Broad Receptive Fields Research Articles

A Hebbian Learning Rule Mediates Asymmetric Plasticity in Aligning Sensory Representations

In the brain, mutual spatial alignment across different sensory representations can be shaped and maintained through plasticity. Here, we use a Hebbian model to account for the synaptic plasticity that results from a displacement of the space representation for one input channel relative to that of another, when the synapses from both channels are equally plastic. Surprisingly, although the synaptic weights for the two channels obeyed the same Hebbian learning rule, the amount of plasticity exhibited by the respective channels was highly asymmetric and depended on the relative strength and width of the receptive fields (RFs): the channel with the weaker or broader RFs always exhibited most or all of the plasticity. A fundamental difference between our Hebbian model and most previous models is that in our model synaptic weights were normalized separately for each input channel, ensuring that the circuit would respond to both sensory inputs. The model produced three distinct regimes of plasticity dynamics (winner-take-all, mixed-shift, and no-shift), with the transition between the regimes depending on the size of the spatial displacement and the degree of correlation between the sensory channels. In agreement with experimental observations, plasticity was enhanced by the accumulation of incremental adaptive adjustments to a sequence of small displacements. These same principles would apply not only to the maintenance of spatial registry across input channels, but also to the experience-dependent emergence of aligned representations in developing circuits.

Proprioceptive and Cutaneous Representations in the Rat Ventral Posterolateral Thalamus

Determining how and where proprioceptive information is represented in the rat ventral posterolateral (VPL) is important in allowing us to further investigate how this sense is utilized during motor control and learning. Here we demonstrate using electrophysiological techniques that the rostral portion of the rat VPL nucleus (rVPL, -2 to -2.5 mm bregma) carries a large amount of proprioceptive information. Caudal to this region is a zone where the cutaneous receptive fields are focal (mVPL for middle VPL, -2.5 to -3.2 mm bregma) with a fine topographic map of the fore- and hindlimbs. The forepaw is represented with digit 1 medial and each subsequent digit increasingly lateral, all of which are dorsal to the pads. The caudal VPL (cVPL, -3.2 to -4.0 mm bregma) has broad receptive fields and is the target of lamina 1 and lamina 2, as well as the dorsal column nuclei, and may represent the flow of nociceptive information through the VPL. Thus we propose that the VPL may be thought of as three subnuclei-the rostral, middle, and caudal VPL-each carrying preferentially a different modality of information. This pattern of information flow through the rat VPL is similar, although apparently rotated, to that of many primates, indicating that these regions in the rat (rVPL, mVPL, and cVPL) have become further differentiated in primates where they are seen as separate nuclei (VPS, VPL, and VPI/VMpo).

Velocity constancy and models for wide-field visual motion detection in insects

The tangential neurons in the lobula plate region of the flies are known to respond to visual motion across broad receptive fields in visual space. When intracellular recordings are made from tangential neurons while the intact animal is stimulated visually with moving natural imagery,we find that neural response depends upon speed of motion but is nearly invariant with respect to variations in natural scenery. We refer to this invariance as velocity constancy. It is remarkable because natural scenes, in spite of similarities in spatial structure, vary considerably in contrast, and contrast dependence is a feature of neurons in the early visual pathway as well as of most models for the elementary operations of visual motion detection. Thus, we expect that operations must be present in the processing pathway that reduce contrast dependence in order to approximate velocity constancy. We consider models for such operations, including spatial filtering, motion adaptation, saturating nonlinearities, and nonlinear spatial integration by the tangential neurons themselves, and evaluate their effects in simulations of a tangential neuron and precursor processing in response to animated natural imagery. We conclude that all such features reduce interscene variance in response, but that the model system does not approach velocity constancy as closely as the biological tangential cell.

