Blind Hemifield Research Articles

The Influence of “Blind” Distractors on Eye Movement Trajectories in Visual Hemifield Defects

There is evidence that some visual information in blind regions may still be processed in patients with hemifield defects after cerebral lesions ("blindsight"). We tested the hypothesis that, in the absence of retinogeniculostriate processing, residual retinotectal processing may still be detected as modifications of saccades to seen targets by irrelevant distractors in the blind hemifield. Six patients were presented with distractors in the blind and intact portions of their visual field and participants were instructed to make eye movements to targets in the intact field. Eye movements were recorded to determine if blind-field distractors caused deviation in saccadic trajectories. No deviation was found in one patient with an optic chiasm lesion, which affect both retinotectal and retinogeniculostriate pathways. In five patients with lesions of the optic radiations or the striate cortex, the results were mixed, with two of the five patients showing significant deviations of saccadic trajectory away from the "blind" distractor. In a second experiment, two of the five patients were tested with the target and the distractor more closely aligned. Both patients showed a "global effect," in that saccades deviated toward the distractor, but the effect was stronger in the patient who also showed significant trajectory deviation in the first experiment. Although our study confirms that distractor effects on saccadic trajectory can occur in patients with damage to the retinogeniculostriate visual pathway but preserved retinotectal projections, there remain questions regarding what additional factors are required for these effects to manifest themselves in a given patient.

Audio-visual stimulation improves oculomotor patterns in patients with hemianopia

Patients with visual field disorders often exhibit impairments in visual exploration and a typical defective oculomotor scanning behaviour. Recent evidence [Bolognini, N., Rasi, F., Coccia, M., & Làdavas, E. (2005b). Visual search improvement in hemianopic patients after audio-visual stimulation. Brain, 128, 2830–2842] suggests that systematic audio-visual stimulation of the blind hemifield can improve accuracy and search times in visual exploration, probably due to the stimulation of Superior Colliculus (SC), an important multisensory structure involved in both the initiation and execution of saccades. The aim of the present study is to verify this hypothesis by studying the effects of multisensory training on oculomotor scanning behaviour. Oculomotor responses during a visual search task and a reading task were studied before and after visual (control) or audio-visual (experimental) training, in a group of 12 patients with chronic visual field defects and 12 controls subjects. Eye movements were recorded using an infra-red technique which measured a range of spatial and temporal variables. Prior to treatment, patients’ performance was significantly different from that of controls in relation to fixations and saccade parameters; after Audio-Visual Training, all patients reported an improvement in ocular exploration characterized by fewer fixations and refixations, quicker and larger saccades, and reduced scanpath length. Overall, these improvements led to a reduction of total exploration time. Similarly, reading parameters were significantly affected by the training, with respect to specific impairments observed in both left- and right-hemianopia readers. Our findings provide evidence that Audio-Visual Training, by stimulating the SC, may induce a more organized pattern of visual exploration due to an implementation of efficient oculomotor strategies. Interestingly, the improvement was found to be stable at a 1 year follow-up control session, indicating a long-term persistence of treatment effects on the oculomotor system.

Neural correlates of hemianopic completion across the vertical meridian

Hemianopic completion refers to the perceptual completion of figures located across the vertical meridian in the context of hemianopia, such that one half of the figure falls within the blind hemifield. It can occur whether the figure is itself complete (veridical completion) or incomplete (paracompletion). Psychophysical evidence suggests that this phenomenon may be a constructive one, and may share features with completion phenomena in normal vision. The neural structures mediating hemianopic completion are unknown. Here we studied the neural activity evoked by hemianopic completion using event-related fMRI in an individual (POV) with a large right visual field homonymous hemianopic scotoma due to left occipital damage. Either a large achromatic circular contour straddling the vertical meridian or a semicircular contour within the left hemifield just crossing the vertical meridian was presented to POV on each trial. POV indicated by button press whether he perceived a semicircular contour, a patchy circular contour or a complete circular contour. On trials where he reported perceiving a complete circular contour despite being presented with a semicircular contour (paracompletion), activity was increased in a region of ipsilateral extrastriate cortex (contralateral to the lesion, ipsilateral to the illusory edge of the circle). These results are discussed in the context of illusory contour completion in healthy subjects and more generally in the recovery of function after brain damage.

Is blindsight just degraded normal vision?

