Author: Donald Doherty

  • Infant and Adult Susceptibility to Others’ Beliefs

    Research has shown that thoughts of events can have similar effects on brain processes as events themselves (for an example see my blog post “Imagine Eating 30 M&M Candies and Eat Less“). Could other peoples’ beliefs have similar effects on our brain processes as do our own beliefs? The new paper “The Social Sense: Susceptibility to Others’ Beliefs in Human Infants and Adults” (published December 24, 2010 in Science) examines this question in 7 month old infants and adults.

    The authors reason that if our abilities to infer what others believe are an innate social sense then these inferences should be spontaneous and automatic and the beliefs of others should be “computed online and effortlessly, just as we compute representations of what we perceive in the environment.” If this is indeed the case then the “representations about others’ beliefs” should affect our behavior.

    The research team used an object detection task to investigate the following two questions:

    • Are belief computations automatically triggered by the mere presence of an agent [with its own beliefs] in adults and in infants as young as 7 months, even when the [agent’s] beliefs are entirely irrelevant to the task participants have to perform?
    • Are beliefs about others’ beliefs stored in a format sufficiently similar to our own representations about the environment that both types of representations can affect our behavior?

    The results of their experiments suggest that both 7 month old infants and adults automatically compute and store the beliefs of others. Also, the beliefs of others appear to be similarly accessible to our own beliefs. Once a belief is computed, it seems to remain active even in the absence of the person who was thought to hold the belief. It’d be interesting to image brain function using functional Magnetic Resonance Imaging (fMRI) during the same tasks carried out by human subjects in this investigation.

    Other related blog posts:

    Imagine Eating 30 M&M Candies and Eat Less

  • How Reindeer Tell Time in the Arctic Where the Sun Neither Rises nor Sets

    Internal clocks are fundamental features of all living things. The circadian rhythm, the 24 hour cycle coincident with a single light and dark (day and night) cycle, is particularly considered fundamental to life on Earth. Internal daily cycles are expressed in the body as daily melatonin cycles. A recent paper asks how it is that reindeer set their internal clock in the arctic where the sun neither rises nor sets.

    The paper titled “A Circadian Clock Is Not Required in an Arctic Mammal?” was published March 23, 2010 in Current Biology.

    The authors found that regulation of melatonin production in reindeer is fundamentally different from in typical animals. Arctic reindeer lack the molecular clocks that normally drive cellular circadian rhythms throughout the body. Instead, melatonin cycles are set directly from exposure to light and dark. Daily light and dark cycles do not occur in arctic reindeer except for during a few weeks around the spring and autumn equinoxes!

  • Sensation and Location in the Hippocampal Formation

    Neurons in the hippocampal formation fire when an animal visits a particular place defined by a small region (firing field) and are known as place cells. An individual place cell may respond to more than one location (firing field) each known as a place field. Previous research has shown that converging input from grid cells in the medial entorhinal cortex, in conjunction with feedback inhibition from inhibitory interneurons in the dentate gyrus, is sufficient to account for the place fields seen in dentate gyrus granule cells.

    Grid cells fire when a freely moving animal moves through a set of small regions (firing fields) which are roughly equal in size and arranged in a periodic triangular array that covers the entire available environment. The axons of many grid cells converge on the dendrites of the granule cells of the dentate gyrus.

    Recent research has shown that the firing rate of a place field may be modified (increased or decreased) based on sensory input. Not only that, but the firing rate in each of multiple place fields of a single neuron may be independently modified due to sensory input. That is, the firing rate of one place field may increase while the firing rate in another place field in the same neuron may decrease due to identical sensory input. The phenomenon is known as rate remapping.

    Grid cells in the medial entorhinal cortex are insensitive to sensory input. Sensory information about the environment is brought to the hippocampal formation by input from the lateral entorhinal cortex. The paper “The Mechanism of Rate Remapping in the Dentate Gyrus” (published December 22, 2010 in Neuron) reports on computer modeling research that asks the following two questions:

    • What is the mechanism of rate remapping?
    • Why do different place fields of the same dentate gyrus cell display independent rate remapping?

    They showed that the lateral entorhinal cortex inputs do not interfere with place cell formation in the dentate gyrus by the medial entorhinal cortex inputs but they do modulate place field responses based on sensory input. The research team identified two processes that cause independent place field rate remapping:

    • Changes in sensory input results in changes in lateral entorhinal cortex cell responses. Since the rate change of lateral entorhinal cortex cells is a function of the location of the animal, the change in lateral entorhinal cortex input is independent for each place field.
    • The change in the excitation of other place cells will determine which place cell is most activate at a given position through a winner-take-all inhibitory mechanism. This process is localized and is therefore independent for each place field.

    In summary, the simulations show that the spatial firing pattern of dentate gyrus cells is determined primarily by the medial entorhinal cortex inputs. The role of the lateral entorhinal cortex is to determine the specific rate at which place cells fire.

    Other related blog posts:

    Do You Know Where You Are? Place Memory

    Nature versus Nurture and Place Memory Development

    Spikelets and Place Cells

    Watching Brain Cells Track Location