Neural Correlates of Attentional Modulation of Prepulse Inhibition

Prepulse inhibition (PPI) refers to the suppression of the startle reflex when the intense startling stimulus is shortly (20–500 ms) preceded by a weak non-startling stimulus (prepulse). Although the main neural correlates of PPI lie in the brainstem, previous research has revealed that PPI can be top-down modulated by attention. However, in the previous attend-to-prepulse PPI paradigm, only continuous prepulse but not discrete prepulse (20 ms) could elicit attentional modulation of PPI. Also, the relationship between the attentional enhancement of PPI and the changes in early cortical representations of prepulse signals is unclear. This study develops a novel attend-to-prepulse PPI task, when the discrete prepulse is set at 150 ms at a lead interval of 270 ms, and reveals that the PPI with attended prepulse is larger than the PPI with ignored prepulse. In addition, the early cortical representations (N1/P2 complex) of the prepulse show dissociation between the attended and ignored prepulse. N1 component is enhanced by directed attention, and the attentional increase of the N1 component is positively correlated with the attentional enhancement of PPI, whereas the P2 component is not affected by attentional modulation. Thus, directed attention to the prepulse can enhance both PPI and the early cortical representation of the prepulse signal (N1).

Induced Positive Affect Reduces the Magnitude of the Startle Response and Prepulse Inhibition

The startle response is a reflex that represents a form of adaptation to environmental changes potentially relevant to survival. Startle magnitude can change depending on a number of factors such as the affective state of the organism during the presentation of the startle-inducing stimulus, or the so-called Prepulse Inhibition (PPI) that occurs when the startling stimulus (or Pulse) is preceded by a low-intensity stimulus (or Prepulse). This paper describes an experiment designed to analyze the impact of an induced positive affect on the magnitude of the startle response and PPI in adult humans. Specifically, each participant received alternating exposures to a picture of a face of a loved person (positive affect condition) or to a picture of a face of an unknown person (control condition) while the startle response and PPI were recorded. The results showed a decrease in both the magnitude of the startle response and percent PPI on the positive affect trials when compared with the control trials. These results are interpreted from psychophysiological and psychological perspectives considering the role of emotions in adaptive behavior.

Understanding the dual-task costs of walking: a StartReact study

The need to perform multiple tasks more or less simultaneously is a common occurrence during walking in daily life. Performing tasks simultaneously typically impacts task performance negatively. Hypothetically, such dual-task costs may be explained by a lowered state of preparation due to competition for attentional resources, or alternatively, by a ‘bottleneck’ in response initiation. Here, we investigated both hypotheses by comparing ‘StartReact’ effects during a manual squeezing task under single-task (when seated) and dual-task (when walking) conditions. StartReact is the acceleration of reaction times by a startling stimulation (a startling acoustic stimulus was applied in 25% of trials), attributed to the startling stimulus directly releasing a pre-prepared movement. If dual-task costs are due to a lowered state of preparation, we expected trials both with and without an accompanying startling stimulus to be delayed compared to the single-task condition, whereas we expected only trials without a startling stimulus to be delayed if a bottleneck in response initiation would underlie dual-task costs. Reaction times of the manual squeezing task in the flexor digitorum superficialis and extensor carpi radialis muscle were significantly delayed (approx. 20 ms) when walking compared to the seated position. A startling acoustic stimulus significantly decreased reaction times of the squeezing task (approx. 60 ms) both when walking and sitting. Dual-task costs during walking are, therefore, likely the result of lowered task preparation because of competition for attentional resources.

Neurophysiological analysis of the clinical pull test

Postural reflexes are impaired in conditions such as Parkinson’s disease, leading to difficulty walking and falls. In clinical practice, postural responses are assessed using the “pull test,” where an examiner tugs the prewarned standing patient backward at the shoulders and grades the response. However, validity of the pull test is debated, with issues including scaling and variability in administration and interpretation. It is unclear whether to assess the first trial or only subsequent repeated trials. The ecological relevance of a forewarned backward challenge is also debated. We therefore developed an instrumented version of the pull test to characterize responses and clarify how the test should be performed and interpreted. In 33 healthy participants, “pulls” were manually administered and pull force measured. Trunk and step responses were assessed with motion tracking. We probed for the StartReact phenomenon (where preprepared responses are released early by a startling stimulus) by delivering concurrent normal or “startling” auditory stimuli. We found that the first pull triggers a different response, including a larger step size suggesting more destabilization. This is consistent with “first trial effects,” reported by platform translation studies, where movement execution appears confounded by startle reflex-like activity. Thus, first pull test trials have clinical relevance and should not be discarded as practice. Supportive of ecological relevance, responses to repeated pulls exhibited StartReact, as previously reported with a variety of other postural challenges, including those delivered with unexpected timing and direction. Examiner pull force significantly affected the postural response, particularly the size of stepping. NEW & NOTEWORTHY We characterized postural responses elicited by the clinical “pull test” using instrumentation. The first pull triggers a different response, including a larger step size suggesting more destabilization. Thus, first trials likely have important clinical and ecological relevance and should not be discarded as practice. Responses to repeated pulls can be accelerated with a startling stimulus, as reported with a variety of other challenges. Examiner pull force was a significant factor influencing the postural response.

