Article summary with Development of visual-spatial attention by Johnson - 2019

How does the visual system work?

The purpose of the visual system is to transducer light reflected from surfaces in the environment into neural signals that are relayed to the brain for processing and action planning. Light goes through the cornea (the outer protective covering) and the lens (to provide focus of near and far objects), and then falls on the retina. The retina is a thin film of tissue covering the back of the eyeball and is comprised of layers of photoreceptors and a network of connections and nonsensory neurons that provide intial processing of visual information. There are different kinds of photoreceptors that are used for different tasks and there are specialized cells and circuits in the retina for color and contrast.

How does the visual system connect to the brain?

The visual system in the brain consists of a interconnected yet functionally segregated network of areas that are specialized in processing different aspects of visual scenes and visually guided behavior. Visual information is processed as follows:

• Information flows from the retina to the lateral geniculate nucleus and then to the primary cortical visual area. Reciprocal connections carry information to secondary visual areas.
• From these primary and secondary visual areas, visual information diverges to two segregated streams: ventral and dorsal.
• The ventral stream is connected to the temporal cortex and is specialized for object recognition, mostly in inferotemporal cortex. The inferotemporal cortex connects to the perirhinal cortex and other areas that are involved in categorization of visual stimuli and formation of visual memories.
• The dorsal stream codes for spatial information and connects primarily to parietal structures that are important for voluntary action planning and coordinating somatosensory, proprioceptive, and visual inputs.

How does the structure of the visual system develop?

The visual system takes shape early in prenatal development. The retina starts to form around forty days after conception. The growth and organization of cells and connections continues well past birth. The distinction between foveal and extrafoveal regions is present early. The topology and patterning of receptions and neurons continue to change throughout prenatal development and the first year after birth. The muscles used for eye movements develop before birth, as well as the subcortical systems to control these muscles. Later developments consist of the physical growth of neurons and the proliferation and pruning of synapses. As soon as neurons are formed and grow, they begin to connect to other neurons.

What does visual attention in newborns look like?

Infants are born with a functional visual system and they may react to visual stimulation with head and eye movements. The functional visual system of infants has the following characteristics:

• Vision is relatively poor, but acuity, contrast sensitivity, color sensitivity, and sensitivity to direction of motion all improve after birth.
• The field of view is relatively small.
• A lack of stereopsis.
• Certain visual preferences: patterned vs. unpatterned, curvature vs. rectilinear, three-dimensional vs. two-dimensional, moving vs. static.

How does gaze control work?

Gaze control in adults functions with a coordinated system comprised of both subcortical and cortical components. Six muscles are connected to the eyeball that are under control of the brainstem. Eye movements are initiated by the frontal eye field, the cortex, and the superior colliculus. These areas are connected to the brainstem from which the signals to drive eye movements originate.

How do infants develop gaze control?

During visual tracking, several types of eye movements that develop over the first six months are made:

• Reflexive eye movements are responses to sudden onset stimuli. They are supported by visual pathways that include the retina, lateral geniculate nucleus, superior colliculus, and potentially primary visual cortex.
• Voluntary eye movements are quick eye movements from location to location. They are planned attention-driven saccades that involve early cortical visual areas, parietal cortex, and frontal eye fields.
• Smooth pursuit eye movements are slower eye movements that track moving targets. They also require attention and are supported by visual pathways involving the medial temporal lobe, middle superior temporal lobe, and parietal cortex.

The pathways for reflexive eye movements are mature at birth, whereas voluntary eye movements mature rapidly during the first six months together with behavioral changes. Smooth pursuit eye movements continue to improve during the first two years.

What is foveation?

Foveation is the bringing of an image in the environment to the fovea, the center of the visual field and the location on the retina producing the highest acuity inputs to the brain. Foveation balances the need for detailed visual information from the world with the need to reduce the metabolic demands of the brain to process the information. The fovea has the highest concentration of photoreceptors. Acuity is best at the point of gaze and then drops off with increasing visual eccentricity into a low-resolution surround. The visual system compromises by shifting the point of gaze with saccadic eye movements and thus reorienting the specific location in the scene that is best represented and processed.

What are the primary eye movement systems?

There are four primary eye movement systems. Each is produced by a separate neural circuitry, channeled through the brainstem, which innervates the ocular musculature.

• Saccades are quick changes of fixation whose function is to place the retinal image of an object of interest on the fovea.
• Smooth pursuit eye movements maintain foveation on a moving item of interest.
• Optokinetic nystagmus is a semi-reflexive eye movement pattern driven by a large visual array that moves with respect to a stationary observer.
• Vestibo-ocular response is a semi-reflexive eye movement pattern driven by a stationary large visual array placed in front of, or surrounding, a moving observer.

