Notes

Computation in Neural Systems: Biological and Computational Vision

The Visual Pathways

There are many structures in the visual system and the majority of them does not give rise to visual perception. We will discuss the conscious vision system.

The retinocortical pathways give rise to conscious vision:

1. Visual field (approx. 180°). The same point is projected onto the nasal and the temporal part of the retina. The retinal image is inverted and reversed right to left compared to the visual field. The less saturated parts (colors) of the visual field are those areas only seen by one eye.
2. The axons of retinal ganglion cells form the optic nerves.
3. At the optic chiasm, axons from the temporal halves of each retina continue into the optic tract on the same side. Axons from the nasal halves cross to the optic tracts on the opposite side. The left hemisphere is analyzed in the right brain hemisphere and viceversa. While the fovea region is represented in both hemispheres.
4. Most axons in the optic tract terminate in the lateral geniculate nucleus.
5. Some axons in the optic tract terminate in the superior colliculus.
6. Axons are sent back and forth between the lateral geniculate and the striate cortex via the optic radiations.
7. Most of the primary visual cortex is on the medial surface of the human brain. It is buried into the calcarine sulcus. And everything below the calcarine sulcus processes the lower quadrant of the visual field.
8. An especially large proportion of primary visual cortex represents the foveal region.
9. The left primary visual cortex gets input from both eyes, but only from the right visual field.

The Human Eye

The Optic Nerve represents a "blind spot" as there are no photoreceptors in its location. However, the brain compensates by "filling" the missing information. The fovea is involved in focused vision and allows only a few degrees of visual angle.

Basic Retinal Circuitry

The retina is the part of the eye that transforms light energy into electrical signals interpretable by the brain (AP). The light arrives from the bottom and passes through all these other cells before reaching the photoreceptors (receptor terminals), so all the circuitry before the photoreceptors has to be transparent.

Light is a noisy source of information (photon noise), but also the retina is a noisy system. Indeed, at each level of processing some noise is added. The brain is capable (via compensation) to process reliably visual information coming from unreliable components featuring noise. The action potentials generated by the retina are then projected through ganglionic cells axons to the lateral geniculate nucleus (LGN) of the thalamus.

The Lateral Geniculate Nucleus (LGN)

From this cross-section frame of LGN in a primate, it is possible to notice that it has a layer structure, where each layer receives input from one eye (three layers per eye), hence each layer is "monocular". The LGN presents Magnocellular (bottom two layers, bigger cells), Parvocellular (top four layers) and Koniocellular (white layers, smallest cells) layers of cells. Magnocellular layers do not care about the color, but rather about light intensity differences. They are extremely contrast sensitive. Parvocellular layers are less contrast sensitive, but they are sensitive to color and are more capable of discriminating fine details than their Magnocellular counterparts. Finally, Koniocellular layers deal with short wavelength information. Hence, there is an anatomical segregation according to the information and the eye from which it is coming from. Notice that there exist two LGN, one for each hemisphere. (Computation done by ganglionic cells is very similar to the computation performed by LGN cells).

Retinotopic Organization of the Brain

The retinotopic organization implies that two points close in the image are represented closely also in the neural representation (Retina, LGN and V1). This peculiarity is common to most animals. Outside of V1, this retinotopical organization is not respected.

The Striate Cortex (V1) - Hubel & Wiesel

The Striate Cortex presents a thick black line, which represents the layer 4 of V1, i.e., the layer where all the connections from LGN are received. In the dark picture: Cat V1 that has been labelled with radioactive label injected in one eye of the cat that has been transported all the way to V1. We can observe that there is an alternation between cells in L4 that receive from one eye and from the other, via the LGN. However, the vast majority of cells in V1 are binocular, even if in L4 the separation of ocular dominated cells is more evident.

You seem the same thing on the surface of L4 (monocular columns):

However, as soon as you get out of L4 (input layer), the cells are not monocular anymore, but rather binocular. Although receiving inputs from both eyes, most of these cells present ocular dominance, i.e., one eye induces an higher excitation of the cell. These cells are organized in ocular dominance columns.

These binocular property of V1 cells outside L4 is important to compare signals from both eyes, which in turns allows depth perception.

About half of the area of the cortex is dedicated to vision (because we are visual animals), however not all animals are visual animals, for example mice have more developed olfactory areas compared to visual ones. Some of these areas are only involved in visual perception and processing, while other are multi-sensory areas that combine signals from different sensory systems.

Each of these areas have a representation of the visual field, some a full one while others only a partial one. For example, V2 has a representation of the visual field which is almost retinotopic. These regions are organized in a hierarchy (right picture), which shows the connections between the areas. The connections between ganglionic cells and LGN are unidirectional, while the connections from LGN onwards are bidirectional (which have been distinguished in feedforward and feedback connections). V1 is strongly connected to V2 and V4, V2 is strongly connected to V4 and V3. The striate cortex of monkeys is very similar to the one of human. All the regions in the left of the hierarchy are part of the parietal lobe, while the right side is part of the temporal lobe. This two part represents respectively the What and Where Pathways.

Two Cortical Functional Streams - Ungerleider & Mishkin, 1982

These two pathways are doing different things and it has been found through lesion studies. Monkeys with parietal cortex lesions were not able to perform tasks associated with object localisation, but they were capable of object identification. On the contrary, monkeys with temporal cortex lesions were still capable of object localization but were defecting of object recognition skills. Hence, the conclusion that temporal and parietal cortices are both involved in processing visual information, but they have distinct roles. The temporal cortex is primarily involved in object discrimination, while the parietal cortex is involved in landmark discrimination. Visual Agnosia: Inability to recognize objects, often associated with face recognition problems (Prosopagnosia). In the case of parietal cortex lesions, it is often associated with neglect symptoms, i.e., patients tend to ignore a complete visual hemisphere.

The Receptive Field (RF)

The Receptive Field of a cell is the region of visual space in which light can affect (increase or decrease) the cell's firing frequency. The further we go away from the retina, the wider the receptive fields of the cells increase. Ganglion cells do not receive light directly, they receive signals from the photoreceptors. The photoreceptors projecting to a single ganglion cell are a round area of the retina. Half of these receptive fields present an On-center surrounded by an Off-region, while the other half presents an opposite. The On-center fires to increase in light intensity, while Off-surround regions fire to decrease in light intensity.

Cortical Receptive Fields Orientation-sensitive cortical cell. This cell responds strongly only when the stimulus is a vertical stripe. Due to the shape of cortical receptive fields, the stimuli that strongly excite the cells are bars rather than spots of lights. They are edge-detectors.