The loss of vision in HH cannot be explained by injury to the eye itself (Hécaen, 1972): the lesion usually occurs in occipital regions that include to the primary visual cortex of the right or left hemisphere. HH can be total or partial (e.g., quadrantopia and scotoma) and have or lack macular sparing (if the region devoted to the central visual field is impaired), as illustrated in Figure 1 (Chokron, 1996; Danckert and Goodale, 2000). Usually, these field deficits are totally homonymous, meaning that the blind portion of each eye can be superimposed.

The most frequent etiology of CVI is stroke (either ischemic or hemorrhagic). According to Marshall et al. (2010), CVI, and especially visual field defects, affect more than 15% of brain-damaged patients. Moreover, in several studies researchers have proposed that 50% of the hospitalizations in neurology departments and rehabilitation centers in England are consecutive to a stroke, with 30% of these patients suffering from HH (Pambakian and Kennard, 1997; Kerkhoff, 2000; Sand et al., 2013). In one striking report, the authors indicated that among 323 stroke patients, only 8% did not show any visual impairment and 49% presented visual field loss (Rowe and VIS Group, 2009). According to a recent study, 60.5% of stroke patients might present a CVI with HH affecting 35% of stroke patients in this database of 11900 cases (Ali et al., 2013).

The aforementioned findings are testament to the urgency of visual field rehabilitation in public health. HH and other visual field defects can also result from brain tumors, cerebral hypoxia, along postchiasmatic visual pathways, occipital lobectomies, trauma, progressive multifocal leukoencephalopathy (Diller and Thompson, 2007), or even degenerative diseases (Levine et al., 1993; Chokron, 1996; Kerkoff, 1999; Zihl, 2000; Meek et al., 2013). However, it is the topography and size of the lesion, rather than the type, that determine the extent and severity of the visual field defect (Tant et al., 2002; Atchison et al., 2006). Among HH, 40% imply lesions of the occipital cortex; 30% result from parietal damage; 25%, from temporal damage; and 5%, from lesions of the optic tract or the lateral geniculate nucleus (LGN; Fujino et al., 1986; Huber, 1992).

In addition to not being able to detect visual stimuli in their contralesional visual field, HH patients suffer from other clinical manifestations, including impaired visual search/orientation in 2D and 3D space, reading difficulties (see below), and slowed and inaccurate performance in functional visual activities (Pambakian et al., 2005; Leff et al., 2006; McDonald et al., 2006). Furthermore, numerous HH patients report locomotion disabilities, especially when outdoors: for instance, they bump into other pedestrians or obstacles in their blind hemifield. Accordingly, these patients are usually not allowed to drive. They also express difficulties in building a global representation of their visual environment (Pambakian and Kennard, 1997). Reading is considerably affected by the visual field defect: patients suffer from omission of letters (in right HH) or lines (in left HH) (Zihl, 2000). Another problem that has been described in these patients is impaired visuospatial exploration: in a study of HH and patients that were asked to generate a saccade towards the blind hemifield, the HH patients exhibited less consistent oculomotor behavior and longer fixation times (Zihl, 1995). Consequently, hemianopes suffer from disorganized, and inefficient visual search strategy that requires a lot of attention to be useful. Moreover, subjective visual assessment indicates that these patients have a reduced quality of life (Papageorgiou et al., 2007). However, and surprisingly, Papageorgiou et al. (2007) found that subjective impairment does not correlate to visual-field assessment, especially when testing the correlation between the extent of the macular spare, and the subjective evaluation of the impairment in everyday activities. This finding suggests that patients with HH should benefit from a complete evaluation including objective visual-field perimetry, evaluation of locomotion and exploration capacities as well as subjective interviews.

