Week 3 : Visual Pathways Flashcards
Ascending visual pathway…
- the optic nerves meet & cross fibres at the optic chiasm
- the reorganized bundles of axons are called optic tracts (90% of these project to the lateral geniculate nucleus LGN)
- then they go up to the visual cortex located in the brain’s occipital lobe
Visual Field Representations…
- axons from right half of right eye (temporal) and right hand of left eye (nasal) combine + form the right optic tract (the left visual world)
- this goes to the right hemifield (contralateral)
- the spatial representation of retinal ganglion cels is maintained in the optic nerve… results in our amazing spatial acuity
Optic Chiasm
- Optic nerve from each eye splits in half here
- this is crucial because it allows for the contralateral hemifield representation
Lateral Geniculate Nucleus LGN
- next stop after the optic chiasm…
- small body of cells comprising the visual centre of the thalamus
- multilayered structure w/ functionally distinct layers (diff. ganglion cells go to diff. layers)
- there is a left and a right LGN because the projections from the 2 eyes do not combine yet
- projections from here usually go to layer 4 in V1 (input layer)
What happens if you cut the left optic nerve?
can’t see the portion only seen by the left eye, but can see the right visual field and the overlap
What happens if you cut the left optic tract (between chiasm and LGN)?
loss of entire right visual field
What happens if you cut the optic chiasm?
Loss of peripheral vision on both sides, only see the overlap of the two
Cortical Magnification…
- there is much more space designated to the fovea on V1 (more than 50%) even though it is only 1% of the retina
- this contributes to exceptional spatial resolution
Cortical Receptive Fields
- neurons in V1 respond preferentially to light bars of particular angles falling at a particular places on the retina
- kind of like the centre-surround organization in retinal ganglion cells
- different tuning curves of different neurons… it is not a simple ‘on’ and ‘off’ station
- there is an optimal orientation that elicits largest response, but other orientations elicit SOME response
- thinner curves are more selective
How do these complex field shapes arise?
- cortical cells undergo a process of convergence in which multiple ascending projections consolidate onto a single cortical cell
- they inherit separate but aligned receptive fields from the retinal ganglion cells, synapsing onto a single cortical neuron
Simple Cells
- there are a few different types of simple cells
(1) edge detector cells = designed to detect the edge of a light stimulus
(2) stripe detector cells - called simple cuz they only can extract a single stimulus (e.g. orientation)
Complex Cells
- neurons in V1 that respond optimally with particular orientations but also to a variety of stimuli across different locations
- less interested in where exactly a stimulus occurs in space (do not have peak location sensitivity)
- respond best to moving stimuli
Cortical Columns
- columnar organization 1st described by Hubel Wiesel
- the cortex has distinct layers of cells w/ distinct functions & is organized into functional columns
- there are separate columns of cortical cells responsive to input from the left & right eye… ocular dominance columns
- within each ocular dominance column are small columns called… hypercolumns
- hypercolumns… preferentially tuned to stimuli of diff. orientations
- blobs… groups of cells responsive to colour
- interblobs… sensitive to orientation
Dorsal ‘where’ pathway…
- how an object in the visual field is moving through space and includes processes relating to how we might move our own bodies in order to interact with/avoid those objects
- neurons carrying info along this pathway leave the primary visual cortex & pass through higher level secondary & tertiary visual cortices, ultimately targeting area MT
- MT is associated with the perception of motion
- then this info proceeds along posterior parietal lobe where it is integrated with info from other senses to form a more complete picture of an objects location & motion
- ultimately it is relayed to motor control areas of the brain where visually guided movements are planned & generated
Ventral ‘what’ pathway…
- info in this pathway pertains specifically to the features that comprise a visual object, including its overall form and colour, which appear to arise via processing in area V4
- involves areas along the inferotemporal cortex
- the inferior surface of the temporal lobe that maintains detailed representations of stimuli w/ which we have a great deal of expertise
Evidence of pathways…
- monkey study
- dorsal = basic landmark task / ventral = object task
- monkeys who got lesioned to parietal cortex (dorsal) couldn’t do landmark task
- monkeys who got lesioned to inferotemporal cortex (ventral) couldn’t do object task
- neither impaired both paths
Mel Goodale & patient DF…
- similar result in humans w/ brain damage
- Mel Goodale described a patient DF w/ bilateral lessons within the ventral streams that left her unable to identify/recognize objects (object agnosia)
- her ability to use visual info to guide her actions was unaffected… dorsal stream
Projections to V2
- in the secondary visual cortex we see the segregation of info that’s destined for the 2 previous pathways
- V2 consists of a series of bands that are the targets of the cells from diff. areas of V1 & which as a result represent diff. stimulus features
- VENTRAL… cells in blobs –> thin stripes (colour)… inter blobs –> inner stripes (object form)
- DORSAL… input layer (V4) –> thick stripes (motion, MT)
Object axis
- looking at the inferotemporal cortex (ventral) we see a patchwork of areas selective for different objects
(1) para-hippocampal place area = important role in encoding environmental
(2) LOC things = familiar things
(3) body area = plays role in encoding parts of human body
(4) Fusiform Face area (FFA) = particularly responsive to face stimuli
FFA
- neuroimaging studies showed that FFA was highly active when participants were shown a pic of a face
- questioned because we have so much experience with facial features
- to test this, researchers developed a group of stimuli called greebles that share features of faces
- concluded that the FFA showed stronger responses to faces first, but after training w/ greebles it was same for both
- Instead, researchers believe that stimuli like faces are encoded in areas like the FFA using a distributed neural code
Remaining 10% goes to the…
- remaining ganglion cells target a midbrain structure called superior colliculus
- here, inputs from the visual, auditory & tactile systems are brought together to allow the eyes to orient toward & follow stimuli of interest in the environment
- structure on top of brainstem, beneath thalamus and it functions to control eye movements
eye movements
- smooth-pursuit = gradual movements of eyes that functions to minimize offset of the retina over time & show very little variability in tracking slowly moving objects
- Saccades = very rapid eye movements that track large, quick object movements & show greater variability due to rapid movement of fovea which renders viewer temporarily blind
- both highly integrated & most natural tracking movements use both