Midterm Two Review.docx

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Giampaolo Moraglia

Rod & Cone Vision Neural Convergence  Signals generated on the receptors travel to bipolar cells, and then to ganglion cells whose axons transmit signals out of retina to optic nerve o Horizontal and amacrine cells connect neurons across retina o Signals can travel between receptors through horizontal cells and bipolar cells, and between ganglion cells through amacrine cells  Higher convergence in rods because about 120 rods pool their singles to one ganglion cells, but only 6 cones send signals to single ganglion cell  Higher convergence in the rod-system makes them more sensitive to light than cones o Convergent rods add up to threshold of ganglion (addition processing) o Non-convergent cones don’t add up therefore need higher stimulus  Less convergent cone are better at sharpness/discreteness (acuity) of vision than the rods o Rods: where light falls doesn’t make a difference (same response) o Cones: depends on which cone gets the light (different responses)  Leads to clarity/sharpness Lateral Inhibition Lateral Inhibition in the Limulus (Horseshoe-Crab)  Each eye of a limulus has hundreds of ommatidia (eyes = 1mm) o Each ommatidium has a cornea, lens, and single receptor o The ommatidia are quite large and can be individually illuminated o The receptor activity can be conveniently studied individually (neurons wide)  Lateral inhibition: inhibition that is transmitted laterally across a neural circuit o What happens in given area depends on neighbouring receptors o Helps determine contrast between light and dark  Lateral plexus: connects neighbouring neurons and causes lateral inhibition o In humans, have horizontal and amacrine cells  Recording activity (action potential) in “A”: o Stimulating “A” alone  significant firing o Stimulating “A” and “B”  when neighbouring receptor is stimulated, firing decreases o Stimulating “A” and “B” (with increased “B”)  decreases firing  In vertebrate-retina lateral inhibition is caused by horizontal cells and amacrine cells 1 Hermann Grid and The “Ghost Images”  Ghost spots seen at the intersection of the streets  Best noticed when looking away from the intersection, and disappear when looking directly at intersection  Disappear because image falling on fovea due to cones being so tightly packed (stimulated equally therefore no lateral inhibition)  Possible mechanism behind the ghost spots o At intersection, lateral inhibition from all 4 directions that leads to increased lateral inhibition therefore see darker areas o In streets, only from two directions therefore lateral inhibition doesn’t increase enough Mach Bands  Mach bands: the areas adjacent to borders between the band appear fainter/darker  A  B looks brighter and C D looks darker because more lateral inhibition from C  D  Where inhibition crosses, see contrast Retinal Lateral Inhibition and Simultaneous Contrasts  Brightness/colour in an area is influenced by the surround o Darker compared to brighter surround o Brighter compared to a darker surround  Better perception of contrast  better spatial resolution/ texture/ sharpness/ acuity of vision  Brighter area gets more lateral inhibition than darker, therefore something in middle has more contrast (ie. looks darker) 2 Light and Dark Adaptation Phototopic and Scotopic Vision  Scotopic = dim light vision/ dark adapted vision  Photopic = bright light vision/ light adapted vision Rhodopsin Cycle  Light adaptation is fast  Dark adaptation takes longer because sensitization/synthesis takes longer Dark-Light Adaptation Curves (Plotted by the Adjustment Method)  Dark adaptation: causes eye to increase its sensitivity in the dark  Dark adaptation curve: a plot of how visual sensitivity changes in the dark, beginning with when the lights are extinguished  2 phases in light sensitivity in the dark adaptation curve: o First rise (5-7 mins) and plateau followed by second rise (20-30 mins)  As soon as lights are extinguished, sensitivity of both rods and cones begin increasing, but since cones are more sensitive they synthesize first  First rise is due to cones  this only contributes to early part of the curve o Cones get synthesized first in dark vision (most sensitive) o After time they stop and become synthesized  Place where rods begin to determine dark adaptation curve is called rod- cone break  Second part is due to rods  this is more significant o Start slow allows for detailed vision  This, rods and cones