Christian Marendaz

The neural substrates and timing of top-down processes during coarse-to-fine categorization of visual scenes: A combined fmri and erp study

Spatial frequencies in an image influence visual analysis across a distributed, hierarchically organized brain network. Low spatial frequency (LSF) information may rapidly reach high-order areas to allow an initial coarse parsing of the visual scene, which could then be “retroinjected” through feedback into lower level visual areas to guide finer analysis on the basis of high spatial frequency (HSF). To test this “coarse-to-fine” processing scheme and to identify its neural substrates in the human brain, we presented sequences of two spatial-frequency-filtered scenes in rapid succession (LSF followed by HSF or vice versa) during fMRI and ERPs in the same participants. We show that for low-to-high sequences (but not for high-to-low sequences), LSF produces a first increase of activity in prefrontal and temporo-parietal areas, followed by enhanced responses to HSF in primary visual cortex. This pattern is consistent with retroactive influences on low-level areas that process HSF after initial activation of higher order areas by LSF.

Cerebral regions and hemispheric specialization for processing spatial frequencies during natural scene recognition. An event-related fMRI study.

It has been suggested that visual scene recognition is mainly based on spatial frequency (Fourier) analysis of the image. This analysis starts with processing low spatial frequencies (LSF), followed by processing high spatial frequencies (HSF). Within the framework of the spatial frequency analysis, the right/left hemisphere would be predominantly involved in LSF/HSF analysis, respectively. The aim of this event-related fMRI study was to evaluate neural correlates and hemispheric specialization of spatial frequency analysis during recognition of nonfiltered (NF) and filtered, either in LSF or HSF, natural scenes. Comparing LSF or NF to HSF scene recognition, significant activation was obtained within right anterior temporal cortex and right parahippocampal gyrus. As these regions are known to be involved in scene processing, we interpret this result as suggesting that scene recognition is mainly based on LSF extraction and analysis. When LSF scene was compared to HSF scene recognition, supplementary activation was obtained within the right inferior parietal lobule that likely reflects attentional modulation on spatial frequency processing. A direct interhemispheric comparison for each particular band of spatial frequencies highlighted predominance within the early visual areas (such as the middle occipital gyrus) to the right for LSF processing and to the left for HSF processing. This result provides supplementary evidence for hemispheric specialization at early levels of visual analysis when spatial frequencies are processed.

Effect of temporal constraints on hemispheric asymmetries during spatial frequency processing

Studies on functional hemispheric asymmetries have suggested that the right vs. left hemisphere should be predominantly involved in low vs. high spatial frequency (SF) analysis, respectively. By manipulating exposure duration of filtered natural scene images, we examined whether the temporal characteristics of SF analysis (i.e., the temporal precedence of low on high spatial frequencies) may interfere with hemispheric specialization. Results showed the classical hemispheric specialization pattern for brief exposure duration and a trend to a right hemisphere advantage irrespective of the SF content for longer exposure duration. The present study suggests that the hemispheric specialization pattern for visual information processing should be considered as a dynamic system, wherein the superiority of one hemisphere over the other could change according to the level of temporal constraints: the higher the temporal constraints of the task, the more the hemispheres are specialized in SF processing.

The importance of low spatial frequency information for recognising fearful facial expressions

A recent brain imaging study (Vuilleumier, Armony, Driver and Dolan 2003, Nature Neuroscience, 6, 624–631) has shown that amygdala responses to fearful expressions are preferentially driven by intact or low spatial frequency (LSF) images of faces, rather than by high spatial frequency (HSF) images. These results suggest that LSF components processed rapidly via magnocellular pathways within the visual system might be very efficiently conveyed to the amygdala for the rapid recognition of fearful expressions, perhaps via a subcortical pathway that activates the pulvinar and superior colliculus, but which bypasses any finer visual analysis of HSF cues in the striate and temporal extrastriate cortex. The purpose of this paper is to analyse the statistical properties of LSF compared with HSF and intact faces. The statistical analysis shows that the LSF components in faces, which are typically extracted rapidly by the visual system, provide a better source of information than HSF components for the correct categorisation of fearful expressions in faces. These results support the idea that visual pathways from the magnocellular visual neurons might be optimal, at a computational level, for the rapid classification of fearful emotional expressions in human faces.

Neural correlates of spatial frequency processing: A neuropsychological approach.

We examined the neural correlates of spatial frequency (SF) processing through a gender and neuropsychological approach, using a recognition task of filtered (either in low spatial frequencies/LSF or high spatial frequencies/HSF) natural scene images. Experiment 1 provides evidence for hemispheric specialization in SF processing in men (the right hemisphere is predominantly involved in LSF analysis and the left in HSF analysis) but not in women. Experiment 2 aims to investigate the role of the right occipito-temporal cortex in LSF processing with a neurological female patient who had a focal lesion of this region due to an embolization of an arterioveinous malformation. This study was conducted 1 week before and 6 months after the surgical intervention. As expected, after the embolization, LSF scene recognition was more impaired than HSF scene recognition. These data support the hypothesis that the right occipito-temporal cortex might be preferentially specialized for LSF information processing and more generally suggest a hemispheric specialization in SF processing in females, although it is difficult to demonstrate in healthy women.

