Steven A. Hackley

Electrophysiological evidence for temporal overlap among contingent mental processes

A series of studies assessed perceptual-motor transmission of stimulus information by measuring lateralization of movement-related brain potentials in a choice reaction task with no-go trials. When stimuli varied in shape and size, lateralized potentials on no-go trials suggested that easily recognized shape information was used to initiate motor preparation and that this preparation was aborted when size analysis signified that the response should be withheld. This indicates that movement preparation can begin once partial perceptual information about a stimulus becomes available, contrary to an assumption of fully discrete models of information processing. By contrast, when stimuli varied only in size, no evidence for preliminary response preparation was obtained, contrary to an assumption of fully continuous models but consistent with asynchronous discrete coding models (Miller, 1982, 1988).

Automatic alerting does not speed late motoric processes in a reaction-time task

When an irrelevant ‘accessory’ stimulus is presented at about the same time as the imperative signal in a choice reaction time-task, the latency of the voluntary response is markedly reduced. The most prominent cognitive theories agree that this effect is attributable to a brief surge in arousal (‘automatic alerting’), but they disagree over whether the facilitation is localized to a late, low-level motoric process or to an earlier stage, the process of orienting to and then perceptually categorizing the reaction stimulus,. To test these alternative hypotheses, we used the onset of the lateralized readiness potential (a movement-related brain potential) as a temporal landmark to partition mean reaction time into two time segments. The first segment included the time required to perceive the visual stimulus and decide which hand to react with; the second included only motoric processes. Presentation of an irrelevant acoustic stimulus shortened the first interval but had no effect on the second. We therefore rejected the motoric hypothesis.

Effects of attending selectively to the spatial position of reflex-eliciting and reflex-modulating stimuli

The question of whether automatic, sensory processes can be modified by selectively directing attention to stimuli was addressed by comparing effects on brainstem reflexes that share a common efferent pathway but have distinct afferent limbs. Subjects judged the duration of brief but intense blink-eliciting tones (Experiment 1) or weak tones preceding a blink-eliciting air puff at interstimulus intervals producing blink inhibition (Experiment 2). Tones occurred unpredictably at left, right, or midline loci; designation of the target location varied across blocks of trials. Latency of blinks to lateralized blink-eliciting targets was facilitated selectively, and the magnitude of blinks evoked by air puff following lateralized prestimulus targets was inhibited selectively. There was no evidence for a midline selective effect. Results appear to support a preset differential processing of stimuli in sensory pathways at low, possibly subcortical, levels and the consequent modification of obligatory, automatic processes.

Which stages of processing are speeded by a warning signal

This article reviews psychophysiological and behavioral studies that attempt to identify which stages of processing are speeded by a neutral warning signal (WS), that is to say, one that conveys no information about the nature of the imperative stimulus or the required response. Experiments involving the lateralized readiness potential (LRP) provide evidence against the widely held view that warning effects on reaction time (RT) are due to facilitation of low-level motor processes. Studies of modality-specific evoked potentials similarly rule out a locus within sensory-perceptual processing. It is concluded that the speeding of RT by a neutral WS is most likely due to nonspecific motor priming within an early phase of response selection. In addition, fast-guess responses, in which subjects choose a response without fully analyzing the stimulus, are assumed to contribute to warning effects.

Visual working memory deficits in patients with Parkinson's disease are due to both reduced storage capacity and impaired ability to filter out irrelevant information

Given that Parkinsons disease broadly affects frontostriatal circuitry, it is not surprising that the disorder is associated with a reduction of working memory. We tested whether this reduction is due to diminished storage capacity or impaired ability to exclude task-irrelevant items. Twenty-one medication-withdrawn patients and 28 age-matched control subjects performed a visuospatial memory task while their electroencephalograms were recorded. The task required them to remember the orientations of red rectangles within the half of the screen that was cued while ignoring all green rectangles. Behavioural and electroencephalogram measures indicated that patients with Parkinsons disease were impaired at filtering out distracters, and that they were able to hold fewer items in memory than control subjects. The results support recent suggestions that the basal ganglia help control access to working memory.