contribute more to dim-light vision, dark-adapted or scotopic vision 3 Experimental Design to Test for Dark-Adaptation Response  Measuring rods and cones  Observer looks at small fixation point while paying attention to flashing light test that is off to side  The image of the fixation point falls on the fovea and the image of the test light falls in the peripheral retina o Causes light to fall on rods and cones Measuring Dark Adaptation  Observer is light adapted  Light is turned off  Once the observer is dark adapted, she adjusts the intensity of light until she can see just see it o Method of adjustment  continually adjust until you see a beam of light  Experiment for cone adaptation o Test light only stimulates cones (use small beam of light to stimulate only fovea) o Results shoe that sensitivity increases for 3-4 minutes and then it levels off  Experiment for rod adaptation o Must use a rod monochoromat (people who have only rods) o Results show that sensitivity increases for about 25 minutes and then levels off Visual Pigment Regeneration  When light hits the light-sensitive retinal part of the visual pigment molecule, it is isomerized and triggers transduction process o It then seperates from opsin which causes retina to become lighter colour (visual pigment bleaching)  Even in the light, molecules that have been split apart are undergoing visual pigment regeneration in which the retinal and opsin become rejoined  Cone pigment regenerates in 6 minutes  Rod pigment regenerates takes over 30 minutes to regenerate o Takes longer to start sensitivity, but contributes the most  Our sensitivity to light depends on the concentration of a chemical (visual pigment) and the speed at which our sensitivity is adjusted in the dark depends on a chemical reaction (regeneration) 4  Detached retina: when part of retina detached, it has become separated from a layer it rests on, the pigment epitheliem, which contains enzymes that are necessary for pigment regeneration o Result is that once pigments are bleached, they can no longer be combined in the detached part of the retina and person becomes blind in this area Absorption Spectra of the Rod and Cone Pigments  Opsin component (protein) part of the 2 pigments of rods and cones are different  Rod overlaps entire graph (more broad)  Dark adapted (rods) most sensitive to blue o Cones = green/yellow o Sensitivity shifts during adaptation Spectral Sensitivity of Rods and Cones  Spectral sensitivity: observer’s sensitivity to light at each wavelength across the visual spectrum  To determine, use flashes of monochromatic light (contains a single wavelength) o Determines threshold for seeing lights across visible spectrum o Convert threshold to sensitivity by: sensitivity = 1/threshold  Difference in spectral sensitivity is due to absorption spectra of visual pigments  Measure rod spectral sensitivity by measuring sensitivity after eye is dark adapted and present test flashes off to side of fixation pt, and measure cones by having people look right at test light  Rod pigment absorbs best at 500nm  Cone pigments absord best at 419nm, 532nm, and 558 nm o Absorption of all cones equals the peak of 560nm in the spectral sensitivity curve  Rod spectral sensitivity shows o More sensitive to short-wavelength light o Most sensitivity at 500nm  Cone spectral sensitivity shows: o Most sensitivity at 560 nm 5  Purkinje shift: enhanced sensitivity to short wavelengths during dark adaptation when the shift from cone to rod vision occurs  Purkinje effect or shift o Blues are bluer after dark adaptation o Reds are redder and greens are greener after light adaptation Nyctalopia = Night Blindness  Due to vitamin A deficiency  Most common preventable form of blindness (most common = catracts)  Readily treated by administering vitamin A  Prevented by diets containing high source of vitamin A o Normal vision restored  Dim light = problem o Can lead to destroyed cornea Receptive Fields of Retina ON, OFF, and ON-OFF System  Yellow bar = light  On ganglion cells = only respond to on  On-off type = only had on/off response  Off ganglion cells = only respond to off  In the developed retina we do not see the on-off type, we have either on or off 6 Sign Conserving and Reversing  Sign reversing for ON system therefore whatever the photoreceptor does (ie. depolarize or hyperpolarize) the bipolar does the opposite  Sign conserving for OFF system therefore whatever the photoreceptor does the bipolar does  Ganglion always does what bipolar does  