Lack of anticipatory gaze-orienting responses in patients with right brain damage

OBJECTIVE

To study eye movements during cervical proprioceptive stimulation by passive body rotation in darkness, with the head held stationary, in patients with right brain damage and hemineglect.

BACKGROUND

At very low frequency, this stimulation is reported to produce an illusion of head turning in space and eye deviations directed opposite to trunk rotation (in the direction of the illusory head rotation).

METHODS

Ten normal subjects and seven patients with unilateral cerebral lesions (five right brain-damaged patients with mild to moderate visuospatial neglect, two left brain-damaged patients without neglect) were included in the study. Subjects were seated on a rotating chair. Stimuli consisted of slow sinusoidal passive trunk rotations (+/-30 degrees, 0.01 Hz) while the head was fixed in space.

RESULTS

Eye movements directed opposite to trunk rotation were typical for normal subjects and for left brain-damaged patients. In contrast, all right brain-damaged patients showed either eye movements in the direction of trunk rotation or no eye deviations at all.

CONCLUSION

This result could characterize a lack of anticipatory coordinating gaze behavior in patients with right brain damage.

Imaging the electrical activity of the brain: ELECTRA

The construction of a tomography of neuronal sources is limited by a lack of information. A possible way around this problem is to change the biophysical model that underlies the statement of the inverse problem, i.e., searching for magnitudes that can be better determined from the available data. In this report, we describe a mathematical characterization of the type of currents that are actually able to produce the scalp-recorded EEG. Considering this characterization, we reformulate the bioelectric inverse problem. This approach, called ELECTRA, yields some advantages over the classical formulation in terms of the current density vector: (1) the number of unknowns can be reduced, which is equivalent to increasing the number of independent measurements, (2) the constraints used to reformulate the problem are undeniable since they do not imply any hypothesis about brain function but are instead based on the character of the measurements, and (3) existing experimental evidence suggests that the proposed source model characterizes the type of currents that arise in excitable tissues. We conclude that if the latter fact proves to be true for brain tissues, then no additional information is added to the inverse problem by using a more general source model than the one proposed here. Images obtained using this method for synthetic data, as well as early and middle components of human visual evoked responses to checkerboard stimuli, are presented to illustrate the characteristics of the reconstructed maps and their interpretation.

Imaging the electrical activity of the brain: ELECTRA

The construction of a tomography of neuronal sources is limited by a lack of information. A possible way around this problem is to change the biophysical model that underlies the statement of the inverse problem, i.e., searching for magnitudes that can be better determined from the available data. In this report, we describe a mathematical characterization of the type of currents that are actually able to produce the scalp-recorded EEG. Considering this characterization, we reformulate the bioelectric inverse problem. This approach, called ELECTRA, yields some advantages over the classical formulation in terms of the current density vector: (1) the number of unknowns can be reduced, which is equivalent to increasing the number of independent measurements, (2) the constraints used to reformulate the problem are undeniable since they do not imply any hypothesis about brain function but are instead based on the character of the measurements, and (3) existing experimental evidence suggests that the proposed source model characterizes the type of currents that arise in excitable tissues. We conclude that if the latter fact proves to be true for brain tissues, then no additional information is added to the inverse problem by using a more general source model than the one proposed here. Images obtained using this method for synthetic data, as well as early and middle components of human visual evoked responses to checkerboard stimuli, are presented to illustrate the characteristics of the reconstructed maps and their interpretation.

Formant pattern ambiguity of vowel sounds

The formant frequencies of a particular vowel vary according to the speaker group and to coarticulation. Therefore, overlapping formant patterns of different vowels are commonly related to sex and age differences and to coarticulation, and are considered to concern mainly the F1-F2 pattern of adjacent vowels. However, several studies have reported indications of a correlation between the lower formant frequencies and F0, as well as of the appearance of different formant numbers relevant to vowel identity. As a consequence, the overlap between formant patterns of different vowels might be more substantial than has traditionally been assumed. Within the present study, therefore, the extent to which a given formant pattern can represent different vowels was investigated for natural Swiss German vowels produced monotonously and in isolation by men, women and children at F0 of 85-870 Hz. Similar formant patterns were found for vocalizations of different vowels with both small and large phonetic distances, and within the entire frequency ranges of the formants relevant for phoneme identity. For vowel sounds displaying ambiguous formant patterns, the main spectral characteristics related to differences in their perceptual identity were found to concern F0 and relative formant amplitudes. Results are given in exemplary vowel series, and consequences for the psychophysics of the vowel are discussed.

