VISUAL PERCEPTION AND PERCEPTUAL DISORDERS

VISUAL PERCEPTION AND PERCEPTUAL DISORDERS

1. VISUAL PROCESSING

   • PHOTORECEPTORS in the eye
     • RODS are responsible for vision at low light levels (SCOTOPIC VISION)
     • CONES are active at higher light levels (PHOTOPIC VISION)
       • Light first hits the photoreceptors, then the information travels back through the BIPOLAR and RETINAL GANGLION cells. The HORIZONTAL cells integrate the information and the AMACRINE cells forms a similar integration on the retinal ganglion cells
         • 130 million photoreceptors on the retina
           • The FOVEA is an indentation on the retina that is packed with cones. There is a one-to-one relationship with bipolar and ganglion cells to increase visual sensitivity and acuity
       • Cones contain one of three photopigments
         • ERYTHROLABE maximise sensitivity to RED light
         • CYANOLABLE maximise sensitivity to BLUE light
         • CHLORLABE maximise sensitivity to GREEN light
     • The OPTIC NERVE sends information to the OPTIC CHIASM where information is split according to visual field
       • The relay stations are the SUPER COLLICULUS and the THALAMUS including lateral geniculate nucleus
         • Processing occurs in the PRIMARY VISUAL CORTEX (occipital lobes)
           • Processing then occurs in the SECONDARY VISUAL CORTEX and VISUAL ASSOCIATION CORTEX (the temporal and parietal lobes)
   • Lightwaves and Photons (particles)
     • The human eye can see wavelengths from 380 - 760 nanometers - from violet to red
     • Some animals have a disproportionate number of rods which allows them to have extra sensitivity in the dark
     • Wavelength = Colour Intensity = Brightness
   • Light enters the eye through the PUPIL (a hole in the iris) and the CORNEA
     • The IRIS consitis of 2 bands of ciliary muscles, and contracts to allow so much light into the eye
       • Adjustment is a compromise between sensitivity and acuity (sharpness)
         • When sharpness is high, sensitivity is less important for greater sharpness
         • When sharpness is low, the pupil lets more light in
     • The image at the back of the eye is upside-down and back to front. The brain converts this image so we see it the right way
   • Transduction occurs when a receptor's photopigment is broken down by light of an appropriate wavelength
     • Photoreceptor proteins in the rods and cones absorb the photons and triggers change in the cell's membrane potential

2. PERCEPTUAL DISORDERS