The main foundations of the various perceptual capacities are built on peripheral (sense organs) and central components of the sensory systems, particularly in the cerebral cortex, which are specialised in the analysis and coding of sensory information, translating the outcome into experience, and making it accessible for use. Figure 2.1 shows a schematic sketch of the brain with the cortical structures relevant for vision, audition, body perception, language, and motor action. The integration of processed information and synthesis of the segregated outputs are essential prerequisites for coherent perception, experience and action (Goldstein 2010; Yantis 2013). A great deal of information about the environment is of a visual nature, i.e. the world within our brains is a predominantly visual one. The prominent role of the visual modality is, however, not only limited to perception. Major elements of experiences, memories and conceptions concerning the external world are based on vision, as are language (e.g. object labels), emotional experiences, and gross (e.g. walking) and fine motor actions (e.g. hand and finger movements). Finally, human curiosity, particularly in more distant space, i.e. beyond the space of grasp and touch, is mainly directed toward visual aspects of the surroundings. Children with cerebral visual disorders are therefore confronted with the very special condition of coping with a world that they can encompass only in part and with great effort, often without access to detailed prior visual experience and knowledge.

According to the level of complexity, perceptual capacities can be classified as “lower” and “higher” visual capacities; another principle of classification refers to the type of visual information and material, which is processed. “Lower” or elementary visual capacities comprise the visual field, visual light and dark adaptation, visual acuity and spatial contrast sensitivity, colour vision, and spatial vision (localisation, distance and direction perception, monocular and binocular depth perception, including stereopsis). “Higher” or more complex visual capacities include visual identification and recognition, topological and geographical orientation, and text and number processing. Moment-to-moment visual mapping of the surrounding environment provides both the substrate for visual search and the principal means of bringing about guidance of movement through our surroundings (Goodale 2011). The various visual capacities are built on the availability and interplay of several visual functions, with varying degrees of involvement of cognition, which make them visual-cognitive abilities, but nevertheless represent genuine visual functions at different levels of organisation.

The visual system can be characterised as a sequence of structures and functions, which transform physical light stimuli into neuronal signals and process them at successively higher levels of organisation. At the most basic level, the visual system operates as an optical measuring system, comprising components that focus optical stimuli as images onto the retina of the left and right eyes with the help of the adaptation system, for mapping the image on corresponding retinal locations so that the two monocular images meld into one image with the help of the vergence system. Accommodation and vergence are coupled: in far vision, both lines of sight are in parallel, i.e. vergence is close to zero, while in near vision vergence increases as well as accommodation. The two retinal images are finally fused in the visual cortex to a coherent percept; the small disparity between the two retinal images is in addition used for finely graduated stereoscopic acuity and stereopsis.

The mobility of the eyes allows a more accurate inspection of objects and their details, respectively, by precise and accurate fixation. Mechanisms in the brain stem, the mid brain and the thalamus and in the cortex are involved in the guidance and control of eye movements, so that we can fixate stationary and moving visual stimuli (Fig. 2.2). Disorders of eye movements can therefore impair visual information processing and thus visual perception, either because oculomotor scanning and fixation shifts in space are impaired or because precise fixation of stimuli for further fine analysis becomes difficult if not impossible. The visual system and the oculomotor systems are intimately connected with each other, and have to co-operate closely to enable fast and reliable visual information processing.

Cognition, in particular, attention and executive functions, and motivation are also involved in visual perception and contribute to the reliability and efficiency of the visual system (see Chap. 3). The close temporal and spatial interplay of the visual system with cognitive, and eye and head movement systems, guarantee high speed, reliable visual information processing and coding even in complex visual task conditions in daily living activities. Table 2.1 shows essential prerequisites for the reliable representation of the visual world in the brain.

In summary, the various functions and capacities of visual perception can hardly be reliably separated from each other, either because they are based on each other (e.g. visual acuity on accommodation; detection and spatial localisation of stimuli on visual field extent; visual identification and recognition, on colour, form, size and shape processing), or they depend on each other (e.g. fixation of stimuli and spatial localisation; visual acuity and fixation accuracy, and vice versa). Any classification of visual functions and capacities is, therefore somehow artificial, but is helpful to denote and define the particular functions and capacities described, and to better explain and understand the interplay between them. Table 2.2 contains a compilation of the visual and oculomotor functions and capacities and their respective functional significance. “Function” refers to basic components of vision. They subserve but do not represent genuine visual-perceptual capacities. Such functions include, for example, oculomotor components, visual adaptation, and contrast sensitivity. “Visual capacities” refer to components of visual perception that support more complex visual abilities, (e.g. visual acuity for form vision, form vision for object and face perception).

