{"id":4448,"date":"2018-10-07T00:38:17","date_gmt":"2018-10-07T04:38:17","guid":{"rendered":"https:\/\/www.amyork.ca\/academic\/zz\/?p=4448"},"modified":"2018-10-07T00:47:26","modified_gmt":"2018-10-07T04:47:26","slug":"motion-perception","status":"publish","type":"post","link":"https:\/\/www.amyork.ca\/academic\/zz\/sensation-and-perception\/motion-perception\/","title":{"rendered":"Motion perception"},"content":{"rendered":"<p>Motion aftereffect (MAE): the illusion of motion of a stationary object (in the opposite direction) that occurs after prolonged exposure to a moving object o Waterfall illusion<\/p>\n<p><em>Computation of visual motion<\/em><\/p>\n<ul>\n<li>Motion detector cells contain two different receptive fields, separated by a fixed distance. The output from the first receptive field is somewhat delayed by an interleaved cell (D). This cell has a fast adaption rate, it fires when the first receptive field detects light, but quickly stops firing if the RF remains lit. The outputs from D and the second RF are connected to a multiplication cell, which can only fire if both D and the second RF are active. By delaying the response from the first RF and then multiplying it by the response of the second RF, we can create a mechanism that is sensitive to motion. Figure 8.3 page 223 o Direction specific o Velocity specific\n<ul>\n<li>If the delay increases, the cell will respond to slower velocities<\/li>\n<li>If the span between the RFs increases, the cell will respond to faster velocities<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p><em>Apparent motion<\/em><\/p>\n<ul>\n<li>Apparent motion: the illusionary impression of smooth motion resulting from the rapid alternation of objects that appear in different locations in rapid succession o Stationary drawings shown at a rapid rate will be perceived as a movie<\/li>\n<\/ul>\n<p><em>The correspondence problem<\/em><\/p>\n<ul>\n<li>Correspondence problem: the problem faced by the motion detection system of knowing which feature in frame 2 corresponds to a particular feature in frame 1 o Figure 8.5 page 225<\/li>\n<\/ul>\n<p><em>The aperture problem<\/em><\/p>\n<ul>\n<li>Aperture problem: the fact that a moving object is viewed through an aperture (or a RF), the direction of motion of a local feature or part of the object may be ambiguous o A variety of contours of different orientations moving at different speeds can cause identical responses in motion-sensitive neurons in the visual system<\/li>\n<li>V1 cells have a limited RF, which can be seen as a small aperture through which the world is viewed, therefor none of the V1 cells can tell with certainty which visual elements correspond to one another when an object moves \u2192 another set of neurons that listen to the V1 neurons integrate the potentially conflicting signals (the global-motion detector that combines the output from the local-motion V1 cells) figure 8.7 page 227<\/li>\n<\/ul>\n<p><em>Detection of global motion in area MT<\/em><\/p>\n<ul>\n<li>The vast majority of cells in MT are selective for motion in one particular direction, but they show little selectivity for form or colour<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<p>Newsome &amp; Pare trained monkeys to respond to correlated dot motion displays. Once trained the monkeys only needed 2-3% of the dots to move in the same direction, to determine what the correlated-motion direction must be. After an MT lesion however, the monkeys need about ten times as many correlated dots in order to correctly identify the direction of motion. The ability to discriminate the orientation of stationary patterns was generally unimpaired.<\/p>\n<ul>\n<li>When the group of cells responsible for the detection of a specific motion direction, for instance to rightward motion, are stimulated the monkey will report the motion as being rightward even if the dots they were seeing actually moved in the opposite direction \u2192 the MT is the site of global-motion detection neurons in the visual system<\/li>\n<\/ul>\n<p><em>Motion aftereffects revisited<\/em><\/p>\n<ul>\n<li>When we view a stationary object, the responses of neurons tuned to different directions of motion are normally balanced, the signals of the different directions are cancelled out and no motion is perceived. When we look at a waterfall