Ch 9

• Sensation


  Sensory information is detected by a sensory receptor, which is a specialized neuron that responds to a specific stimulus
• Types of sensation


  • Vision
  • Audition
  • Gustation
  • Olfaction
  • Proprioception
  • Vestibular
  • Nociception
  • Thermoception
• Perception


  The way sensory information is organized, interpreted, and consciously experienced

  
• Illusions illustrate how our brains may form "incorrect" perceptions
• Illusions are the result of the brain's misinterpretation of a sensory stimulus
• Sensory receptors


  Transduce environmental energy into neural activity

  
• The only input our brain receives from the "real" world is a series of action potentials
• 3 key variables of sensory receptors

  • Receptive fields
  • Receptor density
  • Receptor sensitivity
• Sensory coding and representation

  Neuron discharge rate can encode stimulus intensity or sometimes qualitative changes (redder vs. greener)

  
• Each sensation is processed in its own region of neocortex, represented as a topographic map: a spatially organized neural representation of the body or sensory world

  
• Topographic map distortion - fingertips have 100X more receptors/cm2 than back, so more CNS neurons must be devoted to receiving this sensory info, causing the cortical area representing this information to be huge compared to the back

  
• All sensory systems feed information into the cortex in orderly maps

  
• Sensory systems interact as information is relayed through various brain regions

  
• Perception


  Influenced by context, past experiences, emotional state
• Vision is our primary sensory experience

  
• Far more of the human brain is dedicated to vision than to any other sense
• Understanding the visual system's organization is key to understanding human brain function

  
• Visible light


  Light is the stimulus for vision; enters the eye directly or indirectly
• What is visible is constrained by the properties of our photoreceptors
• Retina


  Light-sensitive surface at the back of the eye consisting of neurons and photoreceptor cells
• Photoreceptors


  Specialized retinal neurons that transduce light into neural activity
• Types of photoreceptors


  • Rods
  • Cones
• Rods


  Specialized to function in low light levels, sensitive to luminance
• Cones


  Specialized for colour and high visual acuity, sensitive to particular wavelengths of light
• Differences between rods and cones

  • Cylindrical outer segment, longer, more numerous, sensitive to low levels of brightness, functional mainly for night vision, all have same light-absorbing pigment
  • Tapered outer segment, mediate colour vision and acuity, only responsive to bright light, fovea has cones only, each contains 1 of 3 pigments
• Visual illuminance


  In low light we rely on our rods, images appear blurrier, ability to see in dim light drops by 50% between ages 20-40
• Three types of cone pigments

  Absorb light over a range of frequencies, approximately equal numbers of red and green cones but fewer blue cones, our colour perception corresponds to the summed activity of the 3 cone types
• Types of retinal neurons

  • Retinal ganglion cells (RGCs)
• Two categories of retinal ganglion cells (RGCs)

  • Magnocellular cell (M-cell)
  • Parvocellular cell (P-cell)
• Magnocellular cell (M-cell)


  Receives input primarily from rods, sensitive to light and moving stimuli
• Parvocellular cell (P-cell)


  Receives input primarily from cones, sensitive to colour and fine detail
• The lateral geniculate nucleus (LGN) of the thalamus receives afferent information from the optic nerve

  
• There are two routes for visual information: the dorsal "where/how" stream and the ventral "what" stream
• The primary visual cortex (V1) maintains the segregation of visual input from the two eyes
• Damage to the ventral "what" stream can cause visual form agnosia, an inability to consciously recognize objects
• Damage to the dorsal "where/how" stream can cause optic ataxia, a deficit in the visual control of reaching and other movements
• Damage to the visual processing streams demonstrates that we are consciously aware of only a small part of the sensory processing that goes on in the brain

  
• 3 key variables


  • RHEO
  • Sensory receptors
  • Receptive fields
• Receptive fields


  Region of sensory space in which a stimulus modifies a receptor's activity
• Receptive fields


  • Aids in localization of sensations in space
  • Cortical patterns/maps forming each person's sensory reality