lecture 11: from eye to (further into) the brain Flashcards
multimodal
all sensory sections project to them (frontal, parietal, temporal)
occipital = purely visual
brain organisation
- 4 lobes = frontal, parietal, occipital and temporal
- cortex = all the convolutions
- subcortex = everything below those convolutions
visual pathway
- eyes –> subcortex (lateral geniculate nucleus - LGN)
- LGN –> primary visual cortex (V1 in occipital lobe)
- cortical visual pathways beyond V1
where is V1 located
occipital lobe
mishkin and ungerleider (1982)
- monkeys were trained on object discrimination task and a landmark discrimination task
object discrimination = involves telling shapes apart
landmark discrimination = involved discriminating the position of a cylinder - once the monkeys learned both tasks half received lesions of the parietal lobe and half received lesions of the temporal lobe
mishkin and ungerleider results
performance on object task:
1. damage to the temporal lobe = impaired
2. damage to the parietal lobe = ok
perfomance on landmark task:
1. damage to temporal lobe = ok
2. damage to parietal lobe = imparied
what is temporal lobe responsible for
distinguishing shapes and not spatial vision
what is parietal lobe involved in
determining where things are in space
ventral stream
–> related to temporal lobe
- involved in pattern visual-also known as the what pathway
dorsal stream
–> related to parietal lobe
- involved in spatial vision also known as the where pathway
what does a change in firing rate of neurons tell us
- all neurons fire at a baseline rate, if a cell is interested in something it will either increase or decrease its firing rate relative to baseline
electrophysiology studies
- drop wires into the brain and listen in to the action potentials of neurons
- all neurons (cells) fire at a baseline rate
- if a cell is interested in something it will either increase or decrease its firing rate relative to baseline
what do rods and cones respond to
changes in illumination
what do retinal ganglion cells respond to
spots of light
eg: could be excited by black spots against a white background and could be inhibited by white spots against a black background
what do LGN cells respond to
spots of light
what do V1 cells respond to
lines of different orientations
what does the IT cortex (beyond V1) respond to
jagged objects
–> the smoother it is the less IT cortex responds to it
grandmother cells
- cells that seem to respond to discrete features (complex objects)
- also known as feature detectors
retinotopic mapping
= point-to-point mapping of external world onto a brain area
V1 and before: there is retinotopic mapping
After V1: there is no retinotopic mapping
what changes as you go higher into the visual system (away from the eyes)
- the info that is processed becomes more complex
- the features that drive a cell change from basic illumination levels (rods and cones) to spots of light (RG cells, LGN cells) to lines (V1) to complex features (IT cortex cells)
- in line with these changes, the complexity with respect to where information has to be changes from small specific areas of space (V1) to very large areas of space (IT cortex cells)
lateral inhibition
- every cell in the brain is not only trying to relay info its recieving, it is also trying to inhibit its neighbour at the same time
- This mechanism enhances the contrast and sharpness of sensory signals, helping the brain to better distinguish between different stimuli
what does it mean when you say that a retinal ganglion cell likes dots
- RG cells has a certain receptive field (centre surround architecture), shows what info has to fall on the receptive field to activate the cell
- something has to happen in the inside which is different to what is happening on the outside = maximal response from a retinal ganglia cell
- the receptive fields of some retinal ganglion cells are organized in a centre surround fashion, where they are more responsive to localized, circular stimuli rather than to larger, more diffuse patterns
what is the point of center-surround architecture
- enhances contrast
- brightness contrast
the hermann grid illusion
–> see black dots in the middle of the intersections where there are none
centre = responds excitatorily (increases firing) when light hits it
surround = inhibits the response when it is stimulated
At the intersections:
centre = exposed to white lines = strong response
surround = exposed to black areas = inhibits response
–> bc white lines around the intersection are surrounded by black, the contrast create a situation where ganglion cells near intersection have increased inhibition in surrounding areas = perceived dark spots
fovea
this is where all your cones and retinal ganglia cells are packed more densely
why does the dot vanish when you foveate the intersection
- at the fovea, the cones are packed more tightly than at the periphery, and so the RGC assembly is smaller, and falls completely within the intersection and street
- bc this region is more compressed you are looking at it with a compressed receptvie field such that now all the centre cells and all those surround cells are bathed in light