Accessory Stimulus Effects on Response Selection: Does Arousal Speed Decision Making?

When an intense but task-irrelevant accessory stimulus accompanies the imperative stimulus in a choice reaction task, reaction times (RTs) are facilitated. In a similar previous study (Hackley & Valle-Incln, 1998), we showed that this effect is not due to a reduction of the interval from onset of the lateralized readiness potential (LRP) until movement onset. In the present study, the RT task was modified to move a portion of the response selection stage into this time interval. The interval remained invariant, indicating that this late phase of the response selection process is not speeded by accessory stimulation. However, we observed amplitude modulation of the LRP on no-go trials in a condition with three alternative responses. This finding suggests that an earlier phase of response selection is influenced by accessory stimulation. In addition, a novel dependent measure was introduced to event-related potential researchthe latency of spontaneous, posttrial blinking.

Waiting to perceive: Reward or punishment?

Neurobiological accounts of the dopaminergic reward system and psychophysiological explanations of the error-related negativity (ERN) both emphasize the comparison of expected versus actual outcome for voluntary actions. The stimulus-preceding negativity (SPN) constitutes a valuable index of that expectation, in that it has high temporal resolution and its anatomical, cognitive and affective correlates have been reasonably well characterized. This review links established findings regarding the SPN to current research on the dorsal and ventral attention systems, somatic marker hypothesis, ERN, the reward system and relevant neurological and psychiatric findings. Special emphasis is given to the pre-feedback SPN and its origin within anterior insular cortex.

The speeding of voluntary reaction by a warning signal

Warning signals can shorten reaction time (RT) via either a top-down mechanism, temporal attention, or a bottom-up one, phasic arousal. The goal of this review article is to identify the locus at which these processes influence RT. Electrophysiological and behavioral evidence indicate that the chronometric locus for both modulatory effects lies mainly within a narrow window at the center of the stimulus-response interval. This interval presumably encompasses late perceptual, response selection, and early motor processes. Phasic arousal is theorized to reduce the threshold for response selection within a circuit involving the supramarginal gyrus. A blind-sight study indicates that conscious, cortical level processing is necessary for temporal attention, at least when the warning signal is visual.

Effects of preliminary information in a Go versus No-go task

A series of studies using a Go versus No-go task examined the question of whether preliminary information available early in the recognition of a stimulus is made available to later processes before stimulus recognition is finished, a question relevant to the controversy between discrete and continuous models. Experiment 1 showed that a Go response is faster following a cue indicating that the response probably would be required than following a cue indicating it probably would not be required. Experiments 2-7 were conducted to find out whether analogous preparation occurred when probability of the Go response was signalled by easily discriminable features of a single stimulus rather than a separate cue. The effect was observed when the easily discriminable features uniquely determined the name of the stimulus letter, but not when they merely indicated that the stimulus name was one of two visually similar letters. These results are consistent with the Asynchronous Discrete Coding model, in which the perceptual system makes available to later processes only preliminary information corresponding to discretely activated stimulus attributes.

Separation of phasic arousal and expectancy effects in a speeded reaction time task via fMRI

Sixteen neurologically normal volunteers performed a 2-choice speeded reaction time (RT) task in which the imperative was the change in color of a clock hand. During trial blocks with low temporal uncertainty (good clock condition), this imperative stimulus occurred at a fixed location (e.g., 2:00). In the bad clock condition, the clock was unpredictive of imperative onset. On half of both good and bad clock trials, a task-irrelevant, cutaneous accessory stimulus accompanied the imperative. The speeding of reactions by the accessory was associated with activation primarily in and near the supramarginal gyrus of the parietal lobe. Contrasts of good- versus bad-clock conditions revealed activation in a variety of perceptual, motor, and executive control regions. Apart from interactions within the cerebellum and left anterior insula, there was little overlap between structures influenced by the arousal and expectancy manipulations.