Strength through Synchrony

Our connection to the external world occurs only through the thalamus, through which all sensory signals (except olfaction) must pass in order to gain access to the neocortex. As Alonso points out in his editorial on Bruno & Sakman's 2005 Science paper, our sensory systems are incredibly sensitive - many observers can detect a single photon in complete darkness, and we'll notice an indentation on our skin of as little as 20 microns (this is about 3 times the diameter of a single red blood cell).

And yet, the neural connections between thalamus and the cortex are incredibly weak both in terms of the strength of their signal (.5 mV, 30 times smaller than average intracortical connections) and in terms of the prevalence of synapses ("thalamocortical synapses account for less than 15% of all synapses onto L4 spiny neurons" in the cortex, according to Bruno & Sakman). So how do we maintain such sensitivity in the absence of any substantial neural signals?

One theory holds that signals from the thalamus are amplified by recurrent connections in cortex. Another theory suggests that thalamic signals don't need a "cortical amplifier" to evoke an action potential in cortex, but that they merely need to synchronize their firing to take L4 neurons over threshold. Bruno & Sakman found evidence that this may be the exact mechanism by which thalamocortical connections evoke action potentials in L4: sensory stimulation, but not sinusoid electrical stimulation to the thalamus, evoked "synchronous discharge of thalamic neurons."

Further analyses showed that as little as 30 thalamic cells firing synchronously could suffice to evoke action potentials in cortex. Therefore, the authors argued that a cortical amplifier in L4 is unnecessary to account for how cortex is capable of receiving inputs from thalamus, and thus, how we are able to perceive the outside world.