Design Review:
Perceptual Processing Systems

Signal Detection Theory
Signal Detection Theory is crucial for perception and decision-making. The eye can distinguish between noise and stimuli, and focus on what is deemed most important (Herzog et al., 2024; Lynn & Barrett, 2014; Mena-Guevara et al., 2024; Sandnes & Zhao, 2015). This theory explains how even weak stimuli can stand out through contrast, luminance, edge detection, and contour. In environments where there is a high level of noise, the stimuli must be stronger in order to stand out because detection thresholds increase (Nesse, 2005). Low-noise conditions allow stimuli to be more subtle because there is a lack of strong distractors. Signal Detection Theory is crucial for design and impacts color, information architecture, and movement. By manipulating contrast, designers are able to make certain stimuli stronger and guide attention to the most important features. 

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Contrast

Contrast is necessary for identifying important stimuli. Key components of contrast are color perception and edge detection (Hattab et al., 2020; Mantiuk et al., 2020; Sandnes & Zhao, 2015). Color perception has three principal categories: luminance, saturation, and hue. The most important element of contrast is luminance, which can be described as “the visible energy of light or the physical light energy weighted according to the wavelength-by-wavelength response of the human visual system” (Yusof et al., 2022, [4]). This is perceived as the intensity of how dark or light a certain color is. These perceptions are detected by the retina's light receptors, cones, and rods (Bohl et al., 2025; Herzog et al., 2024; Mantiuk et al., 2020). Rods are made for low-light conditions and primarily detect luminance. Cones are sensitive to hue and rely on the trichromatic photoreceptors known as the L (red), M (green), and S (blue)-cones to process different light wavelengths (Mantiuk et al., 2020; Jamil & Denes, 2024). As seen in Wuerger et al. (2020), we can compute the luminance mechanism by adding the L- and M-cone responses. Contrast sensitivity increases with stimulus size, which is based on perceived distance from the eye and pictorial cues. Large stimuli are detected by the rods because they are more sensitive in the peripheral vision than the foveal vision; therefore, the eye is drawn toward the larger source of luminance (Wuerger et al., 2020; Mantiuk et al., 2020; Zhang et al., 2024). Voluntary or involuntary saccades can bring these images to the foveal vision (Zhang et al., 2024). This is amplified if the object is moving because new rods are activated, which draws attention. 
Higher luminance contrast, especially blue and yellow combinations, leads to better readability and performance (Hall & Hanna, 2004; Jamil & Denes, 2024). It is necessary for contrast to be detected both in the light conditions and at low light or night conditions because of how the cones and rods process these settings differently. It is recommended to have a luminance contrast of at least 4.5:1 for text and 3:1 for headings. For individuals with 20/80 vision, a ratio of  7:1 for text and 5:1 for headings is needed. For most websites, 3:1 is recommended for components and graphics, and 7:1 is recommended for text (Jamil & Denes, 2024; Sandnes & Zhao, 2015). Hall and Hanna (2004) state, “Colours with greater contrast ratio generally lead to greater readability” (p. 1). Readability is important because in order to identify text, it needs to be distinguishable from the background in different conditions. It is important to note that too much contrast, especially in luminance, can be detrimental. As luminance increases, achromatic contrast sensitivity heightens. When this luminance is higher than 200 cd/m², sensitivity declines; therefore, bright backgrounds make it difficult to detect contrast as they overwhelm the photoreceptors. Fully saturated visuals create fatigue, with red being the least desensitizing hue and blue being the most. An afterimage may result, leading to visual impairment, especially in low-light settings (Nesse, 2005; Wuerger et al., 2020). Hattab et al. (2020) state that there can be a negative interaction of blue and red colors in text, which can cause it to appear blurred and create chromatic aberration, resulting in failure to focus on both colors simultaneously. Contrast plays a major role in visual perception and should be considered heavily in design decisions. Contrast is detected with lateral inhibition, which enhances edge detection and luminance regions in order to identify objects and their boundaries (Chen & Fu, 2010).

