Receptive Field Location Research Articles

Precision and variability of hindlimb representation in cat dorsal horn and implications for tactile localization.

1. One hundred fifty-eight cells were recorded extracellularly in rows of tracks spanning both left and right dorsal horns, at segmental boundaries and midsegment in segments L5-S1, in six anesthetized cats. For each cell the low-threshold cutaneous mechano-receptive field was determined with the use of hand-held probes, and the recording site was marked with a microlesion. Recording sites were reconstructed, and the mediolateral (ML) and rostrocaudal (RC) locations of each cell were recorded along with the location of the cell's receptive field, expressed as distance from tips of toes (D). 2. Ninety-five percent of pairs of cells recorded from bilaterally symmetric locations (+/- 10%) in the same animal had receptive fields on opposite legs that had components that were mirror symmetric. Only 42% of cell pairs deviating from bilateral symmetry by approximately +/- 240 microns had receptive fields with overlapping components. This indicated that there was a substantial bilateral symmetry that was not simply due to large receptive fields. 3. The trajectories of receptive fields of cells in a single row of tracks were plotted in order of mediolateral recording site, going from medial to lateral, combining both sides. These trajectories followed a distoproximal course on the leg. Of 144 adjacent cells used to plot these trajectories, with an average spacing of approximately 120 microns, only 6 reversals of the distoproximal gradient polarity were observed within animals. 4. Data from individual animals were shifted rostrally and caudally, to obtain best agreement of mediolateral somatotopic gradients with the combined data from the other animals in the sample. Best agreement was obtained with shifts ranging from 0.3 segment rostral to 0.4 segment caudal, with an average absolute value shift of 0.22 segment. 5. By comparing cell pairs within the same dorsal horn, on opposite sides of the same animal, and across animals, variability in cell placement given the average map and the receptive field could be calculated. Interanimal variability and bilateral asymmetry were approximately +/- 60 microns, and within-dorsal horn variability was approximately +/- 35 microns. The interanimal variability is equivalent to a variability of distoproximal receptive-field location on the leg of +/- 13 mm, with a smaller variability in areas of high magnification (e.g., the toes), and a larger variability in areas with small magnification (e.g., the thigh). This degree of variability is consistent with the ability of animals with transected dorsal columns to localize tactile stimuli with a normal degree of precision.

Initially Organized Arrangement of Orientation and Direction Sensitivities of LGNd Relay Cells in Decorticate Cats

Since Levick et al. found and studied the orientation biases of retinal ganglion cells of cats, there has been increasing interest in subcortical orientation sensitivity. Shou et al. studied the orientation sensitivity of dorsal geniculate nucleus (LGNd) relay cells of the cat in details. They found that cells with similar preferred orientation are clustered in the LGNd, which is apparent for all cell types (ON, OFF, X, Y) and cells with different receptive field locations in retina. It is suggested that the initial sorting of orientation functional columns in the visual cortex may begin in the LGNd.

Crossed receptive field components and crossed dendrites in cat sacrocaudal dorsal horn.

The hypothesis that sacrocaudal dorsal horn neurons with crossed receptive field components on the tail have dendrites which cross to the contralateral dorsal horn was tested in a combined electrophysiological and morphological study. Dorsal horn cells in the sacrocaudal spinal cord of anesthetized cats were penetrated with horseradish peroxidase-filled microelectrodes. After mapping their low threshold mechanoreceptive fields, cells were iontophoretically injected with horseradish peroxidase. A sample of 16 well-stained cells was obtained in laminae III and IV. Cells with receptive fields crossing the dorsal midline of the tail (n = 8) had somata in the lateral ipsilateral dorsal horn, and some of these cells (5/8) had dendrites which crossed to the lateral contralateral dorsal horn. Cells with receptive fields spanning the ventral midline (n = 2) were located near the center of the fused dorsal horn, and one of these had bilateral dendrites in this region. Cells with receptive fields on the lateral tail, crossing neither the dorsal nor the ventral midline (n = 6), had cell bodies in the middle of the ipsilateral dorsal horn; half had only ipsilateral dendrites, and half had crossed dendritic branches. Although the relationship between cell receptive field (RF) location (RF center, expressed as distance from tips of toes) and mediolateral location of the cell body was statistically significant, the correlation between crossed RF components and crossed dendritic branches was not significant.

