Neuro Exam III

Explain how we are able to distinguish among the senses and modalities, using the concept of labeled lines in your explanation.

- All sensory information from all sensory systems is encoded by action potentials that travel along peripheral nerves to the CNS - The brain recognizes distinct senses (modalities) because each sensory system has a distinct wiring (labeled lines) set up at all levels of neural organization: • Action potentials from receptors of separate modalities travel along separate nerve tracts • Different modalities are processed in different areas of the cortex • We learn to distinguish the senses and modalities - and perceive them differently - through experience

Briefly describe the cause of myopia (nearsightedness) and how it can be corrected with corrective lenses (e.g., glasses, contact lenses) or LASIK surgery.

- In normal vision, refraction bends light rays so they converge, bringing the image into sharp focus on the retina Myopia (nearsightedness) - difficulty seeing distant objects • Develops if the eyeball is too long, causing images to form in front of the retina - A corrective concave lens causes light rays to initially diverge so that the images fall on the retina

Describe the receptive fields of simple cells and complex cells in visual cortex and their responses to stimuli in their receptive fields. Explain how their receptive fields emerge from the input of neurons earlier in the visual pathway.

- LGN neurons send axons to V1, whose neurons have more complicated receptive fields requiring more-specific, elongated stimuli Simple cortical cells - also called bar or edge detectors - respond to a bar or edge of a particular orientation and width within its receptive field location - V1 simple cortical cells project to V1 complex cortical cells, with even more complicated receptive fields Complex cortical cells - respond to a bar or edge of a particular orientation and width, moving in a particular direction within a larger receptive field location

Describe the organization of ocular dominance slabs and orientation columns in V1, as shown in intracranial optical imaging and high-resolution fMRI.

Cortical Modularity - Visualized with high-resolution fMRI (7-Tesla scanner): • Ocular dominance slabs (left) • Red = right eye • Blue = left eye • Orientation columns (right) • Colors indicate preference for bars at different angles

cortical magnification

Cortical magnification - disproportionately large portion of the cortical area in V1 corresponds to the fovea, making high visual acuity in the center of the visual field possible

Information Coding, Receptive Fields, Somatosensory Receptive Fields

Information Coding - How do patterns of action potentials encode the characteristics of particular sensations (e.g., location, color, pitch, intensity)? - Each sensory receptor has a receptive field and/or tuning curve defining stimulus location/features to which it responds • Stimulus location (e.g., for touch and vision) is determined from the spatial receptive field of the activated receptors • Stimulus features (e.g., sound frequency) are determined from the features to which receptive fields of the activated receptors are tuned • Presence of a stimulus in a neuron's receptive field to which it is tuned is encoded as a change in firing rate • Stimulus intensity is encoded by the magnitude (amount) of increase or decrease in firing rate - Topographic mapping to cortex reflects these receptive fields Receptive Fields - The receptive field of a neuron is "what makes it fire" • Spatial location of a stimulus and/or specific features (e.g., tone frequency) that will change that neuron's firing rate from baseline (increase or decrease) • Receptive fields for each neuron in a sensory pathway differ in size, shape, and response to types of stimulation • Experiments sampling the stimulus space test what makes a neuron respond (i.e., change from its baseline firing rate) to define receptive field Somatosensory Receptive Fields - Stimulus location in the somatosensory system is determined from the position of the activated receptors and their topographic mapping to sensory cortex - The somatosensory receptive field is the region of the body surface (skin) in which a tactile stimulus will alter a sensory neuron's firing rate • Donut-shaped

Explain how neurons code for stimulus intensity, including an explanation of range fractionation.

Intensity Coding - A single neuron can convey stimulus intensity by changing the frequency of its action potentials Range Fractionation - allows sensory systems to respond to a wide range of intensities - each sensory receptor cell specializes in just one part of the range • Different cells have different thresholds for firing, over a range of stimulus intensities • Multiple neurons can act in parallel - as the stimulus strengthens, more neurons are recruited

Describe visual deficits resulting from geniculostriate pathway injury or damage (including monocular blindness, hemianopia, quadrantanopia, and scotoma). Identify locations of injury to the geniculostriate pathway that would produce those deficits, and vice versa (given the location of damage, predict the resulting visual deficit).

