Segregated hemispheric pathways through the optic chiasm distinguish primates from rodents.

At the optic chiasm retinal fibers either cross the midline, or remain uncrossed. Here we trace hemispheric pathways through the marmoset chiasm and show that fibers from the lateral optic nerve pass directly toward the ipsilateral optic tract without any significant change in fiber order and without approaching the midline, while those from medial regions of the nerve decussate directly. Anterograde labeling from one eye shows that the two hemispheric pathways remain segregated through the proximal nerve and chiasm with the uncrossed confined laterally. Retrograde labeling from the optic tract confirms this. This clearly demonstrates that hemispheric pathways are segregated through the primate chiasm. Previous chiasmatic studies have been undertaken mainly on rodents and ferrets. In these species there is a major change in fiber order pre-chiasmatically, where crossed and uncrossed fibers mix, reflecting their embryological history when all fibers approach the midline prior to their commitment to innervate either hemisphere. This pattern was thought to be common to placental mammals. In marsupials there is no change in fiber order and uncrossed fibers remain confined laterally through nerve and chiasm, again, reflecting their developmental history when all uncrossed fibers avoid the midline. Recently it has been shown that this distinction is not a true dichotomy between placental mammals and marsupials, as fiber order in tree shrews and humans mirrors the marsupial pattern. Architectural differences in the mature chiasm probably reflect different developmental mechanisms regulating pathway choice. Our results therefore suggest that both the organization and development of the primate optic chiasm differ markedly from that revealed in rodents and carnivores.

A stereotaxic atlas of the brain of the naked mole-rat (Heterocephalus glaber).

The naked mole-rat (Rodentia, Bathyergidae: Heterocephalus glaber) is a strictly subterranean eusocial mammal. These rodents show a suite of morphological and physiological adaptations, including brain specializations, to this underground milieu that they have inhabited since the early Miocene. Recently, naked mole-rats have received considerable attention as the longest living rodent known, and some of these brain specializations may be potentially important to their exceptional longevity. To serve as a basis for future brain studies, we have constructed a stereotaxic atlas of the brain of this species, labeling all major brain structures.

Intrinsic cortical connections in macaque visual area V2: evidence for interaction between different functional streams.

Area V2 of macaque visual cortex represents an important but poorly understood stage in visual processing. To provide a better understanding of the region, we studied the organization of its intrinsic cortical connections by making focal (200-300 microns) iontophoretic microinjections of the tracer biocytin. Alternate tissue sections were tested for biocytin, cytochrome oxidase (CO), or Cat-301 immunoreactivity to localize biocytin label relative to the three stripelike compartments that characterize this area. Biocytin-labeled pyramidal neurons of layers 2/3, and, to a lesser extent, layer 5, provided laterally spreading axon projections that terminated in discrete patches (250-300 microns diameter), primarily in layers 1-3. Any injected locus in V2 projected to 10-15 similarly sized patches, up to 4 mm from the injection site, and distributed in an elongated field orthogonal to the stripe compartments. We noted prominent patchy connections within, as well as between, individual compartments, perhaps reflecting functional substructures within stripes. Each stripe compartment projected to all three compartments but with different relative frequencies; CO-rich compartments projected mainly to other CO-rich compartments (75%), whereas CO-poor compartments projected equally to CO-rich and CO-poor compartments. We therefore emphasize the existence of substantial interconnections among all three V2 compartments. As further evidence for crosstalk between visual channels, we also noted an input to the V2 "thick" CO stripes from V1 cells in layer 4A as a distinct population in addition to the neurons of layer 4B. Thus, the CO stripe architecture may not be a marker for strictly segregated parallel visual pathways through V2.

Topography of pyramidal neuron intrinsic connections in macaque monkey prefrontal cortex (areas 9 and 46).

