The research is abundant and the evidence is clear…visual efficiency problems impact reading and learning. In addition to good visual acuity (eye sight), the American Optometric Association (AOA) defines visual efficiency problems as dysfunctions of visual skills involving binocular vision (eye teaming), accommodation (eye focusing) and/or oculomotor (eye tracking) abilities as well as eye hand coordination and visual perception. The AOA defines these visual skills needed for school success. Problems associated with these areas are usually developmental delays or associated with a neurological event, such as a concussion or traumatic brain injury and can be effectively treated with office-based vision therapy.
For over 10 years it has been my honor and privilege, as an Adjunct Clinical Professor at Michigan’s College of Optometry at Ferris State University (MCO), to present to the 3rd year optometry students on developmental vision and rehabilitation. With the gracious permission of Dr. Mark Swan, Professor of Pediatrics and Developmental Vision at MCO, on October 26, 2018, it was my pleasure to lecture along with my 2 residents, Dr. Jamie Jacobs and Dr. Kelsey Starman in Dr. Swan’s Developmental Vision Course.
Our lecture was entitled: Vision Problems that Impact Reading and Learning. The emphasis in our lecture was to go beyond the academics of these complex issues and provide a level of appreciation for the important role the doctor has of integrating the science and art of vision therapy/rehabilitation in a private practice to obtain the best outcomes for patients.
Our lecture involved 3 cases reports of patients treated by the doctors and vision therapists in our practices. Included were the “before” and “after” visual clinical findings as well as reading performance on a variety of standardized reading tests. In addition, each case report included three separate published papers in peer reviewed journals citing the connection between visual efficiency problems and/or the proven results of office-based vision therapy on treating visual efficiency problems involving binocular vision dysfunction, accommodative dysfunction and/or oculomotor dysfunction.
Therefore, in this lecture we presented not just 1 research paper but 9 research papers from around the world that shows the connection between visual efficiency problems and reading/learning problems.
As parents we all want the best for our children and when they struggle to read, learn or grow academically there can be frustration and worry about what to do to find help.
Since vision is the dominant sensory system through which we read, learn and grow academically, vision care professionals need to be involved to identify any problems involving the eyes and vision and remediate and prevent the struggle that occurs when poorly developed vision or a neurological event, such as a concussion, interferes with learning.
However, reading and learning problems can be multifactorial and therefore, when a child is having academic struggles it is important to consider that for many, a professional “team approach,” working together for the common good of the patient, will help ensure the best opportunity for learning. A partnership is developed between the parents and these professionals.
Help for parents to learn more about the symptoms and behaviors that may indicate a learning related visual problems…and what can be done
Help for professional partners to learn more about well-designed screening protocols and research studies that will help integrate children with learning related visual problems into their clinical practice
Help for vision care providers to gain immediate access to the evidence based, best practice protocols for evaluating children and adults to ensure that they are prepared for optimal learning
What do the Winter Olympic Games have in common with vision-based reading and learning research?
The Winter Olympics brings together the finest athletes from around the globe to compete and show the world how there can be a universal interest in Winter sports and desire to be the best. While you may never have tried to do figure skating or snowboarding the half pipe, when you watch the performance of the these premier athletes in the Winter Games you begin to appreciate the magnitude and complexity of what they are doing, and it touches your mind and heart.
The visual system plays a direct role in the magnitude and complexity of individual performance abilities in every aspect of life. For the Olympic athlete, having good visual efficiency, using both eyes to team, good depth perception, tracking, focus, visual processing and visual motor integration skills are all needed to have peak performance.
But, what about when it comes to the proficiency of a child’s abilities in the day-to-day act of reading and learning? Indeed clarity of sight is important, but there are many visual problems that affects a child’s ability in reading and learning that may not be easily recognizable by just correcting a refractive condition with glasses. For example, vision problems that affect binocular vision (eye teaming) and accommodation (eye focusing) are critical to reading and learning even with 20/20 eye sight. But, some may ask, “Where is the research?”
Awareness for visual efficiency problems, such as Convergence Insufficiency (CI) and its impact on reading and learning, has been reaching the research international stage coming in from around the globe including Canada, India and South Korea, to name a few.