Temporal parameters and patterns of the foot roll over during walking: normative data for healthy adults

Temporal parameters of the gait cycle and foot roll over in 105 healthy adults (75 women and 30 men aged 16 to 63 years) were collected using foot switches. The subjects walked unobserved at their preferred pace and velocity in a hallway 19 m long and 2.8 m wide. After correction for height, a significant gender influence remained on stance parameters and stride duration. For these adults, age had an effect on the forward tilt of the foot and the double support time only. Differences due to side related only to foot patterning and not stride, stance and swing times. Asymmetry coefficient for temporal parameters of gait cycle and foot roll over revealed the greatest asymmetry in metatarsal head and great toe latency and support. These reference data are considered valid in a laboratory using sensors and signal processing comparable to ours.

Psychosocial functioning in chronic epilepsy: relation to hippocampal volume and histopathological findings

MRI volumetric measurements of the ipsi- and contralateral hippocampal volume (HV) were performed in a group of 37 patients with lesional and non-lesional temporal (TLE), and extratemporal lobe (ETLE) lobe epilepsy. Twenty-six patients underwent surgery and the resected tissues subjected to histopathological examination. Psychosocial scores were calculated on the basis of employment, partnership/family status and presence of a positive psychiatric history. Poorest values were obtained for the group with right lesional and non-lesional TLE (RTLE, p < 0.001). In this group, significant positive correlations between psychosocial functioning and the left hippocampal volume or age of onset were observed. We found that the left hippocampal volume in RTLE was smaller with increasing epilepsy duration. We conclude that patients with RTLE represent a particular group with regard to the structural and psychosocial consequences of chronic epilepsy. With regard to histopathological analysis, no particular type of lesion was identified as the major determinant of the results described above.

Evidence for interhemispheric motor-level transfer in a simple reaction time task: an EEG study

Simple visuomanual reaction time tasks require interhemispheric communication when stimuli are presented in the hemifield opposite the responding hand. Although confirmed in many studies, it is still a matter of debate when, at what functional level and at what site this interhemispheric transfer takes place. To address these questions, we recorded event-related potentials (ERPs) in 12 healthy subjects performing such a task and analyzed the data using techniques based on topographic ERP map characteristics. A method which has proved useful for associating ERP map configurations of different time periods with functional states of the brain was supplemented by a source localization procedure. The results suggest that transfer occurs late in time, on a functional motor level and at frontal sites, at least for left-to-right interhemispheric direction of transfer.

Plant-induced seizures: reappearance of an old problem

Several plant-derived essential oils have been known for over a century to have epileptogenic properties. We report three healthy patients, two adults and one child, who suffered from an isolated generalized tonic-clonic seizure and a generalized tonic status, respectively, related to the absorption of several of these oils for therapeutic purposes. No other cause of epilepsy was found, and outcome was good in the two adult cases, but the course has been less favorable in the child. A survey of the literature shows essential oils of 11 plants to be powerful convulsants (eucalyptus, fennel, hyssop, pennyroyal, rosemary, sage, savin, tansy, thuja, turpentine, and wormwood) due to their content of highly reactive monoterpene ketones, such as camphor, pinocamphone, thujone, cineole, pulegone, sabinylacetate, and fenchone. Our three cases strongly support the concept of plant-related toxic seizure. Nowadays the wide use of these compounds in certain unconventional medicines makes this severe complication again possible.

Innate and learned components of human visual preference

BACKGROUND

Recent claims in neuroscience and evolutionary biology suggest that the aesthetic sense reflects preferences for image signals whose characteristics best fit innate brain mechanisms of visual recognition.

RESULTS

This hypothesis was tested by behaviourally measuring, for a set of initially unfamiliar images, the effects of category learning on preference judgements by humans, and by relating the observed data to computationally reconstructed internal representations of categorical concepts. Category learning induced complex shifts in preference behaviour. Two distinct factors - complexity and bilateral symmetry - could be identified from the data as determinants of preference judgements. The effect of the complexity factor varied with object knowledge acquired through category learning. In contrast, the impact of the symmetry factor proved to be unaffected by learning experience. Computer simulations suggested that the preference for pattern complexity relies on active (top-down) mechanisms of visual recognition, whereas the preference for pattern symmetry depends on automatic (bottom-up) mechanisms.

