Half-Wave Rectification

A signal which follows only the positive part of a sinusoid and has zero-response to the negative part. Many simple cells show half-wave rectification to contrast reversing patterns. (See also Full-Wave Rectification).

Haploscopic Stimulation

Different stimuli for the two eyes.

Hemineglect

A condition following damage to the parietal stream of one hemisphere in which individuals are unaware of stimuli from the hemifield that projects to that hemisphere.

Hermann Grid Illusion

Dark blobs appear at the intersections of black rectangles except when viewed foveally. (More on Optical Illusions)

Hollow Face Illusion

A concave hollow mask of a face viewed from a few feet gives the impression of a normal face. The cognitive interpretation overrides the stereoscopic information.

Horizontal Cells

Cells in the retina connected via gap junctions that mediate lateral information transfer over large distances.

Horopter

For a given fixation point, the set of points on the retina that are perceived to have zero retinal disparity. The points sweep out an arc in space that intersects a fixated point in space.

(See also Binocular Disparityand Vieth-Muller Circle ).

Hue

The perception of color. (See also Saturation).

Hyperacuity

Psychophysical tasks involving judgements of relative position that are about ten times finer than the separation of bars at the grating acuity limit.

Hypercolumn

A set of columns in the primary visual cortex (V1) that contains columns responsive to lines of all orientations from a particular region of space and for both eyes. (See also Ocular Dominance Columns and Orientation Columns).

Hypercomplex Cells

(Hubel & Wiesel 1962, 1968)

A third category of striate cells identified by Hubel and Wiesel that have since been classified as subsets of simple and complex cells. They posses inhibitory zones at one or both ends of oriented excitatory regions, thereby responding to bars of preferred orientation only if they are not too long. For a historical perspective, see Simple Cells. (See also Complex Cells , End-Stopped Cell and End-Inhibition).

Hyperopia

An eye is hyperopic when the far point is at a virtual point behind the eye. Generally the hyperopic eye is too short with respect to the refractive state of the standard eye (ie an emmetropic eye or eye requiring no optical correction) or because the optical power of the eye is too low relative to the length of the standard eye. The focus is correctly adjusted using a "plus" lens power or convex lens. Hyperopia is often referred to as "long-sightedness" or "far-sighted". (See also Myopia and Emmetropia).