The effects of localized inactivation of somatosensory cortex, area 3a, on area 2 in cats

Feeling form: the neural basis of haptic shape perception

The tactile perception of the shape of objects critically guides our ability to interact with them. In this review, we describe how shape information is processed as it ascends the somatosensory neuraxis of primates. At the somatosensory periphery, spatial form is represented in the spatial patterns of activation evoked across populations of mechanoreceptive afferents. In the cerebral cortex, neurons respond selectively to particular spatial features, like orientation and curvature. While feature selectivity of neurons in the earlier processing stages can be understood in terms of linear receptive field models, higher order somatosensory neurons exhibit nonlinear response properties that result in tuning for more complex geometrical features. In fact, tactile shape processing bears remarkable analogies to its visual counterpart and the two may rely on shared neural circuitry. Furthermore, one of the unique aspects of primate somatosensation is that it contains a deformable sensory sheet. Because the relative positions of cutaneous mechanoreceptors depend on the conformation of the hand, the haptic perception of three-dimensional objects requires the integration of cutaneous and proprioceptive signals, an integration that is observed throughout somatosensory cortex.

High-resolution functional magnetic resonance imaging mapping of noxious heat and tactile activations along the central sulcus in New World monkeys

This study mapped the fine-scale functional representation of tactile and noxious heat stimuli in cortical areas around the central sulcus of anesthetized squirrel monkeys by using high-resolution blood oxygen level-dependent (BOLD) fMRI at 9.4T. Noxious heat (47.5°C) stimulation of digits evoked multiple spatially distinct and focal BOLD activations. Consistent activations were observed in areas 3a, 3b, 1, and 2, whereas less frequent activation was present in M1. Compared with tactile activations, thermal nociceptive activations covered more area and formed multiple foci within each functional area. In general, noxious heat activations in area 3b did not colocalize with tactile responses. The spatial relationships of heat and tactile activations in areas 3a and 1/2 varied across animals. Subsequent electrophysiological mapping confirmed that the evoked heat and tactile BOLD signals were somatotopically appropriate. The magnitude and temporal profiles of the BOLD signals to noxious heat stimuli differed across cortical areas. Comparatively late-peaking but stronger signals were observed in areas 3b and 2, whereas earlier-peaking but weaker signals were observed in areas 3a, 1, and M1. In sum, this study not only confirmed the involvement of somatosensory areas of 3a, 3b, and 1, but also identified the engagements of area 2 and M1 in the processing of heat nociceptive inputs. Differential BOLD response profiles of the individual cortical areas along the central sulcus suggest that these areas play different roles in the encoding of nociceptive inputs. Thermal nociceptive and tactile inputs may be processed by different clusters of neurons in different areas. To critically bridge animal and human pain studies, human fMRI was related to primate fMRI and electrophysiology of nociceptive processing, examining the functional role of the primary somatosensory cortex in heat nociception and demonstrating that subregion areas 3a, 3b, 1, 2, and M1 are responsive to noxious heat stimuli.

Distribution of neurons immunoreactive for calcium-binding proteins varies across areas of cat primary somatosensory cortex

The primary somatosensory (SI) cortex in the cat contains four cytoarchitectonic areas that appear to contain separate body representations and have different functions. We tested whether functional differences among these areas are reflected in the densities of neurons containing each of three calcium-binding proteins: parvalbumin (PV), calbindin (CB), and calretinin (CR). Colocalization experiments revealed that CR was localized in a population of neurons distinct from those containing PV or CB. The general laminar distributions of the three calcium-binding proteins were similar to those described in other species and cortical areas, but there were significant density differences in layers II and III across SI. The density of PV-immunoreactive neurons was higher in areas 3b and 1 than in areas 3a and 2. CB-immunoreactive neurons were found in higher densities in anterior SI than in posterior SI, and the pattern of CR-immunoreactive neurons was reciprocal to that of CB, with significantly higher densities in posterior regions of SI. Since the firing characteristics of nonpyramidal neurons appear to be related to their calcium-binding protein content, differences in regional distributions of these neurons in layers II and III may contribute to functional differences between the cytoarchitectonic areas of SI cortex.

