Michaël Slama November 1st, 2007HST .722 – Brain Mechanisms of Hearing and SpeechTopic proposalCorticothalamic feedback connectionsIntroductionThe thalamus is the main sensory input to the cerebral cortex, not only for auditory stimuli, but also for visual and somatosensory stimuli (Alitto and Usrey, 2003). On the other hand, approximately half of the neuron input to the dorsal thalamus come from the cortex. These corticothalamic feedback connections seem to be important for sensory processing in the visual, auditory and somatosensory systems. In this topic, we will specifically look at corticothalamic connections in the auditory system. First of all, some background on the auditory thalamus and the corticothalamic feedback connections will be given. Then, we will discuss two papers related to the effects of cortical inactivation and stimulation on thalamic processing. Finally, possible functional roles played by these connections will be discussed, and in particular the idea that the thalamus is an adaptive filter controlled by the auditory cortex, and that the left corticothalamic feedback system may help in speech separation tasks. BackgroundThe Auditory ThalamusSeveral nuclei of the thalamus are involved in the processing of acoustic information, such as the lateral part of the posterior nucleus, the reticular nucleus, and the medial geniculate body. The medial geniculate is an obligatory connection for the inferior colliculus and for the auditory cortex (cf. figure 1). It can be divided into several subnuclei, based on the shape and density of their neurons, as well as on their projections and inputs. Multiple parallel ascending pathways converge to the thalamus from the inferior colliculus (de Ribaupierre, 1997). Anatomical and physiological evidence suggests that these distinct channels are maintained up to the auditory cortex. The ascending pathways between the thalamus and the cortex can be divided into three main systems: a tonotopic system, a diffuse system, and a polysensory system, which connect different parts of the thalamus to different parts of the cortex, with some degree of superposition between the systems. Conversely, information that is processed in the auditory cortex is sent to various subcortical and cortical targets. Among these targets is the auditory 1Figure 1: The Thalamus in the Auditory System(dashed lines are descending pathways)thalamus, which suggests that feedback control from the auditory cortex to the thalamus is functionally important. We will focus on these corticothalamic connections.Anatomical Characteristics of the Corticothalamic ConnectionsIt is thought that the medial geniculate receives as many descending connections from the auditory cortex as ascending connections from the inferior colliculus (de Ribaupierre, 1997). It has been shown with retrograde labeling from the medial geniculate body (Rouiller and de Ribaupierre, 1985) that these descending connections originate from pyramidal neurons in layers V and VI of the auditory cortex. It is interesting to note that these connections respect the tonotopic organization and are reciprocal, i.e. they target the same location in the thalamus from which they receive their input (Winer and Lee, 2007).Effects of cortical inactivation and stimulation on thalamic processingEffects of cortical inactivationOne approach to study corticothalamic connections is to record from thalamic neurons during a temporary inactivation of the auditory cortex, and compare their response to a baseline condition prior to inactivation. This is the approach adopted by Villa et al (1991). They circulated ice-cold water in an aluminum cylindric core in contact with the dura mater of anesthetized cats in order to inactivate the cortical synapses. They monitored cortical inactivation by observing the shape of evoked potentials, and they recorded from single units in various locations of the auditory thalamus. They looked at the changes produced by cortical inactivation on various parameters of interest. First of all, the spontaneous activity in all locations tended to decrease, except for a minority of neurons for which it either decreased or did not change. They also recorded thalamic activity in response to pure tones, and observed a change in frequency response range (either an increase or a decrease) for many units under cortical inactivation condition. They also looked at the best frequency of these units before and after cortical cooling, and noted that for many units, there was a significant change. These results are shown in figure 2a. Finally, they used white noise burst stimuli and observed changes in the peri-stimulus time histograms. For most units, the initial shape was recovered after cortical inactivation was stopped. An example of recording in response to noise bursts is shown on figure 2b. 2Figure 2a: Best Frequency of Tuning before vs. after coolingFigure 2b: Single unit response to 200ms noise burstsTheir experiments suggested that cortical inactivation had an influence on thalamic activity, in a way that was specific to the anatomical subdivision of the auditory thalamus. Tuning as well as temporal processing characteristics were affected in each subdivision. This paper will also be interesting for a discussion of their methodological approach, which could have contributed to alter the neuronal activity in the thalamus.Effects of cortical stimulationHe et al (2002) propose an alternative way to look at the influence of cortex activity on thalamic output. Instead of cooling down the cortex temperature to inactivate it, which is not very selective, they implanted electrodes in the cortex of guinea pigs and stimulated it electrically. A sound stimulus was delivered to the contralateral ear after a delay, following the end of the cortical stimulation. Responses of thalamic neurons were recorded before and after the stimulation condition. They specifically focused on the ventral part of the medial geniculate body.They found, first of all, that a low stimulation current had a facilitatory effect on thalamic activity; nonetheless, this effect was not monotonic with current intensity, until becoming