The question of when a baby becomes conscious is one of the most profound unanswered questions in neuroscience. Consciousness, the state of subjective experience, presents a unique challenge to science because it is an entirely internal, private event. Researchers cannot simply ask a non-verbal infant about their inner world or what it feels like to be them. The difficulty lies in devising objective, measurable tests that can capture the presence of a subjective experience in a developing brain. Determining the onset of consciousness requires scientists to first establish a functional definition and then observe developmental milestones that signify its presence.
Defining Infant Consciousness
To study consciousness in infants, scientists distinguish between two primary components: arousal and awareness. Arousal refers to the state of wakefulness, governed by basic brainstem and thalamic functions. This state determines whether the individual is responsive to the world, such as being awake versus being in a deep sleep or a coma.
Awareness, conversely, is the subjective experience of the self and the environment, processed primarily by the cerebral cortex. A newborn can be aroused, exhibiting reflexes and sleep-wake cycles, without necessarily possessing the rich, integrated awareness an adult experiences.
The scientific community is divided regarding the timeline of infant awareness. The “late-onset” view suggests consciousness requires complex cognitive capacities that emerge long after birth. Conversely, the “early-onset” view suggests a basic form of consciousness is present much earlier, possibly at birth or during late pregnancy. Evidence for a basic conscious state often centers on the development of the thalamocortical connections, which are necessary for complex information processing.
Early Awareness in the Prenatal Period
The development of the nervous system lays the foundation for responsiveness well before birth. The neural structures required for basic conscious processing, the thalamocortical networks, are thought to become established around 24 to 26 weeks of gestation. This structural development creates the potential for the fetus to register sensory information.
The fetus is highly responsive to external stimuli. Touch is the first sense to develop, as the fetus explores its environment by moving its limbs and sucking its thumb as early as the first trimester. Sound is well-developed by the third trimester, and studies show the fetus can react to loud noises and quickly learn to recognize the mother’s voice.
The fetus is largely in a continuous sleep-like state, spending up to 95% of its time asleep in the womb. Sleep-wake cycles, including periods of rapid eye movement (REM) and non-REM sleep, are observed around the seventh month of pregnancy. However, the environment in the womb, characterized by low oxygen levels and sleep-inducing chemicals, keeps the developing brain relatively suppressed. This responsiveness is interpreted as neurological maturation and sensory learning rather than a sustained state of subjective awareness.
Scientific Methods for Measuring Infant Consciousness
Since infants cannot verbalize their experience, researchers rely on non-invasive tools to observe brain function and measure cognitive responses. Electroencephalography (EEG) is widely used because it records the brain’s electrical activity from the scalp with high temporal precision. This technique is useful for tracking the frequency and synchronization of brain waves that correlate with different states of alertness and information processing.
Functional magnetic resonance imaging (fMRI) offers high spatial resolution, showing where in the brain activity is occurring by measuring blood flow, a proxy for neural activity. Combining EEG’s timing data with fMRI’s location data helps scientists map the developing neural networks involved in conscious experience. These brain imaging methods are often performed while the infant is naturally sleeping due to the challenges of movement and noise.
Behavioral studies, such as the habituation-dishabituation paradigm, also provide objective evidence of awareness and memory. In this test, an infant is repeatedly shown a stimulus until their looking time decreases, indicating they have become habituated. If a new stimulus is then presented and the infant looks longer, this renewed interest suggests they can discriminate the new object and remember the old one. This demonstrates the presence of foundational cognitive processes like discrimination and short-term memory.
Postnatal Development of Self-Awareness
The emergence of higher-order cognitive functions in the months after birth provides tangible behavioral markers for a developing, integrated sense of self. One early milestone is the onset of working memory, the ability to hold a small amount of information in mind for a short period. Appreciable working memory begins around the sixth month, allowing infants to track simple objects or locations for a few seconds.
Another foundational development is object permanence, the understanding that objects continue to exist even when they are out of sight. While early research suggested this emerged around eight months, newer studies using more sensitive methods show infants begin to demonstrate a rudimentary understanding between four and seven months of age. The ability to actively search for a hidden object typically solidifies around the eight-to-twelve-month mark, indicating a robust mental representation of the world.
The clearest behavioral sign of self-awareness is demonstrated by the mirror self-recognition test, often called the rouge test. Until about 12 to 18 months, infants typically treat their reflection as another child, smiling or trying to interact with the image. The definitive sign of self-recognition is when the child touches a mark placed on their own nose while looking in the mirror, indicating they recognize the reflection as themselves. This milestone, which marks a significant step toward a subjective sense of identity, is passed by most children between 18 and 24 months.