Here are a few examples of the latest in the “Research Olympics” on Vision and Learning:
Published in the Journal of Optometry, September 2017, entitled:Visual and binocular status in elementary school children with a reading problem, concluded: “The results in this study show that children with an IEP for reading also present with abnormal binocular and/or accommodative test results. To thoroughly investigate the binocular vision system, we recommend that tests of accommodation, binocular vision, and oculomotor function should be performed on all children, especially those with identified reading problems.”
The United States has been the leader in vision therapy research for vision and learning problems thanks to the tireless efforts by the Convergence Insufficiency Treatment Trial (CITT)“Olympic Team”. Over the last 2 decades the US (CITT) Team has lead this research and published papers that have been voluminous showing that accommodative vergence problems, such as Convergence Insufficiency and Accommodative Disorder can have a major impact on near visual performance such as reading and attention.
While, like the Olympics, research is complicated and technical, thanks to the dedication of these “Olympic Research Teams” from around the world, what we know now should touch our minds and our hearts. Vision related reading and learning problems do exist in every country and there is effective treatment with vision therapy. No longer should children who struggle with reading and learning problems be overlooked from having a binocular/accommodative or other visual problems.The research is clear, but the bigger question is how will we respond to end this senseless struggle for those with vision-based reading and learning problems?
Wouldn’t it only make sense for any child who struggles in reading or learning to have of a comprehensive eye and vision evaluation because, if a visual problem exists, effective vision therapy treatment could be a game changer in that child’s life!
When a child struggles to read it can be very frustrating, not just for the child but for the parents and the teachers. No parent wants to see their child having trouble with something as important as reading. The same is true for teachers. In many school systems, teachers are now expected to have their students reach certain reading standards and if they don’t, that child may be faced with repeating a grade level.
Meet Wendy Rosen, a former classroom teacher and educational consultant and author of the new book: The Hidden Link Between Vision and Learning, Why Millions of Learning Disabled Children are Misdiagnosed. In this 6 minute VisionHelp interview, which premiered at the 2017 COVD Annual Meeting, Wendy gives an explanation for why so many children struggle with vision-based learning problems and how to find help they need.
Last week I witnessed the astonished look on a father’s face when his 7 year old little girl (we’ll call Jenny) struggled with some routine chair-side vision tests. His surprise was not during the measurement of her ability to read the eye chart. She read the 20/20 line of letters like a pro. But, when I asked Jenny to visually look at and follow a moving bead on a stick, Jenny responded as if she couldn’t or wouldn’t do it. Observing his daughter’s seeming lack of compliance, Dad began to go into a coaching mode. “Jenny, look at the bead!” , he repeated as I moved it very slowly in front of her face. Then in almost a sense of exasperation he said, “Come on, Jenny, you have to do your best for the doctor”, as if this was a routine personal frustration he had for his 7 year old daughter. Jenny’s face just grimaced as if this was almost a painful process.
So I stopped the visual tracking test and gave Jenny a break. Then I asked her to put on some red/green anaglyph glasses. Sometimes kids call them the 3-D glasses. I asked her to look at a penlight that I positioned about 3 feet in front of her face. Then I brought the light in toward her nose and asked her to tell me when she saw it go double. Jenny reported double at about 14 inches. I repeated the test 2 more times and she consistently reported the light separating into 2 lights further and further away from her face. When her Dad saw this, he thought Jenny was simply “playing around” and said, “Jenny, this is important, you have to try harder!” So, I removed the red/green glasses from Jenny and asked Dad to put them on. I did the same test on him, except he saw the light remain single up to about 2-3 inches from his nose. Now he was starting to get it!
Jenny’s parents situation is not unusual because every family has their own unique side to the story when unaddressed vision problems cause reading and learning problems. It usually begins with a child who is bright and yet struggles in school. In this case their almost 8 yr old daughter, Jenny was having a difficult time reading. She even had extra tutorial support and attention at school. But, her parents were prepared to retain her in the first grade, even though her teachers said Jenny was capable of “just barely” reading 2nd grade material and suggested that she be promoted to 2nd grade. However, while Jenny could read single words at a time, what she struggled with was reading them in a sentence, losing her place and slow to do her work. Her parents felt a bit remorseful of having to make a decision to retain her in 1st grade because Jenny was good in math and was mature enough to handle the transition to 2nd grade. But, because of the reading discrepancy Jenny’s parents thought they should retain her in 1st grade, while on the other hand they were worried that she would get bored and grow to dislike school and feel the emotional sense of failing when her peers were moving forward. They were visibly torn!
