Dissociation of spatial-, object-, and sound-coding neurons in the mediodorsal nucleus of the primate thalamus

The mediodorsal nucleus (MD) is the thalamic gateway to the prefrontal cortex, an area of the brain associated with spatial and object working memory functions. We have recorded single-neuron activities from the MD nucleus in monkeys trained to perform spatial tasks with peripheral visual stimuli and a nonspatial task with foveally presented pictures of objects and faces-tasks identical to those we have previously used to map regional specializations in the dorso- and ventro-lateral prefrontal cortex, respectively. We found that MD neurons exhibited categorical specificity-either responding selectively to locations in the spatial tasks or preferentially to specific representations of faces and objects in the nonspatial task. Spatially tuned neurons were located in parts of the MD connected with the dorsolateral prefrontal cortex while neurons responding to the identity of stimuli mainly occupied more ventral positions in the nucleus that has its connections with the inferior prefrontal convexity. Neuronal responses to auditory stimuli were also examined, and vocalization sensitive neurons were found in more posterior portions of the MD. We conclude that MD neurons are dissociable by their spatial and nonspatial coding properties in line with their cortical connections and that the principle of information segregation in cortico-cortical pathways extends to the "association" nuclei of the thalamus.     

Quantitative assessment of the timing and tuning of visual-related,saccade-related,and delay period activity in primate central thalamus

This study investigates the visuomotor properties of several nuclei within primate central thalamus. These nuclei, which might be considered components of an oculomotor thalamus (OcTh), are found within and at the borders of the internal medullary lamina. These nuclei have extensive anatomical links to numerous cortical and subcortical visuomotor areas including the frontal eye fields, supplementary eye fields, prefrontal cortex, posterior parietal cortex, caudate, and substantia nigra pars reticulata. Previous single-unit recordings have shown that neurons in OcTh respond during self-paced spontaneous saccades and to visual stimuli in the absence of any specific behavioral requirement, but a thorough account of the activity of these areas in association with voluntary, goal-directed movement is lacking. We recorded activity from single neurons in primate central thalamus during performance of a visually guided delayed saccade task. The sample consisted primarily of neurons from the centrolateral and paracentral intralaminar nuclei and paralaminar regions of the ventral anterior and ventral lateral nuclei. Neurons responsive to sensory, delay, and motor phases of the task were observed in each region, with many neurons modulated during multiple task periods. Across the population, variation in the quality and timing of saccade-contingent activity suggested participation in functions ranging from generating a saccade (presaccadic) to registering its consequences (e.g., efference copy). Finally, many neurons were found to carry spatial information during the delay period, suggesting a role for central thalamus in higher-order aspects of visuomotor control.     

Activity of neurons in cerebellar-receiving and pallidal-receiving areas of the thalamus of the behaving monkey.

1. Thalamic neurons that receive synaptic input from the globus pallidus or the cerebellar nuclei were identified in awake monkeys trained to perform an arm-reaching task. The location of electrophysiologically identified cerebellar-receiving (CR) and pallidal-receiving (PR) neurons was used to identify a total of 264 thalamic neurons in cerebellar (CB) or pallidal (GP) regions of the thalamus. 2. Stimulation in the brachium conjunctivum or white matter adjacent to the cerebellar nuclei excited 85 neurons in the thalamus at short latencies. These CR neurons were located in the oral portion of the ventral posterolateral nucleus (VPLo), in caudal portions of the ventral lateral nucleus (VLc), and in area X. 3. Stimulation in the internal globus pallidus (GPi) inhibited 10 thalamic neurons at short latency. These PR neurons were located in rostral portions of VLc, in the oral part of the ventral lateral nucleus (VLo), and in the parvicellular part of the ventral anterior nucleus (VApc). 4. There was no clear single somatotopic organization of neurons in CB and GP regions of the thalamus, as defined by "free-form" responses to passive manipulation and observation of eye movements. There was, in fact, a tendency for two representations, each, of the head/eye/mouth cells and cells with modifications of activity in response to manipulation of the arm. 5. During the hold period before illumination of a visual target, the mean firing rates and variability of discharge of arm-related CR and PR neurons did not differ significantly. This was also true for the total sample of arm-related neurons in the CB versus GP regions. 6. The activity of many neurons in both the CB and GP regions began to change before the reaching movement and, for some, before the earliest recorded changes in electromyographic (EMG) activity. The initial change was an increase in discharge for greater than 75% of the cells studied in both the CB and GP regions. 7. During the reaching task, there also was no significant difference in the time of the initial change in discharge of neurons in the CB versus GP regions of the thalamus. 8. These data are consistent with the hypothesis that the initial task-related change in discharge of PR thalamic neurons is dominated by input from the cerebral cortex and that pallidal input modulates later phases of their movement-related changes in activity.     

