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Perception Summary Notes

Psychology Notes > Perception (2nd year) Notes

This is an extract of our Perception Summary document, which we sell as part of our Perception (2nd year) Notes collection written by the top tier of Durham University students.

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Lateral inhibition 3
= inhibition transmitted across the retina  facilitates edge detection and segmenting scenes into objects and background and ultimately for object recognition
 allows us to see differences in luminance between adjacent surfaces
Property arising from inhibitory connections from receptors to the surround of a G cell
Function of LI: Enhances edges in the visual images  allows better object recognition
Receptive field
= area in visual space where a neuron is responsive
Each cell has a receptive field. Receptive fields tile the visual space
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)
Convergence
= many photoreceptors connect to 1 Ganglion cell centre (100:1)
Convergence increases from fovea to periphery
Little convergence in fovea  high acuity but low sensitivity
High convergence in periphery  low acuity but high sensitivity
Orientation selectivity
Simple cells: orientation selective
 Responds to bars in specific orientations
 Bar length does not matter  just needs to cover excitatory region
Orientation columns = columns in V1 that contain neurons with the same orientation preference

Hubel & Wiesel 3 cells
Simple: orientation selective
 Responds to bars in specific orientations
 Bar length does not matter  just needs to cover excitatory region
Complex: direction selective
 Responds to moving bars in specific orientations
 Responds to stimuli anywhere in RF
 Some prefer certain direction of movement
End-stopped: length selective
 Prefers a bar/edge of particular length
 Decrease response to shorter/longer bar
 Detects corners Retinotopic mapping
= measuring the spatial rep of the retinal image in the cortex  helps to understand how the visual world is represented
Describes where diff locations on the retina are represented
Tootell et al., 1982: V1 is retinotopically organised
 Injected a monkey with deoxiglucose that is taken up by active neurons
 Showed the monkey a flickering grating  looked at the cortical tissue
 Saw a grid-like pattern of activity in the cortical tissue after the monkey had looked at a grating for a prolonged period
 visual stimuli are laid out in an orderly and organised way in the visual cortex = retinotopic organisation
When measuring the RF locations of V1 neurons that are in the same column perpendicular to the cortical surface = they are in approx. the same location
A move of 1mm = separate RF
When moving along the cortical surface = RF locations will change in an orderly fashion,
corresponding to the locations in the visual image

Geons 3
= Visual system uses a limited set of 36 primitives = geons (geometric ions) to recognise objects
Part of Biederman's Rec-By-Components Theory
Properties of a geon = Non-accidental features:
= properties do not change from view to view
 View invariance = geons can be identified from different angles
 Discriminability = geons can be discriminated from each other from almost all viewpoints
When geons can be extracted, the object can be recognised
When geons cannot be extracted, the object cannot be recognised
Eliminating info about the rel between volumes/feature should be detrimental to recognition
Evidence:
Biederman, 1987: removal of contours defining concavities affects object recognition
Recognition accuracy increases with increasing visibility of the geons

Change blindness 4
= failure to notice an obvious change, even when it is in full view
Cannot notice a change in a scene if we are not attending to the location where the change is happening
Levins & Simons, 1997:
Showed Ps video clips n which objects in arbitrary locations and centre of attention were changed
Ps failed to notice changes in both cases

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