W6: Perceptual Constancies

Perceptual Constancies

  • Definition: Perceptual constancy describes our perception that object properties remain constant even when other conditions of stimulation are changed

    • Types include:

      • Lightness Constancy

      • Shape Constancy

      • Size Constancy

      • Colour Constancy

Lightness Constancy

  • Definition: The perceived lightness of a surface remains constant regardless of differing light conditions illuminating it, provided the object is not self-luminous.

    • Depends on the percentage of light reflected by the surface rather than the absolute amount.

Light Constancy

Models of Lightness Constancy

  • Lower-Level Models:

    • Based on physiological mechanisms, such as lateral inhibition, which explains certain lightness illusions like the Hermann grid and Mach bands.

    • these illusions show that lightness perception is not vertical. Lateral inhibition is invoked to help explain these illusions.

    • Lateral inhibition serves to emphasise edges.


Strengths of Herman Grid Illusion:

  • number of intersections

  • regularity in the pattern of the intersections.

Mach Bands

Mach Band:

  • serves to emphasise edges.

  • works similarly to Herman grid with lateral inhibition.

Higher-Level Models:

  • Emphasise the influence of distance and surface perception, and the perception of spatial layout.

  • Cues can be provided by: outline contours, binocular cues, perceived layout of illumination sources, characteristics of shadows, perception of lightness

  • Mach Card - strongest when one eye is shut because depth is ambiguous in monocular images.

    • instead of looking like a roof, it looks like a valley

    • misinterpretation of light input, due to reduction in stereo cues.

  • Surface curvature: Can influence way we perceive lightness and shading.

Physiological Models

  • Lateral Inhibition:

    • A neural mechanism that enhances contrast by emphasizing edges, which can lead to lightness perception illusions.

  • Lightness Illusions:

    • Examples include the Hermann grid illusion, where intersections appear darker due to lateral inhibition effects.

Perceptual Models

  • Influenced by factors such as:

    • Distance Perception: Changes in viewpoint can alter how we perceive lightness.

    • Cues: Outline contours, binocular depth cues, perceived illumination layout.

    • The Mach card demonstrates the effect of lightness perception under varying visual conditions.

Shape Constancy

  • Definition: The perceived shape of an object remains stable despite changes in viewpoint, facilitating recognition from different angles.

  • Depth Information: Requires adequate depth cues for accurate perception.

  • Allows objects to be tilted and perceived from a different perspective and yet still be recognised.

  • retinal image may change, but if we can judge depth we can properly interpret the shape as staying constant.

Shape Constancy in 2-D vs 3-D Objects

  • 2-D Objects: Generally maintain shape constancy, though biases towards symmetrical shapes can occur.

    • If slants are extreme, recognition may be more challenging (e.g., ellipses appearing as circles).

    • perceived shape is biased towards the retinal projection if there are poor distance cues.

  • 3-D Objects: Recognition can decline for unfamiliar objects, especially during rapid viewpoint changes. Familiar objects tend to be recognized better.

Size Constancy

  • Definition: The perceived size of an object remains consistent despite changes in distance or visual angle.

    • Relates to the concepts of visual angle, physical size, and perceived size.

Different types of ‘sizes’

  • visual angle: retinal image size proportional to visual angle

  • physical size: the actual size of the object in space (measured with a rule)

  • perceived physical size: the visual estimate of the physical size.

Size Judgments and Distance Perception

  • Key Study: Holway and Boring's experiment (1941) explored size perception under varying depth cues.

    • Physical size and distance of test circle from observer varied from near (N) to far (F), but visual angle of test object remained constant (1°)

  • Findings showed judgments become less accurate without depth information.

    • Condition 1 - clear binocular

    • condition 2 - monocular

    • condition 3 - monocular + peephole

    • condition 4 - monocular + peephole + drapes to stop shadow

  • As depth cues were reduced (3 and 4), observers’ judgement of the size of object was based more on visual angle

Emmert’s Law

  • Based on demonstration of after-images:

    • Formula: Sp = k × α × Dp

      • Sp = perceived physical size

      • α = visual angle of afterimage

      • Dp = perceived distance

      • k = contast

    • S = R x D (perceived size = retinal image size x perceived distance)

  • The further away the afterimage appears, the larger it is judged to be.

Depth Perception Cues

Pictorial Depth Cues:

  • Monocular cues like interposition, perspective, compression, and shape that convey depth in static images.

    • Interposition: nearer objects occlude more distance object

    • Perspective: Parallel lines converge in the distance.

