Low-level Vision - Sensation and Perception

AFTER THE COGNITIVE REVOLUTION - opening the black box
1. BEHAVIOURIST MODEL - environment leads to sensory perceptions, memories, beliefs etc AND behaviour at the SAME TIME
2. COGNITIVE MODEL - enviornment leads to sensory perceptions, memories, beliefs etc THEN behaviour
3. NEISSER - CONSTRUCTIVIST APPROACH. cognition is orderly stages of mental events that ACTIVELY construct retinal input. PERCEPTION DEPENDS ON COMPLEX MENTAL PROCESSES. e.g. there would be no such thing if GIBSON'S THEORY OF DIRECT PERCEPTION (light hitting the eye). Neisser stated that if our perception was realy just a reflection of the world then impossible figures would never be percieved
HISTORICAL ROOTS OF VISUAL PROCESSING
1. EARLY THEORIES OF VISUAL PERCEPTION
  1. STRUCTURALISM (Wundt) early 1900s
    1. perception arises from combining sensory atoms that are coded on the retina. direct correspondance between retina and conscious awareness
  2. The rise of behaviourism in the 1900s
    1. change to the idea that behaviour is driven by environmental cues rather than internal processes. external not internal
2. 1966 - GIBSON'S THEORY OF DIRECT PERCEPTION. visual behaviour based on detecting STABLE aspects of the environment
  1. Stable aspects of the environment are known as PERCEPTUAL IN VARIANTS. such as OPTIC FLOW (speed and direction) and TEXTURE (depth).
    1. GIBSON says that visual experience is WHOLISTIC rather than the structuralist view of breaking down the retinal image into component parts.
EARLY VISUAL PROCESSING - RECONSTRUCTING the visual world begins AT THE RETINA
1. ELECTROMAGNETIC SPECTRUM - radio waves and x-rays. NEWTON (1672) - LIGHT CAN BE SPLIT INTO MANY COLOURS
  1. colours are captured by three kinds of cone, red, green, blue
    1. CONES and RODS respond to light from different parts of the visual field
2. Birds of prey have the sharpest VISUAL ACUITY due to large eyes to maximise light on the retina, image spread over a large number of cells, they have a high density of retinal receptors
3. RETINA made up of DIFFERENT KINDS OF CELL, and different kinds of PHOTORECEPTORS, RODS AND CONES.
  1. EACH PHOTORECEPTOR HAS UNIQUE RECEPTIVE FIELD
  2. RODS are more numerous (120 million) and are more sensitive than cones, however not to colour
  3. PROCEDURE OF RECEPTIVE FIELDS - light particles enter the eye and travel towards the retina, light is absorbed by retinal photoreceptors, cause an action potential, the action potential transmitted to ganglion cells, optic nerve and brain.
  4. EACH PHOTORECEPTOR IS SENSITIVE TO A VERY SMALL AREA OF THE VISUAL FIELD - RECEPTIVE FIELD
    1. TOPOGRAPHICAL organisation, photoreceptors are sensitive to adjacent areas of the visual field
      1. every area of the visual field is coded by the retina and primary visual cortex
4. CELL TYPES IN THE RETINA. magno and parvo remain segregated in the brain
  1. MAGNOCELLULAR = sensitive to MOTION. magno = faster responses, larger receptive fields, wavelength insensitive
    1. the idea that different cells respond to different things shows that the visual system understands things though component parts (CONSTRUCTIVIST VIEW)
  2. PARVOCELLULAR CELLS = sensitive to changes in LIGHT INTENSITY. colour specific, slow response, colour / shape.
5. EARLY VISUAL PROCESSING looks at BASIC FEATURES (colour, edges, motion) but don't see an object.GESTALT PSYCHOLOGISTS HOWEVER suggest that we use a set of rules to decide if features belong to the same object, such as SIMILARITY, PROXIMITY, CLOSURE CONNECTEDNESS etc... (WERTHHEIMER)
  1. HOWEVER.. such groupings are ruined by camouflage
  2. GESTALTISM - sum more than parts, grouping gives meaning to visual elements
  3. BECK (1982) - TEXTURE defined by colour, orientation, motion and depth. texture shows edges, shape, distance
AUDITION - signals begin in eardums and then signals are processed in the cochlea. sound localisation is processed by interaural time and interaural sound intensity which are coded seperately in the brain
OVERVIEW
1. PHOTORECEPTORS (rods and cones) in the retina translate light into neural signals, rods are sensitive under low levels of illumination BUT are not colour sensitive.
2. the fovea at the center of the retina is densely packed with cones, the outside of the retina contains mostly rods.
3. the visual cortex is made up of distinct regions that carry out specialised processing functions, such as V4 colour and V5 motion.
  1. ACHROMATOPSIA - inability to percieve colour
  2. AKINETOPSIA - inability to process motion
4. visual cortex lesions impair visual acuity
5. MULTIMODAL PERCEPTION - proces information from more than one sensory modality, increases sensitivity and accuracy of perception
6. SYNESTHESIA - people experience a mixing of the senses such as coloured hearing or coloured taste
7. PERCEPTUAL REORGANISATION - when senses are deprived the function of sensory regions of the cortex may become reorganised (plasticity)

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Low-level Vision - Sensation and Perception

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