Somatosensory Representation Research Articles

What can studying musicians tell us about motor control of the hand?

Most standard accounts of human anatomy and physiology are designed to meet the requirements of medical education and therefore consider their subject matter from the standpoint of typical rather than outstanding levels of performance. To understand how high levels of skill are developed and maintained, it is necessary to study elite groups such as professional athletes or musicians. This can lead to the rediscovery of arcane knowledge that has fallen into neglect through a lack of appreciation of its significance. For example, although variability in the muscles and tendons of the hand was well known in the nineteenth and early twentieth centuries, it is through recent studies of musicians that its practical significance has become better appreciated. From even a cursory acquaintance with the training methods of sportsmen and women, dancers and musicians, it is clear that sophisticated motor skills are developed only at the cost of a great deal of time and effort. Over a lifetime of performance, musicians arguably spend more time in skill acquisition than almost any other group and offer a number of unique advantages for the study of motor control. Such intensive training not only modifies cortical maps but may even affect the gross morphology of the central nervous system. There is also evidence that in certain individuals this process can become maladaptive. Recent studies of musicians suggest that intensive training can lead to the appearance of ambiguities in the cortical somatosensory representation of the hand that may be associated with the development of focal dystonia; a condition to which musicians are particularly prone. The realization that changes in cortical maps may underlie dystonia has led to the development of new approaches to its treatment, which may ultimately benefit musicians and non-musicians alike.

Characterization of the effect of associative cortical and peripheral nerve stimulation on the somatosensory evoked potentials and tactile discrimination performance in human subjects

Repetitive median nerve stimulation combined with transcranial magnetic stimulation (TMS) over the contralateral postcentral region (paired associative stimulation, PAS) may induce changes in the amplitude of the P25 component of median nerve somatosensory evoked potentials (MN-SSEP). Here we used 64-channel electroencephalography (EEG) to reveal the early cortical MN-SSEP source(s) that is modified by PAS. In addition, spatial two-point discrimination was tested one day before (day1), immediately after (day2) and one day after (day3) PAS-intervention to assess possible changes of tactile sensitivity corresponding to somatosensory representations inside or outside of the cortical PAS-target, respectively. Eleven healthy volunteers participated in five experiments. MN-SSEPs (1800 trials, ˜3 Hz, 300% perceptual threshold) were recorded before and after PAS intervention. During the intervention the magnetic pulse (180 repetitions at 0.1 Hz, 150% resting motor threshold) was delivered over S1 at one of the following times relative to the subject’s N20 peak: −20, −2.5, 0 and 100 ms (PASN20 − 20, PASN20 − 2.5, PASN20 + 0, PASN20 + 100 ms, respectively). As a control, the TMS pulse was delivered at half time between two median nerve stimulations (PAS+5 s). Following PASN20 − 2.5 and PASN20 + 100 ms, a nonparametric permutation test across all subjects revealed an EEG amplitude increase between 21 and 31 ms. Source analysis showed that the effect of PASN20 − 2.5 ms could be accounted for by an amplitude increase of a tangential source localized at S1. The effect of PASN20 + 100 ms differed in its scalp topography from that of PASN20 − 2.5 ms and may be attributed to a different source at S1. In addition, PAS induced differential effects on tactile performance. Following PASN20 − 20 ms, performance in D2, but not in D5, improved significantly. In contrast, following PASN20 − 2.5 ms, there was a tendency for deterioration in tactile performance of the index finger which was correlated with MN-SSEP changes between 28 and 32 ms (R2 = 0.8). These results lend support to the notion that PASN20 − 2.5 modifies synapses located in superficial cortical layers in Brodmann area 3b and thereby may induce spatially specific differential behavioral changes in tactile performance.

