Computation of color and brightness differences by neurons in the lateral geniculate nucleus of the rabbit

The responses of 51 neurons in the lateral geniculate nucleus of the rabbit to substitution of colored stimuli different brightness and stimuli differing only in intensity were studied. Neurons in the geniculate nucleus, like neurons in the visual cortex, were found to respond with initial phasic discharges at 50–90 msec after stimulus substitution, the magnitudes of these responses correlating with the interstimulus differences; neurons also showed prolonged tonic responses in which the spike frequency depended on the intensity of the stimulus presented. Analysis of phasic responses allowed two groups of neurons to be identified: some were specialized to discriminate stimulus intensity only, while others were specialized to discriminate both the intensity and the color tone of the stimulus. Use of the magnitude of the early phasic discharge as a measure of the difference between stimuli yielded a sensory space for lateral geniculate nucleus neurons. The responses of neurons in the first group (44 cells, 86%) produced a two-dimensional achromatic space with two axes-brightness and darkness; this structure appeared independently of whether stimuli were of the same or different color tones. The phasic responses of neurons in the second group (seven of 51, 14%) generated a four-dimensional space with two color and two achromatic axes. The color and achromatic spaces of lateral geniculate nucleus neurons were analogous to the spaces previously identified for neurons in the rabbit visual cortex using the same stimulation conditions. The sensory spaces reconstructed on the basis of neuron phasic discharges essentially coincided with the spaces obtained from analysis of the N85 component of visual evoked potentials in rabbits, which provides support for the vector information coding principle in the visual analyzer. The tonic discharges of most lateral geniculate nucleus neurons correlated linearly with changes in stimulus intensity and can be regarded as reflecting a pre-detector function for the visual cortex detector neurons. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 56, No. 1, pp. 75–85, January–February, 2006.     

Computation of Color and Brightness Differences by Rabbit Visual Cortex Neurons

Extracellular recording of the activity of 54 neurons in the rabbit visual cortex in responses to substitutions of eight colored and eight monochromatic stimuli in pairs was studied. Stimuli were uniform flashes of light displayed on an SVGA monitor and illuminated the whole retina. The responses of phasic neurons showed an initial discharge (50–90 msec from the moment of the change in stimulus), associated with the brightness or color difference between the stimuli. These “discrimination discharges” were used to construct an 8 × 8 matrix for each neuron, showing the mean number of spikes per sec in responses to changes in different pairs of stimuli. Processing of the matrix by factor analysis identified the major factors determining the axes of the sensory space. A brightness space with only two dimensions, with darkness and brightness orthogonal axes, was seen for 30% of neurons. A four-dimensional color space was seen in 22% of neurons, with two color and two achromatic axes. The sensory space of these neurons was similar to the spaces obtained by analyzing the early components of visual evoked potentials in rabbits induced by changes in color stimuli and behavioral operant responses in conditioned reflex color differentiation. The fundamental coincidence of the sensory spaces obtained by different methods identifies the general nature of the principle of vector coding and the existence of special neuronal mechanisms for detection of color and brightness differences in the visual field. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 55, No. 1, pp 60–70, January–February, 2005.     

Limited plasticity of difference neurons in the visual cortex and hippocampus in rabbits during the oddball (random substitutions) test

The activity of 41 visual cortex neurons and 20 hippocampal field CA1 neurons was studied in rabbits during application of the oddball stimulation paradigm using color stimuli of different intensities. Among these cells, about one third were plastic cells (34% of cortical cells and 37% of hippocampal cells). These neurons showed significant increases in late responses, at times 200–500 and 200–1000 msec for visual cortex neurons and 300–550 msec for hippocampal neurons, to rare deviant stimuli of lesser intensity as compared with responses to the frequent standard stimuli of greater intensity. The initial peak of the response (40–120 msec), the “difference discharge,” remained stable in responses to deviant and standard stimuli throughout the experiment. It is suggested that the strengthening of the late components of neuron responses to rare deviant stimuli (limited plasticity) reflects inclusion of the mechanisms of the orientational reflex. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 55, No. 3, pp. 360–367, May–June, 2005.     