Blindsight phenomenon has been observed in numerous tasks. Numerous experiments performed over the past several decades have shown various residual capacities in the blind field of HH patients. Using forced-choice procedures, researchers have highlighted the capacities of these patients to detect a visual stimulus placed in their blind field (Fendrich et al., 1992); to localize a visual stimulus by an eye jerk (Zihl and von Cramon, 1980) or by manual checking (Perenin and Jeannerod, 1975); to detect stimuli in movement (Riddoch, 1917) or of changing luminous intensity (Barbur et al., 1980); to discriminate among shapes (Weiskrantz, 1986); and to distinguish facial expressions (Pegna et al., 2005). For example, researchers have extensively described an HH patient named GY that was able to compare stimuli according to color attributes, or detect movement in his blind visual field, despite not having conscious vision of the stimuli, but that was unable to compare two degrees of luminance (Ffytche et al., 1996; Morland et al., 1999). Similar behavior has been reported in hemianopic monkeys in numerous studies by Stoerig and Cowey (1989) and Cowey and Stoerig (1995, 1997, 1999). For instance, in some studies hemianopic monkeys were able to distinguish different orientations, wavelengths and colors (for a review, see: Stoerig and Cowey, 1997; Stoerig et al., 2002). Furthermore, in hemianopic monkeys, response times towards stimuli presented in their healthy visual field can be facilitated by first displaying the stimuli in the blind field (Cowey et al., 1998).

Blindsight was not only reported for simple visual tasks such as grasping but also for tasks requiring more complex visual processing. As a matter of fact, some patients were found to be able to analyse the visual stimulus in order to perform category discrimination (Trevethan et al., 2007; Van den Stock et al., 2013, 2014) whereas de Gelder et al. (2008) were able to show that TN, a patient with cortical blindness, was able navigate and avoid obstacles although not being to report their presence. In addition, blindsight was also shown for emotional stimuli including facial and bodily expressions (e.g., Tamietto et al., 2009; Van den Stock et al., 2011) as well as for social cues such as gaze direction (Burra et al., 2013).

In a very recent study on hemianopic patients, Fayel et al. (2014) reported that these patients retained the ability to direct a saccade toward their contralesional hemifield, despite their hemifield defect, but that their verbal detection reports were at chance level. However, saccade parameters (latency and amplitude) were altered by the defect. Saccades to the contralesional hemifield in the patients exhibited longer latencies and shorter amplitudes than did those in the corresponding hemifield in a cohort of healthy subjects. Their findings confirmed previous studies on the direction of saccades in the blind field (for a review and discussion, see Cowey, 2010).

Blindsight has also been described in children. For instance, Tinelli et al. (2013) recently measured sensitivity to several visual tasks in a group of four children with congenital unilateral brain lesions that had left their optic radiations severely damaged, and in a group of three children with similar lesions that they had acquired during childhood. Using functional magnetic resonance imaging (fMRI), the authors measured blood oxygenation level-dependent (BOLD) activity in response to stimulation of each visual field quadrant. They found residual unconscious processing of position, orientation and motion of visual stimuli displayed in the scotoma in the children with the congenital lesions, but not in those with the acquired lesions.

Visual pathways predominantly arise to the primary visual cortex (V1). Considering the global neuronal workspace (GNW) framework (Sergent and Dehaene, 2004), conscious perception would arise through the subsequent activation of several neurons, with a self-amplifying process that leads to consciousness threshold. However, if the system fails to reach said threshold, then conscious vision is not possible. Stoerig and Cowey (1995) proposed that activation of neurons in the primary visual cortex is essential for conscious vision. This premise would explain why hemianopes cannot consciously process visual information. However, visual pathways also project onto several other areas of the brain. Thus, any loss of neurons in V1 could be compensated for by the activity of other visual areas that remain stimulated by visual input. Such compensation would explain the persistence of visual capacities in the absence of conscious vision. Indeed, this idea is the central hypothesis of blindsight literature (de Gelder et al., 1999; Morland et al., 1999; Danckert and Goodale, 2000; Danckert and Rossetti, 2005; Pegna et al., 2005). Thus, researchers proposed that the underlying mechanism behind blindsight is that subcortical pathways bypass V1 to directly project onto secondary visual areas such as V5 (for motion detection), the thalamus, the brain stem, the hypothalamus, and/or the amygdala (for emotional response). In fact, this hypothesis was confirmed based on anatomical data acquired from fMRI studies. Goebel et al. (2001) identified extrastriate activations in the damaged hemisphere of the aforementioned hemianopic patient G.Y., during a forced-choice task known to elicit blindsight. Additionally, bilateral extrastriate cortex activations have been observed in several patients (Nelles et al., 2007). Unfortunately, current neuroimaging techniques are not sufficiently precise to temporally track the temporal course of visual information through subcortical pathways; consequently, the relationship between the activation of subcortical structures and blindsight is presently difficult to establish in humans (Sahraie et al., 1997).