Representation of the urinary bladder in the lateral thalamus of the cat.

1. In alpha-chloralose-anesthetized cats the region surrounding the ventral posterolateral nucleus (VPL) of the thalamus was investigated to locate foci with input from the urinary bladder stimulated by application of intravesical pressure. The locations of the recording sites were verified in Nissl-stained histological sections with reference to electrolytic lesions. 2. Of the 23 visceroceptive thalamic neurons identified, 4 (17%) were located in the periphery of the VPL (VPLp) and 19 (83%) in the lateral and dorsal aspects of the posterior complex (POl and POd, respectively) adjoining VPLp. 3. The neurons responded to noxious intensities of intravesical pressure in the range of 50-100 mmHg. Excitatory responses were elicited in 8 (35%) neurons, "inhibitory" responses in 13 (57%) neurons, and 2 (9%) neurons responded with an increase and a decrease of their discharge to subsequent stimuli. 4. Of the 22 visceroceptive thalamic neurons tested for this parameter, 73% had low-threshold cutaneous receptive fields (RFs). These were located in the region of the lower back, the hip, the thigh, and the proximal tail (12 PO neurons), or covered the entire postcranial contralateral part of the body (3 PO neurons). For only one of the VPLp neurons, a somatic RF was found and this was located on the distal tail. The neurons responded to tap stimuli applied at a low repetition rate. None of the 11 neurons tested with noxious pinching of the skin was activated by this kind of stimulus. 5. It is concluded that the cat's lateral thalamic region, around but not within VPL proper, contains neurons that play a role in the processing of information about noxious events in the urinary bladder. A comparison with results from experiments in the monkey indicates differences in the organization of the visceroceptive systems between both species, regarding the thalamic localization of visceroceptive neurons, the occurrence of convergent low-threshold somatic RFs, and the association of excitatory and inhibitory effects of urinary bladder stimulation with the location of somatic RFs.

Suppression of visual responses of neurons in inferior temporal cortex of the awake macaque by addition of a second stimulus

The responses of neurons, in inferior temporal cortex of the awake macaque, to single stimuli and pairs of stimuli were examined. The responses of most neurons were weaker to pairs of stimuli than to the best single stimulus of that pair presented alone. This ‘suppression by a second stimulus’ did not appear to be stimulus-selective and the suppression was greater than the second stimulus appeared in receptive field locations that exhibited weaker responses. This phenomenon suggests competitive interactions between IT neurons that may be involved in visual attention or learning or both.

The Effects of Localized Inactivation of Somatosensory Cortex, Area 3a, on Area 2 in Cats

In cats and primates, area 3a of the somatosensory cortex is the primary recipient of proprioceptive input (Phillips et al., 1971). Neurons in area 3a project to area 2 (Pons and Kaas, 1986; Porter, 1991), where somatic input relayed from the cortex and the thalamus may be integrated (Iwamura and Tanaka, 1978a,c). The goal of the present study was to determine the effects of area 3a input on neuronal activity in area 2 of cats. Extracellular recording techniques were used to identify neurons in area 2 that responded to deep stimulation of the contralateral forepaw. Neurons in area 3a that responded to the same receptive field modality and location as those in area 2 were also isolated. Single-unit or multiunit responses and evoked potentials to electrical stimulation of the shared peripheral receptive field and spontaneous activity were recorded from areas 2 and 3a. Lidocaine, a local anesthetic, was injected at the area 3a recording site to block neuronal activity. Spontaneous activity and receptive fields were abolished and evoked potentials were considerably diminished at the injection site, immediately after lidocaine was administered. Changes in unit responses, spontaneous activity, and evoked potentials in area 2 were monitored following inactivation of the somatotopically "matched" site in area 3a. Unit activity was recorded at 15 matched sites. In area 2, changes in unit responses to the peripheral stimulation and/or in spontaneous activity were observed at most of the recording sites following inactivation of area 3a. Spontaneous activity rates changed at 63% of the sites (mean change = 85%). Unit responses to the peripheral stimulation changed at 57% of the recording sites (mean change = 47%). The remaining sites in area 2 did not show lidocaine-induced changes. These sites may not have been connected with the matched sites in area 3a. Spontaneous activity and unit responses were not always similarly altered at a given site; sometimes one increased while the other decreased. Decreases in unit responses and spontaneous activity following inactivation of area 3a input were the predominant effects, indicating that area 3a has a facilitatory effect on neuronal activity observed in some regions of area 2. However, increases in neuronal activity at some sites indicated that the effects of area 3a input on area 2 are nonuniform. Evoked potentials were recorded at 19 matched sites, before and after injection of lidocaine. Evoked potentials also changed at some area 2 recording sites following area 3a inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