Fortunately for Jenny, one of the teachers helping to tutor her, spotted some of the hallmark signs of a visual tracking problem and made the referral to our office. At first, Jenny’s dad was convinced that his daughter couldn’t have a vision problem because she had passed the school eye sight test and the pediatricians vision screening. But, he and his wife wanted to make sure so they made the appointment.
I met with Jenny’s parents outlined a treatment plan of twice a week office-based optometric vision therapy and within 4-5 months I expect Jenny will be visually fully functional and able to apply herself in the classroom. Jenny got started last week in treatment, and because we were able to clearly identify a discernible visual problem that can be completely remediated with the proper evidenced-based treatment in a relatively short period of time, Jenny’s parents decided to place her forward into 2nd grade.
If you are like so many who are searching for answers, this story about Jenny helps to give some insight. However, it is probably safe to say that you have questions about how a child, possibly your own child can pass the school or pediatrician vision screening, reportably have normal sight (20/20) and yet have vision problems that cause significant problems in school, such as reading!? Yes, while Dr. Press and I have written about this extensively on the VisionHelp Blog, sometimes it’s better to hear about it from a parent who has been through it before.
With that thought in mind, here is a video of a mother, Michelle, whose son Dimitre had his crossed-eye surgically aligned, did occlusion therapy and had 20/20 visual acuity, but still had serious vision problems that blocked his abilities to read, learn, ride his bike and even make friends. Take a look and see if this helps explain why 20/20 sight is simply not good enough to define vision readiness for reading and learning in the classroom…
For more Facts about vision problems that affect reading and learning, here are some helpful sources:
With the havoc wreaked by COVID you may have missed publication of this article in PLOS ONE last year, as did I: Development of global visual processing: From the retina to the perceptive field. The corresponding author is Uri Polat, who is affiliated with The School of Optometry and Vision Science and The Mina & Everard Goodman Faculty of Life Sciences, of Bar Ilan University in Ramat-Gan, Israel. If the name Uri Polat is familiar to you it may be because he co-authored a seminal paper with Dennis Levi in 1996 that was the first nail in the coffin of the concept that amblyopia can’t be improved after age 7. In 2013 I blogged about the clinical appreciation of developmental crowding, and how that relates to young children with delays in reading acquisition as well as for patients with amblyopia who function as if they have a learning disability.
I like Polat’s use of the term “perceptive field” in this new PLOS ONE article. Although optometry students learn about receptive fields in the visual cortex, Polat and colleagues coin the term perceptive field which has a more developmental ring to it regarding retinal development. In the abstract they write: “Our data suggest that the developmental processes at the retina and visual cortex occur in the same age range. Thus, in parallel to maturation of the PF, which enables reduction in crowding, foveal development contributes to increasing contrast sensitivity.”
The age range where there is typically a big crossover point is around age 6. Clinicians are familiar with this in terms of when children are better able to keep place on the whole line when reading a Snellen Chart rather than requiring pointing or the isolating of letters. It is why developmental saccade tests like the King-Devick and DEM have norms beginning at age 6. Understanding and appreciating crowding serves as the basis for why children’s books have larger print and liberal spacing until age 6, and then each year thereafter progress toward smaller print with tighter spacing, in essence becoming relatively more crowded.
Helping children who lag in retinal and/or cortical development is done through many types of sheets or workbooks that progress from larger font and spacing toward smaller font and spacing. This is the sequence followed in Michigan Letter Tracking (Ann Arbor Series) workbooks, and more recently in a variety of Petrosyan workbooks available through Bernell, or computerized through Anteo. Versions of this are customizable in other computerized programs such as Binovi and Neuro Visual Trainer, and an auto-pacing function built into the ambiNet program. Downloadable Hart Chart Decoding sheets are fun for this as well, zooming the size in or out, as well as Dr. Sarah Lane’s downloadable worksheets.
Although all VT practices deal with “retained crowding” at some level, less common is a parallel approach to training contrast sensitivity.
Doing this with letters likely transfers to reading performance, while doing this with Gabor patches is becoming popular with computerized programs that enhance dynamic visual skills as required for night driving and sports vision. With all of these approaches, using Polat’s terminology, we can say that we are training the perceptive field.