Sharing Receptive Fields with Your Neighbors: Tuning the Vertical System Cells to Wide Field Motion

In the blowfly, the direction-selective response of the 60 lobula-plate tangential cells has been ascribed to the integration of local motion information across their extensive dendritic trees. Because the lobula plate is organized retinotopically, the receptive fields of the tangential cells ought to be determined by their dendritic architecture. However, this appears not always to be the case. One compelling example is the exceptionally wide receptive fields of the vertical system (VS) tangential cells. Using dual-intracellular recordings, Haag and Borst (2004) found VS cells to be mutually coupled in such a way that each VS cell is connected exclusively to its immediate neighbors. This coupling may form the basis of the broad receptive fields of VS cells. Here, we tested this hypothesis directly by photoablating individual VS cells. The receptive field width of VS cells indeed narrowed after the ablation of single VS cells, specifically depending on whether the receptive field of the ablated cell was more frontal or more posterior to the recorded cell. In particular, the responses changed as if the neuron lost access to visual information from the ablated neuron and those VS cells more distal than it from the recorded neuron. These experiments provide strong evidence that the lateral connections among VS cells are a crucial component in the mechanism underlying their complex receptive fields, augmenting the direct columnar input to their dendrites.

Dynamic receptive fields of reconstructed pyramidal cells in layers 3 and 2 of rat somatosensory barrel cortex.

Whole-cell voltage recordings were made in vivo from subsequently reconstructed pyramidal neurons (n = 30) in layer 3 (L3) and layer 2 (L2) of the barrel cortex of urethane-anaesthetised rats. Average resting membrane potentials were well below (15-40 mV) action potential (AP) initiation threshold. The average spontaneous AP activity (0.068 +/- 0.22 APs s-1) was low. Principal whisker (PW) deflections evoked postsynaptic potentials (PSPs) in almost all cells of a PW column but evoked AP activity (0.031 +/- 0.056 APs per PW stimulus 6 deg deflection) was low indicating 'sparse' coding by APs. Barrel-related cells (n = 16) have their soma located above a barrel and project their main axon through the barrel whereas septum-related cells (n = 8) are located above and project their main axon through the septum between barrels. Both classes of cell had broad subthreshold receptive fields (RFs) which comprised a PW and several (> 8) surround whiskers (SuW). Barrel-related cells had shorter PSP onset latencies (9.6 +/- 4.6 ms) and larger amplitude PW stimulus responses (9.1 +/- 4.5 mV) than septum-related cells (23.3 +/- 16.5 ms and 5.0 +/- 2.8 mV, respectively). The dendritic fields of barrel-related cells were restricted, in the horizontal plane, to the PW column width. Their axonal arbors projected horizontally into several SuW columns, preferentially those representing whiskers of the same row, suggesting that they are the major anatomical substrate for the broad subthreshold RFs. In barrel-related cells the response time course varied with whisker position and subthreshold RFs were highly dynamic, expanding in size from narrow single-whisker to broad multi-whisker RFs, elongated along rows within 10-150 ms following a deflection. The response time course in septum-related cells was much longer and almost independent of whisker position. Their broad subthreshold RF suggests that L2/3 cells integrate PSPs from several barrel columns. We conclude that the lemniscal (barrel-related) and paralemniscal (septum-related) afferent inputs remain anatomically and functionally segregated in L2/3.

Auditory Cortical Spatial Receptive Fields

Neurons in the primary auditory cortex (AI) of anesthetized cats were studied for their sensitivity to directions of transient sounds in virtual acoustic space under a variety of conditions. An effective transient sound evokes a single spike or short burst of spikes with a precisely timed onset. The aggregate of effective directions forms a spatial receptive field. Typically, spatial receptive fields are large, often occupying a quadrant or more of acoustic space. Within the receptive field onset latency varies systematically with direction thereby providing information about source direction. This receptive field structure is highly robust, remaining relatively stable under conditions of competing sounds. Maximum likelihood analysis suggests that psychophysical spatial acuity can be achieved with a relatively small ensemble of AI neurons with broad receptive fields having response gradients of latency. Using reverse correlation and white-noise analysis receptive fields were mapped in space and time. This analysis revealed that spatial receptive fields of AI neurons need not be static but may exhibit marked temporal dynamics. This suggests a sensitivity for direction and speed of moving sound sources.

