After staring at a pattern of tilted lines, subsequent lines appear to be tilted in the opposite direction (direct tilt aftereffect, TAE). In a previous fMRI study we have demonstrated a direct TAE solely induced by the mental imagination accompanied by adaptation of orientation-selective neurons located in the extrastriate cortex, supporting the assumption of a perception-like coding of mental images. In this study we enlarge and specify the evidence for a perception-like coding of orientation-imagination. First, we replicated the previously detected direct TAE induced by line imagination with altered design-variations to control possible perceptual task confounds. Second, we tried to induce two other orientation-specific aftereffects: indirect TAE and contrast-threshold elevation aftereffect by mental imagery. The results replicate a robust direct TAE by mental imagery and by visual stimulation, with no influence of attentional resource allocation or perceptual task confounds. We could not induce an indirect TAE, but observed a perception bias in the opposite direction of the indirect TAE. The mental imagery of lines induced no orientation-selective contrast-threshold elevation aftereffect. In general, mental imagery seems to influence visual perception, indicating that perceptual resources are used by mental imagery. However, the utilisation of visual resources seems to be somewhat different from utilisation by perception.

Purpose: To investigate the visual preferences of human infants and toddlers, using tasks borrowed from adult visual search literature. Method: Infants aged between 2 and 12 months and children between 1 and 4 years, grouped in five age groups (3, 6, 10, 18 and 36 months old; 12 subjects/group) were tested with a preferential looking procedure. The stimuli were presented on cardboard cards containing a target item among 15 distracting items. The task of a naive observer was to make a forced-choice judgement on the side of the card preferred by the subject. Correct guesses yielded a positive score, incorrect guesses a negative one. The tasks investigated were “brightness contrast” (a single dark blob amidst white blobs, or a bright blob amidst dark blobs, on a gray background) and “orientation contrast” (a tilted line amidst vertical lines, or a vertical line amidst tilted lines). Results: Three-year-old children always preferred the discrepant target. Their preference showed an asymmetry consistent with adult visual search asymmetry: they had a higher preference for the darker blob and the tilted line than for the brighter blob and the vertical line. In contrast, three-month-olds showed a positive preference for the darker blob, but no preference for the brighter blob or for lines differing in orientation from their surround. Transition from the infantile to the mature pattern of visual preferences occurred around the end of the first year of age. Conclusion: These results corroborate earlier findings (Sireteanu & Encke, IOVS 1999; 40,4:343, Sireteanu, Wagner & Bachert, IOVS 2001; 42,4:122) and show that the human infant enters the world with a visual repertoire dramatically different from that of adult observers. The sharp transition between infancy and toddlerhood points to the emergence of different brain mechanisms, which mediate a qualitatively different pattern of visual preferences.

The purpose of the experiments was to investigate the spatial and especially the temporal distortions in amblyopic vision, and to relate them to the orthoptic status and the contrast sensitivity of the amblyopic subjects, as well as to their performance in psychophysical tasks. In the first experiment participants were asked to describe their perception of the amblyopic eye for patterns of different spatial frequencies (0.4 − 3.2 c/deg). In the second and third experiments, subjects had to reconstruct concentric circles point by point. The radial position of each point was to be recontructed after visual presentation, while the angular position was presented either auditively (second experiment) or visually (third experiment). Twelve out of 14 amblyopes perceived spatial distortions. Five out of 10 strabismic and strabismic-anisometropic, but only 1 out of 4 anisometropic amblyopes perceived temporal instability. Temporal distortions were seen only for spatial frequencies higher than 1.6 c/deg. Temporal instability involved either the whole pattern or only some parts of it. There was no relationship of contrast sensitivity loss to the type or severity of distortion. In experiments 2 and 3, considerable distortions were perceived in the amblyopic eye for strabismic and strabismic-anisometropic amblyopes. In addition, subjects who perceived temporal instability showed a significantly impaired performance in adjusting the auditively presented points. The experiments suggest that strabismus, in addition to amblyopia, is needed to elicit significant spatial and temporal distortions. Temporal instability appears in addition to spatial distortions and has a negative impact on the performance in psychophysical tasks. Strabismic amblyopes show deficits in auditory-to-visual mapping, suggesting an impairment of the dorsal pathway, in addition to the known deficits of the ventral visual pathway.

