HUL 261 Constancy and Plasticity of Perception

Constancy and plasticity of Perception

Snehlata Jaswal

HUL 261

Constancy of PerceptionA unique quality of human perception is constancy the fact that

we perceive objects, events, and patterns, in the same way, despite small changes. In contrast, computers are extremely bad at recognizing objects and patterns that differ in small ways.

Usually, constancy is an adaptive mechanism. However, at times

HUL 261

Usually, constancy is an adaptive mechanism. However, at times it can create problems as well. One of the major reasons that we experience illusions is constancy of perception.

Perceptual constancy is shown for all kinds of percepts: form, depth, movement, size, colour, etc.

Perhaps it is best illustrated and studied in percepts which are relatively more changeable, or can acquire many values; such as size, colour, and brightness.

Constancy of sizeThe size of an object is experienced either through touch

or through vision.

Touch is a very rudimentary way of assessing size, for we are limited by our bodies.

In vision, size is assessed through the image on the

HUL 261

In vision, size is assessed through the image on the retina. Yet, despite continuous changes in the size of the retinal images, we do not perceive objects as changing in size. Mostly we perceive them to be the same size.

In other words, size constancy implies perceiving objects according to assumed or familiar size, not according to retinal size.

Holway and Boring (1941)Studied two crucial factors affecting size constancy: Distance Other referents

HUL 261

Holway and Boring (1941)

Observer sat at the intersection of two corridors, and equated the diameter of a bright disk in one corridor to the diameter of a standard disk in the second corridor.

4 experimental conditions:

HUL 261

Binocular vision

Monocular vision

Monocular through a pinhole in paper (artificial pupil)

Monocular through a pinhole and tunnel (long black tube)

Holway and Boring (1941)Results:

HUL 261

Gibsons theory about size constancyGibson (1950) held that perceived-size constancy can be explained by the unvarying size ratio of objects in a scene. E.g., if we see a man standing next to a house, his height bears a definite size relation to the height of the house. That relation will not change, no matter from how far away we view the man.

The major reason for rejecting the stimulus-relation explanation is

HUL 261

The major reason for rejecting the stimulus-relation explanation is that constancy can be achieved in instances where stimulus relations are not applicable. E.g., in a dark room with only a single luminous object visible, constancy will hold as long as distance information is available, such as is provided at near distances by, accommodation, convergence, and binocular disparity. Yet if the subject is required to view the object with one eye through a small aperture, thus eliminating cues to distance, constancy fails. Then, the objects size appears indeterminate and two such objects are matched on the basis of visual angle alone (Rock, 1984).

Colour constancyIs there a special colour center in the brain? Cerebral

achromatopsia loss of colour vision due to damage to the cortex, while perception of forms is intact, supports this idea

However, a lot of research directly studying the brain has found no such area and suggests that colour processing is

HUL 261

found no such area and suggests that colour processing is distributed all over the brain

Hence colour processing is probably a computational problem

Yet computers are extremely bad at recognizing objects and patterns of different colours

Colour constancyOur perception of the colourof objects remains virtually the same despite changes in the illumination falling on them.

HUL 261

Essentially colour perception depends on the surround.

Colour constancy works best when an object is surrounded by objects of many different colours (Land, 1983, 1986)

Memory colour

Past knowledge affects colour perception

First emphasized by Hering

Each one of us associates a different colour with red depending on our past experience.

HUL 261

depending on our past experience.

Familiar objects are judged to have richer, more saturated colours (Jin and Shevell, 1996)

Contributes to colour constancy

Light from above heuristic

Kleffner and Ramachandran (1992) hold that observers of a scene assume that illumination is always from the above, in fact from the above left (about 26 degrees).

This strong assumption happens because most light in

HUL 261

This strong assumption happens because most light in our environment comes from above.

It also helps us to maintain colour constancy and ignore shadows

Shadows

HUL 261

Shadows are automatically cancelled by our visual system

Constancy of brightness

In fact, perceptual constancy of achromatic colourscontributes a lot to colour constancy.

HUL 261

Perception of brightness is one of the simplest aspects of vision to study

Characteristics of LightLight as a wave has three pairs of attributes:

(1) wavelengths and hue (2) purity and saturation (3) amplitude/intensity and brightness

HUL 261

Amplitude, the height of the light wave, is related to the brightness of a visual stimulus. Light waves that have greater amplitude are perceived as brighter.

Amplitude, in turn, is related to the intensity of light, the amount of radiant energy contained in the light source.

Light and objectsObjects may be classified as luminous and illuminated. Luminous objects are themselves the source of light such as the sun, the stars, an incandescent bulb, etc. the amount of light energy they give off is known as radiance. Most objects, however only reflect light, and are therefore said to be illuminated.

So, the amount of incident light falling on an object or surface is

HUL 261

So, the amount of incident light falling on an object or surface is called illuminance (irradiance). Further, the intensity of light reflected from an illuminated surface is called luminance (reflectance). E.g. the amount of light emanating from the sun or the bulb in the room is radiance, the amount of light falling on this page is illuminance, and the amount of light reflected from the page is luminance.

All these contribute to the psychological experience of brightness.

What if the illumination is different?

Estimate the brightness (whiteness?) of a patch under different amounts of light

HUL 261

Lightness constancyResult:

Lightness unaffected by illumination lightness constancy Visual system computes lightness as a ratio

= intensity of light from the patch intensity of light from background

HUL 261

Brightness and Lightness

Brightness is the subjective estimate of intensity of light from a light source or from a reflecting surface

One important aspect of brightness is lightness. Lightness is the subjective estimate of the whiteness of a surface.

HUL 261

whiteness of a surface.

Changes in the brightness of adjacent areas in the visual field is an important factor in the formation of contours and is thus also the basis of perception of form and image formation.

Simultaneous brightness contrast

Two grey squares of equal intensity

HUL 261

Simultaneous brightness contrast

Difference in perceived brightness due to the surround

HUL 261

Simultaneous brightness contrast

Pattern increases difference?

HUL 261

Remember: Color constancy works best when an object is surrounded by objects of many different colours ( Land, 1983, 1986)

Mach band illusion

HUL 261

Cornsweet illusion

HUL 261

Cornsweet illusion gone?

HUL 261

Plasticity of PerceptionPerceptual constancy prevails in our day to day lives. Nevertheless, over a period of time, experience can modify perceptual processing this is called plasticity of perception.

Nativists vs. empiricists debate

Studies on depth perception

HUL 261

Studies on depth perception

Critical periods / Sensitive period

Visual deprivation during critical periods for monkeys results in permanent problems in visual form perception (Reisen 1966)

Congenitally blind people need special training in form perception after their sight is restored (Held et al., 2011).

Plasticity of neuronsPlasticity is manifested not only at the behavioural level, but also at the neural level.

Brain neurons are the only cells in the animal body which do not reproduce themselves. However, they do grow new connections.

HUL 261

Neuro-plasticity occurs in the brain:1 At the beginning of life: When the immature brain organizes itself.2 In case of brain injury: To compensate for lost functions or maximize remaining functions.3 Through adulthood: Whenever something new is learned and memorized

Thank you

HUL 261