Learning to see

How early visual experience builds reliable brain circuits

To the point

• Development of brain circuits: early visual experiences significantly influence the development of brain circuits responsible for reliable visual perception.
• Two key phases in brain circuit development: the prenatal phase, driven by genetic mechanisms (nature), and the postnatal phase, shaped by experiences (nurture).
• Neural activity in the visual cortex at different developmental stages: early visual experience alters the organization and stability of neural circuits.
• Unreliable visual encoding: without early visual experience, the stability and alignment of activity patterns in brain circuits do not develop.

Brain circuits emerge from a developmental sequence that includes two distinct phases often referred to as ‘Nature and Nurture’.  The period before birth is when genetic mechanisms and spontaneous patterns of neural activity drive the initial formation of brain networks (Nature). The subsequent period, following birth, is when patterns of neural activity driven by experiences further shape neural circuits (Nurture). Despite the clear boundary that defines these two developmental phases, exactly how events at each stage contribute to the function of mature neural circuits remains poorly understood.

In a study reported in the journal Nature Neuroscience, neuroscientists led by David Fitzpatrick and Matthias Kaschube gained fundamental insights into how these different developmental phases contribute to the response properties of the mature functional networks responsible for visual perception.

“Immediately following birth, there is a dramatic change in the brain’s circuitry as the number of connections between neurons increases. This makes it critical to understand the functional state of the networks prior to experience and how the novel patterns of neural activity driven by experience after birth shape the maturation of brain circuits. This knowledge is crucial for addressing a host of neurodevelopmental disorders and has the potential to guide interventions to maximally promote healthy brain development,” described Fitzpatrick.

Visual Circuit Development

To understand how nature and nurture contribute to the maturation of circuits underlying vision, the team of scientists looked at the activity of the neurons in the visual cortex of the ferret at three different stages of development- before any visual experience, during very early visual experience, and when vision was fully mature.

Scientists characterized the patterns of spontaneous activity in networks prior to the onset of visual experience and compared these activity patterns to the very first neural responses driven by sight.  This first visual experience- simple and highly controlled patterns of moving bars of light – produced activity in the visual circuits that were unlike the patterns found in the spontaneous activity before eye-opening. Moreover, although the circuit’s responses to the bars of light were highly robust, the response patterns were not reliable. Each time the same visual image was shown, different activity patterns were produced, and even the activity of individual neurons was not consistent or stable across trials.

This instability was very different from the stable activity the researchers measured in response to the same visual scene in the mature visual processing circuit. Even after just a few days of visual experience, the same scene shown multiple times activates the same neurons in precisely the same pattern, allowing for robust visual encoding. Additionally, after visual experience, the spontaneous patterns of activity exhibited by the circuit align with the patterns of activity driven by various visual scenes. Critically, without early visual experience, the stability of the activity patterns and their alignment with spontaneous activity patterns did not emerge.

With further quantitative analysis of the activity patterns, the scientists were able to show that the stable patterns that emerged following experience were different from both the early evoked patterns and the early spontaneous patterns. This indicated that early visual experience had driven developmental changes in the organization of neural circuits that produced novel patterns of stable activity. Although the nature of the specific network changes responsible for the emergence of reliable responses remains to be determined, a computational model that was developed and compared with the biological data has made specific predictions that are currently being tested.

Fitzpatrick explains, “This study has revealed the powerful role that visual experience plays in transforming the early properties of neural circuits into a mature circuit with novel patterns of activity that reliably encode visual information. Understanding the role of experience in this process advances our goal of uncovering fundamental mechanisms of brain circuit development that we believe are critical for understanding and mitigating developmental disorders.”