Color Perception
Color Vision
Perceptual Closure
Color
Figural Aftereffect
Adaptation, Ocular
Photic Stimulation
Flicker Fusion
Contrast Sensitivity
Visual Pathways
Psychophysics
Encyclopedias as Topic
Retina
Adaptation, Physiological
Spatio-temporal contrast sensitivity, coherent motion, and visible persistence in developmental dyslexia. (1/121)
Three experiments measured spatio-temporal contrast sensitivity, coherent motion, and visible persistence in a single group of children with developmental dyslexia and a matched control group. The findings were consistent with a transient channel disorder in the dyslexic group which showed a reduction in contrast sensitivity at low spatial frequencies, a significant reduction in sensitivity for coherent motion, and a significantly longer duration of visible persistence. The results were also examined by classifying the dyslexic group into dyseidetic, dysphonetic, and mixed (dysphoneidetic) subgroups. There were no differences between the control and dyseidetic groups in contrast sensitivity, in coherent motion and in visible persistence. In comparison to the control group, the mixed (dysphoneidetic) dyslexic subgroup was found to have a significant reduction in contrast sensitivity at low spatial frequencies, a significant reduction in sensitivity for coherent motion, and a significantly longer duration of visible persistence. In comparison to the control group, the dysphonetic group only showed a reduction in contrast sensitivity at low spatial frequencies. Comparisons between the dyseidetic, dysphonetic and mixed dyslexic subgroups showed that there were no substantive differences in contrast sensitivity, coherent motion, and visible persistence. The results support the proposal and findings by Borsting et al. (Borsting E, Ridder WH, Dudeck K, Kelley C, Matsui L, Motoyama J. Vis Res 1996;36:1047-1053) that a transient channel disorder may only be present in a dysphoneidetic dyslexic subgroup. Psychometric assessment revealed that all the children with dyslexia appear to have a concurrent disorder in phonological coding, temporal order processing, and short-term memory. (+info)Enhanced motion aftereffect for complex motions. (2/121)
We measured the magnitude of the motion after effect (MAE) elicited by gratings viewed through four spatial apertures symmetrically positioned around fixation. The gratings were identical except for their orientations, which were varied to form patterns of global motion corresponding to radiation, rotation or translation. MAE magnitude was estimated by three methods: the duration of the MAE; the contrast required to null the MAE and the threshold elevation for detecting an abrupt jump. All three techniques showed that MAEs for radiation and rotation were greater than those for translation. The greater adaptability of radiation and rotation over translation also was observed in areas of the display where no adapting stimulus had been presented. We also found that adaptation to motion in one direction had equal effects on sensitivity to motion in the same and opposite directions. (+info)Colour at edges and colour spreading in McCollough effects. (3/121)
Broerse and O'Shea [(1995) Vision Research, 35, 207-226] proposed that the subjective colours in McCollough effects (MEs) consist of two components: edge colours appearing along the edges of contours, and spread colours radiating from edge colours into adjacent uncontoured regions of test patterns. This proposal was examined in five experiments. First, we demonstrated that fine coloured lines located immediately adjacent to the edges of otherwise achromatic square-wave gratings (i.e. colour-fringed gratings) are sufficient to induce MEs comparable in strength to MEs induced with desaturated versions of traditional uniformly-coloured gratings (Experiments 1 & 2). We then quantified edge and spread colours while varying light/dark duty cycles (white-bar width) in gratings with colour-fringed edges (Experiment 3), uniformly-coloured gratings (Experiment 4), and in achromatic gratings tinged with ME colours after adaptation to colour-fringed gratings (Experiment 5). Whereas the perceived magnitude of edge colours remained constant in all cases, spread colours remained constant only for uniformly-coloured gratings. For both MEs and gratings with colour-fringed edges, spread colours decreased as a function of increasing duty cycle, confirming that conventional MEs may be simulated by gratings with colour-fringed edges. We propose that edge colours arise as a consequence of neural operations correcting for the eye's chromatic aberration, while spread colours reveal a neural filling-in process operating to achieve colour constancy. In seeking to implement these suggestions, we present a putative framework based on the receptive-field properties of single cells described in contemporary neurophysiological investigations of colour. (+info)Afterimages, grating induction and illusory phantoms. (4/121)
