Home About us Contact
|
Home About us Contact | ||
|
|
||
Visual Stream (visual + stream)
Selected AbstractsThe organization of visual object representations: a connectionist model of effects of lesions in perirhinal cortexEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 2 2002Timothy J. Bussey Abstract We have developed a simple connectionist model based on the idea that perirhinal cortex has properties similar to other regions in the ventral visual stream, or ,what' pathway. The model is based on the assumption that representations in the ventral visual stream are organized hierarchically, such that representations of simple features of objects are stored in caudal regions of the ventral visual stream, and representations of the conjunctions of these features are stored in more rostral regions. We propose that a function of these feature conjunction representations is to help to resolve ,feature ambiguity', a property of visual discrimination problems that can emerge when features of an object predict a given outcome (e.g. reward) when part of one object, but predict a different outcome when part of another object. Several recently reported effects of lesions of perirhinal cortex in monkeys have provided key insights into the functions of this region. In the present study these effects were simulated by comparing the performance of connectionist networks before and after removal of a layer of units corresponding to perirhinal cortex. The results of these simulations suggest that effects of lesions in perirhinal cortex on visual discrimination may be due not to the impairment of a specific type of learning or memory, such as declarative or procedural, but to compromising the representations of visual stimuli. Furthermore, we propose that attempting to classify perirhinal cortex function as either ,perceptual' or ,mnemonic' may be misguided, as it seems unlikely that these broad constructs will map neatly onto anatomically defined regions of the brain. [source] Neural correlates of exemplar novelty processing under different spatial attention conditionsHUMAN BRAIN MAPPING, Issue 11 2009Christian Michael Stoppel Abstract The detection of novel events and their identification is a basic prerequisite in a rapidly changing environment. Recently, the processing of novelty has been shown to rely on the hippocampus and to be associated with activity in reward-related areas. The present study investigated the influence of spatial attention on neural processing of novel relative to frequently presented standard and target stimuli. Never-before-seen Mandelbrot-fractals absent of semantic content were employed as stimulus material. Consistent with current theories, novelty activated a widespread network of brain areas including the hippocampus. No activity, however, could be observed in reward-related areas with the novel stimuli absent of a semantic meaning employed here. In the perceptual part of the novelty-processing network a region in the lingual gyrus was found to specifically process novel events when they occurred outside the focus of spatial attention. These findings indicate that the initial detection of unexpected novel events generally occurs in specialized perceptual areas within the ventral visual stream, whereas activation of reward-related areas appears to be restricted to events that do possess a semantic content indicative of the biological relevance of the stimulus. Hum Brain Mapp, 2009. © 2009 Wiley-Liss, Inc. [source] REVIEW: The functional organization of the intraparietal sulcus in humans and monkeysJOURNAL OF ANATOMY, Issue 1 2005Christian Grefkes Abstract In macaque monkeys, the posterior parietal cortex (PPC) is concerned with the integration of multimodal information for constructing a spatial representation of the external world (in relation to the macaque's body or parts thereof), and planning and executing object-centred movements. The areas within the intraparietal sulcus (IPS), in particular, serve as interfaces between the perceptive and motor systems for controlling arm and eye movements in space. We review here the latest evidence for the existence of the IPS areas AIP (anterior intraparietal area), VIP (ventral intraparietal area), MIP (medial intraparietal area), LIP (lateral intraparietal area) and CIP (caudal intraparietal area) in macaques, and discuss putative human equivalents as assessed with functional magnetic resonance imaging. The data suggest that anterior parts of the IPS comprising areas AIP and VIP are relatively well preserved across species. By contrast, posterior areas such as area LIP and CIP have been found more medially in humans, possibly reflecting differences in the evolution of the dorsal visual stream and the inferior parietal lobule. Despite interspecies differences in the precise functional anatomy of the IPS areas, the functional relevance of this sulcus for visuomotor tasks comprising target selections for arm and eye movements, object manipulation and visuospatial attention is similar in humans and macaques, as is also suggested by studies of neurological deficits (apraxia, neglect, Bálint's syndrome) resulting from lesions to this region. [source] Early neural activity in Necker-cube reversal: Evidence for low-level processing of a gestalt phenomenonPSYCHOPHYSIOLOGY, Issue 1 2004Jürgen Kornmeier Abstract Normally we experience the visual world as stable. Ambiguous figures provide a fascinating exception: On prolonged inspection, the "Necker cube" undergoes a sudden, unavoidable reversal of its perceived front-back orientation. What happens in the brain when spontaneously switching between these equally likely interpretations? Does neural processing differ between an endogenously perceived reversal of a physically unchanged ambiguous stimulus and an exogenously caused reversal of an unambiguous stimulus? A refined EEG paradigm to measure such endogenous events uncovered an early electrophysiological correlate of this spontaneous reversal, a negativity beginning at 160 ms. Comparing across nine electrode locations suggests that this component originates in early visual areas. An EEG component of similar shape and scalp distribution, but 50 ms earlier, was evoked by an external reversal of unambiguous figures. Perceptual disambiguation seems to be accomplished by the same structures that represent objects per se, and to occur early in the visual stream. This suggests that low-level mechanisms play a crucial role in resolving perceptual ambiguity. [source] | ||||||||||