Ch 9 (notes)

Psyc 317B

Ecological Perception: the study of perception that begins by determining what information is available to an organism in an environment and (typically) moving through it.

Ecological Optics: description of the stimulus information in terms of ambient optic array

the optic array: information as structure

in the ambient light.• A mirrored sphere may be taken as

an analogy of the optic array.• Imagine the changes in the

reflection on a mirrored sphere moving at eye height through the environment– As environment is nested, so

images on sphere are nested– Projective constancies specify

environmental regularities

• But: this metaphor breaks down: there is no actual ‘image’ in the optical array; there is only structure in the ambient light

• thus one must disregard the ‘real’ existence of an image in this mirrored sphere to get an idea of the optic array.

Gibson (1966): Ambiguity of “Sensation”

• Two sense of the verb to sense– To detect something (senses considered as perceptual systems)– To have a sensation (senses as sensory channels)

• Sensations as experienced effects of receptor stimulation or as signals/messages that are interpreted/decoded/transformed/accepted/ organized/processed by higher brain functions (construction of a percept)

• One can detect something without having a sensation• e.g., “obstacle sense”/”facial vision”; arthropods ommatidia; the horizon

– Perception is direct (not mediated by sensation)– But one cannot detect something without stimulation of

receptors (only that stimulation is not data, not the information for perception)

• “the pattern of the excited receptors is of no account; what counts is the external pattern that is temporarily occupied by the excited recipients as the eyes roam over the world, or as the skin moves over an object” (1966, p. 4)

• An observer may occupy distinct positions (1 and 2) and be at different distances from an object (a cylinder).– Size constancy (SC) is the

phenomenon that the object looks the same size even though it casts smaller retinal image from a greater distance

– If an observer knows the distance, the size constancy can be computed (S=RxD). This is the traditional account of SC, making use of retinal image information.

• But there is information already in the environment as well

Retinal Information vs. Environmental Information

Notice that, regardless of position, the base of the cylinder occupies one square. This information consists in a perceptual invariant.

Ambient Optic Array• The ambient optic array is the

structure of stimulus energy available at any point in an environment.– It can be thought of as the pattern

of light reaching a person’s eye, or as the ‘reflection’ in a virtual mirrored sphere.

• The environment can be thought of as an arrangement of objects, surfaces and textures.

• Different parts of the surfaces of objects in the environment are visible from different positions (as sitting or standing)– Solid lines are visible; dashed

lines are not

OPTIC FLOW:

the effect of observer

movement on the Optic

Array

• When the observer moves through an environment (arrangement of surfaces) there are regular changes in the overall structure of the optic array, depending on the nature of the observer motion. This is optic flow.

• The movement of elements of the environment relative to the observer are continually registered in the ambient structure of light, which is information

Gradient of flow

slow

fast

no flow

FoE

Optic Flow as Self Produced Information

• The relationship between movement and flow is reciprocal:– movement causes flow and flow guides movement– flow is created when the observer moves, and on-going self-

produced movement is made possible by the creation of flow.– this reciprocity reflects the interconnectedness of perception and

action, and (more distantly) the animal-niche mutuality

• Depending on your philosophical bias, this “causal loop” is:– self-contradictory; self-evident; cybernetic; ecological; dialectical;

systems-theoretical; dynamic equilibrium; taoist; etc.

Gibson on Perceiving as a Phase of Acting

“Moving from place to place is supposed to be "physical" whereas perceiving is supposed to be "mental". But this dichotomy is misleading. ... we must perceive in order to move and move in order to perceive.” (Gibson, 1979 p. 223) it’s the only way to achieve perception of some invariants

“Perceiving is not a mental act. Neither is it a physical act. It is a psychosomatic act, not of the mind or of the body but of the living observer.” (Gibson, 1979 pp. 239-40)

“Direct perception is a mode of engagement with the world, not a mode of construction of it.” (1979)

Animal-Environment Mutuality“...the words animal and environment are an inseparable pair.

