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Full year notes Neuroscience.docx

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Department
Psychology
Course
Psychology 2220A/B
Professor
Anthony Skelton
Semester
Fall

Description
Neuroscience Sensation and perception Sensory receptor organs are organs that are specialized to detect a certain stimulus. The receptor cells within the organ convert the stimulus into an electrical signal. Organs very depending on the species. Adequate stimulus is a stimulus that the specific organ is adapted to. Receptor cells convert energy into a change in electrical potential across its membrane. The changing of the signal is called sensory transduction. Receptor cells are transducers that convert energy that leads to sensory perception. Specific nerve energies - Receptors and channels for different senses are independent - Each sense has its own nerve energy All senses use action potentials as their source of energy. Receptor potentials are local changes in membrane potential. Labeled lines – brain recognize distinct senses based on the actions potentials that travel along each nerve tract Pacinian corpuscle is a receptor in the skin that detects vibration and pressure. The stimulus produces an electrical potential, and when the potential is large enough the receptor reaches threshold and generates an action potential. Coding is the patterns of action potentials in a sensory system that reflects a stimulus; neurons can convey stimulus intensity by changing the frequency of its action potential. The pattern of electrical activity in the sensory system must convey information about the original stimulus. Sensory information is encoded by the number and frequency of the action potential. Range fractionation takes place when different cells have different thresholds for firing, over a range of stimulus intensities. Receptor cells can join together and work parallel together, the streght of a stimulus increases and new neurons are recruited and it strengthens due to the number of active cells contributing to the stimulus. Somatosensory system – Detects body sensations, such as touch and pain Adaptation - The progressive loss of a response when the stimulus is present. - Tonic receptors o Show slow or no decline in AP frequency, little adaptation - Phasic receptors o Display adaptation and decrease frequency of action potentials - Prevents the nervous system from becoming overwhelmed by stimuli that offer little news about the world. Accessory structures such as eyelids can reduce the level of input to the sensory pathway. Top down processing – higher brain centers suppress some sensory inputs and amplify others. Pathways: levels of the nervous system process sensory information - Sensory pathway o Chain of neural connections from sensory receptor cells to the cortex o Passes through select stations during processing - Most sensory pathways pass through regions of the thalamus which trades information with the cortex - These pathways are terminated in the cerebral cortex Receptor fields - Space in which a stimulus will change a neurons firing rate - Somatosensory receptive fields have either excitatory center and inhibitory surround or an inhibitory center and an excitatory surround Receptor fields in the cortex - There is a primary sensory cortex in each modality - Primary sensory cortex – the region of the cortex that receive most of the information about the modality from the thalamus or directly from the secondary sensory neurons - Secondary sensory cortex – receives its main input from the primary cortical area for that modality - These are sent through subcortical loops - Secondary cortex exchanges information with the primary cortex. Primary cortex exchanges with the thalamus. Receptors from the brainstem and spinal cord give information to the thalamus - Primary somatosensory cortex (s1) – receives touch information from the opposite side of the body o In the post central gyrus of the parietal cortex - Secondary somatosensory cortex (s2) – maps both sides of the body in overlay Attention - State of selective awareness when a specific stimuli is selected for enhanced processing - Process that allows selection of some inputs over others - Frontal eye field o Involved in attentive visual exploration of space - Cingulate cortex o Motivational aspects of attention - Posterior parietal lobe o Shift in spatial orientation Synesthesia - Stimulus in one modality creates a sensation in another - Example; a person may perceive colors when looking at letters or a taste when hearing a tone. Touch Skin – has 3 layers - Epidermis – outermost layer, thinnest - Dermis – middle layer, contains nerve fibers - Hypodermis – anchors muscles and helps shape body; contains pacinian corpuscles 4 tactile (touch) receptors that detect touch - Pacinian corpuscles – vibration, fast adapting - Meissner corpuscles – touch, fast adapting; texture - Merkel’s discs – touch, slow adapting; light touch - Ruffini’s endings – stretch, slow adapting Dorsal column system carries somatosensory information from the skin to the brain. Dorsal column system carries touch information to the brain. - Receptors send axons in the dorsal column of the spinal cord, where they synapse