The Effects of Localized Inactivation of Somatosensory Cortex, Area 2, on the Cat Motor Cortex

Direct corticocortical afferents to the primary motor cortex (MI) originate in area 2 and area 3a of the primary somatosensory cortex (SI). The functional and morphological characteristics of the two pathways indicate that they relay different sensory signals to MI. The role of area 2 in relaying peripheral information to the cat MI was studied using electrophysiological techniques. Neurons that responded to stimulation of peripheral receptive fields on the contralateral forepaw were identified in MI by extracellular recordings. In area 2 of SI, neurons with the same receptive field modality and location as those in MI were also identified. Field potentials to electrical stimulation of the peripheral receptive field were recorded at the somatotopically matched sites in both MI and SI. Neuronal activity at the recording site in area 2 was blocked by injection of lidocaine, a local anesthetic. Changes in MI and area 2 responses were monitored before and after inactivation of area 2. Neuronal activity near the injection site was abolished, and evoked potentials (EPs) in area 2 were considerably diminished immediately following the injection. In MI, spontaneous activity levels were altered at some sites, but overall these changes were not significant. MI EPs recorded in response to peripheral stimulation were altered, and various patterns of change were noted in the early and late phases of the EPs. Changes often occurred in only one phase of the response. In some EPs, both early and late phases changed, but the direction and magnitude of change in one phase were not always linked to such changes in the other phase. Both increases and decreases in the amplitude and the area of each phase were observed. The morphological characteristics of the projection were reviewed and related to the findings in the study. It is proposed that inherent features of the pathway may account for the variable patterns of change that were observed.

Anatomical pathways from the optic tectum to the spinal cord subserving orienting movements in the barn owl.

Electrical stimulation of the optic tectum in many vertebrate species elicits eye, head or body orienting movements in the direction of the receptive field location recorded at the site of stimulation; in the barn owl, tectal stimulation produces short latency saccadic head movements (du Lac and Knudsen 1990). However, the barn owl, like other avians, lacks a direct projection from the tectum to the spinal cord, implying that less direct connections underlie tectally mediated head movements. In order to determine the pathways by which the tectum gains access to spinal cord circuitry, we searched for overlap regions between tectal efferent projections and the locations of cells afferent to the spinal cord. Tectal efferent pathways and terminal fields were revealed by anterograde labeling using horseradish peroxidase (HRP) or tritiated amino acids injected into the optic tectum. Cells afferent to the spinal cord were identified by means of retrograde labeling using HRP, rhodamine, or rhodamine-coupled latex beads injected into the cervical spinal cord. A comparison of results from the anterograde and retrograde labeling experiments demonstrated several areas of overlap. All of the cell groups that both received heavy tectal input and contained a high proportion of cells projecting to the spinal cord were located in the medial half of the midbrain and rhombencephalic tegmentum, and included the red nucleus, the interstitial nucleus of Cajal, the medial reticular formation, the nucleus reticularis pontis giganto-cellularis, and the nucleus reticularis pontis oralis. All of these cell groups receive their tectal input from the medial efferent pathway, one of three major output pathways from the tectum. The other two output pathways (the rostral and the caudal) project to regions containing no more than a few scattered cells that are afferent to the spinal cord. Based on these data and on the functions of homologous cell groups in other vertebrates, we hypothesize that the medial efferent pathway and its brainstem target nuclei are primarily responsible for tectally mediated orienting head movements in the barn owl.

Area 17 lesions deactivate area MT in owl monkeys.