Monocular blindness - loss of vision from one eye Hemianopia - loss of vision from one-half of the visual field • Homonymous - same side of visual field lost in both eyes • Bitemporal - lost temporal halves of visual field from each eye • Binasal - lost nasal halves of visual field from each eye Quadrantanopia - loss of vision in one-fourth of visual field Scotoma - small blind spot in the visual field - Geniculostriate pathway injury can be diagnosed by deficits in perception due to neuroanatomy and retinotopic mapping

Describe how sensory transduction in Pacinian corpuscles and hair follicle receptors of the somatosensory system leads to the generation of generator/receptor potentials and action potentials.

Pacinian corpuscle - The Pacinian corpuscle (or lamellated corpuscle) is a skin receptor that responds to vibration and pressure • A stimulus to the corpuscle opens mechanically gated sodium channels and produces a graded generator potential • When the generator potential is big enough to reach threshold, the receptor generates an action potential Hair Follicle Receptors - Displacement of hair follicles causes stretch sensitive channels on dendrite to open, allowing an influx of sodium - This sodium influx voltage sensitive sodium-potassium channels to open, producing a nerve impulse

Explain the process of phototransduction, including the names and functions of the molecules involved in the signaling cascade, the resulting voltage change, and the resulting change in neurotransmitter release.

Photoreceptors • Rods have maximal absorption at 496 nm, but do not contribute to color perception • Cones have one of three types of pigments that absorb light over a range of wavelengths, but their maximal absorptions are at: • 419 nm (blue or short wavelength) • 531 nm (green or middle wavelength) • 559 nm (red or long wavelength) Phototransduction • Light particles (quanta, or photons) strike the discs and are captured by photopigment receptor molecules • A cascade of events produces a hyperpolarization of rods or cones • The disc structure of rods and cones contributes to their sensitivity

Define the terms photoreceptors, phototransduction (a.k.a. visual transduction), visual field, point of fixation, visual acuity, saccades, and sensitivity.

Photoreceptors (rods and cones) - in the retina are light- sensitive sensory receptor neurons • Responsible for visual transduction, the process of turning light into electrical neural signals Visual Field - the whole area you can see without moving your head or eyes • Location of objects in visual field are relative to point of fixation Visual Acuity - sharpness of vision • Highest in center of the visual field; falls off towards the periphery • Why you make saccades to fixate on objects of interest Sensitivity - responsiveness; inversely related to perceptual threshold (minimum intensity required for stimulus detection) • High sensitivity = stimulus detected at low intensity = low threshold

Explain three ways that the visual system can adapt to a wide range of ambient (background) light intensities (pupillary reflex, range fractionation, and adjustment of photoreceptor sensitivity through photoreceptor adaptation).

Pupillary reflex - adjustments to size of the pupil (the opening in the iris) in response to changes in background light intensity Range Fractionation - uses different photoreceptors to handle different intensities • Rods = low threshold for low intensity (dim light) • Cones = high threshold for high intensity (bright light) Photoreceptor Adaptation - each photoreceptor adjusts its level of sensitivity to match the average ambient level of light illumination - "recalibrates" to background light intensity

Explain the functioning of center-surround ("donut-like") receptive fields of retinal ganglion cells (RCGs) and LGN neurons. In other words, describe the receptive fields for ON-center and OFF-center RGCs/LGN neurons. Then explain how the corresponding cones, ON-center or OFF-center bipolar cells, RGCs, and LGN neurons would respond to various patterns of illumination within center and surround regions of their receptive field, thus yielding the antagonistic center-surround organization.

Receptive Fields of RGCs and LGN Cells

Describe the relationship between receptor density and relative sensory acuity, and discuss implications for cross-species and intramodal differences.

