Biomedical subjects

Denis Schluppeck

Publications and source records attributed to Denis Schluppeck.

7 recordsLinked to original sources

PubMed2006

Sustained activity in topographic areas of human posterior parietal cortex during memory-guided saccades.

In a previous study, we identified three cortical areas in human posterior parietal cortex that exhibited topographic responses during memory-guided saccades [visual area 7 (V7), intraparietal sulcus 1 (IPS1), and IPS2], which are candidate homologs of macaque parietal areas such as the lateral intraparietal area and parietal reach region. Here, we show that these areas exhibit sustained delay-period activity, a critical physiological signature of areas in macaque parietal cortex. By varying delay duration, we disambiguated delay-period activity from sensory and motor responses. Mean time courses in the parietal areas were well fit by a linear model comprising three components representing responses to (1) the visual target, (2) the delay period, and (3) the eye movement interval. We estimated the contributions of each component: the response amplitude during the delay period was substantially smaller (<30%) than that elicited by the transient visual target. All three parietal regions showed comparable delay-period response amplitudes, with a trend toward larger responses from V7 to IPS1 and IPS2. Responses to the cue and during the delay period showed clear lateralization with larger responses to trials in which the target was placed in the contralateral visual field, suggesting that both of these components contributed to the topography we measured.

Biological Clocks↗

PubMed2005

fMR-adaptation reveals a distributed representation of inanimate objects and places in human visual cortex.

The way information about objects is represented in visual cortex remains controversial. It is unclear, for example, whether information is processed in modules, specialized for different categories of objects or whether information is represented in a distributed fashion across a large network of overlapping visual areas. In this study, we used fMR-adaptation to investigate the extent to which 'specialized' regions of visual cortex are involved in representing information about inanimate objects and places. We found adaptation in the object-selective lateral occipital complex (LOC) following repeated presentations of the same inanimate object. However, we also found fMR-adaptation to inanimate objects in fusiform face area (FFA) and the parahippocampal place area (PPA). Furthermore, this adaptation was not affected by changes in the size of the stimulus. In the second part of the experiment, we found adaptation to repeated images of places in the place-selective PPA, which was both size- and viewpoint-invariant. fMR-adaptation to repeated images of places was also observed in the LOC, but not in the FFA. These results suggest that the representation of inanimate objects and places is not restricted to those regions showing maximal responses to these particular categories of objects, but is distributed across human visual cortex and can include 'face-selective' regions such as the FFA.

Adaptation, Psychological↗

PubMed2005

Topographic organization for delayed saccades in human posterior parietal cortex.

Posterior parietal cortex (PPC) is thought to play a critical role in decision making, sensory attention, motor intention, and/or working memory. Research on the PPC in non-human primates has focused on the lateral intraparietal area (LIP) in the intraparietal sulcus (IPS). Neurons in LIP respond after the onset of visual targets, just before saccades to those targets, and during the delay period in between. To study the function of posterior parietal cortex in humans, it will be crucial to have a routine and reliable method for localizing specific parietal areas in individual subjects. Here, we show that human PPC contains at least two topographically organized regions, which are candidates for the human homologue of LIP. We mapped the topographic organization of human PPC for delayed (memory guided) saccades using fMRI. Subjects were instructed to fixate centrally while a peripheral target was briefly presented. After a further 3-s delay, subjects made a saccade to the remembered target location followed by a saccade back to fixation and a 1-s inter-trial interval. Targets appeared at successive locations "around the clock" (same eccentricity, approximately 30 degrees angular steps), to produce a traveling wave of activity in areas that are topographically organized. PPC exhibited topographic organization for delayed saccades. We defined two areas in each hemisphere that contained topographic maps of the contra-lateral visual field. These two areas were immediately rostral to V7 as defined by standard retinotopic mapping. The two areas were separated from each other and from V7 by reversals in visual field orientation. However, we leave open the possibility that these two areas will be further subdivided in future studies. Our results demonstrate that topographic maps tile the cortex continuously from V1 well into PPC.

Attention↗

PubMed2004

Learning strengthens the response of primary visual cortex to simple patterns.