CONCLUSIONS

Human visual preferences are not fully determined by (objective) structural regularities of image stimuli but also depend on their learned (subjective) interpretation. These two aspects are reflected in distinct complementary factors underlying preference judgements, and may be related to complementary modes of visual processing in the brain.

From cortical plasticity to unawareness of visual field defects

It was long held that, following alterations in sensory input, structural changes in the primary visual cortex take place only in early life, during so-called "critical periods." Recently, however, it has been established that, in adults, cortical maps in the brain are not fixed, and the cortex does not perform stereotyped operations. Instead, neuronal receptive fields in the cortex can reorganize following deactivation or an altered pattern of activation. Plasticity is essential for the normal adjustment of the brain to modifications in the sensory environment, and for improving perceptual skills and sensorimotor performances. It also plays a crucial role in recovery from damage to the visual system. Cortical remapping generates a filling-in of visual field defects. Consequently, it alters the image perceived. Cortical rearrangement following lesions in the visual pathways does not restore function to the destroyed tissue, but it helps to compensate for gaps in perception. In this review article, we focus on effects of plasticity in the adult visual cortex which are of major importance in the daily practice of neuroophthalmology. Cortical reorganization, together with resulting filling-in, affects the early recognition and evaluation of visual field defects. The importance of brain remapping in these matters is still largely underestimated by clinicians.

Spatio-temporal analysis of electric brain activity during semantic and phonological word processing

There is an ongoing debate in cognitive neuroscience about the time course and the functional independence of the different processes involved in encoding written language material. New data indicate very fast and highly parallel language analysis networks in the brain. Here we demonstrate a methodological approach to study the temporal dynamics of this network by searching for time periods where different task demands emphasize different aspects of the network. Multi-channel event related potentials (ERPs) were recorded during a semantic and a phonological reading task from 14 healthy subjects. Signals were analyzed exclusively on the basis of the spatial configuration of the electric potential distributions (ERP maps), since differences in these spatial patterns directly reflect changes in the configuration of the active sources in the brain. This analysis did not reveal any differences of the evoked brain electric fields between the two tasks up to 280 ms post-stimulus. The ERP maps then differed for a brief period between 280 and 380 ms, before they were similar again. The analysis of the maps using a global linear localization procedure revealed a network of areas, active in both tasks, that mainly involved the left postero-temporal and left antero-temporal regions. The left posterior activation was found already around 100 ms post-stimulus, indicating that language-specific functions appear early in time. We therefore conclude that phonological and semantic processing are essentially performed in both tasks and that only late decision-related processes influence the relative strength of activity of the different modules in the complex language network.

A pure case of Gerstmann syndrome with a subangular lesion

The four symptoms composing Gerstmann's syndrome were postulated to result from a common cognitive denominator (Grundstörung) by Gerstmann himself. He suggested that it is a disorder of the body schema restricted to the hand and fingers. The existence of a Grundstörung has since been contested. Here we suggest that a common psychoneurological factor does exist, but should be related to transformations of mental images rather than to the body schema. A patient (H.P.) was studied, who presented the four symptoms of Gerstmann's syndrome in the absence of any other neuropsychological disorders. MRI showed a focal ischaemic lesion, situated subcortically in the inferior part of the left angular gyrus and reaching the superior posterior region of T1. The cortical layers were spared and the lesion was seen to extend to the callosal fibres. On the basis of an extensive cognitive investigation, language, praxis, memory and intelligence disorders were excluded. The four remaining symptoms (finger agnosia, agraphia, right-left disorientation and dyscalculia) were investigated thoroughly with the aim of determining any characteristics that they might share. Detailed analyses of the tetrad showed that the impairment was consistently attributable to disorders of a spatial nature. Furthermore, cognitive tests necessitating mental rotation were equally shown to be impaired, confirming the essentially visuospatial origin of the disturbance. In the light of this report, the common cognitive denominator is hypothesized to be an impairment in mental manipulation of images and not in body schema.