In Neural Science and Vision – Part 5, we mentioned Lea Hyvärinen, MD, PhD and her book What and How Does This Child See, 2nd edition co-authored with Namita Jacob, published at the end of 2019. Our blog introduced Dr. Hyvärinen as a developmental/behavioral ophthalmologist who spoke at the COVD meeting in 2012. Since then, as is apparent in her book, Dr. Hyvärinen has evolved in the direction of neuro-rehabilitative ophthalmology to parallel neuro-rehabilitative optometry. Inventor of the eponymous Lea tests, Dr. Hyvärinen has few if any ophthalmologic peers in the U.S., her closest analog perhaps being Dr. Gordon Dutton in the U.K. In a lecture in the U.K. in 2012 regarding transdisciplinary assessment, Lea presented a slide noting that the role of the optometrist is principally in function and optics, in contrast to the the ophthalmologist who primarily addresses anatomy and disease:
In lectures she has given, as well as in the epilogue to the 2nd edition of her book, Lea emphasizes that the ICD system is useful for defining visual impairment, but not for classifying visual functioning. For the latter, and for clinical purposes of intervention, the ICF (International Classification of Functioning, Disability, and Health, 2001), and its pediatric version, the ICF-CY (ICF for Children and Youth, 2007) is more pertinent. It seems as if there has been movement in this direction internationally with regard to collaboration between Medicine and Education (see this co-presentation from Dr. Hyvärinen and this 2018 publication from Frontiers in Education regarding disability research involving pre-schoolers).
The subtitle of Lea’s book is Assessment of Visual Functioning for Development and Learning, speaks volumes. Let’s take a look at some of the key concepts she embraces.
Regarding fixations: “It is important to observe fixation and other oculomotor functions in situations that require concentration for looking at small or complex pictures and during reading. During these functions, fixations, saccades, and scanning eye movements should be automatic. If a child needs to consciously fixate and focus on a small target, these two simultaneous motor functions may demand too much of the child’s limited capacity and cause overload or lose head control … If reading from the black board or working on a near vision task requires too much motor capacity, posture control may be lost and the child lies flat on the desk. There should be a constant adjustment based on observation of the varying balance between the capacity for using vision and the capacity for postural control so the student can concentrate on his tasks.”
Regarding saccades: “Some children have the greatest difficulties in controlling their eye movements when crossing the midline. Their eyes may close briefly at midline or there is a jerk in the following movement. This rarely reported phenomenon has been observed in children with ‘athetosis’ who often cannot use vision at midline.”
3. Regarding acuity, accommodation, convergence, and the need to consider plus lenses at near: “The sensory task is easier during the short measurement of visual acuity than during the reading of a text which may require so much brain capacity, the motor functions become irregular or weak. Observe the effects of reading glasses: how reading speed and errors in reading and comprehension of the content vary when the child reads with/ without reading glasses.” Lea also stresses the need to maintain balance between the intra-ocular muscles (controlling accommodation) and extra-ocular muscles (controlling versions and vergence), and the need to conduct dynamic retinoscopy on any child with developmental challenges.
4. Visual field as pertaining to reading: “During reading, several functions occur simultaneously. While reading a word, a visual map of the text is created. The map is instantly passed to planning of oculomotor functions in the executive command functions (in the frontal lobes), which get ready to activate the 12 eye muscles as soon as fixation is detached from the word.”
5. As pertains to the visual complexity of reading: “Reading is the most common problem in school referrals. It is a good example of numerous brain functions supporting a demanding task. Reading requires several simultaneous functions in the visual processing and in the visuomotor functions before the information can be used as language. In Chapter 2 we learned that fixation and saccades are more demanding than the conscious fixation and saccadic movements assessed during clinical examinations.”
6. Regarding trans-disciplinary collaboration: “Sometimes changes in motor functions are complex and require close collaboration between the doctors and the school to find optimal devices and ergonomic solutions. The educational resource centers together with the rehabilitation ophthalmologist, orthoptist, optometrist, rehabilitation team, and the school’s occupational and physiotherapists can tackle these problems which are difficult to treat at the hospitals because they lack the special knowledge on inclusive education.”
