Vision V Overview of Topics Chapter 8 in Goldstein Perceiving Movement (chp. 9 in 7th ed.) • Movement is tied up with all other aspects of vision (colour, depth, shape perception...) • Differentiating self-motion from other- motion. • Physiology of motion perception • Higher order motion processing 1 2 1 2 Motion is Everything Motion Makes Colour • J.J. Gibson criticized standard perception research & proposed the “ecological approach” instead. • Gibson: Motion is tied up with all other aspects of vision. Studying reductionistic static stimuli is the wrong way to go. • Example: We don’t stand still while looking at objects but perception researchers still use static images of objects in their research. 3 4 3 4 Motion Defines Depth Motion Defines Shape (and 3D Shape) 5 6 5 6 Five Ways to Perceive Movement Motion is Always There • Real movement • Apparent movement (Even When it Isn’t) • Induced movement (a.k.a. relative movement) • Movement aftereffect • Movement illusions in static stimuli 7 8 7 8 Real Movement Apparent Movement (or is it?) 9 10 9 10 Induced Motion Movement Aftereffects • Movement aftereffect • Observer looks at movement of object for 30 to 60 sec • Then observer looks at a stationary object • Movement appears to occur in the opposite direction from the original movement • Waterfall illusion is an example of this 11 12 11 12 Motion Aftereffects Motion Illusions (Waterfall Illusion) in Static Stimuli 13 14 13 14 Functions of Movement Questions Perception • Survival in the environment What are five ways to perceive motion? • Predators use movement of prey as a primary • means to location in hunting What other aspects of vision does motion • • Prey must be able to gauge movement of predators play a role in? to avoid them. • What phenomena does the “lilac chaser” • Certain kinds of movement trigger reflexive actions stimulus demonstrate? • Navigation: Flow-fields guide one’s sense of movement through the environment. 15 16 15 16 Functions of Movement Motion Agnosia Perception • Motion aids in perceiving objects • Damage to the cortex resulting in inability to perceive movement • Movement of objects or the observer’s movement through objects assists in organization of stimuli • Extremely debilitating and dangerous for the patient • Example: Motion parallax and occlusion-in-motion give cues to depth of different parts of an object, • Does the patient see “choppy” motion, or do they revealing its structure lose the irreducible sense of movement? 17 18 17 18 Three Situations that Lead to Movement Perception Differentiating Self-motion • An object moves, and the observer is stationary: Movement creates an image that moves on the observer’s retina from Other-motion • An object moves, and the observer follows the object with his or her eyes: Movement is tracked so that the image is stationary on the retina • An observer moves through a stationary environment: Image of environment moves across retina but environment is perceived as stationary • What mechanism explains all three situations? 19 20 19 20 Two Approaches to Studying Motion • The question of how we separate self- motion from other motion has been approached from two different perspectives: • Behavioural Approach / Ecological Optics (Gibson) • Physiological Approach 21 22 21 22 Two Explanations of Behavioural Approach to Movement Perception Movement Perception • Behavioral approach/Ecological optics (Gibson) • Local disturbance in the optic array • Information is directly available in the • Object moves relative to background such environment for perception (why represent that it is covered and uncovered and process?) • Global optic flow • Optic array - structure created by surfaces, • Overall movement of optic array indicates textures, and contours, which change as the that observer is moving and not the observer moves through the environment environment 23 24 23 24 • Maria (the observer) is still, so no global • Maria moves her eyes (following George) optic flow. producing a global optic flow. • But George is moving, producing a local • George is moving, producing a local disturbance in the optic array. disturbance in the optic array (relative) 25 26 25 26 • Objects are not moving, producing no local disturbances in the optic array. Questions • Maria moves her eyes (walking down the hall) producing a global optic flow. • Define optic array • Define global optic flow • Define local disturbance of the optic array • Which of the above is associated with self- motion? Which with other-motion? 27 28 27 28 Corollary Discharge Theory • Corollary discharge theory - movement perception depends on three signals • Motor signal (MS) - signal sent to eyes to move eye muscles • Corollary discharge signal (CDS) - copy of the motor signal The corollary discharge model. The motor area sends the motor signal Image movement signal (IMS) - movement of (MS) to move the eyes to the eye muscles and sends the corollary • discharge signal (CDS) to a structure called the comparator. Movement image stimulating receptors across the retina of a stimulus across the retina generates an image movement signal (IMS), which also goes to the comparator. The comparator sends its output to the visual cortex. 