Spatiotemporal EEG analysis and distributed source estimation in presurgical epilepsy evaluation

In the attempts to localize electric sources in the brain on the basis of multichannel EEG and/or MEG measurements, distributed source estimation procedures have become of increasing interest. Several commercial software packages offer such localization programs and results using these methods are seen more and more frequently in the literature. It is crucial that the users understand the similarities and differences of these methods and that they become aware of the advantages and limitations that are inherent to each approach. This review provides this information from a theoretical as well as from a practical point of view. The theoretical part gives the algorithmic basis of the electromagnetic inverse problem and shows how the different a priori assumptions are formally integrated in these equations. The authors restrict this formalism to the linear inverse solutions i.e., those solutions in which the inversion procedure can be represented as a matrix applied to the data. It will be shown that their properties can be best characterized by their resolution kernels and that methods with optimal resolution matrices can be designed. The authors also discuss the important problem of regularization strategies that are used to minimize the influence of noise. Finally, a new kind of inverse solution, termed ELECTRA (for ELECTRical Analysis), is presented that is based on constraining the source model on the basis of the currents that can actually be measured by the scalp recorded EEG. The practical part of the review illustrates the localization procedures with different clinical data sets. Three aspects become important when working with real data: 1) Clinical data is usually far from ideal (limited number of electrodes, noise, etc.). The behavior of inverse procedures in such unfortunate situations has to be evaluated. 2) The selection of the time points or time periods of interest is crucial, especially in the analysis of spontaneous EEG. 3) Additional information coming from other modalities is usually available and can be incorporated. The authors are illustrating these important points in the case of interictal and ictal epileptiform activity. Spike averaging, frequency domain source localization, and temporal segmentation based on electric field topographies will be discussed. Finally, the technique of EEG-triggered functional magnetic resonance imaging (fMRI) will be illustrated, where EEG is recorded in the magnet and is used to synchronize fMRI acquisition with interictal events. The analysis of both functional data, i.e. the EEG in terms of three-dimensional source localization and the EEG-triggered fMRI, combines the advantages of the two techniques: the temporal resolution of the EEG and the spatial resolution of the fMRI.

Visual activity in the human frontal eye field

Although visual information processing in the monkey frontal eye field (FEF) has been well demonstrated, the contribution of its human homologue to vision is still unknown. Here we report a study of intracranial visual evoked potentials (VEPs) recorded from the human FEF which was identified by electrical cortical stimulation. Electrical stimulations and EEG recordings were carried out via subdural grid electrodes placed over the frontal cortex in three epileptic patients. Evoked eye movements were mainly horizontal and always directed to the hemispace contralateral to the stimulation site. Intracranial VEPs showed responses predominately to stimuli in the contralateral visual field. Our findings demonstrate a close relationship between the direction of the electrically elicited eye movements and the visual stimulus location which predominantly leads to neural responses in the FEF. These findings provide evidence for the functional role of the human FEF in the analysis of visual stimuli from the contralateral visual field as well as in the generation of eye movements towards these conspicuous targets.

A clinical study of motor evoked potentials using a triple stimulation technique

Amplitudes of motor evoked potentials (MEPs) are usually much smaller than those of motor responses to maximal peripheral nerve stimulation, and show marked variation between normal subjects and from one stimulus to another. Consequently, amplitude measurements have low sensitivity to detect central motor conduction failures due to the broad range of normal values. Since these characteristics are mostly due to varying desynchronization of the descending action potentials, causing different degrees of phase cancellation, we applied the recently developed triple stimulation technique (TST) to study corticospinal conduction to 489 abductor digiti minimi muscles of 271 unselected patients referred for possible corticospinal dysfunction. The TST allows resynchronization of the MEP, and thereby a quantification of the proportion of motor units activated by the transcranial stimulus. TST results were compared with those of conventional MEPs. In 212 of 489 sides, abnormal TST responses suggested conduction failure of various degrees. By contrast, conventional MEPs detected conduction failures in only 77 of 489 sides. The TST was therefore 2.75 times more sensitive than conventional MEPs in disclosing corticospinal conduction failures. When the results of the TST and conventional MEPs were combined, 225 sides were abnormal: 145 sides showed central conduction failure, 13 sides central conduction slowing and 67 sides both conduction failure and slowing. It is concluded that the TST is a valuable addition to the study of MEPs, since it improves detection and gives quantitative information on central conduction failure, an abnormality which appears to be much more frequent than conduction slowing. This new technique will be useful in following the natural course and the benefit of treatments in disorders affecting central motor conduction.