7. As related to #6: “Children with visual processing disorders may also have other problems in brain functions: in attention, executive functions, motor functions, and auditory processing functions. Many children have atypical peripheral functions in their eyes and ears. The assessment of these children’s many atypical functions and planning of early intervention and education should be supported by information from all medical specialties involved, optometry, and social services. With adequate support, children with complex problems in brain functions may develop, study well, and become the next generation of young workers who can provide first-hand information on what it means to grow and learn using atypical brain functions.”
8. With regard to the importance of communication: “Oculomotor functions can be observed after a short training, but the interpretations are often difficult. A perfect assessment is not the goal. The goal is to understand the information related to the child’s problems and to discuss the child’s functions and functioning at school. Doctors are becoming aware of the need to describe oculomotor functions better and describe behaviors that indicate a problem in oculomotor functions.”
9. Motion perception as related to neurology: “Visual information moves as magnocellular information via the retinal ganglion cell axons to the ‘superior colliculus’ in the tectum and the ‘pulvinar’ in the thalamus. Pulvinar connects to area V5 and through it but also directly to V1, V2, V3, and parietal cortex …The tectopulvinar pathway supports visual functions if the retinocalcarine pathway is damaged between LGN and V1 in the optic radiation, which is a common finding in brain damage around the ventricles. In this damage, form perception may be lost but motion perception may function.”
10. As related to #9: “One of the typical unusual behaviors of children with poor motion perception is walking fast and bumping into large objects. If the child can perceive only very slow movement, objects in their side vision are blurred and uncomfortable when they move at their usual speed. Objects disappear completely if the speed is increased. When the “blurred tunnel” disappears, objects far away become visible because their relative speed is low. Since the objects close by are not seen, the child may bump into large objects and people. This is often misinterpreted as a sign of poor attention, but in reality, the child is functionally blind in relation to the objects he passes close by.
11. Regarding spatial awareness and orientation: “Perception of near and far space need to be assessed as separate functions. Some children function well in the small egocentric space and are quick in building puzzles. They seem to have no difficulties in solving three-dimensional puzzles (if they have good picture perception and recognition) but have major problems perceiving and remembering relationships in large allocentric spaces.”
12. Lea is a proponent of schools using pictures, numbers, on transparent film or plexiglass so that recordings can be made of eye hand accuracy in scanning and localizing, as well as head and eye position as the patient is responding.
Lea’s slim volume doesn’t follow a typical textbook format, and reads more like a hard-bound monograph. But it is well-organized for its purpose, and provides valuable videos as well as a copy of the complete text in its accompanying USB. At $99 for the hardback and thumb drive, it is a worthwhile investment.
Next up is Chapter 52 on Learning and Memory, authored by the seemingly law-firm sounding trio of Shohamy, Schacter and Wagner. Dan Schacter may be the best recognized of the three, having written a book on the seven sins of memory for popular consumption which was an outgrowth of an article on the subject published in the American Psychologist, as well as a video on true vs. false memories featuring Alan Alda of M*A*S*H notoriety.
After reading Chapter 52, I would have been even more pointed in my response, factoring in the role of working memory. The authors write: “In humans, working memory consists of at least two subsystems – one for verbal information and another for visuospatial information. The functioning of these two subsystems is coordinated by a third system called the executive control processes. Executive control processes are thought to allocate attentional resources to the verbal and visuospatial subsystems and to monitor, manipulate, and update stored representations. The visuospatial subsystem of working memory retains mental images of visual objects and of the location of objects in space. The rehearsal of spatial and object information is thought to involve modulation of this information in the parietal, inferior temporal, and occipital cortices by the frontal and premotor cortices.”
Ah … the pervasiveness of vision throughout the brain.
Various forms of implicit memory subserve our daily routines. These include the learning of habits and motor, perceptual, and cognitive skills, and the formation and expression of conditioned responses. Generally speaking these forms of implicit memory are characterized by incremental learning, which proceeds gradually with repetition and aided in many instances by reinforcement and reward.
Research has shown that visual perception and memory are the most important components of vision processing in the brain. It was thought that the perceptual aspect of a visual stimulus occurs in visual cortical areas and that this serves as the substrate for the formation of visual memory in a distinct part of the brain called the medial temporal lobe (MT). However, current evidence indicates that there is no functional separation of areas. Entire visual cortical pathways and connecting medial temporal lobe are important for both perception and visual memory. The relationship between MT and vision is so intimate that it has a dual label of MT/V5 – the middle ground between the dorsal visual pathway above and the ventral visual stream below.