The neurobiological foundation of visual perception is mainly in the retina and geniculo-striate pathway (optic tract, optic radiation), and the striate cortex, which is located in the posterior brain (occipital cortex). The visual cortex consists of the primary visual cortical area (striate cortex, Brodmann area [BA] 17, V1) and the so-called extrastriate or visual association cortex, which consists of more than 30 areas that differ with respect to the processing and coding properties of their neurons, and their connections with other visual and non-visual areas. These ‘higher-order’ visual areas process and code the various dimensions of visual information, e.g. brightness, contour, orientation, form, colour, motion, spatial attributes (position, distance, direction, etc.), and complex visual stimuli, e.g. objects, faces, and places and routes (landmarks) (Goldstein 2010; Yantis 2013; Zihl 2014). The central visual system – like other central sensory systems – shows parallel organisation with hierarchical components. This organisation principle is known as “functional segregation” or “functional specialisation” (e.g. Zeki 1993; Cowey 1994; Rainer and Logothetis 2003; Grill-Spector and Malach 2004), which increases over the course of development (Dobkins 2009). This model implies parallel and serial processing and coding in terms of division of labour, i.e. each area is more or less specialised for the processing and coding of a particular visual stimulus dimension; the aggregated organisation is based on parallel and serial processing modes. The various visual areas are reciprocally interconnected; exchange of information and thus interaction is possible in bottom-up (stimulus-driven) and top-down (intention-driven) directions. Within the extrastriate visual cortex, prominent processing pathways have been identified, which are either specialised in processing and coding spatial information (the occipital-parietal or WHERE-pathway) or object properties (the ventral, occipito-temporal, or WHAT pathway) (Desimone and Ungerleider 1989). This two-route model of visual spatial and visual object information processing has been converted into a two-route model of visual action (dorsal route) and visual perception (ventral route) (Goodale and Westwood 2004; Milner and Goodale 2006, 2008; Goodale and Milner 2010). Although there is some neuropsychological evidence for dissociation of either visual- spatial/visual object processing and visual-action/visual-perceptual deficits after injury to the ventral and dorsal route, respectively, both models may be too simplistic to explain visual perception as a coherent, integrated outcome of processing and coding (Schenk 2006; Schenk and McIntosh 2010; De Haan and Cowey 2011). In particular, the interplay between visual areas and both global and local integration of the various pieces of information into a coherent percept as a whole, and the various parallel and serial convergent and divergent processes within and between the two routes, which allow the processing of a large amount of information within a given time window and in the actual context of perception, intention and action, cannot be fully explained by either model. Co-operation and interaction between the two streams and other components of a distributed network involving also posterior and inferior parietal and prefrontal structures guarantee spatial and temporal coherence of processing of visual information via attention and working memory, also subserving visually guided action and spatial navigation (Goodale and Westwood 2004; Konen and Kastner 2008; Singh-Curry and Husain 2009; Kravits et al. 2011). The dual route model of cortical visual processing in terms of WHERE (dorsal route) and WHAT (ventral route) is still used in the literature and there is evidence, that in the context of focal brain pathology affecting these brain areas this model offers some sense and utility (e.g. Dutton and Jacobson 2001; Dutton 2009; Klaver et al. 2011). For the sake of simplicity, we too refer to this model in the context of describing behavioural manifestations of damage to the visual brain without implying that it exhaustively explains visual perceptual disorders.

Interestingly, early clinical and pathological anatomy observations are in support of the functional specialisation model of the visual brain because of selective loss of single visual capacities, for example, colour vision, visual space perception, and visual recognition (Zeki 1993; Kanwisher 2010; Barton 2011; Zihl 2011). In addition, the visual brain is able to identify and recognise visual stimuli despite changes in their appearance, and thus in the images produced on the retina, concerning, for example, brightness, colour, shape, orientation, location, and perspective of the observer. This capacity is called visual perceptual constancy, and is part of visual concept learning, which requires perceptual organisation and is based on bottom-up and top-down processes (Quinn and Bhatt 2001, 2009). It plays an important role in the adjustment of the visual system to the perception of an object or object property as constant even though our sensation changes (Goldstein 2010; Yantis 2013), and depends on occipito-temporal (ventral) structures (Turnbull et al. 1997; Goldstein 2010; Yantis 2013). For the sake of simplicity, we refer also to this model without implying that it too exhaustively explains visual perception.

