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Psychology Notes Perception (2nd year) Notes

1 Important Concepts In Retinal Processing Notes

Updated 1 Important Concepts In Retinal Processing Notes

Perception (2nd year) Notes

Perception (2nd year)

Approximately 27 pages

Topics include: general revision summary, important concepts, visual pathways, cortical structure & function, and object perception (including theories/models and evaluation). Relevant evidence for each topic is outlined, including methodology and findings.

These notes are informative, to the point, and easy to follow. They are drawn from a wide range of sources utilising additional course reading and independent reading....

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Important concepts in retinal processing

  • Convergence

  • Lateral Inhibition

  • Receptive fields

  • Fovea vs periphery

  • Photopic vs scotopic vision

Sensation = registration of physical stimuli from the environment by the sensory organs

Perception = the interpretation of sensations by the brain

Eye, photoreceptors = converts the stimulus into electrical signals

Retina, visual cortex = interprets the stimulus

Light enters the pupil through retina photoreceptors at back of retina

  1. Cones = active during daylight

Found mainly in the fovea

3 types colour vision: SML cones

  1. Rods = active during low light

Found in the periphery

1 type colour blind

Rods Cones
Absolute number 120 million / eye 6 million / eye
Distribution Retinal periphery Fovea and periphery
Dark adaptation Initially slow, but continue to full adaptation Initially rapid, then plateau

Absolute sensitivity

= ability to detect light in dark-adapted eye

 High Low
Acuity = detail vision Low High

Neural convergence

= no. neurons synapse onto a single neuron

 More Less
Spectral sensitivity Short λ Long λ
Colour vision No Yes (S, M, L cones)
2 types of vision Scotopic vision Photopic vision
Scotopic vision Photopic vision
Rod or cone dominated? Rod Cone
Sensitivity Sensitive in dim light Insensitive in dim light
Visual acuity Poor Good
Maximum sensitivity 510nm 555nm
Colour sensation? No Yes

Photoreceptors convert light E electrical signals which pass onto the ganglion cells via bipolar cells

Ganglion cells = last processing stage in the retina

axons exit the retina via the optic disc which does not have receptors = causes a blind spot in the temporal part of the retina

Receptive fields

= area in visual space where a neuron is responsive

Each neuron in the retina responds only to a limited area in visual space

RFs of neurons also have other properties eg whether the neuron prefers bright/small stimuli

Ganglion cell RFs have a concentric centre-surround structure either on-center and off-surround (prefers bright spots of light onsets), or off-center and on-surround (prefers dark spots of light offsets)

Retinal Convergence

= several photoreceptors connect to 1 Ganglion cell

Greater summation = greater sensitivity

Greater summation = poorer acuity

In the fovea: there is less convergence than in periphery

Foveal ganglion cells receive input from only 1 cone in its centre, and input from several cones in the surround

The inputs to the centre and surround always have different signs

On-centre G cell receives:

  • Excitatory input in its centre

  • Inhibitory input is its surround

Figure 1: prefers small bright spots in RF centre, as light in its surround inhibits the response

Less convergence in the fovea means that the G cell RFs are smaller = signal finer details

More convergence in the periphery means that the G cells = more sensitive to light BUT have larger RFs = cannot signal fine details

hence we move our eyes to read to keep the text in the high-acuity fovea

Lateral inhibition

= inhibition that is transmitted across the retina facilitates edge detection and segmenting scenes into objects and background and ultimately for objet recognition

fundamental property which allows us to see differences in luminance between adjacent surfaces

Property arising from inhibitory connections from receptors to the surround of a G cell

= first characterised in the Limulus (horshoe crab)

  • A compound eye = ommatidium

  • They have lots of little compound eyes with large nerves coming from the photoreceptors

  • Each ommatidium feeds into a single nerve fibre

  • Eyes are bunched up and each is responsible for its own field of view

  • Similar to our cells = each eye is a large scale rep of our visual receptors

Wagner et al., 1956:

  • Measure electrical signals from each eye to different kinds of stimuli

  • Recording each activity = pinpoint which stimuli made the cell react = visual map

Effects of luminance of retinal response:

By activating single ommatidia showed how response changed as a function of light intensity

Firing rate increased with intensity non-linearly (logarithmically)

more light, more response

Each ommatidium has a single output fibre BUT it not independent of its neighbours

When only one eye was stimulated = that nerve gave out a certain response

There are lateral connections that are inhibitory ie shining a light on a neighbouring ommatidium will lead to inhibition of the response of the first fibre

When the...

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