Perception Summary Notes

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

Even when the sole actor in a scene transformed into another person across a instantaneous change in camera angle

Inattentional blindness

= failure to notice existence of an unexpected item because attention is preoccupied

Mack & Rock, 1998:

Ps not able to report which shape had been displayed in an unattended location whilst doing a filler task at fixation

= weren’t paying attention to the object

Attentional blink

= inability to see 2^nd target that is presented as one of a string of briefly flashed stimuli

Ps have difficulty seeing a letter/number in a rapid stream of alphanumeric characters if they have just attended to another character

Eg if Ps are supposed to detect an 8 and P, and the 2 characters are separated by less than 500ms, Ps are usually only able to report seeing the first character

Ie it is easier if there is a long delay between the 2 stimuli

It seems like there is an attentional refractory period during which we cannot re-allocated our attention

Overt and covert attention

Overt = moving your eyes and attention to a stimulus object of interest imaged on the fovea

Covert = moving you attention but not your eyes looking at something from the corner of your eye

Endogenous and exogenous attention

Endogenous attention = voluntarily moving our attention to a specific location/stimulus

Exogenous attention = attention is drawn involuntarily eg when there is an unexpected event/sudden stimulus onset

We are better at processing stimuli when we have been drawn to them by an external cue, but only if the target appears quickly after the cue (<0.3s)

If the target is presented after a longer delay, detection is impaired

Lateral geniculate nucleus

= further organises the info from the retina

LGN cells have similar RF properties to retinal ganglion cells

Inputs from 2 eyes remain separate

Each LGN represents the contralateral visual field right LGN represents L VF

Visual info is passed to LGN and cortex via 2 independent and parallel channels: the M and P channels

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