Biomedical subjects

Guillaume A Rousselet

Publications and source records attributed to Guillaume A Rousselet.

8 recordsLinked to original sources

PubMed2005

Spatial scaling factors explain eccentricity effects on face ERPs.

Event-related potential (ERP) studies consistently have described a strong, face-sensitive response termed the N170. This component is maximal at the fovea and decreases strongly with eccentricity, a result that could suggest a foveal bias in the cortical generators responsible for face processing. Here we demonstrate that scaling stimulus size according to V1 cortical magnification factor can virtually eliminate face-related eccentricity effects, indicating that eccentricity effects on face ERPs are largely due to low-level visual factors rather than high-level cortical specialization for foveal stimuli.

Adult↗

PubMed2004

Spatiotemporal analyses of the N170 for human faces, animal faces and objects in natural scenes.

We assessed the specificity to human faces of the N170 ERP component in the context of natural scenes. Subjects categorized photographs containing human faces, animal faces and various objects. Spatiotemporal topography analyses were performed on the individual ERP data. ERPs elicited by animal faces were similar to human faces ERPs but with a delayed face activity. In the N170 time window, ERPs to human and animal faces had a different topography compared with object ERPs. Such data suggest that N170 generators might process various stimuli with a coarse facial organization and show the care that must be taken in comparing scalp signal to faces and other objects as they are probably generated, at least partially, by different cortical sources.

Adult↗

PubMed2004

Animal and human faces in natural scenes: How specific to human faces is the N170 ERP component?

The N170 is an event-related potential component reported to be very sensitive to human face stimuli. This study investigated the specificity of the N170, as well as its sensitivity to inversion and task status when subjects had to categorize either human or animal faces in the context of upright and inverted natural scenes. A conspicuous N170 was recorded for both face categories. Pictures of animal faces were associated with a N170 of similar amplitude compared to pictures of human faces, but with delayed peak latency. Picture inversion enhanced N170 amplitude for human faces and delayed its peak for both human and animal faces. Finally, whether processed as targets or non-targets, depending on the task, both human and animal face N170 were identical. Thus, human faces in natural scenes elicit a clear but non-specific N170 that is not modulated by task status. What appears to be specific to human faces is the strength of the inversion effect.

Adult↗

PubMed2004

Interaction of top-down and bottom-up processing in the fast visual analysis of natural scenes.

The influence of task requirements on the fast visual processing of natural scenes was studied in 14 human subjects performing in alternation an "animal" categorization task and a single-photograph recognition task. Target photographs were randomly mixed with non-target images and flashed for only 20 ms. Subjects had to respond to targets within 1 s. Processing time for image-recognition was 30-40 ms shorter than for the categorization task, both for the fastest behavioral responses and for the latency at which event related potentials evoked by target and non-target stimuli started to diverge. The faster processing in image-recognition is shown to be due to the use of low-level cues, but source analysis produced evidence that, regardless of the task, the dipoles accounting for the differential activity had the same localization and orientation in the occipito-temporal cortex. We suggest that both tasks involve the same visual pathway and the same decisional brain area but because of the total predictability of the target in the image recognition task, the first wave of bottom-up feed-forward information is speeded up by top-down influences that might originate in the prefrontal cortex and preset lower levels of the visual pathway to the known target features.

Adult↗

PubMed2004

How parallel is visual processing in the ventral pathway?

Visual object perception is usually studied by presenting one object at a time at the fovea. However, the world around us is composed of multiple objects. The way our visual system deals with this complexity has remained controversial in the literature. Some models claim that the ventral pathway, a set of visual cortical areas responsible for object recognition, can process only one or very few objects at a time without ambiguity. Other models argue in favor of a massively parallel processing of objects in a scene. Recent experiments in monkeys have provided important data about this issue. The ventral pathway seems to be able to perform complex analyses on several objects simultaneously, but only during a short time period. Subsequently only one or very few objects are explicitly selected and consciously perceived. Here, we survey the implications of these new findings for our understanding of object processing.

Consciousness↗

PubMed2004

Processing of one, two or four natural scenes in humans: the limits of parallelism.

The visual processing of objects in natural scenes is fast and efficient, as indexed by behavioral and ERP data [Nature 381 (1996) 520]. The results from a recent experiment suggested that such fast routines work in parallel across the visual field when subjects were presented with two natural scenes simultaneously [Nature Neurosci. 5 (2002) 629]. In the present experiment, the visual system was driven to its limits by presenting one, two or four scenes simultaneously. Behavior and ERP reveal a clear cost in processing an increasing number of scenes. However, a parallel-late selection model can still account for the results. This model is developed and discussed with reference to behavioral, single-unit and ERP data.

Electroencephalography↗

PubMed2003

Is it an animal? Is it a human face? Fast processing in upright and inverted natural scenes.

Object categorization can be extremely fast. But among all objects, human faces might hold a special status that could depend on a specialized module. Visual processing could thus be faster for faces than for any other kind of object. Moreover, because face processing might rely on facial configuration, it could be more disrupted by stimulus inversion. Here we report two experiments that compared the rapid categorization of human faces and animals or animal faces in the context of upright and inverted natural scenes. In Experiment 1, the natural scenes contained human faces and animals in a full range of scales from close-up to far views. In Experiment 2, targets were restricted to close-ups of human faces and animal faces. Both experiments revealed the remarkable object processing efficiency of our visual system and further showed (1) virtually no advantage for faces over animals; (2) very little performance impairment with inversion; and (3) greater sensitivity of faces to inversion. These results are interpreted within the framework of a unique system for object processing in the ventral pathway. In this system, evidence would accumulate very quickly and efficiently to categorize visual objects, without involving a face module or a mental rotation mechanism. It is further suggested that rapid object categorization in natural scenes might not rely on high-level features but rather on features of intermediate complexity.

Adult↗

PubMed2002

Parallel processing in high-level categorization of natural images.

Models of visual processing often include an initial parallel stage that is restricted to relatively low-level features, whereas activation of higher-level object descriptions is generally assumed to require attention. Here we report that even high-level object representations can be accessed in parallel: in a rapid animal versus non-animal categorization task, both behavioral and electrophysiological data show that human subjects were as fast at responding to two simultaneously presented natural images as they were to a single one. The implication is that even complex natural images can be processed in parallel without the need for sequential focal attention.

Adult↗