An understanding of the normal organization of prefrontal cortex is essential to the recognition of pathology underlying human behavioral disorders believed to depend on this region. We have therefore studied the pattern of intrinsic intra- and interlaminar pyramidal neuron connectivity in prefrontal areas 9 and 46 (of Walker) in macaque monkey cerebral cortex (anterior to the arcuate sulcus between the principal sulcus and midline). We made focal (200-400 microns) injections of biocytin and mapped the pattern of orthogradely transported label. Injections made into the superficial layers label wide-ranging lateral projections within the same areas of prefrontal cortex. Projections local to such small injections form a narrow band of terminals in layers 1-3 (200-400 microns wide, 2-4 mm long) centered on the injection site. Collateral fibers spread orthogonal to this terminal band, making frequent bifurcations, to establish a series of parallel bands of terminals with uninnervated bands between, spaced regularly across the cortex (center to center 500-600 microns). The entire pattern of terminal label is stripe-like, with occasional narrower interbands and crosslinks between the bands, and can extend over 7-8 mm across the cortex. These projections arise from pyramidal neurons in layers 2, 3, and 5 and terminate in layers 1-3. The stripe-like pattern contrasts with patch-like patterns in other cortical regions (V1, V2, V4, motor, somatosensory) and is smaller in scale than stripe-like zones of corticocortical afferent terminals to this region, reported to be 300-750 microns wide and spaced 1.0-1.5 mm center to center.

Patterns of intrinsic and associational circuitry in monkey prefrontal cortex.

Both local and long-range connections are critical mediators of information processing in the cerebral cortex, but little is known about the relationships among these types of connections, especially in higher-order cortical regions. We used quantitative reconstructions of the label arising from discrete (approximately 350 microns diameter) injections of biotinylated dextran amine and cholera toxin B to determine the spatial organization of the axon collaterals and principal axon projections furnished by pyramidal neurons in the supragranular layers of monkey prefrontal cortex (areas 9 and 46). Both terminals and cell bodies labeled by transport along axon collaterals in the gray matter formed intrinsic clusters which were arrayed as a series of discontinuous stripes of similar size and shape. The co-registration of anterograde and retrograde transport confirmed that these convergent and divergent intrinsic connections also were reciprocal. Transport from the same injection sites along principal axons through the white matter formed associational clusters which were also arrayed as a series of discontinuous stripes. The dimensions of the anterogradely- and retrogradely-labeled associational stripes were very similar to each other and to the intrinsic stripes. These findings demonstrate that divergence, convergence, and reciprocity characterize both the intrinsic and associational excitatory connections in the prefrontal cortex. These patterns of connections provide an anatomical substrate by which activation of a discrete group of neurons would lead to the recruitment of a specific neuronal network comprised of both local and distant groups of cells. Furthermore, the consistent size of the intrinsic and associational stripes (approximately 275 by 1,800 microns) suggests that they may represent basic functional units in the primate prefrontal cortex.

A model for the depth-dependence of receptive field size and contrast sensitivity of cells in layer 4C of macaque striate cortex.

A model of LGN-input to layer 4C of macaque primary visual cortex has been used to test the hypothesis that feedforward convergence of P- and M-inputs onto layer 4C spiny stellate neurons is sufficient to explain the observed gradual change in receptive field size and contrast sensitivity with depth in the layer. Overlap of dendrites of postsynaptic neurons between M- and P-input zones proved sufficient to explain change in the lower two-thirds of layer 4C, while more rapid change in upper 4C was matched by proposing two different M-inputs with partial overlap in upper 4C alpha.

Contrast dependence of contextual effects in primate visual cortex.

The responses of neurons in the visual cortex to stimuli presented within their receptive fields can be markedly modulated by stimuli presented in surrounding regions that do not themselves evoke responses. This modulation depends on the relative orientation and direction of motion of the centre and surround stimuli, and it has been suggested that local cortical circuits linking cells with similar stimulus selectivities underlie these phenomena. However, the functional relevance and nature of these integrative processes remain unclear. Here we investigate how such integration depends on the relative activity levels of neurons at different points across the cortex by varying the relative contrast of stimuli over the receptive field and surrounding regions. We show that simply altering the balance of the excitation driving centre and surround regions can dramatically change the sign and stimulus selectivity of these contextual effects. Thus, the way that single neurons integrate information across the visual field depends not only on the precise form of stimuli at different locations, but also crucially on their relative contrasts. We suggest that these effects reflect a complex gain-control mechanism that regulates cortical neuron responsiveness, which permits dynamic modification of response properties of cortical neurons.

Functional properties of neurons in macaque area V3.

