Retinotopic mapping in awake monkeys suggests a different functional organization for dorsal and ventral V4

Using functional magnetic resonance imaging, we mapped the retinotopic organization throughout the visual cortex of fixating monkeys. The observed retinotopy in V1, V2 and V3 was completely consistent with the classical view. More rostrally in occipital cortex, both areas V3A and MT/V5 had a lower and upper visual field representation split by a horizontal meridian. Both areas were almost completely surrounded by a vertical meridian representa- tion. Ventral, but not dorsal V4 was rostrally bordered by a horizontal meridian. Furthermore, contrary to all other early visual areas including V4v, the eccentricity lines ran almost parallel to the areal boundaries in V4d. These results suggest a different functional organization in dorsal and ventral V4, similar to what has been observed in human

Stereopsis Activates V3A and Caudal Intraparietal Areas in Macaques and Humans

Stereopsis, the perception of depth from small differences between the images in the two eyes, provides a rich model for investigating the cortical construction of surfaces and space. Although disparity-tuned cells have been found in a large number of areas in macaque visual cortex, stereoscopic processing in these areas has never been systematically compared using the same stimuli and analysis methods. In order to examine the global architecture of stereoscopic processing in primate visual cortex, we studied fMRI activity in alert, fixating human and macaque subjects. In macaques, we found strongest activation to near/far compared to zero disparity in areas V3, V3A, and CIPS. In humans, we found strongest activation to the same stimuli in areas V3A, V7, the V4d topolog (V4d-topo), and a caudal parietal disparity region (CPDR). Thus, in both primate species a small cluster of areas at the parieto-occipital junction appears to be specialized for stereopsis

Bases cérébrales de la catégorisation visuelle rapide -Etudes chronométriques et fonctionnelles

Visual processing is generaly understood as resulting from a complex and iterative processing. After discussing six models of visual object recognition, we bring out that the time required to perform complex visual processing is a crucial criterion for understanding vision. We first propose an experimental categorization task in order to determine the time required by subjects to decide wether an animal is present in photographs of natural scenes. Results show that the minimal visual processing time takes less than 150 milliseconds. This speed of visual processing is particularly robust : this latency does not change when presentations are extrafoveal, when the attention is not focused on the stimulus locus, and when the colors of the images are removed. This speed of visual processing does not differ significantly when stimuli are simple shapes, or when subjects have to decide whether the images are in color. Moreover, electrophysiological signs of an implicit categorization of « Animal » scenes appear significantly when new subjects are performing another task. These results show that this task can be achieved by preattentive, parallel, feedforward visual processes. Focused attention, color and foveal acuity are not required to perform this task rapidly.The cerebral bases of this task have been found with dipolar modelisation of ERPs, and with a new event-related fMRI protocol. The task differentialy activates mainly the areas 19 and 31, the fusiform and cingulate gyri. Less activity was found following target trials than distractor ones, which we interpret as inhibition processes, compatible with the competitive attentional models. Visual object recognition could be then considered as simple feature parallel detections. The visual decision (adequation between the actual stimulus and the task performed) could be achieved via inhibition of alternative representations, that could be acts as attentional selection.Après un rapide rappel des principaux résultats de la psychologie et des neurosciences de la vision, illustrés par le schéma de Kosslyn, le parcours de six modèles computationnels de reconnaissance nous amène à discuter des principales alternatives élaborées pour décrire le traitement visuel – généralement compris comme complexe et récurrent. Le temps requis par ce traitement apparaît comme un critère crucial de décision sur son fonctionnement et d'affinement de notre compréhension.Nous constatons que les données d'électrophysiologie disponibles ne permettent pas de disposer clairement de ce critère. Nous mettons alors en oeuvre une tâche expérimentale visant à mesurer le temps nécessaire au système visuel humain pour analyser des scènes naturelles contenant ou non un animal. Les résultats montrent que ce traitement peut être extrêmement rapide, d'une durée inférieure à 150ms. Cette première mesure est complétée par quatre expériences visant à mieux cerner cette contrainte temporelle, en variant les positions des images, leurs couleurs et la tâche. Cette vitesse du traitement visuel des scènes naturelles se montre particulière-ment robuste et constante : lors de présentations par hémichamps parafovéaux, lorsque l'attention n'est pas focalisée sur le lieu d'apparition du stimulus, et en l'absence de couleur comme indice de recon-naissance. Les résultats attenants montrent aussi que les catégorisations d'images contenant des formes simples et la détection de la présence de couleurs ne sont pas plus rapides. La catégorisation "animal" semble d'autant plus résulter d'un mécanisme automatique que sa trace électrophysiologique est encore présente lorsqu'une autre tâche occupe les sujets.Les bases cérébrales de la tâche ont été recherchées à l'aide de modèles dipolaires ainsi qu'avec la création d'un protocole événementiel d'imagerie cérébrale RMN analogue à celui mis en oeuvre en électrophysiologie. Nous montrons que cette tâche decatégorisation implique de manière différentiée les aires visuelles extrastriées 19 et 31, le gyrus fusiforme et les cortex cingulaires postérieurs. Dans les aires visuelles, un effet de suppression d'activité neuronale lié à la présence d'une cible semble mettre en évidence le mécanisme de compétition postulée dans certains modèles.Ces résultats plaident en faveur de mécanismes directs et rapides de la reconnaissance visuelle : traitement essentiellement ascendants (sans boucles) sans recentrage des stimuli latéralisés ; l'attention focalisée, la couleur et une forte acuité nesont pas nécessaires à la reconnaissance d'objets dans des scènes complexes.La reconnaissance visuelle postulée comme mécanisme nécessitant des traitementsrécurrents et des représentations complexes semble ainsi céder la place à de simples détections parallèles de traits visuels, en eux-mêmes suffisants à la représentation mentale des scènes. Dans ce cadre, la décision visuelle - le stimulus présent est adéquat à la tâche prévue – pourrait être l'extraction de ces représentations au moyen de l'inhibition des assemblées neuronales non sélectionnées