The middle temporal visual area, MT, is one of three major targets of the primary visual cortex, area 17, in primates. We assessed the contribution of area 17 connections to the responsiveness of area MT neurons to visual stimuli by first mapping the representation of the visual hemifield in MT of anesthetized owl monkeys with microelectrodes, ablating an electrophysiologically mapped part of area 17, and then immediately remapping MT. Before the lesions, neurons at recording sites throughout MT responded vigorously to moving slits of light and other visual stimuli. In addition, the relationship of receptive fields to recording sites revealed a systematic representation of the contralateral visual hemifield in MT, as reported previously for owl monkeys and other primates. The immediate effect of removing part of the retinotopic map in area 17 by gentle aspiration was to selectively deactivate the corresponding part of the visuotopic map in MT. Lesions of dorsomedial area 17 representing central and paracentral vision of the lower visual quadrant deactivated neurons in caudomedial MT formerly having receptive fields in the central and paracentral lower visual quadrant. Most neurons at recording sites throughout other parts of MT had normal levels of responsiveness to visual stimuli, and receptive-field locations that closely matched those before the lesion. However, neurons at a few sites along the margin of the deactivated zone of cortex had receptive fields that were slightly displaced from the region of vision affected by the lesion into other parts of the visual field, suggesting some degree of plasticity in the visual hemifield representation in MT.(ABSTRACT TRUNCATED AT 250 WORDS)

Expansion of the cortical representation of a specific skin field in primary somatosensory cortex by intracortical microstimulation.

Intracortical microstimulation (ICMS) was applied to a single site in the middle cortical layers (III-IV) in the koniocortical somatosensory fields of sodium pentobarbital-anesthetized rats (Sml) and new world monkeys (area 3b). Low-threshold cutaneous receptive fields were defined in the cortical region surrounding the stimulation site prior to and following 2-6 hr of 5 microA ICMS stimulation. ICMS stimulation did not usually affect the receptive field location, size, or responsiveness to tactile stimulation of neurons at the stimulation site. However, the number of cortical neurons surrounding the stimulation site with a receptive field that overlapped with the ICMS-site receptive field increased in all studied animals, resulting in an enlarged cortical representation of a restricted skin region spanning several hundred microns. The mean size of receptive fields changed in some but not all cases. These results provide evidence that the responses of cortical neurons are subject to change by the introduction of locally coincident inputs into a single location, and demonstrate a capacity for representational plasticity in the neocortex in the absence of peripheral stimulation. These experimental observations are consistent with hypotheses that the cerebral cortex comprises radially oriented populations of neurons that share a common input, and that these inputs are shaped by coincident activity (see Edelman, 1978, 1987; Merzenich, 1987; Merzenich et al., 1990; von der Malsburg and Singer, 1988).

Visual Instruction of the Neural Map of Auditory Space in the Developing Optic Tectum

Neural maps of visual and auditory space are aligned in the adult optic tectum. In barn owls, this alignment of sensory maps was found to be controlled during ontogeny by visual instruction of the auditory spatial tuning of neurons. Large adaptive changes in auditory spatial tuning were induced by raising owls with displacing prisms mounted in spectacle frames in front of the eyes; neurons became tuned to sound source locations corresponding to their optically displaced, rather than their normal, visual receptive field locations. The results demonstrate that visual experience during development calibrates the tectal auditory space map in a site-specific manner, dictating its topography and alignment with the visual space map.

Somatotopic organization of single primary afferent axon projections to cat spinal cord dorsal horn

Horseradish peroxidase injection of identified low threshold cutaneous mechanoreceptor (LTCM) primary afferent axons was used to assess the somatotopic organization of hindlimb projections to laminae III and IV of cat dorsal horn. Multiple injections in the same animals were used to assess bilateral symmetry and precision. Thirty-one axons were injected, with more than 1 axon injected in each of 8 animals (25 axons). Somatotopic relations between their receptive field (RF) centers and the centers of their dorsal horn projections were similar to the somatotopic relations between dorsal horn cell RF centers and cell locations. Very few reversals of mediolateral somatotopic gradients (proximodistal RF location as a function of mediolateral projection center) were observed. Two afferents with nearly identical RFs in 1 animal had nearly identical projections. These observations held for many different combinations of receptor types. A simple mathematical model was used to demonstrate that assembly of dorsal horn cell RFs via passive sampling of the presynaptic neuropil by dorsal horn cell dendrites cannot account for the sizes of dorsal horn cell LTCM RFs. Hypothesized mechanisms for assembly of dorsal horn cell RFs must take into account the functional selectivity of connections required to produce RFs smaller than those predicted by the passive assembly model.

