The effect of image fractal properties and its interaction with visual discomfort on gait kinematics.

Exposure to images of urban environments affords higher cognitive processing demands than exposure to images of nature scenes; an effect potentially related to differences in low-level image statistics such as fractals. The aim of the current study was to investigate whether the fractal dimensions of an abstract scene affect cognitive processing demands, using gait kinematics as a measure of cognitive demand. Participants (n = 40) were asked to walk towards different types of synthetic images which were parametrically varied in their fractal dimensions. At the end of each walk, participants rated each image for its visual discomfort (n = 20) or for its likability (n = 20) as potential confounding factors. Fractal dimensions were predictors of walking speed. Moreover, the interaction between fractal dimensions and subjective visual discomfort but not liking predicted velocity. Overall, these data suggest that fractal dimensions indeed contribute to environmentally induced cognitive processing demands.

The nature effect in motion: visual exposure to environmental scenes impacts cognitive load and human gait kinematics.

Prolonged exposure to urban environments requires higher cognitive processing resources than exposure to nature environments, even if only visual cues are available. Here, we explored the moment-to-moment impact of environment type on visual cognitive processing load, measuring gait kinematics and reaction times. In Experiment 1, participants (n = 20) walked toward nature and urban images projected in front of them, one image per walk, and rated each image for visual discomfort. Gait speed and step length decreased for exposure to urban as compared with nature scenes in line with gait changes observed during verbal cognitive load tasks. We teased apart factors that might contribute to cognitive load: image statistics and visual discomfort. Gait changes correlated with subjective ratings of visual discomfort and their interaction with the environment but not with low-level image statistics. In Experiment 2, participants (n = 45) performed a classic shape discrimination task with the same environmental scenes serving as task-irrelevant distractors. Shape discrimination was slower when urban scenes were presented, suggesting that it is harder to disengage attention from urban than from nature scenes. This provides converging evidence that increased cognitive demands posed by exposure to urban scenes can be measured with gait kinematics and reaction times even for short exposure times.

Rightward bisection errors for letter lines: the role of semantic information.

When bisecting words in their middle, people reveal leftward bisection errors. This tendency might emerge from an attentional bias towards the beginning of the word. However, when longer meaningless letter strings are presented, people reveal a rightward bisection bias. To test the role of semantic information on leftward or rightward bisection biases, we tested letter line bisection performance in healthy right-handed students in four independent experiments. A third of the letter lines contained an embedded four-letter word to the left of true centre, another third contained an embedded four-letter word to the right of true centre, while the remaining lines contained no words. Half of these words were emotional words, the other half were neutral words. Results across experiments revealed a stronger rightward bisection bias: (i) for letter lines containing emotional as compared to neutral words, (ii) for letter lines containing words in the left as compared to right half of the lines, and (iii) for those experiments in which the spatial position of letter lines remained within a narrow body-centred space. Findings from this study suggest that letter line bisection performance might be only minimally determined by visuo-spatial attention. Rather, letter line perception might activate the left hemisphere more than the right hemisphere, shifting the subjective midpoint to the right of true centre. Leftward bisection biases for words only, as had been described in the literature, may thus have resulted from automated reading strategies rather than from attentional biases towards the left hemispace.

Stability of the color-opponent signals under changes of illuminant in natural scenes.

Illumination varies greatly both across parts of a natural scene and as a function of time, whereas the spectral reflectance function of surfaces remains more stable and is of much greater relevance when searching for specific targets. This study investigates the functional properties of postreceptoral opponent-channel responses, in particular regarding their stability against spatial and temporal variation in illumination. We studied images of natural scenes obtained in UK and Uganda with digital cameras calibrated to produce estimated L-, M-, and S-cone responses of trichromatic primates (human) and birds (starling). For both primates and birds we calculated luminance and red-green opponent (RG) responses. We also calculated a primate blue-yellow-opponent (BY) response. The BY response varies with changes in illumination, both across time and across the image, rendering this factor less invariant. The RG response is much more stable than the BY response across such changes in illumination for primates, less so for birds. These differences between species are due to the greater separation of bird L and M cones in wavelength and the narrower bandwidth of the cone action spectra. This greater separation also produces a larger chromatic signal for a given change in spectral reflectance. Thus bird vision seems to suffer a greater degree of spatiotemporal "clutter" than primate vision, but also enhances differences between targets and background. Therefore, there may be a trade-off between the degree of chromatic clutter in a visual system versus the degree of chromatic difference between a target and its background. Primate and bird visual systems have found different solutions to this trade-off.