29 30 29 30 Corollary Discharge Theory • Movement is perceived based on a summation of: • The Corollary discharge signal • The Image movement signal How inputs to the comparator (circle) affect movement • Movement is not perceived when comparator perception. When the comparator receives either the corollary receives equal input from both corollary discharge discharge signal (CDS) alone, as in (a), or the image movement and image movement signals at the same time signal (IMS) alone, as in (b), it sends a movement signal to the cortex, and movement is perceived. When the comparator receives both the CDS and IMS signals, a movement signal is not sent to the cortex, and no movement is perceived. 31 32 31 32 Physiological Evidence for Corollary Discharge Theory • Damage to the medial superior temporal area in humans leads to perception of movement of stationary environment with movement of eyes In all four examples shown in the figure, a signal is • “Real-movement” neurones found in sent to the eye muscles, and a corollary discharge monkeys that respond only when a stimulus is generated. However, no image movement signal moves and do not respond when eyes move is generated, so other-movement is perceived. (note: V1 neurones respond to both self and other motion) 33 34 33 34 Real-Motion Neurones in Monkey Cortex Questions (a) RM neurone fires when bar moves left across the neurone’s receptive field (RF) as the monkey looks at • What is the corollary discharge signal? the fixation point (FP) • What is an image motion signal? (b) neurone does not fire • If the CDS indicates leftward self-motion when monkey moves its eye and the IMS indicates rightward motion on to the right, even though this the retina, what will be perceived in terms of also causes the bar to move motion of self and other? to the left across the receptive field. 35 36 35 36 The Aperture Problem Physiological Basis of • As we saw in Chapter 3, complex cortical cells respond to an oriented bar moving in a specific Motion Perception direction. Is this the basis of motion perception? • Problem: observation of small portion of larger stimulus leads to misleading/incomplete information about direction of movement • Thus, activity of a single complex cell (which intercepts only a small portion of the image) does not provide accurate information about direction of movement 37 38 37 38 39 40 39 40 An illustration of how several V1 The Aperture Problem complex cells can solve the aperture problem by pooling • Solution to aperture problem their responses • Responses of a number of V1 complex cells with different direction sensitivities are pooled • This may occur in the medial temporal (MT) cortex, which is part of the where/how stream • Evidence for this has been found in the MT cortex of monkeys 41 42 41 42 From Chapter 4: The m pathway feeds into the where/how M vs. P Pathway... pathway via motion-processing areas V3 and MT 43 44 43 44 Is MT a Motion Coherent Motion Processor? Stimuli • Evidence that MT is a higher-order motion processing area (compared to V1) comes from coherence experiments by Newsome et al. • Coherence of movement of dot patterns 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 75% Coherence 10% Coherence 45 46 45 46 Is MT a Motion Is MT a Motion Processor? Processor? • Microstimulation experiment by Movshon and Newsome • Lesioning experiment by Newsome and Paré • Monkey trained to indicate direction of fields of moving • Normal monkeys can detect motion with dots coherence of 1 or 2% (i.e., motion • Neurons in MT cortex that respond to specific direction coherence threshold) were activated by this process Monkeys with lesions in MT cortex cannot • Experimenter used microstimulation to activate different • direction sensitive neurones detect motion until the coherence is 10 to 20% • Monkey shifted judgment to the artificially stimulated direction 47 48 47 48 Questions Higher-Level Motion • What is the aperture problem? How is it solved by the visual system? Processing • MT ultimately gets most of its input from the _____ layers of the LGN. • Describe some evidence that MT is a motion-processing centre. www.biomotionlab.ca/Demos/BMLwalker.html 49 50 49 50 STS: A Higher-Order Apparent Motion: The Motion Processor? Occlusion Heuristic • Exp by Ramachandran and Anstis • Neurological studies show biological motion is processed in the superior temporal sulcus (STS) • Question: How does the visual system interpret ambiguous apparent motion situations? Biological motion movement of person or other living • Answer 1: In the most ecologically plausible way.