for a prolonged period, the neurons sensitive to downward motion become fatigued. When we then switch our gaze to a stationary object, the neurons sensitive top upward motion fire faster than the fatigued downward motion neurons, and we therefor perceive the stationary object as going up<\/li>\n<li>With the motion aftereffect, inter-ocular transfer occurs o The aftereffect must reflect the activities of neurons in part of the visual system where info collected from the two eyes is combined\n<ul>\n<li>The MAE in humans is caused by the same brain region shown to be responsible for global-motion detection in monkeys: area MT \u2192 MAEs are due to a population imbalance in area MT (direction-selective adaptation produces a selective imbalance in the fMRI signal in human MT)<\/li>\n<\/ul>\n<\/li>\n<li>First-order motion: the change in position of luminance-defined objects over time<\/li>\n<li>Second-order motion: the change in position of texture-defined\/contrast-defined objects over time<\/li>\n<\/ul>\n<p><em>Second-order motion<\/em><\/p>\n<ul>\n<li>In second-order motion nothing actually moves, furthermore there is nothing to move<\/li>\n<li>Second-order motion proves that matching discrete objects across movie frames is not necessary for motion perception \u2192 motion can define an object<\/li>\n<li>Double dissociation between first- and second-order motion deficits proves that they are present in different brain areas<\/li>\n<\/ul>\n<p><em>Using motion information <\/em><\/p>\n<p><em>Going with the flow: using motion info to navigate<\/em><\/p>\n<ul>\n<li>Optic array: the collection of light rays that interact with objects in the world in front of a viewer<\/li>\n<li>Optic flow: the changing angular positions of points in a perspective image that we experience as we move through the world, used to determine where we\u2019re going o Radial expansion: outward expansion of the optic array (figure 8.12 p232)<\/li>\n<\/ul>\n<p>Focus of expansion: the point in the centre of the horizon, the one place in the visual field that will be stationary, from which, when we are in motion, all points in the perspective image seem to emanate &#8211; \u00a0 Optic flow heuristics:<\/p>\n<ul>\n<li>The mere presence of optic flow indicates locomotion, the lack of flow is a signal that you are stationary<\/li>\n<li>Outflow indicate that you are approaching a particular destination, inflow indicates retreat<\/li>\n<li>The focus of expansion tells you where you are going to<\/li>\n<\/ul>\n<p><em>Something in the way you move: using motion info to identify objects<\/em><\/p>\n<ul>\n<li>People are able to make an immediate and very compelling impression of a living human in action, when all they see is moving dots (attached to joints) in the dark o Biological motion of the moving lights can even be used to identify whether the dots are attached to a male or female<\/li>\n<li>Gender cues such as the average centre of motion and the amount of body sway are used<\/li>\n<li>It take two to tango: when we watch two people interacting, knowing what one is doing helps us understand the actions of the other<\/li>\n<\/ul>\n<p><em>Avoiding imminent collision: the tao of tau<\/em><\/p>\n<ul>\n<li>Time to collision (TTC): the time required for a moving object to hit a stationary object o TTC = distance \/ rate<\/li>\n<li>Tau: the ratio of the retinal image size at any moment to the rate at which the image is expanding, TTC is proportional to tau o Great advantage: it relies solely on info available directly from the retinal image, all you need to do is track the visual angle subtended by the object as it approaches your eye<\/li>\n<\/ul>\n<p><em>Eye movements<\/em><\/p>\n<ul>\n<li>Smooth pursuit: a type of voluntary eye movement in which the eyes move smoothly to follow a moving object<\/li>\n<\/ul>\n<p><em>Physiology and types of eye movements<\/em><\/p>\n<ul>\n<li>Stimulation of a cell in the midbrain results in a movement by a specific amount in a specific direction, every time that cell is stimulated, the same eye movement will result \u2192 the movement is coded<\/li>\n<li>In response to stimulation of some cells in the frontal eye fields, the eyes will move to fixate on a specific spot in space \u2192 the destination is coded<\/li>\n<li>Microsaccades: involuntary, small, jerklike eye movements that