Responses of Rabbit Visual Cortex Neurons to Changes in the Orientation and Intensity of Visual Stimuli

We report here studies of changes in the numbers of spikes in the early phasic discharges (50–90 msec from the moment of stimulus substitution) of neurons in the primary visual cortex of conscious rabbits in response to substitution of lines of different orientations (0–90°) but flashing at constant intensity on a screen, to substitution of lines of constant orientation but different intensities, and to substitutions of complex stimuli in which simultaneous changes were made to the orientation and intensity. Factor analysis of the results showed that the number of spikes in the early phasic discharges of some neurons allowed the two-dimensional sensory space of orientations to be reconstructed. This space was identified in 13 of the 43 neurons studied (30%). Five of the 30 cells studied (16.7%) showed both two-dimensional orientation sensory spaces and two-dimensional intensity spaces. Achromatic spaces were reconstructed by substituting lines of different intensity but constant orientation. On substitution of complex stimuli (intensity + orientation), four stimuli with initial orientations of 0–38.58° (0° corresponding to a vertical line) had an intensity of 5 cd/m², while the other four stimuli (with orientations of 51.44–90°) were presented at an intensity of 15 cd/m². On the plane of the sensory space formed by the first two significant factors, the two groups of stimuli with different intensities were located in opposite quadrants of a circle, while within the groups the stimuli were ordered in a sequence close to the order of increases in their slope angles, from smaller angles to greater. It is suggested that in this version, a single sensory plane space reflects the interaction between the orientation and intensity attributes of the visual stimulus, the intensity factor being predominant. A total of seven such cells were found among the 57 studied (12%).     

Color sensitivity of cells responsive to complex stimuli in the temporal cortex

The inferotemporal (IT) cortex of the monkey lies at the head of the ventral visual pathway and is known to mediate object recognition and discrimination. It is often assumed that color plays a minor role in the recognition of objects and faces because discrimination remains highly accurate with black-and-white images. Furthermore it has been suggested that for rapid presentation and reaction tasks, object classification may be based on a first wave of feedforward visual information, which is coarse and achromatic. The fine detail and color information follows later, allowing similar stimuli to be discriminated. To allow these theories to be tested, this study investigates whether the presence of color affects the response of IT neurons to complex stimuli, such as faces, and whether color information is delayed with respect to information about stimulus form in these cells. Color, achromatic, and false-color versions of effective stimuli were presented using a rapid serial visual presentation paradigm, and responses recorded from single cells in IT of the adult monkey. Achromatic images were found to evoke significantly reduced responses compared with color images in the majority of neurons (70%) tested. Differential activity for achromatic and colored stimuli was evident from response onset with no evidence to support the hypothesis that information about object color is delayed with respect to object form. A negative correlation (P < 0.01) was found between cell latency and color sensitivity, with the most color-sensitive cells tending to respond earliest. The results of this study suggest a strong role for color in familiar object recognition and provide no evidence to support the idea of a first wave of form processing in the ventral stream based on purely achromatic information.     

Responses to Light of Sensorimotor and Visual Cortex Neurons in Rabbits with a Cryptic Focus of Excitation (a defensive dominant) in the CNS

In the sensorimotor cortex of rabbits with a formed cryptic (subthreshold) focus of excitation in the CNS, the spike frequencies of neurons responding to light stimulation were significantly lower (p = 0.01) than the spike frequencies of neurons not responding to light. Similar findings were obtained in the visual cortex of intact rabbits. In this case too, the spike frequencies of neurons responding to stimulation were significantly lower (p = 0.01) than the spike frequencies of neurons not responding to light stimulation. In both intact rabbits and rabbits with a cryptic focus of excitation formed in the CNS, 36 % of neurons in the sensorimotor cortex responded to light stimuli not specific to this area. In the sensorimotor cortex of rabbits with a cryptic focus of excitation formed in the CNS, as compared with intact rabbits, there were significantly more (p = 0.01) cells responding to light stimuli with latent periods of less than 100 msec and significantly fewer (p = 0.02) responding to light stimuli with latencies of 200–300 msec. In the visual cortex of rabbits with a formed cryptic focus of excitation in the CNS, as compared with intact rabbits, significantly fewer (p = 0.01) neurons responded to light stimuli with latent periods of 50–100 msec.     

