Information that Moves from the Eyeball to the Brain

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?