When the blind can see again: A critical question of perception

Our sense of sight is arguably our most important sense.  Imagine how different your life would be if soon after birth, you lost the ability to see.  For over 1.4 million children worldwide, that is their life.  Being blind in developing countries like India has a costly impact: over 90% of blind children do not go to school, less than 50% make it to adulthood, and for those that do, only 20% are employed. But the real tragedy is that many of these cases of childhood blindness are completely avoidable and even treatable.

Why do they go untreated?

In developing countries, children remain blind largely because they do not have easy access to medical care.  Many people do not know that some forms of blindness can be treated, so they do not even seek medical help.  If they do have access to medical facilities, families are often too poor to afford the treatment.  Lastly, many people believe that beyond the first few years of life, intervention is futile because it is past the “critical period” of sight.

What is a critical period?

During early developmental stages, the brain undergoes rapid adjustments, as it soaks in all that you see, hear, taste, and touch.  In a process called synaptic plasticity, neuronal connections are made, erased, strengthened, and weakened, and the brain begins to sort relevant sensory information from extraneous details.  During these “sensitive” or “critical periods,” your interactions with your surroundings profoundly affect brain development and ultimately shape how you perceive the world around you today.

What happens when sensory information is absent during a critical period?

This question was famously answered by two scientists, David Hubel and Torsten Wiesel, in the 1960s when they studied the visual system’s critical period.  In their experiments, one eye of a newborn kitten was kept closed, and after a few months, when the eye was re-opened, they studied the effects of that monocular deprivation.  Not only was the vision in the deprived eye initially fuzzy, but it never improved to normal acuity.  Even though the eye appeared normal, it hadn’t received any visual information early in development during the critical period, so the neurons in the area of the visual cortex devoted to that eye were all but nonexistent.  Interestingly, adult cats who underwent the same procedure experienced no changes in visual performance.  This led Hubel and Wiesel conclude that failure to be exposed to appropriate stimuli during a critical period causes permanent brain changes, but once this critical period has passed, the behavior is largely unaffected by subsequent experiences (or lack thereof).


Around the same time that Hubel and Wiesel were conducting their experiments, physicians were reporting similar results in children who were visually deprived early in life and later had corrective surgery to repair vision.  Cataracts is a clouding of the lens that impairs vision and (if left untreated) can leave a person blind.  It is responsible for half the blindness and a third of the visual impairment in the world.  Thankfully, there is an effective surgical treatment that removes the cloudy lens and replaces it with an artificial lens.  While surgery successfully restores normal vision in adults who develop cataracts, it is less successful in children and they continue to experience vision problems for the rest of their lives.  As in cats, it is essential for humans to receive sensory information early in life for proper visual development.

What are the neurological differences in blind children who become sighted later in life after the visual critical period?

This is the question that the team at Project Prakash has the unique opportunity to try to answer.  Project Prakash (Sanskrit for “light”) works all over India, screening children for treatable eye problems and providing sight-restoring surgeries for free.  Since these children have been blind their whole lives, the team at Project Prakash can study how these children learn to see using tests like the ones highlighted in the figure below.  Their findings provide unique clues into neural plasticity and how the brain integrates different sensory cues to accurately perceive the environment.

examples from Project Prakash

How do newly-sighted children perceive the world?

Consistent with the notion of an early critical period for visual development, some key aspects of vision are permanently compromised in newly-sighted children.  For example, visual acuity (sharpness) and spatial contrast sensitivity (difference in luminosity or color that makes an object distinguishable) are diminished.  Interestingly, newly-sighted children perceive the world very differently from how we experience our visual environment.  For example, they tend to break up shapes into tiny pieces, rather than see holistic objects.  In perceptual oversegmentation, rather than seeing the overlapping circle, rectangle, and pentagon in the image segmentation example above, newly-sighted children see many irregular shapes!

In their most recent study, the Project Prakash team found that newly-sighted children were highly susceptible to visual illusions like the ones shown above.  This is surprising because if the illusion were driven by a learned appreciation for perspective cues, then it would be expected that newly-sighted children would have no difficulty noticing that the two lines are the same length.  Contrary to this hypothesis, they fall for the illusion just like other kids, suggesting that the susceptibility to these illusions is based on processing mechanisms that do not depend on visual experience.

To understand the processes underlying sensory integration, children were asked to visually match an object they had haptically sensed (by touch) soon after sight restoration.  While tactile shape information was not immediately transferred to the visual domain, these children were able to do this task after just a few trials.  This suggests that some cross-modal neuronal connections are left intact until they are used.

The research from Project Prakash offers unique insight into the brain’s limitations and abilities to learn how to see. Longitudinal studies of children who have sight-restoring vision will even further elucidate the mysteries of how the brain organizes the senses to make sense of the world around us.


Project Prakash treats blind children, and with their help, pursues answers to several profound scientific mysteries.  For more information about Project Prakash, visit http://www.projectprakash.org/.



Images by Jooyeun Lee and adapted from Sinha, Pawan, and Richard Held. “Sight Restoration.” F1000 Medicine Reports4 (2012): 17. PMC. Web. 21 May 2016.

Gandhi, Tapan, et al. “Immediate susceptibility to visual illusions after sight onset.” Current Biology 25.9 (2015): R358-R359.

Gandhi, Tapan K., Suma Ganesh, and Pawan Sinha. “Improvement in spatial imagery following sight onset late in childhood.” Psychological science 25.3 (2014): 693-701.

Sinha, Pawan, et al. “Restoring vision through “Project Prakash”: the opportunities for merging science and service.” PLoS Biol 11.12 (2013): e1001741.

Kate Fehlhaber

Kate graduated from Scripps College in 2009 with a Bachelor of Arts degree in Neuroscience, completing the cellular and molecular track with honors. As an undergraduate, she studied long-term plasticity in models of Parkinson’s disease in a neurobiology lab at University of California, Los Angeles. She continued this research as lab manager before entering the University of Southern California Neuroscience graduate program in 2011 and then transferring to UCLA in 2013. She completed her PhD in 2017, where her research focused on understanding the communication between neurons in the eye. Kate founded Knowing Neurons in 2011, and her passion for creative science communication has continued to grow.