Monday, January 23, 2006
11:00 AM
CSB 209
Dana H. Ballard
U. Rochester
Is the Cortex a Digital Computer?
Decades of single-cell recordings in mammalian cortex have revealed the correlation of increased firing rate with behavioral measures, suggestive of a rate-code. But is a rate code the fundamental signalling strategy of the cortex, or is it just a correlate of another quite different strategy? The two possible answers to this question both have ardent proponents. Defenders of the rate code as the basic signaling strategy point to the huge number of experimental studies that use rate code as a basic framework. In addition they argue that the correlations between neuros that are above the number predicted by chance are too small for any synchronous strategy to be a useful phenomena. Proponents of the idea of a more basic code based on spike timing point to the increasing number of data that show spike timing effects. There have been a wide range of models suggesting very different uses of timing, such as signal binding, fast computation, signaling the result of a computation and even consciousness itself. We argue that the cortex has adopted a signaling strategy that makes extensive use of synchrony for fast communication, but does it in a way that is consistent with the rate-code indications. The crux of the argument is that the cortex has to solve a handful of basic computational problems and that these can be expressed as constrants. When these constrants are intersected, one model that fits is that of a handful of simultaneously active digital feedback circuits. This is a very non-standard way of thinking about cortical computation that fits the data.