We investigated the functional properties of neurons in extrastriate area V3. V3 receives inputs from both magno- and parvocellular pathways and has prominent projections to both the middle temporal area (area MT) and V4. It may therefore represent an important site for integration and transformation of visual signals. We recorded the activity of single units representing the central 10 degrees in anesthetized, paralyzed macaque monkeys. We measured each cell's spatial, temporal, chromatic, and motion properties with the use of a variety of stimuli. Results were compared with measurements made in V2 neurons at similar eccentricities. Similar to area V2, most of the neurons in our sample (80%) were orientation selective, and the distribution of orientation bandwidths was similar to that found in V2. Neurons in V3 preferred lower spatial and higher temporal frequencies than V2 neurons. Contrast thresholds of V3 neurons were extremely low. Achromatic contrast sensitivity was much higher than in V2, and similar to that found in MT. About 40% of all neurons showed strong directional selectivity. We did not find strongly directional cells in layer 4 of V3, the layer in which the bulk of V1 and V2 inputs terminate. This property seems to be developed within area V3. An analysis of the responses of directionally selective cells to plaid patterns showed that in area V3, as in MT and unlike in V1 and V2, there exist cells sensitive to the motion of the plaid pattern rather than to that of the components. The exact proportion of cells classified as being selective to color depended to a large degree on the experiment and on the criteria used for classification. With the use of the same conditions as in a previous study of V2 cells, we found as many (54%) color-selective cells as in V2 (50%). Furthermore, the responses of V3 cells to colored sinusoidal gratings were well described by a linear combination of cone inputs. The two subpopulations of cells responsive to color and to motion overlapped to a large extent, and we found a significant proportion of cells that gave reliable and directional responses to drifting isoluminant gratings. Our results show that there is a significant interaction between color and motion processing in area V3, and that V3 cells exhibit the more complex motion properties typically observed at later stages of visual processing.

Intrinsic lattice connections of macaque monkey visual cortical area V4.

We made focal iontophoretic as well as larger pressure injections (n = 30; 19 used for most analyses) of the tracer biocytin in visual area V4 of six macaque monkeys. The resulting transported label enabled mapping of intrinsic inter- and intralaminar connections within the region. We found that pyramidal neurons of layers 2 and 3 make extensive lateral projections within area V4, with oval or circular patches of terminals in layers 1-3. Any small patch of tissue (approximately 250 microns wide) injected in the superficial layers appeared to connect reciprocally to patches scattered up to 3 mm around the injection. The patches of terminal label measure 250-450 microns across, spaced roughly 600 microns (range, 450-1300 microns) center to center, and where most densely packed they occupy 33% of the cortical area with approximately 3-4 patches/mm2. Small injections in layers 4 and 6 did not produce contributions to these patchlike lattice connections, while injections in layer 5 gave very weak rising contributions to the superficial layer patch system. Large pressure injections of biocytin gave wider spread and more densely labeled patches, but their size and spacing appeared much the same as with small injections. The V4 pattern of label was compared to the patterns seen in areas V1 and V2 after similar-sized injections of biocytin; patches in V1 and V2 were slightly smaller (roughly 250-300 microns across) and more closely spaced (425-450 microns center to center), consistent with earlier measures in these areas using HRP. The peak areal occupancy of patches was approximately the same, 27-33% in each region. We interpret these findings as indicating a functional repeat distance of 450-600 microns in area V4 (fixed tissue measures) with a patchy, discontinuous layout. Given the dimensions of the V4 intrinsic connectional system demonstrated here, it seems unlikely that it relates directly to the topography of specific afferent terminations or efferent neuron groups, which appear from other studies to have a larger scale of repeat (2-3 mm). However, patches in V4 tend to aggregate, forming clusters extended in the mediolateral direction, suggesting a possible relation to coarser connection zones.

Independence and merger of thalamocortical channels within macaque monkey primary visual cortex: anatomy of interlaminar projections.