Influence of stimulation of the medial hypothalamus on the interaction of neurons of the rabbit neocortex

The interaction of neurons of the visual and sensory motor areas of the neocortex of the rabbit before and after stimulation of some medial nuclei of the hypothalamus was investigated by plotting cross- and autocorrelation histograms. Stimulation through bipolar electrodes using bursts of biphasic pulses at a frequency of 100 Hz, current strength 50–200 μA, led to the appearance in freely behaving rabbits of the reaction of avoidance of the place of stimulation. Following stimulation, as compared with resting wakefulness, the number of pairs of neurons functioning in correlation increased to 45%; at the same time, discharges of neurons of the sensory motor area ran ahead of discharges of visual neurons in the pairs up to 120 msec; the periodicity of the coupled discharges was mainly in the theta frequency range. A conclusion regarding the reflection of defense motivation in certain indices of the interaction of the cortical cells in the presence of a tonic conditioned reflex is reached on the basis of a comparison of the interaction of neurons following stimulation of the medial hypothalamus and the midbrain reticular formation, in the intersignal periods during the development of a defense conditioned reflex as well. This study was supported by the Russian Basic Research Fund (project No. 94-04-11399 a). Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences, Moscow. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 45, No. 2, pp. 297–304, March–April, 1995.     

Influence of motivational centers of the hypothalamus on the functioning of the visual area of cerebral cortex

It has been demonstrated in chronic experiments on awake rabbits that a solitary stimulation of the middle hypothalamus, its ventromedial and lateral nuclei (VMN and LN), exerts a phasic effect on a formation of the primary response of the visual cortex evoked by a test light flash. In the initial period of their action, the hypothalamocortical responses (1–43 msec for the VMN and 1–10 msec for the LN) completely inhibit the formation of the response of the visual cortex to the light stimulus, while in the second period (43–130 msec for the VMN and 10–150 msec for the LN, respectively), selectively and highly significantly facilitate the formation of the positive phase of the primary response. In the process the negative component is suppressed, and more significantly and longer with stimulation of the VMN than of the LN (140 and 50 msec, respectively). The data obtained make it possible to hypothesize the existence of a highly organized apparatus of phasic hypothalamic (both from the VMN and the LN) control of the function of the visual cortex which is realized both by means of a facilitatory axosomatic mechanism at the level of the dendrites of the basal neurons of layer IV of the cortex and by means of a suppressant mechanism at the level of the apical dendrites of the surface layers of the cortex. Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 43, No. 1, pp. 100–108, January–February, 1993.     

Response suppression induced by afferent stimulation in the superficial and deep layers of the hamster's superior colliculus

Summary Stimulation of the optic chiasm (OX) or visual cortex (VC) elicited a burst of impulses from visual cells in the superficial layers of the hamster's superior colliculus which was followed by a period of response suppression which lasted from 50–200 ms. During this period responses to normally suprathreshold OX and VC shocks, spontaneous activity and even injury discharges were markedly attenuated. For approximately 50% of the visual cells tested VC stimulation also reduced responses to visual stimuli. No correlations between receptive field properties and whether or not VC shocks diminished a given cell's visual responses were noted. Stimulation of either the cervical spinal cord (SC) or somatic sensory cortex (SMCTX) evoked action potentials from somatosensory neurons in the deep tectal laminae. These responses were followed by a period of suppression identical to that seen in the superficial layers after OX or VC shocks. SMCTX stimulation attenuated responses to tactile stimuli for 30% of the cells tested in the deep layers. Again, no correlation was observed between somatosensory response characteristics and whether or not a given cell exhibited response suppression.     

