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Vision. The Visible Spectrum. Light is electromagnetic energy. One nm = one billionth of a meter. Properties of light. hue – determined by wavelength. saturation – relative purity of light. brightness – variation in intensity. Wavelength and Frequency.

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The Visible Spectrum

Light is electromagnetic energy.

One nm = one billionth of a meter


Properties of light

  • hue – determined by wavelength.
  • saturation – relative purity of light.
  • brightness – variation in intensity.

Wavelength and Frequency

As wavelength increase, frequency decreases


The Human Eye

In order to see things in greatest detail our eyes are moved so that the object being looked at falls on the fovea.

Fovea is a central portion of the retina with the greatest visual acuity.

rods and cones contain photopigment that provides input to bipolar and horizontal cells.
  • photoreceptors and bipolar cells do not produce action potentials – instead release neurotransmitters to the ganglion cells.
  • ganglion cells connect with the optic nerve.

Blind spot

Optic disk – where the optic nerve joins the retina – transmits retinal information to the occipital lobes


Blind spot

Close your LEFT eye and move head closer to or further away from the screen until

the central red circle disappears – always fixate the CROSS.








primary geniculastriate pathway
retinaPrimary Geniculastriate Pathway

optic nerve

optic chiasm

optic tract

dorsal lateral geniculate nucleus

optic radiations

striate visual cortex


Lateral Geniculate Nucleus

The LGNd has six layers each of which gets independent input from either the left or the right eye but not both.

There are two major classes of projections, parvocellular (small) and magnocellular (large) projections(known as the P and M pathways).


Large ganglion cells

Small ganglion cells



Colour insensitive

Colour sensitive

Large RFs

Small RFs

Fast, transient

Slow, sustained

High contrast sensitivity

Low contrast sensitivity

Magnocellular Parvocellular


Primary Visual Cortex

The LGNd projects to primary visual cortex (striate cortex or area V1) in the occipital lobe.

The magno and parvo projections are still somewhat segregated in V1.

visual receptive fields
receptive fields of retinal ganglion cells correspond to specific regions in space – hence a retinotopic map of the world in the occipital cortex.
  • receptive fields in visual cortex also respond selectively to other stimulus properties (e.g., orientation, brightness).
Visual receptive fields
centre surround organization
Centre – surround organization
  • tuning – different types of cells are “tuned” to respond to different aspects of visual information
  • e.g., brightness, location, direction of motion, colour etc…
coding information at the retina brightness
centre / surround organization
  • ON, OFF and OFF/ON cells
  • ON/OFF cells project primarily to the superior colliculus (midbrain)
  • the SC is important for directing reflexive saccades
Coding information at the retina - brightness
coding information at the retina colour
trichomatic sensitivity AND colouropponency
  • red – green
  • blue – yellow
  • on/off surround organization
Coding information at the retina - colour

Yellow ON

Blue OFF



Red ON

Green OFF

Green ON


adaptation negative afterimages
after staring at the green Canadian flag you see a red one because the “green” component of red/green cells has adapted to the stimulus.
  • some red/green cells are inhibited for a long period.
  • when looking at neutral light (white light) these cells “rebound” due to the absence of inhibition creating the afterimage.
  • Big Spanish Castle
  • can get afterimages for motion – waterfall illusion .
Adaptation – negative afterimages
striate cortex
6 layers (bands or striations).
  • input from magno and parvocellular information processed at layer IV.
  • disproportionate representation of the fovea (brain would weigh over 30,000 pounds (≈13,600 kg) if the whole visual field had as many neurons dedicated to it as are dedicated to the fovea!!!).
Striate cortex
orientation and movement
cells in striate cortex sensitive to specific orientations.
  • simple cells – opponent system.
  • complex cells – no inhibitory surround – direction specific movement detectors (also in MT).
  • cells organized in columns.
Orientation and movement
spatial frequency

high spatial


  • many of the cells in striate cortex are actually tuned to different spatial frequencies.
  • everything you see in the world can be described in terms of spatial frequency.
Spatial frequency

low spatial


information not lost at low spatial frequencies
Information not lost at low spatial frequencies

Gender and can still be extracted from the low frequency image (right) but identity requires the high frequency image (left).

modularity in vision
Different “modules” sensitive to different visual processes
  • V4 – colour
  • MT – motion
  • FFA – face perception
  • PPA – place recognition
  • IT – object recognition
Modularity in vision

Review Questions

1) A unique feature of the fovea is that it

A) contains mostly rods.

B) contains mostly cone photoreceptors.

C) is devoid of photoreceptors.

D) mediates vision in dim light.

E) has very poor acuity.

2) The reason for a "blind spot" in the visual field is that

A) rods are less sensitive to light than are cones.

B) blood vessels collect together and enter the eye at the blind spot.

C) the lens cannot focus all of the visual field onto the retina.

D) retinal cells die with age and overuse, resulting in blind spots.

E) there are no photoreceptors in the retina where the axons exit the eye.


3) Action potentials in the visual system are first observed in the

A) bipolar cells.

B) horizontal cells.

C) ganglion cells.

D) photoreceptors.

E) axons leaving the internal surface of the retina.

4) Select the correct sequence for processing of information in the primary visual pathway.

A) Retina - > dorsal lateral geniculate (DLG) -> striate cortex

B) Retina -> striate cortex -> extrastriate cortex -> inferior temporal cortex

C) DLG -> retina -> striate cortex -> primary visual cortex

D) Retina -> DLG -> inferior temporal cortex -> amygdala

E) DLG-> frontal cortex -> amygdala -> extrastriate cortex

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