Aging and working memory: early deficits in EEG activation of posterior cortical areas.

Using the n-back task, we recently identified, in young subjects, a positive-negative event related potential component (PN(wm)) in a time-range window between 140 and 280 ms after stimulus onset representing an electrophysiological correlate of working memory load. To evaluate age-related electrophysiological changes in working memory processing, we applied the same neuropsychological paradigm and compared densities of the PN(wm) component in 17 young (mean age: 26) and 17 healthy elderly individuals (mean age: 75). Both age groups displayed a PN(wm) component during the two working memory tasks. For the 1-back task, densities were similar in both young and elderly individuals. In contrast, PN(wm) densities increased with higher memory load (2-back>1-back) in the younger but not in the older group. This difference was mainly observed over parietal electrodes suggesting an impaired activation of neural generators within this brain region. The present results are consistent with the hypothesis of decreased brain reserve in the elderly and provide evidence for age-related deficits in the recruitment of posterior cortical neurons with increasing working memory load.

The role of stimulus type in age-related changes of visual working memory.

Aging is accompanied by increasing difficulty in working memory associated with the temporary storage and processing of goal-relevant information. Face recognition plays a preponderant role in human behavior, and one might therefore suggest that working memory for faces is spared from age-related decline compared to socially less important visual stimulus material. To test this hypothesis, we performed working memory (n-back) tasks with two different visual stimulus types, namely faces and doors, and compared them to tasks with primarily verbal material, namely letters. Age-related reaction time slowing was comparable for all three stimulus types, supporting hypotheses on general cognitive and motor slowing. In contrast, performance substantially declined with age for faces and doors, but little for letters. Working memory for faces resulted in significantly better performance than that for doors and was more sensitive to on-line manipulation errors such as the temporal order. All together, our results show that even though face perception might play a specific role in visual processing, visual working memory for faces undergoes the same age-related decline as it does for socially less relevant visual material. Moreover, these results suggest that working memory decline cannot be solely explained by increasing vulnerability in prefrontal cortex related to executive functioning, but indicate an age-related decrease in a visual short-term buffer, possibly located in the temporal cortex.

Localization of human frontal eye fields: anatomical and functional findings of functional magnetic resonance imaging and intracerebral electrical stimulation.

OBJECT: The goal of this study was to investigate the anatomical localization and functional role of human frontal eye fields (FEFs) by comparing findings from two independently conducted studies. METHODS: In the first study, 3-tesla functional magnetic resonance (fMR) imaging was performed in 14 healthy volunteers divided into two groups: the first group executed self-paced voluntary saccades in complete darkness and the second group repeated newly learned or familiar sequences of saccades. In the second study, intracerebral electrical stimulation (IES) was performed in 38 patients with epilepsy prior to surgery, and frontal regions where stimulation induced versive eye movements were identified. These studies showed that two distinct oculomotor areas (OMAs) could be individualized in the region classically corresponding to the FEFs. One OMA was consistently located at the intersection of the superior frontal sulcus with the fundus of the superior portion of the precentral sulcus, and was the OMA in which saccadic eye movements could be the most easily elicited by electrical stimulation. The second OMA was located more laterally, close to the surface of the precentral gyrus. The fMR imaging study and the IES study demonstrated anatomical and stereotactic agreement in the identification of these cortical areas. CONCLUSIONS: These findings indicate that infracentimetric localization of cortical areas can be achieved by measuring the vascular signal with the aid of 3-tesla fMR imaging and that neuroimaging and electrophysiological recording can be used together to obtain a better understanding of the human cortical functional anatomy.

Human cortical networks for new and familiar sequences of saccades.

