Week 1 Flashcards
How do we study perception
Direct- brain imaging and electrophysiology
Indirect- psychophysics
The visual system is organised
Hierarchically
Where is visual information transmitted
- The retina.
- The LGN.
- The primary visual cortex (V1).
- Other cortical regions specialised for vision
The retina transduce light into neural signals using
Rods and cones
Photosensitive pigment molecules are embedded in
The outer segments of rods and cones
Photosensitive pigment molecules do what
Change their conformation in response to light and begin an intracellular signalling cascade that leads to an electrical signal
How do rods and cones connect to the output cells of the retina
Connect to the bipolar cells
What are the output cells of the retina called
The retinal ganglion cells
How do horizontal cells and amacrine cells process visual information
Laterally
What do bipolar cells do
- Make direct connections with a small number of photoreceptors
- Make indirect connections via the horizontal cells to the surrounding photoreceptors
- The connections cause opposite electrical responses in the bipolar cell
What happens when light falls on the photoreceptors of the on-centre bipolar cells
Causes an excitatory response
Light falling on photoreceptors in the receptive field surround cause
An inhibitory response
A neuron’s receptive field will cause a response when
Photoreceptors at that area are stimulated by light
For off centre bipolar cells, light falling on photoreceptors located at the centre of their receptive field causes
An inhibitory response
or off centre bipolar cells, light falling on photoreceptors in the receptive field surround causes
An excitatory response
What is fading
- Equal amounts of light are received by the receptive field centre and surround.
- Excitatory and inhibitory responses cancel each other out.
- The ganglion cell does not signal the presence of a stimulus
Why don’t solid objects fade easily
Ganglion cells with receptive fields near the edge of the image signal strongly
Retinal ganglion cells signal
Edge information
Two classes of retinal ganglion cells
- Midget cells
- Parasol cells
Are midget and parasol cells independent or connected
Independent parallel processing streams
Midget cells
- Small receptive fields.
- Non sensitive to fast flicker.
- Red-green colour selective.
- ‘what’ stream.
- Connect to parvocellular LGN neurons
Parasol cells
- Large receptive fields
- Sensitive to fast flicker
- Not colour selective
- ‘where’ stream
- Connect to magnocellular LGN neurons
There are at least how many types of retinal ganglion cell
17
Where is the lateral geniculate nucleus (LGN)
Part of the thalamus
What does the LGN do
- Sends visual signals from the retina to the cortex.
- It’s neurons have a centre-surround receptive field
Midget cells connect to
Parvocellular LGN neurons
Parasol cells connect to
Magnocellular LGN neurons
The structure of the receptive fields of neurons in V1 is
- Elongated to wire together retinal cells with neighbouring receptive fields.
- Preferentially respond to oriented lines or edges
Orientation selective cells in V1 are known as
Simple cells
The complexity of receptive fields builds through the visual hierarchy by
Wiring together neurons at lower levels
The theoretical pinnacle is grandmother cells which
Have receptive fields that would be specific for one individual object
Cells in area TEO in the ventral stream have
Moderately complex receptive fields
Quiroga et al 2005 found cells in human temporal cortex that responded
Selectively to images of one person or object
What is sparse code
A network of neurons would represent each context
The neurons in visual cortical areas are
- Retinotopically mapped
- Neighbouring neurons respond to neighbouring regions on the retina
Orientation preference is organised how in the primary visual cortex (V1)
- Systematically.
- Neighbouring columns of neurons have systematically different orientation preferences
What are ocular dominance columns
Inputs from two eyes are systematically interleaved
What is a hypercolumn
A section of V1 that represents all orientations for both eyes
Different populations of V1 neurons are also sensitive to
Different spatial frequencies
The contrast sensitivity function describes
- Human contrast sensitivity as a function of spatial frequency
- Peaks at medium spatial frequencies
The contrast sensitivity function is a result of
The combined responses of a population of different neurons sensitive to different ranges of spatial frequency
Only the adapted neurons show
A reduction in contrast sensitivity
Spatial frequency is encoded by
- Population code.
- Over a population of neurons sensitive to different spatial frequencies
Neurons selective for low spatial frequencies encode
Broad patterns of light and dark
Neurons selective for high spatial frequencies encode
Finer detail
Cortical area V5 (MR) is specialised for
Processing motion
The waterfall illusion reveals that motion is encoded via
- An opponent code.
- After adaption in one direction, illusory motion is perceived in the opposite direction
Reinhardt detectors can detect
- Motion
- Located in a fly
The complexity of receptive fields increases or decreases through the visual hierarchy
Increases
