Changes in pupil size are often thought of as a kind of #reflex, being driven automatically by changes in #luminance. This works shows clearly how even such automatic behaviors are shaped by experience: looking at a picture of the sun – that is DARKER than the preceding image – causes pupil constriction, while equally bright images (of the moon, of pure grey, or of a scrambled sun image) cause the “expected” pupil dilation (due to luminance decrease). #science #neuroscience http://www.journalofvision.org/content/13/6/8.short
from reference 1
It is well known that nervous system plasticity (the mutable quality of brains) underlies animals’ ability to change their behaviors. However, it has been widely thought that such plasticity consists mostly of changes at synapses, the sites of communication between neurons, or in the intrinsic excitability of individual neurons – how likely a cell is to increase or decrease its voltage or to fire action potentials. New research challenges this view by showing that another type of alteration can be at work: single neurons can change the type of neurotransmitter they release in response to stimulation1.
Specifically, this work addresses tadpole camouflage. Tadpoles can rapidly change (increase or decrease) the amount of pigmentation in their skin in order to blend in with their surroundings. The research I here refer to demonstrates that when frog tadpoles (Xenopus laevis) are stimulated with bright light, the number of dopamine-secreting (dopaminergic) neurons in the animals’ brains increases, allowing them to adapt more rapidly to subsequent exposure to light.
When exposed to just hours of direct light, dark-reared tadpoles responded by doubling the number off dopaminergic neurons within a part of their central nervous system called the suprachiasmatic nucleus (SCN). Furthermore, these new dopaminergic neurons seem to integrate into the existing pathways for changing pigmentation. The authors were able to selectively destroy existing SCN dopaminergic neurons, eliminating the tadpoles ability to change pigment, and then rescue this ability with light exposure.
It remains unclear if this phenomenon is purely a developmental one, operating only prior to adulthood, an important caveat. None the less, the ability to change cell type is an as-yet unexplored form of neural plasticity which will have widespread implications for neuroscience research. Furthermore, this work may have even more direct application to human experience.
In human beings, the malfunction of certain dopamine-mediated signalling cascades (cell-level combinations of molecular interactions) have been implicated in seasonal affective disorder (winter depression). It is also well known that dopamine is intrinsically involved in the rewarding feelings delivered by food, sex and drugs. It may thus simply be the case that a lack of light stimulation leads to a lack of dopaminergic neurons, and thus to fewer episodes of rewarding experience during the winter months.
1. Dulcis D, Spitzer NC. Illumination controls differentiation of dopamine neurons regulating behaviour. Nature 456: 195-201, 2008.
Figure 6 from ref. 1
I’ve written several times about Spike-Timing-Dependent-Plasticity (STDP), one method by which the individual neurons in the mammalian brain learn; changing their responses to the signals sent from other neurons.
It is believed that STDP is a major route of such learning, both during development and in the adult animal; for instance potentially underlying the associative conditioning famously demonstrated by Pavlov. Indeed, it is just this sort of patterned external sensory stimulus (bell then food) that represents a candidate for learning through STDP. However, connecting the presence of structured environmental variables and underlying brain changes has proven a difficult experimental challenge.
A recent piece of research has achieved just such a feat in the optic tectum of a non-mammal, the developing frog Xenopus laevis1.
I found the figure above to be the most intriguing result from the paper sumarrizing these experiments, published in Nature Neuroscience. The image represents the finding that if the tadpoles are exposed to repetitive flashes of light with a specific time difference between them (top), the neurons in their optic tecta respond by adjusting the latency (time from stimulus to response) of their spike-reactions to single, isolated flashes (middle: neural activity, bottom: histograms of spike latencies).
I am encouraged by this work because it bridges what is currently a rather formidable gap. That between understanding the brain at the level of single neurons and the behavior of an animal as a whole.
1. Pratt, KG, Dong, W & Aizenman, CD (2008) Development and spike timing–dependent plasticity of recurrent excitation in the Xenopus optic tectum Nature Neuroscience 11, 467-475 | doi:10.1038/nn2076