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Christel Malden
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Prior knowledge regarding the possible identity of an object facilitates its recognition from a degraded visual input, though the underlying mechanisms are unclear. Previous work implicated ventral visual cortex but did not disambiguate whether activity-changes in these regions are causal to or merely reflect an effect of facilitated recognition. We used functional magnetic resonance imaging to study top-down influences on processing of gradually revealed objects, by preceding each object with a name that was congruent or incongruent with the object. Congruently primed objects were recognized earlier than incongruently primed, and this was paralleled by shifts in activation profiles for ventral visual, parietal, and prefrontal cortices. Prior to recognition, defined on a trial-by-trial basis, activity in ventral visual cortex rose gradually but equivalently for congruently and incongruently primed objects. In contrast, prerecognition activity was greater with congruent priming in lateral parietal, retrosplenial, and lateral prefrontal cortices, whereas functional coupling between parietal and ventral visual (and also left lateral prefrontal and parietal) cortices was enhanced in the same context. Thus, when controlling for recognition point and stimulus information, activity in ventral visual cortex mirrors recognition success, independent of condition. Facilitation by top-down cues involves lateral parietal cortex interacting with ventral visual areas, potentially explaining why parietal lesions can lead to deficits in recognizing degraded objects even in the context of top-down knowledge.
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Visual objects in the real world are seen in contextual scenes. These contexts are usually coherent in terms of their physical and semantic content, and they usually occur in typical configurations. Objects can be used to make predictions about probable contexts and about other objects that might be found in the same scene, and contexts can be used to inform the identification of individual objects. A full understanding of object recognition must include a consideration of contextual and associative influences.
'Context frames' might be used as structures of prototypical contexts that represent information about the identity of, and relationships between, objects that are likely to be present in each context (for example, a prototypical bathroom would contain a sink and a mirror, with the mirror typically set above the sink).
These context frames can be viewed as sets of expectations that are derived from exposure to real-world scenes. During recognition, a single object can activate appropriate context frames, and context frames can activate representations of expected objects. Scenes and individual objects can facilitate identification of each other and of other objects that are expected to occur in the same context.
To be useful for facilitating object recognition, the gist of a scene must be extracted and rapidly processed. This rapid extraction might rely on global cues conveyed by low spatial frequencies in an image, with higher spatial frequencies providing details gradually and slowly.
Structures within the medial temporal lobe are thought to be important for associative processing. The prefrontal and retrosplenial cortex also seem to be important for processing contextual information. I propose that the parahippocampal cortex serves as a switchboard-like multiplexer that connects the representations of individual objects in the inferior temporal cortex, according to typical associations represented in context frames.
In the proposed model, a blurred, low-frequency representation of a scene is projected rapidly from the visual cortex to the parahippocampal areas, and a context frame is activated on the basis of an experience-based guess. This context frame activates associated representations of objects in the inferior temporal cortex. Simultaneously, the low-frequency image of a fixated object in the scene is also projected rapidly to the prefrontal cortex, which sensitizes the representations of objects that resemble the fixated object. In the inferior temporal cortex, these two sets of objects intersect and the object can be identified.
We see the world in scenes, where visual objects occur in rich surroundings, often embedded in a typical context with other related objects. How does the human brain analyse and use these common associations? This article reviews the knowledge that is available, proposes specific mechanisms for the contextual facilitation of object recognition, and highlights important open questions. Although much has already been revealed about the cognitive and cortical mechanisms that subserve recognition of individual objects, surprisingly little is known about the neural underpinnings of contextual analysis and scene perception. Building on previous findings, we now have the means to address the question of how the brain integrates individual elements to construct the visual experience.
I would like to thank members of my lab, E. Aminoff, H. Boshyan, M. Fenske, A. Ghuman, N. Gronau and K. Kassam, as well as A. Torralba, N. Donnelly, M. Chun, B. Rosen and A. Oliva for help with this article. Supported by the National Institute of Neurological Disorders and Stroke, the James S. McDonnell Foundation (21st Century Science Research Award in Bridging Brain, Mind and Behavior) and the MIND Institute.