Abnormal retinotopic organization of the dorsal lateral geniculate nucleus of the tyrosinase-negative albino cat.

Visual defects associated with hypopigmentation have been studied extensively in Siamese and albino cats. Previous research on tyrosinase-negative albino cats has shown that (1) approximately 95% of all nasal and temporal retinal fibers cross at the optic chiasm, and (2) ocular dominance columns normally found in cortex are replaced with hemiretinal domains. In this study, we compared the retinotopic organization of the dorsal lateral geniculate nucleus (LGNd) and visual cortex in albino cats. Extracellular recordings were conducted in the LGNd, area 17, and area 18 of six albino cats. Receptive fields (RFs) were plotted for all sites. We find that, as in albino visual cortex, the albino LGNd contains (1) normal cells with RFs in the visual hemifield contralateral to the recording site (RFc), (2) abnormal cells with RFs in the ipsilateral hemifield (RFi), (3) abnormal cells with dual, mirror-symmetric RFs, one in each hemifield (RFd), and (4) abnormal cells with broad RFs that span the vertical meridian (RFb). Our data indicate that lamina A and lamina A1 consist predominantly of normal RFc and abnormal RFi cells, respectively. The C laminae contain a mixture of RFc, RFi, RFd, and RFb cells. The interlaminar zones contained RFd cells, RFb cells, or both. Thus, the albino LGNd is arranged into hemiretinal and not ocular dominance laminae. Finally, the percentage of normal cells is significantly larger in area 17 (84%) and area 18 (70%) than in the LGNd (46%), suggesting a suppression of abnormal activity in albino cat cortex, which could underlie the existing competence of visual function in albinos.

Antennal receptive fields of pheromone-responsive projection neurons in the antennal lobes of the male sphinx moth Manduca sexta.

Stimulation of the antenna of the male moth, Manduca sexta, with a key component of the female's sex pheromone and a mimic of the second key component evokes responses in projection neurons in the sexually dimorphic macroglomerular complex of the antennal lobe. Using intracellular recording and staining techniques, we studied the antennal receptive fields of 149 such projection neurons. An antennal flagellum was stimulated in six regions along its proximo-distal axis with one or both of the pheromone-related compounds while activity was recorded in projection neurons. These neurons fell mainly into two groups, based on their responses to the two-component blend: neurons with broad receptive fields that were excited when any region of the flagellum was stimulated, and neurons selectively excited by stimulation of the proximal region of the flagellum. Projection neurons that were depolarized by stimulation of one antennal region were not inhibited by stimulation of other regions, suggesting absence of antennotopic center-surround organization. In most projection neurons, the receptive field was determined by afferent input evoked by only one of the two components. Different receptive-field properties of projection neurons may be related to the roles of these neurons in sensory control of the various phases of pheromone-modulated behavior of male moths.

A Method of Generating Pattern Descriptions for Robust Image Recognition via Fusion of Visual Information

This paper describes a method which gives new pattern descriptions of observed images for computer vision with robustness to perturbation in the observed images. Although experimental results support that biological pattern vision should extract information about the observed images from signals with a variety of spatial frequencies over its broad receptive field, it is difficult to implement such an extraction due to the uncertainty relationship between the signal and its Fourier Transform; therefore, the biological pattern vision is considered to possess a function for suppression of this uncertainty relationship. We regard this function as fusion and consider that this fusion brings the robustness to the biological pattern vision; then, we propose a method of generating pattern descriptions of the observed images via this fusion. We first describe this fusion as a mathematical constraint. We second introduce a stochastic model satisfying this constraint. We consequently arrive at a Linear Programming; therefore, the pattern descriptions of the observed images can effectively be generated by the simplex method. We have made experiments on pattern matching with the proposed pattern descriptions. Their results have demonstrated that the proposed pattern descriptions are robust to the perturbations in contrast to a conventional method.

