Unit 12 – Movement and Action

Which parts of the fovea are seeing the motion? How Slow and How Fast?

motion thresholds are a function of what parts of the retinae are seeing the motion. We have rather poor motion thresholds. There are simply some stimuli that are too fast or too slow for us to make sense of

▪ Real motion

motion in the world created by continual change in the position of an object relative to some frame of reference

▪ Apparent motion

the appearance of real motion from a sequence of still images

▪ The correspondence problem

how the visual system know if an object seen at time 1 is the same object at time 2

▪ How does the visual system correctly perceive the overall motion of objects?

Local information (= 1 aperture = 1 receptive field) is ambiguous Solution

▪ Induced motion

an illusion whereby one moving object may cause another object to look a if it is moving

▪ Motion Detection in the Retina

The retina plays an important role in the early detection of motion

• Some types of amacrine cells in the retina are sensitive to motion. Thus, the beginnings of analyzing motion occur quite early in the human retinae. The beginning of the M pathway, which codes for motion in V1 and along the dorsal stream of vision. The P pathway also contributes to motion perception, as we see moving objects in color.

▪ Reichardt detectors

neural circuits that enable the determination of direction and speed of motion by delaying input from one receptive field, to determine speed,

• to match the input of another receptive field, to determine direction

▪ Corollary Discharge Theory

the theory that the feedback we get from our eye muscles as our eyes track an object is important to the perception of motion

▪ Eye Movements

we track objects that are moving with our eyes. Vergence refers to eye movements that cause our eyes to resolve a particular object or bring it in to focus

o Saccades

Rapid voluntary movements of the eyes

• in an environment without movement

• most common eye movement

o Smooth-pursuit eye movements

eye movements made to track a moving object

• voluntary eye movements  - only when there is an object in motion in your environment

▪ MT

The Movement Area of the Brain

o Akinetopsia (motion blindness)

a rare condition in which an individual is unable to detect motion despite intact visual perception of stationary stimuli, caused by damage to area MT

o Stimuli used in Newsome and Paré’s (1988) experiment

in the 100% condition, all the dots were moving in the same direction. In the 50% condition, half the dots were moving in a consistent direction. And in the 20% condition, only one fifth of the dots were moving in a consistent direction

o Weigelt et al.’s (2013) results

in the fMRI study, they found that activity in MT was associated with attention to movement. In theses fMRI pictures, the activity in MT is highlighted in color. The results are averaged across 15 participants.

o fMRI results from Kaas et al. (2010)

these diagrams illustrate how MT is activated when people imagine perceived motion

▪ Motion Aftereffects

a motion-based visual illusion in which a stationary object is seen as moving in the opposite direction of real or apparent motion just observed (waterfall illusion)

▪ How do we interpret the point-light walker display?

an experiment in which small lights are attached to the body of a person or an animal, which is then filmed moving in an otherwise completely dark environment

• observers were able to detect human form when the person was walking or running and could distinguish the two motions/directions of motion

▪ What is the concept of Affordance?

information in the visual world that specifies how that information can be used

• for example, seeing a piano affords playing music, seeing an elevator affords going up and down in a tall building

▪ Optic flow

the complex motion of points in the visual field caused by relative movement between the observer and the environment

• provides information about the relative distance of objects from the observer and of the relative direction of movement

▪ Gradient of flow

the difference in the perception of the speeds of objects moving past us in an optic flow display

▪ Focus of expansion

the destination point in an optic flow display, from which point perceived motion derives

o lateral intraparietal (LIP) area

an area of the primate parietal cortex involved in the control of eye movements

o An experiment showing the function of the LIP region of the parietal lobe

monkeys are expected to keep their eyes on a central fixation point. The researchers can then look at the receptive field of a particular neuron in the LIP area. A target then appears in the receptive field of that neuron. When the target disappears, the monkey must make a saccade to that location. Activity in the LIP area precedes the saccades.

o medial intraparietal (MIP) area

an area of the posterior parietal lobe involved in the planning and control of reaching movements of the arms

o anterior intraparietal (AIP) area

a region of the posterior parietal lobe thought to be involved in grasping movements