on the dorsal column nuclei in the medulla - Dorsal column nuclei o Neurons in the medulla that receive somatosensory information from the dorsal columns in the spinal cord o These neurons send axons across the midline and to the thalamus - Dermatome – part of skin innervated by a particular spinal nerve. o Dermatomes that are adjacent tend to overlap a small amount. o Send sensory inputs to different dorsal roots of the spinal cord - If a nerve to the body region is severed, the cortical area will shrink, limiting effectiveness - If the body region is removed, the cortical area for adjacent body regions will expand Pain - Unpleasant sensory or emotional experience that is associate with actual or potential tissue damage - Congenital insensitivity to pain o Syndrome where a person does not experience pain - Short lasting pain causes us to withdraw from the source preventing further damage - Long lasting pain promotes behaviours such as sleeping, feeding and drinking that promote recuperation - Expression of pain such as screaming can cause a signal to others to prevent other objects to engage in the same action Human pain can be measured - Sensory-discriminative quality o Throbbing, gnawing, shooting - Motivational – affective quality (emotional) o Tiring, sickening, fearful - Cognitive evaluative quality o No pain, mild, excruciating - Nociceptors are peripheral receptors that respond to pain - Free nerve endings have specialized receptor proteins on the cell membrane that respond to various signals o Respond to temperature changes, chemicals and pain Hot, cold or cool - Capsacian o Chemical that makes chili peppers spicy hot o Transient receptor potential vanilloid type 1 (TRPV1) binds capsaicin  Detects painful heat  Transmits burning sensation from peppers and normally detects sudden increases in temperature - TRP2 o Detects higher temperatures o Does not respond to capsaicin o Found on As fibres which are large myelinated axons that register pain quickly - TRPV1 o On C fibres which are thin unmyelinated axons that conduct slowly, producing lasting pain o The dull, lasting ache - Cool-menthol receptor 1 (CMR1) o Responds to menthol and to cool temperatures, rather slowly o Located on C fibers as well - Some free nerve endings respond to histamine and use Gastrin- releasing peptide (GRP) to stimulate neurons to provide an itch sensation - SCN9A o Encodes a sodium channel that represents the specific pain receptor protein Mediation of pain (peripheral) - Damaged cells release substances that excite free nerve endings that function as nociceptors - AP generated in the periphery can excite blood vessels and other cells to produce inflammation - Information enters the dorsal root and synapses on the neurons on the dorsal horn - Pain fibers release glutamate as a transmitter and substance P as a neuromodulator in the spinal cord - The dorsal horn cells then send information across the midline to the thalamus Spinal pathways transmit pain information - Pain and temperature is transmitted separately by the anterolateral or spinothalamic systems - Nerve endings, connect to spinal neurons in the dorsal horn - Pain crosses midline in the spinal cord before travelling to the thalamus - Afferent fibres (peripheral) use glutamate to excite spinal cells in the dorsal horn o Also release substance P which is a peptide transmitter that deals with pain transmission o Postsynaptic neurons take up substance P and remodel dendrites that can affect pain perception. Pains pathway to the brain - Pain is carried by the As fibres and slowly through C fibers - Axons of the dorsal horn crosses the midline and travels up the spinal cord through the anterolateral quadrant - Information is provided to various brainstem sites that control pain-related behaviour such as vocalization - Information is distributed to thalamic and cortical areas - Cingulate cortex is activated by pain information The reign of pain is mainly the brain - Neuropathic pain or known as the phantom limb pain is due to inappropriate signaling of pain by neurons - Dorsal horn neurons become hyperexciteable and cause chronic pain - The pain information is integrated into the cingulate cortex and the different sub regions of the cingulate cortex is activated if the person is experiencing or empathizing pain with another Pain can be difficult to control - Analgesia – loss of pain sensation - Opiates are drugs that control pain, and opioids are endogenous opiate like peptides in the brain 3 classes of endogenous opioids – peptide transmitters that have been called the body’s own narcotics - Endorphins - Enkephalins - Dynorphins Periaqueductal gray (PAG) - Area in the midbrain that deals with pain perception - Stimulation in this area produces potent analgesia which relieves pain - Receives input from spinal cord delivering nonciceptive information - Pain information can be blocked by a gating action in the spinal cord Ascending pain pathway - Signal is passed from the skin to the spinal cord - Crosses in the spinal cord - Information from the PG is sent to the S1 - Passed through the reticulothalamic or spinothalamic tract - Delievered to the S1 (parietal cortex), cingulate cortex and the thalamus Descending pain pathway - Signal is