Stephen B. R. E. Brown

Functional significance of the emotion-related late positive potential

The late positive potential (LPP) is an event-related potential (ERP) component over visual cortical areas that is modulated by the emotional intensity of a stimulus. However, the functional significance of this neural modulation remains elusive. We conducted two experiments in which we studied the relation between LPP amplitude, subsequent perceptual sensitivity to a non-emotional stimulus (Experiment 1) and visual cortical excitability, as reflected by P1/N1 components evoked by this stimulus (Experiment 2). During the LPP modulation elicited by unpleasant stimuli, perceptual sensitivity was not affected. In contrast, we found some evidence for a decreased N1 amplitude during the LPP modulation, a decreased P1 amplitude on trials with a relatively large LPP, and consistent negative (but non-significant) across-subject correlations between the magnitudes of the LPP modulation and corresponding changes in d-prime or P1/N1 amplitude. The results provide preliminary evidence that the LPP reflects a global inhibition of activity in visual cortex, resulting in the selective survival of activity associated with the processing of the emotional stimulus.

Speed and Lateral Inhibition of Stimulus Processing Contribute to Individual Differences in Stroop-Task Performance

The Stroop task is a popular neuropsychological test that measures executive control. Strong Stroop interference is commonly interpreted in neuropsychology as a diagnostic marker of impairment in executive control, possibly reflecting executive dysfunction. However, popular models of the Stroop task indicate that several other aspects of color and word processing may also account for individual differences in the Stroop task, independent of executive control. Here we use new approaches to investigate the degree to which individual differences in Stroop interference correlate with the relative processing speed of word and color stimuli, and the lateral inhibition between visual stimuli. We conducted an electrophysiological and behavioral experiment to measure (1) how quickly an individual’s brain processes words and colors presented in isolation (P3 latency), and (2) the strength of an individual’s lateral inhibition between visual representations with a visual illusion. Both measures explained at least 40% of the variance in Stroop interference across individuals. As these measures were obtained in contexts not requiring any executive control, we conclude that the Stroop effect also measures an individual’s pre-set way of processing visual features such as words and colors. This study highlights the important contributions of stimulus processing speed and lateral inhibition to individual differences in Stroop interference, and challenges the general view that the Stroop task primarily assesses executive control.

Noradrenergic and cholinergic modulation of late ERP responses to deviant stimuli.

Researchers have proposed several hypotheses about the neuromodulator systems involved in generating P3 components of the ERP. To test some of these hypotheses, we conducted a randomized placebo-controlled crossover study in which we investigated how the late positive ERP response to deviant stimuli is modulated by (a) clonidine, an α2 agonist that attenuates baseline noradrenergic activity; and (b) scopolamine, a muscarinic antagonist of acetylcholine receptors. We collected EEG data from 18 healthy volunteers during the performance of an auditory oddball task with several active and passive task conditions. We then used temporospatial principal component analysis (PCA) to decompose the ERP waveforms. The PCA revealed two distinct late positive ERP components: the classic parietal P300 and the frontal novelty P3. Statistical analysis of the temporospatial factor scores indicated that in most conditions the amplitude of the classic P300 was increased by clonidine and scopolamine. In contrast, the amplitude of the novelty P3 was decreased by both drugs. The similar pattern of results for clonidine and scopolamine probably reflects the strong interactions between the noradrenergic and cholinergic systems. The results, in combination with previous pharmacological studies, suggest a critical role for both neuromodulator systems in the generation of the P300 and the novelty P3.

Effects of arousal on cognitive control: empirical tests of the conflict-modulated Hebbian-learning hypothesis

An increasing number of empirical phenomena that were previously interpreted as a result of cognitive control, turn out to reflect (in part) simple associative-learning effects. A prime example is the proportion congruency effect, the finding that interference effects (such as the Stroop effect) decrease as the proportion of incongruent stimuli increases. While this was previously regarded as strong evidence for a global conflict monitoring-cognitive control loop, recent evidence has shown that the proportion congruency effect is largely item-specific and hence must be due to associative learning. The goal of our research was to test a recent hypothesis about the mechanism underlying such associative-learning effects, the conflict-modulated Hebbian-learning hypothesis, which proposes that the effect of conflict on associative learning is mediated by phasic arousal responses. In Experiment 1, we examined in detail the relationship between the item-specific proportion congruency effect and an autonomic measure of phasic arousal: task-evoked pupillary responses. In Experiment 2, we used a task-irrelevant phasic arousal manipulation and examined the effect on item-specific learning of incongruent stimulus–response associations. The results provide little evidence for the conflict-modulated Hebbian-learning hypothesis, which requires additional empirical support to remain tenable.