Simultaneous binocular input during a critical period after birth is necessary to maintain and develop the structure of the visual system, otherwise visual processing might be disrupted. If binocular vision is impaired during this period subjects can develop alternating fixation (ensuring normal monocular function of each eye) or amblyopia (vision in one eye is impaired). Testing the interocular transfer of figural adaptation after-effects is one way to investigate binocular integration of the visual input psychophysically. In this study, we investigated binocular integration using orientation-selective fMRI adaptation. We tested 20 normally-sighted subjects and 18 subjects with impaired binocular visual development (10 with alternating fixation and 8 with unilateral amblyopia). In all investigated cortical areas, normally-sighted subjects showed significant monocular orientation-selective adaptation that partially transferred to the non-adapted eye. Observers with impaired visual development showed monocular orientation-selective adaptation, but this adaptation did not transfer to the non-adapted eye. These results demonstrate that fMRI adaptation is a useful tool for the investigation of the neuronal mechanisms of binocular integration in the adult human brain.

Amblyopic subjects show a wide range of changes within the visual system, starting from deficits in simple perceptual processing up to changes of higher functions of the visual dorsal pathway. Recent studies suggest that subjects with amblyopia also demonstrate alterations in visuo-spatial attention. In contrast to normal sighted subjects, who demonstrate a leftward bias ("pseudoneglect") during physical line-bisection, amblyopic subjects show a rightward bias ("minineglect"). Numerous studies have provided considerable evidence that humans represent numbers on a mental number line oriented from left to right, partly analogous to a physical line. In the present study, we investigated whether number representation is also changed in amblyopic subjects. Participants were asked to vocally name the numerical midpoint of two acoustically presented numbers without calculating. Supporting other studies, two control groups of normal sighted participants (each with n=14, group 1 matched for age and gender, group 2 matched for eyedness and education) underestimated the mean number, analogous to a leftward bias ("pseudoneglect"). Interestingly, amblyopic subjects (n=14) estimate the mean numbers less biased. These results support the assumption, that amblyopia also influences higher cognitive processes like number processing. Surprisingly, amblyopia seems to induce a more balanced number processing in contrast to healthy participants.

We devised an experimental strategy for assessing the cortical cross-talk between ocular subsystems. For this purpose we measured the interocular transfer of adaptation (IOTA) at different levels in the human brain, using orientation-selective fMRI adaptation. We tested 10 normally sighted and 10 stereoblind or stereodeficient amblyopic observers by adapting monocularly to phase-reversing, oblique sinusoidal gratings. Following monocular adaptation, cortical activations evoked by the same (monoptic) or the other eye (interocular) were measured for the same and for the orthogonal orientation in a two by two factorial design. In both experimental groups, we obtained significant orientation-selective monocular adaptation in area V1 and in extrastriate regions on the dorsal and ventral visual pathways. In the normally-sighted subjects we found in addition interocular adaptation in V1 and extrastriate visual areas. This interocular adaptation indicates that fMRI adaptation transfers from the adapted ocular subsystem to the non-adapted ocular subsystem, and thus provides a measure of binocular interaction in normally-sighted subjects. In the amblyopic subjects, no interocular adaptation was seen at any of the investigated cortical levels, regardless of which eye was adapted. We suggest that the abnormal pattern of interocular transfer of fMRI adaptation is related to the disturbed integration of binocular signals in amblyopia.

Previous studies have shown that prolonged inspection of a tilted visual pattern leads to changes in perception (“tilt after-effect”, TAE), as well as to a reduction of the neural activation evoked by this pattern (“neural adaptation”). In this fMRI study, we investigated whether such perceptual and neural adaptation can be induced solely by mental imagination. The subjects were asked to mentally generate tilted lines, after which they were presented test lines oriented in the same or the direction orthogonal to the mentally tilted lines. Subjects showed a TAE even after mental imagery (“mental TAE”). Furthermore, a significant orientation-specific adaptation occurred in extrastriate visual areas (V3–V4), showing a decreasing gradient of adaptation from areas V4 to V1. Both this neural adaptation and the individual size of the mental TAE correlated with performance in a behavioural task probing mental imagery. Thus, orientation-selective neurons in visual areas seem to be recruited by mental imagery, and the amount of recruitment correlates with the degree of success of mental imagery operations. The influence of mental imagery on perception and on the neural activity in extrastriate visual areas provides evidence for analogue coding of mental images.