Light ON = hyperpolarized  Light OFF = depolarized Set-up for Mapping the Receptive Field  Receptive field: an area covered by a sensory neuron o Influences firing rate of the neuron o Ex. ganglion cell of retina  Use muscle relaxant for eyes (dilate pupils)  Try to map light at different locations and record firing o Light on centre of ON receptive field = increased action potential firing  Light on OFF centre = decreased action potential o Light in surround = decreased action potential  Excitatory area: causes an increases in firing  Inhibitory area: causes a decrease in firing  Centre-surround antagonism: when centre and surround respond in opposite ways Receptive Fields 7 The Projections of the Retina The Visual Pathways  Nasal field mapped on the temporal fibres and vice versa  Nasal fibres cross and temporal fibres do not  Identify lesions with a detailed map while flashing lights in different visual fields  Right anopia: both nasal and temporal fibres cut = total blindness in one eye  Bilateral hemianopia: midline lesion in optic chiasm that affects the crossed nasal fibres on both sides = temporal fields gones o Pituitary tumors are the most common cause of lesion in the optic chiasm o Pituitary gland close to optic chiasm in a bony cavity o With a tumor, cavity is too small, so tumor expands damaging nasal fibres  Homonymous hemianopia: lesion after chiasm on the optic tract = no left side of both fields o Temporal fibres of right eye and nasal fibres on left affected  Homonymous hemianopia with macular sparing: lesion in optic radiation = entire field except macula is gone o Macular sparing: macular field of vision is commonly spared when there is visual loss due to damage to optic radiations or primary visual cortex o Causes for sparing:  Large cortical representations: huge part of V1 is dedicated to macula whereas peripheral gets a small area  Greater synaptic plasticity: neurons of macula have increased ability for regeneration and plasticity 8 Changes with Accommodation  Convergence of the eyeball  Constriction of the eyeball  Increase in the interior Accommodation Reflex Pathway  Frontal eye field (motor cortex)  III nerve: diffused pathway (not a lot of neurons)  Lost accommodation will still show light reflex pathway Light Reflex Pathway  Don’t need PVC or LGN for light reflex pathway Lateral Geniculate Nucleus: Major “Gateway” for the Visual Pathway  Thalamus: lateral geniculate nucleus and pulvinar are the 2 major relays for vision  Lamellar pattern in LGN: organized in layers  Retinotopicity: mapping of retina on each later (ie. different parts of retina are mapped on individual layers)  Both eyes are represented: o 1,4,6 from contralateral eye (opposite) o 2,3,5 from ipsilateral eye (same side)  M and P segregation: o P = parvocellular fibres  layers 3-6  Small diameter, slow transmission, shows us more detail o M = magnocellular fibres  layers 1-2  Large diameter, looks more dense because bigger neurons, faster transmission  Retrograde fibres to LGN: transmit backwards to LGN 9 o Drivers: allow continuous flow of information  Input into LGN and tell what to do o Modulators: control the driver (activate or inhibit)  Output from LGN o Thus, LGN not only relays functionally distinct, topographically organized, visual signals but also regulates the amount of information reaching the VI o Reticular formation: mesh of neurons from brainstem  thalamus  V1 that activates the extrastriate cortex  Ascending: arousal, attention, and learning  Descending: muscle tone  Information flow in the LGN: LGN receives signals from optic nerve (retina), cortex, brain stem, thalamus, and other nerves in LGN o LGN receives more input back from the cortex than it receives from the retina and the smallest signal of all is from the LGN to the cortex o This decrease in firing that occurs at LGN is to regulate neural information flows from retina to cortex o LGN also organizes information Amygdala via Pulvinar: Fear-Response Pathways  Features of pulvinar: o Largest thalamic nucleus in humans (almost 2/5) o Higher order relays (adds salience to threatening stimuli)  More sophisticated than LGN o Involved in feature binding:  Inter-hemispherical  Intercortical (same cortex)  V1-Amygdala (fast and slow loop) o Important for visual salience: suppression of noise and enhancement of significant stimulus (especially threat) o Key player in affective blindsight  Fast and slow loop: pulvinar adds salience to the emotion evoking responses 10 o Fast loop: quick