Detection of shape orientation depends on salient axes of symmetry and elongation: evidence from visual search.

Three experiments investigated the role of the global spatial structure of two-dimensional (2-D) shapes in terms of symmetry and elongation on visual search for shape orientation. Experiment 1 demonstrated the often reported orientation search asymmetry (i.e., a faster detection of a tilted target among vertical distractors than the reverse) for the global orientation of 2-D polygons that possess a salient, “principal” axis of symmetry or elongation. Moreover, the search asymmetry depended on the orientation of the principal axis, rather than on the orientation of local contours. Further exploration of this effect with polygons (Experiment 2) showed that the search asymmetry for global orientation occurred for shapes containing an axis of symmetry; elongation, on the other hand, did not seem to be crucial. Finally, Experiment 3 demonstrated orientation search asymmetries with shapes composed of curved rather than straight contours: Here, the search asymmetry occurred as a function of the orientation of both axes of symmetry and elongation. Overall, search for global orientation was less efficient than search for local orientation. The results suggest that the perception of the global orientation of shapes is mediated by axis-based descriptions in terms of perceptually salient axes of symmetry and elongation.

Nonhomogeneous resolution of images of natural scenes.

The aim of this research is to model and simulate the loss of visual resolution as a function of retinal eccentricity in the perception of natural scenes. The model of visual resolution is based on a space-variant low-pass filter, having a variable convolution kernel according to retinal eccentricity. The parameters of the model are computed from psychophysical measures of visual acuity as a function of retinal eccentricity. The implementation of the model allowed us to generate images of scenes with nonhomogeneous space-variant resolution, simulating the filtering executed by the eye. These scenes are used to test and optimise the model by means of experiments in static vision (through tachistoscopic presentations) and in dynamic vision where the resolution of the scene is computed, in real-time, as a function of the location of gaze.

Selection of Reference Frames and the ‘Vicariance’ of Perceptual Systems

Interindividual differences in field dependence—independence (FDI) which emerge in situations of vision—posture conflict when subjects are required to orient their bodies vertically were investigated. The first aim was to see whether the same interindividual differences are found in judgements of the orientation of forms in focal vision in which subjects have to deal with conflicting spatial references processed by different sensory modalities. The second aim was to test the idea that the FDI dimension is due to functional habits linked to balancing. Subjects performed Kopfermanns (1930) shape-orientation task in either a stable (experiment 1) or an unstable (experiment 2) postural condition. Results showed that the FDI dimension comes into play in the solution of the Kopfermann shape orientation task, and that there is an interactive link between FDI and postural balance, consistent with theoretical expectations. More generally, it appears that the ‘choice’ of a spatial reference system is the product of both individual and situational characteristics, and that the ‘vicariance’ (or inter-changeability) of the sensory systems dealing with gravitational upright is at the basis of this interaction.

Having to identify a target reduces latencies in prosaccades but not in antisaccades.

In a princeps study, Trottier and Pratt (2005) showed that saccadic latencies were dramatically reduced when subjects were instructed to not simply look at a peripheral target (reflexive saccade) but to identify some of its properties. According to the authors, the shortening of saccadic reactions times may arise from a top-down disinhibition of the superior colliculus (SC), potentially mediated by the direct pathway connecting frontal/prefrontal cortex structures to the SC. Using a “cue paradigm” (a cue preceded the appearance of the target), the present study tests if the task instruction (Identify vs. Glance) also reduces the latencies of antisaccades (AS), which involve prefrontal structures. We show that instruction reduces latencies for prosaccade but not for AS. An AS requires two processes: the inhibition of a reflexive saccade and the generation of a voluntary saccade. To separate these processes and to better understand the task effect we also test the effect of the task instruction only on voluntary saccades. The effect still exists but it is much weaker than for reflexive saccades. The instruction effect closely depends on task demands in executive resources.

Visual search for object orientation can be modulated by canonical orientation.

The authors studied the influence of canonical orientation on visual search for object orientation. Displays consisted of pictures of animals whose axis of elongation was either vertical or tilted in their canonical orientation. Target orientation could be either congruent or incongruent with the objects canonical orientation. In Experiment 1, vertical canonical targets were detected faster when they were tilted (incongruent) than when they were vertical (congruent). This search asymmetry was reversed for tilted canonical targets. The effect of canonical orientation was partially preserved when objects were high-pass filtered, but it was eliminated when they were low-pass filtered, rendering them as unfamiliar shapes (Experiment 2). The effect of canonical orientation was also eliminated by inverting the objects (Experiment 3) and in a patient with visual agnosia (Experiment 4). These results indicate that orientation search with familiar objects can be modulated by canonical orientation, and they indicate a top-down influence on orientation processing.