Chapter 8 Perceiving Motion I Functions of Motion Perception a Motion helps us understand events in our environment II Studying Motion Perception a When do we perceive motion i Motion Agnosia the loss of ability to perceive motion 1 Affects thing such as pouring tea into a cup not being able to perceive fluid rising following dialogue sudden appearance and disappearance of moving people and objects b Motion attracts attention i Attentional Capture the ability of motion to attract attention ii The fact that movement can attract attention plays an important role in animal survival when animals freeze in place c Motion provides information about objects i Movement perceptually organizes all the elements of an object so they create a figure that is separated from the background bird camouflage demonstration ii Our motion relative to objects is constantly adding to the information we have about the objects i Real Motion actual motion of an object perceiving a car driving ii by perceiving a bug scurrying across a tabletop Illusory Motion perception of motion when there actually is none 1 Apparent Motion two stimuli in slightly different locations are alternated with the correct timing allows observer to perceive one stimulus moving back and forth between two locations a Basis for motion we perceive in movies on television and in moving signs used for advertising 2 Induced Motion occurs when motion of one object usually a large one causes a nearby stationary object usually smaller to appear to move a Example when the moon appears to move because clouds surrounding it are moving at a fast pace on a windy night 3 Motion Aftereffects occur after viewing a moving stimulus for 30 60 seconds and then viewing a stationary stimulus which appears to move b Comparing real and apparent motion a Waterfall Illusion different mechanisms ii Larsen and coworkers 2006 i For many years treated as separate phenomena governed by 1 Presented three types of displays to person in an fMRI scanner control condition real motion display apparent motion display 2 Found similarities between the perception of real and apparent motion and between the brain mechanisms associated with the two types of motion 3 Researchers now study both types of motion together c Motion perception cannot simply be explained by considering what is happening on the retina because when the eyes move along with an object the object will remain stationary on the retina III Motion Perception Information in the Environment a J J Gibson i Looked for information in the environment that provides information for perception ignoring the retina altogether ii Optic Array information in the environment the structure created by the surfaces textures and contours of the environment 1 Movement of the observer causes changes in the optic array iii Local Disturbance in the Optic Array occurs when one object moves relative to the environment covering and uncovering the stationary background as Jeremy does when Maria watches him walk across a room 1 This provides information that the object is moving relative to the environment 2 Local disturbance information is available even when an object remains stationary on the retina iv Global Optic Flow when everything moves at once signaling that you are moving but that the environment is stationary as in the case where Maria moves through the room and the objects in the room remain in place v Thus the ecological approach provides a nice solution the problem that we can t explain how we perceive movement in some situations based just on what is happening on the retina IV Neural Firing to Motion Across the Retina a Whereas the ecological approach focuses on environmental information the physiological approach to motion perception focuses on determining the connection between neural firing and motion perception b The Aperture Problem Motion of a Stimulus Across the Retina i Aperture Problem the fact that viewing only a small portion of a larger stimulus can result in misleading information about the direction in which the stimulus is moving ii Visual system seems to solve this problem by pooling the responses of a number of neurons c Motion of arrays of dots on the retina i Newsome and coworkers 1989 iii Medial Temporal MT Cortex 1 Nucleus in the dorsal where or action stream 2 Evidence that MT may be involved in pooling responses from number of neurons 3 Pack and Born 2001 a Determined how neurons in the monkey s MT cortex responded to moving oriented lines b MT s initial response to stimulus determined by orientation c Subsequently neurons began responding to actual direction in which the bars were moving iv Thus visual system can solve this problem by 1 using information from neurons in the MT cortex that pool the responses of a number of directionally selective neurons and 2 by using information from neurons in the striate cortex that respond to the movement of the ends of objects 1 Used computer to create moving dot displays in which direction of motion can be varied 2 Coherence term indicating the degree to which the dots move in the same direction a 0 when dots all moving in separate directions think snowflakes b 50 when at any given point in time half the dots are moving in the same direction c 100 when all dots are moving in the same direction 3 Determined relationship between monkey s ability to judge the direction in which dots were moving and response of neurons in the monkey s MT cortex 4 As coherence increased neurons fired more rapidly in MT 5 Directly measured the relationship between physiology and perception PH2 ii The role of the MT also studied by determining how the perception of motion is affected by 1 lesioning destroying some or all of the MT cortex 2 electrically stimulating neurons in the MT cortex 1 When an MT is lesioned the coherence must be at 10 20 versus when an MT is intact and coherence can be as low as 1 2 2 Evidence of the MT s link to perceiving the direction of neurons iii Microstimulation achieved by lowering a small wire electrode into the cortex and passing a weak electrical charge through the tip of the electrode this weak shock stimulates neurons that are near the electrode tip and causes them to fire just as they would if they were being stimulated by neurotransmitter released from other neurons 1 More evidence linking MT neurons and motion perception V Taking Eye Motions Into Account The Corollary Discharge a Corollary Discharge Theory proposes that the visual system