Low level Vision

Question 1

Define the visual system

Answer 1

It is hierarchically organised. It is passed on from:
1. Retina
2. LGN (lateral geniculate nucleus)
3. Primary visual cortex
4. Through other cortical revisions specially for vision

Question 2

What is perception?

Answer 2

A selective representation of reality= created by the brain + constrained by particular neural systems + sensory organs

Question 3

Examples of direct methods to study perception

Answer 3

Brain imaging and electrophysiology

Question 4

Example of indirect methods to study perception

Answer 4

Psychophysics

Question 5

What do direct and indirect methods to study perceptions help us to do?

Answer 5

Study structure and function of visual systems

Question 6

What does the retina do?

Answer 6

Converts light reflected from objects into neural signals through rods and cones

Question 7

Where can phototransduction be found?

Answer 7

Within the outer segments of rods and cones

Question 8

What is phototransduction?

Answer 8

Photosensitive pigment molecules that can change shape in response to light to start intracellular signalling cascade which causes an electrical signal

Question 9

What is a bipolar cell?

Answer 9

Provides main pathways from photoreceptors to ganglion cells.

They cause an opposite electrical response in the bipolar cell.

They do this through:
Direct connections to surrounding photoreceptors w/ small no. of photoreceptors
Indirect connections via horizontal cells

Question 10

Describe the retinal circuity

Answer 10

1. Cones and rods connect to the bipolar cells
2. Bipolar cells connect to the output cells of the retina (retinal ganglion)
3. Horizontal cells and amacrine cells process visual info horizontally, laterally across the retina

Question 11

Define a neuron’s receptive field

Answer 11

An area on the retina. This causes a response in the neuron when photoreceptors in that area are stimulate by light.

Question 12

What does the retina circuitry look like?

Answer 12

Retina = backwards
Rods and cones = bottom
Dip = optic disc part of the fovea to get ↑ light
Other processing layers = towards the surface of the retina

Question 13

How do bipolar cells get light?

Answer 13

Bc other processing layers, bipolar cells rely on direct connections from the cones above it + indirect connections horizontally

Question 14

Describe the process of on-centre bipolar cells

Answer 14

Light on photoreceptors found in centre of receptive field = excitatory response

Light on photoceptors found in surrounding receptive field = inhibitory response

Question 15

What of on-centre bipolar cells preferred stimulus?

Answer 15

bright spot of light in dark background

Question 16

1. Off centre bipolar cells

Answer 16

Light on photoreceptors found in centre of receptive field = inhibitory response

Light on photoceptors found in surrounding receptive field = excitatory response

Question 17

What is off-centre bipolar cells preferred stimulus?

Answer 17

3. Preferred stimulus = dark spot on a bright background

Question 18

What is a receptive field?

Answer 18

Area on the retina + causes a response in neuron when photoreceptors in that area = stimulated by light.

Question 19

What is a retinal ganglion

Answer 19

The output cells of the retina. They send action potentials to the brain.

Question 20

Describe the fading example

Answer 20

FADING EXAMPLE
If you fixated on a blue dot, the fuzzy blob fades from perception = invisible.

Fuzzy blob = poor stimulus for retinal ganglion cell

Equal amounts of light = received by centre + surrounding receptive field

Causes excitatory + inhibitory responses to cancel + ganglion cell ≠ signal presence of stimulus

Question 21

What are the two classes of the retinal ganglion cell?

Answer 21

Midget cell
Parasol
Represent the start of 2 independent parallel processing streams = remain segregated in what + where pathways all the way through the visual processing hierarchy

Question 22

What is a midget cell?

Answer 22

a. Small receptive fields
b. Not sensitive to fast flicker
c. Red-green colour selective
d. ‘What’ stream

Question 23

What is a parasol stream?

Answer 23

a. Large receptive fields
b. Sensitive to fast flickers
c. Not colour selective
d. ‘Where’ stream

Question 24

What does edge information mean?

Answer 24

Signalled by retinal ganglion cells.
Info sent from the brain = distorted version of the info in the retinal image.

Question 25

What does edge information show about perception?

Answer 25

Shows perception not just pixel mapping of retinal image but + reconstructing of external world from distorted input signals.

