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Is the eyes' movement discrete?

Is the eyes' movement discrete?



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When tired and it was dark, I noticed that if I focused on a dim light source and moved my eyes fairly rapidly sideways, the resulting images that lingered for a short while were not smoothly blurred together, but were discrete.

I assume this is not due to the brain's inability to process the data fast enough or the retinas not repairing fast enough when light breaks part of them down, so I am lead to the conclusion that eyes (if not generally, sometimes) move in very small sudden jolts rather than smoothly.

Is this correct? What is the reason for this (is it due to the eye muscles being unable to sustain motion for a long time, or something to do with the mechanics of the eye)? Was this result anomalous?


There are two types of eye movement: smooth pursuit and saccadic. As the name suggests the latter movement involves quick and discontinuous movements of the eyes. Saccadic movement is used most of the time as the eyes move around analysing the current scene.

According to Wikipedia, these saccades are the fastest movements produced by the human body: peak angular speed of 900 degrees of rotation per second (this would be 2.5 full rotations of the eyeball per second, were such a thing possible).

The processing of information by the eye-brain system during a saccade is complex, and may be responsible for the effect that you report: it causes the phenomenon of saccadic masking explained in this quote from Wikipedia:

A person may observe the saccadic masking effect by standing in front of a mirror and looking from one eye to the next (and vice versa). The subject will not experience any movement of the eyes nor any evidence that the optic nerve has momentarily ceased transmitting. Due to saccadic masking, the eye/brain system not only hides the eye movements from the individual but also hides the evidence that anything has been hidden. Of course, a second observer watching the experiment will see the subject's eyes moving back and forth. The function's main purpose is to prevent smearing of the image.


12.10: Types of Body Movements

Synovial joints allow the body a tremendous range of movements. Each movement at a synovial joint results from the contraction or relaxation of the muscles that are attached to the bones on either side of the articulation. The type of movement that can be produced at a synovial joint is determined by its structural type. While the ball-and-socket joint gives the greatest range of movement at an individual joint, in other regions of the body, several joints may work together to produce a particular movement. Overall, each type of synovial joint is necessary to provide the body with its great flexibility and mobility. There are many types of movement that can occur at synovial joints (Table 1). Movement types are generally paired, with one being the opposite of the other. Body movements are always described in relation to the anatomical position of the body: upright stance, with upper limbs to the side of body and palms facing forward.

Watch this video to learn about anatomical motions. What motions involve increasing or decreasing the angle of the foot at the ankle?

A YouTube element has been excluded from this version of the text. You can view it online here: pb.libretexts.org/aapi/?p=248


What Can You Expect From EMDR?

An EMDR treatment session can last up to 90 minutes. Your therapist will move their fingers back and forth in front of your face and ask you to follow these hand motions with your eyes. At the same time, the EMDR therapist will have you recall a disturbing event. This will include the emotions and body sensations that go along with it.

Gradually, the therapist will guide you to shift your thoughts to more pleasant ones. Some therapists use alternatives to finger movements, such as hand or toe tapping or musical tones.

People who use the technique argue that EMDR can weaken the effect of negative emotions. Before and after each EMDR treatment, your therapist will ask you to rate your level of distress. The hope is that your disturbing memories will become less disabling.

Although most research into EMDR has examined its use in people with PTSD, EMDR is sometimes used experimentally to treat many other psychological problems. They include:


Structure and Function of the Eyes

The structures and functions of the eyes are complex. Each eye constantly adjusts the amount of light it lets in, focuses on objects near and far, and produces continuous images that are instantly transmitted to the brain.

The orbit is the bony cavity that contains the eyeball, muscles, nerves, and blood vessels, as well as the structures that produce and drain tears. Each orbit is a pear-shaped structure that is formed by several bones.

An Inside Look at the Eye

The outer covering of the eyeball consists of a relatively tough, white layer called the sclera (or white of the eye).

Near the front of the eye, in the area protected by the eyelids, the sclera is covered by a thin, transparent membrane (conjunctiva), which runs to the edge of the cornea. The conjunctiva also covers the moist back surface of the eyelids and eyeballs.

Light enters the eye through the cornea, the clear, curved layer in front of the iris and pupil. The cornea serves as a protective covering for the front of the eye and also helps focus light on the retina at the back of the eye.

After passing through the cornea, light travels through the pupil (the black dot in the middle of the eye).