are present even when the eye movements are paralyzed o May be important for very fine spatial judgments, by precisely moving the eye to nearby regions of interest<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<p>Three types of voluntary eye movements:<\/p>\n<ul>\n<li>Smooth pursuit<\/li>\n<li>Vergence eye movements: occur when we rotate our eyes inward or outward to focus on a near or far object<\/li>\n<li>Saccade: a fast jump of the eye that shifts our fixation point from one spot to another\n<ul>\n<li>We tend to fixate on interesting places in an image, thus the eyes are most likely to make saccades in response to contours than broad featureless areas of an image<\/li>\n<li>We make eye movements that are based on the content of a scene and on our specific interest in that scene<\/li>\n<\/ul>\n<\/li>\n<li>Reflexive eye movements: automatic and involuntary o Vestibular eye movements: the eyes move to compensate for head and body movement while maintaining fixation on a particular target\n<ul>\n<li>Operate via the vestibule-ocular reflex (VOR) o Optokinetic nystagmus (OKN): the eyes will involuntary track a continually moving object; the eyes move smoothly in one direction in pursuit of an object moving in that same direction, and then snapping back<\/li>\n<li>Used to move visual acuity in infants<\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<p><em>Eye movements and reading<\/em><\/p>\n<ul>\n<li>Asymmetrical perceptual span: English readers are able to gain info from up to n15 characters to the right of fixation, but only 3-4 characters to the left o Attentional: readers of both Hebrew and English can switch asymmetry<\/li>\n<li>Disappearing text experiments have shown that if words remain on the screen for only 50 ms after it is first fixated, reading proceeds normally<\/li>\n<\/ul>\n<p><em>Saccadic suppression and the comparator<\/em><\/p>\n<ul>\n<li>Saccadic suppression: the reduction of visual sensitivity that occurs when we make saccadic eye movements. Saccadic suppression eliminates the smear from retinal image motion during an eye movement; acts mainly to suppress info carried by the magnocellular pathway<\/li>\n<li>Two copies of each order to move the eyes are send, enabling the motor system to solve the problem of why an object in motion appears stationary. One copy goes to the eye muscles and the other to the comparator<\/li>\n<li>Comparator: compares the image motion signal with the eye motion and can compensate for the image changes caused by the eye movement, inhibiting any attempts by other parts of the visual system to interpret the changes as object motion.<\/li>\n<\/ul>\n<p><em>Development of motion perception<\/em><\/p>\n<ul>\n<li>Reflexive eye movements to moving targets (OKN) are present in new-borns, and the neurons in V1 have adult like sensitivity to visual direction<\/li>\n<li>Sensitivity to global motion, which is thought to reflect processing in MT, appears to develop more slowly, reaching maturity at about 3-4 years of age, while sensitivity to motion-defined form and biological motion takes even longer<\/li>\n<\/ul>\n<p>&nbsp;<\/p>\n<p><em>The man who couldn\u2019t see motion<\/em><\/p>\n<ul>\n<li>Akinetopsia: inability to detect motion o Caused by disruptions to cortical area MT<\/li>\n<\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Motion aftereffect (MAE): the illusion of motion of a stationary object (in the opposite direction) that occurs after prolonged exposure to a moving object o Waterfall illusion Computation of&hellip; <a class=\"continue\" href=\"https:\/\/www.amyork.ca\/academic\/zz\/sensation-and-perception\/motion-perception\/\">Continue Reading<span> Motion perception<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[112],"tags":[],"_links":{"self":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4448"}],"collection":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/comments?post=4448"}],"version-history":[{"count":0,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/posts\/4448\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/media?parent=4448"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/categories?post=4448"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.amyork.ca\/academic\/zz\/wp-json\/wp\/v2\/tags?post=4448"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}