Chapter 7 Summary

Given that paying attention elicits changes at many levels of the nervous system, an obvious question is whether there is an overarching control system (or systems) for the myriad attentional effects on stimulus processing that have been observed using behavioral, electrophysiological, and brain imaging methods.
Clinical data from patients indicate that when certain areas in the parietal and frontal cortices are damaged, the ability to direct and allocate attention is markedly compromised. The characteristics of these syndromes suggest that these areas are involved in the control of attention.
Most studies of attentional control systems in normal individuals have been carried out via endogenous cuing paradigms on subjects cued as to what to attend to on each trial, which they then voluntarily direct their attention to. These studies have yielded abundant evidence that the orienting of attention to locations or stimuli in the external environment is indeed associated with activity in several specific regions of the dorsal frontal and parietal cortices.
In contrast, exogenously induced attention, such as the reorienting of attention to salient or unexpected events, triggers increased activity in cortical regions near the temporoparietal junction and ventral frontal cortex, mainly on the right, which may in turn trigger the involvement of portions of this same dorsal frontoparietal network to implement the attentional orienting itself.
Visual search is the process of inspecting a complex visual scene or array for something of particular interest. Such searches have been roughly divided into those that can be performed in parallel across the visual field (detecting an object distinguished by a simple feature), and those that appear to require sequential focused attention on each item (detecting an object with a conjunction of two different features). Theoretical models and some neural studies have suggested that such searches involve stimulus salience maps, possibly located in parietal or parietal-temporal brain regions, along with activity in portions of the frontal and parietal attentional control circuits.
These various empirical findings have led to the development of models of attentional control that propose modulation via networks of interacting brain areas. A number of experimental studies testing the interactions between brain areas predicted by these models have provided support for these general ideas.
One dominant model holds that in response to both endogenously and exogenously induced attention, the dorsal frontoparietal network, once activated, operates as the attentional orienting control system that sends a biasing signal to the relevant sensory cortices to increase the baseline activity, thus facilitating stimulus processing in those cortices (or other pertinent brain regions). This scheme is consistent with the enhanced sensory activity elicited by attended stimuli, and with the associated improvement in behavioral performance in tasks involving these stimuli.
A complementary network of brain regions, known collectively as the default-mode network, is anticorrelated in activation pattern with the dorsal frontoparietal control network. In particular, the default-mode network, which includes regions in inferior midline frontal cortex and the posterior cingulate cortex, is deactivated when subjects are engaged in an attentionally demanding task, and relatively more activated when they are not. The function of this complementary system is not yet clear, but it may play an overarching interactive role with the dorsal frontoparietal network in brain functions related to attentional control.
Consideration of attentional control and attentional systems leads to questions about levels of arousal and wakefulness, as well as the highly intriguing but difficult problem of consciousness and its neural basis. Levels of arousal and wakefulness are thought to be controlled by subcortical brainstem regions, particularly those collectively termed the reticular activating system.
Studies in both patients and normal subjects have not yet identified a clear neural basis for consciousness in the broader sense of awareness of the world and self. That said, perceptual awareness of sensory stimuli appears to be associated with a modulation of activity in the association sensory cortices, particularly late in processing in a reentrant way, as well as in frontal and parietal cortices that support attention and other higher-level cognitive functions.