Subjective contour illusion: sex-related effect of unilateral brain damage

Feedback from lateral occipital cortex to V1/V2 triggers object completion: Evidence from functional magnetic resonance imaging and dynamic causal modeling

Illusory figures demonstrate the visual system's ability to integrate disparate parts into coherent wholes. We probed this object integration process by either presenting an integrated diamond shape or a comparable ungrouped configuration that did not render a complete object. Two tasks were used that either required localization of a target dot (relative to the presented configuration) or discrimination of the dot's luminance. The results showed that only when the configuration was task relevant (in the localization task), performance benefited from the presentation of an integrated object. Concurrent functional magnetic resonance imaging was performed and analyzed using dynamic causal modeling to investigate the (causal) relationship between regions that are associated with illusory figure completion. We found object‐specific feedback connections between the lateral occipital cortex (LOC) and early visual cortex (V1/V2). These modulatory connections persisted across task demands and hemispheres. Our results thus provide direct evidence that interactions between mid‐level and early visual processing regions engage in illusory figure perception. These data suggest that LOC first integrates inputs from multiple neurons in lower‐level cortices, generating a global shape representation while more fine‐graded object details are then determined via feedback to early visual areas, independently of the current task demands.

Impaired perception of illusory contours and cortical hypometabolism in patients with Parkinson's disease

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We assessed the perception of illusory contours in patients with PD.

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PD patients showed difficulty in perceiving Kanizsa illusory figures.

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Impaired perception of Kanizsa illusory figures was related to LOC hypometabolism.

Neuroimaging evidence suggests that areas of the higher-order visual cortex, including the lateral occipital complex (LOC), are engaged in the perception of illusory contours; however, these findings remain unsubstantiated by human lesion data. Therefore, we assessed the presentation time necessary to perceive two types of illusory contours formed by Kanizsa figures or aligned line ends in patients with Parkinson's disease (PD). Additionally, we used ¹⁸F-fluorodeoxyglucose positron emission tomography (FDG-PET) to measure regional cerebral glucose metabolism in PD patients. Although there were no significant differences in the stimulus durations required for perception of illusory contours formed by aligned line ends between PD patients and controls, PD patients required significantly longer stimulus durations for the perception of Kanizsa illusory figures. Difficulty in perceiving Kanizsa illusory figures was correlated with hypometabolism in the higher-order visual cortical areas, including the posterior inferior temporal gyrus. These findings indicate an association between dysfunction in the posterior inferior temporal gyrus, a region corresponding to a portion of the LOC, and impaired perception of Kanizsa illusory figures in PD patients.

Tracking the completion of parts into whole objects: Retinotopic activation in response to illusory figures in the lateral occipital complex

Illusory figures demonstrate the visual system's ability to integrate separate parts into coherent, whole objects. The present study was performed to track the neuronal object construction process in human observers, by incrementally manipulating the grouping strength within a given configuration until the emergence of a whole-object representation. Two tasks were employed: First, in the spatial localization task, object completion could facilitate performance and was task-relevant, whereas it was irrelevant in the second, luminance discrimination task. Concurrent functional magnetic resonance imaging (fMRI) used spatial localizers to locate brain regions representing task-critical illusory-figure parts to investigate whether the step-wise object construction process would modulate neural activity in these localized brain regions. The results revealed that both V1 and the lateral occipital complex (LOC, with sub-regions LO1 and LO2) were involved in Kanizsa figure processing. However, completion-specific activations were found predominantly in LOC, where neural activity exhibited a modulation in accord with the configuration's grouping strength, whether or not the configuration was relevant to performing the task at hand. Moreover, right LOC activations were confined to LO2 and responded primarily to surface and shape completions, whereas left LOC exhibited activations in both LO1 and LO2 and was related to encoding shape structures with more detail. Together, these results demonstrate that various grouping properties within a visual scene are integrated automatically in LOC, with sub-regions located in different hemispheres specializing in the component sub-processes that render completed objects.

Natural split-brain? Lateralized memory for task contingencies in pigeons

The motion aftereffect (MAE) is an illusory motion in the opposite direction after the sudden halt of a prolonged visual moving stimulus. Birds could perceive the MAE as humans and other mammals. The present study was to investigate whether hemispheric asymmetries of visual processes affect this illusion. To this end, pigeons were trained to discriminate grating patterns which moved up, or down or stood still. The transfer tests were conducted under the binocular or monocular viewing condition. The choice behaviors of pigeons under the binocular and right-eye viewing condition (left hemisphere) were highly indicative for the perception of a MAE. However, the animals under the left-eye viewing condition (right hemisphere) did not change their choice patterns according to the different task displayed on the central stimulus key, but always stuck to the default option of pecking the response key ipsilateral to the open eye. We assume that memory for task contingencies were confined to the left hemisphere and could not be reached by the right half brain due to the absence of the corpus callosum.

Functional neuroimaging findings on the human perception of illusory contours

Illusory contours (IC) have attracted a considerable interest in recent years to derive models of how sensory information is processed and integrated within the visual system. In addition to various findings from neuropsychology, neurophysiology, and psychophysics, several recent studies have used functional neuroimaging to identify the cerebral substrates underlying human perception of IC (in particular Kanizsa figures). In this paper, we review the results from more than 20 neuroimaging studies on IC perception and highlight the great diversity of findings across these studies. We then provide a detailed discussion about the localization ('where' debate) and the timing ('when' debate) of IC processing as suggested by functional neuroimaging. Cortical responses involving visual areas as early as V1/V2 and latencies as rapid as 100 ms have been reported in several studies. Particular issues concerning the role of the right hemisphere and the retinotopic encoding of IC are also discussed. These different findings are tentatively brought together to propose different hypothetical cortical mechanisms that might be responsible for the visual formation of IC. Several remaining questions on IC processing that could potentially be explored with functional neuroimaging techniques are finally emphasized.

Brain State-dependent Functional Hemispheric Specialization in Men but not in Women

Hemispheric specialization is reliably demonstrated in patients with unilateral lesions or disconnected hemispheres, but is inconsistent in healthy populations. The reason for this paradox is unclear. We propose that functional hemispheric specialization in healthy participants depends upon functional brain states at stimulus arrival (FBS). Brain activity was recorded from 123 surface electrodes while 22 participants (11 women) performed lateralized lexical decisions (left hemisphere processing) on neutral and emotional (right hemisphere processing) words. We determined two classes of stable FBS, one with right anterior-left posterior orientations (RA-LP maps) and one with left anterior-right posterior orientations (LA-RP maps). Results show that functional hemispheric specialization is dependent upon the class of FBS and gender. Of those with LA-RP maps, only men showed a strong emotional word advantage (EWA) after left visual field (right hemisphere) presentation, but no EWA after right visual field (left hemisphere) presentation. Subsequent to all other brain states, there was an almost equal EWA after presentation to either visual field. Only about half of the FBS in men led to the pattern of functional hemispheric specialization. We suggest that 'split-brain' research may be marginally describable by a model, but only in exceptional situations, while in connected brains this functional hemispheric specialization is only one of many dynamic states.