Organization of somatosensory cortical areas in the naked mole‐rat (Heterocephalus glaber)

Multiunit electrophysiology was combined with histological analysis of cortical sections to investigate the organization of somatosensory areas in the naked mole-rat. We provide new details for the organization of primary somatosensory cortex (S1) and identify cortical modules and barrels that correspond to the representations of different body parts. In addition, details of the location and organization of secondary somatosensory cortex (S2) are reported, and evidence for a third somatosensory representation, likely the parietal ventral area (PV), is provided and discussed. S1 contained a complete and systematic representation of the contralateral body surface and oral structures. The orientation of S1 was inverted, with the lower body represented medially and the face and oral structures located rostrolaterally. The S2 representation was found in caudolateral cortex forming a mirror image of S1. The two areas were joined at the representation of the vibrissae and snout, so that the orientation of S2 formed an upright representation of the body in cortex. Receptive fields for S2 were consistently larger than those in S1. Evidence for the presumptive parietal ventral area, lateral to S2, suggests that this area may be an inverted mirror image of S2. By aligning the electrophysiological maps of body representations with cytochrome oxidase-reacted cortical sections we were able to identify modules related to the buccal pad, chin, vibrissae, forelimb, hindlimb, trunk, tongue, lower incisor, and upper incisors. The orofacial modules in lateral cortex resemble similar modules reported to relate to oral structures previously described in the laboratory rat, owl monkey, and squirrel monkey.

Aberrant learning in individuals who perform repetitive skilled hand movements: Focal hand dystonia—Part 1

Summary Stressful, repetitive use of the upper limb in work, sports, or musical performance can lead to acute, pain and loss of function. The evidence for tissue mircrotrauma as the underpinnings of this pain is convincing and explains why rest, anti-inflammatory medications, change in movement biomechanics and good ergonomics are usually effective treatment modalities. Unfortunately, some repetitive strain injuries become chronic with degenerative changes found in tendons and muscles, scarring restricting soft tissue and joint mobility and compression of peripheral nerves causing strain and limiting excursion. In other cases, involuntary co-contractions of flexors and extensors lead to painless, uncontrollable, end range twisting movements that interfere with the performance of target tasks. This movement dysfunction is referred to as occupational hand cramps, focal hand dystonia (FHd), golfers yip, keyboarders cramps, or musician's cramps. Research studies report evidence of degradation of the somatosensory, sensorimotor and motor representation of the hand in animals and patients with dystonic hand movements. This aberrant learning requires learning based training to reorganize the brain. In this presentation, the principles of neuroplasticity will be related to the origin, diagnosis, assessment and treatment of FHd grounded on an evidence-based review of the research to support aberrant learning as one etiology for the origin of this disorder and the foundation for a learning based approach to remediate the condition.

Convergence of spinal trigeminal and cochlear nucleus projections in the inferior colliculus of the guinea pig

In addition to ascending auditory inputs, the external cortex of the inferior colliculus (ICX) receives prominent somatosensory inputs. To elucidate the extent of interaction between auditory and somatosensory representations at the level of IC, we explored the dual projections from the cochlear nucleus (CN) and the spinal trigeminal nucleus (Sp5) to the inferior colliculus (IC) in the guinea pig, using both retrograde and anterograde tracing techniques. Injections of retrograde tracers into ICX resulted in cell-labeling primarily in the contralateral DCN and pars interpolaris and caudalis of Sp5. Labeled cells in DCN were either fusiform or multipolar cells, whereas those in Sp5 varied in size and shape. Injections of anterograde tracers into either CN or Sp5 resulted in terminal labeling in ICX primarily on the contralateral side. Most projection fibers from Sp5 terminated in a laminar pattern from ventromedial to dorsolateral within the ventrolateral ICX, the ventral border of IC, and the ventromedial edge of IC (collectively termed "the ventrolateral border region of IC," ICXV). Less dense anterograde labeling was observed in lateral and rostral ICX. Injecting different tracers into both Sp5 and CN confirmed the overlapping areas of convergent projections from Sp5 and CN in IC: The most intense dual labeling was seen in the ICXV, and less intense dual labeling was also observed in the rostral part of ICX. This convergence of projection fibers from CN and Sp5 provides an anatomical substrate for multimodal integration in the IC.

