Edge Detection Flashcards
What is the first place along the visual pathway that has orientation-selective neurons?
the primary visual cortex
What are the 3 main points of the visual pathway?
- retina
- lateral geniculate nucleus (LGN)
- primary visual cortex (V1)
How are retinotopic regions organised?
such that the upper part of the visual field is represented in the ventral retinotopic regions, and the lower part of the visual field is represented in the dorsal retinotopic regions
Describe the receptive fields in the retina and LGN
they are round and respond best to small, round spots of contrast
What does one retinal ganglion cell generally project to?
one LGN cell
What do all V1 neurons have?
oriented receptive fields i.e. they respond best to a bar of particular orientation
What are the 2 types of neurons in V1?
simple and complex cells
What are simple cells?
phase-sensitive i.e. they respond best to either a bright or dark bar, but not both
What are complex cells?
phase-invariant i.e. they respond equally well to a bright or dark bar
What is the neocortex involved in?
higher-order brain functions such as sensory perception, cognition, generation of motor commands, spatial reasoning and language
How does the neocortex appear in light microscopy?
a light, unstained sheet of unmyelinated neurons
How many layers are in the neocortex?
6
What do V1 neurons in the same cortical columns have?
the same preferred orientation
Where do LGN neurons mainly project?
layer 4 of the neocortex where most simple cells are found
What may simple cell orientation selectivity be due to?
selective connections from LGN cells
What may complex cells be due to?
connections from simple cells with varied phase selectivities
For every part of the visual field, what is there?
a different complex cell
In real life, what must there be to make a response relatively constant?
8-10 simple cells signalling to one complex cell
How can a simple cell be created?
an LGN neuron synapsing onto a cortical neuron
How can a complex cell be created?
a simple cell synapsing onto a cortical neuron
How can phase-invariant responses be created?
from phase-selective responses
What are the 2 classical transmitters?
- excitatory glutamate
- inhibitory GABA
What is glycine?
an inhibitory neurotransmitter in the brainstem and midbrain, not present in the thalamus and the cortex
What are the neuromodulators of the cortex?
- dopamine from the ventral tegmental area
- ACh from the nucleus basalis
What is the finest level of synaptic organisation?
monosynaptic connectivity and its synaptic strength
What happens in an excitatory presynaptic neuron?
each AP in P1 causes P1 to release glutamate onto P2, causing an EPSP in P2
What happens in an inhibitory presynaptic neuron?
AP in I1 causes I1 to release GABA onto I2, causing an IPSP in I2
What are the 3 main methods to investigate neural connectivity?
- anatomical methods
- paired intracellular recordings
- correlations between spikes recorded with extracellular electrodes
For dyes that do not cross the synapse, how can post-synaptic neurons be identified?
electron microscopy
What is the main limitation of purely anatomical methods for investigating neural connectivity?
they cannot tell us the strength of the synapse
What is the limitation of paired intracellular recordings?
visual responses from neurons cannot be recorded in a brain slice
What is the order of information in a cortical column?
LGN → L4 → L2/3 → L5/6
What must be recorded to determine the synapse strength?
the membrane potential
What is recorded with extracellular electrodes?
only spikes (APs)
What is the size of the tip length of the electrode used for extracellular recordings?
roughly the size of a neuronal cell body
How can a single AP be recorded using extracellular electrodes?
filter the frequency of signals to between 300-5000Hz and adjust the electrode placement
What is the main idea of extracellular spike correlations?
when a reference neuron excites a target neuron, the target neuron will have an increased probability of spiking for a short time after the reference neuron spikes
How does correlation not imply causation with extracellular spike correlations?
the spikes of A and B could be correlated even if there is no monosynaptic connection from A to B