A new vestibular thalamic area: Electrophysiological study of the thalamic reticular nucleus and of the ventral lateral geniculate complex of the cat

Summary Single unit recordings were carried out in the reticularis thalamic nucleus (RT) and the ventral lateral geniculate body (LGv) of chronically prepared alert cats under sinusoidal vestibular stimulation in the horizontal plane. Optokinetic stimulation was also used.Of the 57 recorded neurons, 12 present vestibular modulation in the dark, analogous to Duensing's and Schaefer's (1958) type I response in the vestibular nuclei. Responses of 26 cells are similar to response of type II vestibular neurons and 14 units have a type III response; the 5 remaining cells were activated by vestibular stimulation in the vertical sagittal plane. The majority of these cells does not present detectable direct visual responses, but 50% can be driven by Optokinetic stimulation.74 % of types I, II and III neurons show saccadic responses to vestibular nystagmic saccades in the dark. About 60% present similar saccadic modulations during Optokinetic nystagmus and 55% keep this response for spontaneous saccades in the dark or in front of a striped background. The saccadic responses are constant for a given neuron in all cases of stimulation with latencies ranging from 30 msec prior to the beginning of the saccade to 120 msec after its onset.The histological localization of these units falls on one hand into the caudal part of the RT nucleus (type III neurons) above the dorsal lateral geniculate nucleus and on the other hand within the internal subdivision of the LGv and its rostral limit (all other types).The significance of this new, saccadic and vestibular focus in the feline thalamus is discussed in relation with the two previously known vestibular thalamic relays in terms of interrelations between the vestibular and the visual systems.     

Projections from the rostral mesencephalic reticular formation to the spinal cord

Summary Eye and head movements are strongly interconnected, because they both play an important role in accurately determining the direction of the visual field. The rostral brainstem includes two areas which contain neurons that participate in the control of both movement and position of the head and eyes. These regions are the caudal third of Field H of Forel, including the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) and the interstitial nucleus of Cajal with adjacent reticular formation (INC-RF). Lesions in the caudal Field H of Forel in monkey and man result in vertical gaze paralysis. Head tilt to the opposite side and inability to maintain vertical eye position follow lesions in the INC-RF in cat and monkey. Projections from these areas to extraocular motoneurons has previously been observed. We reported a study of the location of neurons in Field H of Forel and INC-RF that project to spinal cord in cat. The distribution of these fiber projections to the spinal cord are described. The results indicate that: 1. Unlike the neurons projecting to the extra-ocular muscle motoneurons, the major portion of the spinally projecting neurons are not located in the riMLF or INC proper but in adjacent areas, i.e. the ventral and lateral parts of the caudal third of the Field H of Forel and in the INCRF. A few neurons were also found in the nucleus of the posterior commissure and ventrally adjoining reticular formation. 2. Neurons in caudal Field H of Forel project, via the ventral part of the ventral funiculus, to the lateral part of the upper cervical ventral horn. This area includes the laterally located motoneuronal cell groups, innervating cleidomastoid, clavotrapezius and splenius motoneurons. At lower cervical levels labeled fibers are distributed to the medial part of the ventral horn. Projections from the caudal Field H of Forel to thoracic or more caudal spinal levels are sparse. 3. Neurons in the INC-RF, together with a few neurons in the area of the nucleus of the posterior commissure, project bilaterally to the medial part of the upper cervical ventral horn, via the dorsal part of the ventral funiculus. This area includes motoneurons innervating prevertebral flexor muscles and some of the motoneurons of the biventer cervicis and complexus muscles. Further caudally, labeled fibers are distributed to the medial part of the ventral horn (laminae VIII and adjoining VII) similar to the projections of Field H of Forel. A few INC-RF projections were observed to low thoracic and lumbosacral levels. It is argued that the neurons in the caudal Field H of Forel, which project to the spinal cord are especially involved in the control of those fast vertical head movements which occur in conjunction with saccadic eye movements. In contrast the INC-RF projections to the spinal cord are responsible for slower, smaller movements controlling the position of the head in the vertical plane.Abbreviations Aq aquaduct of Sylvius - BIC brachium of the inferior colliculus - CGL lateral geniculate body - CGLd lateral geniculate body (dorsal part) - CGLv lateral geniculate body (ventral part) - CGM medial geniculate body - CGMd medial geniculate body, dorsal part - CGMint medial geniculate body, interior division - CGMp medial geniculate body, principal part - CM centromedian thalamic nucleus - CP posterior commissure - CS superior colliculus - D nucleus of Darkschewitsch - EW nucleus Edinger-Westphal - F fornix - FR/fRF fasciculus retroflexus - Hab habenular nucleus - HPA posterior hypothalamus area - HT hypothalamus - IN interpeduncular nucleus - INC interstitial nucleus of Cajal - LD nucleus lateralis dorsalis of the thalamus - LHA lateral hypothalamic area - LP lateral posterior nucleus - LV lateral ventricle - MB mammillary body - MC nucleus medialis centralis of the thalamus - MD nucleus medialis dorsalis of the thalamus - ML medial lemniscus - MTN medial terminal nucleus - ND nucleus of Darkschewitsch - NOT nucleus of the optic tract - NOTL lateral nucleus of the optic tract - NOTM medial nucleus of the optic tract - OL olivary pretectal nucleus - OT optic tract - PAG periaqueductal gray - PC pedunculus cerebri - PCN/NPC nucleus of the posterior commissure - PP posterior pretectal nucleus - PTA anterior pretectal nucleus - PTM medial pretectal nucleus - Pul pulvinar nucleus of the thalamus - PV posterior paraventricular nucleus of the thalamus - PVG periventricular gray - R reticular nucleus of the thalamus - riMLF rostral interstitial nucleus of the MLF - RN red nucleus - SM stria medullaris - SN substantia nigra - ST subthalamic nucleus - STT stria terminalis - SUB subiculum - VB ventrobasal complex of the thalamus - VTA ventral tegmental area of Tsai - ZI zona incerta - III oculomotor nucleus On leave of absence from Dept. Anatomy Erasmus University, Rotterdam, The Netherlands     

