How does the brain make sense of what we see, hear, and feel—and transform these separate signals into a coherent perception of the world?

A new study published in Science provides important clues by examining sensory processing in the zebrafish brain. Led by Prof. Emre Yakşi of the Norwegian University of Science and Technology (NTNU) and Koç University Research Center for Translational Medicine (KUTTAM), the research reveals that zebrafish organize and combine sensory information through a hierarchical system that resembles a fundamental feature of sensory processing in mammalian brains.
Separate pathways, a unified perception

Information about the outside world reaches the brain through different sensory pathways. Visual signals, sounds, vibrations, odors, and other stimuli are initially processed separately. Yet our experience of the environment is not divided into isolated sensory channels. The brain eventually combines these signals to form unified perceptions and guide behavior.

In mammals, the thalamus plays a central role in receiving and directing sensory information to different areas of the cerebral cortex. Sensory signals remain partly separated during the early stages of processing and are gradually integrated as they move through cortical networks.

The researchers investigated whether a comparable organizational principle exists in zebrafish, whose evolutionary lineage diverged from that of mammals hundreds of millions of years ago.

Although the zebrafish brain is anatomically different from the mammalian brain, zebrafish are vertebrates and share many fundamental features of nervous-system organization with humans. Their small size and relative optical transparency also allow researchers to observe neural activity across large regions of the living brain.

The team examined how young zebrafish responded to visual stimuli and vibrations in the water. Using advanced neural imaging and circuit-mapping methods, the researchers followed how these signals entered the forebrain and how the activity changed as information moved through different brain regions.

The study identified the preglomerular complex, or PG, as the main source of visual and vibrational information reaching the zebrafish pallium. The pallium is an evolutionarily related counterpart of the vertebrate cerebral cortex.

The PG and its projections to the pallium were organized according to both sensory modality and spatial location. Visual and vibrational signals initially followed distinct pathways and reached different regions of the pallium.

However, the organization became increasingly complex within the pallium. Some neurons responded specifically to one type of sensory input, while others responded to multiple sensory signals.

The researchers also identified neurons that showed nonlinear, coincidence-detecting responses. These cells remained relatively inactive when presented with either light or vibration alone but responded strongly when the two stimuli occurred together. Such activity may help the brain determine whether separate sensory signals belong to the same event.

The findings reveal a spatially organized hierarchy in the zebrafish forebrain. Neurons responding to individual sensory inputs were found alongside progressively more complex neurons capable of integrating information from different senses.

This transition—from sensory-specific responses to multimodal and coincidence-detecting activity—resembles the hierarchical processing observed in mammalian thalamocortical systems.

The anatomical structures involved are not identical in fish and mammals. Nevertheless, both systems appear to follow a similar computational logic: sensory information is initially separated, spatially organized, and then progressively integrated to support more complex perception and adaptive behavior.

The results therefore suggest that hierarchical sensory processing may represent a convergent organizational principle across vertebrate brains. Different evolutionary lineages may have developed distinct anatomical pathways while arriving at comparable solutions to the challenge of interpreting a complex environment.

Understanding these shared principles may provide a broader perspective on how vertebrate brains evolved and how neural circuits transform sensory information into perception and behavior.

The researchers’ next steps include examining the cellular and molecular identities of the circuits involved and investigating how multisensory neurons influence learning, decision-making, and adaptive behavior.