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The Body That Reacts Before Any Thought (An Instantaneous Appraisal)

I was lying quietly when a loud noise from the street entered my room. In a fraction of a second, before any thought arose, before I even knew what it was, my body had already reacted. Almost immediately afterward, I realized that it was only the horn of a passing car.

What interests me here is observing and describing the precise instant at which my body responds to a sudden event—in this case, a loud and unexpected sound.

I am also interested in following the unfolding of this response over the course of the next milliseconds and the first few seconds: how the initial reaction emerges, how it changes as the sound is identified, and how, little by little, the body returns to its previous state.

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The Startle Response from a Third-Person Perspective (Scientific Findings)

The startle response is one of the oldest reactions of the nervous system. From an evolutionary perspective, it exists because it is better to react immediately to a potential threat and be wrong than to fail to react and discover too late that the threat was real.

For this reason, the response is automatic, rapid, and involuntary. It is initiated before any conscious evaluation of the stimulus takes place.

From a physiological perspective, the process begins when an unexpected stimulus—such as a loud sound, a sudden touch, or an abrupt movement—is detected by the sensory systems and processed by brainstem circuits specialized in the detection of potential threats and the generation of defensive responses.

(Here I will focus on the acoustic startle response, as it is the most extensively studied.)

In this case, within a few tens of milliseconds, these circuits activate skeletal muscles.

The response initially manifests in the musculoskeletal system, following an approximately rostrocaudal progression.

It begins with the muscles of the head (orbicularis oculi, approximately 20–40 ms), followed by the neck (sternocleidomastoid, ~60–90 ms), the trapezius and shoulders (~75–100 ms), the arms (~85–120 ms), the trunk (~100–150 ms), and finally the legs (~145 ms or later).

Autonomic responses become detectable shortly afterward, although they partially overlap in time with the muscular response.

Breathing is briefly interrupted almost immediately (around 1 s), followed by an increase in respiratory rate that typically peaks between 3 and 6 seconds.

The cardiac response follows a triphasic pattern, beginning with a brief vagal (parasympathetic) deceleration during the first 2–3 seconds, followed by sympathetic acceleration peaking between 4 and 7 seconds, and then a gradual return toward baseline over the next 15–20 seconds or longer.

The electrodermal response* is the slowest component, with an onset typically occurring between 1 and 3 seconds after stimulus onset and a peak between approximately 1 and 5 seconds.

*The electrodermal response, also known as the galvanic skin response, reflects activation of the sweat glands—particularly in the palms of the hands and the soles of the feet—which slightly increases skin moisture and, consequently, the electrical conductance of the skin.

Sympathetic activation may also produce piloerection—the familiar "goosebumps" caused by contraction of the arrector pili muscles.

This evolutionarily conserved response likely served, in fur-covered ancestors, to increase apparent body size during defensive displays while also improving thermal insulation.

Although less commonly assessed in laboratory studies of the startle response, it appears to occur alongside the sympathetic mobilization phase.

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Description: The Startle Response in First-Person Experience

Science describes this process from a third-person perspective: what happens in the body as measured from the outside. What interests me is something else: what can be observed from within.

I perceive the startle response most distinctly when I am sitting or lying quietly with my eyes closed and a sudden sound enters the environment. In those moments, when I experience a mild startle, I am able to follow more closely what unfolds.

Under these conditions, the startle response no longer appears as a single, simultaneous reaction of the body. Instead, it reveals itself as a sequence of smaller events.

I first notice a tension in my eyes (by this I mean a tension within the eyeballs themselves, rather than in the orbicularis oculi muscles), together with a rapid contraction in the throat—just below the jaw—which seems to direct the tension toward the solar plexus, where I experience a brief sensation of a “cold shock” in the stomach.

It is from this sensation that the state of alertness seems to establish itself in experience: first, a general protective readiness of the body, followed by a search for the source of the stimulus (although, during this practice of sitting quietly, I perceive only a very subtle hint of this global protective contraction).

The moment this contraction arises in the throat, breathing is instantaneously interrupted. The interruption seems to occur simultaneously with the contraction, as though both were part of the same bodily gesture.

Sometimes it is an abrupt pause during exhalation; at other times, it consists of a short, shallow inhalation before the breath momentarily stops, interrupting whatever rhythm it had been following.

(I have the impression that this may depend on the phase of the respiratory cycle in which the startle occurs.)

Once breathing resumes, it typically returns in a faster and more forceful manner, as though the body were compensating for the brief interruption before settling back into its previous rhythm.

When the intensity of the startle is greater—whether because of the nature of the stimulus or because I am more susceptible to reacting at that moment (for example, when I am already feeling anxious)—the response becomes stronger and my entire body trembles.

A kind of electrical wave then seems to travel through the body from head to toe, resembling an electric shock. Occasionally, the hairs on my arms also stand on end, although this is relatively uncommon.