Contextual modulation of central thalamic delay-period activity: representation of visual and saccadic goals

This study examines the influence of behavioral context on the activity of visuomotor neurons in primate central thalamus. Neurons that combine information about sensory stimuli and their behavioral relevance are thought to contribute to the decision mechanisms that link specific stimuli to specific responses. We reported in a previous study that neurons in central thalamus carry spatial information throughout the instructed delay period of a visually guided delayed saccade task. The goal of the current study was to determine whether the delay-period activity of thalamic neurons is modulated by behavioral context. Single neurons were evaluated during performance of visually guided and memory-guided variants of a saccadic choice task in which a cue designated the response field stimulus as the target of a rewarded saccade or as an irrelevant distracter. The relative influence of the physical stimulus and context on delay-period activity suggested a minimum of 3 neural groups. Some neurons signaled the locations of visible stimuli regardless of behavioral relevance. Other neurons preferentially signaled the locations of current saccadic goals and did so even in the absence of the physical stimulus. A third group signaled only the locations of currently visible saccadic goals. For the latter 2 groups, activity was the product of both stimulus and context, suggesting that central thalamic neurons play a role in the context-dependent linkage of sensory signals and saccadic commands. More generally, these data suggest that the anatomical substrate of sensorimotor decision making may include the cortico-subcortical loops for which central thalamus serves as the penultimate synapse.     

Laminar and modular organization of prefrontal projections to multiple thalamic nuclei

The prefrontal cortex projects to many thalamic nuclei, in pathways associated with cognition, emotion, and action. We investigated how multiple projection systems to the thalamus are organized in prefrontal cortex after injection of distinct retrograde tracers in the principal mediodorsal (MD), the limbic anterior medial (AM), and the motor-related ventral anterior/ventral lateral (VA/VL) thalamic nuclei in rhesus monkeys. Neurons projecting to these nuclei were organized in interdigitated modules extending vertically within layers VI and V. Projection neurons were also organized in layers. The majority of projection neurons to MD or AM originated in layer VI (∼80%), but a significant proportion (∼20%) originated in layer V. In contrast, prefrontal neurons projecting to VA/VL were equally distributed in layers V and VI. Neurons directed to VA/VL occupied mostly the upper part of layer V, while neurons directed to MD or AM occupied mostly the deep part of layer V. The highest proportions of projection neurons in layer V to each nucleus were found in dorsal and medial prefrontal areas. The laminar organization of prefrontal cortico-thalamic projections differs from sensory systems, where projections originate predominantly or entirely from layer VI. Previous studies indicate that layer V cortico-thalamic neurons innervate through some large terminals thalamic neurons that project widely to superficial cortical layers. The large population of prefrontal projection neurons in layer V may drive thalamic neurons, triggering synchronization by recruiting several cortical areas through widespread thalamo-cortical projections to layer I. These pathways may underlie the synthesis of cognition, emotion and action.     