Training can significantly improve performance on even the most basic visual tasks, such as detecting a faint patch of light or determining the orientation of a bar (for reviews, see ). The neural mechanisms of visual learning, however, remain controversial. One simple way to improve behavior is to increase the overall neural response to the trained stimulus by increasing the number or gain of responsive neurons. Learning of this type has been observed in other sensory modalities, where training increases the number of receptive fields that cover the trained stimulus. Here, we show that visual learning can selectively increase the overall response to trained stimuli in primary visual cortex (V1). We used functional magnetic resonance imaging (fMRI) to measure neural signals before and after one month of practice at detecting very low-contrast oriented patterns. Training increased V1 response for practiced orientations relative to control orientations by an average of 39%, and the magnitude of the change in V1 correlated moderately well with the magnitude of changes in detection performance. The elevation of V1 activity by training likely results from an increase in the number of neurons responding to the trained stimulus or an increase in response gain.

Humans↗

PubMed2004

Neural responses to Mooney images reveal a modular representation of faces in human visual cortex.

The way in which information about objects is represented in visual cortex remains controversial. One model of human object recognition poses that information is processed in modules, highly specialised for different categories of objects; an opposing model appeals to a distributed representation across a large network of visual areas. We addressed this debate by monitoring activity in face- and object-selective areas while human subjects viewed ambiguous face stimuli (Mooney faces). The measured neural response in the face-selective region of the fusiform gyrus was greater when subjects reported seeing a face than when they perceived the image as a collection of blobs. In contrast, there was no difference in magnetic resonance response between face and no-face perceived events in either the face-selective voxels of the superior temporal sulcus or the object-selective voxels of the parahippocampal gyrus and lateral occipital complex. These results challenge the concept that neural representation of faces is distributed and overlapping and suggest that the fusiform gyrus is tightly linked to the awareness of faces.

Adult↗

PubMed2002

Color opponent neurons in V1: a review and model reconciling results from imaging and single-unit recording.

The signals in visual cortex that ultimately give rise to color perception remain poorly understood. Controversy has particularly surrounded one aspect of color's encoding in the visual system-opponent processing in primary visual cortex. Early single-unit studies suggested that V1 contains relatively few color-opponent neurons. Neuroimaging measurements, however, have suggested that such neurons might be relatively numerous. Here, we reconcile these apparently discrepant results and conclude that V1 contains relatively large numbers of color-opponent neurons. We first review results from each method and find that most neuroimaging studies provide evidence of substantial color opponency in V1, and that despite apparent controversy, most single-unit studies agree that relatively large numbers of V1 neurons show some sort of color opponency. To reconcile the results from different techniques more formally, we used electrophysiological data to predict the outcomes of neuroimaging experiments. We simulated the expected fMRI response in V1 to spatial patterns of different color, based on the neurons' properties, as reported in Johnson, Hawken, and Shapley, (2001). The simulated responses to stimuli used in Engel, Zhang, and Wandell, (1997) agree well with the actually observed fMRI results. The model identifies several factors that led to the apparent discrepancy between techniques, and makes testable predictions about how these factors influence the magnitude of color-opponent signals. fMRI and single-unit data converge to show that large numbers of color-opponent neurons exist in V1.

Color Perception↗

PubMed2002

Activity in the fusiform gyrus predicts conscious perception of Rubin's vase-face illusion.

We localized regions in the fusiform gyrus and superior temporal sulcus that were more active when subjects viewed photographs of real faces than when they viewed complex inanimate objects and other areas in the parahippocampal gyrus and the lateral occipital lobe that showed more activity during the presentation of nonface objects. Event-related functional magnetic resonance imaging was then used to monitor activity in these extrastriate visual areas while subjects viewed Rubin's vase-face stimulus and indicated switches in perception. Since the spontaneous shifts in interpretation were too rapid for direct correlation with hemodynamic responses, each reported percept (faces or vase) was prolonged by suddenly adding subtle local contrast gradients (embossing) to one side or the other of the figure-ground boundary, stabilizing the percept. Under these conditions, only face-selective areas in the fusiform gyrus responded more strongly during the perception of faces. To control for effects of the physical change to Rubin's stimulus (i.e., addition of embossing), we compared activity when the face contours were embossed after the subject had just reported the onset of perception of either faces or vase. Activity in the fusiform face area responded more strongly under the first condition, despite the fact that the physical stimulus sequences were identical. Moreover, on a trial-to-trial basis, the activity was statistically predictive of the subjects' responses, suggesting that the conscious perception of faces could be made explicit in this extrastriate visual area.

Adult↗