Scientists frequently reference the “hard problem” of consciousness, which pertains to the challenge of grasping how neural activity in the brain gives rise to subjective, personal experiences—or qualia.
The precise structures and processes inside our heads that generate the phenomenon of consciousness remain a mystery, meaning there is no consensus on the exact moment this trait manifests in an unborn human.
However, by synthesizing all available neuroscientific research and developmental data, the authors of a new study tentatively propose a model explaining how and when the “light switch” turns on in a developing fetus. The findings of the study were published in the journal Brain Structure and Function.
Based on emerging evidence, the researchers argue that consciousness may not rely on higher cognitive abilities but instead appears to stem from a more primitive capacity to feel and perceive one’s own presence.
According to the authors, the brain structures necessary for this capacity are located not in the cerebral cortex but in the brainstem, which is much older from an evolutionary perspective and oversees some of our most fundamental functions.
Thus, they put forward a “subcortical model” for the origin of consciousness, suggesting that the sense of awareness could begin before the full development of the cerebral cortex. This is the large, wrinkled outer layer of our brain, thought to be involved in perception, thinking, and memory.
Instead, the researchers highlight two components of the brainstem that may trigger consciousness. The brainstem is a stalk-like structure connecting the main brain to the spinal cord.
The first of these components is the ascending reticular activating system (ARAS), which essentially rouses the brain, enabling wakefulness and the maintenance of consciousness. Experiments on cats, for instance, have shown that damage to the ARAS leads to coma, whereas cortical lesions do not.
Meanwhile, the periaqueductal gray (PAG) plays a fundamental role in what is known as valenced affective consciousness, through which the organism can perceive external stimuli as positive or negative. “By integrating interoceptive signals, visceral arousal, and environmental threats, it generates primitive affective qualia, biological values that endow wakefulness with personal significance,” the researchers write.
“The core idea is that ARAS plus PAG constitute the minimal neural architecture for conscious awareness,” explained study author Saul Sal Saria from the University of Oviedo. “ARAS provides the organism’s wakefulness; PAG provides the subject’s sensation of that wakeful state.”
And although it is impossible to pinpoint the exact moment when consciousness activates in a human fetus, the researchers suggest that this may occur during the third trimester of pregnancy. “Rather than emerging suddenly, consciousness unfolds in a continuum rooted in brainstem systems that are already active before birth,” they write. “However, if we must identify a specific time period as the source of these mental experiences, the most likely candidate is the third trimester of pregnancy.”
Such a discovery carries a range of ethical implications. For example, if an unborn child is capable of experiencing “genuine sensations,” then managing fetal pain during invasive procedures becomes a matter of paramount importance.
Nevertheless, the study authors emphasize that this area remains poorly studied and inadequately understood, and that their conclusions are based on a synthesis of existing data rather than on new empirical evidence.
“It is important to note that this is a hypothesis grounded in structural and behavioral correlates, not in direct measurement of subjective experience,” says Sal Saria. “Therefore, we present our findings as ‘evidence-based hypotheses’ rather than as proof.”
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