Spatial resolution of fMRI in the human parasylvian cortex: comparison of somatosensory and auditory activation.

In spite of its outstanding spatial resolution, the biological resolution of functional MRI may be worse because it depends on the vascular architecture of the brain. Here, we compared the activation patterns of the secondary somatosensory and parietal ventral cortex (SII/PV) with that of the primary auditory cortex and adjacent areas (AI/AII). These two brain regions are located immediately adjacent to each other on opposite banks of the Sylvian fissure, and are anatomically and functionally distinct. In 12 healthy subjects, SII/PV was activated by pneumatic tactile stimuli applied to the index finger (0.5 cm2 contact area, 4 bar pressure), and AI/AII by amplitude-modulated tones (800 Hz carrier frequency, modulated at 24-36 Hz). Functional images were obtained with a 1.5-T scanner and were evaluated using SPM99. Sensitivity of fMRI activation in this unselected sample was 71% for tactile and 83% for auditory stimulation. Group analysis showed activation of SII/PV by tactile and activation of three locations in AI/AII by auditory stimuli. Distributions extended to the opposite side of the fissure (19-58% after tactile and 13-14% after auditory stimulation, depending on the side of stimulation/hemisphere). Morphometry of individual sulcal anatomy revealed that the course of the Sylvian fissure varied by 5.3 mm (SD) in vertical direction. Taking this into account, SII/PV was located 5.8 +/- 2.7 mm above the Sylvian fissure, whereas AI/AII was located 6.3 +/- 1.7 mm below the Sylvian fissure. Even in individual analysis, the most significant voxel after tactile stimuli in one subject was found on the "wrong" side of the fissure; this error could be ascribed to the spatial normalization procedure. These data show that fMRI signals may overlap substantially, even if the activated regions are separated by 12 mm across a major sulcus. Spatial normalization to an atlas template can introduce additional variance. Individual sulcal anatomy should be preferred over mean atlas locations.

Absence of a prevalent laminar distribution of IPSPs in association cortical neurons of cat.

Absence of a prevalent laminar distribution of IPSPs in association cortical neurons of cat. J. Neurophysiol. 78: 2742-2753, 1997. The depth distribution of inhibitory postsynaptic potentials (IPSPs) was studied in cat suprasylvian (association) cortex in vivo. Single and dual simultaneous intracellular recordings from cortical neurons were performed in the anterior part of suprasylvian gyrus (area 5). Synaptic responses were obtained by stimulating the suprasylvian cortex, 2-3 mm anterior to the recording site, as well as the thalamic lateral posterior (LP) nucleus. Neurons were recorded from layers 2 to 6 and were classified as regular spiking (RS, n = 132), intrinsically bursting (IB, n = 24), and fast spiking (FS, n = 4). Most IB cells were located in deep layers (below 0.7 mm, n = 19), but we also found some IB cells more superficially (between 0.2 and 0.5 mm, n = 5). Deeply lying corticothalamic neurons were identified by their antidromic invasion on thalamic stimulation. Neurons responded with a combination of excitatory postsynaptic potentials (EPSPs) and IPSPs to both cortical and thalamic stimulation. No consistent relation was found between cell type or cell depth and the amplitude or duration of the IPSPs. In response to thalamic stimulation, RS cells had IPSPs of 7.9 +/- 0.9 (SE) mV amplitude and 88.9 +/- 6.4 ms duration. In IB cells, IPSPs elicited by thalamic stimulation had 7.4 +/- 1.3 mV amplitude and 84.7 +/- 14.3 ms duration. The differences between the two (RS and IB) groups were not statistically significant. Compared with thalamically elicited inhibitory responses, cortical stimulation evoked IPSPs with higher amplitude (12.3 +/- 1.7 mV) and longer duration (117 +/- 17.3 ms) at all depths. Both cortically and thalamically evoked IPSPs were predominantly monophasic. Injections of Cl- fully reversed thalamically as well as cortically evoked IPSPs and revealed additional late synaptic components in response to cortical stimulation. These data show that the amount of feed forward and feedback inhibition to cat's cortical association cells is not orderly distributed to distinct layers. Thus local cortical microcircuitry goes beyond the simplified structure determined by cortical layers.