An important issue in understanding the function of primary visual cortex in the macaque monkey is how the several efferent neuron groups projecting to extrastriate cortex acquire their different response properties. To assist our understanding of this issue, we have compared the anatomical distribution of V1 intrinsic relays that carry information derived from magno- (M) and parvocellular (P) divisions of the dorsal lateral geniculate nucleus between thalamic recipient neurons and interareal efferent neuron groups within area V1. We used small, iontophoretic injections of biocytin placed in individual cortical laminae of area V1 to trace orthograde and retrograde inter- and intralaminar projections. In either the same or adjacent sections, the tissue was reacted for cytochrome oxidase (CO), which provides important landmarks for different efferent neuron populations located in CO rich blobs and CO poor interblobs in laminae 2/3, as well as defining clear boundaries for the populations of efferent neurons in laminae 4A and 4B. This study shows that the interblobs, but not the blobs, receive direct input from thalamic recipient 4C neurons; the interblobs receive relays from mid 4C neurons (believed to receive convergent M and P inputs), while blobs receive indirect inputs from either M or P (or both) pathways through layers 4B (which receives M relays from layer 4C alpha) and 4A (which receives P relays directly from the thalamus as well as from layer 4C beta). The property of orientation selectivity, most prominent in the interblob regions and in layer 4B, may have a common origin from oriented lateral projections made by mid 4C spiny stellate neurons. While layer 4B efferents may emphasize M characteristics and layer 4A efferents emphasize P characteristics, the dendrites of their constituent pyramidal neurons may provide anatomical access to the other channel since both blob and interblob regions in layers 2/3 have anatomical access to M and P driven relays, despite functional differences in the way these properties may be expressed in the two compartments.

Connections between the pulvinar complex and cytochrome oxidase-defined compartments in visual area V2 of macaque monkey.

We examined the distribution of pulvinar afferents to visual area V2 of macaque monkey cerebral cortex in relation to the distribution of the metabolic enzyme cytochrome oxidase (CO). V2 contains three sets of stripelike subregions that are marked by differential staining for CO, and which have different corticocortical connections. The pulvinar provides the major subcortical input to V2, and this input is known to be patchy. We were interested to determine how the pattern of pulvinar afferents relates to the layout of the three stripelike compartments that characterize V2. We made large injections of WGA-HRP into the pulvinar (labelling both the inferior and lateral divisions) and mapped the resulting orthograde terminal and retrograde cell label within V2. We observed pulvinar terminal label mainly in lower layer 3 (at the layer 4 border), with light label in layer 1 as well; terminal label in layers 3-4 was distributed in discrete patches with faint bridges of light label between. Comparison with adjacent sections stained for CO or Cat-301 showed that pulvinar terminal zones aligned precisely with regions of increased CO staining, and targeted both "thick" (Cat-301+) and "thin" CO-rich stripes, avoiding the pale stripes (which aligned with the faint bridges of terminal label). Retrogradely labelled cells were found in layers 5A and 6, but the bulk of the feedback to pulvinar arose from layer 6 rather than layer 5 (unlike V1, where feedback to pulvinar arises primarily from layer 5B). These results show that the increased CO staining in certain subregions of V2 is closely correlated with the presence of thalamic terminals from the pulvinar. Although we cannot rule out the possibility that different sets of pulvinar neurons project to different CO compartments in V2, the presence of a prominent thalamic input shared by the "thick" and "thin" CO stripes (which receive different V1 afferents and make different feedforward projections to other visual cortical areas) could underlie the preferential intrinsic interconnections shown to exist between these V2 subregions and suggests another potential source of integration between the two cortical visual streams.

Comparison of intrinsic connectivity in different areas of macaque monkey cerebral cortex.

We have used small injections of biocytin to label and compare patterns of intraareal, laterally spreading projections of pyramidal neurons in a number of areas of macaque monkey cerebral cortex. In visual areas (V1, V2, and V4), somatosensory areas (3b, 1, and 2), and motor area 4, a punctate discontinuous pattern of connections is made from 200-microns-diameter biocytin injections in the superficial layers. In prefrontal cortex (areas 9 and 46), stripe-like connectivity patterns are observed. In all areas of cortex examined, the width of the terminal-free gaps is closely scaled to the average diameter of terminal patches, or width of terminal stripes. In addition, both patch and gap dimensions match the average lateral spread of the dendritic field of single pyramidal neurons in the superficial layers of the same cortical region. These architectural features of the connectional mosaics are constant despite a twofold difference in scale across cortical areas and different species. They therefore appear to be fundamental features of cortical organization. A model is offered in which local circuit inhibitory "basket" interneurons, activated at the same time as excitatory pyramidal neurons, could veto pyramidal neuron connections within either circular or stripe-like domains; this could lead to the formation of the pattern of lateral connections observed in this study, and provides a framework for further theoretical studies of cerebral cortex function.

Receptive fields and functional architecture of macaque V2.