Receptor Density and Sensory Acuity - Receptor density is important in determining the abilities of a sensory system and relative sensory acuity • Higher acuity means better perceptual discrimination (e.g., high visual acuity means sharp vision) - Differences in receptor density among species determine the special abilities of many animals • Example: Olfactory ability of dogs - Differences in receptor density within a sensory system determine regions of higher or lower relative acuity • Example: More tactile receptors at the fingertips than the back

Describe the basic cellular architecture of the retina, referring to the different cell types and where they are found, including rods, cones (S, M, and L cones), bipolar cells, retinal ganglion cells, horizontal cells, and amacrine cells. Indicate which cell types release neurotransmitter proportional to graded potentials and which cell types generate action potentials.

Retina - Retina is a light-sensitive surface at the back of the eye consisting of photoreceptor cells (rods and cones) and other neurons • Translates light into action potentials (transduction) • Works in a wide range of light intensities (adaptation) • Discriminates wavelengths (colors) Fovea - Fovea is a region at the center of the retina that is specialized for high acuity and color vision • Receptive field at the center of the eye's visual field (point of fixation) Neuronal cell types of the retina: • Photoreceptor cells - rods and cones • Bipolar cells - receive input from photoreceptors • Neural signals in retina converge on retinal ganglion cells (RGC), whose axons give rise to the optic nerve • Lateral connections made by horizontal cells and amacrine cells Retinal Cytoarchitecture • Most retinal cell types only generate graded potentials and communicate via graded NT release • Only retinal ganglion cells (RGCs) fire action potentials • Horizontal, bipolar, and amacrine cells mediate intra-retinal processing that affects nearby areas of the retina, especially lateral inhibition - interconnected neurons inhibit their neighbors, thereby emphasizing contrast

Define, compare, and contrast the scotopic system and the photopic system. Explain how their different patterns of convergence create differences in sensitivity, receptive field size, and visual acuity.

Scotopic system (rods) - allows night vision • High convergence as scotopic retinal ganglion cells receive converging information from many rods -> large receptive fields • High sensitivity (can see in dim light) but low acuity Photopic system (cones) - allows sharp color vision • Little convergence as photopic retinal ganglion cells receive input from only one cone (or very few) -> small receptive fields • Low sensitivity (requires bright light) but high acuity (sharp vision) Visual acuity - measure of how much detail we see; sharpest in the center of the visual field (hits the fovea) • The fovea in the center region of the retina has a high density of smaller, tightly-packed cones with high acuity • This region receives direct light input that does not pass through other cells or blood vessels • The periphery: some cones; mostly rods with low acuity

Define extrastriate cortex and briefly describe the basic flow of information in the extrastriate cortical areas. Identify the basic information coding functions of areas V2, V4 and V5 (hMT), citing experimental evidence from electrophysiology (single-cell recording) and neuroimaging studies

Secondary Visual Cortex (V2-V5; Extrastriate Cortex) • Higher-order extrastriate cortical areas • Neurons demonstrate more complex receptive fields that are not so easily rationalized based purely on "bottom-up" visual input V2 • V3 • V3A • VP (ventral posterior) • V4 = color • V5 = motion • Composed of area MT (middle temporal) and area MST (middle superior temporal)

Differentiate between sensation and perception, describe the "inverse problem" of perception, and explain how perceptual systems attempt to solve it.

