David K. Warland

Mechanisms of Concerted Firing among Retinal Ganglion Cells

Nearby retinal ganglion cells often fire action potentials in near synchrony. We have investigated the circuit mechanisms that underlie these correlations by recording simultaneously from many ganglion cells in the salamander retina. During spontaneous activity in darkness, three types of correlations were distinguished: broad (firing synchrony within 40-100 ms), medium (10-50 ms), and narrow (<1 ms). When chemical synaptic transmission was blocked, the broad correlations disappeared, but the medium and narrow correlations persisted. Further analysis of the strength and time course of synchronous firing suggests that nearby ganglion cells share inputs from photoreceptors conveyed through interneurons via chemical synapses (broad correlations), share excitation from amacrine cells via electrical junctions (medium), and excite each other via electrical junctions (narrow). It appears that the firing patterns in the optic nerve are strongly shaped by electrical coupling in the inner retina.

Retinal waves in mice lacking the β2 subunit of the nicotinic acetylcholine receptor

The structural and functional properties of the visual system are disrupted in mutant animals lacking the β2 subunit of the nicotinic acetylcholine receptor. In particular, eye-specific retinogeniculate projections do not develop normally in these mutants. It is widely thought that the developing retinas of β2−/− mutants do not manifest correlated activity, leading to the notion that retinal waves play an instructional role in the formation of eye-specific retinogeniculate projections. By multielectrode array recordings, we show here that the β2−/− mutants have robust retinal waves during the formation of eye-specific projections. Unlike in WT animals, however, the mutant retinal waves are propagated by gap junctions rather than cholinergic circuitry. These results indicate that lack of retinal waves cannot account for the abnormalities that have been documented in the retinogeniculate pathway of the β2−/− mutants and suggest that other factors must contribute to the deficits in the visual system that have been noted in these animals.

Coding Efficiency and Information Rates in Sensory Neurons

Many sensory neurons code information about the sensory world in discrete spike trains. How much information is carried in sensory spike trains and how close are the information rates to fundamental limits imposed by the spiking statistics? By learning to estimate time-dependent sensory input signals from the spike trains of single neurons, we are able to measure the rate at which the spike trains convey information about the sensory inputs. These information rates are within a factor of two of the absolute information-theoretic bounds imposed by the entropies of the spike trains themselves.

Bits and brains: Information flow in the nervous system

Until recently there have been no convincing quantitative measurements on the rates of information transmission in real neurons. Here we review the theoretical basis for making such measurements, together with the data which demonstrate remarkably high information rates in a variety of systems. In fact these rates are within a factor of two of the absolute physical limits set by the entropy of neural spike trains. Theses observations lead to sharp theoretical questions about the structure of the code and the strategy for adapting the code to different ensembles of input signals.

Reading Between the Spikes in the Cereal Filiform Hair Receptors of the Cricket

Crickets devote a significant percentage of their body to the creation, maintenance, and use of a low frequency sound/air-current detection system known as the cereal sensory system. Although many functions of this system remain elusive, the cereal system can distinguish different air-current stimuli based on the deflection of its filiform hairs. In response to a particularly interesting stimulus (from the cricket’s point of view) such as the wing beat of a wasp [1], or the wind from a striking frog’s tongue [2], the hundreds of filiform afferents send a barrage of spikes representing wind direction and velocity to the next stage of processing in the terminal abdominal ganglion. The goal of this paper is to characterize the information which is carried by just one primaxy sensory afferent.

Measuring the coding efficiency of sensory neurons

All of an organism’s knowledge about the sensory world comes from observation of the spike trains in its own sensory cells. How much information is carried in these spike trains Nand how efficient is the coding? We have recently developed an approach to neural coding which allows us to measure the information a spike train carries about a sensory stimulus without assumptions about how the information is coded[1]Comparing this measured information with an upper bound to the information rate determined by the spiking statistics results in a direct measure of the efficiency of coding. For two different mechanoreceptors from the cricket cercal system and from the bullfrog sacculus we find information rates close to 3 bits per spike (corresponding to 300 bits per sec in the cricket 160 bits per sec in the frog) and coding efficiencies greater than 50%.