1. Visual area V2 of macaque monkey cerebral cortex is the largest of the extrastriate visual areas, yet surprisingly little is known of its neuronal properties. We have made a quantitative analysis of V2 receptive field properties. Our set of measurements was chosen to distinguish neuronal responses reflecting parvocellular (P) or magnocellular (M) inputs and to permit comparison with similar measurements made in other visual areas; we further describe the relationship of those properties to the laminar and cytochrome oxidase (CO) architecture of V2. 2. We recorded the activity of single units representing the central 5 degrees in all laminae and CO divisions of V2 in anesthetized, paralyzed macaque monkeys. We studied responses to geometric targets and to drifting sinusoidal gratings that varied in orientation, spatial frequency, drift rate, contrast, and color. 3. The orientation selectivity and spatial and temporal tuning of V2 neurons differed little from those in V1. As in V1, spatial and temporal tuning in V2 appeared separable, and we identified a population of simple cells (more common within the central 3 degrees) similar to those found in V1. Contrast sensitivity of V2 neurons was greater on average than in V1, perhaps reflecting the summation of inputs in V2's larger receptive fields. Many V2 neurons exhibited some degree of chromatic opponency, responding to isoluminant color variations, but these neurons differed from V1 in the linearity with which they summate cone signals. 4. In agreement with others, we found that neurons with selective responses to color, size, and motion did seem to cluster in different CO compartments. However, this segregation of qualitatively different response selectivities was not absolute, and response properties also seemed to depend on laminar position within each compartment. As others also have noted, we found that CO stripe widths in the macaque (unlike in the squirrel monkey) did not consistently appear different. We relied on the segregation of qualitatively distinct cell types, and in some cases the pattern of Cat-301 staining as well, to distinguish CO stripes when the staining pattern of CO alone was ambiguous. Although all cell types were found in all CO compartments and laminae, unoriented cells were more prominent in layers 2-4 of "thin" stripes, direction-selective cells in layers 3B/4 of "thick" stripes, color-selective cells in the upper layers of thin and pale stripes, and end-stopped cells mainly outside of layer 4 in thin stripes.(ABSTRACT TRUNCATED AT 400 WORDS)

Spatio-temporal interactions and the spatial phase preferences of visual neurons.

We recorded single neuron responses in the cat's lateral geniculate nucleus (LGN) and visual cortex to compound stimuli composed of two sinusoidal gratings in a 2:1 frequency ratio. To probe visual receptive field symmetry, we varied the relative spatial phase of the two components and measured the effect on neuronal responses. We expected that on-center LGN neurons would respond best to gratings combined in positive cosine (bright bar) phase, while off-center LGN neurons would respond best to gratings combined in negative cosine (dark bar) phase. When drifting stimuli were used, cells' phase preferences were roughly 90 deg away from the expected values; when stationary, contrast-modulated stimuli were used, phase preferences were as originally predicted. Computer simulations showed that this discrepancy could be explained by taking into account the cells' temporal properties. Thus, tests using drifting stimuli confound the spatial structure of visual neural receptive fields with their temporal response characteristics. A small sample of data from cortical neurons reveals the same confound.

A model for the intracortical origin of orientation preference and tuning in macaque striate cortex.

We report results of numerical simulations for a model of generation of orientation selectivity in macaque striate cortex. In contrast to previous models, where the initial orientation bias is generated by convergent geniculate input to simple cells and subsequently sharpened by lateral circuits, our approach is based on anisotropic intracortical excitatory connections which provide both the initial orientation bias and its subsequent amplification. Our study shows that the emerging response properties are similar to the response properties that are observed experimentally, hence the hypothesis of an intracortical generation of orientation bias is a sensible alternative to the notion of an afferent bias by convergent geniculocortical projection patterns. In contrast to models based on an afferent orientation bias, however, the "intracortical hypothesis" predicts that orientation tuning gradually evolves from an initially nonoriented response and a complete loss of orientation tuning when the recurrent excitation is blocked, but new experiments must be designed to unambiguously decide between both hypotheses.

Relation between patterns of intrinsic lateral connectivity, ocular dominance, and cytochrome oxidase-reactive regions in macaque monkey striate cortex.

