H1 + H2 Sensation & Perception (Wolfe et al., 5th ed.)

What is sensation and what is perception? - Chapter 1

What are sensation and perception?

Sensation refers to the possibility of noticing a stimulus and sometimes converting it into a personal experience. Perception is the giving of meaning to a perceived sensation. Example: a possible sensation is feeling that a hand touches you; perception is the understanding of this sensation: is it an expression of affection or is it the customs who are looking for suspicious objects? Sensation and perception are central: everything we feel, think and do depends on this. There are five scientific methods to study sensation and perception:

  1. Thresholds.
    For example: What is the softest (loudest) sound that you can hear (without damage)?

  2. Scaling: measuring private experiences.
    Qualia (singular: quale) is the philosophical term for a personal conscious experience of sensation or perception: do you see the same color red as me or do you taste the same taste?

  3. The signal detection theory: measuring difficult decisions.
    For example: Is the abnormality on the mammogram really breast cancer or something benign?

  4. Sensory neuroscience.
    How can our perception of the world depend just as much on the activity of our sensory nerves as on the world itself? For example: After biting a pepper, a burning sensation occurs in the mouth. Yet there is no difference in temperature compared to the time for eating the pepper.

  5. Neuroimaging: an image of the brain
    For example: on one eye a picture of a house can be presented and on the other eye a picture of a face. The result is binocular rivalry; an interesting effect resulting from two images that compete for perception. Sometimes you see the house, sometimes the face, but never both.

What is psychophysics and how and by whom was it developed?

Gustav Fechner (1801-1887) developed 'psychophysics' and is called by some the father of experimental psychology; a title that is normally attributed to Wilhelm Wundt (1832-1920). Fechner is best known for relating changes in the physical world to changes in our psychological experiences. His obsession with the relationship between thought and substance placed him in a classical debate between dualism and materialism. Dualism is the vision that thoughts lead an independent existence; apart from the material world of the body. Materialism is the view that substance is the only thing that exists and that everything - including thoughts and consciousness - is the result of interactions between substances. A modern materialistic position (the majority of scientific psychologists believe in this) states that the thoughts are what the brain does. Fechner came up with ' panpsychism' : the vision that the thoughts exist as a characteristic of all substances, that is, that all substances have consciousness (applicable to humans and animals but also to non-living things). Fechner's goal was to describe the relationship between sensation (thoughts) and the energy (substance) that caused this sensation. He called his theory and methods 'psychophysics': defining quantitative relationships between physical and psychological (subjective) experiences.

Fechner was inspired by Ernst Weber (1795-1878). Weber was interested in touch and studied the accuracy of our idea of ​​touch by measuring the smallest distance between two points to perceive them as separate points instead of one point. Fechner called this the 'two-point touch threshold'.

Weber discovered that people who were given a light weight in a hand were better at detecting a difference in weight with a different weight in their other hand. This was in contrast to a heavy weight as a reference stimulus. He called this the 'just noticeable difference' (JND) or 'difference threshold': the smallest detectable difference between two stimuli, or in other words: the minimal change in a stimulus that is correctly noted to be different from a reference stimulus. JNDs change in a systematic manner and adhere to certain ratios for certain stimuli. This is what Fechner called Weber fractions: the constants of proportionality in Weber's law. Fechner also gave Weber's observations a mathematical formula called Weber's law : the principle that describes the relationship between stimulus and resulting sensation, saying that the JND is a constant fraction of the comparison stimulus. In formula terms, this says that the magnitude of a detectable difference (ΔI) is a constant part (K) of the level of the stimulus (I). In Weber's observations, Fechner found what he was looking for: a way to describe the relationship between thought (mind) and substance. He created Fechner's law : the smallest detectable change in a stimulus (ΔI) can be seen as a unit of thought, because this is the smallest perceptible change. S = k log R , where S is the psychological sensation, k a constant and log R the level of the physical stimulus. Conclusion of this formula: our psychological experience of the intensity of light, sound, smell, touch or taste increases more slowly than the current physical stimulus. This formula has an agreement with Einstein's E = mc² .

An absolute threshold is the minimum amount of stimulation needed for a person to notice the stimulus 50% of the time, the minimum intensity of a stimulus that can be noticed.

Psychophysical methods

What is the absolute threshold of sound, the softest sound you can hear? How do we measure that? There are 3 methods for this. The method of constant stimuli requires that many stimuli be created with different intensities to find the smallest / lowest detectable intensity. Stimuli - ranging from rare to almost always perceptible (or rare to almost always perceptible from a reference stimulus) - are offered one by one. Participants respond to each presentation with "yes / no" or "same / different". The method is easy to use but can be inefficient because much of the time is spent on stimuli that are clearly above the threshold.

