Visual Perception - 1

Magnitude estimation + production tasks
Matching tasks
Detection + discrimination tasks

Pps given standard stimulus with given value and asked to give corresponding value indicating their own perception
BUT can pps actually give accurate numberical values to indicuate the relative strength of their perceptual experience?

Estimate magnitude given a reference stimulus

Set stimulus to a value relative to a reference

1956
Carried out magnitude estimation and production tasks (judging intensity of stimulus)
Created "Stevens' Power Law"

Ψ = subjective magnitude of sensation
C = constant
I = physical intensity of stimulus
m = constant specific to each modality

Set a stimulus to match a standard
Ask pp to choose stimulus that most nearly matches the standard

Estimate sensitivity to weak stimuli
Aim is to find the threshold for detection

Method of adjustment
Method of constant stimuli

Adjust stimulus till it's just visible

One stimulus presented at a time (eg. light flash)
Pp asked if they can see it
Repeat with may different intensities

Prone to response bias

Weakest level of stimulus that can be detected

Minimum detectable change

Intensity level require to make stimulus detectable on 50% of occasions

Context

1830s
Created Weber's law
Measured threshold for detecting light flashes
Varied intensity of stimulus and background

Linear relationship between magnitude of difference threshold and original or background intensity
Require larger intensity change if original already has high intensity

∆I = minimum detectable intensity change
I = background intensity
C = constant

Psychometric function

Relationship between properties of physical stimuli and a behavioual index of perception

When "I" (background intensity) approach 0
Due to internal neural noise unrelated to stimulus

Wall of eye
Tough except for clear, protuding front of eye

Acts as a fixed lens
Anterior chamber inside the cornea (filled with aqueous humour)

Ring of muscle
Controls size of pupil and hence light entering eye

Assists cornea in focusing image on retina

Flattening lens to bring distant objects into focus
OR
Making lens rounder to bring near objects into focus

Manipulates lens shape (see accommodation)

Thin section of neural tissue at back of eye
Responsible for encoding patterns of light and shade

Found on retina
Fovea in the cnetre

High density of cone cells
High acuity due to greater numbers and small receptive fields

Ability to distinguish fine detail within an image

Area of visual field from which cells receives light input

Area of retina where nerve fibres exit eye, projecting to brain
No receptors here: blind spot

Foudn outside photoreceptor layer at back of retina
Unreflective
Possibly helps by 'mopping up' stray light not absorbed by receptors, preventing it being reflected back into retina and blurring retinal image

Photoreceptors
Bipolar cells
Ganglion cells
Horizontal cels
Amacrine cells

~6 million
Greatest density in fovea
Small receptive fields
3 types - sensitive to L/M/S wavelengths of light
Support colour vision

~120 million
Found outside of fovea
Achromatic
Better sensitivity to low light compared to cones
Many rods activate a each ganglion cell

High response to light in receptive field

Photoreceptors transmit signals to bipolar cells
Transmit to ganglion cells then to brain

Integrate info from several photoreceptors

Form links to several different ganglion cells

Graded

All-or-nothing activity
Binary response
Action potenitals

Firing rate (action potentials per second)

Neural response would asymptote (level off)
We would be blind to further increases/decresases in luminance
Retinal ganglion cells have a range of only 0-200 spikes per second (in reality the range of illumination we can perceive is far greater than this)
Adaptation to the average luminance makes it easier to distinguish between light or similar luminance

Pp adapted to high level of light
Measure sensitivity to flashes of light after moving into dark room

1930s
Examined threshold for detecting light flashes

Red flashes: sensitivity to light increased by 2 log units (100x) in first 10mins in darkness but then plateaud
Red = long wavelength (requires cones)
Violet flashes: sensitivity to light increased by 2 log units (100x) in first 10mins in darkness and then improved by further 4 log units (10,000x) in darkness
Violet = short wavelength (rods more sensitive to violet > red)

2 systems for dark adaptation:
1. Phototopic process
Adaptation occurs for 'light seeing' receptors (cones)
Dark adapts quickly but asymptotes after 10mins
2. Scototopic process
Adaptation for seeing in dark, driven by rods
Dark adapts less quickly but to far greater extenet

505nm

550nm

Old world primates (ancestors of Homo sapiens)
Separately in new world primates

1807
Trichromatic theory
Showed that you cannot create all the visible wavelengths of light with only 2 primary colours - must be 3

Matching appearance of any signle wavelength useing mixtures of 3 primary colours

Brown and Wald 1966

Physiological evidence for trichomacy
Used microspectophotometry
Found peak absorptions of cone cells clustered around short, medium and long wavelengths

Shining thin monochomatic beam of light through individual receptors on a dissected retina
Then measure light absorption for different wavelengths

Most mammals have dicromatic vision (only long and short)
Human evolution: long wavelength split into long and medium
Helps to distinguish between ripe fruit and foliage

1870s
Opponent process theory
We perceive 4 primary colours
Red vs green
Blue vs yellow
Can't see reddish-green etc.
Agreed colour vision is trichromatic but later processes confom to our conscious perceptions of colour

Afterimages
Aftereffects
Simultaneous colour contrast

1960s
Studied cells in monkey lateral geniculate nuclei
Found colour opponent cells (+R-G, +B-Y)
Cells responded to green but inhibited by red
Suggests trichrimatic vision at receptor level
Opponent processing at later levels