ON-OFF amacrine cells in cat retina.

We studied the morphology, photic responses, and synaptic connections of ON-OFF amacrine cells in the cat retina by penetrating them with intracellular electrodes, staining them with horseradish peroxidase, and examining them with the electron microscope. In a sample of seven cells, we found two different morphological types: the A19, which ramifies narrowly in stratum 2 (sublamina a) of the inner plexiform layer, and the A22, which ramifies mostly in stratum 4 (sublamina b) but extends some dendrites to sublamina a. Both of these cell types have axon-like processes that extend > 800 microns from the conventional dendritic arbor. ON-OFF amacrine cells in our sample had receptive fields (1.7 +/- 0.3 mm diameter) that were broader than their dendritic arbors (425 +/- 35 microns diameter) and that extended over the region of axon-like processes. In addition, we found many features in common with ON-OFF amacrine cells in poikilotherm vertebrates: a broad receptive field without surround antagonism, two sizes of spike-like events, narrow dynamic range (1 log unit intensity), and excitatory postsynaptic potentials at light on and light off. Two A19 amacrine cells were examined in the electron microscope: most synaptic inputs (93 and 76%, respectively) to either cell were from amacrine cells, with minor inputs from cone bipolar cells. Synaptic outputs were to bipolar, amacrine, and ganglion cells, including the OFF-alpha cell.

Simulation of an anatomically defined local circuit: the cone-horizontal cell network in cat retina.

The outer plexiform layer of the retina contains a neural circuit in which cone synaptic terminals are electrically coupled and release glutamate onto wide-field and narrow-field horizontal cells. These are also electrically coupled and feed back through a GABAergic synapse to cones. In cat this circuit's structure is known in some detail, and much of the chemical architecture and neural responses are also known, yet there has been no attempt to synthesize this knowledge. We constructed a large-scale compartmental model (up to 50,000 compartments) to incorporate the known anatomical and biophysical facts. The goal was to discover how the various circuit components interact to form the cone receptive field, and thereby what possible function is implied. The simulation reproduced many features known from intracellular recordings: (1) linear response of cone and horizontal cell to intensity, (2) some aspects of temporal responses of cone and horizontal cell, (3) broad receptive field of the wide-field horizontal cell, and (4) center-surround cone receptive field (derived from a "deconvolution model"). With the network calibrated in this manner, we determined which of its features are necessary to give the cone receptive field a Gaussian center-surround shape. A Gaussian-like center that matches the center derived from the ganglion cell requires both optical blur and cone coupling: blur alone is too narrow, coupling alone gives an exponential shape without a central dome-shaped peak. A Gaussian-like surround requires both types of horizontal cell: the narrow-field type for the deep, proximal region and the wide-field type for the shallow, distal region. These results suggest that the function of the cone-horizontal cell circuit is to reduce the influence of noise by spatio-temporally filtering the cone signal before it passes through the first chemical synapse on the pathway to the brain.

Nociceptive neurons in area 24 of rabbit cingulate cortex.