Differential projection of the posterior paralaminar thalamic nuclei to the amygdaloid complex in the rat

The thalamic paralaminar nuclei that border the medial and ventral edges of the medial geniculate body, viz. the suprageniculate nucleus (SG), the posterior intralaminar nucleus (PIN), the medial division of the medial geniculate nucleus (MGm), and the peripeduncular nucleus (PP), are regarded as important extralemniscal relay nuclei for sensory stimuli and as an important link for the direct transmission of sensory stimuli to the amygdala. Each of these thalamic nuclei receives a unique pattern of afferent input but an unresolved question is, how each of these thalamic nuclei project to the amygdala and whether there are zones of convergence and/or non-overlapping regions within amygdaloid target nuclei. Small injections of PHA-L or Miniruby, which were made into single thalamic nuclei at different rostrocaudal levels, revealed a non-uniform distribution of anterogradely labeled axons within the amygdaloid complex. Injections into the SG, MGm, and rostral PIN predominantly labeled axons in the laterodorsal and lateroventral portions of the lateral nucleus of the amygdala (LA). Axons from the MGm were located rather in the dorsal part of the LA, whereas SG-derived axons were concentrated in the ventrolateral part of the LA. Injections into the PP labeled axons predominantly in the medial part of the LA, whereas after injections into the caudal PIN axons were seen in the entire LA. In addition, the PIN projects heavily to the anterior basomedial nucleus and medial division of the central nucleus, whereas this projection is virtually absent from the other thalamic nuclei. The lateral part of the central nucleus and the basal nucleus of the amygdala are spared by axons from the thalamic paralaminar nuclei. The present results suggest that, despite a considerable degree of convergence of the thalamoamygdaloid projection in the lateral nucleus, each thalamic nucleus plays a unique role in the transmission of sensory stimuli to the amygdala and in the modulation of intraamygdaloid circuits.     

Subcortical projections to the prefrontal cortex in the rat as revealed by the horseradish peroxidase technique

The sources and distribution of subcortical afferents to the anterior neocortex were investigated in the rat using the horseradish peroxidase technique. Injections into the prefrontal cortex labelled, in addition to the mediodorsal thalamic nucleus, neurons in a total of fifteen subcortical nuclei, distributed in the basal telencephalon, claustrum, amygdala, thalamus, subthalamus, hypothalamus, mesencephalon and pons. Of these, the projections from the zona incerta, the lateroposterior thalamic nucleus, and the parabrachial region of the caudal mesencephalon to the prefrontal cortex have not previously been described.Different parts of the mediodorsal thalamic nucleus project to different areas of the frontal cortex. Thus, horseradish peroxidase injections in the most ventral pregenual part of the medial cortex labelled predominantly neurons in the medial anterior and dorsomedial posterior parts of the mediodorsal nucleus; injections into the more dorsal pregenual area labelled only neurons in the lateral and ventral parts of the nucleus; injections placed supragenually labelled neurons in the dorsolateral posterior part of the nucleus; and injections into the dorsal bank of the anterior rhinal sulcus labelled neurons in the centromedial part of the nucleus.Several other subcortical nuclei had projections overlapping with that of the mediodorsal thalamic nucleus. Five different types of such overlap were distinguished: (1) cell groups labelled after horseradish peroxidase injections into one of the subfields of the projection area of the mediodorsal nucleus (defined as the prefrontal cortex), but not outside this area (parataenial nucleus of the thalamus); (2) cell groups labelled both after injection into a subfield of the projection area of the mediodorsal nucleus and after injections in a restricted area outside this area (anteromedial, ventral and laterposterior thalamic nuclei); (3) cell groups labelled after injections into all subfields of the mediodorsal nucleus projection area, but not outside this area (ventral tegmental area, basolateral nucleus of amygdala); (4) cell groups labelled after injections into any area of the anterior neocortex, including the mediodorsal nucleus projection area (parabrachial neurons of the posterior mesencephalon); (5) cell groups labelled after all neocortical injections investigated (claustrum, magnocellular nuclei of the basal forebrain, lateral hypothalamus, zona incerta, intralaminar thalamic nuclei, nuclei raphe dorsalis and centralis superior, and locus coeruleus).We can draw the following conclusions from these and related findings. First, because of the apparent overlap of projections of the mediodorsal, the anteromedial and ventral thalamic nuclei in the rat, parts of the prefrontal cortex can also be called ‘cingulate’ and ‘premotor’. Second, on the basis of projections from parts of the mediodorsal nucleus, the prefrontal cortex of the rat can be subdivided into areas corresponding to those in other species. Third, the neocortex receives afferents from a large number of subcortical cell groups outside the thalamus, distributed from the telencephalon to the pons; however, the prefrontal cortex seems to be the only neocortical area innervated by the ventral tegmental area and amygdala. Finally, neither the prefrontal cortex nor the mediodorsal thalamic nucleus receives afferents from regions directly involved in sensory and motor functions.     

Corollary discharge circuits for saccadic modulation of the pigeon visual system

A saccadic eye movement causes a variety of transient perceptual sequelae that might be the results of corollary discharge. Here we describe the neural circuits for saccadic corollary discharge that modulates activity throughout the pigeon visual system. Saccades in pigeons caused inhibition that was mediated by corollary discharge followed by enhancement of firing activity in the telencephalic hyperpallium, visual thalamus and pretectal nucleus lentiformis mesencephali (nLM) with opposite responses in the accessory optic nucleus (nBOR). Inactivation of thalamic neurons eliminated saccadic responses in telencephalic neurons, and inactivation of both the nLM and the nBOR abolished saccadic responses in thalamic neurons. Saccade-related omnipause neurons in the brainstem raphe complex inhibited the nBOR and excited the nLM, whereas inactivation of raphe neurons eliminated saccadic responses in both optokinetic and thalamic neurons. It seems that saccadic responses in telencephalic neurons are generated by corollary discharge signals from brainstem neurons that are transmitted through optokinetic and thalamic neurons. These signals might have important roles in visual perception.     

