Human Visual Processing

The brain’s ability to make sense of what you see and the environment in which we live


The importance of visual perception

While technology often provides support to those experiencing visual impairment, **we should not underestimate the importance of visual processing**.

Not only does it ensure our survival when performing risky activities such as crossing roads, but it also allows us to navigate with apparent ease day-to-day activities such as spotting a dropped pencil and turning to the page we want in a book.

Perhaps unsurprisingly, **the value attributed to visual processing is evidenced in the architecture of the brain**, particularly the **occipital lobes**, part of the cerebral cortex found at the rear of the skull. Indeed, more of the brain is dedicated to vision than any other sensory modality and, therefore, warrants a high degree of scientific attention.

In turn, considerable cognitive research now attempts to uncover the potential and the constraints involved in everything from object recognition, face recognition, and depth perception to even, fascinatingly, perception without awareness – where a stimulus is strong enough to perceive, potentially impacting someone’s behavior, and yet they report no knowledge of it.

Visual perception and the brain

**Vision starts with the eye**. The **optic nerve** takes signals from the back of the eye to the back of brain, and, from there, impulses spread out to nearby regions.

Much of the rear of the brain is dedicated to visual perception – this includes all of the **occipital cortex** at the very back of the brain while extending into both the temporal and parietal lobes to the side and top.

The ‘**functional specialization theory**’ of visual processing argues that multiple areas of the brain are engaged in tackling the ‘problem’ of visual processing – each one performing a dedicated function. For example, a red ball, when being caught, is typically recognized as having color, form, and motion. All of these attributes may be processed in physically separate parts of the brain.

And yet, evidence from patients experiencing head trauma and damage to specific brain regions appears conflicting. Those with damage to the area of the brain known as V4, believed to be dedicated to color processing, seem to lose some but not all color perception, while form and motion are left unaffected. There may be more connectivity in visual processing than first thought.

Damage to motion perception

Many of the objects we attempt to identify and interact with are in motion. Perhaps as a result, along with the importance of tracking humans and animals as they move, there appears to be dedicated visual processing separate from that which handles static objects.

Indeed, in the case of a brain-damaged patient, they could see and identify individuals when they were not moving, yet, when in motion, they experienced what they saw as a series of ‘**freeze frames**.’ In comparison, another patient with an injury to a different part of the brain found it almost impossible not to bump into moving people.

Therefore, it has been suggested that **there are 2 visual systems at play**. The first may be ‘**vision-for-perception**,’ which helps us recognize what the object heading toward us is, and the other, ‘**vision-for-motion**,’ to calculate its **direction, orientation, and position**.

Visual illusions

While patients with head trauma offer a great deal of insight into the brain’s inner workings, particularly regarding visual processing, there are other opportunities for gaining a deeper understanding.

Visual illusions such as the **Müller-Lyer illusion**, where the arrow at the bottom seems longer than the one at the top despite being of the same length, show how easily our visual identification system can be fooled.


Follow-up studies teaming illusions with rapid pointing appeared to engage visual movement processing systems, almost completely removing the effect of such illusions.

And yet, the illusory effect seems to return when the subject is asked to reach out and grasp objects. **While research further strengthens the idea that there are 2 systems involved in visual processing, they may be less independent than first thought.**

Depth perception

Depth perception in visual processing is highly complex, involving taking a two-dimensional retinal image and perceiving it as three-dimensional.

‘**Monocular cues**’ take information from a single eye and include texture, shading, and interposition – objects behind others appear further away. ‘**Binocular cues**’ take into account the slight disparity of the images from both eyes.

At the same time, ‘**oculomotor**’ information arises from the muscle contractions involved in the convergence of both eyes on an object and variations in optical power associated with the thickness of the lens to maintain focus.

Ultimately, these different sources of information are combined through complex cognitive processes and provide sufficient information to judge distance and depth.

However, **there are times when one cue dominates another**, especially when they provide conflicting information. Observers, often unconsciously, soon learn which cues to favor in real life, unlike the often somewhat artificial environments found in lab tests.

