Vision Flashcards
Why is Vision our most complex sensory system?
Because it deals simultaneously with many aspects of the visual world. Visible light consists of waves of electromagnetic energy, with slight wavelength variations producing the range of colours we see. Visual receptors for vision in dim light conditions, and cones for colour vision in bright light.
What do Rods and Cones do?
Rods and cones transduce electromagnetic energy into action potentials, which after processing in other retinal cell layers, leave the eye via fibres of the optic nerve. Both eyes project to both hemispheres, with contra lateral fibres crossing over at the optic chiasma. After synapsing and further processing in the lateral geniculate nucleus of the thalamus, the fibres of the optic tract terminate in layer IV of the visual cortex. There are extensive interconnections between cortical cell layers, and between primary visual cortex and surrounding secondary and tertiary areas.
What are receptive fields?
Neurons in the Visual System have characteristic receptive fields. Up to the layer IV of cortical neurons, receptive fields consists of small circular spots of light on the retina. Beyond layer IV neurons respond best to line stimuli on the retina. It seems that a number of neurons early in the system all project to the same higher-order neuron, whose receptive field is then a combination of the separate lower-order fields.
Why is visual perception an active synthesis?
Progressively through the system neurons respond to more complex stimuli, representing a gradual integration of basic visual information. Visual perception is an active synthesis; stimuli are analysed into their basic features early in the system and then reconstructed at higher levels.
What is an orientation column?
All the neurons in a single vertical column in the visual cortex respond to lines of the same orientation on the retina; this is an orientation column. This basic feature detection property is not innate, but determined by early visual experience, which is also crucial to the perception of depth and movement. The visual cortex shows functional plasticity for a short period after birth.
How do we see colours?
The visual system processes shape, colour, and movement separately. Colour perception is based on our possession of three different cone receptors, responding best to red, green and blue light respectively. The relative activation of the three cone populations produces the visible colour spectrum.
How do we produce the integrated perception of a whole object?
Separate areas of secondary and tertiary cortex deal with shape, colour, and movement. Some brain-damaged patients lose their ability to perceive any colour, whereas others may lose the ability to perceive movement. We do not yet know how the results of this parallel and separate processing of visual features are finally brought together to produce the integrated perception of a whole object; certainly there do not appear to be single neurons that respond to complete stimuli.
What is blindsight?
Blindsight is demonstrated when otherwise blind subjects are forced to guess in visual tasks. They can shoe sensitivity to object movement and to some aspects of colour, but without conscious awareness. As these patients often have damage to their primary visual cortex, blindsight implies that pathways outside the main system must exist to connect the retina with higher visual cortical areas.