Sudden events and sharp discontinuities in the external world act as powerful attention attractors in adult humans. Does this reflex-like orienting towards targets deviating from their surround occur in early infancy? Here, we present evidence that, during the first months of life, infants orient preferentially towards repetitive visual patterns, rather than towards uniquely deviating targets. At 3-4 years of age, toddlers show an adult-like pattern of preferences. The transition from the infantile to the adult-like preferences occurs after the end of the first year of age. This development is parallelled by the emergence of novel neural and cognitive mechanisms. These maturational events might reflect the remodeling of the human brain during the transition from infancy to toddlerhood.

Dichoptic stimuli (different stimuli displayed to each eye) are increasingly being used in functional brain imaging experiments using visual stimulation. These studies include investigation into binocular rivalry, interocular information transfer, three-dimensional depth perception as well as impairments of the visual system like amblyopia and stereodeficiency. In this paper, we review various approaches of displaying dichoptic stimulus used in functional magnetic resonance imaging experiments. These include traditional approaches of using filters (red-green, red-blue, polarizing) with optical assemblies as well as newer approaches of using bi-screen goggles.

Neurologically normal observers show a consistent leftward bias when asked to bisect a horizontal line (“pseudoneglect”). In this study, we found that subjects with strabismic and strabismic-anisometropic amblyopia show a consistent rightward bias (“minineglect”) in a line bisection task. The bias was seen in both eyes, but affected more strongly the amblyopic eye. Purely anisometropic amblyopes show a similar bias, affecting only the amblyopic eye. The group of strabismics with alternating fixation did not differ significantly from normal observers. These errors are reminiscent of the attentional neglect of the left extrapersonal space, shown by subjects with lesions in the right posterior parietal cortex. We suggest that an early strabismus might lead to a functional deficit of the dorsal cortical pathway, in addition to the well-known impairments on the ventral visual pathway. We conclude that strabismic amblyopes might show subtle attentional deficits, in addition to their unilateral vision loss.

Visual hallucinations can occur in healthy subjects during prolonged visual deprivation. We investigated the visual percepts and the associated brain activity in a 37-year-old healthy female subject who developed visual hallucinations during three weeks of blindfolding, and then compared this activity with the cortical activity associated with mental imagery of the same patterns. We acquired fMRI data with a Siemens 3T Magnetom Allegra towards the end of the deprivation period to assess hallucination-related activity, and again after recovery from blindfolding to measure imagery-related activity. Detailed subjective descriptions and graphical illustrations were provided by the subject after blindfolding was completed. The subject reported the occurrence of simple and elementary hallucinations, consisting of flashes and coloured and moving patterns during the period of blindfolding. Neural activity related to hallucinations was found in extrastriate occipital, posterior parietal, and several prefrontal regions. In contrast, mental imagery of the same percepts led to activation in prefrontal, but not in posterior, parietal, and occipital regions. These results suggest that deprivation-induced hallucinations result from increased excitability of extrastriate visual areas, while mentally induced imagery involves active read-out under the volitional control of prefrontal structures. This agrees with the subject's report that visual hallucinations were more vivid than mental imagery.

We investigated the patterns of two-dimensional spatial distortions in human amblyopia, using three different psychophysical mapping procedures. Strabismic and strabismic–anisometropic amblyopes showed consistent distortions, consisting in enlargement, shrinkage, or torsion of portions of the tested visual field. Purely anisometropic amblyopes and strabismics with alternating fixation showed increased spatial uncertainty, but no consistent distortions. For all groups of subjects, there was a very good correspondence between the patterns of distortion obtained with the three methods. We conclude that the spatial distortions are robust across different procedures. They might reflect a genuine rearrangement of the cortical topography as a result of strabismus.

In order to test the hypothesis of attentional deficits in dyslexia, we investigated the performance of children with developmental dyslexia on a number of visual search tasks. When tested with conjunction tasks for orientation and form using complex, letter-like material, dyslexic children showed an increased number of errors accompanied by faster reaction times in comparison to control children matched to the dyslexics on age, gender, and intelligence. On conjunction tasks for orientation and color, dyslexic children were also less accurate, but showed slower reaction times than the age-matched control children. These differences between the two groups decreased with increasing age. In contrast to these differences, the performance of dyslexic children in feature search tasks was similar to that of control children. These results suggest that children with developmental dyslexia present selective deficits in complex serial visual search tasks, implying impairment in goal-directed, sustained visual attention.

