Topic 3-perception

• Perception: the processes by which we recognize, organize, and make sense of the sensations we receive from environmental stimuli
• The inverse problem: create a representation (perception) of what is out in the world from what we sense
• Distal stimulus: an object or process out in the world

  Proximal stimulus: the energy or matter that impinges on the sensory receptors
• Sensory receptors: specialized cells to transduce external phenomena into neural signals
• Blindspot: A small, round, dark spot on the retina that is not visible to the eye where the optical nerve exits the eye
• Retina: The light-sensitive layer at the back of the eye. 3 layers of cells:

  1. Photoreceptors (rods and cones)
  2. Bipolar cells
  3. Ganglion cells
• Cones: Sensitive to colour, located in the retina, and are found in the fovea

  • high density at the fovea
  • short, medium, long cones sensitive to different wavelengths
  • used during the day
• Rods: Sensory cells that detect light and dark.

  • used during night
  • less rods at fovea, results in poor colour vision in the dark
• Primary visual pathway:

  1. both visual fields on both retinas (nasal and temporal)
  2. partial crossover at optic chiasm
  3. lateral geniculate nucleus
  4. primary visual cortex (V1)
• Ear:

  • sound = changes in air pressure
  • ear drum converts changes in air pressure into mechanical vibrations
  • vibrations travel through bones of the middle ear to oval window of cochlea
  • hair cells in cochlea detect vibrations
• Bones of the middle ear: Malleus, incus, stapes
• 3 compartments of the cochlea: scala tympani, scala vestbuli, organ of corti
• Basilar membrane: The membrane that vibrates when sound waves hit the eardrum. Location of maximal excitation along the basilar membrane depends on sound frequency
• Hair cells:

  • cilia have ion channels that are mechanically gated and respond to vibration
• Primary auditory pathway:

  • Cochlear nuclei (medulla)
  • Superior olivary nucleus (pons)
  • Nucleus of lateral lemniscus (pons)
  • Inferior colliculus (midbrain)
  • Medial geniculate nucleus (thalamus)
  • Primary auditory cortex
• Types of somatosensation:

  1. Mechanoreception (pressure, vibration)
  2. Thermoception (hot and cold)
  3. Nocioception (pain)
  4. Proprioception (mechanical force)
• Mechanoreceptors for touch:

  1. Messiner corpuscle (RA1)
  2. Merkel cells (SA1)
  3. Pacinian corpuscle (RA2)
  4. Ruffini endings (SA2)
• Primary somatosensory pathway:

  1. dorsal root ganglion
  2. cuneate nuclei (medulla)
  3. ventral posterior nuclei
  4. primary somatosensory cortex
• Sensory adaptation: the proximal stimulus is represented on a relative scale, not an absolute scale
• Visual adaptation: The process by which the visual system adjusts to a change in the lighting conditions.
• Weber's law: the just noticeable difference is the smallest detectable change in a stimulus
• Somatosensory adaptation:

  1. RA: rapidly adapting
  2. SA: slowly adapting
  3. 1: superficial
  4. 2: deep
• Receptive fields: area of sensory surface to which a neuron responds

  • perceptual resolution and acuity are inversely related to sensory receptive field size
• Receptive field of a hair cell: frequency of sound
• Receptive field of a mechanoreceptor: area on skin
• Lateral inhibition: a neuron's response to a stimulus is inhibited by the excitation of a neighbouring neuron
• Topography: spatial organization of sensory surface is generally preserved in (projected onto) primary sensory cortex
• Cortical magnification: area of the cortex is proportional to density of sensory receptors (and inversely related to receptive field size)
• Receptive fields of retinal ganglion cells: combines information from an aggregate of photoreceptors
• Center-surround structure of ganglion cell receptive field:

  • location on ganglion cell determines rate of firing
  • On area: more firing
  • Off area: less firing (but still fires)
• Plasticity: the ability of the brain to change and adapt to new experiences and learning.

  • Synaptic plasticity: changes in the strength of synapses
  • Cortical reorganization: changes in topographic maps
• Hierarchal organization: moving from lower order sensory neurons to higher order sensory neurons, receptive fields gets larger, features get more complex
• Somatosensory homunculus: A region of the brain that is responsible for processing sensory information from the body.
• Hierarchy in visual system:

  1. Primary visial cortex (striate cortex)
  2. Secondary visual cortex (V2,V3,V4,V5)
  3. Tertiary visual cortex
  4. Multidimensional association vortex
• Cortical columns: A group of neurons in the cerebral cortex that are responsible for processing sensory information.

  • organization of orientation feature detectors in V1
  • organized by eye and by orientation
• Interaural time delay: The time difference between the arrival of the sound waves at each ear.
• Somatosensory motion detectors in S1:

  1. Motion sensitive neurons
  2. respond to any motion in receptive field
  3. Orientation sensitive neurons
  4. respond to motion along a particular axis
  5. Direction sensitive neurons
  6. respond to motion in a particular direction
• Where stream:

  • Dorsal pathway: occipital lobe into parietal lobe
  • emphasis on location and motion
  • processing for action
• What stream:

  • Ventral pathway: occipital lobe into temporal lobe
  • emphasis on shape and colour
  • processing for object recognition
• Fusiform face area: A region of the brain that is involved in recognizing faces.