Chapter 3: Receptors and Neural Processing

Receptors: An Introduction

Rods and Cones: The Key to Visual Perception

At the core of our visual experience lie two essential types of photoreceptors: rods and cones. These receptor cells play a pivotal role in translating light into neural signals, setting the stage for visual perception. Rods, the more numerous of the two, are specialized for low-light conditions, enabling us to see in dim environments. On the other hand, cones, although fewer in number, are responsible for our color vision and provide sharpness in well-lit conditions.

Beyond Vision: Diverse Receptors Across Senses

It's essential to acknowledge that our sensory experiences extend far beyond what we see. In our exploration of sensation and perception, we will encounter various receptors dedicated to different senses. For cutaneous senses, mechanoreceptors like Meissner corpuscles, Merkel cells, Ruffini endings, and Pacinian corpuscles play critical roles. In the realm of taste, taste buds become our receptors for detecting flavors and creating our sense of taste. We will investigate all of these, and more, in the chapters ahead.

A Surprise Discovery: Ganglion Cells and Their Unusual Role

Traditionally, we believed that rods and cones were the sole photoreceptors in the eye responsible for visual perception. However, a groundbreaking discovery in 2002 by Samer Hattar and colleagues introduced us to photosensitive ganglion cells. These unique cells possess intrinsic sensitivity to light, but their role lies beyond vision. Instead, these cells are instrumental in regulating our sleep-wake cycles, responding to light continuously during daylight hours.

The Basics of Light and Perception

The Visible Spectrum: A Limited Perception of Light

Our visual perception is confined to a small portion of the electromagnetic spectrum known as the visible spectrum. Ranging from approximately 380 nanometers to 750 nanometers in wavelength, this narrow band of light encompasses the colors we perceive. Notably, different animals perceive various parts of the electromagnetic spectrum, expanding their sensory horizons.

Debunking a Common Misconception: How We "See"

A pervasive misconception is that something leaves our eyes to interact with the objects we look at. In reality, nothing leaves our eyes. Instead, light bounces off objects and enters our eyes, where a cascade of neural processing and the magic of perception unfolds.

Anatomy of the Eye: The Window of Perception

Navigating the Eye: Cornea, Lens, and Iris

The eye's structure holds vital clues to understanding vision. The cornea, a transparent protective layer, primarily focuses light onto the retina, while the lens contributes the remaining 20% of light focusing. The iris, a colored ring of muscles with a central opening (the pupil), regulates the amount of light entering the eye, adapting to varying lighting conditions. Within the eye, we find two fluid-filled chambers: the aqueous humor and the vitreous humor. The aqueous humor nourishes the iris, cornea, and pupil. In contrast, the vitreous humor, a more extensive fluid chamber, may contain floaters—harmless, transparent cells that can cast shadows in our visual field.

Figure 3.1

The Attraction of Pupils

A fascinating study by Hess and colleagues suggests that people might perceive images more attractively when the subject's pupils (rather than irises) are larger. This peculiar finding hints at the interplay between visual cues and human psychology.

Adaptation and Neural Processing

The Mystery of the Inverted Image

When light passes through the lens, it inverts the image that reaches the retina. Surprisingly, we don't perceive the world as upside down. Instead, our brains learn to correct the inverted input, showcasing the brain's role in shaping our perception.

Adapting to Altered Perception: The Power of Brain Plasticity

Our brains exhibit remarkable adaptability when it comes to perception. Experiments have shown that individuals can adapt to altered visual input, demonstrating the brain's capacity to recalibrate sensory information. For example, if a person wears goggles that alter the visual input people will adapt to this in a relatively short period of time and will misperceive the world for a brief period when the goggles are removed. This adaptability underscores the intricate relationship between our sensory experiences and neural processing.

Filling in the Blanks: The Blind Spot

At the point where the optic nerve exits the eye, there are no photoreceptors, resulting in a blind spot. This visual gap might seem like a flaw in our sensory system, but our brains compensate for it by "filling in" the missing information. This intriguing phenomenon highlights the brain's role in processing incomplete sensory input.

The Aging Eye

Presbyopia: The Lens's Aging Woes

As we age, the lens of the eye stiffens, making it challenging to accommodate or focus on nearby objects. This condition, known as presbyopia, is a common part of the aging process. It often necessitates the use of reading glasses to counter the lens's reduced flexibility.

Exploring the Ever-Changing Near Point

The near point, the closest distance at which we can focus on an object, changes throughout our lives. Younger individuals can focus on objects much closer to them than older individuals. This evolving near point exemplifies the dynamic nature of our visual system over time.

Differences in Visual Acuity and Sensitivity

Visual acuity and sensitivity vary between rods and cones, primarily due to differences in their neural connections and response properties.

