Responses of cortical neurones to stimulation of the visual afferent radiations

Summary Extracellular spikes of visual cortical neurones in unanesthetized cats were recorded. The latency after electric stimuli of the optic radiation to the onset of firing of cells with concentric fields, simple receptive fields and complex receptive fields was measured.Simple receptive field neurones had response latencies averaging 0.93 msec longer than neurones with concentric fields. The majority of the latter represented responses of geniculate axones terminating in the visual cortex and a few were action potentials of first-order cortical cells. Cells with complex receptive fields had longer latencies than simple field neurones and often showed prolonged cessation of firing following electrical stimulation.Many neurones showed a period of firing arrest after radiation shock. The duration and variability of this period also varied according to receptive field type.The data were consistent with the hypothesis that neurones with simple receptive fields receive the initial contact of incoming geniculocalcarine afferent fibres. Complex receptive field neurones appear to derive their input from other cortical cells rather than from the direct geniculocortical connections.A preliminary report of this research was presented to the German Physiological Society and an abstract published in Pflügers Arch. ges. Physiol. 294, 56 (1967).     

Neuronal activity in the primary visual cortex of the cat freely viewing natural images

Many studies have now demonstrated that neurons in the visual cortex of cats and monkeys change their activity when stimuli are presented beyond their classical receptive field, and that these responses are not readily apparent from their receptive field properties. However few studies have been conducted to investigate the discharge properties of neurons in the visual cortex of animals when they are allow to freely view natural images. We employ tetrodes, which enable simultaneous and separable recordings of small numbers of neighboring neurons, to record 102 single units from 59 sites from areas 17 and 18 of two alert cats. While the animals viewed either natural images or black screens, they made frequent saccadic eye movements and gaze fixations. Fixations onto an image's location increased neuronal firing peaking at 80-100 ms after the fixation onset, to then decrease steadily with time despite continuous fixation. Saccades trigger a fast decrease in firing rate for both images and darkness. When we examined the incidence of correlated firing, we observed significant synchrony during the initial phases of visual fixations when the animals viewed natural scenes. Such synchrony was absent during saccadic eye movements and during eye movements in darkness. Our data revealed that scanning of natural scenes is associated with a rapid succession of distinct fixation-related activation patterns that included transient rate changes and excess coincident firing. The transient nature of these synchronization phenomena suggests a fast acting mechanism, which is in good agreement with the evidence that basic operations of scene analysis must be accomplished within a few tens of milliseconds in primary visual cortex.     

Comparison of odor receptive field plasticity in the rat olfactory bulb and anterior piriform cortex

Recent work in the anterior piriform cortex (aPCX) has demonstrated that cortical odor receptive fields are highly dynamic, showing rapid changes of both firing rate and temporal patterning within relatively few inhalations of an odor, despite relatively maintained, patterned input from olfactory bulb mitral/tufted cells. The present experiment examined the precision (odor-specificity) of this receptive field plasticity and compared it with the primary cortical afferent, olfactory bulb mitral/tufted cells. Adult Long-Evans hooded rats, urethan anesthetized and freely breathing, were used for single-unit recording from mitral/tufted and aPCX layer II/III neurons. Partial mapping of receptive fields to alkane odors (pentane, heptane, and nonane) was performed before and immediately after habituation (50-s exposure) to one of the alkanes. The results demonstrated that odor habituation of aPCX responses was odor specific, with minimal cross-habituation between alkanes differing by as few as two carbons. Mitral/tufted cells, however, showed strong cross-habituation within the odor set with the most profound cross effects to carbon chains shorter than the habituating stimulus. The results suggest that although mitral/tufted cells and aPCX neurons have roughly similar odor receptive fields, aPCX neurons have significantly better odor discrimination within their receptive field. The results have important implications for understanding the underlying bases of receptive fields in olfactory system neurons and the mechanisms of odor discrimination and memory.     

