ABSTRACT
Cortical neurons receive their inputs only for a small fraction from specific thalamic afferents and for a much larger fraction from other cortical neurons. The same could be said for the sensory thalamus, where the majority of inputs is not from collicular afferents but from centrifugal cortical neurons. Yet, by analyzing single-unit stimulus-response functions, such as the frequency-tuning curve (FTC) or the spectro-temporal receptive field (STRF), one regards the sensory system as a simple single input-single output system. One reason that this works fairly well is because the specific thalamic afferents to cortical neurons are synchronized by the stimulus, whereas the majority of other neural inputs arrive asynchronously. Abeles (1991) was one of the first to draw attention to the power of synchronously arriving inputs in activating cortical neurons. In simplified terms, the amplitude of the post-synaptic potentials (PSP) resulting from synchronized inputs is proportional to the number of inputs, whereas the PSP amplitude from asynchronously arriving inputs is only proportional to the square root of their number. Assume that the synaptic connection strengths and firing rates for thalamic cells combined are a factor 3-4 higher than those of cortico-cortical cells, which is not an unreasonable assumption (Gil et al., 1999). Then, even only 30 synchronously firing specific afferents (the putative number converging on a cortical cell, Miller et al., 2001), which produce compound PSPs that are 3-4 times larger than for cortico-cortical cells and effectively produce 90120 amplitude units of PSP, would still be as effective as 9000 asynchronously
firing non-specific cortico-cortical inputs that produce amplitude units of PSP.