The "thin man" phenomenon: a sign of cortical plasticity following inferior homonymous paracentral scotomas

AIM

To investigate an image distortion, experienced by patients with homonymous paracentral scotomas.

METHODS

Two consecutive patients with right inferior homonymous paracentral scotomas resulting from ischaemic brain insults were examined. Neuro-ophthalmological examination included tangent screen and Amsler grid evaluation. In addition, the patients were asked to describe a figure showing two vertical lines, identical in length and symmetrically located on either side of a fixation point. This figure was presented in such a way that when the subject looked at the fixation point the right line crossed the scotoma. Finally, the patients were asked whether, when looking at the face of an interlocutor, both sides of the body looked the same.

RESULTS

In both patients field defects were markedly smaller when delineated with Amsler grids than using a tangent screen. With the parallel line test, the right line appeared uninterrupted in patient 1, whereas in patient 2 it looked slightly blurred in a two degree long segment corresponding to the middle of the scotoma. To both subjects the right line appeared shorter than the left line. Finally both subjects indicated that, after steadily fixating their interlocutor's face or neck for 5-10 seconds, the left shoulder appeared narrower than the right one, which made him look surprisingly thin. This perceptual alteration was called the "thin man" phenomenon.

CONCLUSIONS

Paracentral homonymous scotomas can be associated with perceptual completion and shape distortion, owing to apparent displacement of images adjacent to the scotoma towards the field defect. Occurrence of such a perceptual change should alert one to the possibility of paracentral homonymous scotomas, which often go undetected when using routine visual field testing procedures.

Semantically-triggered reading epilepsy: an experimental case study

Primary reading epilepsy (PRE) is a rare syndrome in which epileptic seizures are electively provoked by reading. Cognitive neuropsychology has demonstrated the existence of at least two pathways for reading, the sublexical pathway involved in converting graphemes to phonemes, and the lexical pathway used when meaning is conveyed. Which of these specific pathways is relevant in triggering epileptic discharges remains largely unknown. We report the case of a patient suffering from PRE in which the two routes were distinguished on the basis of the reading material employed. Significantly less epileptic discharges were observed when the patient read non-words than words. In view of our findings, we tentatively contrast a lexical form of PRE, triggered by the activation of semantic knowledge structures, with a sublexical form, triggered by non-word reading. Evidence from the literature suggests that the former is characterized by bilateral EEG activating patterns, whereas the latter involves preferentially the left hemisphere.

Frequency domain EEG source localization of ictal epileptiform activity in patients with partial complex epilepsy of temporal lobe origin

The aim of this study was to investigate whether EEG source localization in the frequency domain, using the FFT dipole approximation (Lehmann, D. and Michel, C.M. Electroenceph. clin. Neurophysiol., 1990, 76: 271-276), would be useful for quantifying the frequency content of epileptic seizure activity. Between one and 7 extracranially recorded seizures were analyzed in each of 7 patients with mesolimbic epilepsy, who were seizure-free after temporal lobe resection. The full scalp frequency spectrum for the first 4 s after seizure onset, as well as for subsequent periods, was determined. Power peaks in the spectra were identified, and an instant dipole fit was performed for the frequencies corresponding to these peaks. Ictal frequencies, ranging between 3.5 and 8.5 Hz, showed a variable degree of stability over time in the different patients. For a particular frequency, dipole results were similar during the different phases of seizure development. In patients with more than one prominent frequency, dipole results for the different frequencies were similar. Dipole results were also similar between patients. We conclude that dipole localization of dominant frequencies, as obtained from full scalp FFT analysis, gives quite reproducible results for seizures originating in the mesial temporal area. The method may become a useful tool for the pre-surgical identification of patients with mesolimbic epilepsy.

Hallucinatory experiences in extreme-altitude climbers

OBJECTIVE

This study attempted a systematic investigation of incidence, type, and circumstances of anomalous perceptual experiences in a highly specialized group of healthy subjects, extreme-altitude climbers.

BACKGROUND

There is anecdotal evidence for a high incidence of anomalous perceptual experiences during mountain climbing at high altitudes.

METHOD

In a structured interview, we asked eight world-class climbers, each of whom has reached altitudes above 8500 m without supplementary oxygen, about hallucinatory experiences during mountain climbing at various altitudes. A comprehensive neuropsychological, electroencephalographic, and magnetic resonance imaging evaluation was performed within a week of the interview (8).

