PAIRED-SPIKE INTERACTIONS AND SYNAPTIC EFFICACY OF RETINAL INPUTS TO THE THALAMUS

In many neural systems studied in vitro, the timing of afferent impuls es affects the strength of postsynaptic potentials(1,2). The influence of afferent timing on postsynaptic firing in vivo has received less a ttention. Here we study the importance of afferent spike timing in viv o by recording simultaneously from ganglion cells in the retina and th eir targets in the lateral geniculate nucleus of the thalamus, When tw o spikes from a single ganglion-cell axon arrive within 30 millisecond s of each other, the second spike is much more likely than the first t o produce a geniculate spike, an effect we call paired-spike enhanceme nt. Furthermore, simultaneous recordings from a ganglion cell and two thalamic targets indicate that paired-spike enhancement increases the frequency of synchronous thalamic activity. We propose that informatio n encoded in the high firing rate of an individual retinal ganglion ce ll becomes distributed among several geniculate neurons that fire sync hronously. Because synchronous geniculate action potentials are highly effective in driving cortical neurons(3), it is likely that informati on encoded by this strategy is transmitted to the next level of proces sing.