Perception without awareness

**While ‘blindsight,’ where a sufferer responds to a visual stimuli without being consciously aware of it, may sound paradoxical, it is very real**. British soldiers in the First World War, with damage to their **primary visual cortex**, responded to objects in parts of their visual field while claiming they couldn’t see anything.

Much later, a patient referred to as ‘DB’ had part of their primary visual cortex removed to relieve severe migraines. While able to point to the approximate location of individual objects and identify those that were stationary rather than moving, he reported no conscious experience of what was in his ‘blind’ field.

However, **investigating blindsight patients remains problematic** – we have no way of proving what people are or are not seeing. And yet, functional neuroimaging studies confirm that such individuals have activation predominantly, or even exclusively, in the ‘dorsal stream’, signifying non-conscious processing. Therefore, **the problem may result from a lack of ‘conscious’ awareness of visual stimuli**.

Object recognition

Wherever we look, we see objects – it’s not only unavoidable but essential. And yet, despite appearing to be effortless for humans, attempts to code ‘perceiving’ computers have proved challenging.

After all, many objects in the environment overlap, appear in various rotations, and belong to a wide variety of categories. **Early-stage processing in our visual system seems to rely on detecting an object’s edges and basic features, potentially involving dedicated neurons to perform the processing**.


Subsequent stages most likely incorporate depth and orientation of visible surfaces before ultimately constructing three-dimensional visual representations.

Indeed, neurologist and physiologist **David Marr**’s suggestion that such a series of representations provides increasingly detailed information has inspired many computational models and ongoing research into human and computer vision.

And yet, there may be even further complexity. **While combining low-level features, known as bottom-up processing, is vital, when visual information is incomplete, it is necessary to bring in real-world contextual knowledge, referred to as top-down processing.**

Face recognition

Recognizing who we are talking to is a vital aspect of human communication. Just imagine what it would be like if you couldn’t identify your partner, children, parents, or co-workers.

And yet, for some, it’s a genuine problem. Actor and comedian Stephen Fry admits to experiencing mild ‘**prosopagnosia**’ – sometimes referred to as ‘face blindness.’ It can leave him unable to recognize a colleague in a queue for coffee, having spent all morning with them.

It seems that **facial recognition is happening across multiple layers of brain activity, and typically without effort**. Yet for some, a process within the brain linked to late-stage facial recognition is either not triggered or unable to get the information needed.

Prosopagnosia may be more common than we think. A 2006 study identified that **almost 2.5% of school children and medical students** tested may have some degree of the condition. And yet, it often remains hidden, with sufferers relying on other cues, such as voice, body shape, hairstyle, and behavior, to identify the person standing before them.

Not seeing what’s in front of us

**Visual processing is not simply about seeing; it involves perception**. Even if we ‘can’ see something, it doesn’t mean we will be aware of it.

An iconic study by researchers **Daniel Simons** and **Christopher Chabris** in 1999 involved participants being asked to count the number of passes between a group of basketball players in a specially staged video.

What they didn’t expect, and half of them didn’t see, was, at one point, someone dressed in a gorilla suit walk through the middle of the action.

And it wasn’t a one-off; it’s been repeated in various guises. It seems that such **‘inattentional blindness’ is not uncommon and results from an evolved filtering system that helps our brain from being overwhelmed by stimuli**.

So, while we may comment on what we notice as being unusual in our environment, we may indeed remain unaware of all that we may have missed.

Judging distance

Vision may be crucial to identify what and who we need to engage with in our environment, yet also facilitates our moving around. And it’s vital. Without it, there is nothing to stop us from stepping out in front of a cab heading down the street toward us as we are about to cross.

And yet, calculating time to impact may not involve the complexity of estimating its distance from us and its speed. Professor **David Lee** suggests ‘time to contact’ may be judged based on the size of the image on our eye divided by its rate of expansion – known as its ‘tau.’

If this is correct, **it means our brains can work out our time to contact with any object from variables that are directly measurable directly by the eye**. However, it does not take speed changes and can be challenging with irregularly shaped objects.

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