Senses 3: Visual perception

Question 1

how do we see?

Answer 1

object/person seen against background

perceived 3-D large upright colourful image

Size of projections onto the retina:

Question 2

complex structure of vertebrate retina

Answer 2

functional classes of cells in retina

first stages of visual processing

Question 3

functional classes of cells in the retina

Answer 3

4 classes of photoreceptors (3 cone types and rods)

50-70 classes of horizontal, bipolar and amacrine cells

20-30 classes of ganglion cells

Question 4

first stages of visual processing

Answer 4

Edge detection in visual scenes

Edge enhancement in patterns

Filtering of spatial, wavelength, movement and directional information

Question 5

fundamental visual tasks of imp for many behavs

Answer 5

Seeing and recognising objects, mates, predators or prey

Question 6

major task of visual system

Answer 6

segregate objects and backgrounds, automatically and quickly

Question 7

lateral inhibition

Answer 7

makes edges stand out

H.K. Hartline model of lateral inhibition in the retina (1956)

Neighboring neurons in the same layer of the retina inhibit each other mutually

The photoreceptors stimulated by the right edge of each grey bar are inhibited by the neighbouring photoreceptors stimulated by the lighter bar next door.

Thus, photoreceptors on the right edge report receiving less light than they actually do (i.e., that edge looks darker to us).

Question 8

neighbouring neurons in same layer of retina inhibit each other mutually

Answer 8

If the light falling on the group of retinal neurons is uniform, then their reciprocal inhibitions cancel each other out without further effects.

When an edge (dark and light illumination) is created, the cells on both sides of the edge will influence each other strongly.

This changes their signals such that a much stronger contrast is coded than physically exists.

More distant cells are not affected.

As a result the perception of the edge is enhanced.

Question 9

simultaneous contrast effect

Answer 9

Koffka-ring illusion

A grey ring on a dark and bright background

A white bar separate the two halfs of the ring. Do the two halves of the ring appear to be identical in brightness?

The right half has been moved in a vertical direction. Do the halves look identical?

Question 10

identifying spatial r’ships and properties of objects

Answer 10

Edges and shadows provide context information about the spatial structure of objects (in the picture a three-dimensional object with inclined surfaces) or spatial relationships between objects (identical objects laying sidewise behind the central one)

Without context cues, we perceive the physical reflectance of the surfaces which carries little information

Question 11

segregated rod- and cone-connected pathways in the retina

Answer 11

horizontal connections

vertical connections

Cones (or rods) converging on a bipolar cell form its receptive field.

Similarly, the receptive field of a ganglion cells is formed by all converging bipolar cells.

Receptive fields are large in the periphery (low acuity) and small in the fovea (high acuity)

Question 12

horizontal connections

Answer 12

horizontal cells

amacrine cells

Question 13

vertical connections

Answer 13

Fovea: 1 cone to 1 bipolar

Periphery: Many cones to 1 bipolar, many bipolars to 1 ganglion cell. Same for rods, but connect to rod bipolar cells other classes of ganglion cells.

Question 14

centre-surround (CS) receptive fields

Answer 14

Bipolar and ganglion cells have centre-surround receptive fields (sometimes also classical receptive fields)

Horizontal cells influence bipolar cells either directly or by feeding back information to the cones (probably both)

The bipolar cell integrates EPSPs and IPSPs (spatial and temporal summation)

Signals from several bipolar cells define the activity in a ganglion cell (with additional modulation by amacrine cells)

Question 15

on/off-bipolar cells

Answer 15

In the vertebrate retina, photoreceptors release neurotransmitter (glutamate) when not stimulated.

Exposure to light hyperpolarises the photoreceptor and decreases the release of glutamate.

The bipolar cells invert the receptor signal to the standard: depolarisation when light intensity increases and hyperpolarisation when light intensity decreases

Question 16

the sombrero-shaped response of a cell with a CS receptive field

Answer 16

When a light spot is moved slowly through the receptive field of a bipolar cell, the summation of EPSPs and IPSPs varies

When plotted, the response curve looks like a Mexican sombrero seen from the side

Question 17

retinal ganglion cells also have a sombrero-shaped response curve

Answer 17

Ganglion cells in the retina have long axons that project (through the blind spot of the eye) as optic nerve to the LGN in the midbrain. They generate action potentials (spikes) when transmitting a signal.