RESULTS

All but one subject reported somesthetic illusions (distortions of body scheme) as well as visual and auditory pseudohallucinations (in this order of frequency of occurrence). A disproportionately large number of experiences above 6000 m as compared to below 6000 m were reported (relative to the total time spent at these different altitudes). Solo climbing and (in the case of somesthetic illusions) life-threatening danger were identified as probable triggers for anomalous perceptual experiences. No relationship between the number of reported experiences and neuropsychological impairment was found. Abnormalities in electroencephalographic (3 climbers) and magnetic resonance imaging (2 climbers) findings were likewise unrelated to the frequency of reported hallucinatory experiences.

CONCLUSIONS

The results confirm earlier anecdotal evidence for a considerable incidence of hallucinatory experiences during climbing at high altitudes. Apart from hypoxia, social deprivation and acute stress seem to play a role in the genesis of these experiences.

To see better to the left when looking more to the right: effects of gaze direction and frames of spatial coordinates in unilateral neglect

Unilateral spatial neglect entails a failure to detect or respond to stimuli in the space opposite to a brain lesion. However, the contralesional hemispace can be determined by different frames of spatial coordinates, such as eyes-, head-, body-, or environment-centered coordinates. We observed 2 patients with a right hemisphere stroke whose left spatial neglect was modulated by distinct coordinates systems depending on the task. Four tasks were given in different conditions of central gaze and either the eyes or the head rotated 30 degrees to the right or 30 degrees to the left. While the 2 patients had a retinotopic defect in 1 visual field quadrant that remained the same irrespective of gaze direction (upper or lower quadrant in 1 case each), the other quadranopic field defect improved with eyes rotation to the right but not with head rotation, suggesting a head-centered spatiotopic deficit. Performance on line bisection was influenced both by eyes and head rotation, as well as by the position of the lines with respect to the trunk midline, suggesting the involvement of both head-centered and body-centered coordinates. Visual imagery and auditory extinction were not modified by changing the eyes or head position. These findings suggest that distinct spatial coordinates are brought into play depending on the tasks demands.

Periodic downbeat nystagmus

The authors observed a periodic downbeat nystagmus with a cycle of 3 minutes 30 seconds, beating downward for a period of 90 seconds every 2 minutes. It lasted 86 hours in a patient with severe hypomagnesemia associated with complications from scleroderma. There was no lesion on brain imaging, and an EEG performed during the nystagmus was normal, indicating that the most likely explanation for this periodic nystagmus is metabolic. This form of nystagmus may have resulted from severe hypomagnesemia, possibly associated with thiamine deficiency.

Comprehensive postictal neuropsychology improves focus localization in epilepsy

In recent years, new techniques such as single photon emission computed tomography (SPECT), positron emission tomography (PET) and magnetic resonance imaging (MRI) have been used to improve the localization of epileptic foci during the noninvasive evaluation procedure for epilepsy surgery. Since ictal/or immediate postictal SPECT studies were shown to localize epileptic foci better than interictal SPECT, we addressed the question of whether immediate postictal neuropsychological examination would show the same effect. Neuropsychological examinations were carried out postictally and interictally using a broad range of tests. Postictal results were analyzed with regard to lateralizing and localizing information about the epileptogenic region. Seventeen patients suffering from temporal and extratemporal pharmacoresistant epilepsy were investigated postictally with a subset of tests used for the interictal neuropsychological examination. A significant improvement in focus localization was seen in comparison with interictal neuropsychology (p = 0.014). We conclude that neuropsychology can yield lateralizing and sometimes localizing information, even for extratemporal foci, if carried out during the postictal period and based on a global analysis of the clinical neuropsychological picture.

Illusory contours and spatial neglect

It has been controversial whether the perception of illusory contours arise from higher level cognitive mechanisms that require attention or from early preattentive visual processes. We studied three patients with left spatial neglect who were unable to detect the left inducers of Kanizsa illusory figures in a same/different judgment task but nonetheless showed implicit perception of the figures in a midpoint judgment, in that they made identical bisection for figures with illusory or real contours but very different bisection for other spatially discontinuous figures that did not yield illusory filling-in. Grouping and filling-in mechanisms can thus occur without explicit detection of, or attention to, the inducing features, consistently with the hypothesis that they involve preattentive visual processes.