Segregation and heterogeneity of thalamic cell populations projecting to superficial layers of posterior parietal cortex: a retrograde tracer study in cat and monkey.

The thalamic neurons projecting to the superficial layers of areas 5 and 7 in the cat, and area 5 in the monkey, were investigated by using superficial deposits of either horseradish peroxidase or Fast Blue in one hemisphere. In the contralateral hemisphere injections of the same tracer involving the full cortical depth were made in homotopical locations, and the distribution and soma size of retrogradely labeled thalamocortical neurons in each side of the thalamus were compared. It was found that, in the cat, labeled neurons in the lateral posterior pulvinar complex, and in paralaminar regions of the ventrolateral complex, were fewer in number and smaller in size in cases of superficial deposits than in cases of deep injection. In more lateral portions of the ventrolateral complex, however, there were no size differences. In the monkey, similar differences in number and size appeared in the caudal division of the ventrolateral complex and in the lateral posterior and pulvinar nuclei, whereas no such differences were found for neurons labeled in the oral and medial divisions of the ventrolateral complex, and in the ventral posteroinferior nucleus. In all cases the intralaminar and midline nuclei exhibited retrogradely labeled neurons only when deep layers were injected. These and previous findings point to the existence of a widely distributed layer I-projecting system of neurons which, in most nuclei, are interspersed among neurons projecting mainly to middle or deep layers. In some nuclei, however, as is the case with the ventromedial nucleus proper, layer I-projecting system neurons would make up the whole nucleus. The cell groups located in a paralaminar position, which would be but a part of this system, could provide through their projections to layer I in the posterior parietal and frontal cortical regions a final path for recruiting responses and spontaneous spindling activities.     

Neuronal activity related to saccadic eye movements in the monkey's dorsolateral prefrontal cortex

1. Single-neuron activity was recorded from the prefrontal cortex of monkeys performing saccadic eye movements in oculomotor delayed-response (ODR) and visually guided saccade (VGS) tasks. In the ODR task the monkey was required to maintain fixation of a central spot throughout the 0.5-s cue and 3.0-s delay before making a saccadic eye movement in the dark to one of four or eight locations where the visual cue had been presented. The same locations were used for targets in the VGS tasks; however, unlike the ODR task, saccades in the VGS tasks were visually guided. 2. Among 434 neurons recorded from prefrontal cortex within and surrounding the principal sulcus (PS), 147 changed their discharge rates in relation to saccadic eye movements in the ODR task. Their response latencies relative to saccade initiation were distributed between -192 and 460-ms, with 22% exhibiting presaccadic activity and 78% exhibiting only postsaccadic activity. Among PS neurons with presaccadic activity, 53% also had postsaccadic activity when the monkey made saccadic eye movements opposite to the directions for which the presaccadic activity was observed. 3. Almost all (97%) PS neurons with presaccadic activity were directionally selective. The best direction and tuning specificity of each neuron were estimated from parameters used to fit a Gaussian tuning curve function. The best direction for 62% of the neurons with presaccadic activity was toward the contralateral visual field, with the remaining neurons having best directions toward the ipsilateral field (23%) or along the vertical meridian (15%). 4. Most postsaccadic activity of PS neurons (92%) was also directionally selective. The best direction for 48% of these neurons was toward the contralateral visual field, with the remaining neurons having best directions toward the ipsilateral field (36%) or along the vertical meridian (16%). Eighteen percent of the neurons with postsaccadic activity showed a reciprocal response pattern: excitatory responses occurred for one set of saccade directions, whereas inhibitory responses occurred for roughly the opposite set of directions. 5. Sixty PS neurons with saccade-related activity in the ODR task were also examined in a VGS task. Forty of these neurons showed highly similar profiles of directional specificity and response magnitude in both tasks, 13 showed saccade-related activity only in the ODR task, and 7 changed their response characteristics between the ODR and VGS tasks.(ABSTRACT TRUNCATED AT 400 WORDS)     

Thalamic afferents of area 4 and 6 in the dog: a multiple retrograde fluorescent dye study

