Motion perception revisited: High Phi effect challenges established motion perception assumptions

Optical illusions abound in human visual perception, as demonstrated by the following well-known examples. Although many are static illusions, motion illusions also occur. Recently, scientists at Université Paris Descartes and Centre National de la Recherche Scientifique, Paris, University of Reading, United Kingdom, and Kyushu University, Japan discovered and investigated a new illusory motion effect, termed high phi by the authors, in which we perceive conspicuous large illusory jumps when presentation of motion signals are followed by brief visual stimuli free of detectable motion signals. The researchers found that the size of the illusory jump does not depend on the speed of the motion signals presented, but rather on spatial frequency and transient duration while jump duration depends on motion signal duration. The study’s authors conclude that their findings demonstrate that existing explanations for this illusion – namely, the loss of coherent motion perception above an upper limit and the preference for minimal motion – are incomplete at best.

Lead researcher Mark Wexler describes some of the challenges he and his colleagues – Andrew Glennerster, Patrick Cavanagh, Hiroyuki Ito, and Takeharu Seno – encountered in conducting their study. “We had the idea that these illusory jumps are related to dmax, the supposed upper speed limit on the steps that leads to motion perception, varies between individuals, and must be measured using random textures,” Wexler tells Medical Xpress. “For displacements below dmax you’re supposed to see the motion more or less correctly,” he explains, “while for displacement above dmax you’re just supposed to see noise – and the latter also turns out to be false.” (These illusory jumps are demonstrated in an online supplement to the paper.)

Interestingly, the researchers discovered the illusion as a bug in a computer program whose purpose was to do something else. “The easy thing to do in those kinds of circumstances is to correct the bug and move on,” Wexler comments, “and ignore how strange the effect of the bug actually is. Our key insight was not to move on.”

According to Wexler, the one finding in motion perception that everyone agrees with, for at least 100 years, is the minimal-motion principle. “The minimal-motion principle states that whenever a stimulus is ambiguous and compatible with more than one motion – as it nearly always is – the brain is supposed to prefer the smallest, slowest motion, including stand-still, that is compatible with the stimulus,” he explains. (In fact, he illustrates, many computer vision systems are built around this principle, and neuroscientists have verified it by recording signals from primate neurons.)

"However," Wexler points out, "one consequence of the high phi effect is the minimal-motion principle can be violated! When the stimulus is incompatible with any globally coherent motion, and therefore equally compatible with any motion, people perceive not only a large jump, but the largest possible jump that they can perceive. This maximum jump is the one that steps by dmax, which acts as the speed limit on motion perception."

Another principle that seems to be violated by the high phi illusion, according to Wexler, is dmax itself. “Below dmax, steps should be more or less seen as what they are – and as can be seen in demonstration three, this is what happens. On the other hand, says Wexler, “above dmax you’re supposed to perceive noise, not motion – but this is not what actually occurs.” Rather, you perceive the high phi jump, as can also be seen in demonstration three. “In one of our experiments,” Wexler adds, “we showed that the amplitude of the jump is very closely correlated with the dmax limit, so that people who have higher dmax limits also see a larger high phi jump.”

It’s known, Wexler points out, that the dmax limit depends on spatial frequency: the lower the frequency (that is, the larger the features in the stimulus) the higher the limit. “And indeed,” he notes, “we found that the magnitude of the illusory jump depends on spatial frequency in exactly the same way: the lower the frequency, the farther the jump.” This can be verified, Wexler adds, by viewing high phi demonstration six and demonstration seven.

Discussing the finding that the direction of the jump depends on the duration of the inducing motion signals, Wexler notes, “We think that the preceding – that is, inducing – motion acts like a seed. For brief inducers, the motion itself acts as the seed, and the jump is experienced forwards with respect to the inducer. For longer inducers, vision begins to adapt to the motion – a result known as the motion aftereffect.” Also known as the waterfall illusion, the motion aftereffect occurs when, after viewing a moving object for an extended period of time, and that object then becomes stationary, the object appears to slowly move in the opposite direction. “Many people initially think that what we’ve found is a consequence of adaptation to motion or the motion aftereffect,” he says. “If so, then it’s the fastest motion aftereffect known. We’ve measured that the illusory motion is 10-100 times faster than the inducing motion! We think that for motion inducers, the adaptation acts as the seed of the fast, backward jump.” (Brief and long inducers can be compared directly in demonstration nine.)