Under some conditions (dark or light inspection areas) illusory gratings often appear to be in-phase with the inducing gratings and under others (gray inspection area) illusory gratings often appear to be out-of-phase with the inducing gratings. McCourt reported that point-by-point brightness matches reveal only out-of-phase illusory gratings, no matter what the luminance of the inspection area (McCourt, M. E. (1994). Vision Research, 34, 1609-1617). Since the technique used might have led to afterimages which mimic out-of-phase illusory gratings, the present series of experiments was undertaken to determine how such afterimages might bias illusory grating judgments. Afterimages were induced during fixation with brief flashes of inducing gratings within the inspection area (Experiment 1), or by vertical shifts in the entire stimulus which exposed the retina to real gratings prior to judgments within the inspection area (Experiment 2). Experiment 2 was replicated with drifting inducing gratings (Experiment 3). The subjects were asked to indicate whether illusory gratings appeared in- or out-of-phase. The results of all three experiments reveal that out-of-phase illusory gratings predominate, and that afterimages can only bias judgments with stationary displays. It is suggested that grating induction is perceived when subjects attend to local contrast differences, while phantom visibility is facilitated when attention is captured by the more global aspects of the stimulus. (+info)A diagnostic sign in migraine? (5/121)
At the bedside it was noted that, after ocular fundoscopy, patients with migraine complained more often of an after-image than did non-migraineurs. This phenomenon was then investigated in consecutive patients attending a general neurology outpatient clinic. The relative risk for the diagnosis of migraine in patients reporting an after-image was 2.91 (95% confidence interval 1.96 to 4.34), and the sensitivity, specificity and positive predictive value of this observation for the diagnosis of migraine were 0.63, 0.75 and 0.55 respectively. After-images were equally likely to be reported by migraineurs with and without aura, and by patients with migraine equivalents. The after-image phenomenon probably reflects the heightened sensitivity to visual stimuli of patients with migraine. Although a diagnosis of migraine is primarily established by the patient's history, the presence of an after-image following ocular fundoscopy may support this diagnosis. (+info)Spatial aspects of object formation revealed by a new illusion, shine-through. (6/121)
When a vernier stimulus is presented for a short time and followed by a grating comprising five straight lines, the vernier remains invisible but may bequeath its offset to the grating (feature inheritance). For more than seven grating elements, the vernier is rendered visible as a shine-through element. However, shine-through depends strongly on the spatio-temporal layout of the grating. Here, we show that spatially inhomogeneous gratings diminish shine-through and vernier discrimination. Even subtle deviations, in the range of a few minutes of arc, matter. However, longer presentation times of the vernier regenerate shine-through. Feature inheritance and shine-through may become a useful tool in investigating such different topics as time course of information processing, feature binding, attention, and masking. (+info)Shine-through: temporal aspects. (7/121)
If a vernier stimulus precedes a grating for a very short time, the vernier either remains invisible, but may bequeath some of its properties to the grating (feature inheritance), or might shine through keeping its features - depending on the number of grating elements [Herzog, M. H. & Koch, C., 2001. Seeing properties of an invisible element: feature inheritance and shine-through. Proceedings of the National Academy of Science USA 98, 4271-4275]. Feature inheritance and shine-through represent two different states of feature binding [Herzog, M. H., Koch, C., & Fahle, M., Switching binding states. Visual Cognition (in press)], whereas shine-through depends in subtle ways on the spatial layout of the grating [Herzog, M. H., Fahle, M., & Koch, C., (2001). Spatial aspects of object formation revealed by a new illusion, shine-through Vision Research]. Here, we show that also temporal parameters of the grating influence shine-through. For example, a delayed presentation of certain grating elements can deteriorate performance dramatically. (+info)Are corresponding points fixed? (8/121)
Several investigators have claimed that the retinal coordinates of corresponding points shift with vergence eye movements. Two kinds of shifts have been reported. First, global shifts that increase with retinal eccentricity; such shifts would cause a flattening of the horopter at all viewing distances and would facilitate fusion of flat surfaces. Second, local shifts that are centered on the fovea; such shifts would cause a dimple in the horopter near fixation and would facilitate fusion of points fixated at extreme viewing distances. Nearly all of the empirical evidence supporting shifts of corresponding points comes from horopter measurements and from comparisons of subjective and objective fixation disparity. In both cases, the experimenter must infer the retinal coordinates of corresponding points from external measurements. We describe four factors that could affect this inference: (1) changes in the projection from object to image points that accompany eye rotation and accommodation, (2) fixation errors