    • Compression: texture becomes finer with increasing distance

    • Aerial perspective: contrast is reduced and colours de-saturate with increasing distance

    • Elevation: distance objects are more centred in the visual field (closer to horizon)

    • Lighting and shadows: object closer to a source pass shadows behind themselves.

    • Surface Shading: provides cue to surface shape of the object

      • Bias to assume light from above

    • Specular reflections: provides cue to surface shape of object

    • Size: angular size (smaller objects appear further away), familiar size (can provide accurate distance estimates).

Motion Depth Cues:

  • Changes in visual perspective cue distance through motion parallax and deletion/accretion effects.

    • Motion parallax: nearer objects appear to move faster and to a greater extent than distance objects

      • Nearer objects appear to move faster and to a greater extent than distant objects

      • Objects further than fixation point move in the same direction as subject’s movement

      • Objects nearer than fixation point move against subject’s movement

    • Deletion and Accretion: Occurs when an observer moves in a direction not perpendicular to two objects / surfaces that are at different depths

      • deletion: close objects seem to move and the object at back is covered up

      • accretion: is where the object at the back becomes uncovered.

Binocular Depth Cues:

  • Utilizes retinal disparity and oculomotor cues (convergence and accommodation) to assess depth.

    • Retinal disparity: Slightly difference in location of corresponding points in retinal images in two eyes. Does not require visible object detail.

      • Random dot stereograms

      • magic eye stereograms.

    • Oculomotor cues: convergence and accommodation

      • possibly rely on proprioception

      • not used by everyone

      • only useful at near distances.

    • Equidistant theory: without depth cues, objects are all projected to the same distance, e.g., moon versus sun.

Visual Illusions and Perception

  • Definition: Illusions arise when perceptual interpretation diverges from reality, often due to breakdowns in constancies.

Moon Illusion:

  • Differing perceptions of size. Appears larger on horizon and smaller at its zenith.

    • regardless of its elevation, the distance between the observer and the moon remains constant

    • moon is perceived as growing closer as its elevation increases

  • Theories about the moon illusion: ongoing debate.

    • Misjudgement of distance (apparent distance theory):

      • For moon near the horizon: Moon viewed across terrain which contains depth information may make it look further away

      • For moon at zenith: there are few distance cues

        • Size of moon 3,520 km at 392,000 km

        • Visual angle remains constant regardless of position in sky

      • Emmert’sLaw: S = KxD

        • If the visual angle is the same but the moon appears to be farther away at the horizon than at the zenith it will look larger at the horizon

      • Size-distance paradox: At the horizon the moon looks close

    • Relative size theory (angular size contrast theory): moon appears smaller when it is surrounded by large amount of sky

    • Eyes and Head Tilt theory: zenith moon looks smaller due to tilting of eyes or head upward. Reduced effect if looking through legs, but reduced effect simply if world is upside-down

    • Accommodative micropsia & macropsia: increased accommodation leads to reduced perceived size. Increased accommodation for zenith moon.

  • Factors that may influence moon illusion:

    • Atmospheric effect: looking through atmospheric haze on horizon increases perception of size

    • When the moon is red it looks larger.

Ames Room:

  • People of the same height can appear to be very different sizes.

  • Based upon breakdown of size constancy – resulting from shape of the room:

    • Constructed with sloping walls and floors

  • Two people of same physical size have:

    • different visual angles because they are at different distances

    • but the room is designed so they appear at the same distance thus we perceive them to be different physical sizes.

  • peephole takes away distance cues.

Müller-Lyer Illusion:

  • The central lines of this illusion appear to be of different lengths even though they are the same length and have the same visual angle.

  • Misapplied size-distance scaling theory:

    • Arrows appear either as the inside corner of a room or the outside corner of a building.

    • Lines appear at different distances:

      • Left (outer corner): nearer.

      • Right (inside corner): further away.

    • Using Emmert's Law:

      • Inside corner (right) appears further away, for the same angular size:

      • S (perceived size) = R (retinal size) × D (perceived distance).

      • Perceived size should be greater.

    • Cultural influences on perception of illusion

    • Does not explain dumbbell and 3-D versions of the Muller-Lyer illusion

  • Conflicting cues theory: perception of line length depends on:

    • actual length of the vertical lines and the overall extent of the figure

    • because overall length of right figure is larger due to its outward-orientated fins, vertical line appears longer

    • in lower image, space between dots in (b) appears greater because overall extent of figure is greater than (a)

  • Application of the Muller-Lyer illusion:

    • improving step clearance

    • perception of step height was greater for fin out condition and step clearance were greater when mounted on outdoor steps