Physical measurement of light waves
We can only perceive a small portion of light (Visible spectrum - ~380nm (violet) to 720nm
Structure of the Eye:
Normal sight: Image perfectly focused on fovea
Near-sightedness: Eye too long: Image focused in front of fovea. Concave lens corrects this.
Farsightedness: Eye too short: Image focused behind fovea. Convex lens corrects this.
Major Components of the Visual System
Eye, Optic nerve, Optic chiasm, Lateral geniculate nucleus, visual cortex are major components
of the Visual system.
The image projected onto the retina and received by the visual cortex is upside-down and
How light becomes sight:
Light goes through Ganglion Cell Layer Bipolar Cell Layer Photoreceptor Layer (Cone and
Rod) and detected at the Back of the EyeTransduction of Light by photoreceptors:
Photo strikes photo-pigment Photo pigment split, chemical reactions produce message
Message is transmitted to bipolar cell Message transmitted to ganglion Message is sent to
In what order does neural transmission happen in the retina layers? Answer:
Photoreceptors, bipolar cells, ganglion cells
In what order does light pass through the retinal layers? Answer: Ganglion cells, bipolar
A huge amount of cones at the fovea. (Cones detect colour) few cones in periphery.
There are more rods at the periphery, no rods at the fovea and the Optic disk. Blind spot at the
High light intensity: Cones are better
Low light intensity: Rods are better
Animal has electrode connected to a single cortical visual neuron and animal is watching
something on the screen.
Microelectrodes can be used to record the firing activity of a single cortical visual neuron
Cortical neurons have a background rate of firing (impulses/sec)
This rate of firing can increase or decrease in response to a stimulus
Receptive Field refers to that part of the visual field that the neuron responds to.
The general pattern of the receptive field can be recorded at each level of a sensory
system (example: from a peripheral sensory receptor, the thalamus or the cortex)
On-Center Cell: If you shine a light in the center, you get an on response. If you shine a light in
the surround, you get an off response.
Off-Center Cell: Opposite of On-Center Cell.
This on-off center surround response pattern can result in the local inhibition or reduction of
visual response in certain patterns.Hubel and Weasel (Spelling?): Microelectrode into visual cortex of animal. Stimulus: a line of a
particular slope falling on a particular region on the region. Response to stimulus recorded.
There was little to no firing for horizontal line, big response to a vertical line. This is only for one
cell! Different cells do different things.
Simple Cells (Line detectors): In the cortex, many center cells surround cells may converge on a
Visual Processing in the Cortex:
Hyper complex cells: Respond to additional features such as line length or particular types of
In some other regions of the brain, there are cells that are tuned for very complex stimuli, such
The brain decomposes images and processes different qualities separately (Colour, motion,
Different regions of the primary a