Visual exploration is organized in sequences of saccadic eye movements that depend on both perceptual and cognitive context. Using functional magnetic resonance imaging, we studied the neural basis of sequential oculomotor behavior and its dependence on different types of memory by analyzing cerebral activity during performance of newly learned and familiar sequences of eye movements. Compared to a resting condition, both types of sequences activated a common fronto-parietal network, including frontal and supplementary eye fields, and several parietal areas. Within this network, newly learned sequences induced stronger activation than familiar sequences, probably reflecting higher attentional demands. In addition, specific regions were recruited for the performance of new sequences, including pre-supplementary eye fields, the precuneus and the caudate nucleus. This indicates that in addition to attentional modulation, novelty of saccadic sequences requires specific cortical resources, probably related to effortful sequence preparation and coordination as well as to spatial working memory. For familiar sequences, recalled from long-term memory, we observed specific right medial temporo-occipital activation in the vicinity of the boundary between the parahippocampal and lingual gyri, as well as an activation site in the parieto-occipital fissure. We conclude that neuronal resources recruited by the gaze system can change with the familiarity of the scanpath to be executed. This study is important to better understand how the brain implements memorized scanpaths for visual exploration and orienting.

Conjunctions of colour, luminance and orientation: the role of colour and luminance contrast on saliency and proximity grouping in texture segregation.

To examine whether perceptual grouping on the basis of orientation can be performed simultaneously with or only subsequently to grouping according to colour or luminance, we tested whether subjects are able to segregate arrays of texture elements that differ from surrounding elements by conjunctions of either (i) colour and orientation, or (ii) luminance contrast and orientation, or (iii) luminance contrast polarity and orientation. Subjects were able to use conjunctions between luminance and orientation for segregation but not conjunctions between colour or contrast polarity and orientation. Our results suggest that (i) in agreement with earlier findings, there seem to exist no specific conjunction detectors for colour and orientation or contrast polarity and orientation, and (ii) when orientation defined textures are to be distinguished by virtue of differences in luminance, colour, or contrast polarity, luminance provides a much stronger cue than colour or contrast polarity for saliency-based orientation grouping.

Attention mechanisms in visual search -- an fMRI study.

The human visual system is usually confronted with many different objects at a time, with only some of them reaching consciousness. Reaction-time studies have revealed two different strategies by which objects are selected for further processing: an automatic, efficient search process, and a conscious, so-called inefficient search [Treisman, A. (1991). Search, similarity, and integration of features between and within dimensions. Journal of Experimental Psychology: Human Perception and Performance, 17, 652--676; Treisman, A., & Gelade, G. (1980). A feature integration theory of attention. Cognitive Psychology, 12, 97--136; Wolfe, J. M. (1996). Visual search. In H. Pashler (Ed.), Attention. London: University College London Press]. Two different theories have been proposed to account for these search processes. Parallel theories presume that both types of search are treated by a single mechanism that is modulated by attentional and computational demands. Serial theories, in contrast, propose that parallel processing may underlie efficient search, but inefficient searching requires an additional serial mechanism, an attentional "spotlight" (Treisman, A., 1991) that successively shifts attention to different locations in the visual field. Using functional magnetic resonance imaging (fMRI), we show that the cerebral networks involved in efficient and inefficient search overlap almost completely. Only the superior frontal region, known to be involved in working memory [Courtney, S. M., Petit, L., Maisog, J. M., Ungerleider, L. G., & Haxby, J. V. (1998). An area specialized for spatial working memory in human frontal cortex. Science, 279, 1347--1351], and distinct from the frontal eye fields, that control spatial shifts of attention, was specifically involved in inefficient search. Activity modulations correlated with subjects' behavior best in the extrastriate cortical areas, where the amount of activity depended on the number of distracting elements in the display. Such a correlation was not observed in the parietal and frontal regions, usually assumed as being involved in spatial attention processing. These results can be interpreted in two ways: the most likely is that visual search does not require serial processing, otherwise we must assume the existence of a serial searchlight that operates in the extrastriate cortex but differs from the visuospatial shifts of attention involving the parietal and frontal regions.

Parallel visual search is not always effortless.

Serial visual search can become parallel with practice. We tested whether this parallelisation is accompanied by a transition from an attentive to a pre-attentive and thus effortless mode of visual perception. While psychophysiological indicators of attentional effort (galvanic skin conductance, muscle tonus) reflected the distinction between serial and parallel search modes, they did not follow the perceptual changes during learning. Despite the perceptual parallelisation with practice, the attentional load remained high for initially serial tasks.