The iris—the circular, colored area of the eye that surrounds the pupil—controls the amount of light that enters the eye. The iris allows more light into the eye (enlarging or dilating the pupil) when the environment is dark and allows less light into the eye (shrinking or constricting the pupil) when the environment is bright. Thus, the pupil dilates and constricts like the aperture of a camera lens as the amount of light in the immediate surroundings changes. The size of the pupil is controlled by the action of the pupillary sphincter muscle and dilator muscle.

Behind the iris sits the lens. By changing its shape, the lens focuses light onto the retina. Through the action of small muscles (called the ciliary muscles), the lens becomes thicker to focus on nearby objects and thinner to focus on distant objects.

The retina contains the cells that sense light (photoreceptors) and the blood vessels that nourish them. The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors (the type called cones). The high density of cones in the macula makes the visual image detailed, just as a high-resolution digital camera has more megapixels.

Each photoreceptor is linked to a nerve fiber. The nerve fibers from the photoreceptors are bundled together to form the optic nerve. The optic disk, the first part of the optic nerve, is at the back of the eye.

The photoreceptors in the retina convert the image into electrical signals, which are carried to the brain by the optic nerve. There are two main types of photoreceptors: cones and rods.

Cones are responsible for sharp, detailed central vision and color vision and are clustered mainly in the macula.

Rods are responsible for night and peripheral (side) vision. Rods are more numerous than cones and much more sensitive to light, but they do not register color or contribute to detailed central vision as the cones do. Rods are grouped mainly in the peripheral areas of the retina.

The eyeball is divided into two sections, each of which is filled with fluid. The pressure generated by these fluids fills out the eyeball and helps maintain its shape.

The front section (anterior segment) extends from the inside of the cornea to the front surface of the lens. It is filled with a fluid called the aqueous humor, which nourishes the internal structures. The anterior segment is divided into two chambers. The front (anterior) chamber extends from the cornea to the iris. The back (posterior) chamber extends from the iris to the lens. Normally, the aqueous humor is produced in the posterior chamber, flows slowly through the pupil into the anterior chamber, and then drains out of the eyeball through outflow channels located where the iris meets the cornea.

The back section (posterior segment) extends from the back surface of the lens to the retina. It contains a jellylike fluid called the vitreous humor.

Tracing the Visual Pathways

Nerve signals travel from each eye along the corresponding optic nerve and other nerve fibers (called the visual pathway) to the back of the brain, where vision is sensed and interpreted. The two optic nerves meet at the optic chiasm, which is an area behind the eyes immediately in front of the pituitary gland and just below the front portion of the brain (cerebrum). There, the optic nerve from each eye divides, and half of the nerve fibers from each side cross to the other side and continue to the back of the brain. Thus, the right side of the brain receives information through both optic nerves for the left field of vision, and the left side of the brain receives information through both optic nerves for the right field of vision. The middle of these fields of vision overlaps. It is seen by both eyes (called binocular vision).

An object is seen from slightly different angles by each eye so the information the brain receives from each eye is different, although it overlaps. The brain integrates the information to produce a complete picture.


What is EMDR?

Eye Movement Desensitization and Reprocessing (EMDR) is a psychotherapy treatment that was originally designed to alleviate the distress associated with traumatic memories (Shapiro, 1989a, 1989b). Shapiro’s (2001) Adaptive Information Processing model posits that EMDR therapy facilitates the accessing and processing of traumatic memories and other adverse life experience to bring these to an adaptive resolution. After successful treatment with EMDR therapy, affective distress is relieved, negative beliefs are reformulated, and physiological arousal is reduced. During EMDR therapy the client attends to emotionally disturbing material in brief sequential doses while simultaneously focusing on an external stimulus. Therapist directed lateral eye movements are the most commonly used external stimulus but a variety of other stimuli including hand-tapping and audio stimulation are often used (Shapiro, 1991). Shapiro (1995, 2001) hypothesizes that EMDR therapy facilitates the accessing of the traumatic memory network, so that information processing is enhanced, with new associations forged between the traumatic memory and more adaptive memories or information. These new associations are thought to result in complete information processing, new learning, elimination of emotional distress, and development of cognitive insights. EMDR therapy uses a three pronged protocol: (1) the past events that have laid the groundwork for dysfunction are processed, forging new associative links with adaptive information (2) the current circumstances that elicit distress are targeted, and internal and external triggers are desensitized (3) imaginal templates of future events are incorporated, to assist the client in acquiring the skills needed for adaptive functioning.

