Yoram Bonneh

Motion-induced blindness in normal observers

Cases in which salient visual stimuli do not register consciously are known to occur in special conditions, such as the presentation of dissimilar stimuli to the two eyes or when images are stabilized on the retina. Here, we report a striking phenomenon of ‘visual disappearance’ observed with normal-sighted observers under natural conditions. When a global moving pattern is superimposed on high-contrast stationary or slowly moving stimuli, the latter disappear and reappear alternately for periods of several seconds. We show that this motion-induced blindness (MIB) phenomenon is unlikely to reflect retinal suppression, sensory masking or adaptation. The phenomenology observed includes perceptual grouping effects, object rivalry and visual field anisotropy. This is very similar to that found in other types of visual disappearance, as well as in clinical cases of attention deficits, in which partial invisibility might occur despite the primary visual areas being intact. Disappearance might reflect a disruption of attentional processing, which shifts the system into a winner-takes-all mode, uncovering the dynamics of competition between object representations within the human visual system.

Opposite Neural Signatures of Motion-Induced Blindness in Human Dorsal and Ventral Visual Cortex

Motion-induced blindness (MIB) is a visual phenomenon in which a salient static target spontaneously fluctuates in and out of visual awareness when surrounded by a moving mask pattern. It has been hypothesized that MIB reflects an antagonistic interplay between cortical representations of the static target and moving mask. Here, we report evidence for such antagonism between human ventral and dorsal visual cortex during MIB. Functional magnetic resonance imaging (fMRI) responses in ventral visual area V4 decreased with the subjective disappearance of the target. These response decreases were specific for the cortical subregion corresponding retinotopically to the target, occurred early in time with respect to the perceptual report, and could not be explained by shifts of attention in reaction to target disappearance. At the same time, responses increased in mask-specific subregions in dorsal visual areas in and around the intraparietal sulcus. These opposite responses in ventral and dorsal visual areas occurred only during subjective target disappearance, not when the target was physically removed. Perceptual reports of target disappearance were furthermore associated with a “global” modulation of activity, which was delayed in time, and evident throughout early visual cortex, for both subjective target disappearance and physical target removal. We conclude that awareness of the target is tightly linked to the strength of its representation in ventral visual cortex, and that the mask representation in dorsal visual cortex plays a crucial role in the spontaneous suppression of the target representation during MIB.

Local and non-local deficits in amblyopia: acuity and spatial interactions

Amblyopic vision is thought to be limited by abnormal long-range spatial interactions, but their exact mode of action and relationship to the main amblyopic deficit in visual acuity is largely unknown. We studied this relationship in a group (N=59) of anisometropic (N=21) and strabismic (or combined, N=38) subjects, using (1) a single and multi-pattern (crowded) computerized static Tumbling-E test with scaled spacing of two pattern widths (TeVA), in addition to an optotype (ETDRS chart) acuity test (VA) and (2) contrast detection of Gabor patches with lateral flankers (lateral masking) along the horizontal and vertical axes as well as in collinear and parallel configurations. By correlating the different measures of visual acuity and contrast suppression, we found that (1) the VA of the strabismic subjects could be decomposed into two uncorrelated components measured in TeVA: acuity for isolated patterns and acuity reduction due to flanking patterns. The latter comprised over 60% of the VA magnitude, on the average and accounted for over 50% of its variance. In contrast, a slight reduction in acuity was found in the anisometropic subjects, and the acuity for a single pattern could account for 70% of the VA variance. (2) The lateral suppression (contrast threshold elevation) in a parallel configuration along the horizontal axis was correlated with the VA (R2=0.7), as well as with the crowding effect (TeVA elevation, R2=0.5) for the strabismic group. Some correlation with the VA was also found for the collinear configuration in the anisometropic group, but less suppression and no correlation were found for all the vertical configurations in all the groups. The results indicate the existence of a specific non-local component of the strabismic deficit, in addition to the local acuity deficit in all amblyopia types. This deficit might reflect long-range lateral inhibition, or alternatively, an inaccurate and scattered top-down attentional selection mechanism.