Transgeniculate signal transmission to middle suprasylvian cortex in intact cats and following early removal of areas 17 and 18: a morphological study.

Removal of cat areas 17 and 18 early, but not late, in postnatal development results in the sparing of certain reflexive and nonreflexive visually guided behaviors. These spared behaviors are accompanied by an expansion of geniculocortical projections to middle suprasylvian (MS) cortex. However, little is known about the types of visual signals relayed along these pathways. The purpose of our study was to reveal the morphologies of the neurons participating in the rewired circuits and, by relating them to the morphologies of functionally characterized neurons described by others, infer the types of visual signals transmitted via the lateral geniculate nucleus (LGN) to MS cortex. To do this, we retrogradely labeled LGN neurons from MS cortex with fluorescent microspheres, and subsequently intracellularly filled them with Lucifer Yellow. We then classified well-filled neurons according to a battery of morphological parameters (such as soma size and shape, and dendritic field-form and specializations), and compared them with already defined structure/function relationships. By doing this, we found that the large majority of visual thalamic relay neurons to MS cortex of both normal cats and cats that incurred removal of areas 17 and 18 were types I and IV. These results indicate that visual Y and W signals, respectively, are relayed directly from LGN to MS cortex in both types of cats. Following the early lesions, some of the MS-projecting type I neurons were found in layers A and A1, where they are never found in intact cats. Thus, some layer A and A1 type I neurons redirect axons to MS cortex following early removal of areas 17 and 18. For the type IV MS-projecting neurons in early lesioned cats, the somas were hypertrophied and they had more profuse and broader dendritic arbors than equivalent neurons in intact cats. These results suggest that dynamic interactions take place between inputs and outputs of LGN neurons during development that influence final LGN neuron morphology. Moreover, they suggest that signals transferred to MS cortex by type IV neurons may be modified by early lesions of areas 17 and 18. Overall, these results contribute to our understanding of the types of behaviors that may be spared by early lesions of areas 17 and 18.

Thalamic and cortical projections to middle suprasylvian cortex of cats: constancy and variation.

We investigated the constancy and variability in the numbers of thalamic and cortical neurons projecting to cat middle suprasylvian (MS) visual cortex. Retrograde pathway tracers were injected at a single anatomically and physiologically defined locus in MS cortex. Counts of labeled neurons showed that the visual thalamic projections to MS cortex consistently arose from a fixed set of nuclei in relatively constant proportions. In contrast, counts of cortical neurons revealed that transcortical inputs to MS cortex were much more variable. This differential variability may be linked to the developmental program, which affords greater influence of experiential factors on cortical pathway development than on thalamocortical pathway development. These results have implications for the development of models of cerebral connectivity that include measures of pathway variability.

Thalamic projections to the posterior sylvian and posterior ectosylvian gyri of the sheep brain, revealed with the retrograde transport of horseradish peroxidase.

The auditory area of the sheep cerebral cortex was studied on the basis of its afferents from the medial geniculate nucleus, traced with the horseradish peroxidase retrograde transport method. The results show that the medial geniculate nucleus projects only to the anterior parts of the posterior ectosylvian gyrus and the posterior sylvian gyrus. A small area of the posterior ectosylvian gyrus receives afferents exclusively from the ventral part of the medial geniculate nucleus, while the anterior part of the posterior sylvian gyrus receives also afferents from the posterior nucleus of the thalamus and the pulvinar. In addition, it was found that the medial part of the medial geniculate nucleus projects in a sparse way to the auditory cortex. The middle part of the posterior ectosylvian gyrus receives afferents from the posterior nucleus of the thalamus, the suprageniculate nucleus and the pulvinar, while the posterior part of the posterior ectosylvian gyrus together with the posteriormost part of the posterior sylvian gyrus receive afferents from the pulvinar. Finally, the area located between the anterior and the posteriormost part of the posterior sylvian gyrus receives afferents from both the posterior nucleus of the thalamus and the pulvinar.