A second, perhaps more efficient method is the method of limits, in which specific dimensions of a stimulus, or the difference between two stimuli, are varied step by step until the participant responds differently. A set of stimuli with, for example, tones is presented, starting with the softest sound and becoming slightly louder. The participant is asked to indicate when he / she hears something. It is also possible in reverse order (from loud to soft) and then the participant is asked when he / she no longer hears the sound. The average of such crossover points is seen as the threshold.

A third method is the method of adjustment that is the same as the method of limits, except that the participant himself increases or decreases the intensity of a stimulus step by step. This method is the easiest to understand because it corresponds to daily activities, such as adjusting the volume on a stereo, but is the least used. Because it is difficult to have people reliably adjust intensity to the same value between people and over time. To return to the absolute threshold: the book concludes that there are no hard 'absolute' thresholds. Due to variability in our nervous system, we will sometimes detect stimuli close to the threshold and sometimes not. We will have to do it with a somewhat arbitrary definition of a threshold.

Scaling methods

Scaling methods can be used to find out more about the magnitude of our experiences. Such a scaling method is, for example, the estimation of magnitude: a psychophysical method in which participants must assign values ​​according to their perceived magnitude of the stimuli. Participants, for example, receive a number of solutions with sugar, one sweeter than the other. The participant must assign numbers to each solution in a way that seems logical to them. Sweeter solutions get higher numbers and if A seems twice as sweet as B, the number of A would be twice as large as the number of B. Stevens developed this estimate of magnitude. The relationship between stimulus intensity and sensation is described in Stevens' force law : S = aIb, in which the sensation ( S ) is related to stimulus intensity ( I ) by exponent b . If the exponent is less than 1, the sensation grows less rapidly than the stimulus. That is also what Fechners and Weber's laws predict. Imagine that you have lit candles and light ten more. If you started with one, the change in lightness is great, but when you started with 10,000 candles you will hardly notice the difference. The exponent for light intensity is 0.3. About 0.8 for sweetness. Some values ​​are greater than 1: after all, a 12 cm candle is twice as long as a 6 cm candle. The relationship is only true over an average range of sizes. Adding an inch to the size of a spider changes our sensory perception much more than adding an inch to the size of a giraffe. In the case of electric shocks, the pain grows with I3.5, so a four-fold increase in the electric current is experienced as a 128-fold increase in pain. For comparison:

  • Weber's law concerns an objective measurement. We know how much the stimulus has varied and participants cannot or cannot say that it has changed.

  • Fechner's law has the same kind of objective measurement as Webers, but Fechner's law is actually a calculation based on assumptions about how sensation works. It assumes that all JNDs are perceptually equal to each other. This is an incorrect assumption and leads to places where the 'law' is therefore incorrect.

  • Stevens' power law describes the estimation values ​​quite well, but these are qualitatively different from the data that forms the basis of Weber's law. We can record the participants' ratings and check whether they are reasonable and consistent, but we cannot see whether they are objectively correct or incorrect.

A variant of the scaling method shows that different individuals live in different sensory worlds even though they are exposed to the same stimuli. This method is called cross-modality matching: the ability to match the intensities of sensations that come from different sensory modalities. This skill provides insight into sensory differences. For example, a listener can adjust the light intensity of a light until it matches the loudness of a tone. However, this is not possible with taste. A molecule with the abbreviation PROP is experienced by some as very bitter and as some as completely tasteless, others fall back in between. When asked to match the PROP with sensations that are not related to taste, the same comparison is not found as between sound and light. People who taste no or almost no taste of PROP relate it to weak sensations, such as the sound of a whisper. Supertasters, in contrast, are individuals whose perception of taste sensations is the most intense. They relate the bitterness to the intensity of the sun or experience the worst pain ever experienced.

