Striate Cortex

Question 1

Describe the visual cortex?

Answer 1

• Organised into primary and secondary region in each occipital lobe – located at back of brain in occipital lobe
  o Occupies entire occipital lobe, temporal lobe (IT), posterior parts of parietal lobe and a few small regions in frontal lobe (which are usually to do w/ motor part of eye movements)
• Direct visual signals come into primary cortex
  o Fovea, region of retina w/ highest VA, sends signals directly into primary cortex & is heavily over-represented there compared with peripheral retinal regions
• Secondary visual cortex receives signals secondarily: they are transmitted to these areas for analysis with respect to motion, shape, position etc via intra-cortical pathways
  o Different regions of secondary cortex are responsible for different types of classification and analysis - & depending on “conclusion” actions can be initiated by motor control areas of the cerebrum
• Part of cerebral cortex that’s responsible for processing visual stimuli
• Highly specialised for processing info about static and moving objects – excellent in pattern recognition
• Occupies about 1/3 of surface of cerebral cortex in humans
• Divided into approx. thirty interconnected visual areas
  o 1st cortical visual areas is in calcarine fissure and is the one that receive info directly from LGN = V1
   V1 transmits info to 2 primary pathways called ventral stream and dorsal stream

Question 2

How much of the cortical area is devoted to vision? Which part of cortex is important for motor processing?

Answer 2

1/3
V5

Question 3

Which fibres mostly synapse in primary visual cortex (V1)? What is another name for V? What is overrepresented in V1?

Answer 3

Most LGN fibres synapse in primary visual cortex (V1)
V1 is also named striate cortext as it is stripey
Fovea is overrepresented in V1

Question 4

Which layer of V1 has maximum sensory input and where does it come from?

Answer 4

There are several layers to V1: at layer 4C is where the maximum sensory input from the LGN comes in and is integrated

Question 5

Describe ocular dominance columnns?

Answer 5

• Cells from one LGN layer will project to groups of target cells in layer 4
• These groups of cells form alternating stripes/bands in layer 4, above and below this layer, most cells are driven binocularly, although one eye is usually dominant
• Each neuron getting input from both eyes – integrating info from both eyes
• If one eye pointing inwards or outwards – the info coming from that eye will be suppressed

Question 6

Describe columnar architecture in V1?

Answer 6

• COLUMNAR ARCHITECTURE: Move an electrode vertically through thickness of cortex, find most neurons have same selectivity (same orientation preference and eye dominance).
  o Retinotopically organised
• OCULAR DOMINANCE COLUMNS: Move an electrode tangentially (oblique angle) through cortex, find cells that respond to LE inputs, then binocular (responsive to both/either eye), then RE, then binocular, then LE again etc
• ORIENTATION COLUMNS: Move electrode tangentially in orthogonal direction, find cells selective for vertical, then diagonal, then horizontal etc.
  o Hypercolumn – chunk of cortex about 1mm square by 3mm thick that contains neurons – all approx. same RF location, but with all different orientation selectivities, direction selectivities, both (L&R) eye dominances represented

Question 7

Describe V1 physiology?

Answer 7

• Hubel & Wiesel – discovered 3 different types of neurons – distinguished based on how they respond to visual stimuli: simple cells, complex cells and hypercomplex cells
• V1 neurons transform info (unlike LGN cells whose RFs look just like those of GCs) so that they are orientation selective and direction selective

Question 8

Describe orientation selectivity?

Answer 8

• Most V1 neurons are orientation selective – respond strongly to lines, bars, or edges of a particular orientation (e.g. vertical) but not to orthogonal orientation (e.g. horizontal)
• Static image
• From image can see that middle row – diagonal orientation – evoked greatest response and thus largest no. of action potentials
• Graph shows tuning curve – peak response for one particular orientation and weaker responses for other orientations – falls to 0 when line orientation is ~40° away from neurons preferred orientation

Question 9

Describe direction sensitivity?

Answer 9

• Some V1 cells also direction selective – they respond strongly to orientated lines/bars/edges moving in a preferred direction (e.g. vertical line moving to right) but not at all in opposite direction (e.g. vertical lines moving to left)
• Moving image
• In image, arrows above each electrophysiological recording indicates direction of motion
• This V1 neuron responds best to upward right motion but not at all to downward left motion

Question 10

Describe simple cells?

Answer 10

• Respond best to elongated bars or edges (edge analysis)
• Orientation selective
• Can be monocular or binocular
• Have separate ON and OFF subregions
• Perform length summation (have bigger response with ↑ bar length up to some limit, at which point response reaches a plateau)
• Suggested that each simple cell sums inputs from LGN neurons with neighbouring/aligned RFs to build an elongated RF that is most responsive to elongated bars or edges

Question 11

Describe complex cells?

Answer 11

• Orientation selective
• Spatially homogenous RFs (no separate ON/OFF subregions)
• Nearly all binocular
• Perform length summation

Question 12

What is length summation in simple and complex cells?

Answer 12

↑ firing rate w/ ↑ bar length up to some limit, at which point response reaches plateau

Question 13

Describe hypercomplex cells?

Answer 13

• Like complex cells except there are inhibitory flanks on ends of RD, so that response ↑ with ↑ bar length up to some limit, but then as bar is made longer response is inhibited
• This is called end-stopping
• Graph shows response of hypercomplex cell as a function of bar length

Question 14

What is the receptive field of neuron in striate cortex?

Answer 14

Portion of VF in which presentation of visual stimuli will produce an alteration in firing rate of a particular neuron

Question 15

What is cortical topography & magnification?

Answer 15

Target object is imaged upside down and magnified before brain flips it – fovea is ultra-magnified compared to periphery