Two segmentation mechanisms with differential sensitivity for colour and luminance contrast.

In a texture-segregation paradigm, subjects were asked to detect figures whose elements were segregated from background either because of temporal offset or because of differing orientations. Texture elements were either isoluminant or had high or low luminance contrast. At high luminance contrast, figures could be segregated both on the basis of orientation and temporal cues whereby temporal offsets as short as 10 msec supported detection. At isoluminance, orientation defined figures were as readily distinguishable as in the high contrast condition but temporally defined figures were perceived only for offset intervals > 50 msec. With low luminance contrast, performance for orientation defined figures was impaired relative to the high contrast condition, but for temporally defined figures, it was superior to the isoluminant condition; detection was possible for offset intervals as short as 22 msec. These results suggest that the temporal and orientation cues which support scene segmentation are transmitted by both the luminance and colour sensitive pathways. However, if temporal offsets are < 50 msec, segmentation of temporally defined figures is supported only by the luminance sensitive system.

Selective temporal interactions between processing streams with differential sensitivity for colour and luminance contrast.

Temporal interactions between spatially separated visual stimuli were investigated in human observers. Subjects had to judge whether briefly presented targets consisted of a single or a double flash. Simultaneous presentation of unattended single or double flash distractors impaired performance if target and distractor followed different time courses, confirming previous findings. This interference occurred only when targets had high luminance contrast or were isoluminant and when distractors had high or low luminance contrast, but not when targets had low luminance contrast or when distractors were isoluminant. Low luminance contrast distractors strongly influenced high luminance contrast targets but not vice versa. These results suggest that (i) information about the precise temporal structure of stimuli is conveyed preferentially by the luminance-sensitive magnocellular system; and (ii) that this information influences judgements on the temporal patterning of signals transmitted by the colour-sensitive parvocellular system.

The influence of temporal phase differences on texture segmentation.

Scene segmentation and perceptual grouping are important operations in visual processing. Pattern elements constituting individual perceptual objects need to be segregated from those of other objects and the background and have to be bound together for further joint evaluation. Both textural (spatial) and temporal cues are exploited for this grouping operation. Thus, pattern elements might get bound that share certain textural features and/or appear in close spatial or temporal contiguity. However, results on the involvement of temporal cues in perceptual grouping are contradictory [Kiper et al. (1991). Society for Neuroscience Abstracts, 17, 1209; Fahle (1993). Proceedings of the Royal Society of London B, 254, 199-203]. We therefore reinvestigated the relative contributions of temporal and spatial cues and their interactions in a texture-segmentation paradigm. Our data show that the visual system can segregate figures solely on the basis of temporal cues if the temporal offset between figure and ground elements exceeds 10 msec. Moreover, segregation of figures defined by orientation differences among pattern elements is facilitated by additional temporal cues if these define the same figure. If temporal and textural cues define different figures, the two cues compete and only the more salient pattern is perceived. By contrast, the detection of a figure defined by orientation is not impaired by conflicting temporal cues if these do not define a figure themselves and do not exceed offset intervals of 100 msec. These results indicate the existence of a flexible binding mechanism that exploits both temporal and textural cues either alone or in combination if they serve perceptual grouping but can exclude either of the two cues if they are in conflict or do not define a figure. It is proposed that this flexibility is achieved by the implementation of two segmentation mechanisms which differ in their sensitivity for spatial and temporal cues and interact in a facultative way.

Interocular suppression in normal and amblyopic subjects: the effect of unilateral attenuation with neutral density filters.

In normal subjects, binocular rivalry suppression takes time to build up (Wolfe, 1986a). The time courses of interocular suppression are different and heterogeneous in amblyopic subjects (de Belsunce & Sireteanu, 1991). In the present study, we found that, in normal observers, progressive reduction of one eye's stimulus luminance with neutral density filters produces time courses similar to those of amblyopic subjects. Conversely, in amblyopes, attenuation of the dominant eye's stimulus produces time courses similar to those of normal observers. Under conditions of balancing of the two eyes, amblyopes experience alternating suppression, similarly to binocular rivalry of normals.