Most ganglion cells have centre-surround receptive fields and will show a sombrero-shaped response similar to the bipolar cells

Similar to the bipolar cells, they are specialised to respond to contrast changes in the receptive field which correspond to edges or small bright and dark spots in the visual scene

Question 18

ganglion cells respond to ratios of light/dark

Answer 18

At rest, a ganglion cell fires action potentials (spikes) at a spontaneous rate

Whilst the ON-centre bipolar cell depolarises, the ON-centre ganglion cell responds by increasing its spike rate

Whilst the OFF-centre bipolar cell hyperpolarises, the OFF-centre ganglion cell responds by decreasing its spike rate

Question 19

ganglion cells do not respond to uniform illumination

Answer 19

When the light spot covers the whole ON-centre, the ganglion cell responds with its highest spike rate

When a ring of light covers all of the surround but not the ON-centre, the ganglion cell responds with the lowest spike rate or even no spikes at all

When the whole receptive field is equally stimulated, the ganglion cell is at rest and fires with a spontaneous frequency

Question 20

inverted responses in off-centre ganglion cells

Answer 20

Also respond to light/dark ratios but not to uniform illumination

Responses are inverted

If a spot illuminates the centre of the OFF-centre ganglion cell, the spike rate is reduced

If a spot illuminates the ON-surround of the OFF-centre ganglion cell, the spike is increased

If the whole receptive field is illuminated, the ganglion cell is at rest and fires at its spontaneous rate

Question 21

P- and M- ganglion cells project to diff layers in the LGN

Answer 21

P-ganglion cells (project to Parvocellular layer in LGN) – small RFs, slower conduction speed, high acuity, poor response to transient stimuli, colour sensitive.

M-ganglion cells (project to Magnocellular layer in LGN) – large RFs, higher conduction speed, sensitive to motion, low acuity, no colour discrimination.

Question 22

P- and M-pathways from retina to V1

Answer 22

P- and M- pathways

P – parvocellular (small soma)

M- magnocellular (large soma)

Question 23

spatial layout of retinal ganglion cell projections is preserved

Answer 23

Retinal ganglion cells project retinotopically to each layer of the LGN

Right and left eye projections are also segregated in the LGN

Question 24

LGN neurons project to V1

Answer 24

6 main layers (stripes) in striate cortex (V1)

Question 25

neurons on M- and P-pathway project to diff layers in V1

Answer 25

M-cells project to layer 4cα P-cells project to layer 4c and 2,3β (interblob)

Question 26

columnar structure of V1

Answer 26

In addition to the six horizontal layers, the neurons in V1 are further segregated into functionally distinct hypercolumns (ca 1mm2)

Question 27

Some cells in V1 tuned to orientation of bar stimulus

Answer 27

Neuron responds with maximal spike rate when a slowly rotating bar reaches the neuron’s preferred orientation

Question 28

Hubel and Wiesel's (1977) ice-cube model of the V1

Answer 28

All neurons in an orientation column share the same preference for a particular orientation of a bar stimulus in their receptive field Retinotopic organisation: Signals from co-located ganglion cells in the retina are processed by respectively co-located cortical neurons within each orientation column Hubel and Wiesel (Harvard, USA) were awarded the Nobel prize in 1981 for their work in the 60s and 70s

Question 29

Responses of neurons in orientation columns of V1

Answer 29

When recording from neurons of a particular orientation column, some neurons respond to orientation columns only within a small part of the visual field which corresponds to their receptive field Different to the retinal ganglion cells, these neurons fire at the maximal spike rate when a bar stimulus shows their preferred orientation Other cortical cells in V1 respond with the maximal spike rate to a preferred direction of motion of bars or patterns Microelectrode recordings reveal that cells differ greatly in their receptive fields. (A) Visual cells in the lateral geniculate nucleus (LGN) have concentric receptive fields, like those of retinal bipolar cells and ganglion cells (B) Visual cells in the cerebral cortex are more responsive to bars of light and may show orientation specificity or respond only to motion, or (C) they may respond only to motion in a particular direction