It is a conservative and reasonable suggestion that implicit functioning, as in blindsight, is simply a weakened, degraded form of normal functioning, especially as the parameters of vision in blindsight are themselves weakened--e.g., acuity is reduced, detection thresholds are raised, chromatic discrimination is coarser. But does this mean that maximum performance in blindsight is itself uniformly degraded? The answer is no, because there are many examples of very good performance in blindsight, in the absence of acknowledged awareness, especially if the visual parameters are chosen to be within a selective range. The question then is raised whether the parameters of blindsight vision are qualitatively similar to normal vision, or even to any manifestation of normal vision when it is uniformly degraded, for example, by threshold rises or injection of noise? Evidence is provided that blindsight vision and normal vision, even if it were degraded, are qualitatively different in certain respects. For example, there can be selective loss of colour but not of luminance contrast; visible after-images to unseen stimuli ("prime-sight") can be observed in one subject (DB); there can be super-sensitivity in the blind hemifield that is better than that of the intact hemifield; there can be a change in S-cone retinal inputs. Even when the stimulus contrast is deliberately lowered, blindsight performance itself does not degrade in parallel. Another meaning of "degraded" is that even when blindsight discriminatory or detection ability is excellent, the subject's acknowledged experience is as though the stimuli are weak and degraded to the point of extinction or near-extinction. But this is akin to the very meaning of blindsight itself, and sets the problem to be solved in neural systems terms and philosophical analysis rather than providing a conservative solution.

Cross-modal localization in hemianopia: new insights on multisensory integration

The superior colliculus (SC) has been implicated in the mediation of residual visual function in hemianopic patients, and has been shown to be capable of using multiple sensory cues to facilitate its localization functions. The aim of the present study was to examine the possibility that the SC could effect covert visual processes, via multisensory integration of auditory and visual stimuli in patients with visual field loss. To this aim hard-to-localize auditory targets were presented alone (unimodal condition) or with a visual stimulus (cross-modal condition) in either hemifield and at various spatial (0 degree, 16 degrees, 32 degrees) and temporal (0 ms, 500 ms) disparities. The results showed substantial field-specific differences. As expected, a visual stimulus in the intact hemifield induced a strong visual bias in auditory localization independent of the spatial disparities, and did so even when the two stimuli were temporally offset. In these spatially disparate conditions, the localization accuracy was markedly reduced. In the blind hemifield, however, the visual stimulus affected auditory localization only when it was coincident with that target in both space and time. In this circumstance auditory localization performance was markedly enhanced. This result strongly suggests that covert visual processes remain active in hemianopia, though they differ from those in the normal hemifield. A likely explanation of these differences is that enhancement and visual bias depend on different neural pathways: with the former dependent on circuits involving the superior colliculus, a structure involved in the integration of cues from multiple senses to facilitate orientation/localization; and the latter dependent on geniculo-striate circuits that facilitate more detailed analyses of the visual scene. Overall the present results not only enhance our understanding of the impact of covert visual processes in hemianopic patients, but also enhance our knowledge of how different brain regions areas contribute to processing cross-modal information.

Saccades to the seeing visual hemifield in hemidecorticate patients exhibit task-dependent reaction times and hypometria

In three patients who had one cortical hemisphere removed surgically (hemidecortication), we studied visually-triggered saccades directed contralateral to the intact cortical hemisphere (i.e., ipsilesional saccades). Both saccade reaction times (SRTs) and accuracy of these saccades have been reported as abnormal in hemidecorticate patients, but not monkeys. One explanation for this difference is that deficits in hemidecorticate patients may not have been directly caused by removal of cortical oculomotor structures themselves, but may have been a manifestation of compensatory strategies used to cope with contralesional hemianopia. We hypothesized that deficits in saccade performance to the ipsilesional (seeing) visual hemifield would be directly linked to how easily patients could localize targets in their blind hemifield with searching saccades. To test this hypothesis, we examined how deficits in our patients varied when targets were: (1) randomly presented to either the seeing or blind hemifield for long durations thereby permitting searching saccades in the blind hemifield; (2) presented as in Experiment 1, but briefly flashed thereby removing visual feedback prior to saccade onset thereby rendering searching saccades useless; (3) briefly flashed as in Experiment 2, but at random locations in only the seeing hemifield (blind hemifield irrelevant). Mean SRTs to the seeing hemifield were 165 ms longer than normal in Experiment 2, but only about 40 ms longer in Experiments 1 and 3. Saccade accuracy was characterized by task-dependent hypometria in all three experiments with a mean undershoot of about twice the amplitude variance. The largest undershoots were in Experiments 2 and 3. Our data suggest that deficits resulted from the direct effects of the lesions themselves coupled with context-dependent strategies used to cope with contralesional hemianopia.

