Week 3 : Visual Pathways

Question 1

Ascending visual pathway…

Answer 1

• 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

Question 2

Visual Field Representations…

Answer 2

• 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

Question 3

Optic Chiasm

Answer 3

• Optic nerve from each eye splits in half here
• this is crucial because it allows for the contralateral hemifield representation

Question 4

Lateral Geniculate Nucleus LGN

Answer 4

• 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)

Question 5

What happens if you cut the left optic nerve?

Answer 5

can’t see the portion only seen by the left eye, but can see the right visual field and the overlap

Question 6

What happens if you cut the left optic tract (between chiasm and LGN)?

Answer 6

loss of entire right visual field

Question 7

What happens if you cut the optic chiasm?

Answer 7

Loss of peripheral vision on both sides, only see the overlap of the two

Question 8

Cortical Magnification…

Answer 8

• 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

Question 9

Cortical Receptive Fields

Answer 9

• 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

Question 10

How do these complex field shapes arise?

Answer 10

• 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

Question 11

Simple Cells

Answer 11

• 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)

Question 12

Complex Cells

Answer 12

• 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

Question 13

Cortical Columns

Answer 13

• 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

Question 14

Dorsal ‘where’ pathway…

Answer 14

• 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

Question 15

Ventral ‘what’ pathway…

Answer 15

• 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

Question 16

Evidence of pathways…

Answer 16

• 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

Question 17

Mel Goodale & patient DF…

Answer 17

• 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

Question 18

Projections to V2

Answer 18

• 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)

Question 19

Object axis

Answer 19

• 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

Question 20

FFA

Answer 20

• 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

Question 21

Remaining 10% goes to the…

Answer 21

• 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

Question 22

eye movements

Answer 22

• 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