The role of primate putamen neurons in the association of sensory stimuli with movement

Neuronal activity in the putamen of monkeys was recorded while they performed operantly conditioned body movements. Two categories of neurons were observed. Type I cells had tonic spontaneous discharges and responded to the sensory trigger stimuli for movements with excitation followed by inhibition or with only inhibition. These responses to the trigger stimuli disappeared when the conditioned movement was extinguished. Type II cells were characterized by phasic activity time-locked to the movement. Two subclasses of type II cells were observed. Type IIa cells exhibited phasic discharges before the first movement of a learned, repetitive sequence of arm or orofacial movements that were triggered by the sensory stimuli. Type IIb cells showed phasic activity modulation during each movement in one direction, either flexion or extension, in an unconditioned manner. Activity of the type IIa cells preceded the onset of EMG in prime mover muscles, while most type IIb cells were activated after the EMG had appeared. Thus, in both type I and type IIa cells the activity can be said to be behaviourally contingent. Type I cells show a movement contingent sensory response, and type IIa cells show movement-related activity that is contingent upon the triggering of the movement by a sensory stimulus.     

Mechanical stimulation activates small fiber mediated nociceptive responses in the nucleus gigantocellularis

We characterized nociceptive discharges induced by mechanical stimulation and the modulating effects of orphanin FQ on noxious responses in the rat brain stem gigantocellular reticular nucleus (Gi). A pressure pulse of constant force and rising rate was delivered by a mechanical stimulator with feedback control, allowing responses to be analyzed statistically. A pressure pulse of 300 g, which evoked C-fiber mediated nerve responses, was delivered to the tail. Two excitatory (45/58) and one inhibitory (13/58) types of extracellular unit discharges were recorded in Gi. One of the excitatory types was a phasic discharge (13/45) elicited at the onset and/or the end of stimulation. Latencies of the phasic discharges (0.104±0.1 s) were shorter than those of other type (tonic) discharges (0.43±0.2 s). The tonic discharges (32/45), which frequently persisted past the end of stimulation without adaptation, were classified into two groups. The first group of tonic type units (23/45) was high threshold, like nociceptive specific neurons in the primary sensory cortex, while the second group of neurons (9/45) responded to a wide range of stimulus intensities. The mean frequency, response duration and spike numbers gradually increased with stimulus intensity change in all nine neurons. The neurons encode mechanical stimulus intensity with discharge frequency, response duration and evoked spike numbers. Local injection of orphanin FQ (200 ng/2 μl) changed high threshold tonic type spike numbers in a biphasic manner, i.e., there was an early phase suppression (5–30 min, p=0.016) and a late phase enhancement (30–60 min, p=0.027). In contrast, phasic type discharges did not show an altered discharge pattern in response to orphanin FQ. Thus, orphanin FQ affects small fiber-mediated nociceptive responses and may behave as a complex modulator of pain systems in the brain stem. Electronic Publication     

Activity of rabbit neocortex and hippocampus neurons in orientational-investigative behavior and freezing

Autocorrelation histograms were used to study the nature of spike activity in neurons recorded bilaterally from the visual and parietal areas of the cortex and hippocampal field CA1 in rabbits in free behavior during exposure to emotionally significant stimuli. Active movement orientational-investigative reactions to stimuli were associated with grouping of discharges and periodicity in the spike activity of most neurons in the cortex and hippocampus, this being dominated by the θ frequency (predominantly 4–5 Hz in the cortex and 4–5 and 6–7 Hz in the hippocampus). As compared with active movement reactions, freezing in response to stimulation was associated with increased numbers of neurons with uniform discharge distributions, while the spike activity of neurons with discharge periodicity showed increases in the intensity of the δ frequency (predominantly from 2 to 4 Hz), while θ intensity decreased. The number of neurons with periodic frequency in the δ range was greater in freezing than in the baseline state of calmly sitting rabbits. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 92, No. 11, pp. 1273–1284, November, 2006.     