Making the blindsighted see

A lesion of striate cortex, area V1, produces blindness in the retinotopically corresponding part of the visual field, although in some cases visual abilities in the blind field remain that are paradoxically devoid of conscious visual percepts (“blindsight”). Here we demonstrate that the blindsight subject GY can experience visual sensations of phosphenes in his blind field induced by transcranial magnetic stimulation (TMS). Such blind field percepts could only be induced when stimulation was applied bilaterally, i.e. over GY's area V5/MT in both hemispheres. Consistent with an earlier report [Cowey, A., & Walsh, V. (2000). Magnetically induced phosphenes in sighted, blind and blindsighted observers. Neuroreport, 11, 3269–3273], GY never experienced phosphenes when stimulation was restricted to his ipsilesional V5/MT. To the best of our knowledge this is the first time GY has experienced visual qualia in his blind hemifield. The present report characterizes the necessary conditions for such conscious experience in his hemianopic visual field and interprets them as demonstrating that only via a contribution from GY's intact hemisphere can activation in the damaged hemisphere reach visual awareness.

Restoration of Acoustic Orienting Into a Cortically Deaf Hemifield by Reversible Deactivation of the Contralesional Superior Colliculus: The Acoustic “Sprague Effect”

Removal of all contiguous visual cortical areas of one hemisphere results in a contralateral hemianopia. Subsequent deactivation of the contralesional superior colliculus (SC) nullifies the effects of the visual cortex ablation and restores visual orienting responses into the cortically blind hemifield. This deficit nullification has become known as the "Sprague Effect." Similarly, in the auditory system, unilateral ablation of auditory cortex results in severe sound localization deficits, as assessed by acoustic orienting, to stimuli in the contralateral hemifield. The purpose of this study was to examine whether auditory orienting responses can be restored into the impaired hemifield during deactivation of the contralesional SC. Three mature cats were trained to orient toward and approach an acoustic stimulus (broadband, white noise burst) that was presented centrally, or at one of 12 peripheral loci, spaced at 15 degrees intervals. After training, a cryoloop was chronically implanted over the dorsal surface of the right SC. During cooling of the cooling loop to temperatures sufficient to deactivate the superficial and intermediate layers (SZ, SGS, SO, SGI), auditory orienting responses were eliminated into the left (contracooled) hemifield while leaving acoustic orienting into the right (ipsicooled) hemifield unimpaired. This deficit was temperature-dependently graded from periphery to center. After the effectiveness of the SC cooling loop was verified, auditory cortex of the middle and posterior ectosylvian and anterior and posterior sylvian gyri was removed from the left hemisphere. As expected, the auditory cortex ablation resulted in a profound deficit in orienting to acoustic stimuli presented at any position in the right (contralesional) hemifield, while leaving acoustic orienting into the left (ipsilesional) hemifield unimpaired. The ablations of auditory cortex did not have any impact on a visual detection and orienting task. The additional deactivation of the contralesional SC to temperatures sufficient to cool the superficial and intermediate layers nullified the deficit caused by the auditory cortex ablation and acoustic orienting responses were restored into the right hemifield. This restoration was temperature-dependently graded from center to periphery. The deactivations were localized and confirmed with reduced uptake of radiolabeled 2-deoxyglucose. Therefore deactivation of the right superior colliculus after the ablation of the left auditory cortex yields a fundamentally different result from that identified during deactivation of the right superior colliculus before the removal of left auditory cortex in the same animal. Thus the "Sprague Effect" is not unique to a particular sensory system and deactivation of the contralesional SC can restore either visual or acoustic orienting responses into an impaired hemifield after cortical damage.

Training-induced cortical representation of a hemianopic hemifield

Background: Patients with homonymous hemianopia often have some residual sensitivity for visual stimuli in their blind hemifield. Previous imaging studies suggest an important role for extrastriate cortical areas in such...

