Can Your Eyesight Affect Your Brain? An Exploration of the Visual System And Cognitive Health

Can Your Eyesight Affect Your Brain?

The visual system, which controls eyesight, is the largest system in the brain and can directly affect many functions of thinking, memory and cognition. We acquire information about our environment through our senses and vision plays one of the most important roles in cognition. The visual pathways of the human brain start in the retina and carry sensory information to the primary visual cortex, where it starts being processed, and then to other areas of the cerebral cortex, where complex processing and association take place. 

Visual processing takes up a very large fraction of the human brain—around 27% of the cerebral cortex is predominantly allotted to vision and visual processing in sighted individuals, way above the “space” taken up by other senses [1]. This means that a very significant part of the human brain is involved with analyzing the visual world.  

Being such a significant part of the cerebral cortex, and considering that the retina is actually an extension of the brain, it seems inevitable that healthy visual function must somehow be linked to healthy cognitive function in general. And indeed, that seems to be the case, as there are many studies showing that visual health correlates with cognitive health, particularly as we grow older. 

Figure 1- The human visual system. Source: Wiley Wikimedia Commons. Licence: CC BY-SA 3.0

Visual Health and Cognitive Performance Are Correlated

Even in a context of healthy aging and healthy eye function, there is a decline in visual quality with age, particularly in visual acuity and contrast sensitivity, usually starting between the ages of about 40 and 50. Visual acuity refers to the clarity or sharpness of vision—the ability to discern details of the things we see and detect small, high contrast targets; visual contrast sensitivity refers to the ability to distinguish objects and details from their background or detect subtle differences between similar shades of light and dark [2–4].

There are several age-related neural changes within the visual system that may account for the decline in visual quality. In the eye, there is an increase in the hardness and opacity of the lens, a decrease in the resting diameter of the pupil, and a decrease in the number of photoreceptor cells, which affects the amount of light that may reach the retina and be captured by photoreceptors. In the visual cortex, there is a decreased cortical activation during visual tasks and decreased temporal processing speed, for example [5–7].

It is well documented that poor visual quality and reduced visual health is linked to poorer cognitive function and an accelerated age-related deterioration of cognitive performance [8–17]. Poorer visual quality has been associated with declines in global cognitive function, visual-spatial organization, verbal fluency, and memory, for example [11,16,18]. This association is particularly noticeable in older individuals: those with poor visual performance were shown to be, on average, five times more likely to show poorer cognitive performance compared to those with good vision [9].

Conversely, better visual acuity was found to be correlated with better cognitive function [19,20]. Very good or excellent vision has been linked to better cognitive health outcomes in later life [9].

Studies suggest that poor vision is linked to poorer cognitive performance and that maintaining good vision may be important for maintaining cognitive health.

How Does Vision Impact Cognition?

The exact reason why visual quality and health impacts cognition is still unclear. But a few theories have been put forth.  

For example, it has been hypothesized that the correlation between visual health and cognitive health is a result of the impact of the loss of vision on the performance of mentally stimulating activities (e.g., reading, playing musical instruments, hobbies, socializing, physical activities, tasks of daily living, etc.). In other words, a decrease in sensory capacities such as vision (but also hearing or smelling [14]), particularly in older adults, may lead to a less active lifestyle and a decreased engagement in a number of activities that are essential for maintaining physical and cognitive health [21–24].

As a result of a decreased engagement in such activities, people with declining visual quality will be less mentally stimulated, which may lead to the decline of cognitive performance [23,25–27]. In line with this hypothesis, interventions that support eye health can help to maintain or even increase engagement in such activities, improve general quality of life, and consequently, support cognition [28,29].

A different hypothesis argues that loss of visual quality and cognitive performance may not have a causal relationship, but be two sides of the same coin: aging [30,31]. But the fact that interventions that support visual performance have been shown to support cognitive performance as a secondary outcome support the possibility of a causal link between visual quality and cognition [32,33]. Maybe both play a part.

A decline in vision may affect cognitive performance by decreasing engagement in mentally stimulating activities that rely on vision. This may weaken brain performance in the same way that lack of exercise weakens muscles.

Can Better Vision Support Cognitive Performance?

Supporting vision with something as simple as wearing reading glasses has been linked to better cognitive function. This indicates that even simple methods for supporting visual performance may serve as a protective factor against cognitive deterioration associated with visual deterioration [20].

According to the theory described above, it could be hypothesized that better vision may lead to increased engagement in cognitively stimulating activities, which in turn may result in changes in brain activity and connectivity. In line with this idea, a glimpse into the neurological mechanisms of the association between vision and cognition was given by the finding that, following an intervention for poor visual health, there was an expansion of grey matter volume in the visual cortex. This indicated that supporting visual performance could indeed lead to changes in the brain, i.e. brain plasticity, which is a fundamental property of brain function and cognitive performance [34].