Somatosensory evoked potentials from the human brain-stem: origins of short latency potentials.

Short latency somatosensory evoked potentials (SEPs) were recorded in man from an array of electrodes within the IVth (the pons) and IIIrd ventricles (the thalamus) and the aqueduct of Sylvius (the midbrain). The slow negative (N15 and N20) and positive (P22) waves were preceded by 4 small positive (P9, P11, P13, and P14) waves by approximately 1 msec duration in scalp recordings. The sources of these waves have been differentiated on the basis of their timing and spatial gradients of corresponding intracranial potentials. While P11 is identified as volume conducted from below the brain-stem, P13 and P14 reflect synchronized volleys of the medial lemniscus and its branches to the various pontine and mesencephalic nuclei. In contrast to these earlier positive wavelets arising from fiber tracts, N15 may represent postsynaptic activities within these pontine and midbrain nuclei. N20 and P22 are the initial responses of the juxtarolandic cortex.

Periaqueductal gray inhibition of trigeminal subnucleus caudalis unitary responses evoked by dentine and nonnoxious facial stimulation.

The possible pain inhibitory effects of periaqueductal gray (PAG) stimulation were investigated in cats anesthetized with Nembutal and immobilized with Flaxedil. Unitary responses evoked by electrical stimulation of the upper canine dentine and by cutaneous facial noxious and nonnoxious stimuli were recorded extracellularly from the trigeminal subnucleus caudalis. A bipolar electrode was introduced into the PAG to test the effects of PAG excitation on the trigeminal response to dentine (TRED) and cutaneous nonnoxious stimulation. In some experiments, a similar electrode was lowered into the contralateral posterior thalamus to study the antidromic activation of subnucleus caudalis cells and the effects of thalamic stimulation on the TRED. Dentine stimulation evoked brief (6- to 15-ms) bursts of 1 to 10 spikes with 3- to 25-ms latencies. Most units (88%) were also activated by cutaneous facial stimulation. Stimulation of the posterior thalamus had no effect on the TRED or on responses to cutaneous stimulation, but activated antidromically 10% of the units. In 71% of the units PAG stimulation inhibited the TRED. In some of those cases (12%), the inhibitory effect persisted 30- to 60 s. The PAG stimulation could produce paradoxical effects, potentiating the TRED evoked by threshold intensity and inhibiting the TRED elicited by suprathreshold stimulation. About one-half the PAG points evoked detectable effects. Their location had no clear topographical distribution, although ventral sites were more potent than dorsal sites. Responses evoked by nonnoxious facial stimulation were also inhibited by the PAG.

Single unit activity in cat prefrontal and posterior association cortex during performance of spatial reversal tasks .

Relationships between the performance of complex learning tasks and the firing patterns of single unites in prefrontal and posterior association areas of the neocortex were studied in 9 cats. Recording data from 2 additional cats served as controls for sensory and motor aspects of the tasks employed. During recordings, animals sat in a box, their heads fixed to a stereotaxic instrument, and performed a non-sensory spatial reversal or delayed-alternation task by pressing 1 of 2 lateralized retractable levers. Units obtained were classified into 5 basic types according to their correlation with aspects of the tasks. The first 3 types were related directly to sensory, motor and motivational aspects of the tasks. The fourth type was named associative, as it contained units which changed firing rates in advance or as a consequence of environmental events, rather than in direct relation. Discharge changes of units of the fifth type were rated as non-specific or non-specifiable. Between recording areas and tasks a striking similarity of unit firing patterns was obtained. Differences were observed only between the reactivity of polymodal cells in prefrontal and in posterior fields, and in the proportions of units revealing mnemonic and motor response aspects. It was concluded that a considerable functional overlap between prefrontal and posterior association areas of the cat with respect to processing of complex learning tasks exists on the single neuron level.