Wexler also describes how their findings relate to the activity of neurons in of the primary visual cortex that respond to lines of a certain angle moving in one direction, as first described by Hubel and Wiesel (1959). “In the brain, motion detectors are sensitive to motion in a particular place – the receptive field – a particular direction, and usually a particular speed,” Wexler notes. “When faced with our stimulus, there can be many accidental matches at the local level. In one image there is a dark spot, for example, and in the next, uncorrelated image there happens to be dark spot just next to it. In that case, a local motion detector will react to this false match – so our stimulus actually activates many local motion detectors, but incoherently, in that all of these motion detectors are signaling different motions. The main point is that in all this incoherent mess the brain finally prefers the largest possible motion.”

Commenting on other areas of research that might benefit from their study, Wexler cites computer vision. “The minimal-motion principle is enshrined in a lot of algorithms for extracting motion,” he concludes. “Our study shows that this principle can be violated. Can we find a different way to extract motion?”

"The dependence on transient duration – which can be clearly seen in demonstration five – is, to be completely honest, a mystery, but a very interesting one,” Wexler continues. “The amplitude of the jump is a very linear function of transient duration, at least for small durations. If some perceptual process goes linearly farther for longer durations, then something in the brain must be effectively rotating at constant speed. I have no idea what that something may be, but it’s an interesting challenge for the future.”

Pathway Competition Affects Early Differentiation of Higher Brain Structures

Sand-dwelling and rock-dwelling cichlids living in East Africa’s Lake Malawi share a nearly identical genome, but have very different personalities. The territorial rock-dwellers live in communities where social interactions are important, while the sand-dwellers are itinerant and less aggressive.

Those behavioral differences likely arise from a complex region of the brain known as the telencephalon, which governs communication, emotion, movement and memory in vertebrates – including humans, where a major portion of the telencephalon is known as the cerebral cortex. A study published this week in the journal Nature Communications shows how the strength and timing of competing molecular signals during brain development has generated natural and presumably adaptive differences in the telencephalon much earlier than scientists had previously believed.

In the study, researchers first identified key differences in gene expression between rock- and sand-dweller brains during development, and then used small molecules to manipulate developmental pathways to mimic natural diversity.

“We have shown that the evolutionary changes in the brains of these fishes occur really early in development,” said Todd Streelman, an associate professor in the School of Biology and the Petit Institute for Bioengineering and Biosciences at the Georgia Institute of Technology. “It’s generally been thought that early development of the brain must be strongly buffered against change. Our data suggest that rock-dweller brains differ from sand-dweller brains – before there is a brain.”

For humans, the research could lead scientists to look for subtle changes in brain structures earlier in the development process. This could provide a better understanding of how disorders such as autism and schizophrenia could arise during very early brain development.

The research was supported by the National Science Foundation and published online April 23 by the journal.

“We want to understand how the telencephalon evolves by looking at genetics and developmental pathways in closely-related species from natural populations,” said Jonathan Sylvester, a postdoctoral researcher in the Georgia Tech School of Biology and lead author of the paper. “Adult cichlids have a tremendous amount of variation within the telencephalon, and we investigated the timing and cause of these differences. Unlike many previous studies in laboratory model organisms that focus on large, qualitative effects from knocking out single genes, we demonstrated that brain diversity evolves through quantitative tuning of multiple pathways.”