FOR LAYPEOPLE

EMDR (Eye Movement Desensitization and Reprocessing) is a psychotherapy that enables people to heal from the symptoms and emotional distress that are the result of disturbing life experiences. Repeated studies show that by using EMDR therapy people can experience the benefits of psychotherapy that once took years to make a difference. It is widely assumed that severe emotional pain requires a long time to heal. EMDR therapy shows that the mind can in fact heal from psychological trauma much as the body recovers from physical trauma. When you cut your hand, your body works to close the wound. If a foreign object or repeated injury irritates the wound, it festers and causes pain. Once the block is removed, healing resumes. EMDR therapy demonstrates that a similar sequence of events occurs with mental processes. The brain’s information processing system naturally moves toward mental health. If the system is blocked or imbalanced by the impact of a disturbing event, the emotional wound festers and can cause intense suffering. Once the block is removed, healing resumes. Using the detailed protocols and procedures learned in EMDR therapy training sessions, clinicians help clients activate their natural healing processes.

More than 30 positive controlled outcome studies have been done on EMDR therapy. Some of the studies show that 84%-90% of single-trauma victims no longer have post-traumatic stress disorder after only three 90-minute sessions. Another study, funded by the HMO Kaiser Permanente, found that 100% of the single-trauma victims and 77% of multiple trauma victims no longer were diagnosed with PTSD after only six 50-minute sessions. In another study, 77% of combat veterans were free of PTSD in 12 sessions. There has been so much research on EMDR therapy that it is now recognized as an effective form of treatment for trauma and other disturbing experiences by organizations such as the American Psychiatric Association, the World Health Organization and the Department of Defense. Given the worldwide recognition as an effective treatment of trauma, you can easily see how EMDR therapy would be effective in treating the “everyday” memories that are the reason people have low self-esteem, feelings of powerlessness, and all the myriad problems that bring them in for therapy. Over 100,000 clinicians throughout the world use the therapy. Millions of people have been treated successfully over the past 25 years.

EMDR therapy is an eight-phase treatment. Eye movements (or other bilateral stimulation) are used during one part of the session. After the clinician has determined which memory to target first, he asks the client to hold different aspects of that event or thought in mind and to use his eyes to track the therapist’s hand as it moves back and forth across the client’s field of vision. As this happens, for reasons believed by a Harvard researcher to be connected with the biological mechanisms involved in Rapid Eye Movement (REM) sleep, internal associations arise and the clients begin to process the memory and disturbing feelings. In successful EMDR therapy, the meaning of painful events is transformed on an emotional level. For instance, a rape victim shifts from feeling horror and self-disgust to holding the firm belief that, “I survived it and I am strong.” Unlike talk therapy, the insights clients gain in EMDR therapy result not so much from clinician interpretation, but from the client’s own accelerated intellectual and emotional processes. The net effect is that clients conclude EMDR therapy feeling empowered by the very experiences that once debased them. Their wounds have not just closed, they have transformed. As a natural outcome of the EMDR therapeutic process, the clients’ thoughts, feelings and behavior are all robust indicators of emotional health and resolution—all without speaking in detail or doing homework used in other therapies.

TREATMENT DESCRIPTION

EMDR therapy combines different elements to maximize treatment effects. A full description of the theory, sequence of treatment, and research on protocols and active mechanisms can be found in F. Shapiro (2001) Eye movement desensitization and reprocessing: Basic principles, protocols and procedures (2nd edition) New York: Guilford Press.

EMDR therapy involves attention to three time periods: the past, present, and future. Focus is given to past disturbing memories and related events. Also, it is given to current situations that cause distress, and to developing the skills and attitudes needed for positive future actions. With EMDR therapy, these items are addressed using an eight-phase treatment approach.

Phase 1: The first phase is a history-taking session(s). The therapist assesses the client’s readiness and develops a treatment plan. Client and therapist identify possible targets for EMDR processing. These include distressing memories and current situations that cause emotional distress. Other targets may include related incidents in the past. Emphasis is placed on the development of specific skills and behaviors that will be needed by the client in future situations.

