Marianne Maertens

Retinotopic activation in response to subjective contours in primary visual cortex.

Objects in our visual environment are arranged in depth and hence there is a considerable amount of overlap and occlusion in the image they generate on the retina. In order to properly segment the image into figure and background, boundary interpolation is required even across large distances. Here we study the cortical mechanisms involved in collinear contour interpolation using fMRI. Human observers were asked to discriminate the curvature of interpolated boundaries in Kanizsa figures and in control configurations, which contained identical physical information but did not generated subjective shapes. We measured a spatially precise spin-echo BOLD signal and found stronger responses to subjective shapes than non-shapes at the subjective boundary locations, but not at the inducer locations. The responses to subjective contours within primary visual cortex were retinotopically specific and analogous to that to real contours, which is intriguing given that subjective and luminance-defined contours are physically fundamentally different. We suggest that in the absence of retinal stimulation, the observed activation changes in primary visual cortex are driven by intracortical interactions and feedback, which are revealed in the absence of a physical stimulus.

fMRI Reveals a Common Neural Substrate of Illusory and Real Contours in V1 after Perceptual Learning

Perceptual learning involves the specific and relatively permanent modification of perception following a sensory experience. In psychophysical experiments, the specificity of the learning effects to the trained stimulus attributes (e.g., visual field position or stimulus orientation) is often attributed to assumed neural modifications at an early cortical site within the visual processing hierarchy. We directly investigated a neural correlate of perceptual learning in the primary visual cortex using fMRI. Twenty volunteers practiced a curvature discrimination on Kanizsa-type illusory contours in the MR scanner. Practice-induced changes in the BOLD response to illusory contours were compared between the pretraining and the posttraining block in those areas of the primary visual cortex (V1) that, in the same session, had been identified to represent real contours at corresponding visual field locations. A retinotopically specific BOLD signal increase to illusory contours was observed as a consequence of the training, possibly signaling the formation of a contour representation, which is necessary for performing the curvature discrimination. The effects of perceptual training were maintained over a period of about 10 months, and they were specific to the trained visual field position. The behavioral specificity of the learning effects supports an involvement of V1 in perceptual learning, and not in unspecific attentional effects.

Selective and interactive neural correlates of visual dimension changes and response changes.

In an event-related fMRI study, we investigated the neural correlates of visual dimension and response changes. We used a compound task, which required target selection by a singleton feature, a unique color or motion direction, before the appropriate motor response, which was determined by target orientation, could be selected. Both types of change elicited distinct patterns of activation, with dimension-change-related activation primarily in posterior visual areas and response-related activation primarily in motor-related areas of the parietal and frontal cortices. Response-change-related activation was delayed by about 1 s relative to dimension-change-related activation, suggesting that the latter is elicited by perceptual processes, whereas the former reflects response-related or post-response processes. Although dimension changes and response changes rely on different processes, they are not independent: response facilitation was observed for combined dimension and response repetitions, this facilitation, however, was disrupted by dimension changes.

Shift of activity from attention to motor-related brain areas during visual learning

With practice, we become increasingly efficient at visual object comparisons. This may be due to the formation of a memory template that not only binds individual features together to create an object, but also links the object with an associated response. In a longitudinal fMRI study of object matching, evidence for this link between perception and action was observed as a shift of activation from visual-attentive processing areas along the posterior intraparietal sulcus to hand-sensory and motor-related areas.

Interhemispheric resource sharing: Decreasing benefits with increasing processing efficiency

Visual matches are sometimes faster when stimuli are presented across visual hemifields, compared to within-field matching. Using a cued geometric figure matching task, we investigated the influence of computational complexity vs. processing efficiency on this bilateral distribution advantage (BDA). Computational complexity was manipulated by requiring different types of match decision (physical identity vs. category identity) and processing efficiency was varied by on-task training A pronounced BDA, initially present in both tasks, completely disappeared in the course of training for the less complex and decreased for the more complex task. Thus, the size of the BDA is determined by both, processing efficiency and task complexity.

Illusory Contours Do Not Pass through the “Blind Spot”

Our visual percepts are not fully determined by the physical stimulus input. That is why we perceive crisp bounding contours even in the absence of luminance-defined borders in visual illusions such as the Kanizsa figure. It is important to understand which neural processes are involved in creating these artificial visual experiences because this might tell us how we perceive coherent objects in natural scenes, which are characterized by mutual overlap. We have already shown using functional magnetic resonance imaging [Maertens, M., & Pollmann, S. fMRI reveals a common neural substrate of illusory and real contours in v1 after perceptual learning. Journal of Cognitive Neuroscience, 17, 15531564, 2005] that neurons in the primary visual cortex (V1) respond to these stimuli. Here we provide support for the hypothesis that V1 is obligatory for the discrimination of the curvature of illusory contours. We presented illusory contours across the portion of the visual field corresponding to the physiological blind spot. Four observers were extensively trained and asked to discriminate fine curvature differences in these illusory contours. A distinct performance drop (increased errors and response latencies) was observed when illusory contours traversed the blind spot compared to when they were presented in the normal contralateral visual field at the same eccentricity. We attribute this specific performance deficit to the failure to build up a representation of the illusory contour in the absence of a cortical representation of the blind spot within V1. The current results substantiate the assumption that neural activity in area V1 is closely related to our phenomenal experience of illusory contours in particular, and to the construction of our subjective percepts in general.

Splenial lesions lead to supramodal target detection deficits.

Patients with lesions of the splenium showed higher validity effects of visuospatial cues than did patients with partial lesions of the corpus callosum anterior to the splenium and control participants. Many of the patients tested had also shown a left-ear suppression for consonant-vowel syllables in a previous dichotic listening study. The authors interpret these parallel findings as evidence for the disruption of signals that normally alert the individual to the presence of behaviorally relevant stimuli, possibly originating in the temporoparietal junction area. After splenial lesions, these signals may not reach the contralateral hemisphere, leading to supramodal deficits in target detection, especially under distracting conditions.