# EEG-Based Methods for Diagnosing Color Vision Deficiency: A Comprehensive Review
## 2 Neurophysiological Basis of Color Vision  
As [PERSON] said, \"the rays are not colored\" [23]. This leads to the understanding that the perception of colors occurs due to complex neurons that undergo computations. The human eye's retina comprises specialized cells referred to as cone photoreceptors, as shown in Figure 1[24]. These cells are responsible for color vision. The cone photoreceptors perceive different colors by detecting light wavelengths. While there are many colors, the human retina cannot accommodate a specific photoreceptor for each one. Essentially, there are three types of cones, each sensitive to a specific part of the visible spectrum, as shown in Figure 2. The first type is the S-cone, which is sensitive to short wavelengths; the second type is the M-cone, which is sensitive to middle wavelengths; and the third type is the L-cone, which is sensitive to long wavelengths [25]. The three cones have overlapping sensitivity ranges, necessitating different cortical and retinal computations to perceive a specific color by considering not only the wavelength but also the intensity [26]. Another important factor in cortical computations includes the spatial characteristics of color [27]. All these computations result in the accurate processing of color information. This can help explain why measuring brain signals can provide meaningful insights related to color perception. The mechanism of color vision begins when the cones absorb the light that enters the eye. The brain processes the signals transmitted by the optic nerve and subsequently decodes this information, enabling us to see various hues [25]. Moreover, brain decoding involves, for example, a comparison of two overlapping photoreceptors, depending on the perceived color, to enable the perception of specific colors [26]. Individuals with CVD struggle to perceive red, green, and blue hues due to the lack or abnormalities of the cone cells in the eye [13]. Consequently, this results in the retina's incapacity to adequately respond to various light wavelengths [2], leading to a failure to properly transmit the signals to the brain, which results in an incorrect perception of the visual information. The causes of CVD can be genetic or acquired. Genetic etiologies are linked to the transmission of the X-linked chromosome, whereas acquired etiologies are correlated with chronic illnesses such as diabetes, sickle cell anemia [28], or ocular diseases like cataracts and glaucoma [29; 30; 31]. Other conditions such as Parkinson's disease or Alzheimer's disease could also lead to CVD [32; 33]. Some studies have revealed that even psychological conditions like schizophrenia could affect color vision [34]. Additionally, environmental factors unrelated to health conditions, such as exposure to chemical toxins, can also cause CVD [35; 36; 37]. There are different types of CVD, each depending on the affected cone cell in the eye, as illustrated in Table 1. If one cone is abnormal, anomalous trichromacy occurs. Three types of anomalous trichromacies are distinguished by the affected cone cell. Protonomaly occurs when the L-cone is abnormal, deuteranomaly occurs when the M-cone is abnormal, and tritanomaly occurs when the S-cone is abnormal [13; 38]. Moreover, if a person lacks one cone, meaning they have only two cones, dichromacy occurs, resulting in a total absence of a specific color. There are three types of dichromacy: protanopia, deuteranopia, and tritanopia. Each type is related to the absent cone, which is the L-cone, M-cone, and S-cone, respectively [13; 39]. The most common and widespread type of CVD is red-green CVD. This type encompasses protanomaly, deuteranomaly, protanopia, and deuteranopia [40; 41].