Interactions and Firing Patterns of Rabbit Amygdalar Neurons in Unconditioned Fear Manifested as Freezing

The characteristics of the operation of the amygdalar neural network in unconditioned fear were identified by studying the interactions and nature of the spike activity of individual neurons in the basal and central nuclei of the amygdala in rabbits during freezing (fear), active unconditioned motor reactions (absence of fear), and in response to emotionally significant stimuli, as well as in calm waking. When rabbits froze, there were specific changes in the interactions of close-lying amygdalar neurons as compared with other states; these changes were not seen in the spike activity of individual neurons. Freezing increased the numbers of short-latency (up to 100 msec) excitatory connections and decreased the numbers of long-latency (250–450 msec) inhibitory connections. Neuron interactions were seen at frequencies in the delta2 range (2–4 Hz) more often in this state than in others. When the animals made active motor responses to the stimulus, there were decreases in the numbers of interacting neurons and increases in the numbers of longlatency (200–250 msec) excitatory and short-latency (50–200 msec) inhibitory connections, and a greater proportion of interactions occurred at frequencies of the theta1 range more frequently than in other states. Thus, the balance between the excitatory and inhibitory components of the amygdalar neural network is important for the occurrence of fear.     

Interhemisphere asymmetry of the neocortex and hippocampus during orientational-investigative behavior and freezing in rabbits

Cross-correlation histograms were used to study the discharges of cortical neurons in symmetrical leads in both hemispheres (visual and parietal areas) and in hippocampal field CA1 on the right and left sides in rabbits in conditions of free behavior during exposure to emotionally significant stimuli. During active orientational-investigative responses to stimuli, as compared with baseline, the neocortex showed increases in left-sided influences on cells in the right hemisphere, with delays of up to 100 msec, which led to the appearance of asymmetry in the interhemisphere interaction, with left-sided dominance. On freezing, the left-sided influence weakened and the right hemisphere became dominant. Interhemisphere asymmetry in hippocampal neuron activity was seen, and was reciprocal to the asymmetry observed in the neocortex. The active investigative response increased right-sided influences in the hippocampus with delays of up to 200 msec, leading to right-sided dominance. Freezing was associated with increases in left-sided influences, such that the left side was dominant. The interaction of cells in the hippocampus was largely at the frequencies of the theta rhythm during active movement and in the delta range on freezing. These data lead to the conclusion that the active or passive nature of behavioral movement reactions to emotionally significant stimuli correlates with changes in the asymmetry of interhemisphere neuron interaction at the levels of the cerebral cortex and hippocampus. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel’nosti imeni I. P. Pavlova, Vol. 57, No. 2, pp. 169–180, March–April, 2007.     

Visual, auditory, and somatosensory convergence on cells in superior colliculus results in multisensory integration

Convergence of inputs from different sensory modalities onto individual neurons is a phenomenon that occurs widely throughout the brain at many phyletic levels and appears to represent a basic neural mechanism by which an organism integrates complex environmental stimuli. In the present study, neurons in the superior colliculus (SC) were used as a model to examine how single neurons deal with simultaneous cues from different sensory modalities (e.g., visual, auditory, somatosensory). The functional result of multisensory convergence on an individual cell was determined by comparing the responses evoked from it by a combined-modality (multimodal) stimulus with those elicited by each (unimodal) component of that stimulus presented alone. Superior colliculus cells exhibited profound changes in their activity when individual sensory stimuli were combined. These "multisensory interactions" were found to be widespread among deep laminae cells and fell into one of two functional categories: response enhancement, characterized by a significant increase in the number of discharges evoked; and response depression, characterized by a significant decrease in the discharges elicited. Multisensory response interactions most often reflected a multiplicative, rather than summative, change in activity. Their absolute magnitude varied from cell to cell and, when stimulus conditions were altered, within the same cell. However, the percentage change of enhanced interactions was generally inversely related to the vigor of the responses that could be evoked by presenting each unimodal stimulus alone and suggest that the potential for response amplification was greatest when responses evoked by individual stimuli were weakest. The majority of cells exhibiting multi-sensory characteristics were demonstrated to have descending efferent projections and thus had access to premotor and motor areas of the brain stem and spinal cord involved in SC-mediated attentive and orientation behaviors. These data show that multisensory convergence provides the descending efferent cells of the SC with a dynamic response character. The responses of these cells and the SC-mediated behaviors that they underlie need not be immutably tied to the presence of any single stimulus, but can vary in response to the particular complex of stimuli present in the environment at any given moment.     

