Posts tagged night vision
Articles and news from the latest research reports.
Find a space with total darkness and slowly move your hand from side to side in front of your face. What do you see?
If the answer is a shadowy shape moving past, you are probably not imagining things. With the help of computerized eye trackers, a new cognitive science study finds that at least 50 percent of people can see the movement of their own hand even in the absence of all light.
"Seeing in total darkness? According to the current understanding of natural vision, that just doesn’t happen," says Duje Tadin, a professor of brain and cognitive sciences at the University of Rochester who led the investigation. "But this research shows that our own movements transmit sensory signals that also can create real visual perceptions in the brain, even in the complete absence of optical input."
Through five separate experiments involving 129 individuals, the authors found that this eerie ability to see our hand in the dark suggests that our brain combines information from different senses to create our perceptions. The ability also “underscores that what we normally perceive of as sight is really as much a function of our brains as our eyes,” says first author Kevin Dieter, a post-doctoral fellow in psychology at Vanderbilt University.
The study seems to confirm anecdotal reports that spelunkers in lightless caves often are able to see their hands. In other words, the “spelunker illusion,” as one blogger dubbed it, is likely not an illusion after all.
For most people, this ability to see self-motion in darkness probably is learned, the authors conclude. “We get such reliable exposure to the sight of our own hand moving that our brains learn to predict the expected moving image even without actual visual input,” says Dieter.
Tadin, Dieter, and their team from the University of Rochester and Vanderbilt University reported their findings online October 30 in Psychological Science, the flagship journal of the Association for Psychological Science.
Although seeing one’s hand move in the dark may seem simple, the experimental challenge in this study was to measure objectively a perception that is, at its core, subjective. That hurdle at first stumped Tadin and his postdoctoral advisor at Vanderbilt Randolph Blake after they initially stumbled upon the puzzling observation in 2005. “While the phenomenon looked real to us, how could we determine if other people were really seeing their own moving hand rather than just telling us what they thought we wanted to hear?” asks Blake, the Centennial Professor of Psychology at Vanderbilt and a co-author on the paper.
Years later, Dieter, at the time a doctoral student working in Tadin’s Rochester lab, helped devise several experiments to probe the sight-without-light mystery. For starters, the researchers set up false expectations. In one scenario, they led subjects to expect to see “motion under low lighting conditions” with blindfolds that appeared to have tiny holes in them. In a second set up, the same participants had similar blindfolds without the “holes” and were led to believe they would see nothing. In both set ups, the blindfolds were, in fact, equally effective at blocking out all light. A third experiment consisted of the experimenter waving his hand in front of the blindfolded subject. Ultimately, participants were fitted with a computerized eye tracker in total darkness to confirm whether self-reported perceptions of movement lined up with objective measures.
In addition to testing typical subjects, the team also recruited people who experience a blending of their senses in daily life. Known as synesthetes, these individuals may, for example, see colors when they hear music or even taste sounds. This study focused on grapheme-color synesthetes, individuals who always see numbers or letters in specific colors.
The researchers enlisted individuals from Rochester, Nashville, Fenton, Michigan, and Seoul, South Korea, but, in a lucky coincidence, one synesthete could not have been closer. At the time, Lindsay Bronnenkant was working as a lab technician for co-author David Knill, a professor of brain and cognitive sciences at Rochester.
"As a child, I just assumed that everybody associated colors with letters," says the 2010 Rochester graduate who majored in brain and cognitive sciences. For Bronnenkant, "A is always yellow, but Y is an oranger yellow." B is navy, C burnt orange, and so on. She thought of these associations as normal, "like when you smell apple pie and you think of grandma." She doesn’t remember a time when she did not see numbers and letters in color, but she does wonder if the particular colors she associates with numbers derived from the billiard balls her family had going up. When she donned the blindfold and waved her hand in the experiment, "what I saw was a blur. It was very dim, but it was almost like I was looking at a light source."
Bronnenkant was not atypical in that respect. Across all types of participants, about half detected the motion of their own hand and they did so consistently, despite the expectations created with the faux holes. And very few subjects saw motion when the experimenter waved his hand, underscoring the importance of self-motion in this visual experience. As measured by the eye tracker, subjects who reported seeing motion were also able to smoothly track the motion of their hand in darkness more accurately than those who reported no visual sensation—46 percent versus 20 percent of the time.
Reports of the strength of visual images varied widely among participants, but synesthetes were strikingly better at not just seeing movement, but also experiencing clear visual form. As an extreme example in the eye tracking experiment, one synesthete exhibited near perfect smooth eye movement—95 percent accuracy—as she followed her hand in darkness. In other words, she could track her hand in total darkness as well as if the lights were on.
"You can’t just imagine a target and get smooth eye movement," explains Knill. "If there is no moving target, your eye movements will be noticeably jerky."
The link with synesthesia suggests that our human ability to see self-motion is based on neural connections between the senses, says Knill. “We know that sensory cross talk underlies synesthesia. But seeing color with numbers is probably just the tip of the iceberg; synesthesia may involve many areas of atypical brain processing.”
