Perception and visual cognition



Invented by Weber and Fechner

One way to measure perception

We try to relate a precisely defined physical stimulus with a precisely measured behavioural response

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the agency that causes a visual sensation when it falls on the retina of the eye … it forms a narrow section of the electromagnetic spectrum

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Electromagnetic radiation

·       waves of energy that are caused by the acceleration of charged particles.

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Absolute threshold

the smallest amount of stimulus energy necessary for an observer to detect a stimulus

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Method of constant stimuli

Technique of measuring threshold

-          Very slow

-          No account of observer “decision criterion”, saying yes or no when you haven’t/have seen the flash

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Signal detection theory

Technique for measuring threholds

-          False alarm rate to account for the decision criterion

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Just noticeable difference

-          The smallest difference between two stimuli that a person can detect

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Sensory adaption

reduces sensitivity; can’t see stars in the daytime, can’t hear very well after concerts

Strong, persistent stimulation decreases sensitivity

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Retinal stabilisation

stabilised images fade rapidly

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Retinal adaption

if an image is stabilized on the retina it fades, normally our eyes are constantly in motion to prevent this, which serves to reduce visual clutter and concentrate on changes

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The first place light hits

is perfectly transparent, the skin over the eyes

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Amplitude of accommodation

he ability to focus on objects very close or very far away

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(which consist of rods and cones) detect light and turn light into electricity

This process is called phototransduction

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Duplex theory

Rods neural substrate for night vision (scotopic), cones are neural substrate for day vision (photopic)

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Soundwaves are mechanical motions in a fluid medium

We talk about frequency rather than wavelength

Frequency is associated with pitch

We talk about this in cycles per second or herz

Amplitude of the soundwaves determine the intensity, loudness

As the compressions of the waves change, we perceive it as change in volume

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Theory of audition

frequency is encoded according to position on the basilar membrane; high near the tip of the cochlear spiral and low on the near centre

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Olfactory epithelium

Molecules carried in the air stimulate specific odour receptors in the olfactory epithelium

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true colour blindness

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Subtractive colours

·       Mixing paints

·       Looking through coloured filters

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Additive colours

·       Mixing coloured lights

·       Television

·       Makes the colours lighter

·       Pointillist painting

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Young-Helmholtz theory

Three physiological features that underlie our colour vision

Three types of cone photoreceptor: long-, medium-, short-wavelength-preferring

Problems with the theory:

·       Complementary colours: Have you ever seen a reddish green? Or a yellowish blue?

·       Simultaneous colour contrast: surrounds induce colours

·       Negative after images: a red stimulus gives a green afterimage

The theory doesn’t explain all vision phenomena so we have to adjust it

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Opponent-process theory

L, M, S cone outputs are recoded into six primaries: red, green, blue, yellow, black, white

Primaries are combined in antagonistic pairs: red-green, blue-yellow, black-white

Some people have argued that it is possible to see reddish-greens and yellowish-blues

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A merging of the senses

Experiencing unusual perceptions while going about their everyday lives

Can be triggered by sound, taste, smell, touch, letters, numbers, words, music

Affects over 4 % of the population

95 % get colour sensations

Thought to be happen either because of un-pruned neural connections or reduced neural inhibition

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