Cross-modal warning effects on reflexive and voluntary reactions.

Previous research has shown that warning signals can speed the onset of the startle-blink reflex. To relate this phenomenon to warning effects on voluntary reaction time (RT), the latencies of both reflexive and voluntary responses were measured for nine factorial combinations of warning and reflexogenic stimulus modalities. Previous failures to use factorial manipulations of warning (S1) and reaction (S2) stimulus modalities have led to conflicting results in both the reflex and RT literatures. Using psychophysically matched warning signals, we found a facilitation of reflex latency that was nonspecific with regard to S1 and S2 modality. Furthermore, there was no support for the widely held assumption that visual stimuli are inherently less alerting than auditory and cutaneous stimuli. A between-group comparison showed that simultaneous voluntary reactions do not distort the reflex facilitation effect. These results support the validity of reflex facilitation as a simple model system for studying warning effects on sensorimotor reactions.

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-Inclán, 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 research--the latency of spontaneous, post-trial blinking.

Early visual processing during binocular rivalry studied with visual evoked potentials.

Visual evoked potentials (VEPs) were recorded to probes presented to the dominant and suppressed eyes in a binocular rivalry paradigm. Probes presented to the suppressed eye interrupted the current dominance phase and produced a P300-like deflection (400-700 ms). Probes delivered to the dominant eye increased the duration of the current dominance phase. VEPs to these probes included an endogenous component that overlapped the early exogenous components. The early endogenous component (rivalry-related potential, RRP) started as early as 70 ms and had a broad centroparietooccipital distribution.

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 foreperiod duration on reflexive and voluntary responses to intense noise bursts.

The question of whether a common mechanism underlies the facilitation of voluntary and reflexive reactions by a warning stimulus was investigated in two experiments. In both studies, the foreperiod preceding an intense noise burst was manipulated within and between blocks of trials. Previous reaction time experiments have shown that individuals respond fastest at the shortest foreperiod for between-block manipulations and fastest at the longest foreperiod when foreperiod duration is varied unpredictably from trial to trial. In the present research, this pattern was found for voluntary hand-grip responses, but acoustic startle blinks were facilitated at long foreperiods for both within- and between-block manipulations. Invariance of the trisynaptic postauricular reflex across foreperiod conditions was evidence against any general activation of low-level motor pathways by warning stimuli. Analyses of nonreflexive lid movements subsequent to startle blink suggested that inhibition of spontaneous blinking during the foreperiod may have contributed to the unexpected divergence between voluntary reactions and eyeblink reflexes.

Prepulse effects on the photic eyeblink reflex: evidence for startle-dazzle theory.

Weak visual prestimulation effects on the early (R50, 50-80 ms) and late (R80, 80-200 ms) components of the eyeblink response to bright light flashes were studied in 16 normal individuals. At a lead time of 120 ms, R50 was inhibited relative to no-prepulse control trials, whereas R80 was facilitated. According to the proposed startle-dazzle theory, luminance onset transients trigger an initial response, R50, that is functionally related to startle. Sustained stimulation then activates prolonged eyelid (R80) and pupil responses, which serve to minimize retinal bleaching. Although the sensitivity of the photic blink reflex to attention is controversial, R50 latency showed a pattern suggestive of inhibition of return (Posner & Cohen, 1984, Attention and performance, Vol. X, pp. 531-556, Hillsdale, NJ: Erlbaum). Analyses in a patient with unilateral occipital lobe damage supported previous evidence that inhibition by a visual prepulse requires neocortex, but facilitation does not.

Alerting effects on choice reaction time and the photic eyeblink reflex.

To test the possibility that a common mechanism might be responsible for alerting effects on voluntary and reflexive reactions, choice reaction times (RT) to intense flashes of light were compared with eyeblink reflexes simultaneously evoked by those stimuli. An acoustic accessory stimulus, irrelevant to the RT task, facilitated both voluntary and reflexive reactions. A time uncertainty manipulation also generated facilitation of both responses under conditions in which phasic arousal was presumably greatest. However, there were several dissociations between alerting effects on voluntary and reflexive reactions and between effects on the early and late subcomponents of the photic orbicularis oculi reflex. In conjunction with other research in humans and animals, these data support the assumption that alerting involves the activation of multiple neuromodulatory (e.g. monoamine) systems, each of which is characterized by a distinct behavioral, neuropharmacological, and electrophysiological profile.

Distinct early and late subcomponents of the photic blink reflex: response characteristics in patients with retrogeniculate lesions.

To assess cortical contributions to the photic blink reflex, signal averaged electromyograms (EMG) were compared for responses to strobe flashes presented within the blind and sighted hemifields of 13 patients with occipital lobe lesions. Reflexes evoked by flashes within the scotoma were virtually identical to those evoked by flashes within the intact visual field. This suggests that both the early and late components of this reflex (R50 and R80, respectively) are mediated by subcortical structures that do not require, or benefit from, conscious visual processing. Additional findings included larger R80s at the eyelid contralateral to the lesion, regardless of stimulated hemifield. This presumably reflects the loss of a tonic descending influence of visual cortex onto the motor limb of the reflex arc. The R80 was also larger for stimuli activating the crossed (temporal hemifield) rather than the uncrossed (nasal hemifield) afferent pathway.

Response complexity and precue interval effects on the lateralized readiness potential.