Short-term changes in movement frequency do not alter the spatial tuning of saccade-related neurons in intraparietal cortex

Modulations of the firing rates of neurons in the lateral intraparietal area (LIP) have been observed during experiments designed to examine decision-processing, movement planning, and visual attention. These modulations have been assumed to reflect a uniform scaling of spatially stationary response fields, which describe firing rate as a function of either visual target location or movement metrics. However, because complete response fields are rarely collected, the possibility exists that these modulations may reflect shifts in response field location or changes in response field size. Moreover, many of these observed changes in LIP neuronal activity are also correlated with experimental practices that alter the frequency with which particular visual stimuli are viewed and particular movements are produced. The effects of repeatedly presenting a particular target and eliciting a particular movement on the response fields of LIP neurons warrant closer inspection because manipulations of this type are known to alter both the location and size of the receptive fields of many cortical sensory neurons. To address this issue, we measured the response fields of neurons in intraparietal cortex under two conditions over a period of up to 2 h: one in which each of nearly 200 stimulus locations was equally likely to serve as the saccade target on a trial, and a second in which one stimulus location was up to 750 times likelier to serve as the saccade target on a trial than were any of the other stimulus locations. We found no shifts in response field location or changes in response field size when we altered the frequency with which particular movements were produced or particular visual stimuli were presented. These data suggest that the response fields of intraparietal neurons are stationary over short periods of time and under conditions similar to those typically used to study LIP neuronal activity.     

Receptive field structure of burst and tonic firing in feline lateral geniculate nucleus

There are two recognised modes of firing activity in thalamic cells, burst and tonic. A low-threshold (LT) burst (referred to from now on as 'burst') comprises a small number of high-frequency action potentials riding the peak of a LT Ca²⁺ spike which is preceded by a silent hyperpolarised state > 50 ms. This is traditionally viewed as a sleep-like phenomenon, with a shift to tonic mode at wake-up. However, bursts have also been seen in the wake state and may be a significant feature for full activation of recipient cortical cells. Here we show that for visual stimulation of anaesthetised cats, burst firing is restricted to a reduced area within the receptive field centre of lateral geniculate nucleus cells. Consistently, the receptive field size of all the recorded neurons decreased in size proportionally to the percentage of spikes in bursts versus tonic spikes, an effect that is further demonstrated with pharmacological manipulation. The role of this shrinkage may be distinct from that also seen in sleep-like states and we suggest that this is a mechanism that trades spatial resolution for security of information transfer.     

Object perception in natural scenes: encoding by inferior temporal cortex simultaneously recorded neurons

The firing of inferior temporal cortex neurons is tuned to objects and faces, and in a complex scene, their receptive fields are reduced to become similar to the size of an object being fixated. These two properties may underlie how objects in scenes are encoded. An alternative hypothesis suggests that visual perception requires the binding of features of the visual target through spike synchrony in a neuronal assembly. To examine possible contributions of firing synchrony of inferior temporal neurons, we made simultaneous recordings of the activity of several neurons while macaques performed a visual discrimination task. The stimuli were presented in either plain or complex backgrounds. The encoding of information of neurons was analyzed using a decoding algorithm. Ninety-four percent to 99% of the total information was available in the firing rate spike counts, and the contribution of spike timing calculated as stimulus-dependent synchronization (SDS) added only 1-6% of information to the total that was independent of the spike counts in the complex background. Similar results were obtained in the plain background. The quantitatively small contribution of spike timing to the overall information available in spike patterns suggests that information encoding about which stimulus was shown by inferior temporal neurons is achieved mainly by rate coding. Furthermore, it was shown that there was little redundancy (6%) between the information provided by the spike counts of the simultaneously recorded neurons, making spike counts an efficient population code with a high encoding capacity.     