Question 30

the receptive field of a simple cell in V1 (striate cortex)

Answer 30

Hubel and Wiesel proposed that simple cells in the orientation columns receive input from several neighbouring retinal ganglion cells

Question 31

Low- and high-bandpass filters for diff behavioural responses

Answer 31

Filtering that reveals very low spatial frequencies (left panel) or moderately low spatial frequencies (middle) makes it look as if she is smiling, but the high-spatial-frequency components (right) make her look almost sad. Is this why when we see her from the corner of our eye, she seems to be smiling, but when we look directly at her mouth, it conveys sadness?

Question 32

complex cells respond to bar orientation across visual field

Answer 32

Simple cortical cells (aka bar detectors or edge detectors): respond best to an edge or a bar of particular width, orientation, and location in the visual field. Complex cortical cells: respond best to a bad or particular size and orientation anywhere within a particular area of the visual field

Question 33

perceptual shape constancy

Answer 33

We recognise the same shape from different view points and directions despite the distortions in the retinal projections

Question 34

size constancy

Answer 34

When objects are at different distances, the size of their retinal projection varies Context information allows the brain to judge whether the same object is seen from a different distance or a different object is seen

Question 35

functions of simple and complex cells

Answer 35

Analysis of contours and boundaries analysis of objects Shape and positional invariance Contour enhancement for object identification V1 is fundamentally important for perceptual processing

Question 36

main visual streams in the cortex of primate/human brain

Answer 36

dorsal stream (where system) - interacting with the world ventral stream (what system) - making sense of the world

Question 37

seeing for action: Eye-hand coord

Answer 37

Guiding hand movements requires two processes Deciding which objects to interact with Interacting with objects skillfully These processes require different type of information from both the dorsal and the ventral streams

Question 38

size of projections onto the retina

Answer 38

1° visual angle = 0.288 mm (size of thumb nail when extending your arm = 1.5°), Fovea – 0.6mm, horizontal extension of retina – 32mm

Question 39

where does lateral inhibition occur?

Answer 39

where the neurons in a region—in this case, retinal cells—are interconnected, either through their own axons or by means of interneurons, and each neuron tends to inhibit its neighbours.

Question 40

one group of bipolar cells

Answer 40

contains on-center bipolar cells: turning on a light in the center of an on-center bipolar cell’s receptive field excites the cell because it receives less glutamate, which otherwise inhibits on-center bipolar cells.

Question 41

second group of bipolar cells

Answer 41

contains off-center bipolar cells: turning off light in the center of an off-center bipolar cell’s receptive field excites the cell because it receives more glutamate, which depolarizes off-center bipolar cells

Question 42

what do on-centre bipolar cells have?

Answer 42

an inhibitory synapse (i.e. IPSPs in presence of glutamate) with those cones (or rods) that form the centre of its receptive field (metabotropic glutamate receptors mGluR)

Question 43

what do off-centre bipolar cells have?

Answer 43

an excitatory synapse (i.e. EPSPs in presence of glutamate), ionotropic glutamate receptor (iGluR)

Question 44

the sombrero-shaped response of a cell with a CS receptive field - In an ON-centre bipolar cell:

Answer 44

When the spot is in the OFF-surround, there will be more IPSPs than EPSPs (because no or fewer cones are stimulated in the On-centre than in the OFF-surround) When the spot reaches the ON-centre, there will be more EPSPs than IPSPs because more cones in the ON-centre are stimulated than in the OFF-surround

Question 45

each hypercolumn in V1 composed of

Answer 45

Left and right eye ocular dominance columns (L, R) Orientation columns (rainbow colours) Blobs (drawn as cylinders) Hypercolumns (ca. 1mm2)