|
Natural visual scenes contain many different objects that cannot be processed all at once due to the limited
processing capacity of the visual system. Therefore, attentional mechanisms are needed to select relevant and to
filter out irrelevant information from our cluttered environments. Using fMRI in humans, we have found that neural
mechanisms of selective attention operate at multiple stages in the visual system, including cortical and subcortical
stages (Kastner et al., 1998;
O'Connor et al., 2002). The modulatory effects of attention at each processing stage
appear to be determined by the visual processing capabilities of that stage. Therefore, attention may be described
as a multi-level selection process (Kastner & Pinsk, 2004; Kastner, 2004).
Our studies on the neural basis of visual attention have begun to reveal a complex neural architecture of distributed
brain systems that may serve different functions to mediate the selection of behaviorally relevant information. First, at
the thalamic level, the LGN appears to be the first stage in the processing of visual information that is modulated by
attention, consistent with the idea that it may play a role as an early gatekeeper in controlling neural gain
(O'Connor et al., 2002;
Schneider et al., 2004). Second, in areas at early cortical processing levels, attention may affect contextual
modulation of neural responses, which may serve important functions in basic mechanisms of scene segmentation and grouping.
Third, areas at intermediate cortical processing levels such as V4 and TEO appear to be important sites at which attention
filters out unwanted information by means of intra- and extra-receptive field mechanisms
(Kastner et al. 1998, 2001;
Pinsk et al., 2004). Fourth, the attention mechanisms that operate in
the visual system appear to be controlled by a distributed network of higher-order areas in frontal and parietal cortex,
which generate top-down signals that are transmitted via feedback connections to the visual system (
Kastner et al., 1999; Kastner & Ungerleider, 2000). And fifth, the pulvinar of the thalamus may operate by
integrating and coordinating attentional functions in concert with the fronto-parietal network (
Kastner et al., 2004).
|