during the experimental measurements, (3) non-uniform retinal stretching, and (4) changes in the perceived direction of a monocular point when presented adjacent to a binocular point. We conducted two experiments that eliminated or compensated for these potential errors. In the first experiment, observers aligned dichoptic test lines using an apparatus and procedure that eliminated all but the third error. In the second experiment, observers judged the alignment of dichoptic afterimages, and this technique eliminates all the errors. The results from both experiments show that the retinal coordinates of corresponding points do not change with vergence eye movements. We conclude that corresponding points are in fixed retinal positions for observers with normal retinal correspondence. (+info)An afterimage is a visual phenomenon that occurs when the eye's retina continues to send signals to the brain even after exposure to a stimulus has ended. This can result in the perception of a lingering image, often in complementary colors to the original stimulus. Afterimages can be either positive or negative, with a positive afterimage appearing as the same color as the original stimulus and a negative afterimage appearing as its complementary color.
Afterimages are typically caused by exposure to bright or intense light sources, such as a camera flash or the sun. They can also occur after prolonged exposure to a particular color or pattern. The phenomenon is thought to be related to the adaptation of photoreceptor cells in the retina, which become less responsive to stimuli after prolonged exposure.
Afterimages are generally harmless and temporary, lasting only a few seconds to several minutes. However, they can sometimes be used as a tool for visual perception experiments or to study the mechanisms of visual processing in the brain.
An illusion is a perception in the brain that does not match the actual stimulus in the environment. It is often described as a false or misinterpreted sensory experience, where the senses perceive something that is different from the reality. Illusions can occur in any of the senses, including vision, hearing, touch, taste, and smell.
In medical terms, illusions are sometimes associated with certain neurological conditions, such as migraines, brain injuries, or mental health disorders like schizophrenia. They can also be a side effect of certain medications or substances. In these cases, the illusions may be a symptom of an underlying medical condition and should be evaluated by a healthcare professional.
It's important to note that while illusions are often used in the context of entertainment and art, they can also have serious implications for individuals who experience them frequently or as part of a medical condition.
Color perception refers to the ability to detect, recognize, and differentiate various colors and color patterns in the visual field. This complex process involves the functioning of both the eyes and the brain.
The eye's retina contains two types of photoreceptor cells called rods and cones. Rods are more sensitive to light and dark changes and help us see in low-light conditions, but they do not contribute much to color vision. Cones, on the other hand, are responsible for color perception and function best in well-lit conditions.
There are three types of cone cells, each sensitive to a particular range of wavelengths corresponding to blue, green, and red colors. The combination of signals from these three types of cones allows us to perceive a wide spectrum of colors.
The brain then interprets these signals and translates them into the perception of different colors and hues. It is important to note that color perception can be influenced by various factors, including cultural background, personal experiences, and even language. Some individuals may also have deficiencies in color perception due to genetic or acquired conditions, such as color blindness or cataracts.
Color vision is the ability to perceive and differentiate colors, which is a result of the way that our eyes and brain process different wavelengths of light. In the eye, there are two types of photoreceptor cells called rods and cones. While rods are more sensitive to low levels of light and help us see in dim conditions, cones are responsible for color vision.
There are three types of cone cells in the human eye, each containing a different type of pigment that is sensitive to specific wavelengths of light. One type of cone cell is most sensitive to short wavelengths (blue light), another is most sensitive to medium wavelengths (green light), and the third is most sensitive to long wavelengths (red light). When light enters the eye, it is absorbed by these pigments in the cones, which then send signals to the brain. The brain interprets these signals and translates them into the perception of color.
People with normal color vision can distinguish between millions of different colors based on the specific combinations of wavelengths that are present in a given scene. However, some people have deficiencies or abnormalities in their color vision, which can make it difficult or impossible to distinguish between certain colors. These conditions are known as color vision deficiencies or color blindness.