Visual field recovery after vision restoration therapy (VRT) is independent of eye movements: An eye tracker study

Aim It has been argued that patients with visual field defects compensate for their deficit by making more frequent eye movements toward the hemianopic field and that visual field enlargements found after vision restoration therapy (VRT) may be an artefact of such eye movements. In order to determine if this was correct, we recorded eye movements in hemianopic subjects before and after VRT. Methods Visual fields were measured in subjects with homonymous visual field defects ( n = 15) caused by trauma, cerebral ischemia or haemorrhage (lesion age >6 months). Visual field charts were plotted using both high-resolution perimetry (HRP) and conventional perimetry before and after a 3-month period of VRT, with eye movements being recorded with a 2D-eye tracker. This permitted quantification of eye positions and measurements of deviation from fixation. Results VRT lead to significant visual field enlargements as indicated by an increase of stimulus detection of 3.8% when tested using HRP and about 2.2% (OD) and 3.5% (OS) fewer misses with conventional perimetry. Eye movements were expressed as the standard deviations (S.D.) of the eye position recordings from fixation. Before VRT, the S.D. was ±0.82° horizontally and ±1.16° vertically; after VRT, it was ±0.68° and ±1.39°, respectively. A cluster analysis of the horizontal eye movements before VRT showed three types of subjects with (i) small ( n = 7), (ii) medium ( n = 7) or (iii) large fixation instability ( n = 1). Saccades were directed equally to the right or the left side; i.e., with no preference toward the blind hemifield. After VRT, many subjects showed a smaller variability of horizontal eye movements. Before VRT, 81.6% of the recorded eye positions were found within a range of 1° horizontally from fixation, whereas after VRT, 88.3% were within that range. In the 2° range, we found 94.8% before and 98.9% after VRT. Subjects moved their eyes 5° or more 0.3% of the time before VRT versus 0.1% after VRT. Thus, in this study, subjects with homonymous visual field defects who were attempting to fixate a central target while their fields were being plotted, typically showed brief horizontal shifts with no preference toward or away from the blind hemifield. These eye movements were usually less than 1° from fixation. Large saccades toward the blind field after VRT were very rare. Conclusion VRT has no effect on either the direction or the amplitude of horizontal eye movements during visual field testing. These results argue against the theory that the visual field enlargements are artefacts induced by eye movements.

Temporal sensitivity in a hemianopic visual field can be improved by long-term training using flicker stimulation

Background: Blindness of a visual half-field (hemianopia) is a common symptom after postchiasmatic cerebral lesions. Although hemianopia severely limits activities of daily life, current clinical practice comprises no training of...

Facilitation as well as inhibition of the blink reflex by a visual prepulse requires intact striate cortex

Objective The role of visual cortex in modulation of the human eye blink reflex was assessed. Methods Participants were 13 patients with unilateral striate cortex damage. Nonreflexogenic gratings were presented in their intact or blind hemifield prior to white noise or air puff blink-eliciting stimuli. Results Inhibition of reflex amplitude was observed at asynchronies ranging from about 120 to 600 ms for visible but not invisible prepulses. Facilitation by intact-hemifield gratings was observed for (1) the latency of the acoustic blink reflex, (2) the amplitude of the disynaptic cutaneous blink reflex, R1, and (3) the latency of voluntary hand-grip reactions to the reflexogenic stimuli. These facilitatory effects were absent on trials with blind-hemifield prepulses. Conclusions An intact V1 is required for prepulse facilitation as well as inhibition. Significance These results extend a popular model of sensorimotor gating deficits in schizophrenia.

Non-conscious recognition of emotional body language

Patients with cortical blindness can reliably perceive some facial expressions even if they are unaware of their percept. We examined whether emotional body language may also be recognized in the absence of the primary visual cortex and without conscious stimulus perception. We presented emotional and neutral body images in the blind field of a patient with unilateral striate cortex damage. Using functional magnetic resonance imaging, we measured activation following presentation to the blind hemifield of whole body images (happy, neutral) with the face blurred. Unseen happy body images selectively activated area MT and the pulvinar nucleus of the thalamus, while unseen instrumental neutral body images activated the premotor cortex. Our results show that in the absence of the striate cortex implicit bodily emotion perception may be possible.

