People usually find it easier to see things when they are big and bright, but there are occasionally exceptions. One example concerns moving objects: when they are small, we can identify their direction of motion easily. But this becomes much more difficult for larger objects. This phenomenon is known as spatial suppression http://www.nature.com/nature/journal/v424/n6946/full/nature01800.html.
This decrease in our perception of motion direction with larger objects was documented 15 years ago. Since then, researchers found this effect is much weaker in cohorts of people, the elderly http://www.sciencedirect.com/science/article/pii/S0896627305000139, schizophrenics http://www.jneurosci.org/content/26/44/11403.long, and even those with low IQ http://www.sciencedirect.com/science/article/pii/S0960982213004946. This means people in these groups can perceive the motion direction of larger objects better than the general population.
Despite an outward understanding of motion and perception, there was until recently very little information on the actual neurological mechanism of spatial suppression. Many believed this was occurring at the middle temporal region of the visual cortex, in particular visual area 5 (MT), in the form of center surround antagonism http://www.annualreviews.org/doi/full/10.1146/annurev.neuro.26.041002.131052. However, how this may occur at the level of neuronal firing has not been fully determined.
Now we may be one step closer thanks to Liu D. Liu and Christopher Pack at the Montreal Neurological Institute at McGill University. In collaboration with Ralf Haefner at the University of Rochester, they have documented a neurological basis for visual motion perception and how it can be misinterpreted. Their results are published in eLife http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4882155/.
The team performed their studies in two rhesus monkeys such that their brain functions could be monitored in real time using electrophysiological recordings of the MT region. Before the experiments could begin, the animals were trained to report their perception by making an eye movement. When a stimulus moved, they would respond by looking in the direction they perceived.
The experiment began with a short stimulus that moved in one of the two directions. The monkeys would report their perception as trained and data on neurons firing was collected. The test was performed using a variety of sizes in order to determine how this affected motion perception.
After the tests were complete, the data was analyzed to identify a particular characteristic of neurons: they’re noisy. They never respond to the same stimulus in the same way, which could limit the information coded. If too much noise was detected, the perception of motion would be less than optimal.
When the results came back, the noise factor definitely played a role as larger sized objects led to poor motion perception. This was no surprise. But when the team examined how the brain contributed to this finding, the team found evidence for an unforeseen function.
The number of firing neurons in the MT region increased with size of the object as expected yet, the neurons associated with optical tracking of small objects had lower noise. In contrast, neurons responsible for determining the motion of the surrounding environment were corrupted by noise. As a consequence, the monkeys’ ability to distinguish the motion of small objects was enhanced and their ability to distinguish the motion of large objects was hampered.
For the authors, the results hinted at a possible mechanism for the phenomenon. With increased size, a type of neural desensitization occurred in the MT region that results in this trade-off between the sensitivity for small and large stimuli leading to spatial suppression and poor motion perception. In essence, the animals were desensitized to the large objects and as a result, were unable to follow it in relation to the surrounding area.
While the research offers perspective on the possible neurological mechanism behind the inability to properly perceive motion, the authors also note this type of spatial suppression is weaker in certain populations such as the elderly and also those with schizophrenia. Though there was no correlation made in this study, the authors suggest this may be an area worth investigating in the future.
Text by Jason Tetro, for CAN-ACN