My cardiac interoception is not particularly accurate. Even so, when the stimulus is truly intense and is subsequently appraised as genuinely aversive—transforming the startle into fear—I can perceive my heart beating faster. The same may occur when the startle develops into another intense emotion.

When I realize that the source of the stimulus posed no real danger, I may simply ignore it or experience a sense of relief, depending on the situation.

The increase in perspiration, especially in my hands, is also more difficult to notice because it is usually quite subtle. Nevertheless, on some occasions I have been able to perceive that my hands became slightly more moist immediately after the startle.

Sometimes I can also perceive subtle internal and external tensions around my ears when the sound is particularly loud.

Although the startle response is extremely rapid and short-lived, I feel capable of distinguishing its sequence of events, especially when the startle is very mild.

It is like watching a punch strike one’s own stomach in less than a second: even though everything happens extremely quickly, it is still possible to distinguish the tension in the arm muscles, the rapid movement of the fist through the air, the impact against the abdomen, and the pain that follows. The event appears instantaneous, yet its temporal structure can still be discerned.

Although the startle response usually occurs in response to unexpected stimuli, these moments of quiet—whether sitting or lying down—make it easier to observe.

During such moments, my attention is directed precisely toward transitions between bodily states. It is in the instant when a reaction begins to emerge that I find one of the richest opportunities for phenomenological investigation.

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The Question of Temporal Sequence

There is one particularly interesting aspect that the standard physiological account does not fully capture.

From a scientific perspective, the different components of the startle response are activated almost simultaneously by the central nervous system.

The order in which they become observable largely reflects the different response speeds of the various effector systems, rather than a causal chain in which one component triggers the next. This is what the literature on the neurobiology of the startle reflex suggests.

In experience, however, the phenomenon appears to have a different organization. The responses seem to unfold one after another, as though each component gives rise to the next in a chain reaction.

This may simply be a perceptual artifact: consciousness arriving too late to processes that have already occurred in parallel and subsequently organizing them into a causally ordered temporal sequence.

But it is also possible that this impression reflects a genuine physiological component.

Although the initial activation is central and nearly simultaneous, the earliest muscular responses may alter the state of the body and, even if only subtly, influence the subsequent development of other responses, producing a dynamic coupling among the different systems.

Phenomenological observation alone cannot distinguish between these possibilities. Yet the question itself seems important.

When subjective experience appears to diverge from the physiological account, are we dealing with a retrospective reconstruction of experience, limitations of experimental measurements, or simply two different levels of description of the same phenomenon?

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Voluntary Intervention

There is one final observation that I find particularly intriguing.

Through a kind of attentional training, I am able, in milder startle responses, to attenuate very rapidly the initial tension that arises in my throat and around my eyes.

When this happens early enough, the subsequent autonomic responses seem to diminish substantially. Sometimes they do not fully develop at all. On some occasions, I am even able to prevent the characteristic “cold shock” sensation in my stomach.

At least two hypotheses seem compatible with this observatio:

The first is that the different components of the response are, in fact, partially coupled, such that interrupting an early component attenuates those that follow.

The second is that an extremely early redirection of attention—from the external stimulus toward the emerging bodily reaction itself—prevents the startle response from fully developing.

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Conclusion

The aim of this text was to describe the startle response as it is lived, rather than to propose a new theory about its physiological mechanisms. Even so, the description itself raises interesting questions about the relationship between experience and neurobiology.

If some of the observations presented here reflect genuine regularities of experience, they may serve as a starting point for further experimental investigation.

Regardless of that, this exercise shows that even a phenomenon lasting only a fraction of a second can reveal a surprisingly rich experiential organization when carefully attended to.

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References

Brown, P., Rothwell, J. C., Thompson, P. D., Britton, T. C., Day, B. L., & Marsden, C. D. (1991). New observations on the normal auditory startle reflex in man. Brain, 114(4), 1891–1902. https://doi.org/10.1093/brain/114.4.1891

Koch, M. (1999). The neurobiology of startle. Progress in Neurobiology, 59(2), 107–128.
Link: https://doi.org/10.1016/S0301-0082(98)00098-3

Yeomans, J. S., & Frankland, P. W. (1996). The acoustic startle reflex: neurons and connections. Brain Research Reviews, 21(3), 301–314.
Link: https://doi.org/10.1016/S0165-0173(96)00013-8

Blumenthal, T. D., et al. (2005). Committee report: Guidelines for human startle eyeblink electromyographic studies. Psychophysiology, 42(1), 1–15.
Link: https://doi.org/10.1111/j.1469-8986.2005.00271.x

Yeomans, J. S., & Lee, J. C. (2012). Startle as a tool to study neural circuits of defence. Current Opinion in Neurobiology, 22(4), 737–743.
Link: https://doi.org/10.1016/j.conb.2012.06.007

Lang, P. J., Davis, M., & Öhman, A. (2000). Fear and anxiety: animal models and human cognitive psychophysiology. Journal of Affective Disorders, 61(3), 137–159.
Link: https://doi.org/10.1016/S0165-0327(00)00343-8