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