Electrophysiological study on changes of plasticity of bilateral primary somatosensory cortex following total brachial plexus root avulsion injuries in adult rats

目的 探讨成年大鼠一侧全臂丛根性撕脱伤后双侧初级体感皮层可塑性变化的规律.方法将30只SD雄性大鼠分为6组,分为正常对照组和全臂丛根性撕脱伤术后1 d、7 d、1个月、3个月、1年共5个时间组,每组5只.采用体感皮层诱发电位(somatosensory evoked potential,SEP)记录法,定量评价一侧全臂丛根性撕脱伤后双侧初级体感皮层(Sm1)可塑性变化的时程.结果术后不同时间段电刺激患爪正中神经支配区,在双侧Sm1 均未诱发出SEP.电刺激健侧前爪正中神经支配区,在健爪同侧Sm1未记录到SEP,只在其对侧Sm1记录到SEP;但不同时间段的SEP位点数目均明显多于正常对照组.结论成年大鼠的初级体感皮层仍具有可塑性,一侧全臂丛根性撕脱伤后诱发了双侧Sm1动态的功能重组.两大脑半球相对应的同位区域之间存在着维持半球间平衡和协调的特殊机制.健侧前爪体感代表区的扩大可提高健爪的感觉辨别能力,以部分代偿患肢感觉功能的丧失。

Silent event-related fMRI reveals deficient motor and enhanced somatosensory activation in orofacial dystonia

Previous studies showed cortical dysfunction and impaired sensorimotor integration in primary generalized and focal hand dystonia. We used a whistling task and silent event-related fMRI to investigate functional changes in patients with blepharospasm and patients with a combination of blepharospasm and oromandibular dystonia (Meige's syndrome). Whistling served as a model for a skilful orofacial movement with a high demand on sensorimotor integration. It allowed us to study the oromandibular motor system that is clinically affected in Meige's syndrome but not in isolated blepharospasm. In Meige's syndrome, functional MRI revealed deficient activation of the primary motor and ventral premotor cortex within the mouth representation area during whistling. Compared with healthy controls, both forms of orofacial dystonia had increased activation of bilateral somatosensory areas and the caudal supplementary motor area (SMA) in common. While overactivity of somatosensory areas and caudal SMA in Meige patients was partly reversed by botulinum toxin treatment, impaired motor activation was not. We conclude that impaired motor activation appears to be specific for the clinically affected oromandibular motor system in Meige's syndrome while enhanced somatosensory activation is a common abnormality in both forms of orofacial dystonia independent of the affected motor system. Somatosensory overactivity indicates an altered somatosensory representation in orofacial dystonia while impaired motor activation may be a functional correlate of reduced cortical inhibition during oromandibular motor execution in Meige's syndrome.

Functional imaging of stress urinary incontinence

Stress urinary incontinence (SUI) is defined as an involuntary loss of urine during increases in intraabdominal pressure such as coughing or laughing. It is often a consequence of weakness of the pelvic floor. Treatment of SUI consists of pelvic floor muscle training with EMG-biofeedback (PFMT) or contraction-exercises, with voluntary pelvic contractions in order to strengthen the pelvic floor. We investigated neuroplastic changes comparing PFMT with EMG-biofeedback before and after training in ten female patients with SUI using event-related functional Magnetic Resonance Imaging (fMRI). After a 12-week training a more focused activation in the primary motor and somatosensory cortical representation sites of the lower urogenital tract was found. In addition, reductions in brain activation in the insula, right frontal operculum and the anterior cingulate cortex suggest changes in emotional arousal in micturition after treatment. These changes are related to clinical improvement documented by decreased number of incontinence episodes and increased EMG-activity of the pelvic floor muscles after training. The changes in EMG-activity were correlated with heightened BOLD responses in the primary motor and primary sensory cortical representation sites of the lower urogenital tract.