Effect of stimulation of the lateral hypothalamus on the correlation of neuron spike activity in the rabbit neocortex

Cross-correlation and autocorrelation histograms were constructed with the aim of studying correlated spike activity of neurons in the visual and sensorimotor regions of both hemispheres of the rabbit brain before and after stimulation of the right and left lateral hypothalamic regions, which generates food-motivated responses. Stimulation of the left hypothalamus produced larger rearrangements in correlated neuron firing than stimulation of the right hypothalamus. Stimulation of the left hypothalamus, unlike that of the right hypothalamus, was followed by significant increases in the numbers of pairs of left hemisphere neocortical neurons with linked activity, and also induced the sequential firing of neurons in a particular defined order: sensorimotor cortex cells fired first, followed by visual cortex neurons after delays of up to 120 msec. It is concluded that cortical interhemisphere asymmetry in conditions of hunger is associated with nonuniform functioning of the right and left lateral hypothalamic regions. Institute of Higher Nervous Activity and Neurophysiology, Russian Academy of Sciences, Moscow. Translated from Fiziologicheskii Zhurnal Vysshei Nervnoi Deyatel'nosti, Vol. 46, No. 6, pp. 1068–1075, November–December, 1996.     

Nociceptive responses of midbrain dopaminergic neurones are modulated by the superior colliculus in the rat

Midbrain dopaminergic neurones exhibit a short-latency phasic response to unexpected, biologically salient stimuli. In the rat, the superior colliculus is critical for relaying short-latency visual information to dopaminergic neurones. Since both collicular and dopaminergic neurones are also responsive to noxious stimuli, we examined whether the superior colliculus plays a more general role in the transmission of short-latency sensory information to the ventral midbrain. We therefore tested whether the superior colliculus is a critical relay for nociceptive input to midbrain dopaminergic neurones. Simultaneous recordings were made from collicular and dopaminergic neurones in the anesthetized rat, during the application of noxious stimuli (footshock). Most collicular neurones exhibited a short-latency, short duration excitation to footshock. The majority of dopaminergic neurones (92/110; 84%) also showed a short-latency phasic response to the stimulus. Of these, 79/92 (86%) responded with an initial inhibition and the remaining 14/92 (14%) responded with an excitation. Response latencies of dopaminergic neurones were reliably longer than those of collicular neurones. Tonic suppression of collicular activity by an intracollicular injection of the local anesthetic lidocaine reduced the latency, increased the duration but reduced the magnitude of the phasic inhibitory dopaminergic response. These changes were accompanied by a decrease in the baseline firing rate of dopaminergic neurones. Activation of the superior colliculus by the local injections of the GABA(A) antagonist bicuculline also reduced the latency of inhibitory nociceptive responses of dopaminergic neurones, which was accompanied by an increased in baseline dopaminergic firing. Aspiration of the ipsilateral superior colliculus failed to alter the nociceptive response characteristics of dopaminergic neurones although fewer nociceptive neurones were encountered after the lesions. Together these results suggest that the superior colliculus can modulate both the baseline activity of dopaminergic neurones and their phasic responses to noxious events. However, the superior colliculus is unlikely to be the primary source of nociceptive sensory input to the ventral midbrain.     