Sensation - The registration of physical stimuli from the environment by the sensory organs Perception - Subjective interpretation of sensations by the brain - Our visual experience is not an objective reproduction of what is "out there," but rather is a subjective construction of reality that is manufactured by the brain The Nature of Sensation and Perception - The only input our brains receive from the "real" world are patterns of action potentials passed along the neurons in pathways of our various sensory systems • How sense organs can turn energy, such as light waves, into nerve impulses (sensory transduction) is well-understood • The sensory pathways those nerve impulses take to reach the brain are also well-known • Less well-known is how we end up perceiving these sets of nerve impulses as a representation of the world The Inverse Problem: Perception - Sensory transduction results in a loss of some information from the environment • e.g., environment is in 3-D space; retinal image is only 2-D - Therefore, the source of sensory information is to some degree ambiguous - the inverse problem • Especially apparent with vision in the inverse optics problem • e.g., a retinal image of an object can derive from an infinite number of objects at different distances, sizes, and/or orientations - Due to the inverse problem, perception cannot be a direct readout of the real world • Sensory systems attempt to solve this problem ("disambiguate" stimuli) based on past experience (empirical information) - Perception guided partly by success or failure of previous interpretations of ambiguous stimuli • Past experience of the species influences general developmental wiring plan of sensory systems (DNA changes via evolution) • Past experience of the individual influences interpretations of current stimuli (e.g., learning from "perceptual regularities") - Contextual and top-down influences (such as attention, expectations, etc.) can also influence interpretation

Explain how sensory information can be suppressed or amplified (i.e., modulated) using accessory structures, central modulation, and attention, indicating some important structures and brain regions implicated in these processes. Explain two mechanisms by which pain signals can be "gated" or suppressed.

Sensory Modulation - Other ways to modulate sensory information (can be suppression or amplification): Accessory structures • Removing the stimulus (e.g., closing eyelids) • Accentuating stimulus (e.g., cupping hand to ear) Central modulation of sensory information - CNS structures suppress some sensory inputs, amplify others • For example, endogenous opioid release in the periaqueductal gray of the midbrain can eventually blunt transmission of the pain signal in the spinal cord, thus reducing pain perception Attention - Allows selection of some inputs over others - enhancing perceptual processing, awareness, and memory • Modulates neural activity in sensory cortex • Certain brain regions are important in different types of attentional control Pain Gating: Rubbing the Pain Away

Define the terms sensory organ, sensory receptor cell, adequate stimulus, range of responsiveness, sensory transduction, and generator/receptor potential.

Sensory Organ - All animals have sensory organs containing sensory receptor cells that are specialized to detect some stimuli but not others - Sensory organs are very diverse, reflecting stimulus characteristics and an organism's ecological niche, but all senses ultimately use the same type of energy — action potentials Sensory Receptor Cell - Sensory receptor cells are specialized cells within sensory organs that convert stimuli into electrical signals • Transduce (convert) sensory energy (e.g., light) into neural activity - Each sensory system receptors are designed to respond only to a narrow band of energy: • Vision: light energy (photons) -> chemical energy • Auditory: air pressure changes -> mechanical energy • Somatosensory: mechanical energy • Taste and Olfaction: chemical molecules Adequate Stimulus - An adequate stimulus is the type of stimulus to which a sensory organ is particularly adapted Range of Responsiveness - Sensory systems have a restricted range of responsiveness - Example: the frequency range for hearing, which varies with species Sensory Transduction - Sensory transduction by sensory organs is the conversion of sensory energy from an adequate stimulus into a change of membrane potential in a sensory receptor cell Generator/Receptor Potentials - local, graded changes in membrane potential of receptor cells that are generated by sensory transduction at receptors - For example, in the somatosensory system - which detects body sensations (including touch and pain) - some mechanoreceptors transduce vibration and pressure (Pacinian corpuscle) or touch (hair follicle receptor) into generator/receptor potentials...

Outline the general flow of information in a sensory pathway, primary sensory cortices, and secondary/nonprimary sensory cortices; apply this specifically to the somatosensory system by tracing the pathway of the dorsal column system.

