Interactions Between Attentional Networks and Their Influence on Perception
Interactions Between Attentional Networks and Their Influence on Perception
Attention can be defined as the preparedness to rapidly and accurately respond to stimuli
 coming from the investigators environment and to effectively select between relevant and
 irrelevant information. According to a current model, visual attentional control is based on
 two separate groups of brain regions, so called brain networks. These networks control
 different attentional aspects (e.g., spatial/non-spatial attention) and they interact with
 each other. A disruption of these interactions can lead to attentional disorders such as
 hemispatial neglect. Patients with hemispatial neglect have difficulties directing their
 attention to the left visual field and they act as though the latter does not exist.

 To date, the interactions between the two attentional networks are poorly understood. The aim
 of this study consists in further clarifying different aspects of these interactions and
 their influence on visual perception in healthy participants and in patients with hemispatial
 neglect. Transcranial magnetic stimulation (TMS) will be the principal method applied in this
 study. TMS is a painless and non-invasive method, with which the activity of brain areas can
 be influenced temporarily. This allows us to draw conclusions regarding the functions and
 interactions of these brain areas.

 This study is designed to have a significant impact on the basic understanding of attentional
 control in the human brain and it can benefit the comprehension and treatment of attentional
 disorders, such as hemispatial neglect.
Background

 Attention can be defined as the preparedness to rapidly and accurately respond to stimuli
 coming from the investigators environment and to effectively select between relevant and
 irrelevant information. Attention is a complex cognitive function based on multiple
 components. A recent model postulates that attentional control in the brain is based on two
 discrete neural networks. The ventral (temporo-parietal - ventral frontal) network controls
 non-spatial aspects of attention, such as alertness and vigilance, and stimulus detection and
 reorientation in both hemifields. This network is lateralized towards the right hemisphere
 and plays a role as a 'circuit-breaker'. The dorsal (superior parietal - dorsal frontal)
 network controls spatial attentional shifts, is largely top-down driven, and substantially
 overlaps with eye movement control. This network is not functionally lateralized, each
 hemisphere containing dynamic topographical maps of the contralateral space. Moreover, the
 dorsal networks of the two hemispheres compete to direct attention to the contralateral
 hemispace, thereby exerting reciprocal inhibition.

 Hemispatial neglect - the failure to detect, orient, or respond to stimuli located in the
 contralesional side of space - has been interpreted in terms of pathological changes in the
 interaction between the two attentional networks. Neglect is very common after cerebral
 lesions of the right hemisphere, usually affecting the ventral attentional network. The
 structurally intact right dorsal network would thus receive a weakened input from the right
 ventral network. The ensuing imbalance in the inhibition between the dorsal networks of the
 two hemispheres would then result in the rightward bias in attentional allocation, typically
 observed in hemispatial neglect.

 However, little is known regarding the interactions between ventral and dorsal attentional
 networks and their influence on perception, and the greatest part of the evidence has been
 acquired through correlational approaches. The aim of this study is to further elucidate
 different aspects of these interactions and to investigate their influence on spatial and
 non-spatial attentional performance and on visual perception.

 Objective

 A first objective consists in further elucidating how and through which anatomical structures
 the ventral and the dorsal attentional networks interact. A second objective concerns how the
 interplay of dorsal and ventral attentional networks influences visual perception, in
 particular in cases in which visual stimuli are ambiguous.

 Methods

 Transcranial magnetic stimulation (TMS) is the principal method applied in this study. TMS is
 a non-invasive, painless method which is able to influence cortical areas directing attention
 in an interference approach and to thus establish causal relationships. Furthermore, TMS, in
 combination with the measurement of motor evoked potentials (MEP), allows for the assessment
 of the excitability of cortico-cortical circuits.
Inclusion Criteria:

 -  Written informed consent

 -  Healthy participants:

 -  Age 18-80 years

 -  Neurologically healthy, i.e., with no documented or present neurological disease or
 brain injury

 -  normal or corrected-to-normal visual acuity

 -  Patients:

 -  Age 18-80 years

 -  Showing signs of left hemispatial neglect after a right-hemispheric brain lesion, as
 assessed by previous neuropsychological testing and clinical judgment

 -  Normal or corrected-to-normal visual acuity

 Exclusion Criteria

 -  Healthy participants:

 -  Any instable medical condition, in particular epilepsy (past or present, including
 seizures or febrile convulsions)

 -  Any surgical intervention to the brain

 -  Implanted medical devices (e.g., cochlear implants, infusion pumps, neurostimulators,
 pacemakers)

 -  Presence of metal in the region of the head (excluding fixed dental implants such as
 tooth fillings or fixed dental braces)

 -  Drug or alcohol abuse

 -  Intake of any medication that is likely to lower seizure threshold

 -  For female participants: pregnancy or breast feeding or intention to become pregnant
 during the course of the experiment. All participants of childbearing potential will
 be asked to take a pregnancy test.

 -  Patients:

 -  Any instable medical condition, in particular epilepsy (past or present, including
 seizures or febrile convulsions)

 -  Implanted medical devices (e.g., cochlear implants, infusion pumps, neurostimulators,
 pacemakers)

 -  Presence of metal in the region of the head (excluding fixed dental implants such as
 tooth fillings or fixed dental braces)

 -  Drug or alcohol abuse

 -  Intake of any medication that is likely to lower seizure threshold

 -  For female participants: pregnancy or breast feeding or intention to become pregnant
 during the course of the experiment. All participants of childbearing potential will
 be asked to take a pregnancy test