Orientation bias of neurons in the lateral geniculate nucleus of macaque monkeys

The purpose of this investigation was to analyze the influence of stimulus orientation on the responses of individual neurons in the monkey's lateral geniculate nucleus (LGN). Our specific goals were to assess the prevalence and the degree of orientation tuning in the monkey LGN and to determine if the preferred stimulus orientations of LGN neurons varied as a function of receptive-field position. The primary motivation for this research was to gain insight into the receptive-field configuration of LGN neurons and consequently into the neural mechanisms which determine the spatial organization of LGN receptive fields in primates. In both the parvocellular and magnocellular layers, the responses of the majority of individual neurons to sine-wave gratings varied as a function of stimulus orientation. The influence of stimulus orientation was, however, highly dependent on the spatial characteristics of the stimulus; the greatest degree of orientation bias was observed for spatial frequencies higher than the cell's optimal spatial frequency. On a population basis, the degree of orientation bias was similar for all major classes of LGN neurons (e.g. ON vs. OFF center; parvocellular vs. magnocellular) and did not vary systematically with receptive-field eccentricity. At a given receptive-field location, LGN neurons, particularly cells in the parvocellular laminae, tended to prefer either radially oriented stimuli or stimuli oriented more horizontally than their polar axis. Our analyses of the orientation-dependent changes in spatial-frequency response functions, which was based on the Soodak et al., (1987; Soodak, 1986) two-dimensional, difference-of-Gaussian receptive-field model, suggested that the orientation bias in LGN neurons was due to an elongation of the receptive-field center mechanism which in some cases appeared to consist of multiple subunits. Direct comparisons of the orientation-tuning characteristics of LGN cells and their retinal inputs (S potentials) indicated that the orientation bias in the monkey LGN reflects primarily the functional properties of individual retinal ganglion cells. We conclude that orientation sensitivity is a significant property of subcortical neurons in the primate's geniculo-cortical pathway.

Spatial frequency thresholds of single striate cortical cells in neonatal corpus callosum sectioned cats

Following section of the corpus callosum at 1-6 postnatal weeks in cats, behavioral visual acuity was measured binocularly and monocularly from 6-29 postnatal weeks; physiological determination of spatial frequency thresholds of single striate cortical cells was performed when the cats were at least 8 months old. Results were compared between cats with callosum section at each postnatal week, as well as with normal cats. Cats with callosotomy at 1-3 postnatal weeks had deficits in behavioral visual acuity, and the deficits were greatest in the youngest operated cats. Cats with callosotomy at 1-2 postnatal weeks failed to resolve as high spatial frequencies as did normal cats, and the resolution of the 1 week operated cats was lower than the resolution of the 2 week operated cats. Cats with callosotomy at 3-6 postnatal weeks had spatial frequency thresholds that were equivalent to those of normal cats. To determine what kinds of striate cells had reduced spatial resolution following neonatal corpus callosum section, cells were categorized according to class (Simple, Complex), receptive field location (Central, Peripheral), and monocular behavioral acuity eye performance (Better Eye, Worse Eye). Cats with corpus callosum section during postnatal week 1 had the lowest spatial resolution for all cell categories compared to all groups tested. However, cats with callosum section during postnatal week 2 had normal spatial frequency thresholds for Simple, Central and Better Eye categories. The cats with callosum section in postnatal weeks 3-6 had normal spatial frequency thresholds for all cell categories. For corpus callosum sectioned cats with and without visual deficits, and for normal cats, visual acuity measured behaviorally is significantly related to visual acuity measured physiologically. The results show that neonatal corpus callosum section in cats can affect behavioral visual acuity, as well as the spatial frequency thresholds of many categories of striate cortical cells. However, callosum section at different ages affects different populations of cortical cells. Furthermore, the results suggest that neonatal corpus callosum section may directly affect a single fundamental property of cells in primary visual cortex with a resulting disruption of many visual functions.