Motor But Not Sensory Representation in Motor Cortex Depends on Postsynaptic Activity During Development and in Maturity

The movement representation in the primary motor cortex (M1) of the cat develops between postnatal weeks 7-12. The somatosensory representation in motor cortex is present by the age that the motor map begins to develop. In this study we examined the role of neural activity in development and maintenance of the M1 movement and somatosensory representations. We blocked activity of M1 neurons unilaterally for one month by intracortical infusion of the GABA agonist muscimol during the motor map development period in kittens and in mature cats. After the drug effects were no longer present, we used microstimulation and multiunit recording in the forelimb areas of M1 to determine the motor and somatosensory representations in the infused and noninfused sides. In both kittens and adults, there was a severe reduction or elimination of sites where microstimulation evoked a motor response in the inactivated compared with the control side. In contrast, there was no difference in the percentage, topography or receptive field modality of sites receiving somatosensory inputs on the inactivated and control sides. Moreover, the pattern of somatosensory input to M1 was similar before and after inactivation. This suggests that somatosensory input to M1 is stable after the connections initially develop. Since activity blockade had the same effects on the motor representation of kittens and adult cats, M1 neuronal activity, while possibly important in map development, is equally necessary for map maintenance.

The Sensory Cortical Representation of the Human Penis: Revisiting Somatotopy in the Male Homunculus

Pioneering mapping studies of the human cortex have established the notion of somatotopy in sensory representation, which transpired into Penfield and Rasmussen's famous sensory homunculus diagram. However, regarding the primary cortical representation of the genitals, classical and modern findings appear to be at odds with the principle of somatotopy, often assigning it to the cortex on the mesial wall. Using functional neuroimaging, we established a mediolateral sequence of somatosensory foot, penis, and lower abdominal wall representation on the contralateral postcentral gyrus in primary sensory cortex and a bilateral secondary somatosensory representation in the parietal operculum.

De l'autotopoagnosie à un modèle de représentations des connaissances du corps

Body knowledge can break down in a variety of ways after brain damage. In this article some of these disruptions traditionally associated with left parietal lesion are reviewed. At the beginning of the 20th century, Pick first described autotopoagnosia as an inability to indicate parts of the own body whereas the ability to denominate is preserved. Some years later, Gerstmann introduced the notion of finger agnosia which is the main element of Gerstmann's syndrome (finger agnosia, agraphia, acalculia and left-right confusion). More recently, heterotopoagnosia has been defined as a selective inability to point to another person's body parts while the capacity to point to one's own is intact. Clinical manifestations, cognitive performances and neuroanatomical correlations are exposed and linked.These perturbations are very rare, at least in the pure form (without anomia, apraxia, visuo-spatial or global cognitive dysfunction).They are constant, affecting bilateral parts of the body, without any impact on the daily living. Based on the cases in the literature, the specificity of autotopoagnosia is discussed across different hypotheses. Autotopoagnosia has either been attributed to pure fiction or to a language impairment or to the inability to analyse a whole into its parts. During the last 20 years, further single case studies with circumscribed lesion without aphasia have reported that autotopoagnosia cannot be reduced to a non-specific cognitive disorder. Several accounts propose that autotopoagnosia reflects a human body-specific knowledge impairment. Moreover, implications from the dissociation of disturbed designation and preserved lexico-semantic representations in autotopoagnosia are integrated in a model of the organisation of the body knowledge with multiple levels of representations. The first contains semantic and lexical information about body parts, such as names and functional relationships, strongly linked to the verbal system. The second contains the category-specific visuospatial representations of bodies, such as the structural description of the body, the position and the proximity of the body parts, more strongly linked to the non-verbal visual and somatic sensory systems. The third is a body-reference system that provides a dynamic and actual body image, emerging from various sources of sensory afferences. Finally, motor representations participate to the construction and maintenance of the somatosensory representations. It is proposed that autotopoagnosia is an impairment which predominantly affects body-specific visuospatial representations. Finally, neuroimaging data are suggestive of dysfunction originating in the left dominant parietal regions.