Fundamental properties of an important new tool in cognitive electrophysiology, the lateralized readiness potential (LRP), were examined in two experiments. Experiment 1 resolved an apparent inconsistency in the literature by demonstrating that this response-specific lateralization is larger preceding complex then preceding simple finger movements. In Experiment 2, the foveally presented precue, which indicated hand of response, preceded the go/no-go stimulus by 0, 100, 300, or 1,400 ms. Analyses of LRP latency indicated that hand-specific preparation began earlier with longer foreperiods but was temporally linked to the reaction stimulus as well as the precue. Although the degree of lateralization did not predict reaction speed in either study, a nonlateralized, response-locked negativity was larger prior to faster reactions.

An evaluation of the automaticity of sensory processing using event-related potentials and brain-stem reflexes.

Selective attention effects on reflexes and evoked potentials are reviewed with the aim of evaluating three theories regarding sensory automaticity. (a) The peripheral-gating theory, which assumes that ignored stimuli can be filtered out soon after transduction, was tentatively rejected because neither auditory-nerve nor retinal potentials are reliably affected by attention. (b) At the other extreme, the assumption that sensory analyses are obligatory and cannot benefit from attentional resources (i.e., strong-automaticity theory) was also rejected, because longer latency components were found to be modifiable by attention. (c) An intermediate theory provides the best fit to present electrophysiological data. The earliest sensory analyses are assumed to be strongly automatic and then, at forebrain levels, there is a transition from strong to weak automaticity (i.e., analyses are obligatory but modifiable by attention). This transition can begin as early as about 15 ms for audition and about 80 ms for vision.

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).

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.

The effects of channel-selective attention on the mismatch negativity wave elicited by deviant tones.

The mismatch negativity (MMN) is an event-related brain potential elicited by infrequent, physically deviant sounds in a sequence of repetitive auditory stimuli. Two dichotic listening experiments that were designed to optimize the selective focusing of attention provided a strong test of Näätänen's proposal that the MMN is unaffected by attention and reflects the operation of a strongly automatic mismatch detection system. In Experiment 1, tones were presented at intervals of 120-320 ms, and the deviant tones (intensity decrements) in both the attended and unattended ears elicited negative waves consistent in waveshape, latency, and distribution with previously described MMNs. In contrast to previous reports, however, the MMN elicited by the unattended-channel deviant was markedly reduced (peak amplitude of less than 1 microV) relative to the corresponding negative wave elicited by the attended-channel deviants (3-4 microV), as well as relative to previously reported MMNs (3-6 microV) elicited by comparable deviations in stimulus intensity. In Experiment 2, which employed interstimulus intervals of 65-205 ms, the unattended-channel MMN elicited by the deviant fainter tones was barely discernible, whereas the corresponding attended-channel negativity was again about 3-4 microV. These findings call into question the assertion that the auditory mismatch detection process and the associated MMN wave are wholly independent of attentional influence. Rather, these data provide evidence that the processing of stimuli in unattended channels can be attenuated or gated at an early sensory level under conditions of highly focused auditory selective attention.

Preparation for Donders' type B and C reaction tasks.

Several variants of a precued reaction time experiment were performed to assess preparation for Choice (Donders' type B) or Go/No-Go (type C) reaction tasks. On each trial, the imperative stimulus could necessitate either a keypress with the left forefinger, the right forefinger, or no response. A precue given at lead times of 50-750 ms either informed the subject whether the task was a type B task (left or right forefinger response), a type C task (e.g., left forefinger or No Go), or carried no information (all three possibilities remained open). The noninformative cue produced minor facilitation of reaction time at long lead times only. At lead times of 50-250 ms, the other two precues speeded reactions by about 50 ms. At precue intervals of 400-750 ms, Go/No-Go reactions averaged 25 ms faster than Choice reactions in an experiment which utilized appropriate controls for stimulus and task parameters. The mixed-trial design rules out gross motor (e.g., postural) adjustments as an explanation for the advantage of C over B reactions. An interpretation in terms of a two-stage theory of motor preparation is discussed.

Cross-modal selective attention effects on retinal, myogenic, brainstem, and cerebral evoked potentials.

Short latency evoked potentials were recorded during a cross-modal selective attention task to evaluate recent proposals that sensory transmission in the peripheral auditory and visual pathways can be modified selectively by centrifugal mechanisms in humans. Twenty young adult subjects attended in turn to either left-ear tones or right-field flashes presented in a randomized sequence, in order to detect infrequent, lower-intensity targets. Attention-related enhancement of longer-latency components, including the visual P105 and the auditory N1/Nd waves and T-complex, showed that subjects were able to adopt a selective sensory set toward either modality. Neither the auditory evoked brainstem potentials nor the early visual components (electroretinogram, occipito-temporal N40, P50, N70 waves) were significantly affected by attention. Measures of retinal B-waves were significantly reduced in amplitude when attention was directed to the flashes, but concurrent recordings of eyelid electromyographic activity and the electro-oculogram indicated that this effect may have resulted from contamination of the retinal recordings by blink microreflex activity. A trend toward greater positivity in the 15-50 ms latency range for auditory evoked potentials to attended tones was observed. These results provide further evidence that the earliest levels of sensory transmission are unaffected by cross-modal selective attention, but that longer latency exogenous and endogenous potentials are enhanced to stimuli in the attended modality.

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.