Sensory Pathways - Each sensory system has a distinctive sensory pathway from the periphery to the brain • All receptors connect to the cortex through a sequence of 3 or 4 intervening neurons • Sensory pathways for most modalities pass through specific nuclei of the thalamus before relay to the cortex • Pathways terminate at modality-specific sensory cortical areas in the cerebral cortex - Information can be processed and modified at different stages in the pathway, allowing sensory system to alter or filter incoming sensory information (e.g., pain gating) Primary sensory cortex - first cortical area receiving sensory input; one exists for each modality Non Primary sensory cortex - or secondary sensory cortex — receives main input via direct projections from the primary sensory cortical area for that modality Dorsal Column System - delivers tactile (touch) information • Receptors send axons via the dorsal columns of the spinal cord to synapse onto neurons in the medulla (brainstem) • Axons from those medullary neurons cross the midline, and ascend via the medial lemniscus to the thalamus • Thalamic neurons project to the primary somatosensory cortex (S1)

Identify structures of the human eye on a diagram and describe their functions, including the cornea, iris, pupil, lens, ciliary muscles, retina, fovea, optic disc, optic nerve, blind spot, and extraocular muscles. Include in your descriptions the processes of refraction, accommodation, and fill-in.

Structures of the Human Eye • Cornea - clear outer covering that bends light (refraction) • Forming a backward, inverted image on the light-sensitive retina • Iris - colored part of eye that regulates amount of light reaching retina • Opens (sympathetic) and closes (parasympathetic) to allow more or less light into eye through an opening called the pupil • Lens - adjustable curvature to focus light on retina (more refraction) • Accommodation - focus on near or far objects when ciliary muscles adjust curvature of the lens • Retina - thin layers of neurons in the back of the eye • Where light energy initiates neural activity (visual transduction) • Fovea - central region of the retina • Highest density of photoreceptors (cones only), so sharpest visual acuity • Optic disc - where axons of the optic nerve leave the eye • Forms a blind spot due to lack of photoreceptors • Extraocular Muscles - precisely control eye movements • Three pairs of muscles (innervated by cranial nerves III, IV, VI)

Describe the phenomenon of synesthesia. Explain how we know that colored-grapheme synesthesia reflects actual perceptual experience (rather than metaphorical association). Speculate about the neural cause of synesthesia, with evidence

Synesthesia - Synesthesia ("mixing of the senses") is a condition in which a stimulus in one modality or sensory domain also creates a perception in another • Perceive digits or letters as being differently colored (e.g., "purple numbers") in colored-grapheme synesthesia • Spoken words or musical notes experienced as different colors in colored-hearing synesthesia • Certain words and/or numbers have specific tastes • Numerous other combinations - Psychophysical experiments demonstrate that synesthesia is real perception (not associational) - Cause is unknown, but may result from aberrant neural wiring during early development • Reflected in stronger white matter connectivity (DTI)

Describe several interactions between sensory modalities in typical individuals, including the McGurk effect. Relate these interactions to polymodal neurons and possible locations of these neurons in association areas of the brain.

The McGurk Effect - McGurk effect: Speech perception is altered by mismatching visual information from lip movements Polymodal Perception - Our perceptual experience binds together information across sensory modalities (e.g., dining experience) • Also fundamental to forming long-term episodic memory traces - Sensory systems can influence each other • Information from different senses is normally correlated • Cross-modal illusions when information is discrepant across senses - Polymodal perception indicates an interaction between sensory systems and underlying brain regions • How is sensory information integrated between modalities? - Polymodal neurons allow for different sensory systems to interact - Association areas in the brain process a mixture of inputs from different modalities • Superior colliculus • Association (sensory) cortex

Discuss the significance of the primate visual system, the monumental task of visual perception, and what types of information is emphasized by the visual system (and why).

