Perceptual processes and development The visual system

The eye

Functions of the parts of the eye Pupil Iris Cornea Sclera Lens Aqueous humour retina Vitreous humour Choroid Fluid-filled chamber Optic nerve Fovea (yellow spot) Blind spot

The retina

The structure of the retina

The retina Covers most of the interior of the eye Light is changed in to a NEURAL response (TRANSDUCED).

The structure Ganglion cells Bipolar neurones Photoreceptor cells This is the back of the eye!

Photoreceptor cells Outline some of the key differences between rod and cone cells. Comparisons Rods Cones Number? Distribution? Function?

Rods and cones Questions: They release neurotransmitter molecules to the bipolar cells which in turn connect to the ganglion cells. Axons from the ganglion cells then pass from eye to brain. Questions: The retina is constructed in a ‘back-to-front’ arrangement. Why can the light reach the retina easily? Why is there a small area at the back of the retina which has no photoreceptors? Blind Spot demo!  Why are we not normally aware that we have a blind spot?

The visual pathways…

Visual Pathways… The nasal optic nerve fibres and the temporal optic nerve fibres take different routes… Nasal fibres cross over at the optic chiasm Temporal fibres continue to the same side of the brain. Lateral geniculate nucleus (LGN) – part of the thalamus. Optic radiations lead to the primary visual cortex Visual association cortex (high level processing) 2 pathways: Parietal lobe (visually guided movements) and temporal lobe (nature of the observed object)

Parietal and Temporal Pathways ‘What’ and ‘Where’ pathways…

Problems in perception If either of these two pathways is damaged, is causes severe problems for perception. What symptoms would you expect to see if: The pathway to the temporal lobe was damaged? The pathway to the parietal lobe was damaged?

Sensory Adaptation Sensory systems can adapt to changes in the environment. Sensory adaptation refers to a temporary decrease in sensitivity that occurs when a sensory system is exposed to a stimulus for a long period of time. This occurs without conscious control and does not depend on learning/experience. TASKS Give an example of sensory adaptation. What evolutionary advantage might it serve?

Dark and Light adaptation The visual system has to be able to adapt to changes in light conditions. * Explain why it is difficult to drive in twilight conditions, using your knowledge of photoreceptors. * You will have experienced adaptation when you go in to a cinema (dark adaptation) or when you come out of the cinema in to bright light (light adaptation) * Why does this process take time?

Explaining light and dark adaptation Photoreceptors contain PHOTOPIGMENT. What are photopigments made up of? What happens when a particle of light hits a photopigment? A Rhodopsin molecule, which normally has a pink-purple colour.

Contrast processing What is visual acuity? Describe what we mean by brightness. What is visual contrast? What is the ‘simultaneous lightness contrast’? Give an example of how this might affect your perception of brightness. Using the research by Hartline on horseshoe crabs, explain why we experience the simultaneous lightness contrast effect.