Visual information is transmitted from the retina via the optic nerves, lateral geniculate bodies, optic tracts and optic radiations to the primary visual cortices (V1), from where it is distributed to areas in the visual association cortices (extrastriate visual areas) for further processing (Fig. 2.3). As already mentioned, these visual areas are specialised for the processing and coding of visual stimulus dimensions, for example, colour, form and shape, motion, and spatial aspects (location, distance, direction, and depth). Brain imaging studies in healthy subjects have confirmed the model of functional specialisation of the visual cortex, and have also added additional empirical evidence for the dual-route processing model, showing that processing of spatial information is associated with more dorsal activation, while processing of object properties, objects and faces, etc. is associated with more ventral activation (Corbetta et al. 1991, 1993; Tootell et al. 1996; Kravits et al. 2011; Sewards 2011). Disturbance of one of these areas causes selective loss of the respective visual capacity; thereby visual spatial impairments are typically associated with dorsal injury, while impairments in the perception of colour, shape, objects and faces are associated with ventral injury (Kanwisher 2010; Zihl 2011; see also Sect. 4.​3).

The striate cortex, and extrastriate visual areas are topographically organised, i.e. they possess a representation of the visual field. This topography is very precise in V1, where the central 15° of visual field cover around 37 % of the striate cortex (Wong and Sharpe 1999), and is characterised by high topographical precision, such that a definite retinal position (central visual field) or area (peripheral visual field) corresponds to a definite cortical position or area in V1, that correlates with the diameter of the receptive fields of neurons (Daniel and Whitteridge 1961; Wandell et al. 2007; Benson et al. 2012). The fovea comprises 0.5° on either side of the centre of the visual field (total diameter: 1°), while the macula extends to a radius of 4.5° (total diameter: 9°). There is still no accepted definition on the diameter of the fovea, which according to various authors varies between 0.8 and 2° of visual angle. There is convincing evidence that the central 0.5° of the visual field is represented in both striate cortices; this explains foveal sparing even after total destruction of one striate cortex (Huber 1962; Bunt et al. 1977; Sharpe et al. 1979; Horton and Hoyd 1991). The topographical representations of the visual field in extrastriate visual areas become more and more imprecise, i.e. receptive fields become larger and larger, and eventually cover one quadrant or one hemifield, and may even extend into the other visual hemifield, whereby the magnification factor decreases with eccentricity but remains constant in extrastriate visual areas when related to striate cortex conditions (Dumoulin and Wandell 2008; Harvey and Dumoulin 2011). Top-down (executive, attentional, motivational and emotional) influences modulate selection of particular inputs to visual cortical neurons and enable them to “assume different functional states according to the task being executed”, in an adaptive way by means of feedback pathways (Gilbert and Li 2013, p. 350).

To a major extent, vision is a skill that depends on learning and use, just as other skills such as walking or talking. The development of vision is characterised by a more or less regular sequence. Periods of morphological development are paralleled or followed by functional development, mostly influenced and guided by environmental factors, i.e. the disposition and nature of visual stimuli. While the auditory system already shows an advanced functional developmental stage at birth, the visual system primarily commences its functional development after birth. In the first months, a baby shows rapid development in nearly all visual functions and capacities, although visual acuity and visual contrast sensitivity at distance still need months and even years to fully develop (Granrud 1993; Slater 1998a, b; Atkinson 2000; Braddick and Atkinson 2011). Table 2.3 summarises the normal development of visual and oculomotor functions and capacities in the first year.

The development of the visual system also depends on the stage of maturation of the eye, and takes place in serial steps (Banks and Shannon 1993; Candy 2006; Iliescu and Dannemiller 2008). The critical period of development of the eye and particularly the retina occurs between the second and fourth month of pregnancy. The differentiation of the cellular retinal elements starts in the central retina, the fovea; the ganglion cells develop before the receptors. In the sixth month the eye lids open for the first time and the macula, the inner part of the retina (diameter: 10°) begins to differentiate; this process is completed several months after birth. At the time of birth particularly the receptors in the fovea are wide, have not reached their final density, and need visual stimuli for their functional refinement (Tian and Copenhagen 2003). This fact explains the poor visual acuity in this period of early childhood, which is about 10–30 times lower than that of an adult. At the end of the first year the development of the retina is nearly complete; however, smaller morphological changes can still be observed till the end of the fourth year. It is important to note that the development of visual spatial (and temporal) resolution of the visual system is tightly coupled with the development of the oculomotor system, in particular pupil movement, accommodation, and vergence (Schor 1985; Charman and Voisin 1993; Charman 2004).

