An optical imaging study finds that our ability to see complex motions dates back to our early primate ancestors
By David F. Salisbury
Published: April 13, 2004
When Titans wide receiver Derrick Mason goes up to snag a pass, a part of his brain called the middle temporal visual center (MT) goes into overdrive.
MT, which is located a couple of inches behind and above the ear, specializes in the perception of complex motions and is a distinctive feature in the brains of primates. Neuroscientists have found this special feature in the brains of all the primates that they have searched for it, including humans. However, it is a feature that our closest evolutionary cousins, tree shrews and rodents, lack. (Curiously, cats appear to have developed a similar visual center independently.)
Now a study, published in the Proceedings of the National Academy of Sciences, provides new support for the hypothesis that MT evolved early in the course of primate evolution when our small, long-nosed, bewhiskered and hyperactive ancestors were breaking out of the understory role that they had occupied during the age of the dinosaurs to claim niches that were abandoned as the "thunder lizards” died out.
Using a technique called "in vivo optical imaging,” a team of researchers from Vanderbilt University performed the first detailed analysis of the MT center of a prosimian, a family that diverged from the line of human evolution more than 60 million years ago. The subject of the study was a small nocturnal primate with large eyes called the galago, or bush baby. They found that the bush baby's MT region is organized in a way that is strikingly similar to that of monkeys. The finding provides new support for the argument that this specialized visual center evolved in a common ancestor of prosimians and humans. It also makes it more likely that the human MT is organized in similar fashion.
"The idea is that when a given characteristic is shared between distantly related primates, it is highly likely that it came from a distant ancestor,” says team leader Vivien Casagrande, professor of cell and developmental biology, psychology, ophthalmology & visual science and Kennedy Center investigator. "In making inferences about the human brain, if you see a feature in Old World monkeys and New World monkeys and several other primates, then it probably exists in humans as well.”
The paper's first author is Xiangmin Xu, the postdoctoral fellow who played a key role in organizing the optical imaging laboratory and conducting the experiments. Other participants in the study are graduate students Peter Kaskan and Ilya Khaytin, research associate professor Christine E. Collins and Distinguished Professor of Psychology and Kennedy Center investigator Jon Kaas.
The middle temporal visual area was first identified in 1971 in the bush baby by Kaas and J.H. Allman of the California Institute of Technology. Shortly thereafter it was also identified in owl monkeys, macaques and several other primate species. Using the single-neuron approach, which involves monitoring the activity of individual neurons with electrodes finer than hairs, researchers established that this oval region, located in the temporal lobe, contains a map of visual space and that its neurons are especially responsive to movement in an animal's field of view.
In the last ten years, anatomical evidence and noninvasive brain scanning techniques have also located the MT region in humans. People who have suffered strokes in the area appear to have problems judging movement, but the evidence for this is ambiguous because strokes generally affect a larger region of the brain. Lesions in the MT region of the brains of monkeys exhibit a reduced ability to track complex motions, but unless the injuries are very large, these deficits are temporary and disappear within weeks.
The optical imaging technique that Casagrande's team used to study the organization of MT looks at the activity of different regions on the surface of the brain. It relies on the fact that active neurons consume oxygen. This slightly increases the proportion of hemoglobin that has been stripped of oxygen relative to the amount of hemoglobin carrying oxygen. Because hemoglobin without oxygen reflects less red light than hemoglobin with oxygen, active areas look slightly darker. This is similar to the way in which functional MRI works. A critical difference is that fMRI is noninvasive and so it can be used on people, while optical imaging requires that a portion of the skull be removed to reveal the brain, limiting its use to animal subjects and patients undergoing special types of brain surgery.
The advantage of in vivo optical imaging compared to microelectrode recording is "seeing the forest instead of the trees,” according to Casagrande. "You can see the global pattern of neurons turning off and on in response the same stimulus, rather than studying them one by one.” In addition, the technique has about five times the resolution as fMRI.
When they applied this technique to the MT center of the bush baby, they found that it was organized in a fashion strikingly similar to that of several monkey species, including the owl monkey. As a bush baby was presented with a pattern that filled its entire visual field, neurons throughout the region responded, indicating that it maps the retinas of both eyes.
When the bush baby was presented with moving gratings (groups of parallel lines) in different orientations, the researchers documented the fact that the region is filled with small domains consisting of neurons that respond preferentially to the orientation of the gratings. In addition, they found that different domains responded to the direction of the gratings' motion. Furthermore, they observed that the orientation domains are organized in either pinwheels or linear zones and are divided into territories with opposite movement preference.
"Overall, these results are remarkably similar to optical imaging results reported for MT of owl monkeys, a New World monkey, results that are highly consistent with data from microelectrode recording experiments in New World cebus monkeys and Old World macaque monkeys,” they conclude in the paper.