The aim of our studies is to characterize several attention mechanisms in the human visual system using psychophysical, neuroimaging (fMRI) and causal modelling methods.

Divided attention

There is an ongoing debate in attention research regarding the question whether spatial attention always forms an uniform focus or whether can be split between non-adjacent locations while sparing the space between.

Evidence from psychophysical studies has shown increased detection performance when attention is allocated to selected regions in space. With increasing size of the attended region, information is processed less quickly and less efficiently. Therefore attention is often compared with a zoom lens that highlights a specific region in space and can be adjusted in size. We already investigated neural correlates of this zoom lens within retinotopic visual areas (Müller et al. 2003).
Figure 1: Physiological correlate of the zoom lens (Mueller et al. 2003)


In a current study we investigate the behavioral (Kraft et al. 2005) and neurophysiological basis of spatially divided visual attention – both within as well as across hemifields.
Figure 2: Experimental Design (Kraft et al. 2005)

The aim is to provide further evidences of top-down and bottom-up as well as lateralized processes across hemispheres by combining behavioral measures, functional magnetic resonance imaging (fMRI), the analysis of effective connectivity and computational modelling (Bernstein Center for Computational Modelling).

Currently we ask:

1. Does performance decreases when two separate positions are attended simultaneously within or across hemifields?
2. Is activity in the visual areas at the location “between” affected by attention?
3. Does top-down modulate bottom-up processes in early visual areas?
4. Are different mechanisms of visuo-spatial attention in the human visual system reflected in lateralized processes across hemispheres?

Visual Search

Natural visual scenes confront us with many objects, few of which are relevant for our current behavior. Our brains are endowed with elaborate mechanisms for selecting (i.e. paying attention to) the relevant ones. We are using several variants of the visual search task in combination with fMRI in order to find out how attention works in the human brain during search.

Overlap of Control Mechanisms for Eye Movements and Covert Visual Search

We have recently demonstrated that these areas are situated outside the visual system proper, instead they are located in the posterior parietal (PP) and the prefrontal (PF) cortex (particularly the frontal eye fields), i.e. in brain regions controlling motor actions such as saccadic eye movements (Donner et al., 2000).

Figure 3: The searching brain (Donner et al. 2000)

Similar frontoparietal networks have also been found to control covert shifts of attention in spatial cueing tasks. The relationship between search and eye movement control will be further studied.

Spatial and non-spatial Attention in Visual Search

We are also aiming to understand how the component areas of the frontoparietal network control different types of selection mechanisms employed during search, such as serial selection of object locations and parallel selection of non- spatial object features. We suggest a functional specialization within the network with some components (particularly the frontal eye fields) being predominantly involved in shifts of spatial attention and others being involved in both spatial and non-spatial selection. We are currently testing these hypothesis in a new study.

Selected Publications

Kraft A, Schira MM, Hagendorf H, Schmidt S, Olma M, Brandt SA (2005). fMRI localizer-technique: Efficient acquisition and functional properties of single retinotopic positions in the human visual cortex. NeuroImage, 28(2):453-63.

Kraft A, Mueller NG, Hagendorf H, Schira MM, Dick S, Fendrich RM, Brandt SA (2005) Interactions between task difficulty and hemispheric distribution of attended locations: implications for the splitting attention debate. Cog Brain Res, 24(1), 19-32.

Schira MM, Fahle M, Donner TH, Kraft A, Brandt SA (2004) Differential contribution of early visual areas to the perceptual process of contour processing. J Neurophysiol, 91, 1716-1721.

Mueller NG, Donner TH, Bartelt OA, Brandt SA, Villringer A, Kleinschmidt A (2003) The functional neuroanatomy of visual conjunction search: a parametric fMRI study. NeuroImage, 20, 1578-1590.

Mueller NG, Bartelt OA, Donner TH, Villringer A, Brandt SA (2003) A physiological correlate of the ‘zoom lens’ of visual attention. J Neurosci 23(9), 3561-3565.

Donner TH, Kettermann A, Diesch E, Villringer A, Brandt SA (2003) Parietal activation during visual search in the absence of multiple distractors. Neuroreport, 14(17), 2257-2261.

Donner TH, Kettermann A, Diesch E, Villringer A, Brandt SA (2002) Visual feature and conjunction searches of equal difficulty engage only partially overlapping frontoparietal networks. NeuroImage 15, 16-25.

Brandt SA, Gothe J, Sabel B, Roericht S, Kasten E, Meyer BU (2002) Changes of visual cortex excitability in blind subjects as demonstrated by transcranial magnetic stimulation. Brain 125, 479- 490.

Brandt SA, Brocke J, Roericht S, Ploner CJ, Villringer A, Meyer BU (2001) In vivo examination of human visual system connectivity with transcranial electrical stimulation during functional magnetic resonance imaging prelim. NeuroImage 14, 366–375.

Donner T, Kettermann A, Diesch E, Ostendorf F, Villringer A, Brandt SA (2000) Involvement of the human frontal eye field and multiple parietal areas in covert visual selection during conjunction search. Eur J Neurosci 12, 3407-3414.

Culham JC, Brandt SA, Cavanagh P, Kanwisher NG, Dale AM, Tootell RBH (1998) Cortical fMRI activation produced by attentive tracking of moving targets. J Neurophysiol 80(5), 2657-2670.

Brandt SA, Ploner CJ, Meyer BU, Leistner S, Villringer A (1998) Effects of repetitive transcranial magnetic stimulation over dorsolateral prefrontal and posterior parietal cortex on memory-guided saccades. Exp Brain Res 118, 197-205.

Brandt SA, Stark LW (1997) Spontaneous eye movements during visual imagery reflect the content of the visual scene. J Cogn Neurosci 9, 27-38.