Each term implies the other.” (1979 p. 8)

• The scale at which explanation of perception is couched must be appropriate to the organism (molar not molecular)– The environment is a nested layout of surfaces, a ground, a

sky, an implied horizon, as leaf in tree in a wood in valley…– Affordances too are aspects of this mutuality, are relational

with reference both to the world and the perceiving body

• The animal-organism relationship is reflected in the structure of the optic array.“The optical information to specify the self ... accompanies the

optical information to specify the environment. The two sources of information coexist. The one could not exist without the other... . Self perception and environment perception go together.” (1979 p. 116)

What a Pilot sees during

flyover

• This image depicts the optic flow available to a pilot flying over a runway.– the length of the arrows indicates the speed of flow at different

area; the arrows together indicate overall pattern of flow

• There is greater flow in the foreground; and there is a point in the distance where there is no flow (signifying destination or heading)

Departing Caboose View and Airport Flyover

What a Pilot needs to see

during Landing

• Here the Focus of Expansion is on the runway, and indicates the point where the plane will touch down

• The idea is that ongoing detection of this invariant (FoE) is sufficient information for the pilot to direct the plane to land at that point.– Note: The FoE is not sufficient for safe landing, only for landing. Its use

as guide is consistent with a perfect two-point landing but also with a crash landing.

Angle of Approach

and Texture Gradients

• Texture gradient on the ground provides an additional source of information to a pilot regarding the airplane’s angle of approach– the gradient in (a) indicates steeper approach than gradient in (b)

• A safe landing requires a less steep angle of approach.

Toward and Past the Post • Two questions arise about

Gibson’s new starting point:– Is the information provided by

flow sufficient to determine a person’s heading?

– Do people actually use this information?

• Evidence suggests that flow information is sufficient.– When provided with flow

simulations, people can determine their heading quite accurately.

• Evidence for its use is less compelling: people do use flow to guide walking and driving, but they don’t use only optic flow information

For animations, click here.

Driven round the bend• Lee (1974) notes that the FoE is inadequate for curved roads. • He suggested that we actually use the locomotor flow line, the optic

flow lines that pass from view directly below the driver– the locomotor flow line indicates where the car will go if no change in

steering occurs.• If they are centered on the road going forward, all is well.

– Driver (a) is on course but will need to make adjustments as the curve approaches.

– In (b) the optic flow lines indicate turning, and are lined up with the road even around the corner.

– In (c), flow lines do not line up with road; unless corrective action is taken, the car traveling only those flow lines will leave the road.

Invariants and Direct Perception• Despite changes in sensory stimulation over time, there are

also “higher-order variables” that don’t change, even under movement of observer and intensity of stimulation.

• Perceiving is not based on stimulation of receptors by physical energies, but by the detection (or pick up) of these higher order invariants in ambient light

• Invariants in the energy flux at receptors correspond in lawlike ways to permanent properties of the environment, and are information about that environment (unlike sensations, which specify only intensity, frequency)

• Invariants specify formal properties like size, 3D shape, motion, events and functional significance (meaning)

• Detection of these is a result of organism interacting with environment; it is active (and not just computationally)

What drivers look at• Warren et al. (2001) found evidence

that drivers do use locomotor flow lines, along with other information. – E.g. walkers use landmarks and the visual

direction strategy (keeping their body pointing toward the target)

• Land & Lee (1994) equipped a car to record the steering wheel angle and drivers to record their direction of gaze.– When driving on straight roads, drivers

look close to, but not directly at, the FoE.– When negotiating curves, drivers looked

to the tangent point of the curve.

• Conclusion: Drivers may use optic flow information, but they also use other information.

Blind Walking

• Evidence that people do not use only optical flow information is that we often manage to find our way even when flow information is minimal, as at night or in snowstorms.

• Researchers have shown that people can reach nearby targets even with their eyes closed.

• A target is located up to 12 m away; subjects observed it, then – with covered eyes – walked toward it, often ending within 1 m of target.

• Blindfolded subjects we able to do so quite accurately, even when they were asked to take some steps off to the side before approaching the target (as at right)

• Ongoing visual information is not necessary guide us to nearby targets (that have been seen once)

Notice that overshoot is typical.

Swinging Room• Moving the room forward creates

optic flow pattern (a)• Moving room backward creates optic

flow pattern (b)• Such patterns typically indicated

caused by observer sway in the opposite direction.

