S Cones

Short wavelength sensitive cones (blue). Are most sensitive to a wavelength of approximately 419nm. (See also L Cones and M Cones).

Saturation

The richness of hue. Indicates how much a color has been diluted by grayness. (See also Hue).

Schroder Staircase

(More on Optical Illusions)

Scission Theory

A theory of Lightness perception forwarded by Metelli (1974) in which the visual system . (see also Intrinsic Images)

Scotopic

Dim light conditions where only rods are functional. (See also Photopic and Mesopic).

Second-Order Statistics

Captures differences in granularity and slope.
(See also First-Order Statistics).

Second-Order Motion Mechanisms

Capture motion information from moving properties such as a moving are of flicker in which there is no difference in mean luminance between target and background. (See also First-Order Motion Mechanisms).

Sensitivity

(1) The reciprocal of the minimum stimulus strength required for the stimulus to be detected reliably (1/threshold) May be related to noise and other mechanisms.

(2) Physiologically measured sensitivity averaged over many stimulus/response cycles. A measure of gain. Noise free.

Sensitivity Control

Adjustment of eye sensitivity to compensate for a change in illumination.

Sensory Fusion

The combining of drawing together of two images which fall on different points in the two retinas without changing the vergence of the two eyes. (As opposed to Motor Fusion).

Short-Sightedness

(See Myopia).

Short-Term Visual Store

Visual memory not affected by masking, not in anatomical coordinates, limited in capacity. Less is retained from complex images than simple images.
(See also Iconic Memory).

Silent Substitution

(Estevez & Spekreijse 1982)

If the action spectra of all photoreceptors types are known, spatial or temporal stimulus patterns can be designed which are invisible to some cone types while producing suprathreshold contrast for others. Thus one can study the range of psychophysical performance that is available for solely one type of cone signal.
(See also Null Sets).

Simple Cells

(Hubel & Wiesel 1962, 1968)

One of three cell classifications for cells in the primary visual cortex (V1) proposed by Hubel & Wiesel. Simple cells show rectangular antagonistic on/off zones responding to bars of a particular orientations. They are described as a performing a linear filtering operation followed by a threshold nonlinearity. In a linear system the average response to a drifiting cosine grating will be a cosine at the same frequency. Because of the threshold, response to the negative half-cycles are suppressed.
(See also Complex Cells , Hypercomplex Cells and Half-Wave Rectification).

Historical Note:

A hierarchical model with three cell types, Simple, Complex and Hypercomplex, was proposed by Hubel & Wiesel (1962,1968). Simple cells were thought to form from a convergence of LGN inputs, Complex cells from Simple cell convergence, and Hypercomplex cells from Complex cell convergence. This was later proven to be incorrect as Hypercomplex cells were found to be subsets of Simple and Complex cells. Simple and Complex cells are organized in parallel, not serially, and the orientation tuning of Simple cells is not due to the organization of LGN connections but rather cortical dynamics.

Simultaneous Contrast

An object of moderate reflectance appears relatively brighter or darker according to whether spatially adjacent regions are considerably darker or brighter than the object itself. (More on Optical Illusions)

Singular Value Decomposition

A method for image compression. Defines a linear transformation from a set of image data to a new representation with statistically independent values that range over smaller values. Also know as the Karhunen-Love transform and the Hoteling transform.

Size Constancy

The fact that the perception of the sizes of objects remains remarkably constant at varying distance despite the fact that the image projected on the retina changes.

Size Illusions

(See Table Illusion, Corridor Illusion, Muller-Lyer Illusion and Ebbinghous Illusion)

Snake

(Kass 1987, Terzopoulos 1987)

A deformable closed contour used in image analysis applications. Uses a elastodynamic model with applied forces for edge and curve detection. The energy of a stretchy, flexible contour (a snake) with tension and rigidity is minimized, while the snake itself is attracted to areas of darkness, brightness and/or intensity edges. Can be used to track objects over time as the potential function for the image changes from frame to frame.

Snell's Law

Where is the angle of incidence, ⁱ is the angle of reflectance, and , ⁱ are the refractive indices of the two surfaces.

Spectral Efficiency

The relative luminous efficiency of light as a function of wavelength.

Springer Illusion

A dot pattern produces the perception of faint diagonal lines. (More on Optical Illusions)

Stabilized Retinal Images

Technique for examining perception without eye movement accompanied by loss of color and contour perception. Demonstrates that movement of an image across the retina is vital for perception.
(See also Ganzfeld Phenomenon).

Stereograting

A random-dot stereogram creating the perception of a sinusion in depth. Can be used to obtain a measure of freqeuncy tuning (contrast sensitivity function) of stereopsis. Found to be about 10 times courser than frequency tuning for luminance.

Stereopsis

Perception of depth dependent upon disparity in the images projected on the retinas of the two eyes

Stereoscope

Device for creating a 3-D image of a drawn objects. Works by projects two different images of the objects to the eyes.

Stiles-Crawford Effect

Light entering the eye through the center of the pupil is more effective than light entering at a peripheral point near the edge of the pupil. This is largely to to the directionally sensitive photoreceptors which are oriented towrad the center of the pupil. This helps to minimize the effect of stray light.

Stimulus Equivalence, Problem of

If the stimulus controlling behavior is a pattern of light on the retina than an infinite number of images are equivalent in their effects.

Strabismus

A squint or strabismus is a failure of the two eyes to look at the same object thereby preventing binocular vision. Human binocular vision develops during the first few years of life. Interruption to the motor, sensory or central components, for example nerve or muscle defect, can lead to sensory or central defect. Causes of many squints are not fully understood although the majority are either a hereditary factor or a responsible defect.

Striate Cortex

(See Appendix I: Cortical Areas).

Stroop Effect

When subjects are asked to name the colors of the ink in which the words are printed, their performance is considerably slowed if the words themselves are the names of other colors. (More on Optical Illusions)

Structural Description

A representational format for human pattern and object recognition consisting of a set of propositions about a particular configuration of features.

Subjective Contours

(See Illusory Contours).

Successive Color Contrast

Changes in the appearance of color occur as a result of preceding stimulation.

Successive Contrast Effect

The surface of a given luminance can appear darker or lighter depending on if the preceeding luminance was lower or higher.

Superior Colliculus

A nucleus in the midbrain that coordinate visual, somatic and auditory information adjusting movements of the head and eyes towards a stimulus. contains four types of maps, a visual map, a map of the body surface, a map for sound in space and a motor map. Receives inputs from the retina (via the optic tract), the visual cortex, the somatosensory system and auditory systems. Mediates saccadic eye movements. Is the source of two major descending tracts, the tectospinal tract and the tectopontine tract in addition to regions of the brain stem controlling eye movements.

Superior Temporal Polysensory Area (STP)

(See Appendix I: Cortical Areas).