1. The Visual System
Into. to Neurobiology
2010

2. Types of Retinal Ganglion Cells
M (magnocellular) ganglion cells
Input primarily from rods.
Achromatic (black and white center-surround receptive fields).
Constitute about 10 % of the ganglion cell population.
Larger receptive fields (low spatial frequencies).
Sensitive to the directions of visual motion. High temporal frequencies.
Sensitive to low contrasts (saturate when the the contrast is high).
Most have linear response properties.

3. Types of Retinal Ganglion Cells
P (parvocellular) ganglion cells
Input primarily from cones.
Chromatic.
Constitute about 70 % the ganglion cell population.
Small receptive field centers = high spatial frequencies (resolution).
More sensitive to the form and fine details of the visual stimuli.
Respond poorly to low contrast but do not saturate at high contrasts.
Low temporal frequencies.
Linear response properties.

4. Types of Cat Retinal Ganglion Cells
X Cells
Small receptive fields
Linear response properties (positive transient response to light, adaptation at steady state, drop in firing rate in response to dark).
Good at detecting contrasts.
Y cells
Large receptive fields (X cell * 3)
Non-linear response properties (positive transient response to light, zero steady state response, no response to dark).
On / off cells (respond only to light or only to dark).
Good at detecting motion.
X cell
Y cell
Stimulus
Receptive
Field

5. The Lateral Geniculate Nucleus (LGN)
Located in the Thalamus. Receives convergent input from several optic nerve fibers and feedback from V1.
Left-right, top-bottom organization from Ganglion cells is maintained.
6 layers: layers 1, 4, 6 = information from the contralateral eye ; layers 2, 3, 5 = ipsilateral eye.
Magnocellular/Parvocellular distinction.
Topographically organized projection to V1.
Circular, center-surround receptive fields similar to those of Ganglion cells, but more finely tuned. .

6. Primary Visual Cortex - V1
The primary visual cortex (= the striate cortex = Brodmann area 17 = V1) is divided into 6 layers, differing in cell type and connections with other brain areas.
LGN inputs primarily received at layer 4
- parvocellular to a lower subdivision (layer 4cβ) and magnocellular to an upper subdivision (layer 4cα).
Function: highly specialized for processing information about static and moving objects and excellent in pattern recognition (classification).
V1 outputs to 2 pathways: the dorsal stream and the ventral stream.
dorsal
ventral

7. V1 Organization
Retinotopic map: a very well-defined map of the spatial information. The fovea is represented over a large portion = cortical magnification.
Smallest receptive field size of any visual cortex region.
Occular dominance columns (L4).
Most cells are binocular.
Orientation (most cells) and direction (25-35% of cells) selectivity.
V1 receives feedback connections from higher regions, creating complex responses (for example, to context).

8. V1 Organization : Ocular dominance columns
Input from ipsilateral and contralateral eyes is separated in adjacent columns in layer 4
Adjacent columns make up one hypercolumn
Within each ipsilateral and contralateral ocular dominance column all orientation columns are represented
Output from monocular cells in layer 4C converges on binocular cells in other layers

9. V1 Organization
Another kind of columns was revealed using a stain called cytochrome oxydase.
They are called “blobs”, are spaced at regular intervals and run through layers II, III, V, and VI.
These blobs are arranged in lines, centered on an ocular dominance band in layer 4C. Between the blobs are areas called interblobs whose neurons do not have the characteristics of these blobs.
The blob cells are sensitive to the wave length of light (color) , they are monocular, and they do not have any orientation selectivity; instead, they have circularly symmetrical receptive fields.
Some blob cells have the same center-surround color opposition structure as the P ganglion cells, where this pathway originates.

10. Functional properties of V1 cells
Simple Cells:
Receive direct input from the LGN. Respond to points or bars of light in a particular orientation and location.

11. Cortical Receptive Fields
Simple Cells: “Line Detectors”
© Stephen E. Palmer, 2002

12. Cortical Receptive Fields
Simple Cells: “Edge Detectors”
© Stephen E. Palmer, 2002

13. Constructing a Simple Cell

14. Functional properties of V1 cells
Complex cells:
Respond to bars of light in a particular orientation (but in any location), moving in a specific direction.