• This can lead to disastrous corrective action, notably among children; also to synchronized compensatory swaying among adults

• Constant viewing direction or bearing is an indication of upcoming collision– If the bearing or angle of view

remains constant over time, the boats are on a collision course

• The image below depicts successive images of an approaching square. – Though the square is only

expanding, it appears to be approaching

• The perception of upcoming collision begs the question: when?Collision Course

Tau: a ratio

within a ratio

• It turns out that time to collision may be calculated without having to determine object distance or object or observer velocity.

• Time to Collision=tau=ratio of the object’s retinal size to the rate of change of its retina size over time.

• Tau is the time that will elapse before the observer collides with an object moving at a constant velocity.

• Tau is not available in all circumstances (as when velocity is not constant or collision is with a nearby loved one, rather than with one’s own line of sight)

Gannets use flow

• Lee investigated Hoyle’s “tau ratio” as a detectible feature of optic flow.

• Lee & Reddish (1976) analyzed the diving behaviour of a gannet, a large bird that dives for fish from great heights

• The retraction of its wings just before it enters the water is an indication that it is sensitive to the approaching moment of contact.

• Using the “tau-strategy”, Lee calculated that bird diving from a greater height should begin retracting their wings sooner.– this prediction was born out through

analysis of gannet diving footage

• However, monocular viewing reduces accuracy of human judgments of upcoming time of collision; so for us at least binocularity must be involved too

Figure 9.18, page 314

Getting under that pop fly

How do baseball players manage to run to the exact place where the ball will come down?

What information do they use to guide them over what may be a considerable distance? (Note: tau is not available)

This is like collision course estimation, since the outfielder wants the ball to collide with his/her glove.

One theory is the linear optical trajectory (LOT) strategy, which is to run in the direction that makes the flying ball appear to follow a straight line (successful outfielders ensure the path that the ball takes on their retina is as straight as possible.)

This strategy does not exclude use of other information

Collision-Sensitive Neurons in Pigeons• Wang & Frost (1992) recorded the

response of neurons from the nucleus rotundus, which was known to be involved in perception of space a movement.

• The stimulus expanded in such a way as to appear to be either on a collision course with the pigeon or veering off to its left or right.

• The researchers also manipulated initial size and apparent velocity of the expanding stimulus.

• cells were found that responded about 1 second before apparent collision, regardless of the size or velocity of stimulus.– Heart rate was also measured and found to

almost triple for all/only collision conditions

Somersault

• Professional gymnasts can execute a backward flip within 600 ms.• Is the necessary sequence of movements simply a pre-programmed

routine?• In fact, eye (and ear) input is essential to experts, allowing them to

make in-the-air corrections– This suggests that optic information is used during execution, despite speed.

• Non-experts, interestingly, appear to make less use of it; their performance is proportionately less impaired than experts by closed eyes

Shared World (not private construct)

 ”If a set of observers move around, the same invariants under optical transformations and occlusions will be available to all. To the extent that the invariants are detected, all observes will perceive the same world.” (1979 p. 200)

“It is a mistake to separate the natural from the artificial environment as if there were two environments.... It is a mistake to separate the cultural environment from the natural environment.... There is only one world, however diverse, and all animals live in it.... The fundamentals of the environment ... are the same for all animals.” (1979 p.)

Affordances and Sociality• Even early on, Gibson was trying to bridge “deep schism” between

psychology as natural science and psychology as social science. Costal (1995)

• Affordances are intentional (goal-realizing, coordinate with action) while yet emphasizing the “material conditions” of human action

• “Affordances do not cause behaviour, but constrain behavior” (Gibson )– Use against affordance is possible (may result in tool damage or social

awkwardness)

• “affordance theory address reality of meaning,” beyond meaning a mere convention

• “Only when each child perceives the values [i.e., affordances] of things for others as well as for herself does she begin to be socialized” (Gibson, 1979 p. 141)

• Costal goes farther, towards a “sociality of the material”, suggests all affordances are social

More flow

Monkey Brain

Figure 9.21, page 317

MST Neurons

slant

Neural Map

AIP Neuron Types

Mirror Neurons