In the present study, we compared the distribution of thalamocortical afferents of cortical area 4 to that of cortical area 6 in the dog, using fluorescent tracers. Multiple injections of combinations of two dyes (diamidino yellow dihydrochloride, Evans blue, fast blue, granular blue) were made into either the anterior and posterior sigmoid gyri or into the medial and lateral regions of the anterior sigmoid gyrus in the anesthetized dog. We found that the thalamic afferents of areas 4 and 6 arise from topographically organized bands of cells that traverse several thalamic nuclei and extend throughout the rostrocaudal extent of the thalamus. The most medial band included area 6-projecting neurons in the anterior nuclei, the rhomboid nucleus, the ventral anterior nucleus (VA), ventromedial nucleus (VM) and mediodorsal nucleus (MD). Within this band, cells projecting to medial area 6a tended to be more numerous in the anterior nuclei, anterior parts of VA and VM and anterior and caudal parts of MD. Fewer cells in MD but more cells in caudal parts of VA and VM projected to lateral area 6 a. Lateral bands of cells in central through lateral parts of VA and VL projected topographically to lateral area 4 on the anterior sigmoid gyrus and lateral through medial parts of postcruciate area 4. The most lateral band of cells in VL continued ventrally into the zona incerta. Area 4 also received input from VM and the central lateral (CL) and centrum medianum (CM) nuclei. Within regions of VA, VL and VM, cells from one band interspersed with cells from another, but there were very few double-labeled cells projecting to two cortical sites. When the present results are compared with our previous findings on the distribution of subcortical afferents to the motor thalamus, it appears that separate motor cortical areas may receive predominantly separate but also partially over-lapping pathways in the dog.Abbreviations AV Anterior ventral nucleus - AM anterior medial nucleus - Cb cerebellar nuclei - CeM central medial nucleus - CL central lateral nucleus - CM centrum medianum nucleus - EN entopeduncular nucleus - Hb habenula - LD lateral dorsal nucleus - MD mediodorsal nucleus - mt mammillothalamic tract - MV medioventral nucleus - Pf parafascicular nucleus - R reticular thalamic nucleus - rf retroflex fasciculus - Rh rhomboid nucleus - SN substantia nigra - VA ventral anterior nucleus - VL ventral lateral nucleus, principal division - VLd ventral lateral nucleus, dorsal division - VM ventral medial nucleus - VPL ventral posterior lateral nucleus - ZI zona incerta     

Cyto- and chemoarchitecture of the dorsal thalamus of the monotreme Tachyglossus aculeatus,the short beaked echidna

We have examined the cyto- and chemoarchitecture of the dorsal thalamus of the short beaked echidna (Tachyglossus aculeatus), using Nissl and myelin staining, immunoreactivity for parvalbumin, calbindin, calretinin and non-phosphorylated neurofilament protein (SMI-32 antibody), and histochemistry for acetylcholinesterase and NADPH diaphorase. Immunohistochemical methods revealed many nuclear boundaries, which were difficult to discern with Nissl staining. Parvalbumin immunoreactive somata were concentrated in the ventral posterior, reticular, posterior, lateral and medial geniculate nuclei, while parvalbumin immunoreactivity of the neuropil was present throughout all but the midline nuclei. Large numbers of calbindin immunoreactive somata were also found within the midline thalamic nuclei, and thalamic sensory relay nuclei. Immunoreactivity for calretinin was found in many small somata within the lateral geniculate “a” nucleus, with other labelled somata found in the lateral geniculate “b” nucleus, ventral posterior medial and ventral posterior lateral nuclei. Immunoreactivity with the SMI-32 antibody was largely confined to somata and neuropil within the thalamocortical relay nuclei (ventral posterior medial and lateral nuclei, lateral and medial geniculate nuclei and the posterior thalamic nucleus). In broad terms there were many similarities between the thalamus of this monotreme and that of eutheria (e.g. disposition of somatosensory thalamus, complementarity of parvalbumin and calbindin immunoreactive structures), but there were some unique features of the thalamus of the echidna. These include the relatively small size of the thalamic reticular nucleus and the preponderance of calbindin immunoreactive neurons over parvalbumin immunoreactive neurons in the ventral posterior nucleus.     

Cyto- and chemoarchitecture of the dorsal thalamus of the monotreme Tachyglossus aculeatus, the short beaked echidna

