NATS 1860 Note 14
Visual Cortex: Objects and Actions
- Review of Retina
- Lateral Inhibition and edges
- Neurons in visual cortex
- Orientation and motion
- Migraine visual auras
- Cortical pathway for action.
- Cortical pathways in using vision for object recognition
- Effective chemical THC is a cannabinoid – it’s effective because we have
receptors in several different areas of our brain (protein molecules in nerve
o They are present in the basal ganglia and the cerebellum are involved
in the coordination of motion
There are some people who walk as if they're drunk when
they’re not drunk
o It affects aspect of memory because it affects the hippocampus
o It affects your appetite as the hypothalamus (neurons that release
chemicals into blood to regulate internal organs
- There are receptors in the brain that sense the different levels of chemicals in
the blood in order to release different chemicals into the glands of the body.
Results of Experiment one
- Orientation discrimination: 2o to 3o – orientation selective neurons in V1
o This is good because this shows that there are cells sensitive to
different orientation in our primary visual cortex
- Why were there eight alternatives instead of just two?
o It was to make it harder in order to confuse the students, thus to
reduce the probability of corrective guessing
- Eye movements: two types. You can’t voluntarily decide which you’re going
o One: when there’s something moving in the world you can track it
(smooth pursuit): it was for the tracking motion of M cells.
You need to process the motion to survive.
o Two: jumps (saccades): inspect areas with detail in static scenes of P
When there isn’t anything or nothing is moving, to inspect the
world your brain uses P cells, which are sensitive to fine detail,
to pick an area with high detail and jumps to examine it with
- Face discrimination
o We learned about 6 of 8 faces studied
o We correctly rejected about 6 of 8 new faces o We automatically learned the average of the faces that we saw, and
learned it better than the faces we actually experienced.
We don’t automatically learn when we’re learning things, what
we’re actually learning most effectively is the average of that
Review of the Retina
- Two types of photoreceptors: rods (dim light) and cones (for bright light and
- There are three types of cone photopigments: red, green, and blue.
o Colour blindness (heavily male dominated), are cases where one
pigment is missing.
- The retina is responsible for light adaptation (negative afterimage) this
results from the fact that some retinas are greatly stimulated
- The output of the retina has two ganglia cells
o The M ganglion cells – fast rest response, large receptive field, and no
colour. They see levels of white, gray, and black
o The P ganglion cells: slow response, small receptive field, and colour
HD vision. They pick up fine detail.
- Ganglion cell axons to V1: left side of the world to the right V1
Vanishing of Stationary Images
- When we look at patterns for a long time, sometimes the patterns look like
- Neurons adapt completely to images that are stationary on retina
- To do this properly, you need a video camera in the laboratory and a
computer to find out exactly how the eye moves
o You can get the same effect using blurred image in periphery: large
Retinal Receptive Field
- Record from the retinal ganglion cells
o Shine spot of light at different points on retina to figure out what
stimulates or inhibits it
- Receptive field: the area of the retina within which a spot of light changes the
firing rate of the ganglion cells
o All receptive fields have both excitatory and inhibitory regions
Orientation Selective V1 Neuron
- Cells in the visual cortex: if information from 3 adjacent ganglion cell
provides excitation to a single, that cell is going to respond best to a
o That cell will respond best to vertical line of light.
o For other orientation of bars, there isn’t as much action potential
because it doesn’t fill all of these.
- P cells main input to oriented receptive fields. - Any given cell will respond best to a particular orientation or location on
- Orientation cells sense bars, lines, and edges.
o This is important because the way that an object (face, PC, or car) is
defined as a pattern of lines and edges
o If there were no lines or edges, there would be no lines to see
- Our brain is picking out the most important information when defining
Orientation Selective V1 Neurons
- There are about 12 different orientations represented in V1
- Furthermore, each point in an image is presented by all 12 orientations.
o There more than 12 times as many nerve cells (100 times) that there
are in our retina sending information to the cortex
- They detect lines and edges that form objects.
- These cells are genetically programmed as well as early experienced
o You’re born blind, so you can’t see very far.
o Cells coming in that are active at the same time form synapses that
work on the same time.
Combining Orientation Cell Responses
- What are these images?
- Vertical orientation cell responses – guess what the images are. It’s not much
- Horizontal orientation cell responses
- Afterwards, the two will be combined to recognize the two
o Combining even two orientations is enough information to usually
recognize an object
- This is
- Our brain reorganizes the information into the different orientations,
providing an efficient way of defining objects
o An object is entirely uniform (consider the white area on the screen)
o There’s no valuable information, and the brain needs one piece of
information to say how bright it is
o All the important information in the edges
- It is possible for a person to have damage to a specific orientation
o There’s a condition where at birth, the cornea is not part of a sphere, it
curves more into one direction than the other. Therefore, vertical
orientations will properly work, and horizontal will work.
- This is about 7 times more common in women than in men until menopause
and then men start having migraines.
- A migraine aura is a visual hallucin