Response preparation and execution during intentional bimanual pattern switching

During continuous bimanual coordination, in-phase (IP; 0° relative phase) and anti-phase (AP; 180° relative phase) patterns can be stably performed without practice. Paradigms in which participants are required to intentionally switch between these coordination patterns have been used to investigate the interaction between the performer’s intentions and intrinsic dynamics of the body’s preferred patterns. The current study examined the processes associated with switching preparation and execution through the use of a startling acoustic stimulus (SAS) as the switch stimulus. A SAS is known to involuntarily trigger preprogrammed responses at a shortened latency and, thus, can be used to probe advance preparation. Participants performed cyclical IP and AP bimanual elbow extension-flexion movements in which they were required to switch patterns in response to an auditory switch cue, which was either nonstartling (80 dB) or a SAS (120 dB). Results indicated that reaction time to the switch stimulus (i.e., switch onset) was significantly reduced on startle trials, indicative of advance preparation of the switch response. Similarly, switching time was reduced on startle trials, which was attributed to increased neural activation caused by the SAS. Switching time was also shorter for AP to IP trials, but only when the switching stimulus occurred at either the midpoint or reversal locations within the movement cycle, suggesting that the switch location may affect the intrinsic dynamics of the system. NEW & NOTEWORTHY The current study provides novel information regarding preparation and execution of intentional switching between in-phase and anti-phase bimanual coordination patterns. Using a startling acoustic stimulus, we provide strong evidence that the switching response is prepared before the switch stimulus, and switch execution is accelerated by the startling stimulus. In addition, the time required to switch between patterns and relative limb contribution is dependent upon where in the movement cycle the switch stimulus occurred.

A Flight Simulator Study of the Impairment Effects of Startle on Pilots During Unexpected Critical Events

Recent aircraft accidents have implicated startle as contributory, or directly causal, in situation outcome. The startle reflex is a ubiquitous response to surprising stimuli, which results in aversive movement and attentional orienting. Fear-potentiated startle, where a startling stimulus is experienced in the presence of conditions that are appraised as harmful or threatening, has the effect of initiating and exacerbating the stress response, particularly where threat persists, such as during an aircraft emergency. The deleterious effects of this stress response on cognitive function are discussed. Results from startle research in a B737 flight simulator showed considerable cognitive impairment in approximately one third of participants.

Affective modulation of post-auricular reflexes to serial probes : a test of the nursing hypothesis

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] The post-auricular reflex (PAR) is a vestigial microreflex which is thought to be a component of startle. However, unlike prototypical components of startle (e.g. acoustic blink reflex), the PAR is potentiated during appetitive motivational states and suppressed during defensive ones. An electromyographic (EMG) experiment with 37 men and 47 women tested if the Nursing Hypothesis could satisfactorily explain this anomalous PAR response. The Nursing Hypothesis proposes that infants of an ancestral mammalian species instinctively retracted their pinnae to signal their intention to nurse. Accordingly, appetitive emotions would still prime the ear-retraction musculature of adult humans, thereby causing facilitation of the PAR if a startling stimulus occurred. The present study failed to support the Nursing Hypothesis. Participants viewed one of 14 categories of slides while a series of startle probes was presented. Male participants showed the expected enhancement of PAR amplitude as nursing-related stimuli such as photographs of delicious food were viewed. Female participants by contrast had suppression of PAR amplitude when photographs of healthy breasts or of faces of beautiful women were viewed. Alternate hypotheses of PAR modulation are discussed.

First trial and StartReact effects induced by balance perturbations to upright stance

Postural responses (PR) to a balance perturbation differ between the first and subsequent perturbations. One explanation for this first trial effect is that perturbations act as startling stimuli that initiate a generalized startle response (GSR) as well as the PR. Startling stimuli, such as startling acoustic stimuli (SAS), are known to elicit GSRs, as well as a StartReact effect, in which prepared movements are initiated earlier by a startling stimulus. In this study, a StartReact effect paradigm was used to determine if balance perturbations can also act as startle stimuli. Subjects completed two blocks of simple reaction time trials involving wrist extension to a visual imperative stimulus (IS). Each block included 15 CONTROL trials that involved a warning cue and subsequent IS, followed by 10 repeated TEST trials, where either a SAS (TESTSAS) or a toes-up support-surface rotation (TESTPERT) was presented coincident with the IS. StartReact effects were observed during the first trial in both TESTSAS and TESTPERT conditions as evidenced by significantly earlier wrist movement and muscle onsets compared with CONTROL. Likewise, StartReact effects were observed in all repeated TESTSAS and TESTPERT trials. In contrast, GSRs in sternocleidomastoid and PRs were large in the first trial, but significantly attenuated over repeated presentation of the TESTPERT trials. Results suggest that balance perturbations can act as startling stimuli. Thus first trial effects are likely PRs which are superimposed with a GSR that is initially large, but habituates over time with repeated exposure to the startling influence of the balance perturbation.

Motor preparation of spatially and temporally defined movements: evidence from startle

Previous research has shown that the preparation of a spatially targeted movement performed at maximal speed is different from that of a temporally constrained movement ( Gottlieb et al. 1989b ). In the current study, we directly examined preparation differences in temporally vs. spatially defined movements through the use of a startling stimulus and manipulation of the task goals. Participants performed arm extension movements to one of three spatial targets (20°, 40°, 60°) and an arm extension movement of 20° at three movement speeds (slow, moderate, fast). All movements were performed in a blocked, simple reaction time paradigm, with trials involving a startling stimulus (124 dB) interspersed randomly with control trials. As predicted, spatial movements were modulated by agonist duration and timed movements were modulated by agonist rise time. The startling stimulus triggered all movements at short latencies with a compression of the kinematic and electromyogram (EMG) profile such that they were performed faster than control trials. However, temporally constrained movements showed a differential effect of movement compression on startle trials such that the slowest movement showed the greatest temporal compression. The startling stimulus also decreased the relative timing between EMG bursts more for the 20° movement when it was defined by a temporal rather than spatial goal, which we attributed to the disruption of an internal timekeeper for the timed movements. These results confirm that temporally defined movements were prepared in a different manner from spatially defined movements and provide new information pertaining to these preparation differences.