Frontal cortical areas of the monkey brain engaged in reaching behavior: A 14C-deoxyglucose imaging study

The ( 14C)-deoxyglucose method was employed to study whether different areas of the primate frontal lobe are involved in different aspects of reaching behavior. To this end, we mapped the functional activity of the frontal motor cortical areas in three monkeys performing reaching movements with one forelimb. The first monkey had to capture a peripheral visual target with a saccade and a forelimb-reach together, the second monkey had to reach a peripheral visual target with one forelimb while fixating a central target, and the third one had to reach a peripheral memorized target with one forelimb in complete darkness while the eyes maintained a straight ahead direction. The extent and intensity of activations were compared to those of three respective control monkeys: a saccade-control, a fixation-control, and a dark-control. The primary somatosensory (S1) and motor (F1) forelimb representation, the S1- and F1-trunk representation, the F2-dimple region, areas F3-forelimb, F4, F5-bank of arcuate sulcus, F7-ridge, the dorsal bank of cingulate sulcus, and 24c were activated in all reaching monkeys regardless of accompanying visual stimulation and oculomotor behavior. Interestingly, the S1-forelimb activation in the monkey reaching to memorized targets in complete darkness was more pronounced than that in the monkeys reaching to visual targets in the light, indicating that increased somatosensory processing compensates for the absence of visual feedback. On the other hand, areas F2-periarcuate, F5-convexity, F6, and 23 were preferentially activated by reaching to visual targets and remained unaffected during reaching to memorized targets when no visual feedback was available.

Reduced somatosensory hand representation in thalidomide‐induced dysmelia as revealed by fMRI

The concept of cerebral plasticity suggests that the hand representation in somatosensory cortex is abnormal in congenital malformation disorders. To investigate this issue we studied 11 subjects with different degrees of upper extremity dysmelia due to thalidomide embryopathy in comparison to 10 control subjects. In the affected subjects fingers are typically missing in radio-ulnar order beginning with the thumb. Haemodynamic responses to electrical stimulation of the radial-most and ulnar-most fingers were measured in each subject using functional magnetic resonance tomography. The size of the hand area in the primary somatosensory cortex was estimated by calculating the Euclidian distance between corresponding activation peaks on the lateral postcentral gyrus. The cortical somatosensory hand representation was found to be significantly smaller in dysmelic subjects as compared with the control subjects (P <0.001). The shrinkage of the hand area was not proportional to the number of missing fingers. Furthermore, the cortical representation of the ulnar fingers in the dysmelic subjects was shifted towards the cortical thumb representation of the control group. We suggest that the unproportional reduction of the hand area together with the observed shift may reflect use-dependent rather than malformation-induced reorganization of the somatosensory hand area.

Learning of somatosensory representations for texture discrimination using a temporal coherence principle

In order to perform appropriate actions, animals need to quickly and reliably classify their sensory input. How can representations suitable for classification be acquired from statistical properties of the animal's natural environment? Akin to behavioural studies in rats, we investigate this question using texture discrimination by the vibrissae system as a model. To account for the rat's active sensing behaviour, we record whisker movements in a hardware model. Based on these signals, we determine the response of primary neurons, modelled as spatio-temporal filters. Using their output, we train a second layer of neurons to optimise a temporal coherence objective function. The performance in classifying textures using a single cell strongly correlates with the cell's temporal coherence; hence output cells outperform primary cells. Using a simple, unsupervised classifier, the performance on the output cell population is same as if using a sophisticated supervised classifier on the primary cells. Our results demonstrate that the optimisation of temporal coherence yields a representation that facilitates subsequent classification by selectively conveying relevant information.

