example: you see pin → remember pain from past experience → decide not to touch pin.
  • Perceptual organization (e.g., depth, form, motion, constancy): the process by which our brains structure information into structural units that is easier to visually understand
    • Depth- we perceive how far away an object is, not just a flat 2-D image.
    • Form- our ability to recognize an object's shape/outline. For example: In the dark you are able to see a car, not because you can see it’s colors, but because you can recognize it’s shape by piecing together outlines.
    • Motion- motion perception is our ability to recognize that an object is moving.
    • Constancy- the brain tends to maintain constancy of certain attributes of objects, such as color, size, and shape, even though some of these traits may have changed. For example, as you watch a car approach you from far away it may seem small at first, and increase in size as it approaches you. Even though this is what your eyes are seeing, you know that the car has remained the same size.
  • Gestalt principles: The Gestalt (German word for "whole") principle states that the "whole" object surpasses the singular details that make up an image. Eg, when you look at a flower, you do not first notice the defining lines of the flower, then the pieces that make up the flower, and then the color of the flower, but instead, you see the whole flower as a single image.