Does that mean that most humans are preprogrammed to see themselves in the dark? Not likely, says Tadin. “Innate or experience? I’m pretty sure it’s experience,” he concludes. “Our brains are remarkably good at finding such reliable patterns. The brain is there to pick up patterns—visual, auditory, thinking, movement. And this is one association that is so highly repeatable that it is logical our brains picked up on it and exploited it.”
Whether hardwired or learned, Bronnenkant finds the cross talk between her senses a potent reminder of the underlying interconnectivity of nature. “It’s almost a spiritual thing,” she says. “Sometimes, yeah, I think to myself, ‘I just got this sense from a billiard ball,’ but other times I think that being able to cross modalities actually reflects how unified the world is. We think of math and chemistry and art as different fields, but really they are facets of the same world; they are just ways of looking at the world through different lenses.”
Switching night vision on or off
Neurobiologists at the Friedrich Miescher Institute have been able to dissect a mechanism in the retina that facilitates our ability to see both in the dark and in the light. They identified a cellular switch that activates distinct neuronal circuits at a defined light level. The switch cells of the retina act quickly and reliably to turn on and off computations suited specifically for vision in low and high light levels thus facilitating the transition from night to day vision. The scientists have published their results online in Neuron.
"It was fascinating to see how modern neurobiological methods allowed us to answer a question about vision that has been controversially discussed for the last 50 years", said Karl Farrow, postdoctoral fellow in Botond Roska’s group at the Friedrich Miescher Institute for Biomedical Research. Since the late 1950 scientists debated how the retina handles the different visual processes at low and high light intensities, at starlight and at daylight. Farrow and his colleagues have now identified a cellular switch in the retina that controls perception during these two settings.
At first glance, everything seems clear. The interplay of two photoreceptor types in the retina, the rods and the cones, allow us to see across a wide range of light intensities. The rods are highly sensitive and spring into action in the dark; the cones are activated during the day and in humans come in three diversities allowing us to see color. The rods help us detect objects during the night; while the cones allow us to discriminate the fine details of those objects during the day. The plethora of initial signals originating from the photoreceptors is computed in a system of only approximately 20 neuronal channels that transport information to the brain. The relay stations are the roughly 20 types of ganglion cells in the retina. How they manage the transition from light to dark and enable vision at the different light regimes has remained unclear.
In the retina several cell layers are stacked on top of each other. The photoreceptors are the first to be activated by light; they relay the information to bipolar cells, which in turn activate ganglion cells. The different types of ganglion cells take on distinct tasks during vision. These ganglion cells are embedded in a mesh of amacrine cells that modulate their activity. “Here is where our new genetic tools proofed very helpful,” said Farrow, “because they allowed us to look at individual ganglion cell types and to specifically measure their activities at different light intensities.” Farrow and colleagues could thus show that the activity of one particular type of ganglion cells, called PV1, is modulated like a switch by amacrine cells. The amacrine cells inhibit the ganglion cell strongly at high light intensities and weakly at low ambient light levels. This switch is abrupt and reversible and it occurs at the light intensities where cones are starting to be activated. “We were surprised to see how fast this switch occurs and how reliable we were able to switch between the two states at defined light intensities”, comments Farrow.
While the above experiments were done in a mouse model, the FMI neurobiologists could show that a similar switch operates in human vision. Their volunteers had to look at narrow and broader stripes at different light levels. They could show that there again a switch operates. While the general ability to see all striped patterns improved with increasing light intensity, suddenly, at a certain light level, the volunteers were much better able to detect thinner patterns as compared to the broader ones. Interestingly enough this switch happened at precisely the light level where the volunteers were also able to discriminate between red and blue, hence where the cones spring into action. “We think we have found a regulatory principle that could apply to several processes in the brain”, said Roska, “This principle could explain some situations when gradual changes in the sensory environment leads to abrupt changes in brain computations and perception”
(Source: medicalxpress.com)
Study clarifies process controlling night vision
New research reveals the key chemical process that corrects for potential visual errors in low-light conditions. Understanding this fundamental step could lead to new treatments for visual deficits, or might one day boost normal night vision to new levels.
Like the mirror of a telescope pointed toward the night sky, the eye’s rod cells capture the energy of photons - the individual particles that make up light. The interaction triggers a series of chemical signals that ultimately translate the photons into the light we see.
The key light receptor in rod cells is a protein called rhodopsin. Each rod cell has about 100 million rhodopsin receptors, and each one can detect a single photon at a time.
Scientists had thought that the strength of rhodopsin’s signal determines how well we see in dim light. But UC Davis scientists have found instead that a second step acts as a gatekeeper to correct for rhodopsin errors. The result is a more accurate reading of light under dim conditions.
A report on their research appears in the October issue of the journal Neuron in a study entitled “Calcium feedback to cGMP synthesis strongly attenuates single photon responses driven by long rhodopsin lifetimes.”