What is scanning?

Scanning consists of sequencing of saccades and fixations. It is a form of observable visual attention.

• During a saccade, the point of gaze for both eyes sweeps rapidly across the scene. Its purpose is to direct attention to a different part of the scene for processing.
• During a fixation, the point of gaze is relatively stationary. During fixation information about the scene is acquired and analyzed.

How do infants develop scanning?

Newborns examine their surroundings with a series of fixations, indicating that some of the neural circuitry for saccade generation is already in place. But their fixations do not extend beyond areas of high contrast (such as edges). After three months infants appear to scan in a more exploratory way. As infants get older they also scan between individual stimuli more readily than younger infants. This development shows a shift from reflexive to more purposive scanning, which is consistent with the maturation of cortical pathways.

What is sticky fixation?

Sticky fixation refers to the phenomenon that one- and two-month-old infants exhibit longer looking times than either neonates or four-month-old infants. It is interpreted as a difficulty of disengaging attention and has been tied to tonic inhibition of the superior colliculus by the substantia nigra and basal ganglia, which is later controlled by cortical mechanisms subserving endogenous attentional control and peripheral expansion of the visual cortex. Attentional disengagement continues to improve throughout childhood.

Why are smooth pursuit eye movements limited in infants?

Very young infants may be incapable of engaging in predictive eye movement (so that the future location of a moving target cannot be computed) or they may be unable to track due to limitations in motion processing. Also, the immaturity of retinal photoreceptors may prohibit firm registration of the target on the fovea.

What is orienting?

Orienting can be described as the engagement of visual attention. Young infants seem to attend primarily to a single prominent feature, whereas older infants are more likely to scan between features and to direct saccades with greater accuracy. This may reflect a transition from reflexive to volitional scanning. Infants also become capable of covert orienting: an internal shift of attention that can facilitate saccades to particular spatial locations. Two related phenomena are inhibition of return and spatial negative priming. Inhibition of return is a delay in eye movement toward a previously cued location. Spatial negative priming is a delay in eye movements toward a separate location presented alongside the fixated one.

What is visual selective attention?

Visual selective attention is the ability to select relevant stimuli for processing and to ignore or inhibit competing alternatives. Efficiently allocating visual attention is important for learning in infancy. A child must attend to certain environmental features while at the same time ignore others to be able to organize the world. This requires selection of relevant stimuli for processing and inhibition of those that are irrelevant. Research into visual selective attention has been done by using visual search paradigms, mobile conjugate reinforcement techniques, and visual search tasks.

How does spatial perception develop?

As our heads and bodies move around in space, the gaze moves frequently from place to place in the visual scene and the world is experienced as a stable place. Evidence suggests that visual stability emerges gradually across the first year:

• Young infants have difficulty discriminating optic flow patterns that simulate different directions of self-motion. Optic flow sensitivity may be improved by self-produced locomotion after six months of age or by maturation of the ventral visual stream.
• Young infants´ saccade patterns tend to be retinocentric, rather than body-centered. Retinocentric saccades are programmed without taking into account previous eye movements, whereas body-centered eye movements are programmed while updating the spatial frame of reference or coordinate system in which the behaviors occur. The visual-spatial coordinate system of the infant which is necessary to support perception of a stable visual world may be insensitive to extra-retinal information in planning eye movements. Also, as the child learns to walk, the field of view changes and they are presented with new kinds of visual experiences.
• There are limits in the ability of infants younger than two months to switch attention flexibly and volitionally to consistently maintain a stable gaze.

How does object perception develop?

Object perception follows a developmental pattern from kind of broken to coherent perception of the visual environment. It extends from birth through the first several months afterward, implying a big shift in the infant´s perceptual experience. One way by which infants come to perceive occlusion is by improvements of selective attention. Other advances in object perception come from coordinated visual attention and manual exploration to help infants understand objects as solid in three dimensional space.

What is scene perception?

Visual scenes are complex and characterized by a number of objects at different distances, often overlapping one another. Two scene characteristics are important: perceptual salience (low-level features such as edges, color, and contrast) and semantic relevance. In adults, the role of salience in scene perception is limited and eye movements are more driven by meaning.

How does scene perception develop?

Development of visual-spatial attention in infancy is seen as a shift from relying on exogenous features (perceptual salience) to more endogenous (knowledge), but recent research shows mixed results with regards to the role of salience. Detecting semantically relevant information is partly a function of emerging selective attention skills and age-related effects of perceptual salience on visual-spatial attention may actually be an increase in attention toward meaningful objects. All together the results suggest that development of scene perception is characterized by a growing tendency to look at semantically meaningful regions in scenes, and that this process is gated partially by the emergence of gaze control.