Topic 2 - Visual Processing Flashcards
Pathway from Retina to Cortex
- from retina through optic nerve to:
- Lateral geniculate nucleus (LGN)
- Primary visual receiving area in the occipital lobe (striate cortex)
- To temporal and parietal lobe before arriving at frontal lobe
Lateral Geniculate Nucleus (LGN)
- relay centre in the thalamus for the visual pathway
- has centre-surround receptive field
- Regulates neural information from retina to visual cortex
LGN Layers
- six layers
- 2,3,5 receive signals from ipsilateral eye
- 1,4,6 receive signals from contralateral eye
Retinotopic map
- each place on the retina corresponds to a place on the LGN
- colour coordinated sections - see notes
Cortical magnification
Cortical magnification factor
apportioning of a large area on the cortex to the small fovea
- fovea takes 0.01% of retinal area but 8-10% of cortical maps area
Factor - the size of this magnification
Optic Nerve fibre (Ganglion Cell)
- centre-surround receptive field, responds best to small spots
Simple Cortical Cells
- side-by-side receptive fields
- responds to spots of light
- responds best to vertical bars
Orientation tuning curve
shows response of simple cortical cells for orientations of stimuli
Complex cortical cells
Like simple cells, they respond to bars of light of a particular orientation. However, they respond to movement of bars of light in a specific direction
End-stopped cells
respond to lines of a specific length or to moving corners of angles
- do not respond to stimuli that is too long
Feature detectors
Neurons that fire to specific features of a stimulus
- includes simple cortical cells, complex cortical cells and end-stopped cells
Selective adaptation
Firing causes neurons tuned to specific stimuli to eventually become fatigued, or adapt
It causes a decrease in the neuron’s firing rate, and causes the neuron to fire less when that stimulus is immediately presented again
Measuring selective adaptation
- measure contrast threshold (sensitivity) to range of one stimulus characteristic
- Adapt to one characteristic by extended exposure of high contrast
- Remeasure the contrast threshold of all stimuli from first step
Contrast Threshold
the minimum intensity difference between two adjacent bars that can just be detected
Measuring contrast threshold
- measure contrast threshold by decreasing intensity of grating until person can just see it
- calculate contrast sensitivity by taking 1/threshold
- if threshold is low, person has high contrast sensitivity
Measuring orientation sensitivity
- measure contrast sensitivity to different orientations
- adapt person to one orientation, using high contrast grating
- re-measure sensitivity to all gratings
- selective adaptation for specific orientation if neurons are tuned to this characteristic
Selective rearing
if an animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli will become more prevalent
- Due to neural plasticity
- Blackmore and Cooper kitten in a tube experiment
Sensory coding
refers to how neurons represent various characteristics of the environment
Specificity coding
the idea that an object could be represented by the firing of a specialised neuron that responds only to that object
- unlikely to be correct due to too many objects and faces in the world
Population coding
the representation of a particular object by the pattern of firing a large number of neurons
Sparse coding
occurs when a particular object is represented by a pattern of firing of only a small group of neurons, with majority remaining silent
Contextual modulation
the effect of stimulating outside the receptive field
Location columns
- perpendicular to the surface of the cortex
- receptive fields in the same location on the retina are in the column
Tiling
when columns cover the entire visual field
Orientation columns
- each column contains cells that respond best to a particular orientation
- adjacent columns change preference in an orderly fashion
- 1mm across cortex represents entire range of orientation
Ocular dominance columns
- neurons in the cortex respond preferentially to one eye
- neurons with the same preference are organised into columns
- columns alternate in a left-right pattern every .25 to .50 mm across the cortex
Hypercolumn
a location column with all of its orientation columns (0-180 degrees), and left and right dominance columns
fMRI
Functional Magnetic Resonance Image
- haemoglobin carries oxygen in the blood and contains ferrous (iron) molecule which is magnetic
- Brain activity takes up oxygen, makes haemoglobin more magnetic as it loses oxygen
- fMRI determines activity of areas of the brain by detecting changes in magnetic response of haemoglobin
Lesioning and ablation experiments
Refers to the destruction or removal of tissue in the nervous system
- First animal is trained to indicate perceptual capacities
- Second, specific part of brain is removed
- Third, animal is retrained to determine which perceptual capacities remain
Ungerleider and Mishkin ablation experiment
- Object discrimination problem - monkey shown one object, presented with two-choice task
- Landmark discrimination problem - monkey is trained to remove cover of food closest to tall cylinder
- Removal of parietal or temporal lobe from monkeys
- Removal of temporal lobe resulted in issues with object discrimination (ventral)
- Removal of parietal lobe resulted in issues with landmark discrimination (dorsal)
Double dissociations
One person has function A but not B, other has B but not A, allows us to conclude the two operate independently of each other
Ventral and Dorsal pathways
Ventral pathway - the “what” pathway, responsible for objects identity
Dorsal Pathway - the “where” and “how” pathway, responsible for objects location and how to take action on object
Patient D.F.
Rod and Frame illusion
D.F.
- damage to ventral pathway
- not able to match orientation of card with a slot but able to if placing card in the slot
Rod and Frame
- two tasks, length estimation and grasping (ventral and dorsal)
- Frame orientation affected matching but not grasping
Modularity
the idea that specific areas of the cortex are specialised to respond to specific types of stimuli
Inferotemporal (IT) cortex
- responds best to faces with little response to non-face stimuli
- removal causes monkeys difficulty in telling the difference between objects
Prosopagnosia
- difficulty recognising faces of familiar people
- can be caused by damage to temporal lobe
Fusiform face area (FFA)
Parahippocampal area (PPA)
Extrastriate body area (EBA)
Lateral occipital complex (LOC)
FFA - responds best to faces
PPA - responds best to spatial layout
EBA - responds to pictures of bodies and parts of bodies (but not faces)
LOC - responds to objects
Distributed representation
when a stimulus causes neural activity in a number of areas of the brain, so the activity is distributed across the brain
Mind-Body problem
How to physical processes like nerve impulses become transformed into perceptual experience?
Expertise hypothesis
Our proficiency in perceiving certain things can be explained by changes in the brain caused by long exposure, practice, or training
Evolution and perception
- newborn monkeys respond to direction of movement and depth of objects, babies prefer looking at pictures of assembled parts of faces
- “hardwiring” of neurons plays a part in sensory systems
Experience-dependent plasticity
- brain imaging experiments show areas that respond best to letters and words
- fMRI experiments show that training results in areas of the FFS responding best to objects that people are experts on
