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 (Hattar et al., 2002). 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 (Schmidt et al., 2011).

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 weperceive. Notably, different animals perceive various parts of the electromagnetic spectrum, expanding their sensory horizons.

Debunking a Common Misconception: How We "See"

A common misconception is that something leaves our eyes and travels toward the objects we look at (Winer et al., 2002). It's an understandable idea, after all, it feels as though we're actively directing our gaze at an object, almost as if our eyes are sending something outward. In reality, nothing leaves our eyes when we see. You're not Superman (or Homelander) shooting beams at whatever you're looking at. Instead, the process works in the opposite direction: light reflects off objects and enters your eyes. From there, your brain performs an extraordinary amount of processing, transforming those incoming signals into the rich visual world you experience. There is one small exception; if someone takes your picture with a camera flash, some of that light can reflect off the back of your eye, producing the familiar "red-eye" effect in photographs. But that reflected light isn't what allows you to see; it only happens because the bright flash entered your eye in the first place.

Anatomy of the Eye: The Window to 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

Physiology of the eye.  “Eye diagram” by Kevin David Pointon.  The image is dedicated to the public domain under CC0.

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 (Hess, 1975). Larger pupils may increase perceived attractiveness because pupil dilation is an involuntary physiological response associated with heightened interest, emotional arousal, and attention. Observers may unconsciously interpret dilated pupils as a signal that the person is engaged or interested, making the face appear more appealing. This peculiar finding hints at the interplay between visual cues and human psychology. However, see Cossu et al. (2024) for evidence that faces with more constricted pupils (and therefore larger visible irises) may actually be perceived as more attractive, suggesting that the relationship between pupil size and attractiveness is more complex than originally thought.

Figure 3.2

Examples of faces with naturally large and small pupils adapted from Human Amygdala Sensitivity to the Pupil Size of Others by Demos et al. (2008).  This study found that the human amygdala exhibited greater activation in response to faces with relatively larger pupils than to faces with smaller pupils.

“Examples of big- and small-pupil faces" by Demos et al (2008) licensed under CC BY-NC 2.0.

Life Experiences and Adaptation Influence Perception

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 (Stratton, 1896). 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 (Ramachandran, 1992). This intriguing phenomenon highlights the brain's role in processing incomplete sensory input.

Figure 3.3

To experience your blind spot, close your right eye and focus your left eye on the plus sign. While keeping your gaze fixed on the plus sign, slowly move your head closer to or farther from the image. At a certain distance, the Bates College bobcat logo on the left will disappear. This happens because the image of the bobcat falls on the area of the retina where the optic nerve leaves the eye and where there are no photoreceptors to detect light. Rather than leaving a hole in your vision, your brain fills in the missing area with white, matching the surrounding background.

"Bobcat blind spot." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0 

You can experience a similar effect looking at the two images taken at Bates College (see Figure 3.4). If these simply look like a black and white versus a color image then you need to look more closely.  

Figure 3.4

Both of these images are shown in black and white.  The difference is that the image on the right has a color grid overlaid on top of the black and white image and your brain fills in the rest with color.  

"Black and white door illusion." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0 

This filling in of the color information can be seen more clearly if you zoom in on the image, which has been done in Figure 3.5.

Figure 3.5

This figure shows a zoomed-in view of the black-and-white doorway. At this magnification, it is easier to see that the underlying image in all three panels is displayed in grayscale (with no color). The left panel shows the original grayscale image with no superimposed grid. The center panel has a colored grid overlaid on the grayscale image, while the right panel has a black grid overlaid. You still may need to look closely to see that the majority of the center image is shown in grayscale.

"Closeup black and white doorway." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

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 (Glasser & Campbell, 1998).

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 caused by convergence

Convergence

The neural signal travels from the rods and cones through many layers of cells including horizontal cells, bipolar cells, amacrine cells, and ganglion cells (see Figure 3.6). Convergence refers to how signals from the photoreceptors in the retina (rods and cones) are passed along to the ganglion cells, which send information to the brain. Many rods all connect to a single ganglion cell, so their signals "converge." This pooling makes rods very sensitive to low levels of light, because even a small amount of light hitting many rods can help trigger the ganglion cell to fire. However, since the input is coming from many rods at once, the brain can't tell exactly which rod (or set of rods) was activated, so rods give us poor detail, or low visual acuity (Kolb et al., 2024).

