Responses of single units in the monkey superior colliculus to moving stimuli

1. Single unit responses of pan-directional cells to moving and stationary flashing stimuli were studied in the superficial layers of the superior colliculus in paralysed, anaesthetized rhesus monkeys. The aim of this study was to see how far cell responses to moving stimuli fit in with what would be expected from their responses to stationary flashing stimuli. 2. Both the leading and the trailing edge of a moving stimulus evoke a transient response. If the diameter of moving light spots is increased the strength of the leading edge response increases, reaches a maximum and decreases to a constant value which is similar to the behaviour of the on response when the diameter of flashing spots is increased. The strength of the trailing edge response increases and reaches the same strength as that of the leading edge response. If the width of a long moving slit is increased, the strength of the leading edge response is the same at all slit widths, while the strength of the trailing edge response shows a course similar to that of the trailing edge response if the spot diameter is increased. If the length of a wide moving slit is increased both the leading and the trailing edge responses decrease. These results indicate that the strength of both leading and trailing edge responses is dependent on the degree the inhibitory surround is activated. 3. The leading and the trailing edge of a stimulus evoke their responses at the same position in the receptive field independent of the direction of movement. 4. Increasing the velocity of a moving stimulus shows that in general the leading edge response is present up to higher velocities than the trailing edge response independent of the sign of contrast. The burst duration to moving stimuli decreases with increasing stimulus velocity and appears to be determined by the time a moving edge is present in the receptive field centre. When this time becomes shorter than 10--20 ms, the burst duration for moving stimuli is constant and about the same as for flashing stimuli. This indicates that, although spatial receptive field properties can vary considerably, temporal receptive field properties show a strong similarity among different units. 5. The response latencies to light and dark moving edges are the same, which in turn are about equal to the response latencies to stationary flashing stimuli. 6. Stimulation experiments show that the general response characteristics to moving stimuli can be predicted by using a set of receptive field parameters derived from responses to stationary flashing stimuli. The most important variable of moving stimuli appears to be the period of time a moving contour is present within the receptive field centre, besides the degree of activation of the inhibitory surround.     

Factors forming the edge of a receptive field: the presence of relatively ineffective afferent terminals

A specialized type of spinal cord cell has its cell body in lamina IV and has a small low threshold cutaneous receptive field which is remarkable for its abrupt edge. No signs could be found of a subliminal fringe to this field since its size remains fixed during wide excursions of the cell's excitability. Reversible blocking of peripheral nerves and dorsal roots showed that the afferents responsible for exciting these cells following natural stimuli, run in a restricted area of peripheral nerve and dorsal root. When the fibres necessary to sustain the natural stimulus receptive field were blocked, it was shown that other large myelinated fibres in neighbouring roots were still capable of firing the cell monosynaptically following electrical stimulation of the root or periphery although no natural stimuli were able to change the cell's excitability. It is necessary to divide the afferent synapses on such cells into a class which is highly effective in firing the cell on natural stimulation and a second class which has no effect yet detected following natural stimuli but which can fire the cell monosynaptically if synchronously activated by electrical stimulation. Suggestions are made for possible presynaptic and post-synaptic mechanisms which might divide the effect of arriving impulses into two such classes.     

The inhibitory components in the responses of the lateral suprasylvian area neurons to moving stimuli in cats

The inhibitory components in the neuronal responses of the cat's lateral suprasylvian area (LSA) to moving bright and dark stimuli were investigated. The LSA neurons could be divided into two groups. Neurons of the first group (33%) do not reveal spatial displacement of the inhibitory zones and show displacement of the discharge centers in the receptive field only for one polarity of contrast of moving stimuli, either brighter or darker than the background. The second group (67%) contained the neurons which showed a spatial displacement of the inhibitory components and discharge centers in the receptive field for either polarity of contrasts of the moving stimuli. Tested with stationary flashing stimuli, the majority of neurons in both groups had overlapping ON-OFF discharge regions within their receptive fields. The results obtained with moving stimuli of different speeds and with the masking method suggest the rebound origin of the inhibitory responses in LSA neurons.     

