Reference: Olkoniemi, H., Hurme, M., & Railo, H. (2023). Neurologically healthy humans’ ability to make saccades toward unseen targets. Neuroscience, 513, 111–125.

Guest post by Hisham Ahmed

Shifting our gaze from one word to the next while reading is an intricate process that we typically don’t give much thought. Our eyes are constantly scanning our environment and shifting from one thing to another. This movement is called a saccade, and it is essential for visual perception. Ideally, saccades are quick, accurate, and automatic. Saccades have been investigated widely by neuroscientists to explain the mechanism behind their initiation¹. The phenomenon of blindsight has offered us an interesting experimental context to study saccades.  

Blindsight: Seeing without perceiving 

The human eye is a marvel of nature, capable of sensing the slightest changes in light and color, but sensing information is only the first step in perception. To truly understand what we’re seeing, our brains must process and interpret this information, creating a conscious experience.

Blindsight is a condition where individuals have damage to their primary visual cortex (V1), which is responsible for processing visual input that constructs our conscious vision². Despite the loss of conscious vision in the affected visual field, individuals with blindsight are still able to make saccades toward stimuli presented in the affected visual field. If you want to learn more about blindsight, you can watch this video.

This phenomenon raises an intriguing question: Are saccades influenced by unconscious information? Henri Olkoniemi and colleagues sought to simulate blindsight in healthy people (i.e., those without damage to their V1) and examine the capability of participants to direct their gaze toward visual targets that they reported not consciously seeing.

Simulating blindsight in healthy people

In their first experiment, scientists employed transcranial magnetic stimulation (TMS), a technique in which participants sat in a chair while a technician placed a small coil over the scalp. Then, a stimulus – dots against a light gray background – was presented on a screen, and the coil was used to deliver a brief magnetic pulse to the brain to stimulate specific locations in the primary visual cortex (V1). Prior research has demonstrated that this procedure suppresses conscious perception of the stimuli and produces blindsight-like behavior³. That is, although the stimulus was seen by the participants, it was not consciously perceived.

In a second experiment, researchers employed a technique called metacontrast masking, in which the target stimulus is presented briefly and then followed by a perceptual mask consisting of two black annuli, which reduces the visibility of the target. Transcranial magnetic stimulation disturbed processing in the V1 to test if saccades can be initiated without V1, while meta-contrast masking left V1 intact to investigate whether it’s possible to initiate saccades toward unconsciously seen targets. Researchers instructed participants to keep their eyes directed toward the center of the screen. If participants noticed the target stimulus was presented, they were to move their eyes toward the stimulus. For example, if a target stimulus was presented on the right side of the screen, they were to move their eyes to the right. The precision of saccades was calculated by measuring the distance between saccade end location and the target location. After each trial, the participants rated the target stimuli’s visibility on a four-point scale.

What did they find?

Researchers found similar results across the two experiments. They found that the initiation of saccades toward the target was largely based on conscious perception, and weaker conscious perception was associated with slower and less precise saccades. However, in a small number of trials, the participants were able to initiate saccades toward targets they reported not perceiving. That is, when the participants made saccades and reported not consciously perceiving the stimulus, these saccades were made toward the correct target location.

These findings generally support earlier research on blindsight in humans and monkeys^4,5, and demonstrate that saccades may not be dependent on V1 and may also be influenced by unconscious information. These results show that blindsight saccades are not a consequence of the neuronal reorganization that frequently follows injuries, but rather a characteristic of a neurologically healthy brain.

What do the results mean?

This research highlights the complexity of the brain and the limitations of our conscious perception. We often assume that our conscious experiences are the sole source of information, but these findings demonstrate that unconscious processing can play a significant role in our behavior.

These findings also have implications beyond the realm of vision. Our brain processes information from all our senses, and unconscious processing likely plays a critical role in how we interact with the world around us. In conclusion, the human brain is a complex and mysterious organ, and saccades are just one example of the fascinating mechanisms at play. Although we often assume that our conscious experiences dictate our behaviour, unconscious processing can also play role in our actions and decisions.

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Additional References:

1. Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. Neural Control of Saccadic Eye Movements. https://www.ncbi.nlm.nih.gov/books/NBK10992/

2. Ajina, S., & Bridge, H. (2016). Blindsight and unconscious vision: What they teach us about the human visual system. The Neuroscientist, 23(5), 529–541. https://doi.org/10.1177/1073858416673817
   
3. Amassian, V.E. et al. (1989) “Suppression of visual perception by magnetic coil stimulation of human occipital cortex,” Electroencephalography and Clinical Neurophysiology/Evoked Potentials Section, 74(6), pp. 458–462. Available at: https://doi.org/10.1016/0168-5597(89)90036-1
   
4. Barbur, J. L., Forsyth, P. M., & Findlay, J. M. (1988). Human saccadic eye movements in the absence of the geniculocalcarine projection. Brain, 111(1), 63-82. https://doi.org/10.1093/brain/111.1.63
   
5. Isa, T. and Yoshida, M. (2021). Neural mechanism of blindsight in a macaque model. Neuroscience, 469, pp. 138–161. https://doi.org/10.1016/j.neuroscience.2021.06.022

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Hisham is a student at the veterinary medicine school of Cairo University. After graduation, he aspires to go to a graduate school and study neuroscience. Connect with him on Linkedin.