Cones, on the other hand, usually connect almost one-to-one with ganglion cells, meaning there is little convergence. This makes them less sensitive in dim light because each cone has to provide enough stimulation to trigger the action potential, but it also means that when a cone signal reaches the brain, we know precisely where the light came from. This is why cones provide sharp detail and high visual acuity (Kolb et al., 2024).

A good everyday example is looking at stars at night. If you try to look directly at a faint star that you can see in your periphery, it may disappear because the center of your vision relies on cones, which are in greatest concentration at the center of your vision (see Figure 3.7) and are not very sensitive in dim light. But if you look slightly to the side, the star becomes easier to see because that part of your retina has a large concentration of rods (see Figure 3.7), which are much more sensitive (Hecht et al., 1942). So, visual acuity and sensitivity vary between rods and cones, primarily due to differences in convergence.

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 3.6

Layers of cells in the retina (from rods and cones to horizontal, bipolar, amacrine, and ganglion cells.)

“Layers of the retina as a drawing” by Cenveo is licensed under CC BY 3.0

Figure 3.7

Density of rods and cones in the retina as a function of distance from the fovea. Cones are concentrated at the fovea but are found everywhere while rods only appear in the periphery.

"Density of rods and cones." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

Ganglion Cells and Types

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 (Livingstone & Hubel, 1988).

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 (Livingstone & Hubel, 1988).

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 The Size and Shape of 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 (Kuffler, 1953).
5. 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) (Kuffler, 1953). 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 surrounding 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) (Kuffler, 1953).

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 (Spillmann, 1994).

Lateral Inhibition and Horizontal Cells

Horizontal cells play a role in creating the receptive field of ganglion cells. Ganglion cells in the retina often have what's called an on-center, off-surround receptive field. That means they are most excited when light falls on the center of their receptive field, but they are inhibited when light falls on the surrounding area. This pattern comes about because of lateral inhibition from horizontal cells. When light hits photoreceptors in the center, those photoreceptors activate the bipolar cells and, in turn, excite the ganglion cell (the "on-center" part... excitation). But the photoreceptors in the surrounding area also get activated by light. Instead of directly exciting the ganglion cell, they send signals to horizontal cells. The horizontal cells send inhibitory signals from the surrounding region back to their connected bipolar cells. This reduces the activity of the center pathway, creating the "off-surround" (inhibition) (Hartline et al., 1956).

Figure 3.8

Simplified diagram of a center-surround receptive field. Central photoreceptors connect directly to a bipolar cell; surrounding photoreceptors connect through a horizontal cell that provides inhibitory feedback. Plus signs indicate excitatory input from the center, and minus signs indicate inhibition from the surroundings. Two circular insets at the top illustrate that light in the receptive field center (green at center) increases bipolar cell activity, whereas light in the surrounding region (green in surround) decreases it through lateral inhibition. Labels identify the photoreceptors, horizontal cell, and bipolar cell.

"Center-surround receptive fields." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

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 (although more recent research suggests that cortical mechanisms and receptive-field organization also contribute; Spillmann, 1994).

Figure 3.9

Many people see gray dots at the intersections of the Hermann grid that disappear when you look directly at these locations.

"Gray dots." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

The Hermann grid illusion is often described as being caused by lateral inhibition in the retina. Retinal ganglion cells have center–surround receptive fields (see Figure 3.8), meaning light in the center excites the cell, while light in the surrounding area inhibits it. This arrangement normally enhances contrast, but under certain patterns it produces illusions.

In the Hermann grid illusion, when a ganglion cell’s receptive field is centered on the intersection of two white lines, the center is strongly excited by the bright area, but the surround also receives a lot of light (the 4 light regions; see area labeled A in Figure 3.10). This produces strong inhibition, reducing the cell’s overall firing rate relative to the area between two dark squares. In contrast, when a receptive field is centered along a single white line between two dark squares, its surround falls on less white space (just 2 light regions; see the areas labeled B in Figure 3.10). With weaker inhibition, the cell fires at a higher rate. The brain interprets the lower firing rate at the intersections as if those areas are dimmer, creating the illusion of gray spots at the crossings of the grid.

Figure 3.10

Receptive fields at the intersections (A) includes more inhibitory regions than receptive fields between squares (B); so, A fires less than B. When the center of gaze focuses on the intersection (C) the receptive field is smaller and does not differ from receptive fields between squares (D).

