April

Date: Thursday April 6th
Time: 4pm
Location: HNB small conference room.
Speaker: Eun Jin Lee
Paper: Lin B, Wang SW, Masland RH.Retinal ganglion cell type, size, and spacing can be specified independent of homotypic dendritic contacts Neuron 2004, 43(4), 475-485

In Brn3b(-/-) mice, where 80% of retinal ganglion cells degenerate early in development, the remaining 20% include most or all ganglion cell types. Cells of the same type cover the retinal surface evenly but tile it incompletely, indicating that a regular mosaic and normal dendritic field size can be maintained in the absence of contact among homotypic cells. In Math5(-/-) mice, where only approximately 5% of ganglion cells are formed, the dendritic arbors of at least two types among the residual ganglion cells are indistinguishable from normal in shape and size, even though throughout development they are separated by millimeters from the nearest neighboring ganglion cell of the same type. It appears that the primary phenotype of retinal ganglion cells can develop without homotypic contact; dendritic repulsion may be an end-stage mechanism that fine-tunes the dendritic arbors for more efficient coverage of the retinal surface.

Date: Thursday April 13th
Time: 4pm
Location: HNB small conference room.
Speaker: Ashish Ahuja
Paper: Sekirnjak C, Hottowy P, Sher A, Dabrowski W, Litke AM, Chichilnisky EJ Electrical stimulation of mammalian retinal ganglion cells with multi-electrode arrays J Neurophysiol. 2006 Feb 1

Existing epiretinal implants for the blind are designed to electrically stimulate large groups of surviving retinal neurons using a small number of electrodes with diameters of several hundred microns. To increase the spatial resolution of artificial sight, electrodes much smaller than those currently in use are desirable. In this study we stimulated and recorded ganglion cells in isolated pieces of rat, guinea pig, and monkey retina. We utilized micro-fabricated hexagonal arrays of 61 platinum disk electrodes with diameters between 6 and 25 microns, spaced 60 microns apart. Charge-balanced current pulses evoked one or two spikes at latencies as short as 0.2 ms, and typically only one or a few recorded ganglion cells were stimulated. Application of several synaptic blockers did not abolish the evoked responses, implying direct activation of ganglion cells. Threshold charge densities were typically below 0.1 mC/cm;2 for a pulse duration of 0.1 ms, corresponding to charge thresholds of less than 100 pC. Stimulation remained effective after several hours and at high frequencies. To demonstrate that closely spaced electrodes can elicit independent ganglion cell responses, we utilized the multi-electrode array to stimulate several nearby ganglion cells simultaneously. From these data we conclude that electrical stimulation of mammalian retina with small-diameter electrode arrays is achievable and can provide high temporal and spatial precision at low charge densities. We review previous epiretinal stimulation studies and discuss our results in the context of 32 other publications, comparing threshold parameters and safety limits.

Date: Thursday April 20th
Time: 4pm
Location: HNB small conference room.
Speaker: Joaquin Rapela
Paper: Sharpee TO, Sugihara H, Kurgansky AV, Rebrik SP, Stryker MP, Miller KD Adaptive filtering enhances information transmission in visual cortex Nature Vol 439. 2006. 936-942

Sensory neuroscience seeks to understand how the brain encodes natural environments. However, neural coding has largely been studied using simplified stimuli. In order to assess whether the brain’s coding strategy depends on the stimulus ensemble, we apply a new information-theoretic method that allows unbiased calculation of neural filters (receptive fields) from responses to natural scenes or other complex signals with strong multipoint correlations. In the cat primary visual cortex we compare responsese to natural inputs with those to noise inputs matched fro luninance and contrast. We find that neural filters adaptively change with the input ensem le so as to increase the inforation carried by the neural response about the filtered stimulus. Adaptation affects the spatial frequency composition of the filter, enhancing sensitivity to under-represented frequencies in agreement with optimal encoding arguements. Adaptation occurs over 40 s to many minutes, longer than most previously reported forms of adapatation.