passed from the frontal cortex or hypothalamus - Passes through the PG to the raphe nucleus - When connecting to the dorsal root, opioid releasing cell inhibits the spinothalmic pain signal Placebos which are substances that is ineffective can sometimes relieve pain by releasing endogenous opiates. Acupuncture, which is the insertion of needles at points on the skin, is used to alleviate pain. Stress can activate analgesia systems Hearing - Each part of the ear performs a specific function in hearing - Pathways run from the brainstem to the cortex - Pitch is encoded in two complementary ways - Brainstem auditory systems are specialized for localizing sounds - The auditory cortex performs complex tasks in the perception of sound - Hearing loss is a major disorder of the nervous system The basics of class - Pure tone (sound) can be described by two measures o Amplitude or intensity – loudness o Frequency or number of cycles per second of vibration is measured in hertz (Hz) – pitch - Sound contains a fundamental or basic frequency; harmonics – multiples of that frequency - Timbre – characteristic sound quality of an instrument that deals with intensities of harmonics - Fourier analysis – mathematical process used to analyze sounds as sum of sine waves - Sound intensity is usually expressed in Decibels Auditory system has been shaped to capture biologically important sounds where the force of sound is transduced into neural activity The external ear, the pinna and the ear canal collect sound waves, and transform sound energy The middle ear concentrates sound energies - 3 ossicles contain 3 bones, the malleus, incus and stapes - Occiscles connect the tympanic membrane (eardrum) transmit sound across the middle ear to the oval window(opening from the middle ear to the inner ear Inner ear structures convert sound into neural activity - Fluid-filled cochlea, is a spiral structure with a base and an apex - The base is the nearest to the oval window membrane Cochlea has 3 parallel canals - Scala vestibuli – vestibular canal - Scala media – middle canal - Scala tympani – tympanic canal The round window is a membrane that separates the scala tympani from the middle ear. Organ of Corti has 3 main structures - Sensory cells, or hair cells - Framework of supporting cells - Basilar membrane – vibrates in response to sound Vibrations cause the basilar membrane to oscillate, and different parts respond to different frequencies - High frequency sounds displace the narrow base of the basilar membrane - Low frequency sound displaces the wider apex The corti has two sets of sensory cells - Inner hair cells (IHCs) closer to the central axis - Outer hair cells (OHCs) - Stereocilia, or hairs protrude from each hair cell OHC extend to the tectorial membrane on top of the organ of corti. Afferent nerve fibers carry messages from hair cells to the brain. While, efferent nerve fibers send messages from the brain to the hair cells Thin fibres called tip links run across each hair cells sterocilia. These play a key role in the generation of hair cell potentials. When the sterocilia sway the tip links pop open the ion channels to which they are attached to. These chnnels snap shut again as the hair sways back. These channels are consisting of TRPA1. A sterocilium ion channel is spring-loaded with a hair trigger. The hair cell depolarizes, and calcium influx at the base of the cell causes neurotransmitter release. Tuning curves graph auditory nerve fiber responses that show that additional sharpening takes place. Auditory system pathways run from the brainstem to the cortex - The vestibulocochlear nerve, cranial nerve VIII caontains auditory fibers from the cochlea - Each fiber divides into two branches going to cells in the ventral and dorsal cochlear nuclei. The cochlear nuclei send output to the superior olivary complex - The cochlear nuclei has multiple targets o Superior olivary nuclei – receive bilateral input o Inferior colliculi – in the midbrain, which then send output to the medial geniculate nuclei in the thalamus which sends output to the auditory cortex - All levels of the auditory pathways have tonotopic organization which is when neurons are arranged in a map according to the frequencies to which they respond There are two theories of pitch discrimination - Place coding theory – pitch is encoded in receptor location on the basilar membrane o Activation of receptors near the base of the cochlea signals treble o Activation of receptors near the apex signals bass. - Temporal coding theory – firing rate of auditory neurons encodes the frequency of the auditory stimulus Two theories can be incorporated. The frequency properties of a sound are coded in two ways: - Place coding or tonotopic representation - Temporal patter of firing cells Theories of frequency transduction - Place theory o Location of maximal deflection along the basilar membrane encodes frequency - Temporal(timing) theory o Rate of neurons firing encodes frequency o Could only work for relatively low frequencies o Limited by refractory period/ firing rate - Volley theory o In combination, many neurons could encode frequency through combined firing rates Species may differ in their sensitivity to sounds - Ultrasound – high frequency about 