We have examined the cyto- and chemoarchitecture of the dorsal thalamus of the short beaked echidna (Tachyglossus aculeatus), using Nissl and myelin staining, immunoreactivity for parvalbumin, calbindin, calretinin and non-phosphorylated neurofilament protein (SMI-32 antibody), and histochemistry for acetylcholinesterase and NADPH diaphorase. Immunohistochemical methods revealed many nuclear boundaries, which were difficult to discern with Nissl staining. Parvalbumin immunoreactive somata were concentrated in the ventral posterior, reticular, posterior, lateral and medial geniculate nuclei, while parvalbumin immunoreactivity of the neuropil was present throughout all but the midline nuclei. Large numbers of calbindin immunoreactive somata were also found within the midline thalamic nuclei, and thalamic sensory relay nuclei. Immunoreactivity for calretinin was found in many small somata within the lateral geniculate “a” nucleus, with other labelled somata found in the lateral geniculate “b” nucleus, ventral posterior medial and ventral posterior lateral nuclei. Immunoreactivity with the SMI-32 antibody was largely confined to somata and neuropil within the thalamocortical relay nuclei (ventral posterior medial and lateral nuclei, lateral and medial geniculate nuclei and the posterior thalamic nucleus). In broad terms there were many similarities between the thalamus of this monotreme and that of eutheria (e.g. disposition of somatosensory thalamus, complementarity of parvalbumin and calbindin immunoreactive structures), but there were some unique features of the thalamus of the echidna. These include the relatively small size of the thalamic reticular nucleus and the preponderance of calbindin immunoreactive neurons over parvalbumin immunoreactive neurons in the ventral posterior nucleus.     

The thalamic connections of motor, premotor, and prefrontal areas of cortex in a prosimian primate (Otolemur garnetti)

Connections of motor areas in the frontal cortex of prosimian galagos (Otolemur garnetti) were determined by injecting tracers into sites identified by microstimulation in the primary motor area (M1), dorsal premotor area (PMD), ventral premotor area (PMV), supplementary motor area (SMA), frontal eye field (FEF), and granular frontal cortex. Retrogradely labeled neurons for each injection were related to architectonically defined thalamic nuclei. Nissl, acetylcholinesterase, cytochrome oxidase, myelin, parvalbumin, calbindin, and Cat 301 preparations allowed the ventral anterior and ventral lateral thalamic regions, parvocellular and magnocellular subdivisions of ventral anterior nucleus, and anterior and posterior subdivisions of ventral lateral nucleus of monkeys to be identified. The results indicate that each cortical area receives inputs from several thalamic nuclei, but the proportions differ. M1 receives major inputs from the posterior subdivision of ventral lateral nucleus while premotor areas receive major inputs from anterior parts of ventral lateral nucleus (the anterior subdivision of ventral lateral nucleus and the anterior portion of posterior subdivision of ventral lateral nucleus). PMD and SMA have connections with more dorsal parts of the ventral lateral nucleus than PMV. The results suggest that galagos share many subdivisions of the motor thalamus and thalamocortical connection patterns with simian primates, while having less clearly differentiated subdivisions of the motor thalamus.     

Acetylcholinesterase histochemistry in the macaque thalamus reveals territories selectively connected to frontal, parietal and temporal association cortices

The patterns of histochemical staining for acetylcholinesterase (AChE) activity in the macaque thalamus were analyzed and compared with the distribution of cells and terminals labeled from injections of axonal tracers in the dorsolateral and orbital prefrontal cortex, in area 7a of the posterior parietal cortex and in the polysensory cortex of the superior temporal sulcus. AChE histochemistry is very useful in delineating the thalamic nuclei connected with the association cortex and in uncovering thalamic subdivisions that are barely evident on cytoarchitectonic grounds. Moreover, AChE activity reveals previously unrecognized heterogeneities within several thalamic nuclei, like the ventral anterior (VA), where a new ventromedial subdivision (VAvm) is described, the medial pulvinar (PuIM) or the mediodorsal nucleus (MD). In this nucleus three distinct chemical domains are present: the medial, ventral and lateral sectors characterized by low, moderate and high AChE activities, respectively. The staining pattern of the lateral sector is markedly heterogeneous with patches of intense AChE activity surrounded by a moderately stained matrix. The MD medial sector is connected with the orbitofrontal cortex, whereas the AChE-rich patches in the lateral sector are selectively connected with the dorsolateral prefrontal, parietal and temporal association cortices. In the PulM, a dorsomedial AChE-rich patch is selectively connected with the orbitofrontal cortex, whereas the surrounding territory, which shows moderate AChE activity, is preferentially connected with the parietal and temporal cortices. Chemically specific domains in the anterior, ventral anterior, midline, and intralaminar thalamic nuclei are also connected with the examined association cortices. These findings indicate that the topographic patterns of the thalamo-cortical connections of primate association areas conform to the chemical architecture of the thalamus. This implies that because each cortical area is connected to a particular set of thalamic regions, the influence of the thalamus on cortical function is exclusive for each area, highly diverse among the various association areas, and subject to a wide range of modulation at the thalamic level.     

