How do we see what we see?
- The visual world around us is extremely complex
» Chaotic pattern of light coming from infinite number of angles - Somehow, we have evolved to make sense of this jumbled
pattern, and extract meaning from it - Positions of objects, their identity, and characteristics
- Critical for our survival
We all actually see the world the same, and we aren’t even aware we are doing it. For example, driving down the highway we all perceive and see it the same way and this is critical for our survival.
Two main principles:
- Perception is dynamic - We perceive change (our visual system perceives change)
- We perceive constancies in a changing environment
For an object to be visible, it must:
emit or reflect light
What actually is light?
a series of particles called photons that are emitted by an object. Light is simply one part of a continuum
called electromagnetic radiation
so basically objects are shining little beams of particles and those particles can bounce off surfaces
What is the process of light?
- What we normally consider light is simply one part of a continuum called electromagnetic radiation.
- EMR is generated when an atom emits a “particle” called a
photon. - The energy of photons determines their wavelength.
- The human eye is only capable of detecting light within a narrow range of wavelengths.
- Within this range of visible wavelengths, different wavelengths give rise to the perception of different colours.
What is the basic design of the eye:
The eye is a spherical organ that:
Focuses light at the front (via pupil + lens)
Detects light at the back (via photoreceptors on the retina)
Explain Light Entry & Pupil Control
Pupil: The small aperture at the front of the eye that controls how much light enters.
Bright light → pupil constricts
Dim light → pupil dilates
This is the first active step in vision — the eye dynamically adjusts to manage light intake.
Photoreceptors and Light Sensitivity
Located at the back of the eye (retina).
Paradox:
Photoreceptors detect light but are also damaged by too much light.
Light is toxic to them in excess.
The pupil’s adjustment helps protect these cells from bleaching or death.
Lens Function
The lens focuses incoming light onto the retina.
Purpose: To create a sharp image exactly on the fovea — the most sensitive spot.
Vision problems:
Myopia (nearsightedness): Focus falls short of the retina.
Hyperopia (farsightedness): Focus falls behind the retina.
Glasses or contacts correct these by adjusting the focal point.
Retina, Fovea, and Blind Spot
Retina: The light-sensitive surface at the back of the eye.
Fovea:
Small central area with densely packed photoreceptors.
Responsible for sharp, detailed central vision.
Blind Spot:
Area with no photoreceptors where the optic nerve exits the eye.
The brain fills in missing information → we don’t perceive the gap.
Can be demonstrated with simple visual tests.
Vision as an Active Process
Vision ≠ passive reception — it’s an active process.
The eye adjusts, moves, and focuses continuously.
Micro-saccades: Tiny, constant eye movements even when staring fixedly.
Prevents sensory “fatigue.”
If an image on the retina is perfectly stabilized → it disappears!
This shows we only process change, not static input.
example - snow blindness
What is fixating
When we actually just try to get that thing exactly in the middle of our visual field.
(line of sight should be directly into Fovea)
What are eye movements (saccades)
l Our eyes are constantly moving
» Saccades
- If an image stays stabilised on the retina it disappears (the
photoreceptors stop firing) and the brain fills in the missing space - In fact we all have a stationary blind spot where the optic nerve enters the eye, and we are just unaware of it
example - slow blindness
What is a ciliary muscle
The ciliary muscles control the shape of the lens to
accommodate near or far target
When the lens is bulgy, it focuses light closer, and when it is flatter, it tends to focus light further away
When the ciliary muscle is more bulgy - how does it focus light
focus light a little bit close
When the ciliary muscle is flat- how does it focus light
focuses light further way
What is tirangulation ( Vergence and stereopsis)
The two eyes converge to produce two different but aligned images of the same target
When the two eyes view the same object from slightly different positions, each eye receives a disparate (different) image.
The difference between the two images (binocular disparity) provides precise information about an object’s distance.
The brain uses this disparity to estimate depth automatically — it doesn’t calculate geometry consciously but has evolved to interpret the cues accurately.
Processing Light at the Back of the Eye (The Retina)
The Retina is located at the back of the eye and contains photoreceptors (rods and cones).
It’s not just a passive screen — significant visual processing happens here before signals even reach the brain.
The retina performs early neural processing, filtering and protecting light signals before they’re sent to the brain — showing that vision begins in the eye, not just the brain.
What is a cone (located in retina)
Detect color and fine detail.
Densely packed in the fovea (center of the retina).
Work primarily in bright light (daylight ‘photopic’ vision
Combined activation allows perception of the full color spectrum (trichromatic vision)
What is a Rod (located in retina)
Black and White vision
Highly sensitive to light intensity, not color.
Specialized for night vision and peripheral awareness.
Located mostly outside the fovea.
What are rods and cones?
Two different types of photoreceptors
How Information Travels Through the Retina
- Photoreceptors (rods and cones)
These are the first cells to detect light.
They turn light into electrical signals. - Bipolar cells
Each photoreceptor passes its signal to a bipolar cell.
Bipolar cells act as messengers, carrying signals from the photoreceptors to the next layer. - Ganglion cells
Bipolar cells then send their information to ganglion cells.
Ganglion cells collect and combine information and send it out of the eye via the optic nerve to the brain. - Horizontal and amacrine cells (the connectors)
Horizontal cells connect photoreceptors and bipolar cells to each other.
Amacrine cells connect bipolar and ganglion cells to each other.
These “side connections” help nearby cells share information, so that what happens in one spot of the retina can influence nearby spots — helping detect patterns, edges, and motion. - Convergence (many to one)
As signals move deeper into the retina, many photoreceptors feed into fewer ganglion cells.
On average, about 126 photoreceptors connect to 1 ganglion cell.
This means the retina is compressing and simplifying information before sending it to the brain — this is called information reduction.
Overview of Retinal Physiology
- Receptor cells synapse with bipolar cells.
- Bipolar cells synapse with ganglion cells.
- Horizontal cells connect different receptor or different bipolar cells.
- Amacrine cells connect different bipolar or different ganglion cells.
- These connecting cells allow events at one location to influence events at another.
- Massive convergence as we move deeper into the retina: on average 126 receptors connect to 1 ganglion cell.
- Process of information reduction
In short –> Light hits receptor cells → signals are passed and refined through layers (with help from connecting cells) → the brain receives a simplified but meaningful version of what you see.
What is the gangilion cell and what does it do?
So the ganglion cell acts like a lens, capturing an image over an area of the retina
» ‘receptive field’
- In a primitive way, the ganglion can also enhance the image in its local area.
Simple explanation: it tells you whether anything is happening in that particular part of the visual field. And when you’ve got a whole bunch of these that are kind of overlapping with one another, together they can give you a very strong impression of what’s happening in the visual field. Um As, as I say, they capture information in their receptive field, but they can also act to enhance the information in a particular location.
