Adaptation and the spiking code of retinal ganglion cells. (360G-Wellcome-063414_C_00_Z)

I propose to study the effects of neural adaptation on the spike train code that is used to convey visual information from eye to brain. Adaptation is virtually universal throughout the nervous system and can play a fundamental role in determining the basic functional properties of neural circuits. Many previous studies have explored the consequences of adaptation on time-averaged responses of spiking neurons to stereotyped "classical" stimuli. However, a growing body of evidence suggests that neurons in sensory circuits encode information in the fine temporal structure of spike trains, particularly in response to naturalistic stimuli. Little is known about the consequences of adaptation on the structure and content of these spiking codes. I propose to address this issue by studying the spike train code of retinal ganglion cells. Ganglion cells form the sole connection between eye and brain, and nowhere else in the visual system is the entire scene represented with fewer neurons. Thus it seems likely that this population of cells will reveal clear examples of efficient coding and computation. Adaptation may play a crucial role in achieving this efficiency. The primary aim of the project is to establish the consequences of light, contrast and motion adaptation on the ganglion cell spike trains. I will use a multi-electrode array to record spike trains from up to 100 ganglion cells simultaneously in isolated amphibian and mammalian retinas. Computer generated visual stimuli and digitised natural video sequences will be projected onto the photoreceptor array using a custom display projector. Previous work by the sponsor indicates that the responses of ganglion cells to naturalistic visual stimuli are best described as sparse trains of highly precise firing "events" \- each composed of 1-10 spikes. Each firing event can be viewed as a composite symbol in the neural code. I will first characterise changes in the structure of the ganglion cell spike trains under different states of adaptation. How does the event-like character of the spike trains change following adaptation? How effectively does adaptation act to bring the structure of the spike train back to its preadaptation state? Using analytic tools such as Information Theory, I will then describe the effects of adaptation on the content of the ganglion cell code. How does adaptation alter the meaning of particular patterns of spikes transmitted along the optic nerve? Does this imply that the brain has to alter its strategy for interpreting ganglion cell responses? If so, what cues are available in the spike train for the brain to determine the retina's current state of adaptation? The answers to these questions will further our understanding of the contribution that adaptive mechanisms make to information processing in the retina and in the nervous system as a whole.