The medial temporal lobes are thought to be involved in several kinds of incremental learning. For example, implicit learning of regularities between visual cues is called statistical learning. This is essential to how learning takes place through repetition. Skill learning moves from a cognitive stage, where knowledge is represented explicitly, and the learner must pay a great deal of attention to performance, to an autonomous stage where the skill can be executed with a minimum of conscious attention. In essence, the learning of sensorimotor skills depends on numerous brain regions that vary depending on the specific associations being learned.
On this visual processing tour from Principles of Neural Science we’ve passed through low-level processing in the retina parsing patterns of light in part 3 to intermediate levels of processing so-called visual primitives in part 4, and now arrive at high-level visual processing which integrates information from a variety of sources as the final stage in the visual pathway leading to visual perception. In the authors’ words: “High-level visual processing is concerned with identifying behaviorally meaningful features of the environment and thus depends on descending signals that convey information from short-term working memory, long-term memory, and executive areas of cerebral cortex.” In figure 24-1 the authors use the properties of a horse to represent high-level visual processing. (Take note of emotional valence, a feature we previously blogged about regarding strabismus.)
The late, great Dr. Irwin Suchoff, in the early days of the Optometric Center of New York, would often speak about “the invariant” in this regard. Albright and Freiwald, co-authors of chapter 24, do a nice job of addressing the subject as follows:
“The ability to recognize objects as the same under different viewing conditions, despite the sometimes markedly different retinal images, is one of the most functionally important requirements of visual experience. The invariant attributes of an object – for example, spatial and chromatic relationships between image features or characteristic features such as the stripes of a zebra – are cues to the identity and meaning of objects. For object recognition to take place, these invariant attributes must be represented independently of other image properties. The visual system does this with proficiency, and its behavioral manifestation is termed perceptual constancy.”
If you haven’t read Dr. Suchoff’s monograph on visual-spatial development in the child before, it is now available through OEPF bundled together with Dr. Gerry Getman’s monograph on optometric care of children’s vision.
Neurologically, the center of object recognition has been localized to the inferior temporal cortex. Object recognition relies on past experience. Perceptual learning can improve the ability to discriminate between complex objects and refine neural selectivity in inferior temporal cortex. This and many other aspects of Chapter 24 are summarized at NeuPsyKey.com.
It is also worth pointing out that the parsing of visual information processing we have overviewed is consistent with the clinical model advanced by Dr. Lea Hyvärinen with regard to visual cognition, depicted here:
Chapter 9 – Processing of Visual Information 9.1 Brain structures involved in visual processing 9.2 Typical Behaviours 9.3 Assessment, general considerations 9.3.1 Functions to be tested 9.4 Early visual processing functions 9.4.1 Direction and length in the early processing of visual information 9.5 Ventral stream functions 9.5.1 Pictures as representations of objects and activities 9.5.2 Copying basic forms, texts, and pictures as visual tasks 9.5.3 Face blindness, prosopagnosia 9.5.4 Perception and recognition of facial expressions 9.5.5 Recognition of concrete objects and landmarks 9.6 Reading 9.7 Mathematical spatial, memory and recognition problems 9.8 Dorsal stream functions 9.8.1 Spatial awareness and orientation 9.9 Depth perception 9.10 Simultaneous perception, simultanagnosia 9.11 Eye-hand coordination 9.12 Integration of sensory and motor information 9.13 Mirror neuron system 9.14 Visual and auditory overload 9.15 Reporting on visual processing problems 9.15.1 Profile of visual functioning 9.15.2 Dual sensory processing losses
In surveying previous chapters of Principle of Neural Science, we saw how sophisticated retinal circuitry compresses light information into signals representing contrast and movement. These signals are transmitted through the fiber optic cable we know as the optic nerve, ultimately reaching the visual cortex which uses that information along with other sources of information to analyze objects of regard. This process of using line segments to represent specific objects in known as contour integration, and is the subject of Chapter 24 which is titled: “Intermediate-Level Visual Processing and Visual Primitives“.
