Cog & Bio Vision Flashcards
What would happen if our eyes produced an accurate copy of the external world?
Illusions would not exist
What is the basis of seeing?
Light rejected into our eyes from objects is the basis of seeing
What is light sometimes defined as?
Waves of electromagnetic energy
What is the length of waves of electromagnetic energy?
Between 380 and 760 nanometers in length - human visual system respond to these
What is the iris?
Bands of contractile tissue that regulate the amount of light reaching the retina
What is the pupil?
Hole in the centre of iris that allows light to enter the retina
What happens in bright light?
Small pupil, better acuity (detail, focus resolution) - image is sharper
What happens when it is dim?
Bigger pupil to let more light in, worse acuity, more sensitivity
The amount of light reaching the retina is regulated by what?
The donut-shaped bands of contractile tissues & the irises
What gives our eyes their characteristic colour?
The iris
What represents a comprise between sensitivity and acuity?
Adjustment of pupil size in response to changes in illumination
What happens when the level of illumination is high (and the sensitivity is thus not important) ?
This visual system contradicts the pupils - image falling on each retina is sharper
What does the lens do?
Focuses light on the retina
What do ciliary muscles do?
Alter the shape of the lens needed
What is accommodation?
The process of adjusting the lens to bring images into focus
The positions of the images in your retina can never what?
Correspond
What is binocular disparity?
The difference in the position of the same image on the two retinas - greater for closer objects
What do visual systems use to make 3D percept from the 2D retina images?
Binocular disparity
What does the retina do with light?
Converts light into neural signals, conducts them towards the CNS and participates in the processing of the signals
What’s at the back of the eye and what do they do?
Cone receptors - convert light into neuro signal and takes that to rod receptors
What happens once the receptors have been activated?
The neural message is transmitted back out through the retinal layers to the retinal ganglion cells
The inside-out arrangement create what 2 visual problems?
- Incoming light is distorted by the retinal tissue through which it must pass before reaching the receptors
- For The bundle of retina ganglion cell axons to leave the eye, there must be a gap in the receptor layer, this gap is called the blind spot
What is photopic vision?
Good light, high acuity, colour (cone-mediated)
What is scotopic vision?
Dim light, low acuity, high sensitivity, lacks colour (rod-mediated)
More convergence in rod system does what?
Increases sensitivity whilst decreasing acuity
What is only in the fovea?
Only cones
More rods in the where compared to where
More rods in the nasal hemiretina than at temporal hemiretina
What are the 3 involuntary fictional eye movements?
- Tremour
- Drifts
- Saccades
What are saccades?
Small jerky movements or flicks
What is the striate cortex also known as?
Primary visual cortex or V1 (visual area 1)
What 2 ways do signals from the left visual field reach the right primary visual cortex?
- Ipsilaterally from the temporal hemiretina of the right eye
- Contralaterally (via the optic chiasm) from the nasal hemiretina of the left eye - opposite is true of all signals from the right visual field
How many layers does each lateral geniculate nucleus have?
6 layers
Each layer of the geniculate nucleus receives what
Input from all parts of the contralateral visual field of one eye
Where do the lateral geniculate neurons that project the primary visual cortex terminate?
In the lower part of the cortical layer IV
What are the 4 top layers of Magno and Parvo LGN channels?
- Parvocellular layers (small cell bodies) - colour, fine pattern details, stationary or slowing moving objects, majority of input provided by cones
What are the 2 bottom layers of magno and parvo LGN channels?
Magnocellular layers (large cell bodies) - movement, majority of input provided by rods
At least how many channels of communication flow through each lateral geniculate nucleus?
2 parallel channels
Why are there 4 parvocellular layers?
2 from each eye
Parvo LGN cells receive input from what?
Small midget ganglion cells
Magno LGN cells receive inputs from what?
Large parasol ganglion cells
What proportion of the primary visual cortex is dedicated to the analysis of the fovea output?
25%
What are receptive fields?
Area of the visual field within which it is possible for a visual stimulus to influence the firing of that neuron
What does the fovea mediate?
Fine grained (high acuity) vision
All of the neurons (retinal ganglion cells, lateral geniculate neurons & lower layer IV neurons) all have what
Circular RFs
All neurons are monocular - what does this mean?
Each neuron has a RF in one eye but not the other
The neurons at each of the 3 levels of the retina-geniculate-striate system have RFs that what?
Comprise an excitatory area and an inhibitory area separated by a circular boundary
RF of other layers of V1 can be divided into what?
On / off regions - straight lines (rectangular)
What do on-centre and off-centre cells respond best to?
Contrast
What is the most effective way to influence the firing rate of an on-centre or off-centre cell?
Maximise the contrast between the centre and the periphery of its receptive field by illuminating either the entire centre or the entire surrounding (periphery) while leaving the other region completely dark
What is one function of many of the neurons in the retina-geniculate-striate system?
To respond to the degree of brightness contrast between the two areas of their receptive fields
What is the difference between complex cells and simple cells in V1?
More numerous than simple cells
What is similar between simple and complex cells?
Complex cells have rectangular RFs, respond best to straight line stimuli in a specific orientation, are unresponsive to diffuse light
What is binocular?
Respond to stimulation of either eye
Complex cells have larger…
RFs than simple cells & no static on off regions
Complex cells respond to?
Straight edge stimuli of a particular orientation regardless of position within RF
What do the simple and complex neurons not include
Neurons of lower layer IV
What does V1 have?
A columnar organisation
Where does the combination of information to be able to see take place?
In the visual association cortex
The major flow of visual information in the cortex is from where to where?
From V1 to the various areas of secondary visual cortex to the areas of association cortex
What happens as one moves up the visual hierarchy?
The neurons have larger receptive fields and the stimuli to which the neurons respond are more specific and more complex
Most areas of secondary visual cortex are located in what 2 general regions!
Prestriatecortex and the inferotemporal cortex
What is the prestriate cortex?
The band of tissue in the occipital lobe that surrounds the primary visual cortex
What is the inferotemporal cortex?
The cortex of the inferior temporal lobe
Where is the largest single area that receives visual input
In the posterior parietal cortex
What did patient DF suffer from?
Hypoxia
What is hemispatial neglect
Lack of awareness of the side of space opposite to the side of a brain lesion
How do right hemisphere stroke patients behave?
As if the left side of the world ceased to exist
What are the conventional measures used to assess neglect?
Pencil & paper tests - such as cancellation tasks, line bisecting & figure copying tasks
In line bisection tasks, patients with left neglect will typically what?
Mark the centre more to the right than the true centre
Pooled evidence of lesion location found what
The most common sites of damage was in the inferior parietal lobule and the superior temporal gyrus in the right hemisphere
What is spatial neglect?
Radar system on left hand side no longer working well