The level of abstraction that carries the most information, and at which objects are typically named most readily. For example, subjects would recognize an Australian Shepherd as a dog (that is, basic-level) more easily than as an animal (that is, superordinate-level) or as an Australian Shepherd (that is, subordinate-level).
An experience-based facilitation in perceiving a physical stimulus. In a typical object priming experiment, subjects are presented with stimuli (the primes) and their performance in object naming is recorded. Subsequently, subjects are presented with either the same stimuli or stimuli that have some defined relationship to the primes. Any stimulus-specific difference in performance is taken as a measure of priming.
(MEG). A non-invasive technology for functional brain mapping, which provides superior millisecond temporal resolution. It measures magnetic fields generated by electric currents from active neurons in the brain. By localizing the sources of these currents, MEG is used to reveal cortical function.
Originally described as a negative deflection in the event-related potential waveform occurring approximately 400 ms following the onset of contextually incongruent words in a sentence. It has consistently been linked to semantic processing. Although it is probably one of the best neural signatures of contextual processing, its exact functional significance has yet to be elucidated.
Use a priori probability distributions derived from experience to infer optimal expectations. They are based on Bayes' theorem, which can be seen as a rule for taking into account history information to produce a number representing the probability that a certain hypothesis is true.
Builds on Hebb's learning rule that the connections between two neurons will strengthen if the neurons fire simultaneously. The original Hebbian rule has serious limitations, but it is used as the basis for more powerful learning rules. From a neurophysiological perspective, Hebbian learning can be described as a mechanism that increases synaptic efficacy as a function of synchrony between pre- and postsynaptic activity.
I'd like to be able to place more than two colors on the wheel, but I'm not sure that it's possible with visual.RadialStim. Looking through the documentation, I can't see anything that helps, though, I did come across this old thread where Jon seems to suggest that it is possible, but I frankly can't make heads or tails of it and I think the OP left it unresolved as well. Does anyone know if my suspicions about RadialStim are correct (i.e., can't use more than two colors)? Alternatively, does anyone have another recommended solution to replacing it so that I could maybe get 3 or 4 colors modeled on this larger circle?
PLEASE NOTE - for those unfamiliar with PsychoPy, it is a collection functions specially built for creating reseach studies and requires any code that uses it to be run in a PsychoPy terminal (rather than just any old Python terminal). PsychoPy can run any Python package, but a Python terminal cannot run PsychoPy code, so if you tried to run this on your own without PsychoPy, it likely will not work.
Try as I could, I could not get the texture approach to work, but I settled on a far less eloquent solution. By reducing the opacity of the RadialStim and overlaying another RadialStim of a complementary color at half opacity, and situated at a 30-degree angle, I was able to more or less create the appearance of four colors. Not thrilled, but it'll do for now. Looking forward to someone else showing me up.
Recent studies in human adults have shown that they are sensitive to crossmodal spatiotemporal dynamics between external objects and the body. In fact, a number of authors now argue that the special status of representations of peripersonal spatial events in the brain and behaviour (shown, e.g., in speeded responses to stimuli close to the body) may be explained by the predictive mechanisms at play when somatosensory processing is modulated by prior visual, auditory or audiovisual stimuli that move towards the body1,11,27. Evidence for this account comes from a number of studies demonstrating that responses to tactile stimuli can be modulated by predictive but spatially or temporally distant stimuli in a different sensory modality (i.e. vision)9,10. The key novelty of these findings is their focus on crossmodal interactions via predictive relations between visual and somatosensory events, which cannot be mediated via exogenous crossmodal effects due to colocation or synchrony between the visual and tactile stimuli28,29. These findings support the existence of predictive mechanisms using visual motion cues to make judgments about the time and location of an impending tactile stimulus and enhancing tactile processing at the time and location of impending contact10.
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