Attention-dependent suppression of metabolic activity in the early stages of the macaque visual system

Attention-dependent suppression of distracter visual input can be cross-modally cued as indexed by anticipatory parieto-occipital alpha-band oscillations

Recent studies show that in addition to enhancing neural processing for attentionally relevant stimuli, selective attention also operates by suppressing the processing of distracter stimuli. When subjects are pre-cued to selectively deploy attention during voluntary (endogenous) attentional tasks, these mechanisms can be set up in advance of actual stimulus processing. That is, the brain can be placed in a biased attentional state. Two recent cueing studies have provided evidence for the deployment of such biased attentional states [J.J. Foxe, G.V. Simpson, S.P. Ahlfors, Neuroreport 9 (1998) 3929-3933; M.S. Worden, J.J. Foxe, N. Wang, G.V. Simpson, J. Neurosci. 20:RC63 (2000) 1-6]. Specifically, these studies implicated oscillatory activity in the alpha frequency-band (8-14 Hz) as an anticipatory mechanism for suppressing distracter visual stimulation. The current study extends these findings by showing that this alpha-suppressive effect is also invoked by cross-modal cues. Auditory symbolic cues were used in an intermodal attention task, to direct subjects' attention to a subsequent task in either the visual or auditory modality. Cueing attention to the auditory features of the imminent task stimuli resulted in significantly higher parieto-occipital alpha amplitude in the period preceding onset of this stimulus than when attention was cued to the visual features. Topographic mapping suggests that this effect is generated in regions of the inferior parietal cortex, areas that have been repeatedly implicated in the engagement and maintenance of visual attention. Taken together, the results of this series of studies suggest that these parietal regions are capable of integrating sensory cues from multiple sensory modalities in order to program the subsequent deployment of visual attention.

Integrating electrophysiology and neuroimaging of spatial selective attention to simple isolated visual stimuli

Visual-spatial attention involves modulations of activity in human visual cortex as indexed by electrophysiological and functional neuroimaging measures. Prior studies investigating the time course and functional anatomy of spatial attention mechanisms in visual cortex have used higher-order discrimination tasks with complex stimuli (e.g. symbol matching in bilateral stimulus arrays, or letter discrimination), or simple detection tasks but in the presence of complex distracting information (e.g. luminance detection with superimposed symbols as distractors). Here we tested the hypothesis that short-latency modulations of incoming sensory signals in extrastriate visual cortex reflect an early spatially specific attentional mechanism. We sought evidence of attentional modulations of sensory input processing for simple, isolated stimuli requiring only an elementary discrimination (i.e. size discrimination). As in prior studies using complex symbols, we observed attention-related changes in regional cerebral blood flow in extrastriate visual cortex that were associated with changes in event-related potentials at a specific latency range. These findings support the idea that early in cortical processing, spatially-specific attentional selection mechanisms can modulate incoming sensory signals based on their spatial location and perhaps independently of higher-order stimulus form.

Imaging image processing in the human brain

Functional imaging in humans reveals the interplay of the many components of the human visual system: how they process the various types of information contained in the image to recover characteristics of the three-dimensional world surrounding us, but also how, in the course of this process, the retinal image is gradually integrated with non-retinal signals to provide information about the outside world in a format useful to other non-visual brain regions.

Context, state and the receptive fields of striatal cortex cells

Visual cortical cells are commonly characterized by their receptive-field structure. Originally, a visual receptive field was defined in a purely spatial way as that retinal area from which a change in spiking response of the regarded cell could be elicited by visual stimulation. The first attempts to understand receptive-field structure were based entirely on the anatomical connectivity of the primary visual pathway. More recently, however, it has been discovered that the spatial and temporal context in which a stimulus is presented to a cell can strongly influence its receptive field, and this in turn is dependent on the state of arousal and attention. Accordingly, new concepts recognize that cortical receptive fields are highly dynamic entities embracing more than the sum of the full spatial and temporal response properties of a cell.