The Auditory Midbrain Major Integrating Centers

The inferior and superior colliculi collectively form the roof of the midbrain. Both are major integrating centers, receiving converging input from a wide variety of sources representing the auditory, visual, and somatosensory systems.

Anatomically, the IC is subdivided into a central nucleus (ICC) and a surrounding cortex (divided into external and pericentral nuclei). The ICC forms an essential link in the mainline lemniscal auditory system and thus is critically involved in the transmission of auditory sensory information to the forebrain from lower auditory centers. The ICC also receives a rich supply of afferents from many other sources and directs its outputs to a variety of targets. This results in numerous feedback loops (Fig. 5) that involve essentially all major auditory areas of the forebrain, midbrain, and brain stem of the same side of the brain. In addition, several major axonal bundles (commissures) connect auditory areas of one side of the brain with their counterparts on the other. Hence, it is not surprising that essentially all information transmitted to the auditory forebrain is first transformed within the ICC. Transformations involve changes in the balance of excitation and inhibition exerted by converging afferent inputs—changes that are reflected in such fundamental attributes of coding as frequency tuning curves, timing of spike discharges, spontaneous background activity, sensitivity to modulations in frequency and intensity, binaural interactions, and spatial tuning. Remarkably, the diverse parallel input and output, and the transformations associated with them, are highly organized within the ICC tonotopic map, which is best described as being made up of frequency-specific layers or bands. Within each band are overlapping functional maps related to a wide array of response features and stimulus attributes such as amplitude modulation, pitch, intensity, threshold, onset latency, sharpness of frequency tuning, and binaural sensitivity. It is thought that these functional maps may provide the early substrates from which sound percepts are eventually derived. Information transformed by the ICC is carried to the auditory forebrain via the brachium of the IC.

The superior colliculus (SC) is a layered structure involved primarily in the control of movements of the eyes to an auditory or visual target. Thus, in the classical sense it is not a "sensory" structure. Auditory input carries information on the direction of a sound source to deep SC layers. Sound-source direction is represented by SC neurons having spatial receptive fields arrayed to form a map of auditory space. Visual input is received in the upper layers, and information on target direction is provided by its position on the SC visuotopic map. The auditory and visual maps are normally in alignment, presumably to coordinate visual and auditory directional information so that the eyes can move quickly and accurately to an object that appears in the visual and sound fields.

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Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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