Hand somatosensory cortex activity following selective dorsal rhizotomy: report of three cases with fMRI

Selective dorsal rhizotomy (SDR) is an effective treatment for lower extremity spasticity in cerebral palsy. Cortical organization in sensory cortex may be abnormal in cerebral palsy, and deafferentation is known to lead to cortical reorganization in many situations. We used functional magnetic resonance imaging (fMRI) of hand sensory stimulation to determine if the partial deafferentation of the lower extremity sensory system, associated with SDR, led to any alterations in the cortical somatosensory representation for the upper limbs. Three patients with spastic diplegia were studied with blood oxygen level-dependent (BOLD)-fMRI before and after SDR. fMRI during tactile stimulation of the digits of the right hand was used to map hand somatosensory cortex. Comparison of the cortical maps devoted to the hand before and after SDR assessed for cortical reorganization following partial deafferentation of the lower extremity. In the one patient with upper extremity involvement, the hand sensory representation was markedly enhanced following SDR. In the other two patients, a normal pattern, but with diminished activity, was seen compared with preoperative findings. SDR for lower limb spastic diplegia does not lead to extensive reorganization of cortex dedicated to the representation of the upper limb. An essentially normal pattern of activation was seen both before and after SDR. The relief of attention demands associated with spasticity may explain the modulation in intensity seen after SDR in the patients who exhibited no upper extremity involvement despite lower limb spasticity.

The Somatosensory Hand Representation is Shrunken in Thalidomide-Induced Dysmelia

It is well established that functional representations in the cerebral cortex expand due to excessive use]. Little is known, however, about the reverse. In this study we investigated the relationship between congenitally missing fingers and the size of the somatosensory hand representation in thalidomide embryopathy. Twenty-one subjects participated in our study, ten with normal, and eleven with malformed upper extremities due to intrauterine exposure to thalidomide. The number of developed fingers varied between two and four in the dysmelic subjects. Fingers were missing systematically from radial (D1 first finger to be missing) to ulnar (D5 last finger to remain). The somatosensory hand representation was studied using electrical stimulation of the most distant fingers of each hand in combination with MEG and fMRI. The localization of the finger representations was based on equivalent current dipoles (ECD) with localization in SI (MEG) and significant (p<0.001) activation peaks on the lateral surface of the postcentral gyrus (fMRI). To estimate the size of the somatosensory hand area and the effect of the malformation disorder, Euclidian distances between the representations of the most distant fingers and correlations between the size of the hand representation and the number of fingers were calculated. Both methods showed an agreement with similar results: The mean size of the hand area was significantly smaller in the dysmelic subjects (5.67 and 8.6mm, respectively) as compared to controls (11.26 and 20.4mm). There was a significant rank correlation (p<0.05) between the number of developed fingers and the Euclidian distance between the cortical representations of the most distant fingers (rs=0.62 and 0.69). Both methods showed a somatotopic arrangement of cortical finger representations. In the dysmelic subjects, however, the representation of the ulnar finger was shifted towards the thumb representation of the controls. Our data show an effect of congenitally missing fingers on the size of the sensory hand area. In contrast, when fingers were experimentally amputated or webbed, the size of the sensory hand area appeared unchanged. We suggest that our findings reflect the negative counterpart of lifelong use-dependent plasticity related to dysmelia-induced impairment in hand use.

Observation of action: grasping with the mind's hand

Engagement of the primary motor cortex (MI) during the observation of actions has been debated for a long time. In the present study, we used the quantitative 14C-deoxyglucose method in monkeys that either grasped 3-D objects or observed the same movements executed by humans. We found that the forelimb regions of the MI and the primary somatosensory (SI) cortex were significantly activated in both cases. Our study resolves a debate in the literature, providing strong evidence for use of MI representations during the observation of actions. It demonstrates that the observation of an action is represented in the primary motor and somatosensory cortices as is its execution. It indicates that in terms of neural correlates, recognizing a motor behavior is like executing the same behavior, requiring the involvement of a distributed system encompassing not only the premotor but also the primary motor cortex. We suggest that movements and their proprioceptive components are stored as motor and somatosensory representations in motor and somatosensory cortices, respectively, and that these representations are recalled during observation of an action.

Multiple sensory afferents to ferret pseudosylvian sulcal cortex.