Collateral sprouting of the central terminals of cutaneous primary afferent neurons in the rat spinal cord: Pattern, morphology, and influence of targets

The capacity of the central terminals of primary afferents to sprout into denervated areas of neonatal spinal cord and the morphology of any novel terminals has been investigated. In rats which had undergone sciatic nerve section on the day of birth, 12 of 18 physiologically characterized intact saphenous hair follicle afferents (HFAs) were labelled intra-axonally with horseradish peroxidase (HRP) were shown to sprout up to 2,000 microns into the deafferented sciatic terminal field. The morphology of these sprouts depended on which area of the sciatic nerve territory was invaded by the afferent sprouts. Six HFAs sprouted into areas normally innervated by glabrous skin afferents and the morphology of the collateral sprouts in this region resembled that of rapidly adapting (RA) afferents. The other six saphenous HFAs had sprouted into sciatic "hairy" skin areas and the morphology of these sprouts, although abnormal, was flame shaped. In rats whose sural, saphenous, and superficial peroneal nerves were cut at birth, 4 of 7 single HRP labelled RA afferents had central terminals that had sprouted into regions of cord normally devoted to "hairy" input. These showed clear signs of HFA morphology despite their peripheral receptive fields remaining in the glabrous skin. The results show collateral sprouting of single cutaneous sensory afferent axons into adjacent inappropriate central target regions following neonatal deafferentation. Such plasticity may provide some compensation following neonatal injury. The morphology of the sprouted terminals is appropriate to the new target area rather than to its functional class and is also independent of the peripheral receptive field location providing an example of central rather than peripheral control over afferent growth patterns.

Self-calibrated collinearity detector

The human visual system can make remarkably precise spatial judgements. There are reasons to believe that this accuracy is achieved and maintained by using processes that calibrate and correct errors in the system. This work investigate this problem of self-calibration and describes an adaptive system for detecting the collinearity of points and the straightness of lines. The system is initially inaccurate, but, by using an error correction mechanism, it eventually becomes highly accurate. The error correction is performed by a simple self calibration process named proportional multi-gain adjustment. The calibration process adjusts the gain values of the system input units. The process utilizes statistical regularities in the input stimuli. It compensate for errors due to noise in the input units receptive fields location and response functions by ensuring that the average deviation from collinearity offset detected by the system is zero. As a by product of the error correction, the system exhibits also adaptation and aftereffect phenomena, similar to those observed in the human visual system.

Responses of medullary neurons to moving visual stimuli in the common toad

The concept of coded 'command releasing systems' proposes that visually specialized descending tectal (and pretectal) neurons converge on motor pattern generating medullary circuits and release--in goal-specific combination--specific action patterns. Extracellular recordings from medullary neurons of the medial reticular formation of the awake immobilized toad in response to moving visual stimuli revealed the following main results. (i) Properties of medullary neurons were distinguished by location, shape, and size of visual receptive fields (ranging from relatively small to wide), by trigger features of various moving configural stimulus objects (including prey- and predator-selective properties), by tactile sensitivity, and by firing pattern characteristics (sluggish, tonic, warming-up, and cyclic). (ii) Visual receptive fields of medullary neurons and their responses to moving configural objects suggest converging inputs of tectal (and pretectal) descending neurons. (iii) In contrast to tectal monocular 'small-field' neurons, the excitatory visual receptive fields of comparable medullary neurons were larger, ellipsoidally shaped, mostly oriented horizontally, and not topographically mapped in an obvious fashion. Furthermore, configural feature discrimination was sharper. (iv) The observation of multiple properties in most medullary neurons (partly showing combined visual and cutaneous sensitivities) suggests integration of various inputs by these cells, and this is in principle consistent with the concept of command releasing systems. (v) There is evidence for reciprocal tectal/medullary excitatory pathways suitable for premotor warming-up. (vi) Cyclic bursting of many neurons, spontaneously or as a post-stimulus sustaining event, points to a medullary premotor/motor property.

Evidence for restricted central convergence of cutaneous afferents on an excitatory reflex pathway to medial gastrocnemius motoneurons

