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Stefan Pollmann

Publications and source records attributed to Stefan Pollmann.

17 recordsLinked to original sources
PubMed2007

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, 1553-1564, 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.

Adult↗
PubMed2006

Perception modulates auditory cortex activation.

Event-related functional magnetic resonance imaging signal change in Heschl's gyrus and the planum temporale was found to reflect sensory decisions about target presence. In a dichotic listening task, activation was higher for target present responses, irrespective of actual target presence. In fact, activation was highest for false alarms, that is, 'present' responses in the absence of a target stimulus, and lowest for missed targets. This shows that activity at the earliest stage of cortical auditory processing reflects subjective perceptual decisions. Whether this activation is driven by bottom-up or top-down factors remains to be investigated.

Acoustic Stimulation↗
PubMed2006

Neural basis of redundancy effects in visual object categorization.

Bilateral redundant information often leads to improved detection performance compared to only unilateral input. This experiment investigates redundancy effects at the level of object processing in behavioral and functional imaging data. Subjects received different combinations of categorical information presented to both visual hemifields simultaneously. For pictures of faces as well as for pictures of buildings, response latencies were faster when two pictures from the same object category were presented compared to two pictures from different categories. This behavioral advantage was accompanied by increased activation in the respective object-selective areas, i.e. in the Fusiform Face Area (FFA) for pictures of faces and in the Parahippocampal Place Areas (PPAs) for pictures of buildings. These results suggest an involvement of visual object-selective areas in the behavioral redundancy gain.

Adult↗
PubMed2005

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.

Adult↗
PubMed2005

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.

Adult↗
PubMed2005

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.

Adult↗
PubMed2005

Differential activation of object-selective visual areas by passive viewing of pictures and words.

Functional imaging has shown that pictures of faces (N. Kanwisher, J. McDermott, M.M. Chun, The fusiform face area: a module in human extrastriate cortex specialized for face perception, J. Neurosci. 17 (1997) 4302-4311) and buildings (R. Epstein, N. Kanwisher, A cortical representation of the local visual environment, Nature 391 (1998) 598-601) activate different regions within the lateral occipital complex (LOC). To investigate effects of stimulus format on activation in these areas, we used event-related fMRI to measure brain activity during the passive viewing of pictures showing buildings and faces, and of words identifying these pictures. Consistent with earlier findings, pictures of faces activated bilateral regions in the lateral fusiform gyri, whereas pictures of buildings activated bilateral regions in the parahippocampal gyri. Analyzing the activation elicited by visually presenting the written names of the pictures, however, we did not find an effect of word meaning on the fMRI signal change in these areas: fMRI signal changes for the names of faces and the names of buildings did not differ in any of the areas selectively activated by the corresponding pictures. In general, both word conditions and non-preferred picture conditions elicited similar signal amplitudes. While presentation of words did not lead to strong activation in object-specific areas, activation for words of both categories was found in the left occipito-temporal cortex, close to the location which has tentatively been called 'visual word form area' (L. Cohen, S. Dehaene, L. Naccache, S. Lehéricy, G. Dehaene-Lambertz, et al., The visual word form are: spatial and temporal characterization of an initial stage of reading in normal subjects and posterior split-brain patients, Brain 123 (2000) 291-307), revealing that words were processed effectively during the experiment. Taken together, these results show that names of pictures do not automatically activate the corresponding object-selective areas.

Adult↗
PubMed2005

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.

Adult↗
PubMed2004

Auditory target detection in dichotic listening involves the orbitofrontal and hippocampal paralimbic belts.

In dichotic listening, two similar, yet different stimuli are presented simultaneously to the left and right ear. When two syllables are presented in this way, they seem to blend and discrimination of syllables presented to one ear is only possible with uncertainty. In this event-related fMRI study, we found that the orbitofrontal and paralimbic belts were involved in target detection in dichotic listening. The posterior orbital gyri bilaterally, the left amygdala, hippocampal formation and the left pregenual paracingulate area (PPA) were activated more strongly during dichotic target detection than during correct rejection of target presence. The right posterior orbital gyrus also showed stronger activation during dichotic compared to diotic target detection. Further analyses showed that the blood oxygen level dependent (BOLD) responses in these areas, with the exception of the right hippocampal formation, varied with the subjective decision on target presence rather than with the physical target presence itself. The left PPA, amygdala and hippocampal formation responded differently to left and right ear target detection, suggesting their involvement in the right ear advantage observed in this task. The data show the importance of the orbitofrontal and hippocampal paralimbic belts for auditory stimulus decision processes based on ambivalent sensory information.

Adult↗
PubMed2004

Anterior prefrontal cortex contributions to attention control.

In a series of event-related functional magnetic resonance studies, we consistently found activation in anterior prefrontal cortex related to visual dimension changes in singleton search tasks. I review these data and discuss possible contributions of anterior prefrontal cortex to attention control in visual search. It is proposed that anterior prefrontal cortex may detect task-relevant stimulus changes when the target is ambiguously defined. This process may occur in the absence of awareness and may support visual dimension weighting by inhibition of the old relevant dimension in favor of the new dimension.

Attention↗
PubMed2003

The neural basis of the bilateral distribution advantage.