Fast ventral stream neural activity enables rapid visual categorization

International audiencePrimates can recognize objects embedded in complex natural scenes in a glimpse. Rapid categorization paradigms have been extensively used to study our core perceptual abilities when the visual system is forced to operate under strong time constraints. However, the neural underpinning of rapid categorization remains to be understood, and the incredible speed of sight has yet to be reconciled with modern ventral stream cortical theories of object recognition.Here we recorded multichannel subdural electrocorticogram (ECoG) signals from intermediate areas (V4/PIT) of the ventral stream of the visual cortex while monkeys were actively engaged in a rapid animal/non-animal categorization task. A traditional event-related potential (ERP) analysis revealed short visual latencies (< 50–70 ms) followed by a rapidly developing visual selectivity (within ~ 20–30 ms) for most electrodes. A multi-variate pattern analysis (MVPA) technique further confirmed that reliable animal/non-animal category information was possible from this initial ventral stream neural activity (within ~ 90–100 ms). Furthermore, this early category-selective neural activity was unaffected by the presentation of a backward (pattern) mask, generalized to novel (unfamiliar) stimuli and co-varied with behavioral responses (both accuracy and reaction times). Despite the strong prevalence of task-related information on the neural signal, task-irrelevant visual information could still be decoded independently of monkey behavior. Monkey behavioral responses were also found to correlate significantly with human behavioral responses for the same set of stimuli.Together, the present study establishes that rapid ventral stream neural activity induces a visually selective signal subsequently used to drive rapid visual categorization and that this visual strategy may be shared between human and non-human primates

Early interference of context congruence on object processing in rapid visual categorization of natural scenes

International audienceWhereas most scientists agree that scene context can influence object recognition, the time course of such object/context interactions is still unknown. To determine the earliest interactions between object and context processing, we used a rapid go/no-go categorization task in which natural scenes were briefly flashed and subjects required to respond as fast as possible to animal targets. Targets were pasted on congruent (natural) or incongruent (urban) contexts. Experiment 1 showed that pasting a target on another congruent background induced performance impairments, whereas segregation of targets on a blank background had very little effect on behavior. Experiment 2 used animals pasted on congruent or incongruent contexts. Context incongruence induced a 10% drop of correct hits and a 16-ms increase in median reaction times, affecting even the earliest behavioral responses. Experiment 3 replicated the congruency effect with other subjects and other stimuli, thus demonstrating its robustness. Object and context must be processed in parallel with continuous interactions possibly through feed-forward co-activation of populations of visual neurons selective to diagnostic features. Facilitation would be induced by the customary co-activation of “congruent” populations of neurons whereas interference would take place when conflictual populations of neurons fire simultaneously

All-or-none activity as a correlate of object awareness in monkey visual cortex

Recurring activity in visual areas has been argued to have an essential role in object aware recognition. However, this has been hard to prove, mainly due to the difficulty in dissociating low-level feature extraction from the actual object recognition activity. Here we used an innovative technique called Semantic Wavelet-Induced Frequency-Tagging (SWIFT), where cyclic wavelet-scrambling allowed us to isolate neural correlates of the semantic extraction from low-level features processing of the image. Electrocorticogram electrodes placed intracranially over ventral visual areas from V2 to TEO allowed us to record neural activity with both high temporal and spatial resolution. One macaque monkey was trained to perform an animal/non-animal categorization task. In each trial a SWIFT sequence containing either a target (an animal) or a distractor (a landscape, object or meaningless texture) was presented. The monkey reported the presence or absence of a target by a go or no-go manual response respectively. In each session, one third of the trials corresponded to new images, making the task quite challenging (about 65% correct responses on targets). Event-related potential (ERP) analysis of local sources revealed two ERP components in ventral visual areas. A first positive (P1) component, representing the feed-forward sweep, peaked around 100 ms; while a second positive (P2) component, likely representing recurring reactivation, appeared from 200 ms after the semantic onset. The P1 component was present either the target was recognized or not and its amplitude was modulated by stimulus category (low amplitude for meaningless texture distractors, medium amplitude for object distractors and high amplitude for animal targets). On the other hand, the P2 component was only present when the target was recognized or when a distractor elicited a false alarm, but totally absent otherwise, either when the target was not recognized or when a distractor was correctly rejected, thus being modulated in an all-or-none fashion by image recognition as a target of the task. Importantly, this P2 modulation was observed when comparing the same images before and after being recognized as a target, demonstrating that the P2 component is a specific feature related to aware image recognition