![]() Together, these processes modulate neural activity within the visual system and thus shape how we perceive our environment. However, the apparant dichotomy of top-down and bottom-up attention has been challenged recently, since evidence suggests that there might be other factors controlling visual selection, such as reward-based history effects ( Awh et al., 2012 Failing and Theeuwes, 2018). For example, it was found that top-down beta band influences causally increased bottom-up gamma-band influences from early visual areas ( Richter et al., 2017). Both attention modes have been associated with distinct neural processes, but seem to be closely connected ( Katsuki and Constantinidis, 2014). These terms describe if attention is allocated based on events in the sensory periphery (bottom-up) or from higher cortical areas (top-down). Top-down refers to the voluntary guidance of attention by internal goals, whereas bottom-up attention refers to the involuntarily capture of attention by salient events in the environment. One of the most common distinctions is made between top-down and bottom-up attention ( Corbetta and Shulman, 2002). ![]() This biasing of input is implemented through different types of attention. Even though there are many definitions of attention, the following review will mainly be concerned with visuospatial selective attention. Since the capacities of the perceptual system are limited, focusing on a certain aspect of the visual field enables us to prioritize relevant information and ignore irrelevant information. The term “visual attention” refers to cognitive processes that allow us to selectively process the vast amount of information we are confronted with every day. Introduction Definition of Visual Attention In doing so, we hope to shed some light on the current understanding of the role of neurochemistry in shaping neuron properties contributing to the allocation of attention in the visual field. We will not only review the roles of widely accepted neuromodulators, such as acetylcholine, dopamine and noradrenaline, but also the contributions of other modulatory substances. Here, we summarize the importance and relative contributions of different neuromodulators and neurotransmitters to the neural mechanisms of top-down and bottom-up attentional control. Since for most humans vision is the dominant sense, visual attention is critically important for higher-order cognitive functions and related deficits are a core symptom of many neuropsychiatric and neurological disorders. This selection is usually controlled by bottom-up and top-down attentional biasing. Visual attention is the cognitive process that mediates the selection of important information from the environment.
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