We have explored the manner in which the population of retinal ganglion cells collectively represent the visual world. Ganglion cells in the salamander were recorded simultaneously with a multielectrode array during stimulation with both artificial and natural visual stimuli, and the mutual information that single cells and pairs of cells conveyed about the stimulus was estimated. We found significant redundancy between cells spaced as far as 500 mum apart. When we used standard methods for defining functional types, only ON-type and OFF-type cells emerged as truly independent information channels. Although the average redundancy between nearby cell pairs was moderate, each ganglion cell shared information with many neighbors, so that visual information was represented approximately 10-fold within the ganglion cell population. This high degree of retinal redundancy suggests that design principles beyond coding efficiency may be important at the population level.

To understand a neural circuit completely requires simultaneous recording from most of the neurons in that circuit. Here we report recording and spike sorting techniques that enable us to record from all or nearly all of the ganglion cells in a patch of the retina. With a dense multi-electrode array, each ganglion cell produces a unique pattern of activity on many electrodes when it fires an action potential. Signals from all of the electrodes are combined with an iterative spike sorting algorithm to resolve ambiguities arising from overlapping spike waveforms. We verify that we are recording from a large fraction of ganglion cells over the array by labeling the ganglion cells with a retrogradely transported dye and by comparing the number of labeled and recorded cells. Using these methods, we show that about 60 receptive fields of ganglion cells cover each point in visual space in the salamander, consistent with anatomical findings.

II.Selectivity for multiple stimulus features in retinal ganglion cells. J Neurophysiol 96: 2724 -2738, 2006. First published August 16, 2006 doi:10.1152/jn.00995.2005. Under normal viewing conditions, retinal ganglion cells transmit to the brain an encoded version of the visual world. The retina parcels the visual scene into an array of spatiotemporal features, and each ganglion cell conveys information about a small set of these features. We study the temporal features represented by salamander retinal ganglion cells by stimulating with dynamic spatially uniform flicker and recording responses using a multielectrode array. While standard reverse correlation methods determine a single stimulus feature-the spike-triggered average-multiple features can be relevant to spike generation. We apply covariance analysis to determine the set of features to which each ganglion cell is sensitive. Using this approach, we found that salamander ganglion cells represent a rich vocabulary of different features of a temporally modulated visual stimulus. Individual ganglion cells were sensitive to at least two and sometimes as many as six features in the stimulus. While a fraction of the cells can be described by a filter-and-fire cascade model, many cells have feature selectivity that has not previously been reported. These reverse models were able to account for 80 -100% of the information encoded by ganglion cells.

Segev, Ronen, Jason Puchalla, and Michael J. Berry II. Functional organization of ganglion cells in the salamander retina. J Neurophysiol 95: 2277-2292, 2006. First published November 23, 2005 doi:10.1152/jn.00928.2005. Recently, we reported a novel technique for recording all of the ganglion cells in a retinal patch and showed that their receptive fields cover visual space roughly 60 times over in the tiger salamander. Here, we carry this analysis further and divide the population of ganglion cells into functional classes using quantitative clustering algorithms that combine several response characteristics. Using only the receptive field to classify ganglion cells revealed six cell types, in agreement with anatomical studies. Adding other response measures served to blur the distinctions between these cell types rather than resolve further classes. Only the biphasic OFF type had receptive fields that tiled the retina. Even when we attempted to split these classes more finely, ganglion cells with almost identical functional properties were found to have strongly overlapping spatial receptive fields. A territorial spatial organization, where ganglion cell receptive fields tend to avoid those of other cells of the same type, was only found for the biphasic OFF cell. We further studied the functional segregation of the ganglion cell population by computing the amount of visual information shared between pairs of cells under natural movie stimulation. This analysis revealed an extensive mixing of visual information among cells of different functional type. Together, our results indicate that the salamander retina uses a population code in which every point in visual space is represented by multiple neurons with subtly different visual sensitivities. I N T R O D U C T I O NIn now classic work, Wässle and colleagues showed that the alpha ganglion cell in the cat, which is distinguished by its large soma size, formed two plexuses with their dendritic arbors-one with ON-type light responses and the other with OFF-type-that precisely covered visual space (Wässle and Boycott 1991). The somas of neighboring cells tended to be spaced one dendritic diameter apart from one another, the tips of their dendrites just barely touching. A similar territorial organization and dendritic coverage factors close to one have been found in other, prominent morphological types of ganglion cells both in the monkey and the rabbit (Dacey 1993;Vaney 1994). Tight correspondences between anatomy and function have also been found (Wässle and Boycott 1991), along with an increasingly intricate functional segregation of axons and dendrites in the inner plexiform layer (MacNeil et al. 1999;Pang et al. 2002;Roska and Werblin 2001;Sterling 1983;Wassle 2004;Zhang et al. 2004). For all these reasons, tiling has come to be regarded as a fundamental principle of retinal organization.However, this simple picture of efficient tiling does not hold for all of the ganglion cells. Several other anatomical types have coverage factors greater than two (Rodieck ...

Deterministic lateral displacement arrays have shown great promise for size-based particle analysis and purification in medicine and biology. Here, we demonstrate that the use of an array of triangular rather than circular posts significantly enhances the performance of these devices by reducing clogging, lowering hydrostatic pressure requirements, and increasing the range of displacement characteristics. Experimental data and theoretical models are presented to create a compelling argument that future designs of deterministic lateral displacement arrays should employ triangular posts. The effect of practical considerations, such as vertex rounding, post size, and shape, is also discussed.