Wk5: Cortical organisation and development

hubel and wiesel initial model (wrong): 

cones/rods → RG (retinal ganglion cells) → LGN → simple → complex

multiple cones and rodes make RG, multiple RG makes LGN etc. 

wrong because:

• would lose sensitivity to luminance polarity (light vs dark)

  • gets the same response to light and dark stimuli

• reduced sensitivity to the location of the stimulus

  • same response to the sitmulus anywhere in its RF

feature-detection model

• moving up the visual system, cells are tuned to more complex features

• reaching a stage where the cell is tuned to a very complex set of features/stimulus

problems

• required no. of cells

  • not feasible to have a specific cell to detect each object

  • have diff views of the same object, need many cells to encode

  • e.g. grandmother encoded but now she’s wearing a hat

• how do you encode a new object?

• cortical cells respond to many stimulus properties

cells respond to combinations of features

cortical cells respond to a combination of features, not just a single feature

hence, they are not feature detectors

• simple cells respond to stimulus: 

  • orientation

  • width

  • length (if shorter than the the RF)

  • luminance contrast

  • luminance polarity

  • location within the RF

• complex cells respond to stimulus: 

  • orientation

  • width 

  • length (if shorter than the the RF)

  • luminance contrast 

  • speed and direction (motion)

alternative models

• network response

  • info encoded by the response of a network of cells

  • local network response to sepcific features

• spatial-freq detectors

wk5: cortical organistion and orientation processing

topographic and retinotopic mapping

topographic mapping

• topology

• spatial layout of the world

• how the world is mapped out/represented in an area

retinotopic mapping

• in the retina

• how the retina is mapped onto/represented in an area

organisation: topographic mapping in the retina

• outside world mapped in the retina

  • topographic

  • neighbouring regions of the world project onto neighbouring cells in the retina

• retinotopic mapping in v1

  • retina is mapped onto v1

  • neighbouring cells in the retina project to neighbouring cells in v1

• topographic mapping in v1

  • topographic retina → v1

  • hence topographic also in v1

organisation columns: orientation, ocular dominance, hyper columns

cortical magnification (eccentricity effects)

• zooming in on the center

• how does the brain allocate more space to center vision

  • size of each hypercolumn stays the same'

  • each hypercolumn covers a larger chunk of visual space

  • RG get bigger

  • the amount of cortex devoted to that area gets smaller

1. orientation columns

   • perpendicular to the surface of the cortex are tuned to the same orientaiton

   • progressive change in preferred oreination across the cortex

2. ocular dominance columns

   • binocular cells can favour input from one eye → some cells respond more strongly to one eye than the other4

3. hypercolumns

   • a “complete toolkit” for analysing a tiny patch of your visual field

   • contains all possible orientaion columns

   • all neurons in a hypercolumn look at the same spot in space

organisations: blobs and interblobs

blobs: concentric cortical cells

• special spots in v1 to help see colour

• contain cells that respond to colour and shades of grey

interblobs: orientation-sensitive cells

• area between blobs

• detects edges and shapes

• contains cells sensitive to contours and orientations

• e.g. lines, angles, edges

orientation processing 

hypercolumns and orientatioin steps

• in v1, cells are grouped into hyperolumns, each one covers a small patch of visual space

• within a hypercolumn, cells are tuned to diff orientations (10 degrees diff)

• how to detect smaller diff? e.g. 6 degrees

  • population response → network of cells

  • relative activity: cells w nearby orientation preferences fire at diff strengths

  • interpolation: brian compares these signals to estimate angles between 10 degree steps

tilt aftereffect

supports brain using populaiton response

tilt aftereffect uses adaptation

adaptation: 

• brain adjusts sensitivity after looking at something for a while

• may result in reduced response

• e.g.

  • stare at tilted line 15 degrees

  • at first cell that’s tuned to it fire a lot

  • over time becomes less sensitive

oblique effect

The oblique effect is the phenomenon in vision where humans can better discriminate or perceive details of stimuli oriented horizontally or vertically than those oriented diagonally (obliquely).

why would this non-uniform sensitivity occur? 

• greater exposure to vertical and horizontal contours in the environment

• leads to the question of what effect does the environment have on the development of the tuning properties of cells

cortical development: nature vs nurture

questions

1. is the initial tuning of cells genetically determined and/or depends upon visual experience

   • test for genetically determined tuning→visual deprivation study

2. can the initial tuning of cells be modified

   • test if tuning can be changed → selective visual environment

visual deprivation

permanent loss of visual inputs in humans from early age

→input from other modalities can take over

→so that v1 can involve in processing diff tasks

initial properties of cells are genetically determined

can tuning be changed

• binocular cells

  • integrate input from both eye

• monocular cells

  • respond to input from one eye only

test: selective visual environment

• reduce range of visual stimulation → monocular stimulation

note. gratings: patterns of alternating light and dark stripes, used to study how visual neurons respond to orientation

tuning can be changed later in life