The Signal Detection Theory (SDT)

The signal detection theory (SDT) is a theory that states that the stimulus that you are trying to notice (the signal) is always noticed in the presence of noise. A distinction is made between internal noise (for example, the fact that when you close your eyes in a dark room, you still see a gray pattern with occasional lighter spots) and external noise (for example, the noise in a mammogram can also resemble cancer). Sometimes it is easy to distinguish between noise and signal. The theory helps us understand what happens when we make decisions in uncertain situations. The measurements obtained from a number of presentations are the sensitivity (d ') and the criterion of the participant. The criterion is an internal threshold, determined by the person. If the internal response is above this criterion, the person gives a different automatic response than when it is below this criterion. There are two normal distributions: one of the noise and one of the noise together with the signal. A vertical dotted line is drawn where the criterion is. There are four options:

  1. Correct rejection: there is no signal and you say you have not noticed a signal

  2. Hit: there is a signal and you say you have noticed a signal

  3. False alarm: there is no signal, but you say you have noticed a signal

  4. Miss: there is a signal, but you say you have not noticed a signal

If the two normal distributions overlap, it is very difficult or impossible to distinguish the signal from the noise. The further apart the tops of the distributions are, the greater the sensitivity. The distance between two peaks is called the sensitivity or d-prime (d '). Sensitivity is a value that represents how easily a person can notice the difference between the presence and absence of a stimulus or the difference between stimulus 1 and 2. If the sensitivity is fixed, you can only change the pattern of errors by changing your criterion shift. All the way to the left: if you don't want to miss a single signal, the chance of more frequent false alarms is greater. All the way to the right: you will not make false alarms, but more misses. Receiver operation characteristic (ROC) curve is the graphical plot of the number of hits as a function of the number of false alarms. If these are the same, all points fall on the diagonal line, indicating that the person cannot indicate the difference between the presence or absence of the signal. As the sensitivity of the person increases, the diagonal line curves into a bulging line that bends from the bottom right towards the top left corner and bends back to the top right corner. This indicates that a good distinction can be made between signal and noise.

Fourier analysis

Joseph Fourier (1768-1830) developed analyses whereby complex sounds such as music and speech, complex head movements and complex images such as objects and situations can be broken down into a set of simpler signals that are easier to describe. One of the easiest types of sound is a sine wave: in hearing this is also called a pure tone and it is a wave for which the variation as a function of time is a sine wave function. In vision, a sine wave is a pattern for which variation in a property such as color or brightness as a function of space is a sine function. The air pressure changes continuously on one frequency. The time (or space) required to complete one cycle of a repeating wave is called a period. The distance required for a full cycle of oscillation for a sine wave is called wavelength. The height of a wave is the amplitude. The phase of the wave is its position relative to a set marker and is measured in degrees. Sine waves are rare because only a few vibrations in the world are so pure. However, the Fourier analysis states that there is a mathematical procedure whereby every complex signal can be broken into sine waves at different frequencies. The complex signal is the sum of these sine waves. Noises are described as changes in air pressure over time; images can be described as changes in light and dark over space. The images are broken into components that tell how often changes from light to dark occur within a specific space, this is called spatial frequency and is often specified in cycles per degree: the number of pairs of light and dark vertical stripes ('bars') per degrees of the visual angle. The greater the contrast in the picture, the greater the contrast between black and white stripes.

What is meant by sensory neuroscience and the biology of perception?

An important assumption regarding nerves and brains is that studying the nerves of animals tells us something about human nerves, so there is a kind of continuity. The strongest argument for continuity between animals and humans came from Darwin's theory of evolution. At the same time psychologist wrote Johannes Müller (1801-1858) are 'Handbook of Physiology, which he doctrine of specific nerve energies (doctrine of specific nerve energies) formulated. This doctrine states two things: (1) that the nature of a sensation does not depend on how sensory fibers are stimulated, but which fibers are stimulated and (2) that we are only aware of the activity in our nerves and we are not immediately aware can belong to the world itself. There are twelve pairs of nerves (each one nerve from one body half and one from the other body half) that originate in the brainstem and reach to sensory organs and motor systems (muscles) through openings in the skull. These pairs of nerves are called cranial nerves . Three of the cranial nerves - smell, optical and auditory - are exclusively dedicated to sensory information. Odor nerves are the first pair of cranial nerves. The axons of these form bundles after they have passed through the cribriform plate, after which they form the olfactory nerve. The olfactory nerve led impulses from the olfactory epithelium in the nose to the olfactory bulb. The optic nerves are the second pair of cranial nerves and open up the retina. They bring visual information to the thalamus and other parts of the brain. The auditory nerves are the eighth pair and connect the inner ear with the brain. The nerve carries information involved in hearing and spatial orientation. The auditory nerve is also called the 'vestibulocochlear' nerve, because it also serves vestibular (equilibrium) purposes and it is made up of the auditory nerve (cochlear nerve) and the vestibular nerve. Three other nerve pairs - 'oculomotor', 'trochlear' and 'abducens' - are dedicated to muscles that move the eyes. Eye movement nerves (oculomotor nerves) are the third pair of cranial nerves and provide nerve action to all extrinsic muscles of the eye except the lateral rectus muscle and the superior oblique muscles. They do, for example, provide the lift muscle of the upper eyelid called the ciliairy muscle, the circle muscle of the jet-shaped body in the eye and the muscle of the pupil called the 'sphincter' muscle. The fourth pair of nerves, the pulley nerves, supplies the superior oblique muscles of the eyeball with nerves and the sixth pair of nerves, the draining nerves (abducens nerves) do this for the lateral rectus muscle. The other six cranial nerves are either motor-exclusive or process both sensory and motor signals. The first six are discussed later. Müller's leather does not only talk about cranial nerves, but also about warm and cold fibers that regulate the temperature of your skin. This is possible both via external outside temperature and through substances. Menthol, for example, lets cold fibers fire, leaving your skin feeling colder without changing the temperature. Substances such as in peppers (called capsaicin) do the opposite with heat fibers. Paradoxically, capsaicin and menthol are used in ointments for pain, while too many of these substances actually cause pain receptors to fire in the skin.