A transition between eye and object rivalry determined by stimulus coherence

Two orthogonal patterns presented to the two eyes, respectively, are perceived as alternating in time, a phenomenon often assumed to reflect competition between neuronal activities corresponding to the two eyes, presumably in the primary visual cortex. Recent evidence supports a competition between neuronal activities corresponding to the two patterns (objects) at some higher cortical processing stage after inputs from the two eyes have converged. Here, using textures made of Gabor signals, we present psychophysical data showing that the level of visual processing at which competition takes place and is resolved, is determined by the degree of stimulus coherence. Moreover, depending on stimulus parameters, competition may occur at several levels of processing at the same time.

Abnormal Speech Spectrum and Increased Pitch Variability in Young Autistic Children

Children with autism spectrum disorder (ASD) who can speak often exhibit abnormal voice quality and speech prosody, but the exact nature and underlying mechanisms of these abnormalities, as well as their diagnostic power are currently unknown. Here we quantified speech abnormalities in terms of the properties of the long-term average spectrum (LTAS) and pitch variability in speech samples of 83 children (41 with ASD, 42 controls) ages 4–6.5 years, recorded while they named a sequence of daily life pictures for 60 s. We found a significant difference in the groups average spectra, with ASD spectra being shallower and exhibiting less harmonic structure. Contrary to the common impression of monotonic speech in autism, the ASD children had a significantly larger pitch range and variability across time. A measure of this variability, optimally tuned for the sample, yielded 86% success (90% specificity, 80% sensitivity) in classifying ASD in the sample. These results indicate that speech abnormalities in ASD are reflected in its spectral content and pitch variability. This variability could imply abnormal processing of auditory feedback or elevated noise and instability in the mechanisms that control pitch. The current results are a first step toward developing speech spectrum-based bio-markers for early diagnosis of ASD.

ADHD subjects fail to suppress eye blinks and microsaccades while anticipating visual stimuli but recover with medication.

Oculomotor behavior and parameters are known to be affected by the allocation of attention and could potentially be used to investigate attention disorders. We explored the oculomotor markers of Attention-deficit/hyperactivity disorder (ADHD) that are involuntary and quantitative and that could be used to reveal the core-affected mechanisms, as well as be used for differential diagnosis. We recorded eye movements in a group of 22 ADHD-diagnosed patients with and without medication (methylphenidate) and in 22 control observers while performing the test of variables of attention (t.o.v.a.). We found that the average microsaccade and blink rates were higher in the ADHD group, especially in the time interval around stimulus onset. These rates increased monotonically over session time for both groups, but with significantly faster increments in the unmedicated ADHD group. With medication, the level and time course of the microsaccade rate were fully normalized to the control level, regardless of the time interval within trials. In contrast, the pupil diameter decreased over time within sessions and significantly increased above the control level with medication. We interpreted the suppression of microsaccades and eye blinks around the stimulus onset as reflecting a temporal anticipation mechanism for the transient allocation of attention, and their overall rates as inversely reflecting the level of arousal. We suggest that ADHD subjects fail to maintain sufficient levels of arousal during a simple and prolonged task, which limits their ability to dynamically allocate attention while anticipating visual stimuli. This impairment normalizes with medication and its oculomotor quantification could potentially be used for differential diagnosis.

