J. Schummers, J. Mariño and M. Sur.Picower Center for Learning and Memory, MIT, Cambridge, MA 02139 and Neurocom, Universidade da Coruña Spain

The mechanisms of synaptic integration involved in computing orientation selectivity are not fully understood. We have previously shown that neurons near pinwheel centers (PWs) in the orientation preference map receive broadly tuned inputs, yet produce sharply tuned outputs1. Here, we have examined the role of rapid membrane potential (Vm) fluctuations (“synaptic noise”) in shaping the spike tuning. It has been shown that Vm fluctuations in the gamma band preferentially lead to spikes, and that the tuning of power in the gamma range is more sharp than the mean Vm 2,3. One possibility, therefore, is that larger Vm fluctuations in the gamma (or other) range at preferred orientations compared to orthogonal generate sharp spike tuning in neurons near PWs. We combined optical imaging of intrinsic signals with whole cell recordings of Vm in adult cat area 17, and analyzed the tuning of different frequency bands, including gamma band Vm fluctuations, in neurons near PWs and in orientation domains. We find that gamma band power is generally, but not always, more sharply tuned than mean Vm, but less sharp than spike tuning. Cells in orientation domains show sharper tuning of power than cells at PWs, not only in the gamma range but also in frequency ranges below and above the gamma band. In some pinwheel neurons, gamma power is only weakly tuned. Overall, our analysis suggests that it is unlikely that differences in the temporal structure of Vm fluctuations between different stimulus orientations play a dominant role in producing sharp spike tuning in neurons near PWs. Other mechanisms, including strong orthogonal inhibition, may be more important in generating sharp tuning near PWs.

1 Schummers J, Mariño J, Sur M., Neuron. 2002 Dec 5;36(5):969-78.
2 Azouz R, Gray CM., Neuron., 2003 Feb 6;37(3):513-23.
3 Volgushev M, Pernberg J, Eysel UT., Eur J Neurosci. 2003 May;17(9):1768-76.

Supported by EY07023 (MS), MECD (JM) and HHMI (JS)