Four different types of sensory information reach the cortex in different places. Visual perception uses both cortex that runs anterior to the parietal lobe and cortex that runs ventral to the temporal lobe. Hermann Ludwig Ferdinand von Helmholtz (1821-1894) was one of the most important scientists ever, he discovered many important things and was very influenced by Müller. However, what Hermann did not like about Müller was his idea of vitalism : the idea that there is a force in life that is different from physical entities. Helmholtz stated that everything could be explained by physical forces. He demonstrated that the activity of neurons followed physics and chemistry rules and was the first to effectively measure the speed at which neurons transmit their signals.

Neural connections

Santiago Ramón y Cajal (1852-1934) conducted important research into the organization of neurons. He stated that neurons do not touch each other, there is a space between the cells. Sir Charles Sherrington (1857-1952) called that space a synapse: the space between neurons that allows the transfer of information. Sherrington also found that the speed of neural transmission decreased as the transmission went through more and more synapses. For a long time it was thought that the transfer of information from one neuron to another (arrival at the dendrites) went via an electric wave, but Otto Loewi (1873-1961) was convinced that this was not the case: some neurons increase the activity (exciting ) and some decrease the activity of the next neuron (inhibitory). Loewi introduced the idea that a chemical transmission should take place. These chemical molecules were called neurotransmitters. There are many different types and neurons are selective in allowing which neurotransmitters excite or inhibit them.

Neural firing: The action potential

Sir Alan Hodgkin (1914-1998) and Sir Andrew Huxley (1917-2012) conducted experiments in which they could isolate neurons and in this way they could test how an impulse spread across the axon. They discovered that firing a neuron is electrochemical. The voltage across the axon increases as a result of changes in the membrane of the neuron, this allows sodium ions (Na +) to flow in, this increases the voltage, and this is called the action potential. Hence, the action potential can also be viewed as the "firing of neurons". Potassium ions (K +) then flow out of the membrane, which reduces the voltage. This process happens along the entire length of the axon, until it reaches the end.


Methods for creating images of the structure or / and function of the brain are collectively called neuroimaging . These methods help to examine the brain.

Electroencephalography (EEG) is a technique where dozens of electrodes are placed on the head. The electrodes measure electrical activity of populations of neurons. EEG tells with great accuracy where groups of neurons are and their activity. The activity of one electrode is low, because it is variable, even if you repeatedly present the same stimulus (such as a light flash). However, if you take the average of many signals, you will see a pattern of electrically positive (P) and negative (N) waves. The resulting average wave is an event-related potential (ERP) : a measurement of electrical activity of a subpopulation of neurons in response to the presentation of specific stimuli. For this, the average of EEG measurements must be taken.

An EEG-related method is the magnetoencephalography (MEG) , this megahelm measures changes in magnetic activity between populations of neurons. It has an advantage and disadvantage compared to EEG. An advantage is that MEG gives a better idea as to which groups of neurons are most active, because MEG uses extremely sensitive means to measure the smallest changes in the magnetic field. A disadvantage is that these drugs are very expensive and rarer than EEG drugs.

Computed tomography (CT) is an imaging technique that uses X-ray to get an image of layers of the brain in this case. They are horizontal slices of brain that you get to see. Dark areas are filled ventricles, the white is blunt and the gray are the brain.