Intermediate-level visual processing involves assembling local elements of an image into a unified percept of objects and background. Each relay in the visual circuitry of the brain has built-in logic that allows assumptions to be made about the likely spatial relationships between elements. In certain cases, these inherent rules can lead to the illusion of contours and surface that do not actually exist in the visual field. Illusory contours are purposeful properties of the visual system that allow us to make predictions based on past experiences and contexts. Yet it does expose us to potential ambiguities.
The left image below is the Kanizsa triangle illusion, in which one perceives continuous boundaries extending between the apices of a white triangle, even though the only real contour elements are formed by the Pac-Man-like figures and the acute angle. The image on the right shows that the inside and outside of the illusory pink square are the same white color as the screen background, but a continuous transparent pink surface within the square is perceived.
Three interacting features of visual processing help overcome ambiguity in the signals from the retina and are vital to the visual analysis of complex scenes:
The way in which a visual feature is perceived depends on everything that surrounds it. The response of a neuron in the visual cortex is context-dependent, depending as much on the presence of contours and surfaces outside the cell’s receptive field as much as on the attributes within it.
The functional properties of neurons in the visual cortex can be altered by visual experience or perceptual learning.
Visual processing in the cortex is subject to the influence of cognitive functions, specifically attention, expectation, and perceptual task – which is the active engagement in visual discrimination or detection.
The balance of this chapter focuses on how depth perception helps segregate objects from background. An important cue for the perception of depth is the difference between the two eyes’ view of the world, which must be computed and reconciled by the brain. The integration of binocular input begins in the primary visual cortex, the first level at which individual neurons receive signals from both eyes. The balance of input from the two eyes varies among cells in V1.
Binocular neurons in many visual cortical areas other than V1 are also selective for depth, which is computed from the relative retinal positions of objects at different distances and angles from the observer creating binocular disparity. Individual neurons can be selective for a narrow range of disparities. Some are selective for objects within the plane of fixation whereas others respond only when objects lie in front of the plane (near cells) or behind the plane (far cells).
Depth plays an important role in the perception of the 3-D properties of a scene. The same type of perceptual filling-in that occurs in an illusory sense described above now becomes purposeful in the real world. A surface passing behind an object is perceived as continuous even though its 2-D image on each retina represents two surfaces separated by the occluding object. When the brain encounters a surface interrupted by gaps that have appropriate alignment and contrast, and lying in the near depth plane, it fills in the gaps to create a continuous surface. Here the perceptual filling-in can be conceived as 3-D visual closure.
Whereas the depth of a single object can be established relatively easily, determining the depth of multiple objects with a scene is a much more complex task. The disparity calculation now becomes global rather than local. The calculation in one part of the visual scene influences and disambiguates information in other parts of the visual scene. This process is known as disparity capture. It is believed that the type of foreground emergence from background underlying local stereopsis occurs at the level of V1, as distinct from figural emergence in global stereopsis which occurs at the level of V2.
I believe this is why it is incorrect to insist on random dot stereopsis as the sine qua non of depth perception. True, RDS requires simultaneous bifoveal comparisons between the two eyes, but it is not necessarily the most useful form of depth perception. Context is the key. RDS is marvelous for figure-ground identification in complex scenes; but this type of global stereopsis is not as relevant when dealing with local primitives. Give me global RDS when I need to parallel park, but good ol’ high level Wirt Circle stereoacuity when hammering a nail. Give me RDS to confirm the absence of strabismus, but let me pinch the wings of the fly to tell you about the utility of binocular summation for most activities of daily living (ADLs).
Speaking of ADLs, intermediate level vision is an aspect of acquired brain injury (ABI) that tends to be glossed over. In our chapter on Spatial Vision in Suter & Harvey’s book on Vision Rehabilitation, Bob Sanet and I refer to the fragmentation or dis-integration of vision that occurs, and our role in guiding the patient toward re-assembly or re-integration. After reading Chapter 24 in Principles of Neural Science, I would propose that what occurs to various degrees in ABI is a loss of disambiguation. The regression to a more primitive state of ambiguity understandably stymies the individual through uncertainty that restrains or inhibits self-guided action. Part of what we do through lenses, prisms or active therapy is set conditions for exploration that restore these crucial intermediate levels of visual disambiguation, thereby aiding awareness and confidence in visually guided action. Motion parallax and stereoscopic judgment are merely two overt examples in our toolbox of cues used at intermediate levels of visual processing.