Download COLOUR VISION Newton`s Prism Experiments: a white light beam

COLOUR VISION
Newton’s Prism Experiments: a white light beam refracts into all colours of the visible light spectrum when passed through a
prism – can only be split once but can be converged to form white again
The Electromagnetic Spectrum: a continuum of all wavelengths (not discrete groups) – visible wavelengths range from 400700nm
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Small = dangerous (dangerous -> pretty -> useful)
Wavelength: difference between two troughs for crests – objective (cannot be argued with)
Colour:
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Subjective – not absolute, purely in your head, “colour is an experience”
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Colour is a continuum not a series of discrete categories (categories came from newton – 7 categories because he wanted
to match the number of colours to the number of notes in a music scale)
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Objects appear to be whichever colour they reflect i.e. white is reflecting all wavelengths, black is absorbing all
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Colour is an experience resulting from a perceptions of wavelengths therefore black and white qualify as colours
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Light – Additive colour mixtures i.e. areas have an addition of more wavelengths so white is “seen”
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Paint – Subtractive colour mixtures i.e. areas have less wavelengths being reflected because more wavelengths are
being absorbed by the addition of more paint so a mix of all results in black being “seen”
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More than one wavelength is reflected from an object to create “colour”
Photoreceptors:
Rods (NIGHT – SCOTOPIC)
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Only one type – as a result, they are not tuned to
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Several types, each tuned to a different wavelength
Principle of Univariance: a receptor’s activity
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Mostly in fovea
is related to the number of photons it catches
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Lower sensitivity
not the type (wavelength) of photon
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Day vision: photopic – rods are all over-stimulated so its
wavelengths so colour is not “seen” just brightness
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Cones (DAY – PHOTOPIC)
Probability of absorbing photons is heighted
down to cones during the day
None in central fovea, high sensitivity, night vision
(scotopic)
Young’s Colour Theory
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Logic says that the number of cones should be limited
Metamers: two different stimuli’s appear to be identical due to the equal ratios of wavelengths
If there were 2 cone types:
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We could match any given wavelength by adjusting the intensities of almost any two other wavelengths (metamers) and
there would be a single wavelength appearing grey (“neutral point”), where the firing of both channels is equal
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This isn’t how it works in humans
Colour Matching Experiments (Young, 1802)
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For humans, metamers are created by mixing 3 or fewer primary wavelengths to create an intensity matching that of a
single wavelength so they are seen as the same “colour”
MULTISENSORY PROCESSING
Important Principles:
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Spatial Correspondence: info to different senses from the same event from SAME SPACE
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Temporal Synchrony: info to different senses from the same event comes SAME TIME
Evidence:
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McGurk Effect (Stimulus identification): mismatching information changes perception -> auditory component paired
with visual component of a sound -> third sound created. Demonstrates: spatial and temporal synchrony.
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Crossmodal Cueing: irrelevant sounds influence visual detection
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Task: is there a visual target? (Present vs. absent), Conditions: irrelevant auditory cue (in/valid)
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Finding: irrelevant auditory stimuli can influence the detection of a visual target
Ventriloquist Effect: vision effects auditory – demonstrates temporal synchrony: the brain infers that the visual and
auditory information must be from the same source – which should be in the same location
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The Rubber Hand Illusion: visual-tactile integration – illustrates the importance of temporal synchrony
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Summary: this evidence behaviourally reflects multisensory interactions
Mechanisms:
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Multisensory Convergence Zones (Feedforward Processing)
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Initial processing in sensory specific zones
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Subsequent (later/higher) processing by multisensory ‘convergence
zones’
Standard View Of Multisensory Processing:
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Initially in unimodal sensory cortex – subsequent in multisensory
convergence zones = Feedforward connections/processes
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Evidence for multimodal responses in primary sensory area =
feedback and lateral connections
Anatomical Tracing Studies: Highly interactive network that integrates information from the senses for conscious perception
(Not just Feedforward / strictly hierarchical)
Mechanisms Summary: Several key areas of the brain contain neurons that respond to sensory inputs from two or more
modalities (e.g., superior colliculus, parietal, temporal and prefrontal cortex) and are superadditive. Needs revision to include
direct connections between sensory areas, & more extensive feedback connections.
Cross-Modal Mappings & Synaesthesia
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Synaesthesia: perception of a specific stimulus induces a concurrent and distinct experience within a modality/s
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Measures
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Direct: subjective report, consistency over time – can characterise phenomenon but doesn’t say how it occurs
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Indirect: look at the effect it has on other tasks – objective and may indicate mechanisms
Implications: synaesthetic congruency effects give objective measures of the experiences