Acute effect of an incision on mechanosensitive afferents in the plantar rat hindpaw

The purpose of this study was to examine which primary afferent fibers are sensitized to mechanical stimuli after an experimental surgical incision to the glabrous skin of the rat hindpaw. Afferent fibers teased from the L(5) dorsal root or the tibial nerve were recorded in anesthetized rats. The mechanical response properties of each fiber were characterized before and 45 min after an incision (or sham procedure) within the mechanical receptive field. Sensitization is characterized by an expansion of the mechanical receptive field, an increase in background activity, an increase in response magnitude, or a decrease in response threshold. After incision, the background activity and response properties of Abeta-fibers (n = 9) to mechanical stimuli were unchanged. Four of 13 mechanosensitive Adelta-fibers exhibited sensitization after the incision; response threshold decreased, response magnitude increased, or receptive field size increased. Background activity of Adelta-fibers was not increased by the incision. Sensitization was observed in 4 of 18 mechanosensitive C-fibers 45 min after the incision. Background activity of C-fibers was not increased by the incision. In a group of mechanically insensitive afferent fibers (MIAs), 3 of 7 Adelta-fibers and 4 of 10 C-fibers sensitized 45 min after incision. Response threshold was decreased in only 2 of 17 MIAs; receptive field size increased in 7 of 17 MIAs. Abeta-fibers did not sensitize after the incision, and only 8 of 31 (26%) mechanosensitive Adelta- and C-fibers gave evidence of sensitization. In a group of MIA Adelta- and C-fibers, a greater percentage of 17 fibers studied (41%) were sensitized after incision. In this model, the principal effect of an incision, when examined 45 min after the insult, is an increase in receptive field size of the afferents, particularly those characterized as MIAs. To the extent that the mechanical hyperalgesia characterized in the same model is initiated in the periphery, it would appear that spatial summation of modestly increased response magnitude is important to the development of hyperalgesia.     

Indirect, across-the-midline retinotectal projections and representation of ipsilateral visual field in superior colliculus of the cat

1. In agreement with previous work, we have found that the ipsilateral visual field is represented in an extensive rostral portion--from one-third to one-half--of the superior colliculus (SC) of the cat. This representation is binocular. The SC representation of the ipsilateral visual field can be mediated both directly, by crossed retinotectal connections originating from temporal hemiretina, and indirectly, by across-the-midline connections relaying visual information from one-half of the brain to contralateral SC. 2. In order to study the indirect, across-the-midline visual input to the SC, we have recorded responses of SC neurons to visual stimuli presented to either the ipsilateral or the contralateral eye of cats with a midsagittal splitting of the optic chiasm. Units driven by the ipsilateral eye, presumably through the direct retinotectal input and/or corticotectal connections from ipsilateral visual cortex, were found throughout the SC, except at its caudal pole, which normally receives fibers from the extreme periphery of the contralateral nasal hemiretina. Units driven by the contralateral eye, undoubtedly through an indirect across-the-midline connection, were found only in the anterior portion of the SC, in which is normally represented the ipsilateral visual field. Receptive fields in both ipsilateral and contralateral eye had properties typical of SC receptive fields in cats with intact optic pathways. 3. All units having a receptive field in the contralateral eye had also a receptive field in the ipsilateral eye; for each of these units, the receptive fields in both eyes invariably abutted the vertical meridian of the visual field. The receptive field in one eye had about the same elevation relative to the horizontal meridian and the same vertical extension as the receptive field in the other eye; the two receptive fields of each binocular unit matched each other at the vertical meridian and formed a combined receptive field straddling the vertical midline of the horopter...     

The conduction velocity of lateral inhibition in the cat's retina

Summary By means of microelectrodes action potentials of single optic nerve fibers from on-center neurons of the cat's retina were recorded. The latency of the inhibition of the neuronal activity elicited by light stimuli in the inhibitory receptive field periphery was measured. The inhibiting light stimuli (rings or bars) were projected into different parts of the receptive field periphery. From the spatial separations of the inhibiting light stimuli and the differences of the corresponding inhibition latencies the conduction velocity of lateral inhibition within the receptive field was measured. A value of 90–110 mm · sec^–1 in the retina was found, which corresponds to about 400–480 degrees · sec^–1 in the visual field.Mit Unterstützung der Deutschen Forschungsgemeinschaft (Gr. 161/5-12). FrauA. Thiele, Frl.R. Sasowski und Frl.I. Korten danken wir für sorgfältige technische Assistenz. Das Computerprogramm schrieb Frl.J. Vierkant.     