Lateral Inhibition

Lateral inhibition enhances contrast through cones, rods, and cells.  After light is absorbed by the cones and rods, the bipolar cells receive and transfer signals. The horizontal cells enhance contrast by mediating and suppressing the photoreceptors around the photoreceptor of the stimuli. The rod and cone bipolar cells are on if light sensitivity is increasing and off if it decreases. These on-and-off cells signal to ganglion cells the light and dark regions, creating edge detection (Herzog et al., 2024). Amacrine cells make synapses in the eye to inhibit signals and enhance edge detection by delaying responses. These cells also carry motion and brightness signals to ganglion cells (Chen & Fu, 2010; Herzog et al., 2024). Ganglion cells receive contrast and edge-detected signals from the bipolar and amacrine cells, which are converted into neural signals. They then send information through the optic nerve to refine the information from cones and rods. Some ganglion cells (ipRGCs) regulate pupil dilation based on the light signals detected by melanopsin, rods, and cones. The ipRGCs are most used in bright lighting and are active longer than cones and rods. These cells determine lighting from the receptive fields of the retina (Bohl et al., 2025; Chen & Fu, 2010). Mirault & Grainger (2025) found that vertically aligned information can create a crowding effect where excess information slows object information. The researchers suggested this is primarily a visual effect rather than cognitive inhibitory control. Lateral inhibition is necessary for edge clarity and visual perception. Factors that make lateral inhibition less efficient are increased receptors, weaker inhibitory signals, and complex signals. Chen & Fu (2010) state that lateral inhibition enhances edges and broader contours enhance lateral inhibition but can lose fine details and lengthen processing time. These cells and photoreceptors are crucial in identifying and processing signals, as well as contributing to the detection speed, known as onset. 
 

Onset

Onset refers to the speed at which a stimulus is first detected. This can be explained in how the eye’s rods, cones, and ganglion cells process stimuli. Rods are optimal under low light conditions and have slower response times than cones. Therefore, there is a faster onset in conditions with more luminance (Kuo & Chao, 2025). Visual systems are especially sensitive to abrupt onset, and onset singletons can be used as a tool to make stimuli more abrupt, as they are difficult to ignore. Therefore, using motion or visual light stimuli is more effective than color-singleton (Kuo & Chao, 2025). Hall & Hanna (2004) stated that there is better detection with black text on grey and yellow backgrounds, with blue backgrounds resulting in lower performance. This is consistent with findings that high contrast leads to faster reaction times. Onset can also be heightened by saturation, and there is a strong positive relationship between saturation and arousal (Hall & Hanna, 2004). For signal detection to be most effective on a webpage, it's important to have something that stands out but doesn't overpower the senses. Nesse (2005) stated, “Repeated exposures to certain kinds of danger may adaptively lower response thresholds, making systems vulnerable to runaway positive feedback” (p .1). When the eye detects everything as a threat, then it is more difficult to differentiate what is important. Therefore, it's necessary not to have excess noise that has too high a contrast. It is important to have lower contrast for lower-order items, as having to allocate attention fatigues and leads to a worse user experience. Performance and attention can be enhanced by the design of visual stimuli, but it's equally important to consider how accessible these designs are.

Accessibility

Designing with all kinds of users in mind can create platforms where all people are able to engage and use products equally. Over 300 million people experience color vision deficiency (Jamil & Denes, 2024). This deficiency can impair contrast perception, as two colors may appear the same to someone with this syndrome. It is crucial to design websites that are accessible to all users for this reason. There are high-contrast modes that computers can implement, but the website has to be compliant, ensuring that colors and layouts function with accessibility modes (Jamil & Denes, 2024). 
Age affects the ability to recognize contrast, especially hue and luminance (Mena-Guevara et al., 2024). Wuerger (2013) found that there is a decline in elderly people's ability to differentiate between similar colors, and they need higher S cone input to perceive certain hues under low light conditions. They also state that as humans age, the lens yellows and causes a reduction of short-wavelength light detection in cone receptor mechanisms (Wuerger, 2013). The lens also loses flexibility as the proteins in the eye become stiffer. This makes it more difficult to see objects up close as the lens needs to be flexible to focus short range, known as presbyopia (Fang-Fang et al., 2020). Cataracts cloud the lens and can contribute to vision problems. Fang-Fang et al. (2020) found that contrast sensitivity varies with age, spatial frequency, and noise levels. It was also found that “smaller pupil size, lower optical density of the ocular media, and decreased light absorption in the aged population may have a strong influence over contrast sensitivity at high spatial frequency” (Fang-Fang et al., 2020, [5]). Sandnes & Zhao (2015) state that in the W3C Web Accessibility Guidelines, the criterion requires that web pages be readable by people with color blindness, 20/40 vision, and 20/80 vision. This demonstrates the relationship between contrast, text size, and visual acuity. Visual acuity declines with age, so it is important to include these three criteria on all web pages.