Spatial interactions in amblyopia: Effects of stimulus parameters and amblyopia type

Adults with amblyopia were recently shown to perform abnormally in tasks requiring integration of local features into global percepts. Moreover, spatial interactions in amblyopic patients, though often found to be abnormal, showed marked variability. Here we measured collinear lateral interactions using Gabor patches in a large number of amblyopic (N=75) and normal subjects (N=25), testing four spatial frequencies (1.5, 3, 6, 9 cpd). We used the lateral masking paradigm, in which the contrast-detection threshold is measured in the presence of high-contrast flankers at different distances from a central target. Whereas in normal subjects spatial interaction patterns were evident across all spatial frequencies, amblyopic subjects showed abnormal spatial interactions and increasing deficiencies with increasing spatial frequencies. These abnormalities depended on the axis of astigmatism (in meridional amblyopia) and were more pronounced in strabismic than in anisometropic amblyopia. Spatial interactions were independent on the contrast-detection thresholds. Thus, adults with amblyopia might perform as well as normal observers for some stimulus parameters and abnormally for others. Our results indicate a close relationship between abnormal visual input to the visual cortex during development and abnormal functionality of the collinear spatial interactions in adults with amblyopia.

Development of Sensitivity to Texture and Contour Information in the Human Infant

Texture discrimination and bounding contour extraction are essential parts of the object segmentation and shape discrimination process. As such, successful texture and contour processing are key components underlying the development of the perception of both objects and surfaces. By recording visual-evoked potentials, we investigate whether young infants can detect orientation-defined textures and contours. We measured responses to an organized texture comprised of many Gabor patches of the same orientation, alternated with images containing the same number of patches, but all of random orientation. These responses were compared with a control condition consisting of the alternation between two independently random configurations. Significant difference potentials were found as early as 25 months, as were significant odd harmonics in the test conditions. Responses were also measured to Gabor patches organized either as circles (all patches tangent to an imaginary circular path) alternated with pinwheels (all patches having a fixed orientation offset from the path). Infants between 6 and 13 months also showed sensitivity to the global organization of the elements along contours. Differential responses to our texture and contour stimuli and their controls could only have been generated by mechanisms that are capable of comparing the relative orientation of 2 or more patches, as no local information at a single patch distinguished the random and organized textures or the circle and pinwheel configurations.

Collinear interactions and contour integration.

The visibility of a local target is influenced by the global configuration of the stimulus. Collinear configurations are a specific case in which facilitation or suppression of the target has been found to be dependent on the contrast threshold of the target. The role of collinear interactions in perceptual grouping, especially in contour integration, is still controversial. In the current study, the role of collinear interactions in noise was investigated using experimental conditions similar to those utilized in studies of contour integration. The contrast detection paradigm in the presence of similar Gabor elements presented in the background was used. The results show that contrast detection threshold of the target alone is increased (suppression) when it is embedded in randomly oriented background elements. However, when the target is flanked by two collinear Gabor elements, the target is facilitated even at higher target contrast levels. Facilitation is not found for orthogonal configurations. The results suggest that the response to a local element in a contour is modified by lateral facilitative and suppressive inputs from elements comprising the smooth contour and randomly oriented background elements, respectively. Thus, detection of elements along a contour should be considered as integration of global neuronal activity rather than as the output of local and individual neurons.

Abnormal binocular rivalry in unilateral neglect: evidence for a non-spatial mechanism of extinction

Unilateral spatial neglect (USN) is considered to be an attention deficit, which is primarily related to space. Recent evidence points to the relevance of non-spatially lateralized mechanisms, with impairments found in rapid stimulus presentation conditions. Here we used the phenomenon of binocular rivalry (BR) to explore a non-spatial deficit over long temporal intervals. Six right-hemisphere damaged (RHD) patients with contralesional neglect (USN+), five RHD patients without neglect (USN−) and six normal controls were tested on the basic properties of BR induced by dichoptic presentation of orthogonal gratings at fixation. USN+ patients had much slower perceptual alternations compared to the USN− and normal groups (factors 2.5 and 4, respectively), and were much more sensitive to inter-ocular changes in relative stimulus contrast, which, unlike normals, altered both the suppression and dominance phases. Most notably, a small advantage of one monocular stimulus caused a long-term extinction of the other stimulus in the USN+ group alone. We explain the results in terms of impaired habituation to dominant and attended stimuli, which normally prevents a winner-takes-all behavior and extinction of the weak. This impaired habituation may in turn contribute to inappropriate environmental monitoring and attenuated novelty-seeking behavior.