Chapter 4: The Lateral Geniculate Nucleus (LGN) and Primary Visual Cortex (V1)

Introduction

In our quest to unravel the intricacies of visual perception, we have journeyed through the complex layers of the eye and ventured into the neural pathways that translate light into vision. In this chapter, we embark on a closer examination of a critical relay station in this intricate visual network: the Lateral Geniculate Nucleus (LGN) before moving to a discussion of neural processing in the Primary Visual Cortex (V1 or striate cortex).

The Lateral Geniculate Nucleus (LGN)

Before delving into the specifics of retinal connections, let's reintroduce the LGN, a key component of the thalamus. Nestled near the center of the brain, the thalamus acts as a gateway for sensory information, funneling it to various cortical regions for further processing. When it comes to vision, the LGN plays a pivotal role in this relay process.

Retinal Connectivity to LGN: Nasal and Temporal Portions

To comprehend the intricate connections between the retina and LGN, it's crucial to divide the retina into nasal and temporal regions:

• The nasal portion of the retina is situated closer to the nose and projects information to the contralateral hemisphere. In simpler terms, the nasal portion of the right eye sends signals to the left hemisphere, while the nasal portion of the left eye connects to the right hemisphere.

• In contrast, the temporal portion of the retina is closer to the temples or sides of the head and follows an ipsilateral pathway. This means that the temporal portion of the right eye projects information to the right hemisphere, and the temporal portion of the left eye sends signals to the left hemisphere.

This unique connectivity pattern explains why information shown in the left-visual field will be transmitted to the right hemisphere and information from the right visual field will end up in the left hemisphere.

Figure 4.1

LGN Layers and Pathway

With the retinal connection in mind, let's explore the LGN's internal structure and how it processes visual information. The LGN consists of six distinct layers that are connected to the retina in a specific format:

1. Layers 1, 4, and 6 predominantly receive input from the nasal portion of the retina (of the contralateral eye).
2. Layers 2, 3, and 5 primarily receive input from the temporal portion of the retina (of the ipsilateral eye).

This division ensures a systematic processing of visual information, with each LGN layer specialized in receiving data from specific regions of the retina.

Receptive Fields in LGN

Just as we've encountered in the retina, the cells within the LGN have receptive fields— specific areas in visual space where light stimulation elicits a response in a given neuron. Within the LGN, these receptive fields follow a center-surround arrangement, similar to those observed with ganglion cells.

Magnocellular and Parvocellular Layers

As we delve deeper into the LGN's layers, we encounter two distinctive cell types: the magnocellular and parvocellular layers. These cells contribute to diverse aspects of visual processing:

1. Magnocellular Layers (Layers 1 and 2): These layers receive input from the M ganglion cells, which are larger and sensitive to motion. The magnocellular layers are primarily involved in processing motion-related information and the location of objects in space.
2. Parvocellular Layers (Layers 3, 4, 5, and 6): These layers receive input from the P ganglion cells, which are smaller and play a more significant role in color perception and perception of fine details.

The Primary Visual Cortex (V1)

The information processed within the LGN serves as the foundation for higher-level visual processing in the primary visual cortex, known as V1. Located in the occipital lobe at the back of the brain, V1 is where the initial shaping of our visual perception takes place. Here, the segregated magnocellular and parvocellular inputs continue to influence how we perceive the visual world.

To comprehend the processing within V1, we must first consider the nature of the receptive fields for cells in this part of the brain. These are distinct from the receptive fields found in ganglion cells or LGN cells, as V1 cells exhibit a specific response pattern. Instead of a center-surround arrangement, V1 cells respond most effectively to edges, particularly lines with specific orientations. In essence, V1 cells are finely tuned to detect edges in the visual scene. There are three primary types of cortical cells identified by Hubel and Wiesel that are instrumental in this process.

1. Simple Cortical Cells: These cells respond to edges with specific orientations. They are highly selective and will fire in response to an edge at a particular location and orientation. The orientation preferences can vary across different simple cortical cells, covering all possible angles. The firing pattern of these cells is primarily static, and they are not sensitive to the movement of the edge.
2. Complex Cortical Cells: Unlike simple cortical cells, complex cortical cells require motion for activation. They respond to edges with specific orientations, but this response is contingent upon the edge's movement in a particular direction.
3. End-Stop Cells: These cells are specialized in responding to lines of a specific length, and the length of the line matters in their activation. Additionally, some end-stop cells are sensitive to corners or angles in the visual input.

