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Michael A. Freed, Ph.D.
Research Associate Professor of Neuroscience
Dept of Neuroscience
123 Anatomy Chemistry Building
Phone 215-573-3211, Lab: 215-898-7536, Fax: 215-573-9871
Email: michael@retina.anatomy.upenn.edu
For more information: http://retina.anatomy.upenn.edu/%7Emichael/
Click here for selected publications since Dr. Freed's arrival at Penn
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