The accessory stimulus effect is mediated by phasic arousal: A pupillometry study

People usually respond faster to a visual stimulus when it is immediately preceded by a task-irrelevant, auditory accessory stimulus (AS). This AS effect occurs even in choice reaction time tasks, despite the fact that the AS carries no information about the correct response. Researchers often assume that the AS effect is mediated by a phasic arousal burst evoked by the AS, but direct evidence for that assumption is lacking. We conducted a pupillometry study to directly test the phasic arousal hypothesis. Participants carried out a demanding choice reaction time task with accessory stimuli occurring on 25% of the trials. Pupil diameter, a common index of arousal, was measured throughout the task. Standard analyses of task performance and pupil diameter showed that participants exhibited the typical AS effect, and that accessory stimuli evoked a reliable early pupil dilation on top of the more protracted dilation associated with the imperative stimulus. Moreover, regression analyses at the single-trial level showed that variation in reaction times on AS trials was selectively associated with pupil dilation during the early time window within which the AS had an effect, such that particularly large AS-evoked dilations were associated with especially fast responses. These results provide the first evidence that the AS effect is mediated by AS-evoked phasic arousal.

Neurobiological Foundations of Action Planning and Execution

The successful planning, initiation, and execution of actions depend on an intact functional loop that encompasses the frontal, premotor, and motor cortices, as well as the basal ganglia and the cerebellum. After having sketched how the brain is built and how it works, we will provide a brief overview of the major cortical and subcortical areas involved in action planning and action execution.

Perception and Action

This chapter discusses the relationship between perception and action. Adaptation studies show that conscious perception can be dissociated from action control, and there is evidence that perception and action interact along several concurrent processing pathways. Perception has a systematic impact on action control, but action control processes also affect the perception of, and attention to external stimuli. The available evidence suggests that perception and action are more interwoven and interdependent than commonly thought, and a theoretical framework is presented that explains how perception-action interactions may operate.

Noradrenergic and Cholinergic Modulation of Belief Updating

To make optimal predictions in a dynamic environment, the impact of new observations on existing beliefs—that is, the learning rate—should be guided by ongoing estimates of change and uncertainty. Theoretical work has proposed specific computational roles for various neuromodulatory systems in the control of learning rate, but empirical evidence is still sparse. The aim of the current research was to examine the role of the noradrenergic and cholinergic systems in learning rate regulation. First, we replicated our recent findings that the centroparietal P3 component of the EEG—an index of phasic catecholamine release in the cortex—predicts trial-to-trial variability in learning rate and mediates the effects of surprise and belief uncertainty on learning rate (Study 1, n = 17). Second, we found that pharmacological suppression of either norepinephrine or acetylcholine activity produced baseline-dependent effects on learning rate following nonobvious changes in an outcome-generating process (Study 1). Third, we identified two genes, coding for α2A receptor sensitivity (ADRA2A) and norepinephrine reuptake (NET), as promising targets for future research on the genetic basis of individual differences in learning rate (Study 2, n = 137). Our findings suggest a role for the noradrenergic and cholinergic systems in belief updating and underline the importance of studying interactions between different neuromodulatory systems.

Intentions and Action Goals

How exactly do you tie your shoelaces? Very few people can give a sensible answer to that without using one of these two strategies: either they actually start to tie their shoelaces, observe what they do, and report on that, or they imagine the actions involved and report on what they see before their “mind’s eye.” Both strategies demonstrate the same: we know surprisingly little about our own actions and appear to have no privileged access to the knowledge that allows their execution. Although we are able to perform goal-directed, intentional actions, we actually do not know how or why we can do so. The present chapter attempts to resolve this mystery.

Controlling and Coordinating Multiple Actions

Everyday actions often overlap in time and require frequent shifts between different tasks. This chapter explains how multiple tasks are controlled and coordinated, how and to what degree performance suffers from shifting and multitasking, and which particular cognitive processes are affected.