The two-dimensional spatial structure of simple receptive fields in cat striate cortex

1. A reverse correlation (6, 8, 25, 35) method is developed that allows quantitative determination of visual receptive-field structure in two spatial dimensions. This method is applied to simple cells in the cat striate cortex. 2. It is demonstrated that the reverse correlation method yields results with several desirable properties, including convergence and reproducibility independent of modest changes in stimulus parameters. 3. In contrast to results obtained with moving stimuli, we find that the bright and dark excitatory subregions in simple receptive fields do not overlap to any great extent. This difference in results may be attributed to confounding the independent variables space and time when using moving stimuli. 4. All simple receptive fields have subregions that vary smoothly in all directions in space. There are no sharp transitions either between excitatory subregions or between subregions and the area surrounding the receptive field. 5. Simple receptive fields vary both in the number of subregions observed, in the elongation of each subregion, and in the overall elongation of the field. In contrast with results obtained using moving stimuli, we find that subregions within a given receptive field need not be the same length. 6. The hypothesis that simple receptive fields can be modeled as either even symmetric or odd symmetric about a central axis is evaluated. This hypothesis is found to be false in general. Most simple receptive fields are neither even symmetric nor odd symmetric. 7. The hypothesis that simple receptive fields can be modeled as the product of a width response profile and an orthogonal length response profile (Cartesian separability) is evaluated. This hypothesis is found to be true for only approximately 50% of the cells in our sample.     

Characteristics of surround inhibition in cat area 17

 The effects of stimuli falling outside the ’classical receptive field’ and their influence on the orientation selectivity of cells in the cat primary visual cortex are still matters of debate. Here we examine the variety of effects of such peripheral stimuli on responses to stimuli limited to the receptive field. We first determined the extent of the classical receptive field by increasing the diameter of a circular patch of drifting grating until the response saturated or reached a maximum, and by decreasing the diameter of a circular mask in the middle of an extended grating, centred on the receptive field, until the cell just began to respond. These two estimates always agreed closely. We then presented an optimum grating of medium-to-high contrast filling the classical receptive field while stimulating the surround with a drifting grating that had the same parameters as the central stimulus but was varied in orientation. For all but five neurons (of 37 tested), surround stimulation produced clear suppression over some range of orientations, while none showed explicit facilitation under these conditions. For 11 cells (34% of those showing suppression), the magnitude of suppression did not vary consistently with the orientation of the surround stimulus. In the majority of cells, suppression was weakest for a surround grating oriented orthogonal to the cell’s optimum. Nine of these cells (28%) exhibited maximum inhibition at the optimum orientation for the receptive field itself, but for 12 cells (38%) there was apparent ’release’ from inhibition for surround gratings at or near the cell’s optimum orientation and direction, leaving inhibition either maximal at angles flanking the optimum (9 cells) or broadly distributed over the rest of the orientation range (3 cells). This implies the existence of a subliminal facilitatory mechanism, tightly tuned at or near the cell’s optimum orientation, extending outside the classical receptive field. For just two cells of 13 tested the preferred orientation for a central grating was clearly shifted towards the orientation of a surrounding grating tilted away from the cell’s optimum. The contrast gain for central stimulation at the optimal orientation was measured with and without a surround pattern. For nine of 25 cells tested, surround stimulation at the cell’s optimum orientation facilitated the response to a central grating of low contrast (≤0.1) but inhibited that to a higher-contrast central stimulus: the contrast-response gain is reduced but the threshold contrast is actually decreased by surround stimulation. Hence the receptive field is effectively larger for low-contrast than for high-contrast stimuli. Inhibition from the periphery is usually greatest at or around the cell’s optimum, while suppression within the receptive field has been shown to be largely non-selective for orientation. Inhibition by orientations flanking the optimum could serve to sharpen orientation selectivity in the presence of contextual stimuli and to enhance orientational contrast; and it may play a part in orientation contrast illusions. Received: 28 July 1996 / Accepted: 30 January 1997     