Initial EMDR processing may be directed to childhood events rather than to adult onset stressors or the identified critical incident if the client had a problematic childhood. Clients generally gain insight on their situations, the emotional distress resolves and they start to change their behaviors. The length of treatment depends upon the number of traumas and the age of PTSD onset. Generally, those with single event adult onset trauma can be successfully treated in under 5 hours. Multiple trauma victims may require a longer treatment time.

Phase 2: During the second phase of treatment, the therapist ensures that the client has several different ways of handling emotional distress. The therapist may teach the client a variety of imagery and stress reduction techniques the client can use during and between sessions. A goal of EMDR therapy is to produce rapid and effective change while the client maintains equilibrium during and between sessions.

Phases 3-6: In phases three to six, a target is identified and processed using EMDR therapy procedures. These involve the client identifying three things:
1. The vivid visual image related to the memory
2. A negative belief about self
3. Related emotions and body sensations.

In addition, the client identifies a positive belief. The therapist helps the client rate the positive belief as well as the intensity of the negative emotions. After this, the client is instructed to focus on the image, negative thought, and body sensations while simultaneously engaging in EMDR processing using sets of bilateral stimulation. These sets may include eye movements, taps, or tones. The type and length of these sets is different for each client. At this point, the EMDR client is instructed to just notice whatever spontaneously happens.

After each set of stimulation, the clinician instructs the client to let his/her mind go blank and to notice whatever thought, feeling, image, memory, or sensation comes to mind. Depending upon the client’s report, the clinician will choose the next focus of attention. These repeated sets with directed focused attention occur numerous times throughout the session. If the client becomes distressed or has difficulty in progressing, the therapist follows established procedures to help the client get back on track.

When the client reports no distress related to the targeted memory, (s)he is asked to think of the preferred positive belief that was identified at the beginning of the session. At this time, the client may adjust the positive belief if necessary, and then focus on it during the next set of distressing events.

Phase 7: In phase seven, closure, the therapist asks the client to keep a log during the week. The log should document any related material that may arise. It serves to remind the client of the self-calming activities that were mastered in phase two.

Phase 8: The next session begins with phase eight. Phase eight consists of examining the progress made thus far. The EMDR treatment processes all related historical events, current incidents that elicit distress, and future events that will require different responses


6 Cataract Risk Trends And Eye Color

Although it is tempting to assume we are all the same, seemingly superficial traits can directly affect our health predispositions in sometimes counterintuitive ways. People with light eyes tend to have light skin that burns easily in sunlight, yet scientific findings published following research in Sydney, Australia, indicate an increased incidence of certain cataracts in individuals with dark eyes at a rate of 2.5-fold. It appears that those with dark eyes are simply significantly more vulnerable to this form of harmful change over time than light-eyed humans.

Robert Cumming, PhD, points out that a certain type of cataract was significantly more likely to develop in darker-eyed individuals than those with blue or hazel eyes. Sunlight was identified as a potential factor through speculation that dark eye colors absorb sunlight in the same way black surfaces do. While sunlight may be a factor, increased cataract risks were found even in those who were not spending an inordinate amount of time in the sun. This might suggest that internal factors are also at play.


1 Introduction

The nervous system faces a dual challenge in shaping behavior. To induce changes in the external world, it is necessary to contract muscles or secrete chemicals. Such processes necessarily involve the manipulation of continuous variables—muscle length or chemical concentration. In addition, animals must make decisions. To do so, they must entertain several different possible courses of action, or policies. Ultimately, they must select one of these actions or policies that are necessarily discrete. We draw on recent work that considers the interactions between the neuronal processing of discrete and continuous quantities (Friston, Parr, & de Vries, 2017). To make this more concrete, we focus on the oculomotor system. Sampling the visual world entails decisions about where to look and the implementation of these decisions by contraction of the extraocular muscles.

We use perceptual inference performed by the networks supporting eye movements as a way of motivating and illustrating the theoretical challenge we want to address. However, the treatment we offer generalizes to any system that involves the physical implementation of categorical decisions. The ideas presented in this article complement previous treatments of cognitive time (VanRullen & Koch, 2003), including the notion of a perceptual moment (Allport, 1968 Shallice, 1964 Stroud, 1967) and the suggestion that brain oscillations act as discrete clocks to support this type of computation (Buschman & Miller, 2009, 2010). They also resonate with recent developments in machine learning (Linderman et al., 2017) and some of the problems faced in modern robotics (Cowan & Walker, 2013 Schaal, 2006). In short, the coupling of categorical decision making and dynamic perception (and motor control) raises some deep questions about the temporal scheduling of perception (and action).