The brain structures involved in visual information processing begin their development by weeks 13–15 of gestation (Chi et al. 1977; Table 2.4). The visual midbrain (lateral geniculate body, LGB) and the primary visual cortex (striate cortex, V1) develop rapidly; the thickness of the striate cortex and the number of interneurons and fibre connections within and between areas in the visual cortex, and between these areas and other structures of the brain increase. Visual modularity also increases over the course of development (Dobkins 2009), but development of visual cortical areas depends on stimulus-specific visual experience (Quin et al. 2013). The two main visual information processing pathways in the brain show different developmental timing. The ventral (i.e. occipito-temporal) visual pathway shows prolonged development and appear less “plastic” compared to the dorsal (i.e. occipito-parietal) visual pathway; in contrast, fibre connections within the ventral pathway mature earlier (Klaver et al. 2011; Dormal et al. 2012). The morphological development of the visual brain is the crucial basis for the functional development of the various substructures, and their co-operativity, as well as their connectivity with other functional systems, for example the attention, memory, executive and motivation and reward systems. Learning by selection is a fundamental principle in visual development; it depends on an efficient attentional mechanism for information processing (Amso and Johnson 2006). Attention also plays an important role in recognition memory development during early childhood (Rose et al. 2001); it modulates active vision in (spatial) selection and processing, but also in coding and representation of visual information (Berman and Colby 2009). Emotional evaluation of visual stimuli supports vision by enhancing selection of stimuli, based on the affective impact of past visual sensation and experience (Barrett and Bar 2009). ‘Affective learning’, i.e. repeated systematic experience with task-relevant affective information, for example, colours or faces, not only modulates visual association cortical areas and connected prefrontal structures, but also enhances response activity and functional connectivity within the primary visual cortex (Damaraju et al. 2009) indicating that stimulus-emotion associations are powerful learning conditions even at low-level visual processing stages. Interestingly, facial expression (“emotional faces”) in particular captures attention, i.e. children preferentially direct spatial attention in the direction of such faces (Elam et al. 2010).

It appears that the prenatal development of the visual system occurs at a basic level without visual stimulation and associated experience. In contrast, the postnatal development of the visual system is both, environment- and practice-dependent; it is environment and action – “anticipatory”. Early visual experience is required for further development of the structures and functions of the visual system to make use of the visual information provided by the environment, and to develop the necessary adjustments to this visual environment (Berardi et al. 2000; Maurer et al. 2008; Lewis and Maurer 2009). This principle applies also to the development of the so-called receptive fields, i.e. the region of the visual field that is covered by a neuron, in which a stimulus can elicit the best response of this neuron, regarding its size as well as its specialisation of processing, and coding properties, and its interactions. Monocular visual deprivation, for example, caused by improper alignment of the eyes (known as strabismus, squint, or heterotropia), or anisometropia (a greater refractive error developing in one eye), causing amblyopia is associated with underdevelopment of the corresponding visual cortex: the number of functionally intact neurons decreases, the receptive fields do not fully develop or can shrink, and neurons do not arrive at the normally occurring high degree of specialisation in processing and coding of the finer details of visual patterns (Bi et al. 2011). Similarly, bilateral amblyopia can compound untreated bilateral refractive error (Abrahamsson et al. 1990) and developmental disorders affecting the peripheral part of the visual system, where the associated sensory deprivation can cause amblyopia, therefore lead to delayed or even impaired morphological and functional development of the central components of the visual system (Kiorpes and McKee, 1999; Sireteanu 2000; Maurer et al. 2005, 2008; Lewis and Maurer 2009). This delay or impairment may affect both the differentiation of the visual system as well as the specialisation of the neurons with regard to their processing and coding properties. Consequently, incoming sensory information may be processed only incompletely, imprecisely or incorrectly, and integration of information may be impaired, despite the central structures of the visual system being intact. This condition of additional bilateral amblyopia may thus induce or compound difficulties with the development of a sufficiently valid and reliable representation of the visual world in terms of fine differentiation within visual categories, the identification and recognition of visual stimuli, the guidance and control of motor activities and of complex actions, and the development and optimisation by perceptual learning.