15. Constructing a Complex Cell
Detection of motion:
A. When the axons of many simple cells with the same orientation and adjacent but not identical receptive fields converge on a complex cell, it can detect movement from the differences between these fields.
B. Temporal summation: if a cell that has already been excited once is excited again shortly afterward, its membrane is still depolarized enough that a stimulus that would not normally suffice to trigger another action potential can do so. Thus, when a moving light beam activates several simple cells in succession, the temporal summation of the stimuli applied to them causes the complex cell to respond to the movement

16. The Visual Cortex V1 – primary visual cortex
V2 – prestriate cortex. First association area. Receptive fields: orientation, spatial frequency, color. Has dorsal and ventral regions
V3 – cells similar to V2, but some are sensitive to color and movement. Has dorsal and ventral regions.
V4 – located in extrastriate cortex. First ventral region showing strong attentional modulation. Tuned for object features of intermediate complexity, like simple geometric shapes. Receives info. From blobs and interblobs.
V5 / MT (middle temporal) – located in extrastriate cortex. Perception of motion.
LO – Lateral Occipital complex
FFA - Fusiform Face Area
PPA – Parahippocampal Place Area
STS – Superior Temporal Sulcus
(Pl = places, O = objects, F = faces, P-U = ventral, P-D = dorsal )

17. Dorsal and Ventral Streams
The dorsal, or M-pathway (WHERE) ends up in the parietal cortex.
The ventral, or P-pathway (WHAT) ends up in the temporal cortex.
The two pathways are not totally independent – there are many cross-connections at every level, and information also flows in the reverse direction.

18. Dorsal and Ventral Streams
Ventral visual pathway:
Concious perception, recognition and identification of objects by processing “intrinsic” propertis (shape, color, etc.)
Dorsal visual pathway:
Allows visual-motor control over objects by processing their “extrinsic” properties necessary to handle them (size, location, position and orientation in space)

19. V1 and Beyond: Depth Perception
Depth is analyzed through a combination of monocular and binocular cues.
1. Monocular Cues:
Perspective - The property of parallel lines converging at infinity allows us to reconstruct the relative distance of two parts of an object, or of landscape features.
Relative retinal size – more distant = smaller.
Familiar size - use of previous knowledge.
Loss of detail in distance – more distant = less luminance contrast + accomodation (hard to focus).
Occlusion - "ranking" of relative nearness.
Relative apparent movement as you move your head – more distant = more slowly moving.

20. V1 and Beyond: Depth Perception
2. Binocular Cues: Stereopsis
When an observer fixates on a visual object the image of this object is positioned on corresponding regions of the two retinae.
Human eyes are horizontally separated by about mm (between pupils). Thus, each eye has a slightly different view of the world. Objects more near or far than the fixation point fall on non-corresponding areas on both retinae.
The degree to which the images are non-corresponding (as measured by difference scores in retinal eccentricity for instance) is defined as binocular disparity.
The ability to use binocular disparity to determine the distance of an object from oneself, and its relation to the fixation plane, is called stereopsis, or depth perception.

21. Optical Illusions Geometric Illusions:
Optical illusions give us a better understanding of how human visual perception works.
They demonstrate that what we see of the world is not a simple physical record, like a photograph.
Some of these mechanisms arise in the retina, but most of them result from the way that the images captured with the eyes are reconstructed by the visual cortex.
Geometric Illusions:
Produced by the arrangement of points, lines, and simple shapes in ways that make you misinterpret these elements when you see them. Many geometric illusions involve two objects that are actually identical but look different because of their surroundings.
Zöllner's illusion

22. Optical Illusions Size-relationship Illusions:
The proximity of a test element to larger / smaller inducing elements causes the size of the test element to be underestimated / overestimated.
The result is that though two test elements are identical, they can look different to us, because of the context effect.
The presence of lines suggesting perspective can also create size illusions. Given two objects of equal size, if one of them looks farther away because of perspective, we will perceive it as being larger.

23. Optical Illusions Motion Illusions:
Some images can give the illusion that their elements are moving when you move yourself slightly relative to them.
For other motion illusions, the particular arrangement of the graphic elements in the picture is enough to create the appearance of movement as you look at it, because the pattern makes it hard for your eye to determine the contours of the circle in the center.

24. Optical Illusions Artistic Illusions:
It is not that the human visual system interprets reality incorrectly, but rather that the reality itself is deliberately ambiguous. Using various tricks of drawing, the artist creates an object that looks realistic but could never actually be built in the real world.

25. Optical Illusions Artistic Illusions:
Another type of artistic illusions uses ambiguous cues – the drawing can be interpreted in more than one way.