20,000 Hz - Infrasound – very low frequency – 20 Hz Binaural cues signal sound location, two ears: - Intensity differences o Differences in loudness at the two ears - Latency differences o Differences between the two ears in the time of arrival of sounds o There are two kinds of latency differences  Onset disparity – difference in hearing at the beginning of a sound  Ongoing phase disparity – continuous difference between ears in arrival of parts of a sound wave The duplex theory shows that sound localization requires processing of both intensity and latency differences. At low frequencies there are no intensity difference, the only cue comes from the time of arrival. In mammals, the superior olivary nucleus is the main localization nucleus for sound with two divisions: - Lateral superior olive processes intensity difference - Medial superior olive (MSO) processes latency differences byt encodes sound by relative activity of the left and right sides The lateral and medial superior olives react to differences in what is heard by the two ears. Medial refers to arrival time differences and lateral refers to amplitude differences. Both project to the superior colliculus. The deep layers of the superior colliculi are laid out according to auditory space which allows the location of sound sources in the world the shallow layers are laid out reinotopically. In sound localization in the superior olive, LSO neurons measure intensity differences and MSO neurons measure timing differences. Barn owls - Exceptional sound localization - Ears improve location in azimuth as well as horizontal plane - Critical model for understand neural basis of sound localization Interautal timing difference - Jeffress proposed that delay lines could produce time difference coincidence detectors which senses the co-occurrence of two events - Konishi found anatomical evidence in barn owls for just such coincidence detectors External ear structure selectively reinforces some frequencies called spectral filtering. Spectral cues provide critical information about elevation. Auditory cortex performs complex tasks in the perception of sound. They are analyzed in two main streams: - Dorsal stream – in parietal lobe, involved in spatial location - Ventral stream – in temporal lobe, analyzes components of sound - Processing these streams is for located where and what the sound is Two streams Auditory signals are conducted to two areas of association cortex - Prefrontal cortex - Posterior parietal cortex Anterior auditory pathway may be more involved with identifying sounds (what), while posterior auditory pathway may be more involved in locating sounds (where). There are interactions between the auditory and visual systems. For example some neurons have visual and auditory receptive fields. McGurk effect – phonemes vary continuously and are perceived categorically. Visual and auditory input of two phonemes can give rise to perception of intermediate phoneme. What you are seeing clashes with what you are seeing. What we see overrides what we believe we hear. When you experience sounds of a particular frequency , it can cause a rapid retuning of auditory neurons. Heschl’s gyrus, is the part of the auditory cortex that processes music - it is greater in musicians, where amusia is the inability to discern tunes. DTI or diffusion tensor imaging uses MRI to show fewer connections between frontal cortex and temporal lobe in tone deaf people. Experience contributes to development of sound localization and plasticity into adulthood. Hearing loss is a major disorder of the nervous system - Decreased sensitivity to sound ranging from moderate to sever Deafness – loss of hearing - 3 main causes o Conduction deafness – disorder of the outer or middle ear, preventing sound to the cochlea o Sensorineural deafness – from the cochlear to auditory nerve lesions o Central deafness – hearing loss caused by brain lesions such as a stroke - Ototoxic effects are ear damaging effects that may be due to drugs, loud sounds, or noise pollution - Damage to the hair cells can result in tinnitus which is a sensation of noises or ringing in the ears Central deafness - Word deafness – cannot recognize spoken words - Cortical deafness – difficulty in recognizing verbal and nonverbal auditory stimuli - Cochlear implants are used to treat deafness when there is a loss of hair cells Vision The visual system extends from the eye to the brain. The visual pathway starts at the retina and goes to the primary visual cortex and other cortices. Neural signals in the retina converge on ganglion cells. Their axons form the optic nerve and terminate in multiple brain regions. The eye has camera like features: - Cornea and lens focus light o Cornea bends light rays and forms image on retina o Lens focuses the image on the retina - Refraction which is the bending of light rays is done by the cornea and forms the image - Ciliary muscles in the eye adjust the focus, by changing the shape of the lens - Accommodation is the process of focusing the lens - The pupils formed by the iris controls the amount of light entering the eye. - Extraocular muscles are used to control eye movement Visual processing - Begins in the retina - Contains two types of cells o Photoreceptor