6: The Visual System Flashcards
Light…
Sometimes described as electromagnetic waves or photon-particles.
Infrared light waves…
Humans cannot see them because they are too long.
Intensity/brightness…
= Wavelength/colour.
What length wave is bright red?
= 700nm wave.
High illumination…
+ acuity, - sensitivity.
Low illumination…
- acuity, + sensitivity.
What is the role of ciliary muscles?
Hold lens in place with ligaments.
Accommodation…
Changes in focus made my lenses.
Prey have eyes on the sides of their heads because…
It allows for a wider visual field to spot approaching predators.
Predators have eyes side-by-side because…
It allows for better accuracy, such as better depth perception for determining the distance of their prey.
Binocular disparity…
Difference in position of an image on eye retinas due to eyes not being in exact same position.
Retina has 5 layers of neurons (Re, Ho, Bi,Am,ReG)…
(Back-front) Retina receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells.
Blind spot…
The gap in the receptor layer that affects exiting ganglion cells due to the retina being back-to-front.
Fovea helps…
Minimise distortion of light due to retina being back-to-front.
Completion…
Helps compensate for blind spot.
Surface interpolation…
A type of completion process that helps complete the appearance of a large surface by using information such as edges and contrast.
Nocturnal creatures…
Have rod-only retinas (rod-type of receptor).
Daytime creatures…
Have cone-only retinas (cone-type of receptor).
Duplexity Theory…
Vision is influenced by rods and cones.
Photopic (cone) vision…
Used in good lighting, high acuity provided.
Scotopic (rod) vision…
Used in poor lighting, high sensitivity provided.
Fovea only contains…
Cones, hence superior acuity.
Spectral sensitivity curve (SSC)…
A graph displaying relative brightness of lights at different wavelengths.
Photopic SSC…
Relative brightness of light shone on fovea.
Scotopic SSC…
Relative brightness of light shone on retina.
Purkinje effect…
When the relative brightness of objects changes due to changes in scotopic and photopic spectral sensitivity.
Summation…
A process whereby our eyes build on the last ‘scan’ it makes of the seen environment, enabling us to have a continual perception of our environment.
Pigment…
Any substance that absorb light; in this case, rhodopsin, a red pigment.
Light effects on rhodopsin…
It becomes bleached when it absorbs too much bright light, and regains colour in darkness.
Rhodopsin is…
A G-protein that responds to light, not neurotransmitters.
Retina-Geniculate-Striate pathways…
Conduct signals from the retinas to the primary visual cortex via the lateral geniculate nuclei in the thalamus. Its system is retinotopic.
25% of the primary visual cortex…
Is for the analysis of fovea input.
Parvocellular layers…
A channel of communication via RGS pathways. Cones provide most input to these.
Magnocellular layers…
A channel of communication via RGS pathways.
Rods provide most input to these.
Edges…
The most important piece of information in any visual display, as it establishes object separation and their positions.
Mach Bands…
A.K.A the Chevreul illusion. Stripes used to emphasises differences of a colour along a spectrum.
Lateral inhibition…
+ intense light = + ommatidium axon firings = + lateral inhibition
Inhibition of rate of firing of surrounding cells.
Receptive field…
The area of a visual field of a neuron in which a visual stimulus can influence the firing rate.
Fovea receptive fields…
Are smaller than in periphery visual components of eye. Supports idea of small area-high acuity.
Neurons are monocular…
Meaning their receptive field only accounts for one eye.
On-centre cells…
Illumination on central region = excitatory.
Illumination on peripheral region = inhibitory.
Off-centre cells…
Illumination on central region = inhibitory.
Illumination on peripheral region = excitatory.
Simple cells…
On-off response, receptive fields are rectangular shaped, so respond best to bar-shaped features.
Complex cells…
On-off response, except its ‘on’ and ‘off’ regions aren’t static, enabling continuous response to a stimulus. Good for recognising moving objects.
Organisation of cells…
Determines its preference for the orientation of features, e.g. | / - \ | , horizontal or columnar organisation.