Oscillatory rhythms in measured brain activity have been linked to cyclical perceptual processes (Buzsaki, 2006), with theta and alpha cycles as the most popular hypothesized units of perceptual time (VanRullen, 2016). In endorsement of this, the timing of processing relative to the phase of certain oscillations appears to be important (Buzsáki, 2005). While there is some controversy concerning the frequency of the perceptual clock, an advantage to focusing on the oculomotor system is that we can evade this issue. The frequency of spontaneous saccadic sampling is around 4 Hz, allowing us to commit to a theta rhythm. Conveniently, this is the frequency often associated with attentional and central executive (decision) functions (Chelazzi, Miller, Duncan, & Desimone, 1993 Duncan, Ward, & Shapiro, 1994 Hanslmayr, Volberg, Wimber, Dalal, & Greenlee, 2013 Landau & Fries, 2012 VanRullen, 2013), as opposed to sensory processes associated with faster frequencies (Drewes & VanRullen, 2011 Dugué, Marque, & VanRullen, 2011 Ergenoglu et al., 2004 van Dijk, Schoffelen, Oostenveld, & Jensen, 2008).

The oculomotor system is a distributed network that includes brain stem, cortical, and subcortical regions (Parr & Friston, 2017a). An important point of contact between the cortical oculomotor networks and those in the brain stem is the superior colliculus (Raybourn & Keller, 1977), found in the midbrain. This structure receives a dual input from the cortex (Fries, 1984) and the basal ganglia (Hikosaka & Wurtz, 1983) and provides an important input to the brain stem oculomotor nuclei. In the following, we argue that the connectivity implied by active inference is consistent with a role for the superior colliculus as an interface between the discrete and continuous processing of the oculomotor system.

This article is organized as follows. In section 2, we review the principles of active inference, their application to discrete and continuous state spaces, and the relationship between the two. In section 3, we relate the computational anatomy implied by active inference to the neuroanatomy of oculomotion. In section 4, we illustrate oculomotor behavior, and its neural correlates, through simulation. Section 5 presents the discussion, and section 6 concludes.


Nervous System Parts

The anatomy of the nervous system in humans consists of the brain and spinal cord, along with the primary sense organs and all the nerves associated with these organs. The brain and the spinal cord form the central nervous system (CNS). All other neuronal tissue is brought under the umbrella of the peripheral nervous system (PNS). Therefore, the PNS includes neurons within sense organs, other sensory nerves, and all motor nerves that deliver messages to different parts of the body.

Functionally, the organs of the nervous system can be further divided into different parts. For instance, the brain is situated within the cranial cavity and weighs less than 1.5 kgs. However, it is the seat for many higher-order mental functions, such as planning, consciousness, perception, and language. It is broadly divided into the cerebrum, cerebellum, and medulla. The cerebrum is the largest part and is the section that is seen most obviously in external pictorial representations of the organ. It contains two hemispheres of nearly equal size and each hemisphere has four lobes. These lobes, called the parietal, temporal, frontal and occipital, have distinct functions, being involved in impulse control, problem-solving, visual perception, hearing, language, and speech. Though the hemispheres of the brain have some extent of plasticity, specific tasks remain localized to specific sections of the cerebral cortex.

Neurons form the basic functional unit of the nervous system. They can be afferent or efferent neurons based on whether they carry information towards the CNS or transmit signals from the CNS. Some, called interneurons, are important to integrate information from different stimuli and to create a unified response.


Role of eye movements in the retinal code for a size discrimination task.

<p>The concerted action of saccades and fixational eye movements are crucial for seeing stationary objects in the visual world. We studied how these eye movements contribute to retinal coding of visual information using the archer fish as a model system. We quantified the animal's ability to distinguish among objects of different sizes and measured its eye movements. We recorded from populations of retinal ganglion cells with a multielectrode array, while presenting visual stimuli matched to the behavioral task. We found that the beginning of fixation, namely the time immediately after the saccade, provided the most visual information about object size, with fixational eye movements, which consist of tremor and drift in the archer fish, yielding only a minor contribution. A simple decoder that combined information from <or=15 ganglion cells could account for the behavior. Our results support the view that saccades impose not just difficulties for the visual system, but also an opportunity for the retina to encode high quality "snapshots" of the environment.</p>


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