Perceptual closure, also known as "closure perception" or "gestalt perception," is not a term that has a specific medical definition. It is a concept in the field of psychology and perception, particularly in gestalt psychology.
Perceptual closure refers to the ability of the brain to recognize and complete incomplete patterns or shapes by filling in the missing information based on context and past experiences. This allows us to perceive and understand complex stimuli even when they are partially occluded, distorted, or incomplete. It is a fundamental aspect of how we process visual information and helps us quickly and efficiently make sense of our environment.
While there may not be a specific medical definition for perceptual closure, deficits in this ability can have implications for various medical conditions, such as neurological disorders that affect vision or cognitive function.
In the context of medical terminology, 'color' is not defined specifically with a unique meaning. Instead, it generally refers to the characteristic or appearance of something, particularly in relation to the color that a person may observe visually. For instance, doctors may describe the color of a patient's skin, eyes, hair, or bodily fluids to help diagnose medical conditions or monitor their progression.
For example, jaundice is a yellowing of the skin and whites of the eyes that can indicate liver problems, while cyanosis refers to a bluish discoloration of the skin and mucous membranes due to insufficient oxygen in the blood. Similarly, doctors may describe the color of stool or urine to help diagnose digestive or kidney issues.
Therefore, 'color' is not a medical term with a specific definition but rather a general term used to describe various visual characteristics of the body and bodily fluids that can provide important diagnostic clues for healthcare professionals.
"Figural aftereffect" is not a widely recognized or established term in medical or clinical neuroscience literature. However, it seems to be related to the concept of "perceptual aftereffects," which are well-documented phenomena in visual and other sensory perception. Here's a definition that may help you understand figural aftereffects within this context:
Perceptual aftereffect is a phenomenon where exposure to a specific stimulus for a certain period can temporarily alter the perception of subsequent stimuli, making them appear different from what they would have been without the initial exposure. This effect arises due to neural adaptation in response to the prolonged exposure.
In the case of "figural aftereffect," it likely refers to a specific type of perceptual aftereffect where the perception of figures or shapes is affected by prior exposure. For example, if someone stares at a curved line for a while and then looks at a straight line, they might initially perceive the straight line as being more curved than it actually is due to the lingering influence of the initial stimulus.
However, since "figural aftereffect" isn't a standard term in medical or neuroscience literature, I would recommend consulting original research articles or experts in visual perception for a more precise definition and context.
Ocular adaptation is the ability of the eye to adjust and accommodate to changes in visual input and lighting conditions. This process allows the eye to maintain a clear and focused image over a range of different environments and light levels. There are several types of ocular adaptation, including:
1. Light Adaptation: This refers to the eye's ability to adjust to different levels of illumination. When moving from a dark environment to a bright one, the pupils constrict to let in less light, and the sensitivity of the retina decreases. Conversely, when moving from a bright environment to a dark one, the pupils dilate to let in more light, and the sensitivity of the retina increases.
2. Dark Adaptation: This is the process by which the eye adjusts to low light conditions. It involves the dilation of the pupils and an increase in the sensitivity of the rods (specialised cells in the retina that are responsible for vision in low light conditions). Dark adaptation can take several minutes to occur fully.
3. Color Adaptation: This refers to the eye's ability to adjust to changes in the color temperature of light sources. For example, when moving from a room lit by incandescent light to one lit by fluorescent light, the eye may need to adjust its perception of colors to maintain accurate color vision.
4. Accommodation: This is the process by which the eye changes focus from distant to near objects. The lens of the eye changes shape to bend the light rays entering the eye and bring them into sharp focus on the retina.
Overall, ocular adaptation is an essential function that allows us to see clearly and accurately in a wide range of environments and lighting conditions.
Photic stimulation is a medical term that refers to the exposure of the eyes to light, specifically repetitive pulses of light, which is used as a method in various research and clinical settings. In neuroscience, it's often used in studies related to vision, circadian rhythms, and brain function.
In a clinical context, photic stimulation is sometimes used in the diagnosis of certain medical conditions such as seizure disorders (like epilepsy). By observing the response of the brain to this light stimulus, doctors can gain valuable insights into the functioning of the brain and the presence of any neurological disorders.