Plasticity…
Is a key function in the visual cortex, and contradicts the key assumption that receptive fields are static properties of neurons.
Black…
Absence of light.
White…
Intense and equal mixture of a wide range of wavelengths (colours).
‘Component’ or ‘Trichromatic’ theory…
Young (1802)
- 3 types of colour receptors, and stimulus is perceived in varying ratio by these receptors.
- Can be any 3 wavelengths provided that the colours are mutually exclusive.
- Supported by findings that there are 3 types of cones that are mutually exclusive.
‘Opponent Process’ theory…
Hering (1878)
- 2 cell classes for colour coding, and another class for brightness coding.
- Each class has 2 complementary colour perceptions (red-green, blue-yellow, and black-white for brightness).
- Supported by idea that there is no such thing as ‘reddish-green’ or ‘bluish-yellow’.
Dichromats…
Most mammals are this, lacking perception of red hues.
Quadromats…
Some birds and reptiles are this. Can detect ultraviolet light.
Colour constancy…
- Tendency to perceive an object as same colour in spite of the major variations occuring in the wavelengths said object reflects.
- Trichromatic and Opponent Process theories cannot explain this.
Colour constancy remains providing that…
The illumination contains a mixture of short, medium, and long wavelenghts, and that the object is perceived as part of the visual scene.
‘Retinex’ theory…
Land (1977)
Object colour is dictated by its reflectance, which is the ratio of different wavelengths of light reflected by its surface.
Dual-opponent colour cells…
Fire ‘on’ when red is on periphery and green is on centre, and ‘off’ for the reverse.
‘Blobs’…
Peglike, cytochrome, oxidase-rich, dual-opponent colour columns. This supports discovery that visual cortex organisation is largely columnar.
Visual cortex is divided into 3 parts…
1: Primary visual cortex (V1) <— Secondary visual cortex information.
- The further away from V1 neurons are, the larger their receptive fields are, and the more complex and specific the stimuli they respond to.
V1’s location…
Hidden in posterior of the occipital lobes, surrounded by the prestriate cortex.
Scotoma…
- An area of blindness induced by V1 damage.
- Tested for using the ‘perimetry test’, which creates a visual map for each eye, which is indicative of areas of damage.
Hemianopsic…
A scotoma in half of one’s visual field.
Blindsight…
When patients with scotomas can still respond to visual stimuli that pops up in scotomas, even though they don’t have any conscious awareness of it.
Perception of motion…
Is hardiest visual ability when visual damage occurs. Probably because it is one of our most basic perceptual abilities.
First theory for blindsight…
Remaining cells of striate cortex are still alive,and there is a sufficient amount of them to continue carrying out visual abilities without conscious awareness.
Second theory for blindsight…
Visual pathways between secondary visual cortex and visual structure that don’t go through V1 are still able to carry out visual abilities as they don’t need cognitive awareness.
Visual pathways are mainly part of two streams…
- Dorsal stream
- Ventral stream
- Same as sensorimotor system!
Dorsal stream…
V1 —> dorsal prestriate cortex —> posterior parietal cortex. This responds to spatial stimuli.
Ventral stream…
V1 —> ventral prestriate cortex —> inferotemporal cortex. This responds to colour, shape, size etc. of stimuli.
‘Where-vs-What’ theory…
Organises roles of the 2 streams as:
- Dorsal - where?
- Ventral - what?
‘Control of Behaviour-vs-Conscious Perception’ theory…
Organises roles of the 2 streams as:
- Dorsal - control of behaviour
- Ventral - conscious perception
Prosopagnosia…
- Visual agnosia for faces.
- Damage to an area of secondary visual cortex that is responsible for face recognition.
- Can detect faces, but cannot recognise.
- Not just specific to face recognition, but recognition of objects, i.e whose chair/dog/house/car?
Akinetopsia…
- Inability to see movement in smooth progression.
- Nefazodone (drugs) can induce temporary akinetopsia.
- Damage to middle temporal area of cortex (MT).
- MT function is likely the perception of motion.