Cones:

• Concentrated mainly in the fovea, which is responsible for high visual acuity.
• One-to-one connections with ganglion cells, resulting in better spatial detail (visual acuity).
• Require a more substantial stimulus (brightness) to respond.
• Essential for color vision, with different types of cones responding to different parts of the color spectrum.

Rods:

• Predominantly located in the peripheral regions of the retina.
• Multiple rods converge onto a single ganglion cell, leading to lower spatial detail.
• Highly sensitive to dim light, allowing for vision in low-light conditions (scotopic vision).
• Less involved in color vision, as they are more sensitive to shorter wavelengths of light.

Figure 3.2

The visual information captured by photoreceptors (rods and cones) is transmitted to ganglion cells, which play a crucial role in relaying this information to the brain. Ganglion cells exhibit distinct characteristics and can be categorized into different types. Two primary types of ganglion cells discussed in this chapter are P cells and M cells.

P Cells (Parvocellular Cells):

• Connect to parvocellular layers in the lateral geniculate nucleus (LGN) of the thalamus.
• Smaller receptive fields.
• Sustained response to stimuli, meaning they continuously fire as long as the stimulus is present.
• High sensitivity to color and texture.

M Cells (Magnocellular Cells):

• Connect to magnocellular layers in the LGN of the thalamus.
• Larger receptive fields.
• Transient response to stimuli, responding strongly to the onset and offset of stimuli.
• Specialized for motion detection.

While P cells and M cells are the main focus, there are also K cells that connect to intermediate layers of the LGN. However, we won't delve into the details of K cells in this class.

Receptive Fields

Receptive fields refer to specific regions that influence the firing rate of a neuron. In the context of visual perception, let's explore how receptive fields are determined and what they signify.

Determining Receptive Fields:

To measure a neuron's receptive field, scientists first identify a cell that they want to investigate (e.g., a retinal ganglion cell). In an experiment involving a cat, for instance, the following steps are then taken:

1. Anesthesia and Eye Focus: The cat is anesthetized, and its eyes are directed toward a screen. This ensures that the cat's eyes remain stationary during the experiment.
2. Stimulus Presentation: Various stimuli, often in the form of light, are presented on the screen. Importantly, each point on the screen corresponds to a specific point on the cat's retina due to the stationary eye position. For example, a stimulus at point A on the screen corresponds to point A' on the retina, and so forth.
3. Stimulus Effects: When a small spot of light is flashed on the screen, the response of the neuron is recorded. If the light is flashed in specific areas on the screen and the neuron changes its firing rate then this is recorded as either increasing (excitatory) or decreasing (inhibitory) the neuron's firing rate.
4. Defining the Receptive Field: The receptive field of the neuron is determined by identifying the areas on the screen that, when stimulated, influence the neuron's firing rate.

Receptive Field Characteristics:

When the receptive field has two circular regions where one region is enclosed inside the other, it is categorized as a "center-surround receptive field" because it comprises a center region that responds in one way (excitatory) and a surrounding region that responds in the opposite way (inhibitory). In this specific example, it's referred to as an "excitatory center-inhibitory-surround receptive field."

Center-Surround Antagonism:

The distinct responses of the center and surround regions within the receptive field lead to a phenomenon called "center-surround antagonism." This phenomenon is illustrated when the size of the stimulus presented to the receptive field changes. For instance, a small spot of light presented to the excitatory center of the receptive field causes a small increase in nerve firing, while increasing the light's size to cover the entire center of the receptive field intensifies the cell's response. However, if the light also stimulates the inhibitory surround region then the response rate will decrease. If the two regions (center and surround) are entirely filled with light then the areas might cancel each other out (rather than either an increase or decrease in firing).

Center-Surround Arrangement and Lateral Inhibition

Receptive Field Sizes

In our previous section, we discussed the concept of receptive field sizes. Receptive fields are the areas in the visual field that trigger the activity of a specific sensory neuron, such as a ganglion cell. It turns out that ganglion cells in the periphery have larger receptive field sizes compared to those in the fovea, which have smaller receptive field sizes and this might help to explain illusions like the Hermann grid.

Lateral Inhibition and Perceptual Illusions

Horizontal cells in the retina play a crucial role in lateral inhibition, modulating the signals transmitted to bipolar and ganglion cells. Lateral inhibition enhances edge detection and helps us perceive differences in lightness and darkness within our visual field.

The Hermann Grid: Ghostly Gray Dots at Intersections

Imagine gazing at a grid of black squares that are evenly spaced on a white background, commonly known as the Hermann Grid. As you fixate your gaze on the intersections of these lines, you might notice the peculiar appearance of ghostly gray dots that seem to appear and disappear. This phenomenon can be mystifying at first, but lateral inhibition offers a compelling explanation.