Various hypotheses have been proposed to explain blindsight, including the presence of spared islands in V1 and the projection of visual information from the superior colliculus (SC) or LGN to preserved visual areas. We discuss these mechanisms in the following section.

An early but now-defunct substitution technique entailed the use of optical aids (e.g., mirrors or Fresnel prisms) to shift visual information from the blind visual field to the central or the ipsilesional, preserved visual field. However, there were only anecdotal reports of the positive effects of this approach; furthermore, such tools were reported to cause diminished acuity, confusion or even diplopia in patients (for a review, see: Pambakian and Kennard, 1997; Grunda et al., 2013). Therefore, this technique is no longer used in rehabilitation of HH patients.

Currently, compensatory techniques for HH are used principally to enlarge and reinforce visual search, by training patients in oculomotor strategies. Indeed, there are extensive reports that hemianopic patients have difficulties in visual scanning for object detection, as well as in identifying people. These problems can lead to omission of important parts of a scene and consequently, to poor comprehension and to social misunderstanding. Parafoveal visual-field defects also affect reading (hemianopic alexia), due to a reduction in the “perceptual window” involved in letter identification and in saccades planification and thus compromizing guidance of eye movements along text (Poppelreuter, 1971[1990]; Zihl, 1995; McDonald et al., 2006). These deficits can selectively be trained in hemianopic patients (Schuett et al., 2012).

Compensation therapies can be proposed regarding the fact that recovery from a very severe perceptual deficit can be difficult to obtain. Therefore, they typically involve using and modifying the patient’s preserved capacities to sidestep the impairment or render it less disabling. Accordingly, compensation strategies for HH require the use of the ipsilesional hemifield or of the central visual field to compensate for the blind area in the contralesional hemifield.

In classical oculomotor rehabilitation, patients are trained to search for a stimulus projected into their blind hemifield, and then respond to it as quickly as possible. The target can be presented alone or amongst distracters (Zihl and Werth, 1984). Researchers usually record reaction times (RTs) and error rate, given that an inefficient search will lead to longer RTs. These techniques are generally efficient, with shorter RTs, but sometimes lead to longer RTs (Pambakian et al., 2004). Although longer RTs could be considered as being detrimental, in some cases, they have been described as an improvement: the authors of these studies affirm that longer RTs might reflect underlying compensatory mechanisms (i.e., the development of a new strategy) that probably need more time to progress and reach maximum efficacy. These types of compensation techniques are principally based on top-down mechanisms, because they train patients to focus their attention on the blind hemifield.

Another compensation technique involves a bottom-up strategy based on multisensory stimulation and integration (Bolognini et al., 2005). According to these authors, this therapy does not require the patient’s attention, which can be impaired in some patients and therefore, enables more interesting perspectives for rehabilitation. They based their rehabilitation technique on the existence of neurons that encode information coming from different sensory modalities, in the superior colliculi and other parts of the brain. As the superior colliculi are involved in gaze orientation, and the visual modality is impaired in these patients, Bolognini et al. proposed that a non-visual modality (e.g., auditory) could be harnessed to reinforce gaze orientation towards the blind hemifield. They presented their patients with audio-visual stimulation to help them find a subsequently presented visual target. The authors observed a greater increase in visual oculomotor exploration upon the addition of an auditory cue together with the visual stimulus than without it. Thus, multisensory stimulation enabled the capacities of a fully functioning sense (hearing) to be transferred to a deficient sense (vision).