The available data indicate that enhancing cognitive performance as an outcome of supporting vision is a possibility. But human studies have only established a correlation between visual and cognitive performance, which does not imply causation. Nevertheless, because there have been interventions shown to support both vision and cognition, even if we assume there is no causal link between them, we could safely assume there are shared mechanisms underlying both visual and cognitive health and performance—keep in mind that the retina is a neural tissue and an extension of the brain.

Research suggests that enhancing cognitive performance by supporting vision is a possibility. Whether it’s because of a causal link or of shared mechanisms is unclear.

Shared Mechanisms of Visual and Cognitive Health

The brain and the retina share a couple of characteristics (at least) that make them particularly vulnerable to oxidative stress, when compared to other tissues of the human body.

First, both the brain and the retina have very high energetic demands that require a very high metabolic activity.

Despite being no more than 2% of our body weight, the brain is responsible for around 20% of our body’s daily energy expenditure due to the high energetic demand of maintaining neurotransmission, consciousness, and cognition [35]. Similarly, the retina has very high energetic demands for phototransduction and neurotransmission and its metabolic rate is among the highest in the human body [36].

Our cells and tissues use energy in the form of ATP, which is produced in mitochondria in a metabolic process known as oxidative phosphorylation. This process takes place in the mitochondrial electron transport chain (ETC), a chain of enzyme complexes located in the inner mitochondrial membrane that use the energy of electron transfers to power the production of ATP.

But the ETC is also a major site of reactive oxygen species (ROS) production as a byproduct of energy metabolism. At balanced levels, ROS are important cellular signaling molecules. But when ROS levels fall off balance and accumulate, they cause oxidative stress and become potentially harmful [37]. That’s why the brain and the retina have potent antioxidant defense mechanisms designed to keep ROS levels under control.

The brain and retina require high amounts of energy. The cost of the high metabolic activity is oxidative stress, which can be a problem.

Second, the structural and functional integrity of cellular membranes is a very important aspect of both brain and retinal function. But both the brain and the retina have a particularly high lipid content in their cellular membranes, especially polyunsaturated fatty acids, which are more prone to oxidation (and hence oxidative stress).

Figure 2 - General structure of a cell membrane—a phospholipid bilayer containing different molecular components that vary according to cell type. Source: OpenStax, Anatomy and Physiology; 3.1 The Cell Membrane. License CC BY 4.0

About 50% of the brain’s dry weight is actually made up of lipids, mostly from neuronal membranes and the myelin sheath. Pretty much every aspect of neuronal function is linked to healthy membranes: the generation of action potentials (i.e., neuronal firing); the fast conduction of action potentials, which requires good electrical insulation given by the myelin sheath; and the transmission of information between neurons at synapses [38].

Retinal neuronal function is also highly dependent on membrane health, not only for neuronal communication, but also for the actual initiation of the visual process. Vision starts with the conversion (or transduction) of light energy into a neuronal electrical signal in the retina through the activity of two types of visual photoreceptors: rods and cones. Light is captured in a region of photoreceptors called the outer segment, which contains a stack of membrane disks where light-sensitive photopigments are found. These pigments convert light energy into a change in membrane potential, which in turn generates a neuronal signal that is conveyed to the brain [38].

Cell membranes in the brain and the retina have characteristics that make them particularly susceptible to oxidative stress.

Even ROS production ties in with membrane health since it is in mitochondrial membranes that the ETC is located. Unhealthy mitochondrial membranes can affect the efficiency of the ETC, which can lead to an increased production of ROS. This can tip the balance of ROS production and generate a state of oxidative stress [39]. This, coupled with the high levels of oxidizable lipids in neuronal membranes, is a recipe for both retinal and cognitive dysfunction.

This is why both the brain and the retina require potent antioxidant defenses that protect membranes and cellular function in general. Most of these defenses are enzymes and molecules with antioxidant activity. And some of them are actually dietary nutrients, including vitamins or carotenoids, for example. Let’s focus on the latter.

Lutein and Zeaxanthin Support Vision and Cognition

Lutein and zeaxanthin are most recognized for being the xanthophyll carotenoids that accumulate in the macula of the retina—known as macular pigments, which also include meso-zeaxanthin—and protect it from blue light-induced photooxidative stress. But lutein and zeaxanthin, which cross both the blood-retina barrier and the blood-brain barrier, also accumulate in the human brain, where lutein is the predominant carotenoid [40,41]. Lutein and zeaxanthin are more than just blue light filters: they play important parts in both visual function and cognitive performance.