In order to test the hypothesis of attentional deficits in dyslexia, we investigated the performance of children with developmental dyslexia on a number of visual search tasks. When tested with conjunction tasks for orientation and form using complex, letter-like material, dyslexic children showed an increased number of errors accompanied by faster reaction times in comparison to control children matched to the dyslexics on age, gender, and intelligence. On conjunction tasks for orientation and color, dyslexic children were also less accurate, but showed slower reaction times than the age-matched control children. These differences between the two groups decreased with increasing age. In contrast to these differences, the performance of dyslexic children in feature search tasks was similar to that of control children. These results suggest that children with developmental dyslexia present selective deficits in complex serial visual search tasks, implying impairment in goal-directed, sustained visual attention.

PURPOSE. To investigate the relationship between the subjectively experienced misperceptions and the objectively determined two-dimensional spatial displacement maps in subjects with strabismic and anisometropic amblyopia.METHODS. Seventeen experimental subjects were asked to describe and sketch their perception of simple geometric pattern, as perceived through their amblyopic eyes. A subgroup of 15 subjects participated in a psychophysical experiment, in which the two-dimensional displacement maps were determined by asking the subjects to reconstruct, point-by-point, memorized circles of different radii. The results of these displacement maps were related to the clinical characteristics and the perceptual descriptions of the same subjects.RESULTS. Twelve of the 17 investigated subjects experienced spatial distortions; six subjects perceived temporal instabilities, either in addition, or in the absence of spatial distortions. Objectively determined spatial displacement and spatial uncertainty were significantly larger in subjects with a history of strabismus and a deep acuity loss than in subjects with refractive etiology and a mild acuity loss. Subjects experiencing temporal instability showed more spatial uncertainty in the amblyopic eye than did subjects with a stable perception.CONCLUSIONS. These results suggest that a history of strabismus and a deep amblyopia are more likely to be associated with temporal misperceptions than a refractive etiology and a mild acuity loss. A temporally unstable perception may be related to a more profound disorganization of the central neural pathways connected to the amblyopic eye.

BACKGROUND:The aim of this study was to provide a description of the spatial distortions and temporal instability in amblyopic vision, and to attempt to define a cortical substrate of the spatial distortions in strabismic amblyopia.MATERIAL AND METHODS:The perceptual distortions and instabilities occurring in amblyopic vision were investigated psychophysically, by asking 17 subjects to describe and sketch their percepts. This was then visualised with an animated computer programme and validated by the subjects. In a second experiment, the cortical responses of normal observers to patterns corresponding to the spatial distortions reported by amblyopic subjects were investigated using functional magnetic resonance imaging.RESULTS:Spatial distortions were more marked in strabismic than in anisometropic amblyopes or in strabismic subjects with alternating fixation. Temporal instability occurred mainly in strabismic amblyopes and affected mainly patterns with higher spatial frequencies. Experiments with functional magnetic resonance imaging showed that the patterns with the highest spatial distortions yield increased activation in the primary visual cortex of normally-sighted observers.CONCLUSIONS:The results of the imaging experiment suggest that the occurrence of spatial distortions might explain the higher activation in the primary visual cortex of some amblyopic subjects. The occurrence of temporal instability in strabismic amblyopia suggests an involvement of higher-order, extrastriate visual areas of the dorsal, "where" visual pathway in amblyopia, in addition to the known deficits in the ventral, "what" visual pathway.

The aim of this study was to investigate the spatial and temporal distortions that occur in strabismic and anisometropic amblyopic vision. Twelve subjects with strabismic (n = 4), anisometropic (n = 4), mixed amblyopia (n = 3) and bilateral refractive amblyopia (n = 1) were asked to describe and sketch their subjective percept of different geometrical patterns, as seen with their amblyopic eye. Based on their descriptions, computer-animated patterns were generated, which were then validated by the subjects. Both spatial distortions and temporal instability were perceived mainly by strabismic and strabismic-anisometropic amblyopes. Temporal instability occurred mainly at high spatial frequencies. Our data suggest that strabismus, in addition to amblyopia, is needed to elicit significant spatial and temporal distortions. The occurrence of these distortions may be related to the early history of each subject.