Perception Chapter 9: Event Perception Event Perception: an event is defined as a change in both time and space. Thus far we have discussed how our visual system processes very simple static qualities of the environment such as color, distance, pattern, etc. But, the visual system was not designed just to encode and understand these static events, instead, and even more impressively, our visual system was designed to encode and understand the meaning of "events", changes that are dynamic, that visually unfold over time and space. Structure from motion: perceiving three-dimension shape from object movement Biological motion Gunnar Johansson (1975) point-light studies Biological Motion: it appears that the human visual system is keenly sensitive to certain properties of movement which indicate biological movement vs. inanimate movement. What cues to biological motion are present here? self-propelled- regularity predictability - fluidity Certain parts moving in certain ways in relationship to other parts are indicators of the movement of a living organism 200ms exposure; 4month infant preference; Brain area: Visual cortex, MT and STS. Visually guided locomotion Optic flow: changing pattern of visual stimulation associated with movement through a textured environment Focus: least expansion, target of movement; expansion increases further into periphery Retinal image size/expansion At a distance, focus corresponds to target of movement, minimal retinal image expansion As target nears, rate of retinal image size expansion increases rapidly. Visually system uses this expansion rate to calculate time of arrival or collision. Eye movements Types of eye movements: 1) saccadic: these are the jerky, jumping movements made by the eye in order to bring a target into the foveal region for intense detailed encoding. It takes about 200 milliseconds to initiate a saccade, fifth of a second; saccades are ballistic -- cannot be readjusted during movement; take about 100 milliseconds to complete, 10thof a second during a second, the visual world is blurry If the visual world is blurry during a saccade, and thousands of saccades are made in a typical hour, why do we not constantly see a blurry world? Visual Masking: the ability to see one stimulus is reduced by the presence of another stronger stimulus presented temporally close to the original. Visual masking • Backward masking: the masking stimulus is presented after the original • Forward masking: the masking stimulus is presented before the original. • In laboratory studies (which more often use backward masking) the interval in which masking takes place is around 100 milliseconds, just about exactly the time of the typical saccade. Since the blurry visual scene is sandwich between two more stable images, it is not surprising that it is masked and never noticed. Experiments have shown that if you turn on lights only during saccade, blurry world will be seen. Eye movements and perceived motion • Corollary discharge hypothesis: Eye movements signals automatically cause a second signal to be sent to visual system to discount scene movement in opposite direction of eye movement. Eye movements: 2nd type 2) Tracking movements: (or pursuit or smooth movements) these are movements made to maintain foveation on a target as it moves. Tracking movements require two types of information for successful execution – sensory information about speed and direction of movement. Successful up to 1/3 of millimeter per second across retina. Faster require predictive saccadic movements. Motion sensitive cells Probably in PVC Response strongest to equally intense visual stimulus that moves at certain rate across visual scene. Delay line in earlier stimulated region specifies speed of movement to which cell response most strongly. In diagram on right, delay line would be leftmost connection from white-coded cell to “on” motion detecting cell. Apparent motion: perceiving movement from static images Note that motion sensitive cells are based on timing, no actual movement. Thus they can be fooled. Role of MT lobe in movement perception Possess motion sensitive cells tuned to optic flow patterns. As little as 3-5% correlated movement in random-dot stimuli will result in motion perception. Damage to MT requires 10x greater correlation Motion After Effect Selective adaptation or fatigue of motion sensitive cells can produce illusion of motion in opposite direction; waterfall effect Lesson: movement is always a aggregate response across different motion sensitive cells or systems.