The Task of Visual Perception - Vision is our primary sensory experience • Humans and non-human primates - Far more of the human/primate brain is dedicated to vision than to any other sense - Understanding the visual system's organization is therefore key to understanding human brain function - In a fraction of a second, our visual system must: • Capture light energy of different wavelengths reflecting off objects in all different directions • The eye • Transduce this light energy into a 2-D array of numbers (representing neuron activity) • Photoreceptors in the retina • Process visual information; perceive important visual features such as shape, color, motion, location, distance; and recognize the identity of objects in our dynamic 3-D world • Other retinal neurons, visual pathways, and the cerebral cortex - The visual system emphasizes relevant information in the environment (especially changes): • Perception of lightness and color is not a direct readout of the photons reaching your eye, but the visual system emphasizes contrast (Δ luminance and/or Δ color) - and therefore edges • Aids boundary identification and object recognition • Flashes of light (Δ luminance/time) or motion (Δ position/time) can enhance the perception of objects in the periphery of our visual field, thus capturing our attention • We only detect motion within the range of the biologically relevant speeds of animal movement

Compare and contrast the responses of tonic receptors and phasic receptors, including a definition of the term sensory receptor adaptation and an explanation of its purpose.

Tonic receptors - show slow or no decline in action potential frequency to a maintained stimulus Phasic receptors - display adaptation and decrease frequency of action potentials to a maintained stimulus Sensory Receptor Adaptation - Adaptation — the progressive loss of receptor sensitivity as stimulation is maintained • Sensory systems often shift away from accurate portrayal — noting changes can be more important than exactness • Allows sensory system to emphasize change in the external world, which is most relevant for behavior and survival

Describe the concept of a topographic map and apply it to the somatotopic map and organization of receptive fields in the primary somatosensory cortex (S1). Note: You do not need to memorize the exact layout, but understand concepts such as the relative cortical area for different body regions and the interpretation of this pattern.

Topographic Map - A topographic map is a neural-spatial representation of the body, spatial location, or stimulus characteristics perceived by a sensory organ • Relative cortical surface area for each region of stimulus space (e.g., body part in somatosensory system) is roughly proportional to sensory acuity • Reflects relative receptor density in sensory organ S1 Somatotopic Map - spatially organized neural representation of the body Primary somatosensory cortex (S1, postcentral gyrus) - receives touch information from the contralateral (opposite) side of body

Describe how information about eye of origin (referring to ocular dominance columns) and retinal ganglion cell type (photopic vs. scotopic system, referring to parvocellular vs. magnocellular) is preserved through the primary visual pathway from the retina to the LGN to V1.

ocular dominance

Charles Bonnet Syndrome

• Consider patient "Nancy" with a small scotoma, about 10 degrees in her left visual field • Dozens of times a day, her brain would fill in her scotoma in a most curious manner: • "On some occasions I see Disney cartoons... Mostly what I see is just people and animals and objects. But these are always line drawings, filled in with uniform color like comic books. It's most amusing." • Her brain was filling in her scotoma with visual hallucinations!

Outline the anatomical targets and basic functions of the four major visual pathways detailed in class (geniculostriate, tectopulvinar, retinohypothalamic, and pupillary).

• Geniculostriate system (conscious vision) • Retina -> lateral geniculate nucleus (LGN) of the thalamus -> striate cortex (primary visual cortex, V1) -> other visual cortical areas • Tectopulvinar system (attentional orienting, saccades) • Retina -> superior colliculus (of the tectum) -> pulvinar (of the thalamus) -> other visual cortical areas • Retinohypothalamic pathway (circadian rhythms) • Retina -> the suprachiasmatic nucleus (SCN) of the hypothalamus • Pupillary pathway (pupillary light reflex) • Retina -> Edinger-Westphal nucleus - Contralateral organization due to crossing of nasal RGC axons

Phototransduction 3

• In darkness, photoreceptors are depolarized, so they are continually releasing glutamate • Light triggers a hyperpolarization of the cell, so it releases less glutamate when illuminated • Graded NT release: the magnitude of hyperpolarization determines the reduction in glutamate release • No action potentials!

Phototransduction 2

• Photopigments consist of two parts: RETINAL and an opsin • When light activates a photopigment, RETINAL changes shape (isomerizes) and the opsin is activated, setting a G protein - mediated signaling cascade in motion • The specific opsin determines absorption spectra of bound RETINAL • The transducer molecule of rods is the photopigment rhodopsin (peak sensitivity at 496 nm) • Cones use similar pigments