response o Slow loop: longer route (important for recognizing threat)  One type of agnosopias is affective blindsight: blindsight for emotional faces especially fearful faces o Pulvinar is rapidly activated along with amygdala with fearful faces are presented in contrast with happy, sad, angry, and neutral faces Hypothalmus, Pineal Gland, and Circadian Rhythm  Rods and cones – not the exclusive receptors of the retina  The link for photo-entrainment resides in retina o Photo-entrainment: alignment of biological rhythms to ambient light- dark cycles  Vertebrates – inverted retina o Receptors are away from light source o Ganglion cells = photoreceptors  Melanopsin: axons of melanopsin ganglion cell traced to suprachiasmatic nucleus (thalamus)  Melanopsin associated with photoreceptive system (not all ganglion cells have it) o Has its own biological nucleus o Biochemical osscilatonsin if showing a 24 hrs like cycle o Has connection to light of day/night 11  Melanopsin-melatonin and circadian rhythm o Noctural animals: during light have decreased melanopsin and increased melatonin = sleep during day o Diurnal animals: during light have decreased melanopsin and melatonin = awake during day  Melanopsin knockouts: poor photo-entrainment in melanopsin knockout animals  became crazy o Normal photo-entrainment in rod/cone pigment knockouts (blind)  biological rhythms maintained well  Circadian-entrainment actograms: way of recording activity  Restoration of visual function in mice with retinal degeneration by transfecting melanopsin o Melanopsin is transfected using a viral vector into mice with degenerated rods and cones (totally blind) o Papillary light-reflex is restored o Mice show normal light-avoidance response o Show light-dark discriminatory response o Use open field to get mice to avoid light, then in visual discrimination task to chose between light and dark, and choses dark area because safer  Restoration of visual function in retinal degeneration mice by ectopic expression of melanopsin  Melanopsin related “second sight” actually came first in evolution and appears first in development  Treat circadian disorders with melanopsin-related medications  Use melanopsin without primary vision for some survival 12 M and P Cell-Pathway  Receptive field of M and P cells of LGN body o M cells (magnocellular): large size = large field o P cells (parvocellular): small size = small field  Ganglion cells connected with M cell pathway have long dendrites  Distribution of M and P cell pathways in the retina o Many fibres = fovea connected with small fibres of LGN o M cell pathways = periphery  Properties of M and P cell pathways relate to their size  Properties of M and P cells  Taxonomy of cells in LGN NOTE: These types are P NOT related to layers of M the retina. Neurotransmitters of Retina  OFF: only NT transmitted is glutamate o NT brings about depolarization in bipolar cell when light off o Releases more glutamate  ON: when on, hyperpolarizes bipolar cell o Releases less glutamate  Lights turned off: two different types of glut receptors (one in off and one in on) o ON: Cl- ions enter glut therefore hyperpolarizes 13 o OFF: Na+ and Ca+ ions enter with glutamate therefore depolarizes The Rod/Cone Plan  Rods only have ON bipolar cells (cones have ON and OFF bipolar cells)  Cones have bipolar cells responding to increments (and decrements) in light intensities  Rods: convergent system therefore lacks detail o Can only detect brightness  Cones: divergent system therefore detailed o Can detect changes in darkness and lightness o 10 types of on and 10 types of off with different thresholds to detect different intensities ON-OFF Receptive Field Circuitry and Lateral Inhibition  Lateral inhibition: inhibition transmitter laterally across a nerve circuit  Amacrine also contributes to the lateral inhibition  Horizontal and amacrine  important NT is GABA  Lateral inhibition is due to GABA Why: ON and OFF System?  