preferential looking

A method used to assess visual acuity in pre-verbal infants and young children. It involves presenting two side-by-side stimuli: one with black and white stripes (gratings) of varying spatial frequencies, and the other a homogeneous gray field matched for luminance. An observer judges which stimulus the infant preferentially looks at by monitoring their eye and head movements. The finest grating the infant can resolve provides an estimate of their visual acuity.

primary angle of deviation

In incomitant strabismus, the primary angle of deviation is the angle of misalignment of the non-fixating eye from the point of fixation when the normal, unaffected eye is fixating. It represents the maximum angle of deviation when the affected eye is not fixating.

primary position (of gaze)

The orientation of the eyes when looking straight ahead with the head erect and still. It refers to the position where the visual axes are parallel, aligned with the head, and the eyes are in their most relaxed state with minimal extraocular muscle activity.

prism diopter

A unit that quantifies the deviating power of a prism. 1 prism diopter causes a 1 cm deflection of a light ray at a distance of 1 meter from the prism. Prism diopters differ from lens diopters and are used clinically to measure the angle of ocular misalignment and prescribe prism correction for strabismus.

progressive-addition lens

A  type of multifocal spectacle lens with a gradual change in power from the distance portion at the top to the near portion at the bottom. This smooth transition allows wearers to see clearly at all distances without abrupt image jumps. PALs have an aspheric, progressive surface, but may cause peripheral distortion due to varying power across the lens.

proximal convergence

Proximal convergence is the inward movement of the eyes triggered by the perception of an object’s proximity, independent of accommodation or fusional stimuli. It is a reflexive convergence response elicited by the awareness of a nearby target, even in the absence of blur or diplopia cues. Proximal convergence contributes to the overall convergence response, along with accommodative and fusional components, to maintain binocular single vision at near distances.

pseudostrabismus

Pseudostrabismus is the false appearance of misaligned or crossed eyes when the visual axes are actually properly aligned. It is caused by facial features like a wide nasal bridge, prominent epicanthal folds, or angle kappa abnormalities that create an illusion of inward or outward eye deviation.

psychogenic amblyopia

Also known as functional or non-organic vision loss, psychogenic amblyopia is a decrease in visual acuity or visual field without an identifiable organic cause. It is a diagnosis of exclusion, made after ruling out other ocular or neurological disorders. Patients may present with bilateral vision loss, tubular visual fields, or spiral visual fields. Psychogenic factors such as stress, anxiety, or depression are often associated with this condition. Treatment involves a multidisciplinary approach, including reassurance, psychotherapy, and addressing underlying psychological issues.

psychometric visual acuity

A method of measuring visual acuity that uses psychophysical techniques and forced-choice responses. It involves presenting optotypes like Tumbling E or Landolt C in varying orientations, and the patient indicates the perceived orientation. Psychometric acuity testing reduces crowding and contour interaction effects, providing more reliable measure of acuity for amblyopia.

pursuits

Voluntary, smooth tracking movements that allow the eyes to fixate on a slowly moving target by matching eye velocity to target velocity. They are initiated by visual motion signals to maintain the image of the moving object on the fovea.

Pursuits differs from rapid re-fixating saccades and have an inherent delay before initiation due to visual processing time. At higher speeds, catch-up saccades aid pursuit in maintaining foveation.

random-dot stereogram

A pair of images containing only random patterns of dots or pixels with no recognizable monocular shapes or contours. When the two images are viewed with proper binocular fusion, the visual system processes the slight horizontal shift between the patterns to create a perception of depth and extract any hidden 3D forms due to binocular disparity..

recession

An extraocular muscle surgery where the muscle is detached from its original scleral insertion and reattached further back on the globe to weaken its pulling force. First, an absorbable suture secures the tendon. The muscle is then disinserted and reattached at a measured posterior distance. This reduces the muscle’s field of action and ability to rotate the eye.

resection

An extraocular muscle surgery that tightens an underacting muscle by removing a segment of its distal tendon and belly, then reattaching the remaining stump at the original scleral insertion. This shortens the muscle path, increasing its pulling force. It is often paired with recession of the antagonist muscle for larger misalignments.

retinal rivalry

Also known as binocular rivalry, retinal rivalry is the alternating perception that occurs when the two eyes are simultaneously presented with dissimilar stimuli such as contrasting colors, orientations, or patterns. Instead of fusing the two images into one percept, perception fluctuates irregularly between the two rival images, with one eye’s view being consciously perceived while the other is suppressed from awareness.

saccades

 Abrupt, voluntary shifts in fixation from one target to another, rapidly moving the fovea to a new object of interest. Saccades are very fast, ballistic, conjugate eye movements that alternate with smooth pursuit to scan a visual scene. Their velocity depends on the amplitude of the movement.

saccadic velocity

The speed of saccadic eye movements, typically measured using techniques like electro-oculography that record the electrical potentials corresponding to eye movements. Saccadic velocity follows a predictable relationship with saccade amplitude, peaking around 20-30 degrees. Abnormally slow velocities can indicate muscle paresis, helping differentiate it from mechanical restrictions.

second-degree fusion

The ability to combine portions of two slightly dissimilar images into a single percept by fusing areas with common borders or contours. It represents an advanced level of binocular fusion beyond fusing identical images, occurring over Panum’s fusional area where some horizontal disparity is tolerated. Second-degree fusion allows stable single binocular vision and stereopsis by integrating binocular inputs with minor differences.

secondary angle of deviation

In incomitant strabismus, the secondary angle of deviation is the angle of misalignment of the normally fixating eye from the point of fixation when the affected eye with limited motility is fixating. It is typically smaller than the primary deviation measured when the normal eye fixates.

secondary position

Any position of the eye represented by a vertical or horizontal deviation of the line of sight from the primary position of straight ahead gaze.

sensory fusion

The process by which the brain combines the slightly different images from each eye into a single coherent visual percept, allowing for depth perception and stereopsis. It occurs over Panum’s fusional area, where some horizontal disparity between the retinal images is tolerated by the visual system’s binocular neurons.