Induced gamma-band oscillations correlate with awareness in hemianopic patient GY

In normal vision gamma oscillations are involved in object perception, are modulated by attention, and have been linked to awareness by way of their putative role in perceptual integration, arguably as a mechanism for synchronizing activity in separate neural assemblies. We tested the hypothesis that the presence of gamma oscillations (∼30–80 Hz) signal the entry of a neural representation into awareness (as indexed by direct report), while attempting to control for other measures of neural information processing such as discrimination accuracy and reaction time. Hemianopic patient GY sometimes reports an awareness “that something happened” in his blind visual hemifield, in response to stimuli of sufficiently high contrast, although he may deny “seeing” anything. At lower contrast levels GY denies any awareness, but may continue to exhibit greater-than-chance accuracy (blindsight). Using a near-threshold level of contrast offers a unique way to test hypotheses concerning correlates of perceptual awareness, since GY's accuracy on certain tasks is independent of awareness. We tested GY on an orientation–discrimination task using stationary stimuli at a fixed near-threshold level of contrast, to which GY sometimes responded “aware” and sometimes “unaware”. We recorded brain activity using magnetoencephalography (MEG) in order to determine the relationship between local induced gamma-band oscillations and awareness. GY's accuracy was significantly greater than expected by chance and no different whether or not he reported awareness of the stimulus. Oscillatory activity in the gamma band (44–66 Hz) over the left occipito-parietal region correlated significantly with awareness (but not accuracy), whereas activity in the alpha band (8–12 Hz) did not.

Audiovisual Integration in Patients with Visual Deficit

In the present study, we investigated the possibility that bimodal audiovisual stimulation of the affected hemifield can improve perception of the visual events in the blind hemifield of hemianopic patients, as it was previously demonstrated in neglect patients. Moreover, it has been shown that "hetero-modal" and "sensory-specific" cortices are involved in cross-modal integration. Thus, the second aim of the present study was to examine whether audiovisual integration influences visual detection in patients with different cortical lesions responsible of different kinds of visual disorders. More specifically, we investigated cross-modal, audiovisual integration in patients with visual impairment due to a visual field deficit (e.g., hemianopia) or visuospatial attentional deficit (e.g., neglect) and patients with both hemianopia and neglect. Patients were asked to detect visual stimuli presented alone or in combination with auditory stimuli that could be spatially aligned or not with the visual ones. The results showed an enhancement of visual detection in cross-modal condition (spatially aligned condition) comparing to unimodal visual condition only in patients with hemianopia or neglect; by contrast, the multisensory integration did not occur when patients presented both deficits. These data suggest that patients with visual disorders can enormously benefit the multisensory integration. Moreover, they showed a different influence of cortical lesion on multisensory integration. Thus, the present results show the important adaptive meaning of multisensory integration and are very promising with respect to the possibility of recovery from visual and spatial impairments.

Repression of unconscious information by conscious processing: evidence from affective blindsight induced by transcranial magnetic stimulation.

Some patients with a lesion to the primary visual cortex (V1) show "blindsight": the remarkable ability to guess correctly about attributes of stimuli presented to the blind hemifield. Here, we show that blindsight can be induced in normal observers by using transcranial magnetic stimulation of the occipital cortex but exclusively for the affective content of unseen stimuli. Surprisingly, access to the affective content of stimuli disappears upon prolonged task training or when stimulus visibility increases, allegedly increasing the subjects' confidence in their overall performance. This finding suggests that availability of conscious information suppresses access to unconscious information, supporting the idea of consciousness as a repressant of unconscious tendencies.

Restitution of visual functions in cerebrally blind children

In adult patients who suffer from a visual field defect due to cerebral lesions, visual functions can be restored by systematic visual field training. Such visual field training is not feasible in young, brain-damaged children, who are unable to cooperate like adults. We have already shown earlier [Werth, R., & Moehrenschlager, M. (1999). The development of visual functions in cerebrally blind children during a systematic visual field training. Restorative Neurology and Neuroscience, 15, 229–241.] that systematic visual field training, which requires no ability to understand instructions or to cooperate, is successful in brain-damaged children and may lead to complete recovery of the visual field within 3 months. The present study provides more behavioral evidence and a control of intraocular light scatter. In addition to the earlier study, the luminance difference thresholds in the recovered visual field were compared with those of a normal control group. Seventeen children aged 1–4 years who had been blind for more than 1 year after perinatal asphyxia and two children suffering from homonymous hemianopia due to asphyxic-ischemic lesions participated in systematic visual field training. The functional visual field was assessed with a specially designed arc perimeter. Visual functions developed within a training period of 3 months in 11 children who received visual field training, whereas there was no recovery in the control group ( N = 37). The findings support the assumption that systematic visual field training facilitates the development of vision in cerebrally blind children. In two children who recovered from blindness, it was shown in functional magnetic resonance imaging (fMRI) that brain tissue in the area of the visual cortex contralateral to the blind visual hemifield was activated by light. In two children suffering from asphyxic-ischemic lesions who did not recover only brain tissue in the area of the visual cortex contralateral to the good visual hemifield could be activated by light. These results support the assumption that activity in spared tissue of the striate and extrastriate visual cortex are a necessary condition for recovery of the visual field in children suffering from cerebral blindness.