Magnetic resonance imaging (MRI) is an imaging technique that uses the reaction of atoms to strong magnetic fields. This forms an image of the brain structures. The atoms go wild with the change of the magnetic field. The result is a vertical image of half the brain. In functional magnetic resonance imaging (fMRI) , active brain tissue is hungry brain tissue: it uses oxygen and other supplies supplied by blood. The result is a blood oxygen level-dependent signal (BOLD signal) that can be measured. BOLD is the ratio between oxygen-rich and oxygen-poor hemoglobin and ensures that the neurons that are most involved in a task can be located. The image is a horizontal image of half of the brain showing a BOLD signal. Areas in warm colors (red / orange / yellow) are areas where the BOLD signal is increased by the presence of a visual stimulus. Blue areas show a decrease in BOLD activity.

Positron emission tomography (PET) is an imaging technique that shows where neurons are mainly active in the brain. PET measures brain cell metabolism by injecting a safe amount of radioactive isotopes (the 'tracer') into the blood stream. The idea is the same as fMRI: detect activity in neurons by searching for increased metabolic activity. A commonly used tracer is an unstable form of oxygen, 15O, which is active for two minutes. An advantage is that it is a silent technique, which is useful in studies of brain activity related to hearing. A disadvantage is that it is not the easiest technique. 

What three questions are important to keep in mind regarding development over the life span?

Recall that no sensory system has fixed properties over the course of the life span. In other words, development is not a method (like PET or scaling). Rather, it is an approach of thinking about sensation and perception. In the next chapters, development will be mentioned a lot. When reading about development in these chapters, it is worth keeping the following three big questions in mind:

  1. What comes with the system?
  2. What has to be learned?
  3. What changes with age?

Practice questions

  1. Which five ways are there to study sensation and perception?

  2. What is the difference between the absolute threshold value and the difference threshold value (or JND)?

  3. What does the estimation of magnitude entail and which formula is included?

  4. What are the four options for responding to signal detection theory after setting the criterion?

  5. How did Helmholtz and Müller disagree?

  6. What does an action potential entail?

  7. Name a number of neuroimaging techniques.

How can our eyes perceive light? - Chapter 2

How can light be described from the perspective of physics?

Light is a form of electromagnetic radiation or energy produced by vibrations of electrically charged material. You can conceptualize light in two ways. First, light can be viewed like a wave: a fluctuation that passes through one medium, this by transferring energy from the particle to the other particle without causing permanent displacement of the medium. Secondly light can be viewed as a stream of photons: a quantity of light or another form of electromagnetic radiation that shows properties of both the particle and the wave. A photon is a small particle of light that contains an amount of energy. Visible wavelengths are between 400 and 700 nanometers. What happens to the light of a star? Electromagnetic radiation travels in a straight line with the speed of light to the atmosphere where some photons are absorbed by dust and water, among other things. Absorbing refers to absorbing light, sound or energy and not transferring it. In contrast, some photons will be scattered (sometimes called: diffracted) by these particles. The scattering of light in an uncontrolled way. However, most photons pass through the atmosphere and reach a surface / object. When it touches a light-colored surface, it reflects: when light hits a - mainly light, 'sound' or - hot surface, it is sent back to its origin. If it hits a dark surface, it is absorbed. If light is not reflected or absorbed, it is transmitted by the surface: it is transferred from one place to the next. When we stand behind a window and look at a star, some rays of light will break (refract): (1) the direction of an energy wave is changed as it passes through another medium, in this case through glass as light passes through; (2) measuring the degree of refraction in a lens or eye.

How do eyes capture light?

The first eye tissue that encounters the light from the star is the cornea: the transparent 'window' to the eyeball. This lets light through without interruption; the light is not absorbed or reflected, but transmitted. Here the light is deflected for the first time and also the strongest (it contributes two thirds to the focus power of the eye). It is transparent because it contains highly ordered fibers and no blood vessels that can absorb light. There are many sensory nerve endings that close the eyes or produce tears if the cornea is damaged, this often recovers within 24 hours. The space immediately behind the cornea, the anterior chamber, is filled with aqueous humor, a fluid that comes from blood. It provides oxygen and nutrients to the cornea and lens and removes waste from both. Here the light is also slightly bent. The lens allows you to focus on something else, which is also transparent and is controlled by the ciliary muscle. Light is also broken in the lens. To get to the lens, light must pass through the pupil : the dark round opening in the center of the iris of the eye where the light reaches the eye. The iris is the colored part of your eye and regulates the allowable amount of light by increasing or decreasing the pupil. As illustrated by the will light (for the fourth and last time) are deflected pupil and has come through the lens through the vitreous humor, a transparent liquid that fills the back room, which is 80% of the eyeball. The light bounces against the retina: a membrane sensitive to light in the back of the eye that contains cones and rods, they receive an image from the lens and send it to the brain via the optic nerve. Not all the light from the star comes to the retina, as mentioned earlier, not everything passes the atmosphere and much is lost in the eyeball: about 50% survive.

