RESEARCH SUMMARY

RESEARCH INTERESTS

•  Detailed structure of the retina's neural circuitry
•  Encoding of visual information by the retina

RESEARCH TECHNIQUES

•  Whole cell and extracellular recording from an intact in vitro retinal preparation
•  Noise analysis of vesicular release at chemical synapses
•  Information theoretical analysis of spike trains and postsynaptic currents.

RESEARCH SUMMARY

A single retinal ganglion cell transmits information to the brain at rates of 10 to 100 bits per second. Because the human retina contains about 10⁶ ganglion cells, the total information transmitted is more than 10⁷ bits per second. This is equivalent to the capacity of a dedicated internet connection to the University of Pennsylvania !

These considerations raise a key question: how is all of this information combined by the ganglion cell and transmitted to the brain? Ultimately, the answer will require understanding in detail: (1) how much information is carried by a synaptic vesicle when its contents are released upon the ganglion cell; (2) how much information a vesicle conveys after its postsynaptic currents are combined in the ganglion cell with those from other vesicles; (3) how much of the information from the vesicle reaches the spike train.

We have recently estimated that each synaptic vesicle's postsynaptic current encodes less than a bit (< 0.4 bits) of information, significantly less than encoded by a spike (about 3 bits). Thus a vesicle encodes less information than a spike, or even a single gate in a digital computer, which we attribute to the stochastic (noisy) nature of vesicular release.

Our research attempts to connect basic elements of the retinal circuit (synapses, neurons, spikes, vesicles) to information processing. Our research is relevant to retinal diseases that degenerate photoreceptors but spare ganglion cells. A prosthetic device that stimulates the remaining neurons might restore sight to its original quality, but would need to match the original information rate. Thus providing the right amount and kind of information to each neuron will be critical to prosthetic design.

Supported by NEI grant to M.F. 2R01EY013333  and 2P30EY001583

KEY WORDS: Neural noise, retina, coding, synapse.


KEY REFERENCES

Freed, M. A. Quantal encoding of information in a retinal ganglion cell. Journal of Neurophysiology 94: 1048-1056, 2005.

Dhingra, N. K., Freed, M. A., and Smith, R. G. Voltage-gated sodium channels improve contrast sensitivity of a retinal ganglion cell. Journal of Neuroscience 25: 8097-8103, 2005.

Koch, K., McLean , J., Berry , M., Sterling , P., Balasubramanian, V., and Freed, M. A. Efficiency of information transmission by retinal ganglion cells. Current Biology 14: 1523-1530. 2004

Freed MA, Smith RG, Sterling P: Timing of quantal release from the retinal bipolar terminal is regulated by a feedback circuit. Neuron 38:89-101, 2003.

Freed, M.A. Rate of quantal excitation to a retinal ganglion cell evoked by sensory input. Journal of Neurophysiology 83:2956-2966, 2000.

Freed, M.A. Parallel cone bipolar pathways to a ganglion cell use different rates and amplitudes of quantal excitation. Journal of Neuroscience 20: 3956-3963, 2000.