Visual Perception

• Retina


  Light-sensitive surface on ~75% of inner eye
• Sclera


  Outer layer with tough fibrous coat protecting jelly inside
• Fovea


  Opposite lens, controlling central vision with the highest acuity
• Macular


  Area around fovea, degeneration leads to loss of central vision
• Visible light


  400-700nm, with short to long wavelengths and high to low electro magnetic energy
• 1st transformation results in the inverted object representations on the retina
• Accommodation


  Focusing on object, bringing object representations onto the fovea. Near targets tighten muscles to curve the lens and cause much light bending. Far targets cause muscles to relax, leading to a slim lens and little light bending (ciliary muscles, lens, fovea (macula))
• Near point


  Accommodation limit
• Myopia


  Nearsightedness, requires concave glasses
• Hyperopia


  Farsightedness, requires convex glasses
• 2nd transformation


  Retinal image is transformed into electrical signals (transduction), which is achieved by photoreceptors (rods and cones)
• Rods


  • Absolute number of 120m per eye, distributed in retinal periphery, dark adaptation is initially slow but continues to full adaptation, absolute sensitivity (light detection) is high, acuity is low, neural convergence is more, spectral sensitivity is short wavelengths (Purkinje shift - increased sensitivity to short wavelengths in dark adapted eye), no colour vision, and is scotopic vision
• Cones


  • Absolute number of 6m per eye, distributed across fovea and periphery, dark adaptation is initially rapid then plateaus, absolute sensitivity is low, acuity is high, neural convergence is less, spectral sensivity in long wavelengths, colour vision with S, M, and L cones, and is photopic vision
• Electrical signals travel through layers of the retina

  Until it reaches ganglion cells
• Retinal layers


  • Photo receptors
  • Horizontal connectors
  • Bipolar cells
  • Horizontal connectors
  • Ganglion cells
• Light travels through the eye ball and then through layers 5 to 1
• Electricity travels from layer 1 to 5
• Centre-surround/on-off receptive fields


  • On - excitatory area, increasing firing
  • Off - inhibitory area, decreased firing
• Types of ganglion cells

  • M cells - magnocellular, parasol, input from rods, not colour specific
  • P cells - parvocellular, midget, input from L and M cones, colour specific (green/red on/off)
  • K cells - koniocellular, input from 5 cones, colour specific (blue on)
• Electric signal leaves the eye

  Via the optic nerve (axons of the ganglion cells) resulting in a blind spot (optic nerve and optic disc)
• Optic nerve reaches optic chiasm

  Nasal axons cross the side and combine with temporal axons from retrospective other eye to optic tract, resulting in complete contralateral representation of the visual field after the optic chiasm
• Optic tract reaches the lateral geniulate nucleus


  On either side of the thalamus. There are 6 layers, the top 4 layers contain axons from P cells, the bottom 2 layers contain axons from M cells, and axons from K cells are interlayers. The receptive fields are centre surround
• Optic radiations reach V1 (primary visual cortex/striate cortex)


  Simple cortical cells with elongated receptive fields. Orientation-selective neurons, motion-direction neurons, colour-selective neurons (opponent neurons with single-opponent or double-opponent receptive fields), disparity-selective neurons
• General rule - the less a stimulus matches the preferred feature of the neuron, the less the neuron fires
• V1 projects into V2


  Neurons in V2 like in V1, but increased in complexity, including end-stopped cells
• Dorsal Stream


  Identifies object location and projects into parietal lobe → V3 → V5 = MT (middle temporal area) - Motion-direction-selective neurons (real motion neurons) and Depth-selective neurons (disparity neurons)
• Sensation


  The uninterupted sensory impressions created by the detection of a stimulus
• Perception


  The psychological and cognitive processes of making sense of the sensations
• Bottom-up theory


  • Assumes that perception = sensation, that perception is automatic and is determined by the retinal image, that no cognition occurs, perception is directly measurable, and it is effortless, fast, and accurate
• The main critique of bottom-up theories is that perception is not always accurate, the retinal image is not sufficient for perception, and this can be shown through illusions and ambigious figures
• Bottom-up - Structuralism


  • Wundt (1879) and Titchener (1901) - describing elements of consciousness, that conscious experience can be broken down into basic elements which can be combined to describe all human experiences. Studied through introspection which is a method to reduce perception into its basic element
• Bottom-up - Psychophysics


  • Measuring elements of perception through threshold measurements. Fechner (1890) Measurement of the absolute threshold, which is the just noticeable intensity threshold to detect a stimulus. Method of adjustment, method of limits, method of constant stimuli
• Bottom-up - psychophysics


  • Measurement of the difference threshold (Weber, 1834) - the just noticeable intensity difference to tell two stimuli apart. Weber's law - the difference threshold is a constant proportion of the respective stimulus. Magnitude estimation/scaling (Stevens, 1957) - relation between the objective intensity and subjective perceived intensity
• Bottom-up - direct perception/Ecological theory

  • Gibson (1966) - the environment delivers everything necessary for perception. Perception should be measured in a natural environment (ecological approach). Perception takes place in the optic array (light in the environment) and is directly based on the invariant information (optic flow pattern, texture gradient, horizon ratio, gravity, straight lines, reflectance of areas) in the visual field which is extracted by the observer's movement (making optic array ambient revealing invariant information
• Top-down theories

  • Assume that perception > sensation, as the retinal image needs to be interpreted to create perception. They assume that perception is not automatic, that cognitive processing is involved, perception is not effortless and can be erroneous or equivocal due to illusions etc
• Top-down - gestalt psychology

  • The whole is more than the sum of its parts, perceptional organisation is needed to achieve Gestalt/the whole. Grouping and segmentation occurs through its principles (proximity, similarity, common fate, good continuation, closure, relative size, surroundedness, horizontal/vertical orientation, asymmetry). Law of Pragnanz determines which principle is applied
• Top-down - Constructed perception

  • Gregory (1970) - active and interpreted perception, meaning that information from the retina is insufficient, and perception is a matter of interpretation. Indirect perception is the interaction between sensation and cognition. Perception > sensation. Perception is hypothesis-guided on the basis of stored knowledge
• Visible light spectrum


  400-700nm. Short and Medium wave lengths lead to blue, Medium wave lengths are green, medium and long wave lengths are yellow, long wave lengths are red, and long, medium, and short wavelengths are white
• Chromatic colours


  Result of selective reflection where some wavelengths are reflected and some are absorbed
• Achromatic colours


  Result of equal reflection where all lightwaves are reflected to the same degree