Focusing light onto the retina

The force with which the room moisture and glassy body bend the rays of light is much smaller than that of the cornea. Because the power is so low, they cannot focus on objects that are very close. The lens can do this, it can change its bending power by changing the shape of the lens, this process the accommodation: the change of focus. For example, the lens thickens when the viewing direction is focused on an object that is close by. Accommodation is achieved by ciliary muscle contractions. The lens and ciliary muscle are connected by fibers called 'zonules'. At rest the lens is fairly flat, the zonules are stretched out and there is tension on it. You see distant objects. When focusing on an object that is close by, the zonules relax so that the lens can bend / thicken. Presbyopia literally means 'old vision' and inevitably occurs in people between 40 and 50 years old. Due to insufficient accommodation, people are less able to see nearby. The cause is that the lens becomes harder and that which causes the lens to change shape loses elasticity. The lens is transparent, everything that causes it to become opaque is called opacity (opacities). Opacities of the lens is cataract (cataracts) referred to, this is caused by irregularities of the crystals in the lens. Cataracts can start at any age and there are different types, especially noticeable after age 50 and at age 70, everyone has lost some transparency. In order for the light from a distant star to reach the retina, the bending power of the four optical components of the eye (cornea, chamber fluid, lens, glassy body) must match the length of the eyeball perfectly. The perfect match is called emmetropia. If the length of the eyeball is too long, the image of the star is focused just before the retina, this is called myopia . This can be corrected by negative lenses that cause the light rays to widen slightly just before hitting the lens, so that they touch the lens a little wider and later precisely meet on the retina. The image of the star can also be focused just behind the retina, this is called hyperopia. If it is not serious, it can be compensated by accommodation, otherwise with positive lenses that bring the light rays closer together before they go into the eye. The average length of an adult eyeball is 24 millimeters. You can also have a slightly longer or shorter eye ball that is emmotropic, because the eye generally matches the innate power of the optical components.

When the cornea is not spherical in shape, astigmatism occurs, a visual defect due to the uneven bending of the cornea. Vertical lines can be focused just before the retina and horizontal lines just behind, or vice versa. Or some lines appear out of focus and others sharp. Lenses with two focal points can correct this.

The retina

The process of seeing starts with the retina, where light energy is converted into neural energy that can be interpreted by the brain. Converting from one type of energy to another is called ' transduction' . For example, light energy from a star is transduced into neural energt that can be interpreted by the brain.

What the doctor saw

The fundus is the posterior layer of the retina, ophthalmologists can see this through their ophthalmoscope. There is a white circle called the optical disk, blood vessels and veins coming in that feed the retina and ganglion cells leave the eye through the optic nerve. The fundus is the only place in the body where you can see blood vessels and veins directly, so it is an important framework for doctors to see how your vascular system is doing. However, even with an ophthalmoscope you do not get a detailed view of the retina, photomicrography is required for that. This shows that the retina is layered with a few layers of clear neurons with another layer of dark cells (the 'pigment epithelium'). These neurons together contribute to the process of interpreting the information in visual images. The conversion of light energy into neural energy starts at the very back of the retina, the layer that consists of cells called photoreceptors . Before light reaches the photoreceptors, it must pass through ganglion, bipolar, horizontal and 'amacrine' cells. Most of these are transparent, but cells in the pigment epithelium, which provide important nutrients to the photoreceptors, are opaque.

Retinal geography and function

There are approximately 100 million photoreceptors in the retina. They receive light and produce chemical signals. There are two types: cones and rods. Cones (cones) specialize in seeing daylight, fine visual sharpness and color. Rods specialize in night vision. Because the retina has both types, it is a duplex retina, consisting of two parts that function under different conditions. The cones are concentrated in the center of the retina (fovea). With retinal eccentricity , this concentration reduces the distance from the retina. Fine details can be seen with the retina.

How can the visual system adapt itself to changes in lighting?

There are four ways in which the visual system adapts itself to changes in lighting. Pupil size, photo pigment regeneration, the duplex retina and the neural circuit.

Pupil size

If bright light shines into your eye, your pupil shrinks to approximately 2 mm. This is in contrast to when you enter a dark room in bright sunlight where your pupil increases to 8 mm. Because the amount of incoming light in your eye is proportional to the size of the pupil, a four-fold increase in diameter equals a sixteen-fold improvement in sensitivity. So 16 times more light can enter the eye when your pupil is fully enlarged. In this way your visual system adapts to dark and light.