Accurate bidirectional saccade control by a single hemicortex.

Anatomical, electrophysiological and lesion studies indicate that each cortical hemisphere normally generates saccades directed to the contralateral side. In contrast, in patients who had an entire cortical hemisphere removed surgically (hemidecortication), the remaining hemicortex can generate both contraversive and ipsiversive saccades. However, current evidence indicates that ipsiversive saccades are grossly inaccurate. The obvious reason for this is that hemidecorticate patients are blind in the hemifield ipsilateral to the remaining hemicortex, and therefore normal visual signals are not available to drive ipsiversive saccades. However, absent vision also implies that visual error signals are not available to calibrate ipsiversive movements. Furthermore, the innate anatomical substrate needed to support accurate ipsiversive saccade control, in addition to the normal contraversive control, appears sparse. We show here that, in spite of these obstacles, hemidecorticate patients could generate accurate ipsiversive saccades in a task that dissociated hemianopia from saccade direction. In this task, while the patients fixated a central fixation target (FT), saccade targets (STs) were briefly presented to the intact visual hemifield contralateral to the intact hemicortex. The FT was then moved towards and beyond the former location of the ST which evoked tracking eye movements that moved the eyes towards and then beyond the ST, thereby moving the goal, ST, into the blind visual hemifield ipsilateral to the intact hemicortex. When the FT was extinguished, the patients generated, in the dark, ipsiversive saccades that moved their eyes to the remembered location of the ST with the same accuracy as normal control subjects. This indicates that a single hemicortex can mediate accurate bidirectional saccade control via fully functional bilateral connections from cortex to brainstem oculomotor structures. The mechanisms whereby visual signals can calibrate ipsiversive saccades remain elusive.

Brain potentials associated with conscious aftereffects induced by unseen stimuli in a blindsight subject.

The study is of brain activity in a blindsight subject (D.B.), who reports conscious visual afterimages of stimuli of which he is unaware when they are presented. This contrast offered a unique opportunity to study event-related potential recordings of conscious versus unconscious visual phenomena generated by the very same stimulus in the identical locus of the visual field. The behavioral results confirmed the reliability of the difference in the subject's report for inducing stimuli versus their aftereffects. The rationale of the event-related potential analysis was to subtract "on" signals from "off" signals, the latter associated with the onset of conscious events and the former for events that remained unconscious. Because there are inherent differences in on and off potentials, the subtractive resultants for the blind hemifield were compared with the same subtractions for the good hemifield when the subject was aware both of the stimuli and their afterimages. A differential pattern in subtractive resultants emerged with a strong anterior left frontal focus for the blind field and a posterior focus for the intact field. The results are compared with other studies suggesting an anterior focus for conscious visual events.

Nasotemporal overlap of retinal ganglion cells in humans: a functional study.

A zone of overlap along the vertical retinal meridian where ipsi- and contralaterally projecting ganglion cells intermingle has been demonstrated histologically in nonhuman primates. The widening of the zone of overlap in the foveal region was thought to produce a foveal sparing extending 1.5 degrees in the blind hemifield in human hemianopia. The functional relevance of the nasotemporal overlap is still unclear and cannot be shown definitely by conventional perimetry, because of insufficient spatial accuracy, light-scattering effects, and insufficient fixation control. Therefore, this study was undertaken to investigate the vertical field border by a perimetric method that does not have these shortcomings. A scanning laser ophthalmoscope (SLO) was used to scan vertical triplets of dots along the vertical field border in 20 patients (36 eyes) with homonymous hemianopia without macular sparing. Stimuli and fundus were imaged simultaneously for fixation monitoring. None of the patients showed a field border that coincided exactly with the vertical midline. In 34 eyes, the seeing area extended from the vertical meridian into the blind hemifield and formed a vertical strip of perception. None of the patients showed additional foveal sparing. Twenty-two eyes showed a concave shape of the seeing area within the foveal region of the blind hemifield. Our results show that the nasotemporal overlap exists in humans. It consists of a strip of intact perception reaching into the blind hemifield. The concave shape can be explained by the size and distribution of the receptive fields of the retinal ganglion cells.