Macular pigments support many aspects of visual performance and their actions depend on their levels in the macula. Higher macular pigment levels have been correlated with less visual discomfort in bright light conditions (i.e., glare) [42–44], faster photostress recovery (i.e., the time it takes vision to recover from a bright flash of light) [44], enhanced speed of dark adaptation and visual performance in dim lighting conditions [45,46], and enhanced visual processing speed and accuracy [47–49]. Lutein and zeaxanthin supplementation has also been shown to support many aspects of visual performance, including reduced glare disability (i.e., “washed out” vision) [50–52], faster photostress recovery [52], and enhanced contrast sensitivity [53,54].

Macular carotenoids support retinal health and visual function through multiple mechanisms. Blue light absorption and antioxidant activity are the most significant, but there are also benefits, such as supporting visual processing speed and contrast sensitivity, that are better explained by a support of neuronal communication between visual neurons [55] and/or enhanced visual neurophysiology [53].

Macular carotenoids are more than blue-light filters: they support neuronal physiology and neuronal communication.

What’s particularly interesting about these mechanisms is that they are also relevant for contexts of neuronal activity other than vision. Given that lutein and zeaxanthin also accumulate in the brain, this means that there is a context for these carotenoids to potentially benefit non-visual aspects of brain function. And indeed, lutein and zeaxanthin have been positively associated with different measures of cognitive function in individuals of all ages [56].

[Note: The levels of lutein and zeaxanthin in the macula correlate with their levels in the brain, which means that macular carotenoid levels provide an indirect measure of their brain levels [57,58]. This is important because, unlike brain carotenoid levels, macular carotenoid levels can be easily assessed through a non-invasive eye exam.]

In older adults, macular carotenoid levels have been linked to enhanced global cognition, executive function, visual-spatial processing, reaction time, processing speed, language and speech, attention, and attention switching [49,57,59–63].

Studies that compared the actual levels of lutein and zeaxanthin in the brain (using post-mortem brain tissue) with cognitive performance (that had been previously assessed) also found a correlation between brain carotenoid levels and cognitive measures such as global cognition, memory recall and learning, intelligence quotient (IQ), and executive function [41,64].

In children and adolescents, macular carotenoid levels also correlated with cognitive performance, namely with academic performance and intellectual ability, including reading, math, and written language [65,66]. Furthermore, children with higher macular carotenoid levels had to use “less brain” (i.e., had lower brain activation) and made less errors when conducting the same cognitive tasks as children with lower macular carotenoid levels, who had to use “more brain” (i.e., had poorer neural efficiency) [67].

Lutein and zeaxanthin levels are positively associated with cognitive function throughout our lifespan. They may help the brain ‘think’ more efficiently.

Because lutein and zeaxanthin must be obtained from the diet (meso-zeaxanthin can be produced in the retina from lutein), their levels in the macula and the brain may show considerable variation depending on our dietary patterns.

Lutein and zeaxanthin supplementation is well known to be able to enhance the levels of these carotenoids in the macula [68], which, as we’ve seen, are a marker of their brain levels. Accordingly, several studies have also shown that lutein and zeaxanthin supplementation supports cognitive function.

For example, lutein and zeaxanthin supplementation was shown to support complex attention and cognitive flexibility in older adults [69]. In healthy young adults, lutein and zeaxanthin supplementation supported spatial memory, reasoning ability, and complex attention [70]. Lutein supplementation in adults also supported verbal fluency scores [71]. And in line with these findings, consumption of lutein-rich vegetables (green leafy vegetables and cruciferous vegetables) was associated with a prolonged cognitive healthspan [72].

How Lutein and Zeaxanthin Support Cognition

Lutein and zeaxanthin support neuronal function through several mechanisms: they support antioxidant defenses, neuroprotective mechanisms, neuronal structure, neuronal function, and neuronal communication, for example [73–75]. As mentioned above, most of these mechanisms are relevant not only for visual health, but also for other aspects of healthy neuronal activity in the brain, and consequently, for cognitive function.

An important characteristic of lutein and zeaxanthin is that they are potent membrane antioxidants and are thus able to offset two vulnerabilities of brain function: susceptibility to membrane oxidation and high ROS production as a result of high metabolic activity.

In neurons, lutein and zeaxanthin accumulate in cell membranes and axonal projections. They have structural characteristics that give them a high solubility in cellular membranes and allow them to influence membrane properties such as fluidity and stability, as well as axon structure and neuronal communication [76–78].

