In:- The Neuroscience of Attention, Ed. George R. Mangun
Oxford University Press (2012)
Summary and review of the above chapter
Mechanisms of attention are seen as crucial for the brain to select relevant information. This chapter discusses the type of attention that we are compelled to give rather than the type of attention where we are looking out for particular types of stimuli. This is sometimes described as exogenous attention, meaning that the trigger comes from outside the brain, as distinct from endogenous, meaning that the trigger comes from inside the brain. Typical of events producing involuntary attention are loud bangs or flashes of light.
It has been demonstrated that visual attention, either voluntary or involuntary, does not require direction of the gaze or movement of the head. This is referred to as ‘covert attention’. One distinction between voluntary and involuntary attention is that involuntary attention takes only about 50 ms after the initial input of a stimulus to register data, while involuntary attention takes 300 ms or longer. However, involuntary attention lasts for only a few hundred milliseconds, after which, either voluntary attention is attracted to the stimulus, or there is an involuntary inhibition of attention to the stimulus. In contrast, voluntary attention can be focused for long periods.
The methodology of the chapter concentrates on event related potentials (ERPs) where the early peaks and troughs of the wave-form relate to early sensory processing, but later components relate to cognitive processing. With visual ERPs, a C1 component can peak in the early or striate visual cortex (V1) 70-100 ms after the initial input to the retina. The second component is P1, which peaks after 100-130 ms in the extrastriate cortex. This is followed by the N1 component, of which the posterior component peaks 170-200 ms after the initial stimuli to the retina. A long-latency component known as P3 peaks after 300 ms or longer. Different P3 components are thought to reflect different types of cognitive processing.
The chapter discussed here relates to decisions about stimuli, and the integration of these decisions into the perception of the environment. P3 is not just a motor preparation component; it is active even if no response is required. However, the stimulus has to be relevant to understanding the environment. In contrast, other ERP components are only linked to physical responses. The entire ERP response happens within half a second of the initial input.
In practice, voluntary and involuntary attention can be active at the same time. However, the mechanisms for orientating voluntary and involuntary attention may not be the same, and responses may be different according to whether it is voluntary or involuntary attention that is engaged. Only voluntary attention is found to modulate the cognitive N1 wave-form, and this modulation is also influenced by the type of task involved and the difficulty of the task. In general, voluntary attention has more influence on the later stages of processing. The two systems, voluntary and involuntary are regarded here as separable. In the case of involuntary attention, the initial response is seen as being automatic, but the relationship to pre-existing voluntary attention may influence how long attention is paid to the initially automatic stimulus.
Top-down expectations are more important for the later stages of processing, and in determining how long attention is kept on a particular stimulus. Early processing of involuntary attention is seen as bottom-up in the early sensory areas, while the later stages of processing are more subject to top-down control. The authors consider that the length of time of holding attention may be particularly important in determining subsequent actions and behaviour, and that the importance of this aspect may have been under estimated in earlier studies.
CONCLUSION: In indicating that voluntary and involuntary attention are separate mechanisms, this is yet another neuroscience study that goes against some of the consciousness studies of the last century, in showing that consciousness is not a blanket aspect of neural processing, but something which arises in more specialised areas of the brain, with voluntary attention intimately connected to perception, understanding of the environment, and to the determination of actions and behaviour.