1. Single-unit responses in area 24 of cingulate cortex were examined in halothane-anesthetized rabbits during stimulation of the skin with transcutaneous electrical (TCES, 3-10 mA), mechanical (smooth or serrated forceps to the dorsal body surface or graded pressures of 100-1,500 g to the stabilized ear) and thermal (> 25 degrees C) stimulation. 2. Of 542 units tested in cingulate cortex, 150 responded to noxious TCES (> or = 6 mA), 93 of 221 units tested responded to noxious mechanical (serrated forceps) and 9 of 47 units tested responded to noxious heat (> 43 degrees C) stimuli. Twenty-five percent of the units that responded to noxious mechanical stimuli also responded to noxious heat stimuli. The only innocuous stimulus that evoked activity in cingulate cortex was a "tap" to the skin and this was effective for 11 of 14 tested units. 3. In 74 units that produced excitatory responses to TCES of the contralateral ear, response latency was 166 +/- 11.3 (SE) ms and response duration was 519 +/- 52.1 ms. 4. Twenty of the 150 units that responded to noxious TCES were initially inhibited. These responses were usually < 1 s in duration (17 of 20 units), whereas responses in the other 3 lasted for over 20 s. 5. Most units had broad receptive fields, because noxious mechanical stimuli anywhere on the dorsal surface of the rabbits, including the face and ears, evoked responses. A small number of units for which the entire body surface was tested (3 of 15 units) had receptive fields limited to the ears, rostral back, and forepaws. 6. Fifteen of 33 units tested had no preferential responses to noxious TCES of the ipsilateral and contralateral ears. Of the remaining units, 10 had a greater response to contralateral and 8 had a greater response to ipsilateral stimuli. 7. The locations of 186 units were histologically verified. Most nociceptive cingulate units were in dorsal area 24b in layers III (n = 35), II (n = 13), or V (n = 9). 8. Cortical knifecut lesions were made in five rabbits to determine if the responses in area 24 were dependent on lateral or posterior cortical inputs. These lesions did not alter the percentage of units driven by noxious stimuli nor response latency. 9. Injections of lidocaine were made into medial parts of the thalamus in six animals and injection and recording sites analyzed histologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative analysis of the recurrent collaterals derived from Purkinje cells in zone X of the cat's vermis

Intracellular electrodes filled with 4% horseradish peroxidase (HRP) were used to impale Purkinje cells in zone x of the cat's cerebellum. Subsequent to recording responses to bilateral stimulation of the ulnar, radial, and sciatic nerves, the cells were injected with HRP. Intracellularly labeled Purkinje cells were located and correlated with the physiological data. Purkinje cells in zone x respond primarily to stimulation of the ipsilateral forelimb. However, they have broad receptive fields in that most also receive inputs from stimulation of the contralateral forelimb or from more than one forelimb nerve; a few respond to activation of the hindlimb. The distribution of the recurrent collaterals of zone x Purkinje cells has several features that are similar to those from zones a,b, and c: 1) they have a similar number of varicosities within the plexus; 2) they distribute primarily in the sagittal as compared to the transverse plane; and 3) most varicosities in the plexus are located within the Purkinje cell layer. However, several characteristics distinguish the collaterals of zone x Purkinje cells. First, they have a greater transverse extent than those derived from Purkinje cells in zones a,b, and c. Second, they have a higher percentage of collaterals that extend to more superficial aspects of the molecular layer as well as to deep levels of the granule cell layer. The rostral extent of zone x has not been clearly defined in previous studies. On the basis of anatomical and physiological data derived from the present study it appears that Purkinje cells located in lobule Va have physiological and morphological properties similar to those observed for Purkinje cells in more caudal folia of lobule V. Thus, on the basis of the organization of local circuits, it appears that zone x extends at least to the most rostral folia of lobule V. In conclusion, these data suggest that Purkinje cells in zone x may not be involved in controlling the movements of specific muscles, as may be the case in other cortical zones. Rather, it could be hypothesized that Purkinje cells in zone x have a more generalized role in coordinating agonist and antagonist muscle groups in one limb or regulating patterns of movement between limbs.

Modulation of raphe-spinal neurons by supraspinal sensory system. Periaqueductal gray and thalamic relay nucleus (author's transl)