Organization of the thalamo-cortical connexions to the frontal lobe in the rhesus monkey

Summary In 25 rhesus monkeys horseradish peroxidase was injected in different parts of the frontal cortex. The retrogradely labelled thalamic neurons formed longitudinal bands, some of which crossed the internal medullary lamina, and extended from one thalamic nucleus into another. On the basis of these findings the frontal cortex was subdivided into seven transverse cortical strips which receive afferents from seven longitudinal bands of thalamic neurons. The most rostral transverse strip receives afferents from the most medial thalamic band which is oriented vertically and extends through the most medial part of the MD into the medial pulvinar. Progressively more caudally located transverse strips receive afferents from progressively more laterally located thalamic bands which in part are situated in the VL and show an increasing tilt towards the horizontal. Moreover, those parts of the various bands which are situated along the dorsal and lateral margin of the thalamus project to the medial portions of the transverse cortical strips, i.e. along the medial margin of the frontal lobe, while the other parts situated ventromedially in the thalamus project to the lateral portions of these strips, i.e. along the lateral margin of the frontal lobe.These data provide an alternative view of the organization of the thalamus and suggest that this structure contains a matrix of longitudinal cell columns which in some cases extend across specific nuclear borders and may represent the basic thalamic building blocks in respect to the thalamo-cortical connexions.     

Re-examination of the thalamostriatal projections in the rat with retrograde tracers

Topographical arrangements of thalamostriatal projection neurons was examined in the rat by the retrograde tract-tracing method. After injecting Fluoro-Gold (FG) and/or cholera toxin beta-subunit (CTB) in different regions of the caudate-putamen (CPu), distribution of retrogradely labeled neurons was observed in the thalamus. The main findings were as follows: (1) Retrogradely labeled neurons were seen in the midline-intralaminar thalamic nuclei in all rats examined in the present study.Neurons in the ventral lateral and posterior thalamic nuclear groups were also labeled in the rats which were injected with the tracer into the dorsal part of Cpu, but not in the rats which were injected with the tracer into the nucleus accumbens (Acb) and its adjavent regions in the ventromedial part of the Cpu. (2) Topographical organization was observed in the projections from the midline-intralaminar thalamic nuclei to the CPu. After the tracer injection into the dorsal part of the CPu or the ventral part of the CPu (including the Acb), labeled neurons in the midline-intralaminar thalamic nuclei were distributed predominantly in the lateral part of the intralaminar nuclei or the midline nuclei, respectively. On the other hand, after the tracer injection into the medial or the lateral part of the CPu, labeled neurons in the midline-intralaminar nuclei were distributed mainly in the dorsal or the ventral part of these nuclei, respectively. (3) Topographical organization was also observed in the thalamostriatal projections from the ventral and Pos. After the tracer injection into the rostral part of the CPu, labeled neurons were distributed mainly in the rostral part of the ventral nuclear group. On the other hand, after the tracer injection into the caudal part of the CPu, labeled neurons were distributed mainly in the caudal part of the ventral nuclear group, as well as in the posterior nuclear group.     

An autoradiographic study of topographical relationships between pallidal and cerebellar projections to the cat thalamus

Summary Injections of ³H-leucine were made in the entopeduncular nucleus or dentate nucleus of the cerebellum in eight cats. The terminal projection zones of both pathways in the thalamus were studied using the sagittal plane and their relationships to one another as well as to cytoarchitectural boundaries of thalamic nuclei were compared. The data indicate that the territories controlled by the two projection systems are almost entirely segregated. The segregation is mainly along the antero-posterior axis as the main pallidal projection zone occupies the medio-ventral VA while the main dentate projection zone lies posterior to it in the VL. Furthermore, the dorsolateral part of the VA not occupied by pallidal projections receives dentate projections. In the VM, both afferent systems terminate in the lateral part of the nucleus with pallidal territory located anteriorly and dentate territory located posteriorly, again without overlap. As the delineations of nuclear subdivisions in the ventral thalamus of the cat have been a subject of some controversy, it is suggested that the boundaries of the VA, VL and VM in the cat thalamus be defined on the basis of basal ganglia and cerebellar projection zones.Abbreviations used in the Text and in Fig. 5 AM anterior medial nucleus - AV anterior ventral nucleus - BC brachium conjunctivum - CA anterior commissure - CC crus cerebri - CP posterior commissure - CD caudate nucleus - CE centrum medianum - CLN central lateral nucleus - DN dentate nucleus - EPN entopeduncular nucleus - FF Forel's field - FN fastigial nucleus - FR fasciculus retroflexus - HL lateral habenular nucleus - HM medial habenular nucleus - INA anterior interposite nucleus - INP posterior interposite nucleus - IC internal capsule - LD lateral dorsal nucleus - LG lateral geniculate body - MD medial dorsal nucleus - MTT mamillothalamic tract - NR red nucleus - OT optic tract - PAC paracentral nucleus - PF parafascicular nucleus - PV pulvinar - RT reticular thalamic nucleus - SM submedian nucleus - SN substantia nigra - SNr substantia nigra pars reticularis - STN subthalamic nucleus - VF ventral posterior nucleus - VA ventral anterior nucleus - VL ventral lateral nucleus - VM ventral medial nucleus - ZI zona incerta Supported in part by a grant from the American Parkinson Disease Association and NIH grant R01NS19280