Illumination-contrast sensing: most perception by detecting edges and sharpness  Better spatial resolution (texture)  Perception of increments in brightness as well as in darkness Retina and Ribbon Synapses  “Ribbons” are dense presynaptic plate/disc-like bodies containing high concentration of neurotransmitter vesicles o Projections of presynaptic neurons that are ribbon-like and vesicles attached o Vesicles dock close to specific site (clustered in one region)  They cause graded neurotransmitter release o Depends on how much depolarization  Seen in cones, rods and bipolar cells of retina o Also seen in the hair calls of cochlea  Tethered with hundreds of synaptic vesicles  Anchored perpendicular to presynaptic membrane like a flag  Rod has one ribbon, cones and bipolar cells have varying numbers (2-10)  Strategically located at the invaginations of the synapse  Usher’s syndrome: most common hereditary blindness-deafness-vestibular disorder due to defective ribbon synapses 14 o Problems with equilibrium and posture o Most likely causes mental retardation Plasticity in Retina: Activity-Dependent Modeling of Retina  Visual stimulation is required for refinement of ON and OFF pathways in postnatal retina o Like brain tissue therefore shows plasticity  Stratification of ON and OFF ganglion cell dendrites o Developed retina: inner plexiform layer (IPL) of ganglion cells has 2 layers (A and B)  Those that synapse at layer A are off type and those that in layer B are on type  Light-induced refinement of ON and OFF system: need light for ganglion cells to separate into 2 layers o No on/off system without exposure to light o In neonatal animals, show on and off responses o By 28 days after birth, open eyes and develop on and off system  Dark adaptation and “plasticity” in outer plexiform layer of retina o Spinules (protrusions of dendritic spines) of horizontal cells quickly retract within minutes of dark adaptation within their cone synapses o Project outwards with light o Shows plasticity depending on environment Electroretinogram  Electroretinogram (ERG) technique: summed up activity of the retina o Record activity of retina o Have one electrode on cornea (contact lens) which records activity between cornea and retina o Can take spontaneous activity or shine light  ERG and origin of ERG waves o Negative wave  a  A-wave contributed by receptors o 2 positive waves  b and c  B-wave contributed by “on” bipolar cell  C-wave contributed by pigment epithelial (outermost layer) Leber Congenital Amaurosis (LCA): Major Characteristics  Definition: amaurosis is blindness without clearly discoverable lesion in the eye structures o Retina looks perfectly healthy when born  Characteristics of LCA: o 5% of World’s congenital blindness (of varying degree) 15  Early and severe visual loss o Absence of any gross pathology of retina or any eye structure o Flat pattern on ERG o One of the common causes is mutation in the gene for retinal pigment epithelium related to isomerase RPE65  Enzyme defective because gene not there  Success in gene therapy achieved  gene introduced and vision almost fully recovers  In humans RPE65 deficiency requires early intervention o Over time retina destroyed even though originally healthy Retinal Implants and Other Applications  Common diseases that affect the retina o Retinis pigmentos  Genetic disease  Rods are destroyed first  Foveal cones can also be attacked  Severe cases result in complete blindness o Macular degeneration  Fovea and small surrounding area are destroyed (macula destroyed in patches)  Creates an additional blind spot on retina  Most common in older individuals  Taking advantage of an orderly architecture in the retina in therapeutics o Electrical stimulation of mammalian retinal ganglion cells with multi- electrode arrays  Sits on ganglion cell layer and directly stimulates cells  Use 6 small electrodes  Recover significant amount of vision  No sharpness o Miniature intraocular telescopes  Remove lens and replace with telescope  No sharpness of vision, but can navigate Vision for Cognition and Vision for Survival: Organization of Visual Cortex Visual Cortex (Striate and Extrastriate)  Significant part of cortex devoted to visual cortex  Extrastriate: involved in vision, but outside primary visual cortex (ex. areas in temporal lobe)  Striate: primary visual cortex o One of layers quite dense and only seen in PVC o Thick band is reason called striate cortex (defining feature)  Superior colliculus: area involved in controlling eye movements and other visual behaviours 16 o Receives about 10% of fibres from optic nerve Visual Areas  Striate: where LGN makes 1 synapse/relay  Prestriate cortex: directly in front of striate  About 30 different vision areas  Most LGN fibres go first to V1, then V2 (doesn’t necessarily continue up or start at V1)  Cortical and subcortical visual areas: there are 32 areas in the cortex besides several sub-cortical areas that need to be successfully found for visual experience (all interconnected) Organization of PVC  Lamellae: layered arrangement