Furthermore, lutein and zeaxanthin localize preferentially in membrane domains rich in polyunsaturated fatty acids (PUFAs) including, for example, docosahexaenoic acid (DHA), where their antioxidant activity is most needed [79], and their orientation and distribution within the lipid bilayer maximizes their antioxidant action. Inhibition of DHA oxidation, which is a major fatty acid in brain phospholipids and the retina, helps to maintain membrane structure and fluidity but also preserves DHA so it remains available for other functions (e.g., neuroprotection, modulation of membrane stability and function, and neuronal communication) [80].

So, in addition to their strong antioxidant activity, the efficiency of these carotenoids as neuroprotective molecules can also be attributed to their membrane localization and how they interact with those membranes.

There are a number of additional mechanisms through which carotenoids within cellular membranes may support neuronal function in brain regions associated with cognitive function: by influencing the activity of receptors and the signaling pathways they activate; by influencing synaptic communication; by influencing mitochondrial function and ETC efficiency; by influencing the formation of the myelin sheath around axons that promote neuronal impulse speed, and consequently, neuronal communication [41,56].

Figure 3 - Lutein and Zeaxanthin in visual and cognitive health. Source (adapted): Loskutova E, et al. HRB Open Res 2019. Licence: CC BY 4.0

Designing Qualia Vision to Support Eye & Brain Performance

The macular pigments lutein and zeaxanthin were included in Qualia Vision because of their key role in supporting visual and retinal function and health. Although this was the main goal of their inclusion, it wasn’t the only one. Because we were aware that the support of cognitive function may be a secondary outcome of supporting vision, we wanted to work that into our formula.

Therefore, in addition to lutein and zeaxanthin, included in Qualia Vision along with meso-zeaxanthin as Lutemax® 2020, we also included a few other ingredients that not only support vision, but also cognitive function and health.

Astaxanthin, another carotenoid with potent antioxidant activity that can cross the blood-retina barrier and the blood-brain barrier, supports visual function, performance, and health [81–89]. It also supports cognitive function, including learning and memory [4–8], and brain processes known to support cognitive performance, including sleep [9,10], hippocampal neurogenesis [6], neural stem cell function [18,19], brain-derived neurotrophic factor (BDNF) levels [11–16], neural mitochondrial function [21,26,27], and neuroprotective functions [17,20–26], for example.

Goji fruit extract is rich in bioactive antioxidant compounds, including zeaxanthin (it’s considered one of the richest food sources of this macular pigment), but also polysaccharides, believed to be responsible for many of Goji’s health benefits, including the support of vision and eye health [90–106]. Goji also supports neuroprotective functions [107–113], cognitive function [114], and a number of processes and mechanisms that underlie healthy cognition, including synaptic plasticity [113,115,116], neurogenesis [112,113,116], and cerebral blood flow [111].

Taurine, an amino acid found in all ocular tissues, supports visual function, retinal neuroprotection, and resistance to visual fatigue and stress [117–131]. Taurine also supports a number of neuronal processes and brain functions that underlie and support cognitive performance, including short-term memory [132], synaptic long-term potentiation [133], GABAergic neurotransmission [132,134–136], hippocampal neurotransmission [137], BDNF [132], cerebral blood flow [138], and neuronal mitochondrial function [138].

The spice saffron is rich in bioactive compounds among which are zeaxanthin and other carotenoids, such as crocin, responsible for saffron's color. Saffron extract supports healthy eye function and visual performance [139–144]. Saffron also supports cognitive health and performance (e.g., focus and attention [145]) by supporting neurotransmitter signaling [146,147], neuroprotective functions [144,147–151], long-term potentiation [152], sleep [152–154], and healthy brain aging [155–157].

Ginger, one of the most widely used spices in the world, contains many bioactive compounds that support visual health [158–161], neuroprotective functions [162–165], and cognitive function [166,167].

Bilberry extract is rich in antioxidant polyphenols that support vision [168–178], as well as brain health and neuroprotective functions, including protection of cognitive health [179–182].

Amla is one of the most important fruits in Ayurveda, where it is classified as a rejuvenator used for healthy aging and is the primary eye tonic medicine. Amla supports healthy visual function and retinal antioxidant defenses [183–186]. Amla supports cholinergic neurotransmission, brain mitochondrial function, neuroprotective functions, and cognitive health [187–194].

Methylcobalamin is a coenzyme form of vitamin B12, which is essential for the healthy function of nerves. Methylcobalamin supports visual health and optic nerve function [195–204]. In the brain, vitamin B12 is essential for the maintenance of healthy neuronal and axonal structure and function. Because vitamin B12 deficiency has been associated with poor neuronal and cognitive health [205], it’s important to support healthy brain levels of this vitamin.

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