cells – rods and cones - Bipolar cells o Receive input from photoreceptors and synapse on ganglion cells whose axons form the optic nerve, which carries the information to the brain - Horizontal cells in the retina contact both photoreceptors and bipolar cells - Amacrine cells contact bipolar and ganglion cells - Ganglion cells are the only cells in the eye that don’t generate graded potentials, they just fire action potentials Rods and cones are used for two different systems. Rods are used in the scotopic system which works in dim light. The photopic system uses cones, and it requires more light and allows color vision. The basics of light - Visual system responds to electromagnetic radiation measured in quanta - Each quantum has a wavelength - Quanta of light energy with visible wavelengths are called photons. In rods, the quanta of light is captured by the photo pigment rhodopsin cones use similar pigments. Each photo pigment, consists of two parts - Retinal and opsin o Similar structures as G proteins - When light activates rhodsopsin, Retinal dissociates and the opsin is activated o When retinal is actrivated it activates about 500 molecules of the G protein transducin - This cascade of events produces hyperpolarization of rods and cones - The magnitude determines the reduction in glutamate, neurotransmitter release - Visual system responds to changes in light Visual receptors must be stimulated to account for three characteristics of the visual system - Sensitivity o Weak stimuli are amplified to produce physiological effects - Integration o The stimulus over time makes vision relatively slow but increases its sensitivity - Adaptation o Visual system is adapted to a wide range of light intensities o They deal with these intensities by adjusting pupil size and range fractionation, which is when receptors with different thresholds handle different intensities o Rods contain low thresholds, while cones contain high thresholds Photoreceptor adaptation is the ability of photoreceptors to adjust sensitivity to prevailing levels of illumination. Calcium regulation and the amount of photo pigment available are two factors that contribute to photoreceptor adaptation. The visual field is the whole area you can see without moving your head or eyes. While visual acuity is the sharpness of vision, it tends to fall off towards the periphery of the visual field. Acuity is the best in the fovea, which is the central portion of the retina that is packed with the most photo receptors and is the centre of our gaze. Fovea also contains high concentrations of cones, Optic disc, which is on the nasal side of the fovea, is where the blood vessels and ganglion cell axons leave the eye. There are no photoreceptors in the optic disc, creating a blind spot that we don’t notice. Rods are not present in the fovea, because they are more numerous in the periphery and more sensitive to dim light then cones. Rod input converges on ganglion cells in the scotopic system. The tapetum lucidum is the reflective layer at the back of the retina, that gives a double change of the photon hitting the photoreceptive cell. Brightness is created by the visual system - Lateral inhibition – the process where interconnected neurons inhibit their neighbors and produce contrast. The ganglion cell axons form the optic nerve and cross at the optic chiasm, which is the point at where the two optic nerves meet. After passing the optic chiasm, the axons are called the optic tract. - Binocular depth perception is correlated with the amount of the visual field overlap Most of the axons synapse on cells in the lateral geniculate nucleus (LGN) of the thalamus. This area then sends its information to the visual areas in the occipital cortex. Axons of the postsynaptic cells in the LGN form the optic radiations which terminate in the primary visual cortex (V1) or the striate cortex of the occipital lobe, which is the region where most visual information first arrives. Extrastriate cortex – the cisual cortical areas outside the striate cortex In these two cotrexes there is a topographic projection of the retina, which means there is also a topographic projection of the visual field. Most of this space is devoted to the foveal region. When part of your visual field is being mapped in an orderly fashion it is called retinotopic mapping, no matter what level of the visual system it occurs in. Damage to parts of the visual system can be diagnosed from defects in perception of the visual field. By finding the site of the visual injury in the visual pathway we can predict the location of the scotoma, or perceptual gap, which is a spot where nothing can be perceived. This is known as blind sight, because the person cannot consciously perceive visual cues, but there is still some visual discrimination in this region. Evolution of eyes with lenses - Concentrating light sensitive cells - Clustering light receptors into a pit like depression - Narrowing the top of depression - Closing the opening with transparent skin or filling the cup with a transparent substance - Forming a lens Neurons at different levels of the visual system have very different receptor fields. Receptor field – the stimulus region and features that affect the activity of