To help understand the role of long-range, clustered lateral connections in the superficial layers of macaque striate cortex (area V1), we have examined the relationship of the patterns of intrinsic connections to cytochrome oxidase (CO) blobs, interblobs, and ocular dominance (OD) bands, using biocytin based neuroanatomical tracing, CO histochemistry, and optical imaging. Microinjections of biocytin in layer 3 resulted in an asymmetric field (average anisotropy of 1.8; maximum spread--3.7 mm) of labeled axon terminal clusters in layers 1-3, with the longer axis of the label spread oriented orthogonal to the rows of blobs and imaged OD stripes, parallel to the V1/V2 border. These labeled terminal patches (n = 186) from either blob or interblob injections (n = 20) revealed a 71% (132 out of 186) commitment of patches to the same compartment as the injection site; 11% (20 out of 186) to the opposite compartment, and 18% (34 out of 186) to borders of blob-interblob compartments, indicating that the connectivity pattern is not strictly blob to blob, or interblob to interblob (p < 0.005; chi(2)). In injections placed within single OD domains (n = 11), 54% of the resulting labeled terminal patches (43 out of 79) fell into the same OD territories as the injection sites, 28% (22 out of 79) into the opposite OD regions, and 18% (14 out of 79) on borders, showing some connectional bias toward same-eye compartments (p < 0.02; ANOVA). Individual injection cases, however, varied in the degree (50-100% for CO patterns, 22-100% for OD patterns) to which they showed same-compartment connectivity. These results reveal that while connectivity between similar compartments predominates (e.g., blob to blob, right eye column to right eye column), interactions do occur between functionally different regions.

Cells and circuits contributing to functional properties in area V1 of macaque monkey cerebral cortex: bases for neuroanatomically realistic models.

Anatomical and physiological data obtained from investigations of area V1 of the macaque monkey visual cerebral cortex have been used in 3 models outlining possible circuitry underlying functional properties of the region. The 3 models use, respectively, a fully implemented computer neural network, a mathematical formulation of interactions in a descriptive model of anatomical circuitry and a purely descriptive account of circuitry that could underlie particular functions. The 1st 2 models involve as part of their design an interpolation principle where afferents of opposite physiological property establish spatially offset but adjacent terminal fields and the postsynaptic neurons' dendrites have a continuum of different degrees of overlap into the 2 afferent pools and therefore different synaptic weights from the 2 afferents; this creates a functional and spatial gradient of response properties in the postsynaptic neurons between the properties of the different sets of afferents. The 3rd model examines lateral excitatory and inhibitory interactions in such gradients. Model 1 addresses the transformation of distinct thalamic axon properties to a gradient of response properties in postsynaptic spiny stellate neurons in layer 4C of V1. Model 2 proposes circuitry producing orientation specificity in V1 that begins by generating specificity of responses to orthogonal orientations; this is achieved by means of orthogonally oriented lateral axon projections made by the layer 4C spiny stellate neurons; this is followed by generation of a full cycle of orientation specificities by means of pyramidal neuron dendritic overlap across spatially separated fields of spiny stellate neuron axons responding preferentially to orthogonal orientations. Model 3 describes a circuitry to explain inhibitory and facilitatory interactions observed to occur in single unit responses when the classical receptive field is stimulated concurrently with the surround region. All the proposed models make predictions that can be tested by further anatomical and physiological experiments in the real visual cortex.

Inhibitory synapse cover on the somata of excitatory neurons in macaque monkey visual cortex.

Electron microscopy was used in macaque monkey cortical area V1 to investigate what factors might determine the proportion of somatic membrane covered by inhibitory type 2 synapses. In a sample of 4654 excitatory neurons, synapse cover did not correlate consistently with cell variety (pyramid or spiny stellate), soma size, synaptic apposition length or thalamic input. There were significant differences in somatic synapse cover per layer, but the pattern of differences in cover among layers differed significantly between animals, suggesting that laminar environment alone is not a generally applicable determinant of amount of inhibitory synapse cover. The pattern of cover for cells in different layers was, however, similar between the two hemispheres of an individual monkey. Measures of inhibitory synapse cover on four sets of pyramidal neurons in layers 5 and 6, each with different efferent projection targets, showed that the sets differed significantly from other cells in their respective layers, and differed significantly from each other. These findings demonstrate that there is unique circuitry for different subsystems within single layers of cortex and provide a rationale for the rich variety of cortical GABAergic interneurons within single layers.