Photopigment regeneration

The second mechanism is because of the way photopigments are used and placed in receptor cells. Photo pigments in bars react better in dark rooms. If a photopigment molecule has been bleached (used to detect a photon), it must be regenerated before it can be reused. When it suddenly becomes very light, photo pigments cannot be regenerated quickly enough to detect all the photons that hit the photoreceptors. This slow regeneration is good for increasing our sensitivity. If photons are rare, we use them all to see, if we have too many photons, we throw something away and use what is left over.

The duplex retina

The compensation mechanism just discussed is assisted by the duplex retina. Cones are much less sensitive than dimmed bars, but their range is much larger to as many as thousands of photons per second. We use rods when there is little light and cones when there is a lot of light. Cones regenerate faster.

Neural circuitry

The main reason that we are not disturbed by all variations in light levels has to do with the neural circuitry of the retina. Ganglion cells fire maximum when the center of their receptive fields is strongly lit and the environment is dark (or vice versa). However, they still fire more than can be expected based on probability if the light falls on the entire receptive field, as long as the light is brighter in the ON part of the field than in the OFF part. Ganglion cells respond to the contrast between adjacent retinal regions and do their best to ignore any variation in the overall light level.

Age-related macular degeneration (AMD) is a disease that is associated with aging and affects the yellow spot (macula). The yellow spot is the central part of the retina with a high concentration of cones. Reading and recognizing faces becomes difficult because there is a gray round spot in the middle of the view. There is wet AMD and dry AMD.

Retinitis pigmentosa (RP) is a progressive degeneration of the retina that affects night vision and peripheral vision. It can be caused by defects in a number of recently identified genes. It is a hereditary disease characterized by the progressive death of photoreceptors and the degeneration of the pigment epithelium. The result is blindness.

How does the retina process information?

The retina consists of five major classes of neurons: (1) photoreceptors; (2) horizontal cells; (3) bipolar ces; (4) amacrine cells, and; (5) ganglion cells. Here, we take a closer look at the functions of each of these cell types.

Light transduction by rod and cone photoreceptors

Both types of photoreceptors consist of an outer segment: adjacent to the pigment pithelium and contain photopigment molecules, and an inner segment: located between the outer segment and the cell nucleus and a synaptic terminal, where the transmission of information is converted into the release of a chemical carrier. The visual pigments are created in the inner segment and stored in the outer segment. Visual pigment molecules consist of (1) a protein that determines which wavelengths of light they absorb and (2) a chromophore that captures the light photons. Each photoreceptor has one of the four types of visual pigments. The rodopsine pigment is only found in bars. Each cone has one of the other three pigments that respond to long, medium and short wavelengths. Recent research suggests that there may be a third type of photoreceptor that matches our biological rhythm with day or night. These are sensitive to the light from outside and contain the photo pigment melanopsin . These receptors with melanopsin send signals to the suprachiasmatic nucleus (SCN): the center of the brain's biological clock. When light from a star reaches the outer segment of a rod and it is absorbed by a rodopsin molecule, the energy will be transmitted to the chromophore part of the visual pigment molecule. This process is called photo-activation, activation by light. It initiates a set of biochemical processes that result in the closing of the cell membrane channels that normally allow ions to flow into the outer segment of the rod. The inside of the cell is now being charged more and more negatively, this is hyperpolarization . Decreasing the calcium concentration also reduces the concentration of glutamate molecules. This change sends signals to the bipolar cell that the rod has received a photon. Cones work the same way. The information to bipolar cells goes via ' graded potentials' : an electrical potential that can vary continuously in amplitude.

As mentioned earlier, rods are mainly for night vision and therefore function relatively well in situations where dim light is (scotopic). Cones need more / stronger light (photopic). In such scotopic conditions you therefore have a blind spot of approximately 1 degree. There is another important functional difference: all bars have the same photo pigment, so they cannot send differences in color. Cones each have one of three different photo pigments that differ in the wavelengths in which they absorb the light most efficiently. Cones can therefore transfer information about such wavelengths. This makes them the basis of color vision.

The photo pigments of the cones are not distributed fairly. Cones sensitive to short wavelengths (S-cones, S for Short) cover approximately 5-10% of the total cone population and on average there are twice as many L-cones (long wavelength) than M-cones (medium wavelength). The ratio between L-cones and M-cones can vary greatly per individual.

