Visual perception I: Lower-level vision

Question 1

Distal vs Proximal senses

Answer 1

Distal sense: based on gathering remote information which in not direct. E.g. Vision is based on distal stimulus in the form of light entering our eyes.

Proximal sense: based on information we have direct contact with the. E.g. Tactile sense.

Question 2

Vision as an Active sense

Answer 2

We can choose where we look via 6 eye muscles :

Medial/lateral rectus> toward/away from nose;
Superior/inferior rectus: raise/lower eyes
Superior/inferior oblique: rotate eyes

+ our visual perception is also affected by our expectations about the environment

> > > First Eye-tacking experiments (Yarbus): using eye movements as a component showing our visual attention (Eg., web surfing)

Question 3

Anatomy of eye

Answer 3

Cornea: 80% of light refraction (fixed).

Iris: colored muscles of the eye which control the size of pupil.

Pupil: a hole shaped by iris. Control light intensity.

Lens: 20% of light refraction (variable).

Sclera: white fatty part around the eye.

vitreous chamber: light passes through this chamber to receive retina

Optic nerve/disc: the axons of retinal ganglions which travel to the brain (LGN)

Question 4

Inversion in Retina

Answer 4

1. Light passes the transparent neural organizations of retina to get to the black end of eyes (pigment epithelium: dark colored to avoid light reflection) > then come backward through the neurons to get to optic nerve.
2. picture is inverted in retina due to the light aberration via lens

Question 5

Retina’s organization

Answer 5

Pigment epithelium >
Rods & Cones (graded potentials) >
[Horizontal Cells] >
Bipolar Cells >
[Amacrine Cells] >
Ganglion Cells (action potentials) >
Nerve Fiber

Forward Processing:
photoreceptors (rods & cones) > bipolar > ganglion

Contextual Processing:
- Horizontal cells: located between the photorecptors and the bipolar cells.
- Amacrine cells: located between the bipolar cells and the ganglion cells.

They generally help with contrast detection and generally differences between neighbouring photoreceptors.

Question 6

Cones

Answer 6

Cones: color receptors, dense in fovea
● short-wave cone (blue)
● middle-wave cone (green)
● long-wave cone (red)
- photopic vision: color
- has high spatial acuity

• 1 cone = 1 ganglion cell in fovea (so more acuity in fovea)

Question 7

Rods

Answer 7

black & white under dim light (low light levels)
dense in the periphery.
- scotopic vision: scoto = darkness
- achromatic
- has low spatial acuity
- slowly adapts to the dim light, because it gets bleached in bright light
- more convergent e.g. 1:10 bipolar cell/rod ratio (more rods are connected to just one bipolar cell)

Question 8

Ganglion cells

Answer 8

fires with action potential, gathers info and carries info with their axon all the way to the brain through the optic nerve.
Axons of Ganglions form the optic nerve which goes to LGN (via blind spot, 15 degree temporal from Fovea)

Question 9

FOVEA

Answer 9

Fixation = seeing with fovea.
We would need to fixate, so we can use our fovea piece by piece to create the nice resolution of the environment (top-down processing).

High level of Cones in fovea, no rods=> In low light levels we don’t see the environment with our fovea.
(E.g., using peripheral vision when tracking a star in dark, using rods)

Question 10

Purkinje Shift

Answer 10

The Purkinje shift is a shift in perceived brightness of colours when going from light conditions to low-light conditions. Due to the difference in wavelength sensitivity between rods and cones, high wavelength colours (Red and Yellow) appear bright in photopic vision(Light), whereas low wavelength colours (blue and green) appear bright in skotopic vision (Low-light)

Question 11

Density of Cones

Answer 11

• S-cones are less dense (only %10)
• Density: L-cones> M-cones> S-cones

Why? Chromatic Aberration: failure of a lens to focus all colors to the same point
Axial aberration occurs when different wavelengths of light are focused at different distances from the lens or due to the fact that different colors of light hit the lens at different speeds

Low resolution in blue; best resolution in green.

Question 12

Blind spot

Answer 12

Where the optic nerve leaves the eye (15 degrees off the fovea), there is a blind spot, because no photoreceptors are present. What’s interesting is that even though nothing is perceived in this spot, it is filled in by what the brain deems most likely based on the surroundings.

Question 13

Classical visual field

Answer 13

the region of visual space where a stimulus can directly cause an increase or decrease in spike rate.

[in non-classical visual field the effects may be modulatory not direct and linear. On/center cells as an example]

Question 14

On-Center & Off-center cells

Answer 14

• On-center type:
  If you stimulate center => response goes up.
  If you stimulate surround => response goes down.
• Off-center type
  If you stimulate center => response goes down.
  If you stimulate surround => response goes up.

Question 15

Mexican hat! model

Answer 15

Mexican hat model: excitation model for on center off surround ganglion cells. ( general property of cortical information processing).
This model is applicable in lots of the related concepts in this field: like in salience & pop-out in attention.

Question 16

Linearity of Ganglion Cell Receptive Fields

Answer 16

the response to the center stimulus alone plus the response to the surround stimulus alone equals the response to simultaneous stimulation of center and surround. That is, this ganglion cell obeys the linear additivity rule.

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