Functional plasticity in primate somatosensory thalamus following chronic lesion of the ventral lateral spinal cord

The long-term consequences of thoracic spinothalamic tract lesion on the physiological properties of neurons in the ventral posterior lateral nucleus of the thalamus in monkeys were assessed. Neurons responding to both compressive and phasic brush stimuli (multireceptive neurons), but not brush-specific (low-threshold) neurons, in the partially deafferented thalamus showed increased spontaneous activity, increased responses evoked by cutaneous stimuli and larger mean receptive field size than the same types of cells in the thalamus with intact innervation. The spike train properties of both the spontaneous and evoked discharges of cells were also altered so that there was an increased incidence of spike-bursts in cells of deafferented thalamus. These changes were widespread in the thalamus, and included cells in both the fully innervated forelimb representation and the partially denervated hindlimb representation ipsilateral to the lesion. The spontaneous and evoked spike trains in the ipsilateral thalamus also show increased frequency of both spike-burst and non-burst events compared to the intact thalamus. These results indicate that chronic spinothalamic tract lesion produces widespread changes in the physiological properties of a discrete cell population of the thalamus.The findings in this study indicate that the thalamic processing of somatosensory information conveyed by the lemniscal system is altered by transection of the spinothalamic tract. This change in sensory processing in the thalamus would result in altered cortical processing of innocuous somatosensory inputs following deafferentation and so possibly contribute to the generation of the central pain syndrome.     

Neuron discharges in the rat auditory cortex during electrical intracortical stimulation

Studies were carried out in rats anesthetized with ketamine or nembutal, with recording of multicellular activity (with separate identification of responses from individual neurons) in the primary auditory cortex before and after electrical intracortical microstimulation. These experiments showed that about half of the set of neurons studied produced responses to short tonal bursts, these responses having two components—initial discharges arising in response to the sound, and afterdischarge occurring after pauses of 50–100 msec. Afterdischarges lasted at least several seconds, and were generally characterized by a rhythmic structure (with a frequency of 8–12 Hz). After electrical microstimulation, the level of spike activity increased, especially in afterdischarges, and this increase could last up to 4 h. Combined peristimulus histograms, cross-correlations, and gravitational analyses were used to demonstrate interactions of neurons, which increased after electrical stimulation and were especially pronounced in the response afterdischarges. Department of Neurosciences, Faculty of Medicine, University of Pennsylvania, Philadelphia, USA. Laboratory of Auditory Physiology, I. P. Pavlov Institute of Physiology, 6 Makarov Bank, 199034 St. Petersburg, Russia. Translated from Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 82, No. 5-6, pp. 3–17, May–June, 1996.     

Multiple temporal components of optic flow responses in MST neurons

 Neurons in monkey medial superior temporal cortex (MST) respond to optic flow stimuli with early phasic, tonic, and after-phasic response components. In these experiments we characterized each response component to compare its potential contributions to visual motion processing. The early responses begin 60–100 ms after stimulus onset and last between 100 and 250 ms, the tonic responses begin 100–300 ms after stimulus onset and last for as long as the evoking stimulus persists, and the after-responses begin about 60 ms after the stimulus goes off and last for 100–350 ms. A neuron’s tonic responses were evoked by specific optic flow stimuli: over two-thirds of the 264 neurons showed tonic responses evoked by two to five stimuli, whereas only 15% responded to either all or none of the stimuli. The tonic responses continued with stimulus presentations as long as 15 s, with their directional preferences being maintained throughout stimulation. However, the tonic response to a given stimulus was seen to change in amplitude when it was presented in random sequence with different sets of other stimuli. Thus, the tonic responses might convey substantial information about optic flow patterns, which continue with prolonged stimulation, but can be modified by the visual context created by other visual motion stimuli. Only about one-third of the 264 neurons had early responses that were selective for specific stimuli. In neurons yielding at least one early response, that neuron was most often activated by all the visual motion stimuli. After-rsponses occurred in only half the neurons, but they were more often specifically related to particular optic flow stimuli, regardless of whether those stimuli had evoked tonic excitatory or tonic inhibitory responses. The presence of early and after-responses complicates the interpretation of activity evoked when one stimulus immediately follows another. However, under those conditions, early responses and after-responses might contribute to signaling changes in the ongoing pattern of optic flow. We conclude that several components of MST responses should be recognized and that they potentially play different roles in the cortical analysis of optic flow. Tonic responses show the greatest specificity for particular optic flow stimuli, and possess characteristics which make them suitable neuronal participants in self-movement perception. Received: 1 April 1996 / Accepted: 1 October 1996