across the cortical thickness (I to VI are the lamellae) o 6 layers of visual cortex o Layer 1-3: output to neighboring cortical areas o Layer 4 is very thick therefore striate area  Divided into IVa, IVb, IVc  Reason so dense because gets fiber relays from thalamus (LGN) to IVc o Layer 5: output to subcortical areas o Layer 6 gives signals back to thalamus  Location columns: perpendicular to surface of cortex so that all of the neurons within a location column have their receptive fields at same location on retina  Orientation column: column contains cells that respond to certain orientations  Ocular dominance columns: columns separate input from right and left eye in alternating pattern o 1 make synapse in layer IVc o Radioactive amino acid study: injected radioactive proline into one of the eyes  Proline transported by axon transporter  LGN  visual cortex  Found dark, bright, dark, bright alternating columns in visual cortex (bright = proline)  discovered columns  Takes few days to occur o 17 Development of columns: injected tetradoxin in both eyes to stop neurons from firing during critical period, see same pattern as with one eye  Therefore need continuous activation of neurons to maintain columns  Take healthy newborn and find left/right forms synapses in different areas  Take newborn with only one working eye and find one eye synapses less  Orientation sensitive cells: orientation columns pick up activation of only particular orientations o Record action potentials by moving on an angle towards cortical surface  Orientation-tuning curve will be missed if the electrodes are advanced vertical to cortical surface  Each ocular dominance column has all orientations o Hypercolumn: a 1mm block of striate cortex representing all orientations and both eyes, and a single location column  Map hypercolumns using voltage sensitive dyes  Size might vary between monkeys and humans o Receptive fields in the PVC are elongated  All concentric cells go to same neuron therefore integrate into one receptor  Helpful in detecting edges/contrast with this design o Simple cortical cells: have side by side receptive fields o Complex cells are sensitive to orientation and direction of movement  Discovered by moving slides up and down (saw edge line)  Respond to more than one feature  Found in PVC o Hypercomplex cells: some neurons in striate cortex responded beast to bars of particular orientation + direction of movement + length of bar  Also called “end-stopped” cells  Ideal for detecting corners moving in particular direction o Not born with ocular dominance or orientation columns o Selective adaptation: if neurons fire for long enough, they become fatigued or adapt  Adaptation causes 2 physiological effects: neurons firing rate decreases and the neuron fires less when that stimulus is immediately presented again  Adaptation is selective because only neurons that respond to verticals or near-verticals adapt, others do not  Grating stimuli are alternating bars  Contrast threshold is the difference in intensity at which the bars can just barely be seen 18  Measure by contrast thresholds of different orientations, adapt person to one orientation by having person view high contrast adapting stimulus, and re- measure  With vertical bars, when re-measure need to have higher contrast to see (decreased response to verticals) o Selective rearing: animal is reared in an environment that contains only certain types of stimuli, then neurons that respond to these stimuli become more prevalent  Follows from neural plasticity – the idea that the response properties of neurons can be shaped by perceptual experience  Results indicate “use it or lose it” phenomena – if reared in vertical, lost horizontal responding neurons  Cytochrome oxidase – colour blobs are colour opponent (occupy 50% of layer 2 and 3 of V1) Map in the Striate Cortex  Information about objects near each other in the environment is processed near each other in the cortex  Cortical magnification factor: apportioning of small fovea with a large portion on the cortex o Not the same for all observers o Those with more cortical space had better acuity Streams of Visual Cortex  Ventral stream (vision for perception): features (what stream)  Dorsal stream (vision for action): movement (where stream)  M-fiber pathways that are ventral in the LGN connect with the dorsal stream in the visual cortex o M-fibers = action  P-fiber pathways that are dorsal in the LGN connect with the ventral stream in the visual cortex o P-fibers = details  Visual fields in humans: most of visual side on medial side, above and below calcrine suclus o Macular has large representation 19 General Properties of Two Streams  Feed forward (V1  higher association areas) o Progressively more complex feature-sensitivity (ie. hierarchy) o Progressively wider distribution (ex. face represented in many areas) o Progressively poor retinotopicity o Antagonist surround gets weaker  Around area of activation  Probably don’t want too much because of conjunctional binding  Feed back pathways (higher association areas  V1) o Pre-attentive search to V1 o Attentive serial search and conjunctional binding beyond V1  Binding pathways (among neurons of equal hierarchy and between ventral and dorsal streams) o Depending on stimulus complexity, dorsal and ventral stream connected o Ex. dorsal stream identifies hammer by movement to help ventral stream identity by shape Special Attributes of Ventral Stream  Columns for complex contours, angles, textures, and colours  V4  Columns for face  right inferior temporal cortex (fusiform gyrus) o Right more associated with face perception o Extrastriate body area (EBA) responds selectively to body parts  Weak response to object location (compared to dorsal stream) o Only involved in recognizing  Tonic responses to objects (slower response that stays longer) o When object moves, cannot identify in time because of longer refractory period due to being depolarized longer o Electrophysiological response characteristics  Strong perceptual learning (compared to dorsal stream)  Weak-luminance contrast sensitivity compared to dorsal stream o Change in contrast in brightness poor 20 Ventral Stream and Further Division of Dorsal Stream  Important location  intraparietal sulcus o Superior = dorsal – dorsal o Inferior = ventro – dorsal Special Attributes of Dorsal Stream  Motion detection: o Object pursuit: superior parietal lobule  Also called dorsal-dorsal pathway  Fast in detection of motion (online) o Space/depth perception: especially medial temporal (MT) and inferior parietal lobule  Also called ventro-dorsal pathway  Detection of coherence in motion  Firing and coherence experiment  Coherence = objects moving in same direction  Coherence of movement of dot pattern was varied  Monkeys were taught to judge direction of dot movement and measurements were taken from MT neurons  Results showed that as coherence of dot movement increased, so did the firing of the MT neurons and the judgment of movement accuracy  Coherence as low as 1-2% elicits a significant response in the MT and a behavioral response from the monkey  Lesioning experiment  Normal monkeys can detect motion with coherence of 1-2%  Monkeys with lesion (by electric pulse) cannot detect motion until the coherence is 10-20%  Microstimulate experiment  Monkey trained to indicate direction of fields of moving dots  Neurons in MT cortex that respond to specific direction were activated 21  Experimenter used microstimulation to activate different direction sensitive neurons  Monkey shifted judgment to the artificially stimulate direction  Strong luminance-contrast sensitivity compared to ventral stream  Strong response to varying locations of objects  Detection of binocular disparity (in MT) o In right eye sees something slightly different than left o Brain computes small difference and creates 3D image o Intraparietal sulcus = response to 3D depending on plane  Area selectively sensitive to different gradients/plane  Attentional search light o Attentional feedback that highlights neural information:  In the V1 visual inputs come in from the LGN and  Along the ventral stream o This reduces information overload by filtering and facilitates serial search  Transient responses to visual stimuli o Quick responses (phasic response)  Short refractory because of short depolarization  Electrophysiological response characteristics o Important for attention scanning  Weak perceptual learning (compared to ventral stream) Perception of Biological Motion  Biological motion: movement of person or other living organism  Point-light walker stimulus: biological motion made by pacing lights in specific places on a person  Structure-form-motion takes place with point-light walkers  Neurological studies show biological motion is processed by superior temporal sulcus (STS) and fusiform face area (FFA)  Transcranial magnetic stimulation applied to STS caused a decrease in ability to detect bio
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