Presentation on theme: "Motion Perception. Motion perception has many functions - it plays a role in segregating figure from ground in encoding depth in helping us avoid collisions."— Presentation transcript:
Motion perception has many functions - it plays a role in segregating figure from ground in encoding depth in helping us avoid collisions with objects and to control our locomotion and posture
Motion Perception is Important “She had difficulty, for example, in pouring tea or coffee into a cup because the fluid appeared to be frozen, like a glacier. In addition, she could not stop pouring at the right time since she was unable to perceive the movement in the cup (or a pot) when the fluid rose.... In a room where more than two other people were walking she felt very insecure and unwell, and usually left the room immediately, because "people were suddenly here or there but I have not seen them moving."... She could not cross the street because of her inability to judge the speed of a car, but she could identify the car itself without difficulty. "When I'm looking at the car first, it seems far away. But then, when I want to cross the road, suddenly the car is very near.” (Zihl, von Cramon, and Mai, 1983)
When do we perceive movement 2 general cases 1) - something moves in the world- image moves across our retina (However, movement on the retina can’t be the whole story - when we move our eyes and the world is stationary there is movement on the retina, but we don’t perceive movement) 2) - something move and we track it - doesn’t move across our retina
To perceive motion, the nervous system begins by extracting from the retinal image the spatial displacements of features over time. This defines motion as a spatio-temporal event.
Addams - 1834 - Edinburgh Philosophical Magazine and Journal of Science, 5, 373-374 "Having steadfastly looked for a few seconds at a particular part of the cascade, admiring the confluence and decussation of the currents forming the liquid drapery of waters, and then suddenly directed my eyes to the left, to observe the face of the sombre age-worn rocks immediately contiguous to the water-fall, I saw the rocky surface as if in motion upwards, and with an apparent velocity equal to that of the descending water, which the moment before had prepared my eyes to behold that singular deception."
A spot against a background is exposed briefly and then is followed a short time later by exposure of a second spot a short distance away We see the spot as moving from one spot to another - apparent motion Phi Phenomenon - Apparent Movement
Random dot kinematograms are a motion analogue to the random-dot stereograms - sterograms appear to be a random collection of dots unless viewed properly with both eyes. Similarly, random dot kinematograms appear to be a random array of dots unless they are viewed properly - that is, moving. Random Dot Kinematograms
The Aperture Problem A B Moving a square behind a small, circular aperture creates an an ambiguous perception of movement Although the squares are moving in different directions in A and B, through the aperture the square appears to move horizontally to the right
The aperture problem occurs because each basic direction selective unit “sees” what is happening within its own receptive field. The aperture problem tells us that perceived motion is not determined solely by individual responses to the stimulus velocity generated within isolated, local regions of the field. Instead, similar local measurements of velocity are integrated, producing an overall response. This integration points to some place in the visual system at which the local velocity-related signals from Area V1 are brought together. One area that may be involved in solving the aperture problem is the mediotemporal area (Area MT) or V5.
Area MT may help in ‘solving’ the aperture problem
Receptive fields in area MT are considerably larger in size than those in Area V1, implying that an array of spatially distributed V1 neurons contribute to the synthesis of individual MT receptive fields. Also, in Area V1 only a minority of neurons exhibit direction- selectivity, but in Area MT nearly every neuron is directionally- selective. Some neurons in MT receive input from V1 neurons with different preferred directions of motion. The result? The directional-selectivity of these MT neurons differs qualitatively from that of their predecessors in V1. And these differences enhance the importance of MT’s contribution to the perception of motion.
When two gratings that are moving in different directions are superimposed, the resulting plaid pattern appears to be moving as a whole and in a direction different from the movement of each grating when seen alone
Response of component (V1) cells and pattern (MT) cells to a plaid. Both cells respond to ‘movement to the right’ In A. the component cell fires to horizontal movement of one of the gratings and the pattern cell does not fire In B. the component cell does not fire, but the pattern cell fires to rightward movement of the pattern. A B
Converging lines of evidence implicate area MT in motion perception. For example, damage to MT impairs motion perception. The motion-blind patient D.L. whose symptoms were described earlier in the lecture has brain damage centered in the area where MT is located. We also know that motion perception in normal individuals can be transiently impaired by a brief, strong pulse of magnetic energy applied to the region of the scalp overlying MT.
The role of Area MT in motion perception has been fruitfully studied using one particular class of animation sequences: computer-generated displays consisting of moving dots. Such displays make it possible to control precisely the strength of motion signals present in animation sequences.
MT’s key role in motion perception was most dramatically shown in a study done by Salzman, Murasugi, Britten and Newsome (1992). Used tiny, localized electrical currents to induce a temporary change in the responses of selected MT units. These currents boosted the response of particular directionally-selective neurons and thereby altered perception of visual motion. The study began by establishing the direction preferences of various MT neurons. This purely physiological phase of the study was followed by a phase that was purely behavioral: while they viewed random dot displays, monkeys moved their eyes in one direction or another to signal the perceived direction of those displays. Finally, Salzman and his colleagues combined behavior and physiology, eliciting perceptual judgments (i) while electrical stimulation was being applied to particular MT neurons, or (ii) without accompanying electrical stimulation. Electrical stimulation of neurons that share a direction preference biased the monkeys’ perceptual judgments of random dot displays.
Motion Sensitive Mechanisms Beyond MT Normal motion perception depends upon activity distributed over many areas of the brain, each extracting somewhat different information from the retinal image. To emphasize this point, consider the computations carried out in another region of the cortex, Area MST (Medial Superior Temporal)
Motion Perception and Eye Movements When our eyes track a moving object - there is no movement of the image on the retina but we see movement. Also, what about the case in which the world is stationary but we move our eyes - there is image movement on the retina but the world appears to be stationary.
Over the years, people have proposed two different mechanisms by which eye movements could be monitored. According to one view, the visual system tracks actual changes in the extraocular muscles (Inflow theory); according to an alternative view, the visual system tracks the command signals that go to the extraocular muscles (Outflow Theory).
M = motor signals from brain S = sensory movement of the image on the retina C = corollary discharge (efferent command signal)
2 Predictions: 1) If eye moves but no C, then S will not be cancelled and the scene should appear to move. 2) If there is C but no S to cancel it scene should to move - no sensory input from retina, but movement signal sent from brain.