The effect of stimulation of peripheral and supraspinal sensory system on the activities of raphe-spinal neurons were studied extracellularly in chloralose-anesthetized cats. Among 300 raphe neurons recorded from the brainstem of cats, a total of 74 raphe-spinal neurons were activated antidromically by stimulation of the cervical dorsolateral funiculus. The mean conduction velocity of these neurons, calculated from the stimulation-recording distance and latencies, was 25.4 m/sec. These raphe-spinal neurons were found to originate in the caudal raphe nuclei, i.e., 56 in raphe magnus, 14 in raphe obscurus, 3 in raphe pallidus and 1 in raphe pontis. All of them were shown to respond to innocuous and/or noxious peripheral mechanical stimuli, all with broad receptive fields. Single pulse periaqueductal gray (PAG) stimulation evoked driven firing with the spike per stimulus ratio ranging from 1 to 4 and latencies ranging from 5 to 15 msec in 57 (95%) of 60 units examined. Driven firing was followed by inhibition with a period ranging from 100 to 200 msec. Repetitive PAG stimulation at 20 Hz induced powerful facilitation of firing. The facilitation persisted for several minutes after termination of the stimulation when the repetitive stimulation was continued for 5 to 30 sec. Single pulse VPL or VPM stimulation evoked driven firing with the spike per stimulus ratio ranging from 1 to 4 and latencies ranging from 10 to 20 msec in 58 (97%) of 60 units tested. Long lasting powerful facilitation was also provoked in those units by repetitive stimulation at 20 Hz for 5 to 30 sec. This facilitation was not affected by removal of the bilateral sensorimotor area. PAG-induced facilitation was apparently and reversibly avoided by the administration of naloxone (0.1-0.3 mg/kg, i.v.) in 7 (70%) of 10 units investigated. On the other hand, the administration of naloxone (0.3 mg/kg, i.v.) was found not to affect VPL-induced facilitation of firing in any of ten units examined. Effects of sustained repetitive stimulation of the thalamic relay nucleus on activities of raphe-spinal neurons were studied in 17 units. After sustained stimulation for 3 hours, facilitation of firing by the thalamic relay nucleus stimulation was shown to completely disappear. Facilitatory response to single pulse thalamic relay nucleus stimulation was also found to be reduced. Nevertheless, facilitation of firing induced by single pulse or repetitive PAG stimulation was apparently unchanged. These results suggest that both thalamic relay nucleus stimulation and PAG stimulation for pain-relief may activate a final common path, i.e., raphe-spinal neurons, which then inhibit spinal neurons sensitive to pain, as a part of the mechanism of pain-suppression. Although the thalamic relay nucleus-induced facilitation of raphe-spinal neurons was shown to be as powerful as that of PAG, it did not appear to relate to the opiate mechanism, unlike that of PAG.

Diencephalic modulation of activities of raphe-spinal neurons in the cat

The modulatory effects of diencephalic stimulation on the activities of raphe-spinal neurons were studied extracellularly in cats. Among 240 raphe neurons recorded, 57 neurons were activated antidromically by stimulation of the cervical dorsolateral funiculus. These raphe-spinal neurons were found in the caudal raphe nuclei, i.e., the raphe magnus (43 neurons), raphe obscurus (11), raphe pallidus (2), and raphe pontis (1). All of them responded to innocuous and/or noxious peripheral mechanical stimuli with a broad receptive field. The activities of the majority of these neurons were facilitated by trains of pulse stimulation of the rostral periaqueductal gray and the thalamic relay nucleus but not of the thalamic center median nucleus. The facilitation of firing persisted for more than 3 min after the cessation of train pulse stimulation when the stimulation was applied at 20 Hz for 5 to 30 s. This facilitation was not affected by decortication of the sensorimotor area bilaterally. The facilitatory response to periaqueductal gray stimulation was markedly suppressed by systemic administration of naloxone. On the other hand, that of the thalamic relay nucleus stimulation was found to be unaffected. Based on these findings, the mechanisms of pain relief by stimulation of the rostral periaqueductal gray and thalamic relay nucleus reported in human intractable pain appear to relate, at least partly, to the activation of raphe-spinal neurons. However, the paths to raphe-spinal neurons of stimuli from the periaqueductal gray and the thalamic relay nucleus are thought to be independent from each other based on the different effects of naloxone.

Synaptic transmission to the horizontal cells in the retina of the larval tiger salamander.