In examining the fish from embryos to adulthood, the researchers found that the mbuna, or rock-dwellers, tended to exhibit a larger ventral portion of the telencephalon, called the subpallium – while the sand-dwellers tended to have a larger version of the dorsal structure known as the pallium. These structures seem to have evolved differently over time to meet the behavioral and ecological needs of the fishes. The team showed that early variation in the activity of developmental signals expressed as complementary dorsal-ventral gradients, known technically as “Wingless” and “Hedgehog,” are involved in creating those differences during the neural plate stage, as a single sheet of neural tissue folds to form the neural tube.

To specifically manipulate those two pathways, Sylvester removed clutches of between 20 and 40 eggs from brooding female cichlids, which normally incubate fertilized eggs in their mouths. At about 36 to 48 hours after fertilization, groups of eggs were exposed to small-molecule chemicals that either strengthened or weakened the Hedgehog signal, or strengthened or weakened the Wingless signal. The chemical treatment came while the structures that would become the brain were little more than a sheet of cells. After treatment, water containing the chemicals was replaced with fresh water, and the embryos were allowed to continue their development.

“We were able to artificially manipulate these pathways in a way that we think evolution might have worked to shift the process of rock-dweller telencephalon development to sand-dweller development, and vice-versa. Treatment with small molecules allows us incredible temporal and dose precision in manipulating natural development,” Sylvester explained. “We then followed the development of the embryos until we were able to measure the anatomical structures – the size of the pallium and subpallium – to see that we had transformed one to the other.”

The two different brain regions, the dorsal pallium and ventral subpallium, give rise to excitatory and inhibitory neurons in the forebrain. Altering the relative sizes of these regions might change the balance between these neuronal types, ultimately producing behavioral changes in the adult fish.

“Evolution has fine-tuned some of these developmental mechanisms to produce diversity,” Streelman said. “In this study, we have figured out which ones.”

The researchers studied six different species of East African cichlids, and also worked with collaborators at King’s College in London to apply similar techniques in the zebrafish.

As a next step, the researchers would like to follow the embryos through to adulthood to see if the changes seen in embryonic and juvenile brain structures actually do change behavior of adults. It’s possible, said Streelman, that later developmental events could compensate for the early differences.

The results could be of interest to scientists investigating human neurological disorders that result from an imbalance between excitatory and inhibitory neurons. Those disorders include autism and schizophrenia. “We think it is particularly interesting that there may be some adaptive variation in the natural proportions of excitatory versus inhibitory neurons in the species we study, correlated with their natural behavioral differences,” said Streelman.

Longer Days Bring ‘Winter Blues’—For Rats, Not Humans

Most of us are familiar with the “winter blues,” the depression-like symptoms known as “seasonal affective disorder,” or SAD, that occurs when the shorter days of winter limit our exposure to natural light and make us more lethargic, irritable and anxious. But for rats it’s just the opposite.

Biologists at UC San Diego have found that rats experience more anxiety and depression when the days grow longer. More importantly, they discovered that the rat’s brain cells adopt a new chemical code when subjected to large changes in the day and night cycle, flipping a switch to allow an entirely different neurotransmitter to stimulate the same part of the brain.

Their surprising discovery, detailed in the April 26 issue of Science, demonstrates that the adult mammalian brain is much more malleable than was once thought by neurobiologists. Because rat brains are very similar to human brains, their finding also provides a greater insight into the behavioral changes in our brain linked to light reception. And it opens the door for new ways to treat brain disorders such as Parkinson’s, caused by the death of dopamine-generating cells in the brain.

The neuroscientists discovered that rats exposed for one week to 19 hours of darkness and five hours of light every day had more nerve cells making dopamine, which made them less stressed and anxious when measured using standardized behavioral tests. Meanwhile, rats exposed for a week with the reverse—19 hours of light and five hours of darkness—had more neurons synthesizing the neurotransmitter somatostatin, making them more stressed and anxious.

“We’re diurnal and rats are nocturnal,” said Nicholas Spitzer, a professor of biology at UC San Diego and director of the Kavli Institute for Brain and Mind. “So for a rat, it’s the longer days that produce stress, while for us it’s the longer nights that create stress.”