"Receptive fields at the intersections." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

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 (small receptive field) or information from the periphery (large receptive field).

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 that is  centered on an intersection (see A in Figure 3.10) will include more white regions than a ganglion cell centered between two squares (see B in Figure 3.10). 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. However, a ganglion cell that responds to information in the fovea (small receptive field) and is centered on an intersection (see C in Figure 3.10) will include the same amount of white area as a ganglion cell with a small receptive field between two squares (see D in Figure 3.10). In this situation cell C and D will fire at the same rate and the two regions will not differ in perceived brightness.

Mach Bands: Enhanced Edge Perception

Mach Bands are another intriguing perceptual phenomenon that lateral inhibition helps explain. These bands consist of alternating light and dark stripes. The bands appear to exaggerate the differences in light intensity at their boundaries, making the edges seem sharper than they truly are (Mach, 1959).

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.

Figure 3.11

The rectangles of Mach bands are evenly shaded yet appear uneven toward the edges.

"Rectangles of Mach bands." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

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).

Figure 3.12

Mach bands can be explained by lateral inhibition from neighboring areas.

"Mach bands." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

If the mathematical explanation (given above) is not clear, another way of conceptualizing this is to image receptive fields that are spread across the area between dark and light regions (see regions A, B, C, and D in Figure 3.13). Ganglion cells respond to light, with the center being excitatory and the surround being inhibitory. At a uniform region (all light or all dark), excitation and inhibition balance out, so the response is fairly stable. But at an edge, there's an imbalance.

On the lighter side of the edge (see region A in Figure 3.13), the center of the receptive field is strongly excited, while part of the inhibitory surround falls on the darker region (which results in less inhibition). With less inhibition, the cell fires more strongly, so that area looks brighter than it really is. So, region A looks brighter than region B in Figure 3.13.

On the darker side of the edge (see region D in Figure 3.13), the center is weakly excited, but part of the inhibitory surround lies in the brighter region (which results in more inhibition). With more inhibition, the cell fires less strongly, so that area looks darker than it really is. So, region D looks darker than region C in Figure 3.13.

Figure 3.13

Ganglion cells with excitatory centers (+) and inhibitory surrounds (–) respond differently when their receptive fields lie entirely within a uniform region (B and C) versus when they span a light–dark edge (A and D). Reduced inhibition on the light side of the edge (see A relative to B) increases neural activity, while increased inhibition on the dark side (see D relative to C) decreases neural activity, exaggerating the perceived brightness difference at the boundary and producing the Mach band illusion (Ratliff, 1965).

"Mach bands explained by receptive fields." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

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

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 (Wallach, 1948). 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. In Figure 3.14 the two central gray squares are identical.  However, because the target region on the left (3) reflects only 1/3 of the light of its background (90), your brain interprets this low ratio as a darker gray surface. By contrast, the target region on the right (30) reflects 3 times more light than its background (10), so your brain interprets this high ratio as a light gray surface.

Figure 3.14

Two physically identical gray squares (30 units of light) appear different because they are viewed against different backgrounds. The gray square on the white background (90 units; ratio = 1/3) appears darker, whereas the gray square on the black background (10 units; ratio = 3) appears lighter. According to Wallach's Ratio Theory, perceived lightness depends on the luminance ratio between an object and its surrounding background rather than on the object's absolute luminance.

"Simultaneous lightness contrast." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

White's Illusion

White's Illusion is another intriguing example where our perception of lightness is influenced by the surrounding context (White, 1979). Based on lateral inhibition, a gray rectangle surrounded primarily by a black background should receive less inhibition and therefore appear lighter, while the same gray rectangle surrounded by a white background should receive more inhibition and appear darker. However, observers perceive exactly the opposite.  If you look at Figure 3.15 you will see that the gray rectangle surrounded by black stripes (left side of Figure 3.15) appears darker, and the identical rectangle on the white stripes (right side of Figure 3.15)  appears lighter. This reversal suggests that lightness perception depends not only on local retinal interactions, such as lateral inhibition, but also on higher-level processes that organize the scene into surfaces and determine which regions belong together. The dominant explanation for this illusion is based on the Gestalt principle of belongingness, where elements are grouped together to form a holistic pattern (two rectangles). 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 (Gilchrist et al., 1999).