Finally, other researchers have focused on the reading impairment caused by HH. For instance, Spitzyna et al. (2007) induced an optokinetic nystagmus in order to facilitate reading in hemianopic patients. They presented patients with right-to-left moving text that the patients had to read. The right-to-left visual movement induced a left-to-right nystagmus that increased reading speed.

Although the aim of these compensatory techniques is not to restore per se the impaired visual field, they nevertheless improve the quality of life of patients. Indeed, overall, in these studies the authors report subjective improvement in everyday life, according to subjective questionnaires (Bolognini et al., 2005). Therefore, compensation therapies seem to be a first step that should be systematically proposed for hemianopia treatment when restoration therapy is unavailable. However, and as we previously mentioned, since these techniques are compensatory in nature, the visual impairment remains. In addition, there are reports that the ipsilesional hemifield of hemianopic patients, when thought to be intact, can actually also be impaired—not in terms of visual-field loss, but in terms of quality of vision (Paramei and Sabel, 2008; Bola et al., 2013). Thus, relying exclusively on the “intact” visual field may not be the best option in rehabilitation techniques. Furthermore, special care for visual-field improvement, and restoration therapies of the blind hemifield, are needed whenever possible. In the following section we discuss restoration therapies.

Attempts to enlarge the visual field appear to be a more encouraging way to help hemianopic patients. Unfortunately, as we explained in the previous section, restoration of visual function may seem impossible in HH patients. This might be linked to the fact that, even if a few studies demonstrated in particular condition (see above) that visual experience is possible even in the absence of V1, generally V1 is considered to be crucial for visual consciousness (Weiskrantz et al., 1974; Perenin and Jeannerod, 1975; Tamietto et al., 2010).

Thus, recovery of explicit conscious visual detection in the absence of V1 would imply a degree of neural plasticity that is known to be unattainable after a normal rearrangement during the first 3 months post-lesion. Moreover, whether any reorganization that did occur would be powerful enough to activate conscious vision is unknown. However, studies in animals and humans have shown that perceptual learning is possible in hemianopia (i.e., training can improve visual perception) (Fahle and Poggio, 2002). A few researchers have hypothesized that the size of the visual field could be increased to enable recovery from blindness (Chokron et al., 2008). In the next section we describe oculomotor rehabilitation techniques, followed by restorative techniques using blindsight.

Plasticity is a fundamental mechanism for the brain to adjust to sensory changes in the surroundings, to improve perception and to recover from damage to the visual system. Studies on neural plasticity have shown that the brain can react to environmental inputs after a lesion, in infants, children and even seniors. Thus, researchers have demonstrated that part of the areas adjacent to a lesion can replace the function of the affected one, as can occur after sensory-motor defects (Liepert et al., 2000), or that the healthy (undamaged) hemisphere can reorganized itself to take over part of the damaged hemisphere’s functions (Netz et al., 1997; Johansen-Berg et al., 2002; Rossini and Dal Forno, 2004). In related work, Safran and Landis (1999) suggested that even in adult brains, cortical maps are not fixed.

Studying blindsight enables researchers to determine, in the absence of the primary visual system, whether the spared or recovered visual ability stems from an existing alternative pathway or from newly formed pathway. As described above, Bridge et al. (2008), using DTI, and Silvanto et al. (2009), using transcranial magnetic stimulation (TMS), each observed pathways in patients with visual-field defects that they did not observed in control patients. Their findings suggest that after a brain lesion, specific connections can be created. Nelles et al. (2007) conducted a related study in thirteen hemianopic patients that had suffered a unilateral ischemic stroke in the striate cortex. Stimulation in the blind visual field led to bilateral activation in the extrastriate areas, and this activation was stronger in the ipsilateral (healthy) hemisphere. This result suggests that activation had been transferred from the damaged hemisphere to the healthy one.