While the ferret cerebral cortex is being used with increasing frequency in studies of neural processing and development, little is known regarding the organization of its associational sensory and multisensory regions. Therefore, the present investigation used neuroanatomical methods to identify non-primary visual and somatosensory representations and their potential for multisensory convergence. Tracer injections made into V1 or SI cortex labeled axon terminals within the pseudosylvian sulcal cortex (PSSC). These inputs were distributed according to modality, with visual inputs identified in the lateral aspects of the posterior dorsal bank, and somatosensory inputs found anterior along the dorsal bank, fundus and ventral bank. Somatosensory inputs showed a topographic arrangement, with inputs representing face found more anteriorly than those representing trunk regions. Overlap between these different sensory projections occurred posteriorly in the PSSC and may represent a zone of multisensory convergence. These data are consistent with the presence of associational visual, somatosensory, and multisensory areas within the PSSC.

Spatial distribution of functional superficial–deep connections in the adult ferret superior colliculus

Numerous studies have identified connections between the superficial visual and deeper multisensory layers of the superior colliculus (SC), but the functional distribution of the superficial–deep projection has not been mapped. This question was assessed in the present study using extracellular electrophysiological stimulation and recording techniques. In vitro slices from adult ferrets were used to functionally map the rostro-caudal, medio-lateral, and dorso-ventral distribution of these superficial–deep connections. For each coronal ( n=6) or parasagittal ( n=10) slice, single and multi-unit responses to electrical stimulation of a point in the superficial layers were systematically recorded at different locations along a grid (approximately 300 μm intervals) across the slice. Recording sites with similar activation thresholds were grouped on the histological reconstruction of each slice to plot the functional access of superficial stimulation site to the deeper layers. Low intensity stimulation (defined as a current threshold ≤75 μA) activated areas of the subjacent intermediate layers in most cases (75%; 12/16). Higher intensity stimuli (>75–600 μA) accessed larger areas which, in 50% of the slices, extended into the deepest layers of the SC. However, regardless of the rostro-caudal or medio-lateral position of the superficial layer stimulation site, the proportion of the deeper layers activated remained remarkably constant, although the volume of activated deep layer tissue was shifted in each case toward the central regions of the SC. This last observation argues against the precise alignment of the superficial and deep layer visual maps, suggesting instead that the arrangement of the superficial layer projection may more closely relate to the organization of deep layer auditory and/or somatosensory representations.

Connection from the dorsal column nuclei to the superior colliculus in the rat: Topographical organization and somatotopic specific plasticity in response to neonatal enucleation

Somatosensory stimuli from the body to deep and intermediate strata of the superior colliculus (SC) are relayed from the dorsal column nuclei (DCN), gracile (GrN) and cuneate (CuN). Electrophysiological studies have shown that the somatosensory representation in SC is arranged into a map-like pattern. However, there is a lack of studies confirming a morphological correlate of such an organization. On the other hand, after neonatal enucleation in rodents, somatosensory inputs ascend from their normal termination territory in intermediate and deep collicular strata to invade the more dorsally located visual strata. However, the origin of these reactive afferents has not been specified. By using anterograde (biotinylated dextran amine 10,000; BDA) and retrograde (Fluoro-Gold; FG) tracers, we studied separately the connection from GrN and CuN to the intact and neonatally deafferented SC. GrN-collicular afferents were found to terminate mainly within the periphery of the caudomedial SC quadrant, whereas CuN-collicular fibers innervated primarily the lateral part of the rostrolateral and caudolateral collicular quadrants, in a way consistent with previously described functional data. Retrograde tracing experiments using FG injected in SC confirmed this topographical arrangement. Injections of BDA in GrN or CuN of neonatally enucleated rats showed that reactive fibers reaching superficial strata are only those CuN-collicular fibers innervating the caudolateral SC quadrant, where the forelimb is represented. The present results provide an anatomical substrate for the known somatotopic organization of tactile representation in SC and further reinforce the previous proposal that the plastic reorganization of DCN-collicular afferents following neonatal enucleation constitutes a functional compensatory response.