1. The receptive field diameter for most horizontal cells far exceeds the lateral spread of processes for any cell. Therefore horizontal cells probably receive synaptic input from neighbours as well as from the photoreceptors. The electrical effects of these two synaptic inputs were studied. 2. We have characterized the electrical properties of the horizontal cell inputs by determining the current-voltage curves in dark and light. These curves were compared with those obtained in the presence of Co2+ or Mg2 was nearly identical to the curve in the light. 4. The putative transmitter substances glutamate, aspartate and GABA depolarized the cells by increasing conductance. Current-voltage curves measured in the presence of these substances intersected the dark and light curves at +50 mV, the same level at which the dark and light curves intersect. 5. The light response of cells uith broad receptive fields, between 1.0 and 2.0 mm, showed little or no change in conductance associated with the light response. The input resistance was near 20 Momega, and the current-voltage curves intersected at an extrapolated potential level near 200 mV. 6. In the presence of ACh, electrical properties of the broad field cells reverted to those of the narrow field cells: the receptive field was reduced to 0.5 mm, the imput resistance increased, and the current-voltage curves intersected near +50 mV. Thus ACh appeared to interrupt synaptic input from neighbouring horizontal cells. 7. The results confirm the suggestion that horizontal cells receive a tonic excitatory input from the photoreceptors which is decreased by light. They show that horizontal cells receive an additional input from their neighbours, not associated with a measurable conductance change. The input from neighbours is selectively interrupted by ACh, but the nature of this synapse and of the cholinergic action is not known.

Neuronal activity in the lateral vestibular nucleus of the cat

The synaptic input to Deiters neurones evoked by stimulation of peripheral somatic nerves was measured by intracellular recordings. EPSPs with broad receptive fields and latencies which indicate polysynaptic connexions were commonly evoked from the FRA. In other cells, low threshold cutaneous afferents were effective at rather short latencies suggesting oligosynaptic connexions from fast ascending fibres. One example was found of EPSPs due to low threshold muscle afferents. IPSPs due to climging fibre activation of Purkinje cells as observed in most of the neurones were evoked by cutaneous volleys above 1.5-2.0T and muscle volleys above 5T (above 3-3.5T in case of Q). Often, IPSPs were evoked by stimulation of nerves, to the segmental level of which the the vestibulospinal neurone under investigation projected. A small proportion of cells received short latency IPSPs involving direct fast mossy fibre tracts, which were evoked from low threshold cutaneous afferents. IPSPs due to polysynaptic mossy fibre activation of Purkinje cells were evoked from the FRA bilaterally and from ipsilateral cutaneous afferents at 1.5-2.0T ("prolonged inhibition"). Prolonged excitatory/inhibitory events mediated by mossy fibre pathways may be involved in quadruped locomotion or other processes making use of a broad motor integration.

Activity patterns of interneurons in the caudal ganglion of the crayfish.

Responses of ascending interneurons from the caudal ganglion of crayfish have been recorded from single units isolated by dissection from the ventral nerve cord; in addition, post-synaptic activity within the ganglionic neuropile has been studied with intracellular micropipettes. The following classes of interneurons have been found: (1) Large fibers which responded to tactile stimuli with single spikes or phasic bursts. These units usually showed broad receptive fields; and spontaneous activity, when present, showed transitory depressions following responses to natural stimuli. (2) A group of fibers, including many small ones, which responded to proprioceptive stimuli with tonic discharges of varying adaptation rate. (3) Interneurons which showed responses both to tactile stimuli and to activation of the sixth ganglion photoreceptor; and (4) units with constant frequency discharges which were unmodifiable by any of the above afferent inputs. Intracellular recording of post-synaptic activity has shown (1) that widely graded excitatory post-synaptic potentials occur; (2) that multiple firing from single synaptic potentials is usual; (3) that the post-synaptic responses to phasic natural stimuli and to electrical stimulation of ganglionic roots are similar. The existence of widely graded post-synaptic potentials and of extensive receptive fields suggests a high degree of convergence from primary afferents to interneurons. The activation of such post-synaptic units involves integrative synaptic transfer, without 1:1 correspondence between pre- and post-fiber activity.