Identification of cone mechanisms in monkey ganglion cells

1. Blue, green, and red sensitive cone mechanisms have been studied in two types of on-centre ganglion cells in the Rhesus monkey's retina.2. One type of cell receives signals from both green and red sensitive cone mechanisms, both of which excite in the centre and inhibit in the periphery of the cell's receptive field. These cells discharge transiently to maintained stimuli of any wave-length and are called phasic.3. The second type of cell receives excitatory signals from only one cone mechanism, either blue, green or red sensitive, in the centre, and inhibition from another cone mechanism in the periphery of its receptive field. These cells discharge continuously to maintained stimuli of appropriate wave-length and are called tonic.4. Tonic cells outnumber phasic cells although both are found adjacent to one another throughout the retina. Phasic cells are relatively more common toward the periphery and tonic cells relatively more common toward the fovea.     

Nitric oxide controls the light adaptive chromatic difference in receptive field size of H1 horizontal cell network in carp retina

In carp retina, the receptive field size of the H1-type horizontal cell (HC) network is known to be chromatically selective, as electrophysiological signals are generated by short-wavelength (SW) light stimuli, which spread much less than those for long-wavelength (LW). We have shown previously that the signalling mechanism underlying this chromatic difference operates only in the light-adapted retina and that it involves cGMP as an intermediary messenger. In the present study, the possible role of nitric oxide (NO) as such a control mechanism was investigated. Application of a NO donor (SNP or SNOG) to dark-adapted retinae produced a chromatic difference in the receptive field size, such as in the light-adapted state. This effect was due mainly to a reduction in the spread of signals generated by SW stimuli; LW signalling spread was not altered. No such effect was observed in light-adapted retinae where a chromatic difference in receptive field size was already present. On the other hand, application of the NO 'scavenger' haemoglobin to light-adapted retinae suppressed the chromatic difference. These results are consistent with NO being a light-adaptive retinal neuromodulator involved in the generation of the chromatic difference in H1 cell receptive field size. These results are discussed in the context of two different hypotheses.     

The time course of perisaccadic receptive field shifts in the lateral intraparietal area of the monkey

Neurons in the lateral intraparietal area of the monkey (LIP) have visual receptive fields in retinotopic coordinates when studied in a fixation task. However, in the period immediately surrounding a saccade these receptive fields often shift, so that a briefly flashed stimulus outside the receptive field will drive the neurons if the eye movement will bring the spatial location of that vanished stimulus into the receptive field. This is equivalent to a transient shift of the retinal receptive field. The process enables the monkey brain to process a stimulus in a spatially accurate manner after a saccade, even though the stimulus appeared only before the saccade. We studied the time course of this receptive field shift by flashing a task-irrelevant stimulus for 100 ms before, during, or after a saccade. The stimulus could appear in receptive field as defined by the fixation before the saccade (the current receptive field) or the receptive field as defined by the fixation after the saccade (the future receptive field). We recorded the activity of 48 visually responsive neurons in LIP of three hemispheres of two rhesus monkeys. We studied 45 neurons in the current receptive field task, in which the saccade removed the stimulus from the receptive field. Of these neurons 29/45 (64%) showed a significant decrement of response when the stimulus appeared 250 ms or less before the saccade, as compared with their activity during fixation. The average response decrement was 38% for those cells showing a significant (P < 0.05 by t-test) decrement. We studied 39 neurons in the future receptive field task, in which the saccade brought the spatial location of a recently vanished stimulus into the receptive field. Of these 32/39 (82%) had a significant response to stimuli flashed for 100 ms in the future receptive field, even 400 ms before the saccade. Neurons never responded to stimuli moved by the saccade from a point outside the receptive field to another point outside the receptive field. Neurons did not necessarily show any saccadic suppression for stimuli moved from one part of the receptive field to another by the saccade. Stimuli flashed <250 ms before the saccade-evoked responses in both the presaccadic and the postsaccadic receptive fields, resulting in an increase in the effective receptive field size, an effect that we suggest is responsible for perisaccadic perceptual inaccuracies.     

