Implantation of artificial whiskers on the ears of newborn mice induces visual re-mapping in the superior colliculus

Population coding strategies and involvement of the superior colliculus in the tactile orienting behavior of naked mole-rats

Even simple behaviors of vertebrates are typically generated by the concerted action of large numbers of brain cells. However, the mechanisms by which groups of neurons work together as functional populations to guide behavior remain largely unknown. One of the major model systems for exploring these mechanisms has been mammalian visuomotor behavior. We describe here experiments that establish a new model system for analyzing the sensory control of behavior by neuronal populations using a mammalian somatosensory response: orientation to touch cues in a rodent. We found that the CNS mechanisms used to direct these orientation responses to touch can be delineated from behavioral experiments. In this study we demonstrate that the superior colliculus, a component of the vertebrate midbrain most often thought of as a visual structure, is an essential component of the naked mole-rat's unique tactile orienting behavior. Furthermore, the information processing that underlies this behavior displays striking parallels with that used for visual orientation at anatomical and computational levels.

Differential formation of topographic maps on the cerebral cortex and superior colliculus of the mouse by temporally correlated tactile- tactile and tactile-visual inputs

Coincident electrical activity of nerve fibres seems to play a fundamental role in the development of ordered connections in the CNS. To test this hypothesis on the formation of topographic maps we connected two cutaneous regions of the body of newborn mice by implanting an artificial bridge of pig hair. Through this procedure we produced the mechanical fusion of the ear with either the shoulder or the nose. In these conditions the ear not only was connected with shoulder or nose, but was also in relation with the nasal or the inferior portion of visual space. Therefore the probability of temporally correlated tactile-tactile inputs (ear-shoulder or ear-nose) as well as tactile-visual inputs (ear-inferior or ear-nasal visual space) increased. By recording from the primary somatosensory cortex and superior colliculus, we found that the formation of topographic maps was based on different principles. The somatosensory cortex developed in terms of tactile-tactile correlated inputs, showing somatosensory neurons with receptive fields extending through the fused parts of the body. Conversely, the superior colliculus processed tactile-visual correlated inputs; we found somatosensory-visual bimodal neurons with visual receptive fields extending into the portion of visual space where the artificial bridge was directed. These results suggest that the fusion of two body parts is represented in terms of cutaneous coordinates in the cortex and external world (visual) coordinates in the superior colliculus. Therefore the differential tactile-tactile and tactile-visual coincident activity seems to be correlated to the different meaning of information processing of these two brain regions.

Orienting behaviour and superior colliculus sensory representations in mice with the vibrissae bent into the contralateral hemispace

When a tactile stimulus touches the body on one side, animals show an orienting response toward that side with the eyes, the head or the entire body. This movement requires the transformation of sensory information into motor commands. The superior colliculus is supposed to be a fundamental part of the brain where this sensorimotor transformation occurs and where one of the possible mechanisms could be the alignment among sensory and motor topographies. We changed the body shape of mice in order to analyse the development of new orienting responses following tactile stimulation. To do this, we bent the left vibrissae from left to right such that they were located in the right portion of the visual hemifield. If left-right inversion was performed in adults, tactile stimulation of the left vibrissae performed from the right produced wrong orienting movements to the left. Conversely, if left-right inversion was performed in newborns, mice learned to respond correctly to the right. By recording from superior colliculus multisensory neurons of mice whose vibrissae were displaced at birth, we found a shift of visual and auditory receptive fields from left to right in those multisensory neurons receiving tactile input from the displaced vibrissae. These results show the strict relation existing between the neuronal modifications in the superior colliculus and the changes in orienting behaviour. These findings also suggest two important conclusions. First, sensory mapping in the superior colliculus depends on sensory inputs coming from the same portion of space.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of early postnatal modification of body shape on the somatosensory-visual organization in mouse superior colliculus

Different regions of the body of an animal have their own shape and location within visual space. Accordingly, in the superior colliculus there are somatosensory-visual bimodal neurons receiving tactile and visual input from the same region of space. In newborn mice, we changed the position of some body parts within visual space in order to see what happened to the alignment of the somatosensory and visual receptive fields of superior colliculus bimodal neurons. To do this, we modified the shape of the head by displacing the superior vibrissae and the ears, normally in the superior portion of visual space, into the inferior visual space. Analogously, we bent the inferior vibrissae into the superior visual space. At the sixth postnatal week we recorded from somatosensory-visual bimodal neurons of the deep layers of the superior colliculus and found that the tactile and visual receptive fields were aligned. Neurons receiving tactile input from the downward-displaced superior vibrissae and ears showed visual receptive fields in the inferior portion of visual space, whereas neurons receiving input from the upward-displaced inferior vibrissae showed visual receptive fields in the superior visual space. These results show that an experience-dependent interaction between visual and somatosensory inputs occurs during development, and that early exposure to abnormal visual-somatosensory experience modifies the organization of multisensory neurons in the superior colliculus.