Tectum
The Tectum (aka the Optic Tectum) also known as the Superior Colliculus
The brain of mammals, birds, and other animals creates spatial maps based on input from its various sensory modalities. Visual maps typically develop with an innate 1-to-1 correspondence to the animal’s retina (a retinotopic map). Likewise, the brain also develops maps of auditory space by organizing neurons according to their tuning for sound location cues (such as ITD and IID). One particular brain structure, the optic tectum (in birds) or superior colliculus (in mammals) integrates auditory and visual input into one unified map of space. This means that auditory and visual stimuli that originate from the same spatial location activate the same tectal neurons.
Several critical experiments have shown how these visual and auditory maps are integrated in the tectum (and will be talked about below), but to get a deeper understanding about why they develop in such a way, it’s helpful to understand the primary function of the tectum in guiding actions. The tectal system has evolved to direct behavioral responses toward a specific location in egocentric space. Each tectal layer represents a topographic map of the physical world in retinotopic coordinates with a major role in directing eye saccades or head movement toward objects of interest. That is, when sensory stimuli cue the organism about something in the world they should be attending to, the tectum is ultimately responsible for informing the eyes of the spatial location that needs to be foveated. So whether the animal sees something move in it’s periphery, or hears something off to it’s side, if the head and eyes move quickly and efficiently to foveate this object, the tectum has done its job. If they eyes initially miss this object and have to perform a search before they finally foveate this object, the tectum tries to correct these errors by altering its neural representations of space.
Imagine a very young owl that has just opened its eyes for the first time, and it hears a mouse off to its right. These auditory cues enter its (still underdeveloped) tectum and inform the owl to foveate 10O azimuth to the right. However, if the mouse is actually located at 20O to the right, the owl learns that its auditory map is skewed to the left, and over time makes the necessary modifications in the tectal spatial map. After several weeks, the same auditory signal (once thought to be a cue from 10O to the right) now causes the owl to foveate at exactly 20O to the right; and it’s now dead-on. Experiments using barn owls have demonstrated that it is indeed vision has an instructive influence on the developing auditory spatial map in the tectum. Owls that wore prism goggles that shifted the visual world 20O to the left (for 70 days) caused their auditory map to shift left by 20O as well. Thus, when they heard a sound coming from 20O to the left they would foveate at exactly 0O (center), which perfectly matched where they’d see the object through these goggles. To demonstrate that this remapping process is performed in the tectum, experimenters recorded from tectal neurons (again after wearing the leftward shifting goggles for 70 days). With the goggles still on, they found a group of neurons that was responsive to visual stimuli presented at 20O azimuth to the left. With the goggles now off, they found the same neurons were activated by visual stimuli at 0O azimuth. Still recording from these same neurons, they found that auditory stimuli centered around 20O to the left caused these neurons to be most active. Thus showing that vision has an instructive influence on the development auditory space map in the optic tectum.