Lateral inhibition through horizontal and amacrine cells

Horizontal cells act perpendicular to the photoreceptors, they make contact with photoreceptors and bipolar cells in the neighborhood. These connections play an important role in lateral inhibition , the antagonistic neural interaction between adjacent regions of the retina. Amacrine cells are also part of this lateral path. Like horizontal cells, they run perpendicular to photoreceptors in the inner layer of the retina, where they receive input from bipolar cells and other amacrine cells. They send signals to bipolar, amacrine and retinal ganglion cells. Amacrine cells are thought to be involved in contrast and temporal sensitivity (noticing changes in light patterns over time), but the precise function remains unclear.

Convergence and divergence of information via bipolar cells

Photoreceptors, bipolar cells and ganglion cells are part of the vertical path. Bipolar cells are the intermediaries. They are cells in the retina that have a synapse with a cone or rod (not with both) and with horizontal cells. Bipolar cells receive from many photoreceptors, polarize the input and send it to the ganglion cells. Because there is input of about the convergence of 50 photoreceptors to one bipolar cell (called a diffuse bipolar cell) is a characteristic of the rod path. A diffuse bipolar cell therefore receives input from several cones. Reversing the input increases the visual sensitivity or (1) the ability to perceive through the senses (2) the extreme response to radiation especially to light of a specific wavelength and (3) the ability to respond to transmitted signals. Midget bipolar cells, on the other hand, receive input from a few cones. These types of bipolar cells are only in the retina. This explains why images are most evident when they fall on this part of the retina. Each cone in the retina consists of two bipolar cells (which stands for a divergence of information): one that responds to an increase in light received by the cone (ON bipolar cell) and one that responds to a decrease thereof (OFF bipolar cell).

Communicating with the brain via ganglion cells

By the time signals arrive at the last layer of the retina - ganglion cells - much processing has already taken place, some information has been pooled during convergence and some information has been propagated by lateral paths. A ganglion cell is a cell in the retina that receives visual information from photoreceptors via two intermediate neuron types (bipolar cells and amacrine cells) and transmit information to the brain and midbrain. A P ganglion cell is a small ganglion cell that receives exciting information from dwarf bipolar cells in the central retina and feeds the 'parvocellular' (meaning 'small cell') layer of the LGN (lateral geniculate nucleus). An M ganglion cell is a ganglion cell that is shaped like an umbrella by the large dendrites. This receives exciting input from diffuse bipolar cells and nourishes the 'magnocellular' (meaning 'large cell') layer of the LGN. Both types of ganglion cells get larger dendrites as retinal eccentricity, but M ganglion cells will always have more dendrites than P ganglion cells. Of all ganglion cells, approximately 70% are P ganglion cells and 10% M ganglion cells. The remainder are koniocellular cells : neurons between the magnocellular and parvocellular layers of the LGN.

Ganglion cells fire action potentials simultaneously, about once per second in the absence of visual stimulation. Each ganglion cell has a receptive field, a region on the retina in which visual stimuli affect the number of times the neuron fires. This influence can be exciting or inhibiting. Kuffler concluded from research that this field is concentrated: a small circular area in the middle responds to an increase in light and a surrounding ring responds to a decrease in light. A certain type of ganglion cell fires the fastest when light falls precisely on the exciting area of ​​the field and it fires less quickly as the light focus overlaps both with the center and with a part of the environment. Such a cell it is an ON-center cell. A ganglion cell that does exactly the opposite does it an OFF center cell. This organization (center-surround organization) has two functional consequences. First, each ganglion cell will respond best to a light beam of a certain size and less well to rays that are larger or smaller, so that they serve as a filter by adjusting the information they send to the brain. Secondly, most ganglion cells are sensitive to differences in light intensity between the center and the environment and relatively insensitive to average light intensity. The average intensity of the light on the retina varies a lot (depending on, for example, day or night, inside or outside). Contrast is the difference in lighting between an object and the background or between lighter and darker parts of the same object.

P cells have smaller receptive fields than M cells. On the one hand because M cells have larger dendrites and on the other because M cells are more sensitive in low light conditions. In addition, P cells and M cells differ in temporal responses. P cells fire for a long time when light falls on them and M cells fire calmerly, with a brief burst of impulses. P cells mainly provide information about contrast and M cells about changes over time.

Practice questions

  1. In which two ways can light be conceptualized?

  2. What is accommodation and how does this process work?

  3. What are the two types of photoreceptors?

  4. Which two types of bipolar cells do the cones in the retina consist of?

  5. What are the three types of ganglion cells?

  6. In which four ways does the visual system adapt itself to changes in lighting?


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Summaries & Study Note of HannekevanderHoef
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