Vision

Light is electromagnetic energy.One nm = one billionth of a meter

The Visible Spectrum

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

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.

Photoreceptors• 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 spotOptic 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 untilthe central red circle disappears – always fixate the CROSS.

Visual Fields

temporal nasal

VISUAL FIELD

RETINA

temporal nasal

Primary geniculostriate visual pathway

Primary Geniculastriate Pathway

retina

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).

Magnocellular Parvocellular

Large ganglion cells Small ganglion cells

Centre/Surround Centre/Surround

Colour insensitive Colour sensitive

Large RFs Small RFs

Fast, transient Slow, sustained

High contrast sensitivity Low contrast sensitivity

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.

Retinotopic map in striate visual cortex

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).

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 - colour

Coding information at the retina - colour• trichomatic sensitivity AND colour opponency

• red – green• blue – yellow• on/off surround organization

Yellow ONBlue OFF

Blue ONYellow OFF

Green ONRed OFF

Red ONGreen OFF

Coding information at the retina - colour• impossibility of seeing a redish green colour!

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 .

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!!!).

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.

Spatial frequency

• 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.

low spatial frequency

high spatial frequency

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

1) A unique feature of the fovea is that itA) 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 thatA) 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.

Review Questions

3) Action potentials in the visual system are first observed in theA) 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 cortexB) Retina -> striate cortex -> extrastriate cortex -> inferior temporal cortexC) DLG -> retina -> striate cortex -> primary visual cortexD) Retina -> DLG -> inferior temporal cortex -> amygdalaE) DLG-> frontal cortex -> amygdala -> extrastriate cortex

Recommended web page

http://www.tutis.ca/Senses/index.htm