cognitive psychology --colour vision

colour vision -- types of cones

Depends on three types of cone
They are sensitive to short, medium and long wavelengths of light

EEG (electroencephalogram)

recording of brain's electrical activity at the surface of the skull

VEP (Visual Evoked Potentials)

measures the signals from your visual pathway

fNIRS (Functional near-infrared spectroscopy)

non-invasive brain imaging technique that measures blood oxygenation changes similar to fMR

observation / deprivation

Behaviour → habituation, novelty preference, eyetracking, VoE

visual acuity

Poor at birth
20/20 vision at about 36 months
General pattern for CSF is very similar to adults
Immaturity of the infant visual system may provide the best learning ground for discriminating faces
Ratio of input of cones is the same across the lifespan

critical periods

period in development when perceptual systems are sensitive to environmental stimuli

colour perception

Colour discrimination thresholds half with every doubling of age

natural scene statistics

We expect that our perception is shaped by fixed adaptations and plastic adaptations
May vary in predictable and measurable ways
Spatial information - fractal dimension, spatial frequency
Colour information - saturation, range of colour

how is colour information encoded in the retina?

by midget cells and small bistratified ganglion cells → send info in 2 further parallel processing streams

where are the Genes encoding the L+M photosensitive pigments?

X chromosome

what does colour vision depend on?

bottom-up signals sent from the retina AND higher level perceptual processes → conception of the illumination and 3D interpretation of scene

normal trichromacy

One copy of L photopigment gene
Midget cells = signal red-teal differences
Bistratified ganglion cells = signal lime-violet differences
One or more copies of the M photopigment gene

dichromacy

Either the L or M photopigment gene missing
Retina has only 2 types of cone
Colours vary along only one dimension

anomalous trichromacy

Photopigment gene is hybrid - contains sections of L and M photopigments
3 cone types but 2 have very similar sensitivities
Red-teal signal carried by the midget cells is weaker
Colour perception of red-teal colour difference is muted

what is colour constancy?

we perceive the colours of objects as table despite changes in the colour of the illumination

what does the light reflected from objects depend on?

both the object's reflectance spectrum and the illumination spectrum

refelctance spectrum

proportion of light reflected at each wavelength

problems with colour constancy

Signals sent by the 3 classes of cone when exposed to the light reflected from the object are different under the two different illuminants

finding the colour of illumination

3D structure of the scene and shadows
Colour memory
grey world
highlights

Wallach's (1948) ratio law

Perception of lightness is determined by the ratio of luminance of a surface and its background

Gilchrist's (2006) experiment

Showed the importance of perception of 3D geometry and the scene for the visual system's perception of lightness

Emmert's law

Perceived size is proportional to the product of retinal image size and perceived distance