However, it's important to note that photic stimulation should be conducted under the supervision of a trained healthcare professional, as improper use can potentially trigger seizures in individuals who are susceptible to them.
Flicker Fusion is the frequency at which an intermittent light stimulus appears to be completely steady or continuous to the average human observer. In other words, it is the rate at which a flickering light source transitions from being perceived as distinct flashes to a smooth and constant emission of light. The exact threshold can vary depending on factors such as the intensity of the light, its size, and the observer's visual acuity.
Flicker Fusion has important implications in various fields, including visual perception research, display technology, and neurology. In clinical settings, assessing a patient's flicker fusion threshold can help diagnose or monitor conditions affecting the nervous system, such as multiple sclerosis or migraines.
Size perception in a medical context typically refers to the way an individual's brain interprets and perceives the size or volume of various stimuli. This can include visual stimuli, such as objects or distances, as well as tactile stimuli, like the size of an object being held or touched.
Disorders in size perception can occur due to neurological conditions, brain injuries, or certain developmental disorders. For example, individuals with visual agnosia may have difficulty recognizing or perceiving the size of objects they see, even though their eyes are functioning normally. Similarly, those with somatoparaphrenia may not recognize the size of their own limbs due to damage in specific areas of the brain.
It's important to note that while 'size perception' is not a medical term per se, it can still be used in a medical or clinical context to describe these types of symptoms and conditions.
Contrast sensitivity is a measure of the ability to distinguish between an object and its background based on differences in contrast, rather than differences in luminance. Contrast refers to the difference in light intensity between an object and its immediate surroundings. Contrast sensitivity is typically measured using specially designed charts that have patterns of parallel lines with varying widths and contrast levels.
In clinical settings, contrast sensitivity is often assessed as part of a comprehensive visual examination. Poor contrast sensitivity can affect a person's ability to perform tasks such as reading, driving, or distinguishing objects from their background, especially in low-light conditions. Reduced contrast sensitivity is a common symptom of various eye conditions, including cataracts, glaucoma, and age-related macular degeneration.
Binocular vision refers to the ability to use both eyes together to create a single, three-dimensional image of our surroundings. This is achieved through a process called binocular fusion, where the images from each eye are aligned and combined in the brain to form a unified perception.
The term "binocular vision" specifically refers to the way that our visual system integrates information from both eyes to create depth perception and enhance visual clarity. When we view an object with both eyes, they focus on the same point in space and send slightly different images to the brain due to their slightly different positions. The brain then combines these images to create a single, three-dimensional image that allows us to perceive depth and distance.
Binocular vision is important for many everyday activities, such as driving, reading, and playing sports. Disorders of binocular vision can lead to symptoms such as double vision, eye strain, and difficulty with depth perception.
Visual pathways, also known as the visual system or the optic pathway, refer to the series of specialized neurons in the nervous system that transmit visual information from the eyes to the brain. This complex network includes the retina, optic nerve, optic chiasma, optic tract, lateral geniculate nucleus, pulvinar, and the primary and secondary visual cortices located in the occipital lobe of the brain.
The process begins when light enters the eye and strikes the photoreceptor cells (rods and cones) in the retina, converting the light energy into electrical signals. These signals are then transmitted to bipolar cells and subsequently to ganglion cells, whose axons form the optic nerve. The fibers from each eye's nasal hemiretina cross at the optic chiasma, while those from the temporal hemiretina continue without crossing. This results in the formation of the optic tract, which carries visual information from both eyes to the opposite side of the brain.
The majority of fibers in the optic tract synapse with neurons in the lateral geniculate nucleus (LGN), a part of the thalamus. The LGN sends this information to the primary visual cortex, also known as V1 or Brodmann area 17, located in the occipital lobe. Here, simple features like lines and edges are initially processed. Further processing occurs in secondary (V2) and tertiary (V3-V5) visual cortices, where more complex features such as shape, motion, and depth are analyzed. Ultimately, this information is integrated to form our perception of the visual world.
Psychophysics is not a medical term per se, but rather a subfield of psychology and neuroscience that studies the relationship between physical stimuli and the sensations and perceptions they produce. It involves the quantitative investigation of psychological functions, such as how brightness or loudness is perceived relative to the physical intensity of light or sound.
