Tag: Brain Science

The recent paper “Decorrelated neuronal Firing in Cortical Microcircuits” published January 29, 2010 in Science takes a new look at correlated response fluctuations among simultaneously recorded neurons in the cerebral cortex.

This convincing study used very high quality simultaneous recordings from nearby neurons in the primary visual cortex to overcome technical challenges that may have contributed to a large body of literature that concludes that nearby neurons share significant common input based on correlated trial-to-trial variability in activity.

They saw correlations near zero using a number of different stimulus protocols and experimental conditions. They then went on to artificially introduce into their experiments some of the possible confounding factors and reproduced the significant correlations observed in earlier papers. The authors conclude that either 1) “adjacent neurons share only a few percent of their inputs” or 2) “their activity is actively decorrelated.”

At the back inside of each of your eyeballs is a sheet of nerve tissue known as the retina. Signals move from the retina to the rest of the brain through only one kind of nerve cell (neuron) known as the retinal ganglion cell.

Retinal ganglion cells communicate with the rest of the brain through a structure known as the thalamus and, more specifically, the part of the thalamus known as the lateral geniculate nucleus. So brain signals, known as spikes or action potentials, related to vision originate in the eyeball’s retina and are sent by retinal ganglion cells to the lateral geniculate nucleus of the thalamus from where they may travel to other brain structures.

A new paper asks if spikes in the retina that carry the most visual information are being preferentially funneled through the thalamus. The research team also looked at the role that spike timing may play in the information transfer process from eyeball to brain.

(The paper “Spike Timing and Information Transmission at Retinogeniculate Synapses” was published October 13, 2010 in the Journal of Neuroscience.)

The authors define a spike in a retinal ganglion cell followed after a short period (around 2.5 milliseconds) by a spike in an associated thalamic neuron as a relayed spike. Only a subset of spikes in the retina are relayed to the thalamus.

The conclusion reached in the paper was that spikes carrying the most information were selectively relayed from the eyeball to the brain. They base this conclusion on the following results:

• relayed spikes had a stronger correlation with the visual stimulus than non-relayed spikes.
• short intervals between spikes were more effective at driving thalamic neuron responses than spikes that followed longer intervals.
• relayed spikes occurred with significantly greater spike timing precision (less variance) than non-relayed spikes.
• relayed spikes had significantly less variance in spike number than non-relayed spikes.
• relayed spikes carried significantly greater information than non-relayed spikes.

While the techniques and analysis appear sound I find myself ill at ease with this paper and its conclusions. It seems to me that at least the first of these findings is circular. A receptive field is a foundational idea in the brain sciences. Sensory system brain cells exhibit receptive fields that by definition show greatest response and shorter latencies to stimuli in particular parts of sensory space. The second finding follows directly from fundamental aspects of our understanding of how neurons process input. And the last three findings are straight forward consequences of filtering out the spikes that are less relevant to the receptive field of the thalamic neuron.

Did we really learn anything new from this paper? Is the relay between the eyeball and the brain really acting like a filter or is there something more subtle going on where both “non-relayed” and “relayed” spikes are contributing to the code found in the thalamus? Do the authors present evidence to support the conjecture that retinal ganglion cells “appear to use discrete firing events as the symbols with which to encode the visual world?”

Am I missing something?

Spikelets are a small all-or-none signal in brain cells (neurons) thought to be distinct from spikes (technically called action potentials). A paper from earlier this year suggests that spikelets are important participants in signal processing in the brain.

The paper is “Impact of Spikelets on Hippocampal CA1 Pyramidal Cell Activity During Spatial Exploration” published January 22, 2010 in Science.

The research team recorded from inside neurons of awake and behaving rats to assess the role of spikelet activity while the animals moved through and explored their environment. Some evidence suggested that spikelets sometimes induced a spike to occur. They also saw spikelet activity show preference for place like with action potentials.

Intriguing but where are these spikelets coming from? Are these correlations without causation? Various suggestions for how spikelets are generated include:

• they result from direct action potential transmission through electrical coupling between cells
• they are spikes being recorded long distance from the electrode in the cell body. Perhaps from the cell’s dendrites or axon.

Spikelets may be important signals or they may be epiphenomena. What do you think?

Other related blog posts:

Do You Know Where You Are? Place Memory

Nature versus Nurture and Place Memory Development