a cell in a sensory system At rest, photoreceptors steadily release glutamate, which either hyperpolarizes, or depolarizes bipolar cells depending on the type of glutamate receptor the cell has. There are two types of receptive field cells. There are on center bipolar cells, that turning on light excites them, causing them to receive less glutamate, which normally inhibits on center bipolar cells. Off center bipolar cells, are when turning off light in the center of the field excites the cells. This causes them to receive more glutamate, and become depolarized. Bipolar cells release glutamate, which always depolarizes ganglion cells. - ON center bipolar cells excite on center ganglion cells when light is trned on - Off center bipolar cells excite off center ganglion cells when light is turned off Neurons in the retina and the LFN have concentric receptive fields. - On center/ off surround - Off center/ on surround - On or off, relates to if the center or surrounding area of the circular file inhibits or excites the cell of interest The LGN of mammals has six main layers - Parvocellular o Four dorsal layers of the LGN – small cells , small receptive fields - Magnocellular o Two ventral layers – large cells and large receptive fields Two main types of retinal ganglion cells - Small ganglion cells (input from single cones) project to the parvocellular layer of the LGN and discriminate color - Large ganglion cells (input from MANY cones) project to the magnocellular layer of the LGN which cannot discriminate color Neurons in the visual cotex have more complicated receptive fields - Simple cortical cells or bar/edge detectors o Respond to edge or par of a particular with, orientation and location - Complex cortical cells o Respond to a bar of a particular width and orientation but may be located within a larger area of the visual field Spatial frequency filter model emphasises Fourier analysis of visual stimuli. The spatial frequency of a visual stimulus is the number of light/ dark or color cycles per degree of visual space. Cortical cell responses are tuned to spatial frequency grids more accurately than bar widths. Each cell’s receptive field has an excitatory axis and bands of inhibition on the side, and the spacing shows the frequency tuning. This creates multiple channels that are tuned to different special frequencies. - V1 (primary visual cortex) perceives objects and events, and also forms mental images. - V2, V4 and the inferior temporal lobe are involved in perception of form. - V5 or known as the medial temporal area (MT) is specialized for motion perception V2 is beside V1 and has similar properties. V2 cells respond to illusory boundaries and perceive complex relations in among parts of the receptive fields. V4 cells have strong response to concentric and radial stimuli. Cells in the inferior temporal (IT) area show an intermediate stage between V1 and V2 processing and pattenr and form recognition. V4 is also involved in color perception. The cells in IT respond to more complex forms, that the subject has learned to recognize. Receptive fields in IT probably develop through experience and learning. While the prefrontal cortex responds to faces V1 - Contains representation for four parts of a visual stimulus o Location in the visual field o Ocular dominance o Orientation o Color - Ocular dominance column o Region of cortex with greater synaptic input from one eye - Vertical columns are arranged in ocular dominance slabs, which are neurons in all layers that respond better to one eye - Stimulus orientation in V1 is also organized into columns - Orientation column responds to rod shaped stimuli of a particular orientation Colour vision depends on special channels from the retinal cones through cortical area V4 The visual system creates color in 3 dimensions - Brightness – dark to light - Hue – varies in colours - Saturation – caries from full colours to gray Trichromatic hypothesis of color perception - 3 different types of cones - They are excited by different regions of the spectrum - They each have their own separate pathway to the brain Opponent process hypothesis of color perception - 4 unique hues and 3 opposed pairs of colors o Blue vs yellow o Green vs red o Black vs white - Three systems that produce opposite responses to different wavelengths Each human cone has one type of pigment – with a different peak of sensitivity - Short (s) – peak sensitivity at 420 nm - Medium (m) – 530 nm - Long (L) – 560 nm Color blind humans only have dichromatic vision that cannot distinguish short wave length stimuli (blue) from long-wavelength stimuli (not blue). Color vision capabilities among animals can be separated into 4 categories - Excellent trichromatic color vision - Robust dichromatic color vision (2 kinds of cones and lots of cones) - Feeble dichromatic color vision (2 kinds of cone pigments but few cones) - Minimal color vision (single kind of cone pigment that relies on interactions between rods and cones to discriminate wavelength Colour blindness is usually due to the absence of cones sensitive to medium wavelength cones. Men are more likely to have this defect because it occurs on the x chromosome and males on
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