Analgesia from rostral brain stem stimulation in the rat.

Rats implanted with bipolar stimulating electrodes in the rostral medial brain stem were tested for brain stimulation-produced analgesia using tail-flick, pinch and hot-plate tests. Potent analgesia across all three tests was obtained from stimulation of sites in the gray matter surrounding the aqueduct and the caudal portion of the third ventricle, the posterior hypothalamus, the midline area of the caudal thalamus and the pretectal region of the meso-diencephalic junction. The analgesia obtained from these sites was comparable to that produced by stimulation of the previously studied caudal periaqueductal gray matter: it outlasted the period of brain stimulation, was not due to a generalized motor debilitation of the animal, and was not correlated with changes in electrographic activity. Stimulation of sites in the caudal thalamus and pretectal area yielded analgesia without stimulation-induced aversive reactions, confirming the potential of these sites for use in the relief of clinical pain in man.

[Stimulation of the periaqueductal gray matter for pain : electrical activity and evoked potentials (author's transl)]. / Effets de la stimulation de la substance grise périaqueductale chez l'homme sur l'activité spontanée et évoquée.

There is some evidence that stimulation of the periaqueductal gray matter produced analgesia is accomplished by the activation of neuronal systems where endorphins are thought to be transmittors. However, the neurophysiological mechanism of this stimulation is not elucided. In this preliminary paper, the spontaneous and the evoked electrical activity has been studied in 3 patients operated on with implantation of an electrode in the periaqueductal gray matter for chronic pain. Particularly interesting are the pronounced decrease of the evoked potentials in the sensory nucleus of the thalamus after stimulation of the periaqueductal gray matter.

Brain stem reticular units: synaptic responses to stimulation within the ascending reticular pathways.

When the ascending reticular axonal system is stimulated, the responses of distal structures (e.g., the cerebral cortex) appear to outlast the stimulus; these longlasting effects could reflect the intrinsic nature of the distal structure, or the response could reflect an interaction among the reticular cells which tends to prolong the effects of stimulation. To examine the latter hypothesis, single units with ascending axons (projecting units) were recorded in the cat rostral rhombencephalon in acute experiments conducted under halothane-nitrous oxide anesthesia. Stimulation of areas to or through which axons of reticular neurons projected (midbrain tegmentum and lower tectum, medial thalamus, and basal forebrain) produced a consistent and specific response which was elicited only from these areas: suppression of spontaneous activity which was typically elicited from several areas having ascending axons. One-half of these responses were accompanied by a short latency-single spike synaptic excitation. Stimulating areas more than 1.0 mm from the ascending trajectory never produced this response, whereas the number of responses was directly related to the number of projecting axons identified in any one experiment from a given site. Thus, the predominant effect of stimulating within the ascending axonal trajectory was suppression of spontaneous activity in the projecting units, not an 'en cascade' activation of these units; on the contrary, the only type of excitation encountered was a single, short latency spike. Therefore, any effects of stimulation within the ascending reticular pathway which appear to outlast the stimulus (as previously described in the literature) cannot be ascribed to a reverberating (excitatory) circuit among projecting units. A possible source of the synaptic responses of projecting units is a retrograde activation of collaterals interconnecting the reticular cells. If such interaction exists, it is specifically distributed among cells with ascending axons, as the responses were only observed in a very few units not identified by antidromic excitation; however, other evidence is adduced to support the belief that these few units were projecting units whose axons were beyond the reach of the stimulating electrodes. Futhermore, the axons may be bundled such that units with axons nearest that of a given projecting unit give rise to the most extensive synaptic interactions; the activation of these nearby axons suppresses spontaneous activity, while axons farther away have a greater possibility of being excitatory in nature. Should such a medium for interaction exist, reticular collateral interactions might be seen to exist specifically for the purpose of decreasing the activity of cells destined for similar rostral target structures.