Interestingly, in most of the published studies concerned with cortical reorganization in hemianopic patients, the authors did not address the side of the lesion. This issue was tackled by Perez et al. (2013), who sought to determine the effects of the lesion side on cortical reorganization in brain-damaged patients. They demonstrated that the pattern of cortical activation during a visual detection task and a visual categorization task depends on the side of the occipital lesion. Indeed, RHH patients showed activation predominantly in the right (intact) hemisphere (occipital lobe and posterior temporal areas), whereas LHH patients showed more bilateral activation (in the occipital lobes). Thus, lateralization of an occipital lesion is not without consequence for the subsequent cortical reorganization. Along those lines, the impact of the lesion lateralization might be crucial to the cortical reorganization and consequently, to the choice of rehabilitation scheme for left or right hemianopic patients.

The main question when studying brain reorganization after recovery is: Does recovery involve recruitment of existing pathways, establishment of new neural connections or both? We address this question in the following section.

As explained above, various techniques are now available to compensate for damage, or restore vision, in the hemianopic visual field. Regarding the functional basis of these rehabilitation techniques, some studies have suggested that visual training can induce neural modifications in V1 (Furmanski et al., 2004; Maertens and Pollmann, 2005). For example, Raninen et al. (2007) demonstrated that intensive visual training can improve abilities in the blind visual field. After training, their patients achieved a level of performance in the blind visual field similar to that in the healthy visual field. Furthermore, this gain was underlined by changes in cortical activation. Indeed, after training, the patients exhibited an ipsilateral response to blind visual-field stimulation. In a related fMRI study, Marshall et al. (2008) were the first to report changes in cerebral responses to stimuli after visual training in hemianopic patients. They trained the patients with repetitive stimulation of the border zone. After 1 month of training, the patients exhibited increased BOLD activity for border-zone detection compared to detection in the healthy part of the visual field. After visual training, the most activated areas were the right inferior and lateral temporal areas, the right dorsolateral frontal areas, the bilateral anterior cingulate cortices and the bilateral basal ganglia. Moreover, these authors observed a correlation with behavioral data obtained out-of-scanner, which revealed an improvement in response times for detecting stimuli in the border zone. The authors concluded that visual training induced a shift of attention from the non-trained seeing field to the trained border zone, and stated that the effect seemed to be mediated by frontal regions and other higher-order visual areas. In related work, Ro and Rafal (2006) trained patients, and then observed an ipsilateral response to blind visual-field stimulation. They proposed that the visual pathway used in blindsight—in this case, the sub-cortical pathway—is probably the same one that underpins recovery after training.

Henriksson et al. (2007) trained patients, and then studied them by fMRI, observing that visual information in both hemifields was processed in the intact hemisphere. Thus, training patients for visual detection in the hemianopic field induced cortical representation of this visual field in their ipsilateral, healthy hemisphere. As such, after training, the visual areas of the healthy hemisphere (in particular, V5) might have represented not only the contralateral field, but also the ipsilateral visual field. This premise is consistent with the findings of Nelles et al. (2007), in their study of hemianopic-field stimulation in patients with occipital strokes: they found that stimulation induced bilateral activation within the extrastriate cortex, and this activation was stronger in the ipsilateral (contralesional) hemisphere. Furthermore, the activation pattern in the patients was different to that observed in normal subjects, who exhibited contralateral activation of the striate and extrastriate regions (as similarly observed by Silvanto et al., 2007, using TMS).

Recently, Plow et al. (2012) studied a 3-month regime of VRT in two patients suffering from a left occipital lesion (Right HH). They coupled sessions of visual training with TMS of the occipital area. They observed better recovery in the patients (who had received TMS) compared to control patients (who had received sham stimulations). Using fMRI, they also recorded activation around the lesioned area and bilateral activation of the associative visual areas.

Taken together, the aforementioned studies suggest that visual training in the blind visual field, regardless of whether it is coupled to TMS, can induce cortical reorganization. Nevertheless, more studies are needed in order to standardize these rehabilitation programs as well as to understand the neural bases of recovery. Furthermore, psychophysical studies are required to determine the extent to which the recovered vision in the hemianopic visual field resembles the vision in the ipsilesional field.