Each intersection you fixate on becomes the center of attention for specific ganglion cells in your retina. These ganglion cells have receptive fields that may include white areas of stimulation or black areas where there is no light. In addition, these receptive fields will differ in size depending on whether the ganglion cell is responding to information at the fovea or information from the periphery.

It is important to understand that in determining whether something appears white or dark gray comparisons are made (i.e., this area of space looks darker than some other area). In the case of the Hermann grid, a ganglion cell that responds to information in the periphery and is centered on an intersection (see A in the figure) will include more white regions than a ganglion cell centered between two squares (see B in the figure). Looking at this example you should see that cell A will have more inhibitory surrounding area stimulated than cell B and for this reason the intersection will appear dark gray (i.e., cell A will fire less rapidly than cell B). However, a ganglion cell that responds to information in at the fovea and is centered on an intersection (see C in the figure) will include the same amount of white area as a ganglion cell with a small receptive field between two squares (see D in the figure). In this situation cell C and D will fire at the same rate and the two regions will not differ in perceived brightness.

Figure 3.4

Perception

Figure 3.5

Mach Bands: Enhanced Edge

Mach Bands are another intriguing perceptual phenomenon that lateral inhibition helps elucidate. These bands consist of alternating light and dark stripes along a gradient in luminance. The bands appear to exaggerate the differences in light intensity at their boundaries, making the edges seem sharper than they truly are.

If the bands are arranged such that the darker region is to the left of the lighter region, as shown in the example here, people will perceive the right- hand side of the darker bar as darker than it truly is, and the left-hand side of the lighter bar as brighter than it truly is.

This can be explained with lateral inhibition. Imagine the dark area has 60 units of light and the light area has 80 units of light. If horizontal cells send 10% of this to neighboring bipolar cells then the middle portion of the dark bar will be inhibited by 12 units (6 units of inhibition from each side), cells that respond at the border will be inhibited by 14 units (6 units from the darker side and 8 units from the lighter side), and the middle portion of the light bar will be inhibited by 16 units (8 units from each side). This will result in the right- hand portion of the dark bar (60-14 = 46) appearing darker than the rest of that bar (60-12 = 48). This will also result in the left-hand portion of the light bar (80-14 = 66) appearing lighter than the rest of that bar (80-16 = 64).

These phenomena remind us of the intricate neural processes that shape our visual perception.

Figure 3.7

Contextual Influences on Lightness Perception

Context and Lightness Perception

Moving beyond lateral inhibition, we delve into the role of context in lightness perception. Our brains interpret the brightness or darkness of an object based on a comparison to its surroundings.

Simultaneous Lightness Contrast

One powerful example discussed is Simultaneous Lightness Contrast, where two identical gray regions appear to have different lightness depending on their surrounding context. This phenomenon is explained by Wallach's Ratio Theory. The central idea behind Wallach's Ratio Theory is that our perception of an object's lightness is influenced not only by the absolute amount of light it reflects but also by the relative amount of light it reflects compared to its surroundings. In other words, the perceived lightness of an object depends on the ratio of its luminance (brightness) to the luminance of its background.

White's Illusion

White's Illusion is another intriguing example where our perception of lightness is influenced by the surrounding context. Despite lateral inhibition predicting the opposite patter of results, this illusion may be explained by the Gestalt principle of belongingness, where elements are grouped together to form a holistic pattern. Here, when the white regions “belong” to the rectangle the gray regions are perceived to be lighter than when the black regions “belong” to the rectangle.

Figure 3.8

Visual Perception and Social Context

Our perception of the world is not only influenced by physical context but also by social context. For example, Ellen Langer recruited MIT ROTC students who aspired to be pilots and had good vision. She had some of them engage in a flight simulation exercise where they were instructed to imagine themselves as pilots actively flying. Later, she tested their vision using a disguised eye chart (serial numbers on aircraft wings). The "pilots" showed greater improvement in vision compared to the control group who merely sat in a cockpit, even though no mention was made of vision during the simulation. This fascinating insight suggests that our brain's perceptual processes are responsive to social situations.

Conclusion

In conclusion, our perception of brightness, darkness, and lightness is a complex interplay between our sensory receptors, neural processing, and the context in which we view the world. Receptive field sizes and lateral inhibition mechanisms help us understand how our brain extracts information from the visual environment, leading to phenomena like perceptual illusions. Additionally, social and environmental contexts play a significant role in shaping our visual perception. This chapter has provided insights into the intricate processes that underlie our sensory experiences, emphasizing the importance of both neural mechanisms and contextual factors in our perception of the world around us.