1. We previously reported that excitatory postsynaptic potentials (EPSPs) produced by low-threshold electrical stimulation of the caudal cutaneous sural nerve (CCS) occur preferentially and with the shortest central latencies in the medial gastrocnemius (MG) portion of the triceps surae motor nuclei. The present study employs the spatial facilitation technique to assess interneuronal convergence on the short-latency excitatory pathway from CCS to MG by several other ipsilateral hindlimb afferents [the lateral cutaneous sural (LCS), caudal cutaneous femoral (CCF), saphenous (SAPH), superficial peroneal (SP), posterior tibial (TIB), and posterior articular (Joint) nerves]. 2. Spatial facilitation of CCF EPSPs in MG motoneurons was demonstrated with conditioning stimulation of the LCS, CCF, SAPH, SP, and TIB nerves, but was most readily and consistently observed with CCF conditioning. Facilitation of CCS and CCF EPSPs was obtained in individual MG motoneurons with a wide range of condition-test intervals. 3. CCF EPSPs in MG motoneurons produced by twice threshold (2T) afferent stimulation had a mean latency of 4.8 ms and often appeared as slowly rising, asynchronous potentials. On the other hand, 2T CCS EPSPs had a mean latency of 2.8 ms and appeared as sharper rising, less variable depolarizations. The optimum condition-test interval for facilitation of CCS and CCF EPSPs was found to be 5.2 ms on average, with CCS stimulation delayed from that of CCF. The longer latency of CCF EPSPs and the finding that the minimum condition-test interval was on the order of 3.9 ms suggests that convergence occurs late in the excitatory CCF pathway to MG motoneurons. 4. Convergence between excitatory pathways to MG from CCF and CCS afferents is discussed with regard to the original observations of Hagbarth on the location of cutaneous receptive fields and excitation of ankle extensors. In addition, evidence for the segregation of these specialized reflex pathways from those involved in general flexion reflexes is discussed.

Demonstration of discrete place‐defined columns—segregates—in the cat SI

The SI forelimb area of cats was examined with receptive field (RF) mapping techniques. Arrays of closely spaced, near-radial microelectrode penetrations were inserted into the crown of postsigmoid gyrus of ketamine anesthetized subjects and minimal RFs were obtained at several depths. The minimal RF was defined as the skin site providing the strongest input to each recorded cluster of neurons. Data analysis showed that all studied cortical territories contained groups of discrete cortical columns, 300-400 microns in diameter. The columns were regarded as topographic entities because no change in minimal RF location could be observed within their boundaries. The boundaries of columns were sharp and could be unequivocally distinguished because the minimal RFs sampled on opposite sides of a boundary occupied displaced, nonoverlapping positions. Pair-wise comparison of single neuron maximal RFs (defined as the entire skin area providing input to the recorded neuron) further clarified the nature of the SI place-defined columns: (1) no systematic differences in maximal RF position could be demonstrated for different parts of the same column (even though the maximal RFs in most columns varied extensively in size and skin areas covered), and (2) at the boundary between neighboring columns maximal RFs shifted en masse to center on a new skin locus. These minimal and maximal RF observations strongly support our recent proposal that body surface is represented in SI by a honeycomblike mosaic of discrete place-defined cortical columns, segregates.

Physiological changes in the somatosensory forepaw cerebral cortex of adult raccoons following lesions of a single cortical digit representation

The purpose of this study was to determine whether restricted lesions within primary somatosensory (SmI) cortex cause changes in the functional organization of cortical areas bordering on the site of injury. Focal ablations of cortical tissue were made in the representational area for digit 3 within the SmI forepaw cortex of adult raccoons. Electrophysiological mapping experiments done 15–17 weeks later showed that significant alterations had occurred in the response properties of clusters of neurons within those representational zones adjoining the lesion—the zones for digit 2, digit 4, and the palmar pads. These three cortical areas were modified by the appearance of new, usually weaker secondary inputs and changes in some properties of the normal primary inputs from the forepaw. (i) Many neurons responded to stimulation of previously ineffective skin regions; the new inputs often originated from digit 3 but frequently involved other digits or the pads as well. (ii) Neuronal receptive fields (RFs), mapped at a standard suprathreshold stimulus intensity, were larger than normal. (iii) Skin type and submodality sensitivity typically were less specific than normal; more neurons had RFs that included both glabrous and hairy skin or claws and displayed mixtures of responsiveness to skin touch, hair deflection, or claw touch. (iv) The representation of RF location, skin type, and submodality sensitivity was more variable as a function of horizontal and vertical distance through the cortex. In general, the physiological changes were found to degrade the somatotopic order and response specificity of the intact cortical areas adjoining the lesion. The relatively mixed nature of the novel inputs following cortical damage was qualitatively similar to that produced by digital nerve transection described previously by Kelahan and Doetsch ( Somatosens. Res., 1984, 2: 49–81). The data suggest that both cortical and peripheral somatosensory injuries may produce their effects by unmasking synaptically weak neural connections. Synaptic unmasking may involve facilitation or disinhibition of neurons with convergent properties—especially those located within the heterogeneous subdivisions of SmI cortex.