Neural responses in motor cortex and area 7a to real and apparent motion

The neural activity in area 7a and the arm area of motor cortex was recorded while real or path-guided apparent motion stimuli were presented to behaving monkeys in the absence of a motor response. A smooth stimulus motion was produced in the real motion condition, whereas in the apparent motion condition five stimuli were flashed successively at the vertices of a regular pentagon. The stimuli moved along a low contrast circular path with one of five speeds (180–540 deg/s). We found strong neural responses to real and apparent motion in area 7a and motor cortex. In the motor cortex, a substantial population of neurons showed a selective response to real moving stimuli in the absence of a motor response. This activity was modulated in some cases by the stimulus speed, and some of the neurons showed a response during a particular part of the circular trajectory of the stimulus; the preferred stimulus angular locations were evenly distributed across this neuronal ensemble. It is likely that these neural signals are continuously available to the motor cortex in order to generate responses that demand immediate action. In area 7a, two overlapping populations of neurons were observed. The first comprised cells the activity of which was tuned to the angular location of a circularly moving stimulus in the real motion condition. These cells also responded to apparent motion at high stimulus speeds. A visual receptive field analysis showed that the angular tuning in most of the area 7a neurons did not depend on the spatial location of the stimulus in relation to their receptive field. The second population was selective to apparent moving stimuli and showed a periodic entrainment of activation with the period of the inter-stimulus interval of the flashing dots. Both the angular location and the inter-stimulus interval neural signals can be used to generate precise behavioral responses towards real or apparent moving stimuli.     

Receptive fields of cerebellar cells receiving exteroceptive input in a Gymnotid fish.

Single neurons in the caudal lobe of the cerebellum of the weakly electric fish Apteronotus albifrons respond to distortions in the normal electric field produced by the animal. Moving plastic or metal objects as well as a simpler stimulus, a moving electrical dipole, produce adequate distortions of the fish's field to cause the cerebellar cells to respond. The moving dipole stimulated small enough areas of the fish's skin, as determined by the responses of single electroreceptors, to allow maps of the receptive fields of single cerebellar cells to be produced. The receptive fields seen varied widely in complexity from relatively small excitatory or inhibitory areas to larger fields containing multiple excitatory and inhibitory areas usually bordering one another. Most cells studied displayed directional responses. Usually qualitatively different responses resulted from opposite directions of movement, and less frequently units were seen in which no response resulted from movement opposite the direction which caused responses; Varying the rate of stimulus movement caused only small changes in the responses of cerebellar cells; however, motionless stimuli applied over areas of skin known to respond to moving stimuli produced weaker responses of the appropriate sign for that area. Movement seems to be an important component of the stimulus for these cells. Cells were also seen which responded to visual as well as to electroreceptive input. Responses to each of these two modalities presented above were quite different. The cells recorded from frequently displayed burst discharges similar to those produced by Purkinje cells in other lower vertebrates, and most of the cells studied are believed to be Purkinje cells. A somatotopic relationship was found between the position of the center of a receptive field on the fish's body and the position of the cell in the brain. All of the results obtained are compatible with the hypothesis that the caudal lobe of the cerebellum is processing electroreceptive information related to object detection.     

Visual response properties of zebrafish tectal cells

The visual properties of zebrafish tectal cells have been studied with a variety of stimulation routines. These include illumination of the whole receptive field and of the surround, use of moving edges, very small spots, bars of varing orientations, moving spots with varying direction and speeds, growing discs, and pairs of spots whose presentation varies in position and sequence. A number of properties correlate with the classification scheme set forth in the preceding paper. Type B cells, unlike other types, are insensitive to moving stimuli. Experiments involving surround stimulation show that type S cells have inhibitory surrounds while those of type I do not. Type I cells, however, exhibit several properties which are consistent with an intratectal delayed inhibitory mechanism operating within the receptive field.These properties include the response to moving edges and growing stimuli, and the dependence of response duration on the size of a flashed stimulus. Various explanations of these properties are considered, and a specific model is proposed which states that cells of type I receive inhibitory input from neighbouring tectal cells of the same physiological type. The properties involved may be of direct importance in the visual behavior of the fish.     

