Low-level visual processing Flashcards
What is ‘accommodation’?
The adjustment of the lens to focus light on the retina.
What percentage of the brain comprises the visual cortex?
20%
How many steps are there before visual stimuli reach the visual cortex?
3
Which steps have to occur before visual stimuli reach the visual cortex?
- Reception: absorption of physical energy by receptors in the eye.
- Transduction: the conversion of physical energy to electrochemical patterns in the neurons.
- Coding: One to one mapping between the physical stimulus and neural activity. One ‘block’ of light is dealt with by one neuron.
What are cone cells responsible for in the eye?
- colour vision
- visual acuity
What are rod cells responsible for in the eye?
- vision in low light
- movement
Which types of receptors are more numerous in the eye?
Rod cells.
What are the two (important) visual pathways?
Parvocellular and magnocellular.
What does the parvocellular pathway involve?
- Input comes from cone cells
- Deals with colour and fine detail
What does the magnocellular pathway involve?
- Input comes from rod cells
- Movement/motion
Describe the 5 components of the pathway from the eye to the brain.
- Retina
- Optic nerve
- Optic chiasma
- Lateral geniculate nucleus
- Cortical area V1
What is the retina?
A layer of light sensitive cells at the back of the eye.
What is the optic nerve?
A pair of nerves which transmit impulses from the retina to the brain.
What is the optic chiasm?
The location at which the optic nerve crosses over.
What is the lateral geniculate nucleus?
Two bodies in the thalamus where the optic nerve terminates.
What is the receptive field?
The region of sensory space in which light will trigger the firing of that relevant neuron.
What is lateral inhibition?
A reduction in the activity of one neuron caused by activity in a neighbouring neuron.
What is the center-surround receptive field?
Specific neurons are linked to specific patterns of light in a receptive field. (e.g dark middle or light outside)
What is retinotopic organisation?
The arrangement of neurons being directly linked to the section of the visual field that they respond to.
What is V1 responsible for?
Basic information from the visual field, such as edges, orientations and wavelengths of light.
Several LGN cells combine to form…
Simple cells
Several simple cells combine to form…
Complex cells.
What is cortical blindness?
A condition which is diagnosed when patients cannot consciously report objects presented in a region of space. Caused by damage to V1.
What is the ‘what’ pathway involved in?
Colour and form processing. Temporal/ventral.
What is the ‘where’ pathway involved in?
Movement processing. Parietal/dorsal
Who devised functional specialisation theory?
Zeki (1992, 1993)
What does the functional specialisation theory suggest about the visual cortex?
That different parts of the visual cortex are specialised for different functions.
According to functional specialisation theory, what are the roles of each visual area?
V1 & V2 - shapes
V3 (V3a) - form & moving objects
V4 - colour
V5 - visual motion
What evidence is there for V4 and V5?
Zeki (1991) conducted a PET study and found that:
- V4 was more active for coloured images compared to greyscale images.
- V5 was more active for moving dots compared with static dots.
What evidence is there against V4 being linked to colour?
Patients with cortical achromatopsia, who can’t see colours due to V4 damage. However, damage also to V2 and V3.
What is the name of the condition that arises when there is damage to visual area V5 (MT)?
Akinetopsia.
What evidence is there for V5?
Patient LM had damage to V5, was good a locating stationary objects, but could not detect motion in objects.
What is the synchronisation hypothesis?
Form, colour, motion etc is bound together across the different areas.
What are the three different aspects of colour?
- Hue
- Brightness
- Saturation
What is hue?
The actual colour category. (red, blue, yellow, etc)
What is brightness?
The perceived intensity of light
What is saturation?
The paleness of the colour.
What is trichromatic colour theory and who devised it?
Hermann von Helmholtz postulated that there must be 3 types of colour receptors in the eye, each responding to different wavelengths of light.
What were the three colours and wavelengths that Trichromatic theory proposed?
- Blue (short)
- Yellow/green (medium)
- Red (long)
What does the opponent-process theory suggest?
There are three ‘opponent processes’ - pairs of colours. Suggests that we cannot perceive both colours in an opponent-process at once.
Which two studies have investigated the opponent-process theory?
Hering (1878)
DeValois & DeValois (1975)
What does the dual-process theory suggest?
One achromatic channel combines activity of medium and long-wavelength cones to measure the intensity of the light.
A blue yellow channel measures the difference between medium and long wavelength cone signals
A red-green channel represents the difference between these signals.
What is colour constancy?
The tendency for a surface or object to appear to have the same colour, despite the fact they have changed slightly.
What is chromatic adaptation?
The decrease in the sensitivity to light of any given colour over time.