Furthermore, it's crucial to note that cells within V1 are arranged in a retinotopic map, which means that the spatial arrangement of cells in V1 corresponds to the spatial layout of the retina. This retinotopic map allows V1 cells to encode information about the specific location of the stimulus in the visual world from which the information originated. Additionally, the number of V1 cells dedicated to processing information from the fovea, the small central part of the retina responsible for high acuity vision, is greater than that for other retinal areas even though these peripheral areas of the retina are larger than the fovea. This illustrates the concept of cortical magnification. Cortical magnification, refers to the disproportionate representation of different regions of the visual field in the primary visual cortex (V1) of the brain. This means that certain areas of the visual field, particularly those that are of high importance for visual perception, are allocated more cortical space or neurons than other less important regions.

Interestingly, V1 plays a pivotal role in our conscious experience of vision. Damage to V1 can result in a person reporting blindness, even though other visual pathways may remain intact and allow the person to perceive aspects of the visual world, like movement. Most of the visual information from the eyes is routed to the LGN and subsequently to V1. However, some information goes to the superior colliculus located at the top of the brainstem. From there, an alternate pathway ascends to the dorsal processing pathway. This means that an individual with V1 damage may still exhibit "blindsight," wherein they report blindness but retain the ability to perceive and respond to motion.

This overview of visual processing in V1 provides a foundation for understanding the intricate mechanisms that underlie our perception of the visual world. The distinctions among simple, complex, and end-stop cells, as well as the retinotopic map, cortical magnification highlight the complexity and sophistication of the human visual system, and the concept of blindsight highlights the importance of early visual brain areas in conscious perception.

2DG and column-like organization

The 2-DG technique is a method used to map functional activity in the brain, like the primary visual cortex (V1). This method is based on the use of a radioactive glucose, 2- deoxyglucose, to trace areas of the brain that are most active during a given task or in response to specific stimuli. The idea is that neurons that are active during the experiment uptake both glucose and the 2-DG. After the administration of 2-DG and the completion of the experimental condition (such as exposure to visual stimuli), the brain tissue is processed for autoradiography. This involves slicing the brain thinly and placing the slices on photographic films. The areas of the brain that took up more 2-DG appear darker on the film. When the 2-DG technique is applied while an animal is exposed to visual stimuli (like oriented lines), it reveals a pattern of stripes or bands in V1. These bands correspond to cortical columns that are selectively responsive to specific orientations (or to one eye in the case of ocular dominance stripes). This supports the idea that V1 has a column-like organization.

Neural Fatigue

If you stare at the large red rectangle on the left for 1 minute and then shift your attention to the small red dot on the right (see figure below) you will experience an illusion where the lines on the right do not appear equally spaced. Instead, the top bars on the right will look more closely spaced and the bottom bars on the right will look mores spread out.

In the top area the cells that are most active (have the greatest response) are cells that respond to low spatial frequency. In the bottom area the cells that are most active (have the greatest response) are cells that respond to high spatial frequency.

Then, when you shift your attention to the small red dot the cells that had been most active are now fatigued. This neural fatigue is depicted with the red arrows in the figures. So, when you look at the medium spatial frequency bars you get an illusion. Now the top bars look more closely spaced and the bottom bars look mores spread out.

Conclusion

In conclusion, our exploration of the lateral geniculate nucleus (LGN) and primary visual cortex (V1), has unveiled the remarkable intricacies of visual processing. We have traced the journey of visual data from its inception in the eye through the LGN, where initial stages of filtering and organization occur, and finally to V1, where the foundation for our conscious perception of the visual world is laid. Within V1, we have discovered a complex tapestry of cortical cells, each finely tuned to detect specific visual features, be it edges, motion, or unique configurations. The concept of a retinotopic map and cortical magnification underscore the precision with which V1 encodes spatial information as well as information of high importance. Furthermore, the enigmatic phenomenon of blindsight highlights the profound role V1 plays in our conscious visual experience.