Binocular single vision and depth discrimination. Receptive field disparities for central and peripheral vision and binocular interaction on peripheral single units in cat striate cortex

Summary Of binocularly-activated striate neurons only a proportion have their two receptive fields in exactly corresponding positions in the contralateral hemifield. Those which are not corresponding are said to show receptive field disparity. Because the eyes diverge in the anaesthetized and paralyzed preparation, the binocular receptive fields are horizontally separate. With increasing retinal eccentricity there is a gradual decrease in this horizontal separation as well as progressive changes in the local receptive field disparities. With increasing horizontal retinal eccentricity there is a progressive increase in horizontal receptive field disparities together with a smaller decrease in vertical disparities. Receptive field disparities are relatively unaffected by increasing vertical retinal eccentricity. A neurophysiological theory for binocular single vision and depth discrimination is put forward as a theoretical framework for the construction of the horopter for the cat as well as a region analogous to Panum's fusional area in man. Observations have been made on the responses, particularly to moving slit stimuli, of units with peripherally-located receptive fields. For several binocular units it was possible to study the full range of the binocular interaction when the two receptive fields were moved from exact correspondence to positions of increasing non-alignment.     

Comparison of receptive field properties of neurons in area 17 of normal and bilaterally amblyopic cats

Receptive field properties of extracellularly recorded units in the visual cortex (area 17) of cats made bilaterally amblyopic by a variety of rearing conditions were measured and compared with the properties of units in normal cats. Properties studied included sensitivity to vernier offset, response facilitation to increasing bar length, receptive field size, responsiveness to moving and flashed stimuli, orientation tuning, the relation between mean firing rate and its variance, the amount of overlap of regions of on and off responsiveness in simple and complex cells, and, for flashed stimuli, latency to response onset, time to peak response, and response decay time constant. Behavioural testing of the amblyopic animals showed that spatial resolution was 2–4 times lower and vernier acuity thresholds 10–20 times greater than normal. Despite this, several neuronal response properties did not differ significantly from those in normal animals. These included peak responsiveness to moving stimuli, widths of orientation tuning curves, response variability, and latency to initial response for flashed stimuli. Other properties showed small but significant changes. Sensitivity to vernier offset (impulses per degree of offset) was reduced to nearly half its normal level; receptive field sizes increased by about 24% and an incomplete segregation of regions of on and off responsiveness was found in some cells, which made them hard to classify as simple or complex. Responses to flashed stimuli were smaller and more persistent. Their statistical significance notwithstanding, it seems unlikely that these relatively small response abnormalities in area 17 can fully account for the observed behavioural deficits.     

Spatial arrangements of responses by cells in the cat visual cortex to light and dark bars and edges

Summary Detailed examination is made of the responses of visual cortical cells (area 17, border 17–18 and adjacent area 18) in the anaesthetized cat to stationary flashing bars and to bars (lines) and edges moving at their optimal velocities. Particular attention is given to the receptive field organization of cells in the simple family. While there is good general agreement between the main receptive field subregions revealed by stationary and moving stimuli, the responses to moving light and dark bars, supplemented by the responses to moving light and dark edges, provide a much more rapid, accurate and complete guide to the spatial organization of the receptive fields than do the response profiles to a stationary flashing bar. Moving light and dark bars between them generally reveal more subregions in the receptive fields of simple cells than is evident from the response profiles to a stationary flashing bar, particularly when the receptive fields have many subregions. In addition the responses to moving edges provide a rapid guide to spatial summation across the width of a subregion and the possible antagonistic effects of the next subregion in sequence.Two subclasses of cells in the simple family have been recognized: ordinary simple and fast simple cells. Two cell classes (A-cells and silent periodic cells) having properties intermediate between simple and complex types are discriminated and their properties described.     

Functional properties of neurones in the posterior part of area 7 in awake monkey

The posterolateral part of the parietal association cortex (area 7 of Brodmann) was investigated in three awake, behaving macaque monkeys using transdural microelectrode recording technique. Only one of the 114 cells isolated remained unidentified. Of the cells 79% responded to somesthetic (39%) or visual (23%) or both somesthetic and visual (17%) stimulation. The somesthetic receptive fields were large, covering, for instance, the whole arm or hand. Most receptive fields were on the upper extremities. Half of the somesthetically drivable cells had bilateral receptive fields. The visually drivable cells responded only to stimuli moving in a certain direction. The effective stimuli for most of the cells responding both to visual and somatosensory stimulation were: touching of the skin and visual stimuli moving towards the cutaneous receptive field. Of the cells 20% were active only during the monkey's own movements, most often during grasping and manipulation with fingers. The results indicate that the area studied is specialized in the control of hand movements, e.g. grasping, grooming and somesthetic recognition of the forms of objects.     