In medical contexts, psychophysical methods may be used in research or clinical settings to understand how patients with neurological conditions or sensory impairments perceive and respond to different stimuli. This information can inform diagnostic assessments, treatment planning, and rehabilitation strategies.
An encyclopedia is a comprehensive reference work containing articles on various topics, usually arranged in alphabetical order. In the context of medicine, a medical encyclopedia is a collection of articles that provide information about a wide range of medical topics, including diseases and conditions, treatments, tests, procedures, and anatomy and physiology. Medical encyclopedias may be published in print or electronic formats and are often used as a starting point for researching medical topics. They can provide reliable and accurate information on medical subjects, making them useful resources for healthcare professionals, students, and patients alike. Some well-known examples of medical encyclopedias include the Merck Manual and the Stedman's Medical Dictionary.
The retina is the innermost, light-sensitive layer of tissue in the eye of many vertebrates and some cephalopods. It receives light that has been focused by the cornea and lens, converts it into neural signals, and sends these to the brain via the optic nerve. The retina contains several types of photoreceptor cells including rods (which handle vision in low light) and cones (which are active in bright light and are capable of color vision).
In medical terms, any pathological changes or diseases affecting the retinal structure and function can lead to visual impairment or blindness. Examples include age-related macular degeneration, diabetic retinopathy, retinal detachment, and retinitis pigmentosa among others.
Physiological adaptation refers to the changes or modifications that occur in an organism's biological functions or structures as a result of environmental pressures or changes. These adaptations enable the organism to survive and reproduce more successfully in its environment. They can be short-term, such as the constriction of blood vessels in response to cold temperatures, or long-term, such as the evolution of longer limbs in animals that live in open environments.
In the context of human physiology, examples of physiological adaptation include:
1. Acclimatization: The process by which the body adjusts to changes in environmental conditions, such as altitude or temperature. For example, when a person moves to a high-altitude location, their body may produce more red blood cells to compensate for the lower oxygen levels, leading to improved oxygen delivery to tissues.
2. Exercise adaptation: Regular physical activity can lead to various physiological adaptations, such as increased muscle strength and endurance, enhanced cardiovascular function, and improved insulin sensitivity.
3. Hormonal adaptation: The body can adjust hormone levels in response to changes in the environment or internal conditions. For instance, during prolonged fasting, the body releases stress hormones like cortisol and adrenaline to help maintain energy levels and prevent muscle wasting.
4. Sensory adaptation: Our senses can adapt to different stimuli over time. For example, when we enter a dark room after being in bright sunlight, it takes some time for our eyes to adjust to the new light level. This process is known as dark adaptation.
5. Aging-related adaptations: As we age, various physiological changes occur that help us adapt to the changing environment and maintain homeostasis. These include changes in body composition, immune function, and cognitive abilities.
Eye movements, also known as ocular motility, refer to the voluntary or involuntary motion of the eyes that allows for visual exploration of our environment. There are several types of eye movements, including:
1. Saccades: rapid, ballistic movements that quickly shift the gaze from one point to another.
2. Pursuits: smooth, slow movements that allow the eyes to follow a moving object.
3. Vergences: coordinated movements of both eyes in opposite directions, usually in response to a three-dimensional stimulus.
4. Vestibulo-ocular reflex (VOR): automatic eye movements that help stabilize the gaze during head movement.
5. Optokinetic nystagmus (OKN): rhythmic eye movements that occur in response to large moving visual patterns, such as when looking out of a moving vehicle.
Abnormalities in eye movements can indicate neurological or ophthalmological disorders and are often assessed during clinical examinations.
The occipital lobe is the portion of the cerebral cortex that lies at the back of the brain (posteriorly) and is primarily involved in visual processing. It contains areas that are responsible for the interpretation and integration of visual stimuli, including color, form, movement, and recognition of objects. The occipital lobe is divided into several regions, such as the primary visual cortex (V1), secondary visual cortex (V2 to V5), and the visual association cortex, which work together to process different aspects of visual information. Damage to the occipital lobe can lead to various visual deficits, including blindness or partial loss of vision, known as a visual field cut.