Figure 3.15

Gray rectangles in White’s illusion appear lighter when surrounded by white areas and darker when surrounded by black areas.

"Gray rectangles in White’s illusion." by Kahan, T.A. is licensed under CC BY-NC-SA 4.0

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 (Langer et al., 2010).

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.

References

Cossu, M., Trupia, M. G., & Estes, Z. (2024). Beauty is in the iris: Constricted pupils (enlarged irises) enhance attractiveness. Cognition, 250, 105842. https://doi.org/10.1016/j.cognition.2024.105842

Demos, K. E., Kelley, W. M., Ryan, S. L., Davis, F. C., & Whalen, P. J. (2008). Human amygdala sensitivity to the pupil size of others. Cerebral Cortex, 18(12), 2729–2734. https://doi.org/10.1093/cercor/bhn034

Gilchrist, A., Kossyfidis, C., Bonato, F., Agostini, T., Cataliotti, J., Li, X., Spehar, B., Annan, V., & Economou, E. (1999). An anchoring theory of lightness perception. Psychological Review, 106(4), 795–834. https://doi.org/10.1037/0033-295X.106.4.795

Glasser, A., & Campbell, M. C. W. (1998). Presbyopia and the optical changes in the human crystalline lens with age. Vision Research, 38(2), 209–229. https://doi.org/10.1016/S0042-6989(97)00102-8

Hartline, H. K., Wagner, H. G., & Ratliff, F. (1956). Inhibition in the eye of Limulus. The Journal of General Physiology, 39(5), 651–673. https://doi.org/10.1085/jgp.39.5.651

Hattar, S., Liao, H.-W., Takao, M., Berson, D. M., & Yau, K.-W. (2002). Melanopsin-containing retinal ganglion cells: Architecture, projections, and intrinsic photosensitivity. Science, 295(5557), 1065–1070. https://doi.org/10.1126/science.1069609

Hecht, S., Shlaer, S., & Pirenne, M. H. (1942). Energy, quanta, and vision. The Journal of General Physiology, 25(6), 819–840. https://doi.org/10.1085/jgp.25.6.819

Hess, E. H. (1975). The tell-tale eye: How your eyes reveal hidden thoughts and emotions. Van Nostrand Reinhold.

Kolb, H., Fernandez, E., & Nelson, R. (Eds.). (2024). Webvision: The organization of the retina and visual system. University of Utah Health Sciences Center. https://webvision.med.utah.edu/

Kuffler, S. W. (1953). Discharge patterns and functional organization of mammalian retina. Journal of Neurophysiology, 16(1), 37–68. https://doi.org/10.1152/jn.1953.16.1.37

Langer, E., Djikic, M., Pirson, M., Madenci, A., & Donohue, R. (2010). Believing is seeing: Using mindlessness (mindfully) to improve visual acuity. Psychological Science, 21(5), 661–666. https://doi.org/10.1177/0956797610366543

Livingstone, M. S., & Hubel, D. H. (1988). Segregation of form, color, movement, and depth: Anatomy, physiology, and perception. Science, 240(4853), 740–749. https://doi.org/10.1126/science.3283936

Mach, E. (1959). The analysis of sensations and the relation of the physical to the psychical (C. M. Williams, Trans.). Dover Publications. (Original work published 1865)

Ramachandran, V. S. (1992). Blind spots. Scientific American, 266(5), 86–91. https://doi.org/10.1038/scientificamerican0592-86

Ratliff, F. (1965). Mach bands: Quantitative studies on neural networks in the retina. Holden-Day.

Schiller, P. H., & Carvey, C. E. (2005). The Hermann grid illusion revisited. Perception, 34(11), 1375–1397. https://doi.org/10.1068/p5447

Schmidt, T. M., Chen, S.-K., & Hattar, S. (2011). Intrinsically photosensitive retinal ganglion cells: Many subtypes, diverse functions. Trends in Neurosciences, 34(11), 572–580. https://doi.org/10.1016/j.tins.2011.07.001

Spillmann, L. (1994). The Hermann grid illusion: A tool for studying human perceptive field organization. Perception, 23(6), 691–708. https://doi.org/10.1068/p230691

Stratton, G. M. (1896). Some preliminary experiments on vision without inversion of the retinal image. Psychological Review, 3(6), 611–617. https://doi.org/10.1037/h0072918