module 3

• Rene Descartes

  Supported the concept of dualism, thought the brain communicated with the mind by transmitting signals out into the ether via the pineal gland
• Sensation and perception

  Thought to be products of the mind, considered irrevocably mysterious, and beyond measure
• Sensation


  An ability to detect sensory input
• Perception


  Your subjective experiences of sensory input
• Wundt and von Helmholtz were aware that our brains/minds generate experiences that are additional to the retinal images that reach our eyes</b>

  
• Our brains have no direct access to information concerning the external visual environment
• Visual perception

  1. Detect electromagnetic radiation

  2. Convert it into neural events

  3. Brain infers what these neural events are signalling regarding our surrounds

  
• Transduction

  The conversion of electromagnetic radiation into neural events, involves light sensitive chemicals - visual pigments
• Visual pigments

  • Contained in outer segments of photoreceptors that form the retina
  • Absorb photons of light, beginning a process that changes photoreceptor membrane conductance, causing depolarisation and action potentials

  
• Visible spectrum

  The range of wavelengths that can be absorbed, the 'light' that you can see

  
• Transduction

  1. Action potentials propagate to retinal Ganglion cells, via Horizontal, Bipolar & Amacrine cells, forward through retina

  2. Output from retinal ganglion cells combine to form the optic nerve - that carries signals out of your retinae to your brain

  
• Optic disc

  The point at which the optic nerve leaves your eye, where there can be no photoreceptors - so you are blind to images that project to that position on your retinae

  
• Physiological blindspot

  The area of the retina where the optic nerve leaves the eye, where you are blind to images that project to that position

  
• The human visual system seems to assume that the same things that surround the blindspot are also within the blindspot - a process called perceptual filling in

  
• Neon colour spreading

  • A blue tinged square, surrounded by white, which is illusory
• Neon colour spreading and perceptual filling-in across the blind spot are similar - your brain is constructing an experience based on inference about what is probably there
• Coloured aftereffects

  Situations where your brain causes you to see things that are not present on the retinae

  
• Opponent process theory

  After protracted viewing of certain colours, you can see oppositely coloured afterimages, indicating the human visual system contains competitive mechanisms tuned to opposite colours
• The fact that you can see colours when none are physically present shows that you are seeing activity in your brain, rather than retinal images, or a veridical impression of the external world

  
• Your brain can easily mistake opposite colour aftereffect signals for real colours in images
• Coloured aftereffects helped reveal what wavelengths of light cones are maximally sensitive to many decades before we had the requisite technology to study photoreceptors
• Sensory adaptation

  Can impact perception of complex visual features - like the gender of human faces
• The gender face aftereffect shows that even our experiences of well known complex forms are subject to change - shaped by visual adaptation
• Perception is a dynamic construction of your brain/mind
• Colour constancy

  Colour perception is not simply determined by the physical wavelengths refracted from a given surface, the human brain facilitates a sense of colour constancy by estimating what the prevailing light source is, and subtracting its influence from your impression of object colours

  
• A range of wavelengths are reflected from any given surface, and this range is shifted by the light source

  
• The brain's tendency to estimate the prevailing light source can be used to reverse apparent colour and contrast polarity (brightness)

  
• We can see colours other species can't, and some humans can't see colours that other humans can see
• Normal human daytime vision

  Humans use 3 types of 'photoreceptor' which catch photons carried by different wavelengths of light, and turns these into signals that are sent to your brain

  
• Colour vision deficiencies

  • Some humans only have 2 types of photoreceptor (missing L-cones, M-cones or S-cones)
  • A very small number of people (girls) have an extra class of day time photoreceptor

  
• Some people may experience green colours as brighter than red, and others the reverse, but we all learn colours by association, so normally we could never tell if this were true
• All your experiences are the product of brain activity, visual experiences can be a product of inferential processes that are additional to retinal images, and some of us have different sensory experiences
• Functional architecture of human vision

  • Specialization - cells respond selectively to specific stimulus characteristics
  • Modularity - cells that respond to similar stimulus characteristics are clustered into specific brain regions
  • A hierarchy of processing structures - cells respond to progressively complex stimulus characteristics as we follow the anatomical connections from the retina through to higher level visual brain regions

  
• Processing of visual input

  1. Begins in the retinae

  2. Signals from photoreceptors propagate to retinal Ganglion cells via Bipolar and Amacrine cells

  3. Each ganglion cell receives input from many photoreceptors

  
• Receptive field

  The region of retina to which a visual cell is responsive

  
• Retinal ganglion cells

  • Have circular receptive fields, can respond positively to input from a central region and negatively to input in a surrounding region (On Centre / Off Surround)
  • Or respond negatively to input from a central region and positively to input in a surrounding region (Off Centre / On Surround)
  • Receptive field sizes scale with distance from fixation
• Because of the scaling of receptive field sizes, visual acuity is much higher for input that projects onto your fovea - in the centre of your retinae

  
• The action of retinal ganglion cells, and changes in receptive field size, can explain the Hermann Grid Illusion
• In retinae, visual cells have a simple response selectivity - they respond to circular blobs of contrast

  
• How visual signals get to your brain

  1. Visual signals exit your eye via the optic nerve

  2. Signals from the Temporal sides of your retinae project to the LEFT side of your brain, and signals from the Nasal sides of your retinae project to the RIGHT side of your brain

  3. Visual signals propagate from your Optic chiasma to the Lateral geniculate nucleus (LGN), and then to the Primary Visual Cortex, at the back of your brain