Letters can be matched by their physical identity (i.e., a-a: same/A-a: different) or by their name (both a-a and A-a: same). The latter, more demanding task has in previous experiments led to an advantage of bilateral over within-hemifield matches, which was not observed in the former. We have investigated the neural basis of this bilateral distribution advantage (BDA) in letter name matching with event-related fMRI. Unilateral, compared to bilateral, name matching led to increased activation in the contralateral fusiform and lateral occipital gyri. This increase went along with an ipsilateral increase of activation in homologous areas. Such a hemispheric resource sharing was not observed for letter shape matching. This pattern of activation shows that letter name matching induces hemispheric resource sharing in visual areas, which occurs when task demands in the hemisphere of input reach a critical level. Activation in anterior cingulate complex and posterior cingulate/retrosplenial cortex showed a task x visual field interaction with lower activation for bilateral than unilateral name matches but higher activation for bilateral than unilateral shape matches, which fits the interhemispheric transfer demands in these tasks.

Adolescent↗
PubMed2003

Determining subprocesses of visual feature search with reaction time models.

After the classic serial/parallel dichotomy of visual search mechanisms has been increasingly doubted, we investigated what search mechanisms are used between the two poles termed "pop-out" and "strictly serial search" in an overt feature search paradigm. Since reaction time slopes do not contain sufficient information for this purpose, we developed a novel technique for analyzing reaction times. Individual reaction times are modeled as sums of the durations of successive search steps. Model parameters are task characteristics (similarity, number and arrangement of target and distractors) and processing characteristics of the participant (e.g., attention dwell and shift durations). In Experiment 1, several model variants were fitted numerically to empirical reaction times. The best fitting model suggested that more than one item can be processed in a single fixation, movement of attention is abrupt and not continuous, and even in pop out search, attention is often explicitly moved to the target. In Experiment 2, we measured the central model parameter, the so-called range of attention, more directly and thereby validated the model. The model provides an explanation for the strong variation in the slope of reaction time functions, which is not based on an explicit distinction between parallel and serial search processes.

Adult↗
PubMed2003

Left and right occipital cortices differ in their response to spatial cueing.

We investigated cue and target-related laterality effects with event-related fMRI. Both left and right occipital areas responded maximally when both cue and target were presented in the contralateral visual hemifield (VF), and minimally when cue and target were presented in the ipsilateral VF. However, whereas signal increases in right ventromedial and lateral occipital cortex were intermediate in those trials in which the cue appeared in the VF contralateral to the target (invalid cue trials), signal strength in left occipital cortex was almost identical for valid and invalid cues, i.e., high for RVF cues, and low for LVF cues, independent of the VF of the target. These data support theories which postulate a greater ability of the right hemisphere for bilateral processing. However, these laterality effects were observed earlier in the visual pathway than previously thought, leading to the question whether the hemispheric differences observed in occipital cortex are generated in the activated areas or are the effect of reentrant processes from more anterior areas, potentially in parietal cortex.

Adult↗
PubMed2003

Division of labor between the hemispheres for complex but not simple tasks: an implemented connectionist model.

When stimuli have to be matched in a complex task (such as whether 2 letters have the same name), then performance is better when stimuli are presented across the hemispheres of the brain, whereas for simpler tasks (such as whether 2 letters have the same shape), better performance is achieved when stimuli are presented unilaterally. The authors show that this bilateral distribution advantage effect emerged spontaneously in a neural network model learning to solve simple and complex tasks with separate input layers and separate, but interconnected, resources in a hidden layer. The authors show that relating computational models to behavioral and imaging data proves fruitful for understanding hemispheric processing and generating testable hypotheses.

Brain↗
PubMed2002

Covert reorienting and inhibition of return: an event-related fMRI study.

Using event-related fMRI, we analyzed the functional neuroanatomy of covert reorienting and inhibition of return (IOR). Covert reorienting to a target appearing within 250 msec after an invalid contralateral location cue elicited increased activation in the left fronto-polar cortex (LFPC), right anterior and left posterior middle frontal gyrus, and right cerebellum, areas that have previously been associated with attentional processes, specifically attentional change. In contrast, IOR, which leads to prolonged response times to targets that appear at the cued location at a stimulus-onset-asynchrony (SOA)>250 msec, was accompanied by increased activation in brain areas involved in oculomotor programming, such as the right medial frontal gyrus (supplementary eye field; SEF) and the right inferior precentral sulcus (frontal eye field; FEF), supporting the oculomotor bias theory of IOR. Pre-SEF and pre-FEF areas were involved both in covert reorienting and IOR. The supramarginal gyri were bilaterally involved in IOR, with the right supramarginal gyrus additionally involved in covert reorienting.

Adult↗
PubMed2002

Dichotic listening in patients with splenial and nonsplenial callosal lesions.

The authors found splenial lesions to be associated with left ear suppression in dichotic listening of consonant-vowel syllables. This was found in both a rapid presentation dichotic monitoring task and a standard dichotic listening task, ruling out attentional limitations in the processing of high stimulus loads as a confounding factor. Moreover, directed attention to the left ear did not improve left ear target detection in the patients, independent of callosal lesion location. The authors' data may indicate that auditory callosal fibers pass through the splenium more posterior than previously thought. However, further studies should investigate whether callosal fibers between primary and secondary auditory cortices, or between higher level multimodal cortices, are vital for the detection of left ear targets in dichotic listening.

Adult↗