Spatial and temporal parameters of cortical inactivation by GABA

Inactivation by GABA is a powerful tool for studying the function of specific cortical regions. It is especially useful in electrophysiology, because inactivation is reversible within short time periods, and because the extent of the inactivated region can be accurately controlled. Iontophoresis of GABA inactivates neurons up to 300 microm around the micropipette. Pressure injection of GABA inactivates neurons further away, but the spatial and temporal characteristics of inactivation by this method have been poorly studied. In order to address this question, we built devices made of micropipettes and microelectrodes glued at various distances. We experienced that repetition of small injections of 100 mM GABA inactivate cortex in a more homogenous way than bolus injections. Diffusion of GABA after pressure injection does not seem to follow a point spread diffusion model as in the case of iontophoresis: GABA probably goes up along the micropipette shaft, and the volume of inactivation has an ellipsoidal form. In order to precisely determine the extent of the inactivated region, we built a mathematical model to fit the experimental data of inactivations obtained above and below the pipette tip. The model provides estimates of the inactivated region for volumes smaller than 60 nl of GABA 100 mM. Limits of inactivation are between 250 and 500 microm lateral to the tip of the pipette. The geometry of inactivation is difficult to predict beyond 60 nl and it seems hazardous to try to inactivate neurons beyond 800 microm with pressure injections of GABA 100 mM.

Glutamate decarboxylase immunoreactivity in corticocortical projecting neurons of rat somatic sensory cortex

Combined retrograde tracing and immunocytochemical experiments were carried out on rats to ascertain whether corticocortical projecting neurons in the somatic sensory areas are immunoreactive to an antiserum against glutamate decarboxylase. Injections of a retrograde tracer (colloidal gold-labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase) in the first somatic sensory area labelled neurons in the injected area, in the second somatic sensory area, and in the parietoventral area of the ipsilateral hemisphere. The topographical and laminar distribution of these retrogradely-labelled corticocortical neurons in the first and second somatic sensory areas and in the parietoventral area was in line with a previous description (Fabri M. and Burton H. (1991b) J. comp. Neurol. 311, 405-424). In sections processed for the simultaneous visualization of the retrograde tracer and glutamate decarboxylase immunoreactivity, a number of neurons were double-labelled. Double-labelled neurons were most numerous in the first somatic sensory cortex, where they accounted for 5% of all retrogradely-labelled neurons. Outside this region, double-labelled cells were observed in the second somatic sensory cortex and in the parietoventral cortex, where they amounted respectively to 2.8% and 2.3% of all corticocortical neurons labelled in these two areas. Glutamate decarboxylase-immunopositive corticocortical neurons were mainly concentrated in the infragranular layers (73.8% of all double-labelled neurons in the first somatic sensory area, 81.7% in the second somatic sensory area, and 76.5% in the parietoventral area). The results indicate the presence of a small but significant contingent of GABAergic inhibitory neurons in the associative connections of the somatic sensory areas.

Cytoarchitecture and responsiveness of the medial ansate region of the cat primary somatosensory cortex

To understand its relationship to somatosensory areas in other species, we studied the rostral bank of the medial ansate sulcus in adult cats. Neurons in the shoulder and upper part of the sulcal wall responded to low-threshold cutaneous stimuli much like neurons on the crown of the gyrus, whereas neurons in some deeper portions of the sulcus required more intense but innocuous somatic stimuli. Because we found much of the body surface re-represented in this area, we suggest that, besides the representation in area 3b, there is another cutaneous representation of the hindlimb and trunk located on the gyral crown near the medial end of the medial ansate sulcus and of the forelimb and trunk within the medial ansate sulcus. Posterior to this second cutaneous representation, many parts of the body were also represented in regions activated by more intense stimuli and having a different cytoarchitecture, suggesting that they were part of another body representation. Area 3b and the shoulder of the gyrus were distinguished by relatively intense acetylcholinesterase staining of layers III and IV. In the wall of the sulcus, all layers except layer I were uniformly stained to a point where electrophysiological recordings showed the cortex to be unresponsive, whereupon the outer two-thirds of layer I became very pale. Neurons activated by afferents from knee joints were found only in a small area; we did not find a mediolateral band serving joint afferents as is reported in primates. These data suggest that cat somatosensory cortex differs in some ways from primates but that it contains multiple representations of the body, as do most other mammals.