Question 26

Define lateral geniculate nucleus (LGN)

Answer 26

Part of the thalamus. Relay station, sending visual signals from retina to cortex. Neurons have a centre-surround receptive field structure similar to retinal ganglion cells

Question 27

Define the Lateral geniculate nucleus

Answer 27

Separate visual streams from the retina are separated to diff. anatomical layers in the LGN.

Question 28

Define the Primary visual cortex

Answer 28

Discovered by Hubel and Wiesel. Structure of receptive fields of neurons in V1 is elongated + preferentially respond to orientate lines/ edges

Question 29

What are simple cells ?

Answer 29

They are orientation selective cells in V1. The receptive fields of V1 (primary visual cortex) cells = elongated bc they wire together retinal (+ LGN) cells w/ neighbouring receptive cells.

Question 30

What are grandmother cells?

Answer 30

A neuron that responds only to a highly specific stimulus

Question 31

What is a criticism of the evidence supporting grandmother cells?

Answer 31

Quiroga et al. (2005) found cells in human temporal cortex responded selectively to images of one person or object Only presented 16 diff. objects to each P = statistically unlikely to hit grandmother cell Results = consistent w/ a sparse code = a network of neurons represent each concept, w/ connections between the networks of associated concepts.

Question 32

What does retinotpy mean?

Answer 32

The primary map in the visual cortex.

Question 33

What are operation channels?

Answer 33

Information about diff. orientations – signalled by diff. pops of neurons

Question 34

What does spatial frequency coding mean?

Answer 34

Diff. pops of V1 neurons = sensitive to diff. spatial freq. Human sensitivity to contrast varies w/ spatial freq. if the visula system is adapted by exposure to a particular freq. = reduction in contrast specificity

Question 35

if there is a reduction, does sensitivity to other spatial frequencies change?

Answer 35

No

Question 36

What type of neurons show a reduction in contrast sensitivity?

Answer 36

Adapted neurons

Question 37

What does contrast sensitivity function mean?

Answer 37

Describes human it as a function of spatial freq. and peaks at medium spatial freq. Caused by combined responses of no. of diff. neutrons that are sensitive to diff. ranges of spatial frequency

Question 38

How does the receptive field structure increase in complexity?

Answer 38

Builds through the visual processing hierarchy via wiring together neurons at lower levels Theoretical pinnacle = grandmother cells w/ receptive fields that are spec. for 1 indvdl object Cells in area TEO (near IT) in the ventral stream = moderately complex receptive fields

Question 39

How are neutrons in the visual cortical areas mapped?

Answer 39

Retinotopically mapped a. Neighbouring neurons respond to neighbouring regions on the retina (or visual field) Similar to other systematic organisations in the cortex e.g a. Somatotopy b. Tonotopy

Question 40

How is spatial coding and population linked

Answer 40

It is encoded by a population code = over a pop. of neuronns sensitive to different spatial. freq

Question 41

What are spatial frequency channels?

Answer 41

Neurons selective for low spatial frequencies encode broad patterns of light and dark, while neurons selective for high spatial frequencies encode finer detail.

Question 42

What is V1?

Answer 42

Primary visual cortex

Question 43

Where is motion processed?

Answer 43

Cortical area (V5)

Question 44

What is the waterfall illusion?

Answer 44

Motion is encoded via an opponent code. After adaptation in one direction, illusory motion is perceived in the opposite direction.

Question 45

What are Reichardt detectors?

Answer 45

Detecting motion in the fly, and motion in humans is probably encoded via a similar mechanism.

Question 46

Describe the process of Reichardt detectors starting from location 1

Answer 46

Location 1 on the retina is connected to neuron A via a delay + location 2 is directly connected to neuron A. If a stimulus moves from location 1 to location 2, a response = triggered. Combined inputs from both connections, neuron A is active = excite the output neuron.

Question 47

Describe the process of Reichardt detectors from location 2

Answer 47

Neuron B has connections of the opposite pattern and is not activated when a stimulus moves from location 1 to location 2. If a stimulus moves from location 2 to location 1, neuron B receives the converging inputs. Neuron B activated = inhibits the output neuron. opponent code = waterfall illusion – after exposure to rightward motion neuron A adapts and its response fatigues. When no motion, inputs to the output neuron = unbalanced + overall inhibition from neuron B, consistent with leftward motion. Illusory leftward motion is perceived.