Because rats explore and search for food at night, while humans evolved as creatures who hunt and forage during the daylight hours, such differences in brain chemistry and behavior make sense. Evolutionary changes presumably favored humans who were more active gatherers of food during the longer days of summer and saved their energy during the shorter days of winter.

“Light is what wakes us up and if we feel depressed we go for a walk outside,” said Davide Dulcis, a research scientist in Spitzer’s laboratory and the first author of the study. “When it’s spring, I feel more motivation to do the things I like to do because the days are longer. But for the rat, it’s just the opposite. Because rats are nocturnal, they’re less stressed at night, which is good because that’s when they can spend more time foraging or eating.”

But how did our brains change when humans evolved millions of years ago from small nocturnal rodents to diurnal creatures to accommodate those behavioral changes?

“We think that somewhere in the brain there’s been a change,” said Spitzer. “Sometime in the evolution from rat to human there’s been an evolutionary adjustment of circuitry to allow switching of neurotransmitters in the opposite direction in response to the same exposure to a balance of light and dark.”

A study published earlier this month in the American Journal of Preventive Medicine found some correlation to the light-dark cycle in rats and stress in humans, at least when it comes to people searching on the internet for information in the winter versus the summer about mental illness. Using Google’s search data from 2006 to 2010, a team of researchers led by John Ayers of San Diego State University found that mental health searches on Google were, in general, 14 percent higher in the winter in the United States and 11 percent higher in the Australian winter.

“Now that we know that day length can switch transmitters and change behavior, there may be a connection,” said Spitzer.

In their rat experiments, the UC San Diego neuroscientists found that the switch in transmitter synthesis in the rat’s brain cells from dopamine to somatostatin or back again was not due to the growth of new neurons, but to the ability of the same neurons there to produce different neurotransmitters.

Rats exposed to 19 hours of darkness every 24 hours during the week showed higher numbers of dopamine neurons within their brains and were more likely, the researchers found, to explore the open end of an elevated maze, a behavioral test showing they were less anxious. These rats were also more willing to swim, another laboratory test that showed they were less stressed.

“Because rats are nocturnal animals, they like to explore during the night and dopamine is a key part of our and their reward system,” said Spitzer. “It’s part of what allows them to be confident and reduce anxiety.”

The researchers said they don’t know precisely how this neurotransmitter switch works. Nor do they know what proportion of light and darkness or stress triggers this switch in brain chemistry. “Is it 50-50? Or 80 percent light versus dark and 20 percent stress? We don’t know,” added Spitzer. “If we just stressed the animal and didn’t change their photoperiod, would that lead to changes in transmitter identity? We don’t know, but those are all doable experiments.”

But as they learn more about this trigger mechanism, they said one promising avenue for human application might be to use this neurotransmitter switch to deliver dopamine effectively to parts of the brain that no longer receive dopamine in Parkinson’s patients.

“We could switch to a parallel pathway to put dopamine where it’s needed with fewer side effects than pharmacological agents,” said Dulcis.

Decision Making: From Neuroscience to Psychiatry

Adaptive behaviors increase the likelihood of survival and reproduction and improve the quality of life. However, it is often difficult to identify optimal behaviors in real life due to the complexity of the decision maker’s environment and social dynamics. As a result, although many different brain areas and circuits are involved in decision making, evolutionary and learning solutions adopted by individual decision makers sometimes produce suboptimal outcomes. Although these problems are exacerbated in numerous neurological and psychiatric disorders, their underlying neurobiological causes remain incompletely understood. In this review, theoretical frameworks in economics and machine learning and their applications in recent behavioral and neurobiological studies are summarized. Examples of such applications in clinical domains are also discussed for substance abuse, Parkinson’s disease, attention-deficit/hyperactivity disorder, schizophrenia, mood disorders, and autism. Findings from these studies have begun to lay the foundations necessary to improve diagnostics and treatment for various neurological and psychiatric disorders.