Cutaneous masking. II. Geometry of excitatory andinhibitory receptive fields of single units in somatosensory cortex of the cat.

1. The responses of single neurons in the primary somatosensory cortex of the cat to brief air-pulse stimuli were quantitatively examined. These controlled natural stimuli activated almost exclusively rapidly adapting hair units which, on systematic movement of the stimulus through the receptive field, gave unit-response profiles that showed the classical unimodal tent-shaped distribution. 2. Conditioning stimulus-induced inhibition of a response evoked by a fixed test stimulus was measured by systematically moving the conditioning stimulus through the receptive field. The spatial distribution of in-field inhibitory activity was unimodal and highly covariant with that of the conditioning excitation, the peak inhibition corresponding to the functional center of the excitatory receptive field. 3. Nearly one-half of the units studied evidenced inhibition extending beyond the excitatory receptive field, forming a "surround" inhibitory region; but these were usually restricted areas with rather weak inhibitory effects. 4. Time-course measuring revealed, on the average, inhibition effects measureable from 10 ms before to some 70 ms following conditioning stimulation, with peak inhibition delayed some 10--15 ms from the conditioning stimulus onset. We showed the backward inhibition, occurring with the test stimulus delivered before the onset of the conditioning stimulus, to be a property of the test response duration. Inhibition measured in the surround areas had essentially the same time course as the inhibition calculated from measurements made within the receptive fields. 5. The spatial and temporal profiles of the excitatory and inhibitory cortical unitary activity are thus very similar to the parametric features of psychophysical enhancement and masking. These findings suggest that the excitatory and inhibitory activities related to individual stimuli interact in multipoint stimulus paradigms so that simple unimodal composite profiles are synthesized.     

The effects of strychnine on neurons in cat somatosensory cortex and its interaction with the inhibitory amino acids, glycine, taurine and beta-alanine

In area 3b of primary somatosensory cortex, neurons may be classified as either rapidly adapting or slowly adapting to sustained stimuli and may be differentiated further by the presence or absence of a receptive field and by their threshold of activation. It is also possible to use the rate of adaptation of the background activity to a sustained stimulus to divide the cortex into slowly adapting regions or rapidly adapting regions. By blocking GABA-mediated inhibition with iontophoretically administered bicuculline methiodide, others have observed an increase in receptive field size in rapidly adapting regions but not in slowly adapting regions. The present study was designed to look for a different inhibitory transmitter which might control receptive field size in slowly adapting regions. Iontophoretically delivered strychnine was employed as an antagonist because it interferes with glycine-like inhibitory transmitters such as glycine, taurine and beta-alanine. Pharmacological tests were performed on 157 neurons in two series of experiments. In the first series three effects were documented. (i) In rapidly adapting regions, the size of the receptive field increased in 11 out of 25 cases whereas none of the 20 receptive fields tested in slowly adapting regions enlarged. (ii) In 13 of 24 cases a receptive field was revealed for previously unresponsive neurons in rapidly adapting regions whereas only 5 of 22 unresponsive cells tested in slowly adapting regions developed a receptive field. (iii) In 15 of 25 cells with receptive fields tested in rapidly adapting zones, strychnine reduced the threshold for somatic stimuli but only 8 of 20 cells isolated in slowly adapting zones showed this effect. In a second series of experiments, the effect of beta-alanine, glycine and taurine was examined on neurons of the rapidly adapting regions. beta-Alanine and taurine reduced the excitability of all neurons tested. Glycine inhibited most neurons. However, strychnine only antagonized the inhibitory effects of beta-alanine on responses to peripheral stimuli (9 of 11 cases). When neurons could not be driven by peripheral stimuli, the inhibition of spontaneous or glutamate-induced activity could not be blocked by strychnine (0 of 18 cases). We suggest that glycine-like amino acids contribute to the control of receptive field size and the control of neuronal excitability in rapidly adapting regions but not in slowly adapting regions. Our data suggest that strychnine-sensitive synapses are limited only to a subset of cortical neurons driven by somatic inputs.