Coding of tactile response properties in the rat deep cerebellar nuclei

In the lateral hemispheres of the cerebellar cortex, somatosensory responses are represented in a finely grained fractured somatotopy. It is unclear, however, how these responses contribute to the ultimate output of the cerebellum from the deep cerebellar nuclei (DCN). Robust responses of DCN neurons to somatosensory stimuli have been described, but a detailed examination of their somatotopic arrangement and stimulus coding properties is lacking. To address these questions, we recorded extracellular, single-unit activity in the DCN of ketamine-anesthetized rats in response to air-puff stimuli aimed at six different orofacial and forelimb locations. In additional experiments, the duration and intensity of air-puff stimuli to the ipsilateral upper lip were systematically varied. Overall, we found that DCN neuron responses to air puff stimuli showed combinations of three distinct response components: a short-latency spike response, a pronounced inhibition, and a long-latency increase in firing. Individual neurons responsive to air-puff stimulation exhibited any combination of just one, two, or all three of these response components. The inhibitory response was most common and frequently consisted of a complete cessation of spiking despite a high spontaneous rate of baseline firing. In contrast to published findings from cerebellar cortical recordings, the receptive fields of all responsive neurons in the DCN were large. In fact, the receptive field of most neurons covered the ipsi- and contralateral face as well as forepaws. Response properties of individual neurons did not reliably indicate stimulus intensity or duration, although as a population DCN neurons showed significantly increasing response amplitudes as air-puff intensity or duration increased. Overall, the responses were characterized by a distinct temporal profile in each neuron, which remained unchanged with changes in stimulus condition. We conclude that the responses in the DCN of rats to air-puff stimuli differ substantially from cerebellar cortical responses in their receptive field properties and do not provide a robust code of tactile stimulus properties. Rather, the characteristic temporal response profile of each neuron may be tuned to control the timing of a specific task to which its output is linked.     

Constraints on the source of short-term motion adaptation in macaque area MT. I. the role of input and intrinsic mechanisms

Neurons in area MT, a motion-sensitive area of extrastriate cortex, respond to a step of target velocity with a transient-sustained firing pattern. The transition from a high initial firing rate to a lower sustained rate occurs over a time course of 20-80 ms and is considered a form of short-term adaptation. The present paper asks whether adaptation is due to input-specific mechanisms such as short-term synaptic depression or if it results from intrinsic cellular mechanisms such as spike-rate adaptation. We assessed the contribution of input-specific mechanisms by using a condition/test paradigm to measure the spatial scale of adaptation. Conditioning and test stimuli were placed within MT receptive fields but were spatially segregated so that the two stimuli would activate different populations of inputs from the primary visual cortex (V1). Conditioning motion at one visual location caused a reduction of the transient firing to subsequent test motion at a second location. The adaptation field, estimated as the region of visual space where conditioning motion caused adaptation, was always larger than the MT receptive field. Use of the same stimulus configuration while recording from direction-selective neurons in V1 failed to demonstrate either adaptation or the transient-sustained response pattern that is the signature of short-term adaptation in MT. We conclude that the shift from transient to sustained firing in MT cells does not result from an input-specific mechanism applied to inputs from V1 because it operates over a wider range of the visual field than is covered by receptive fields of V1 neurons. We used a direct analysis of MT neuron spike trains for many repetitions of the same motion stimulus to assess the contribution to adaptation of intrinsic cellular mechanisms related to spiking. On a trial-by-trial basis, there was no correlation between number of spikes in the transient interval and the interval immediately after the transient period. This is opposite the prediction that there should be a correlation if spikes cause adaptation directly. Further, the transient was suppressed or extinguished, not delayed, in trials in which the neuron emitted zero spikes during the interval that showed a transient in average firing rate. We conclude that the transition from transient to sustained firing in neurons in area MT is caused by mechanisms that are neither input-specific nor controlled by the spiking of the adapting neuron. We propose that the short-term adaptation observed in area MT emerges from the intracortical circuit within MT.     

Linear integration of convergent visual inputs in an oculomotor reflex pathway

The functional connectivity between identified visual interneurons [sustaining fibers (SF)] and oculomotor neurons was assessed by simultaneous recording and cross-correlation analysis. A small group of SFs exhibit excitatory functional connections to an identified tonic oculomotor neuron. The excitatory interactions are found exclusively between SFs and oculomotor neurons with similar and/or overlapping excitatory receptive fields. A second group of SFs exhibit inhibitory connections to motor neurons. The excitatory receptive fields of these SFs correspond to the inhibitory receptive fields of the motor neurons. The collective action of the SFs is sufficient to produce all of the steady-state visual behavior of the motor neurons including the increment in firing rate elicited by illumination, unique features of the motor neuron receptive field, and differential sensitivity to blue light and polarized light. Pairs of SFs that converge on the same motor neuron sum their effects linearly. Thus the joint interaction of two SFs on a motor neuron is equal to the sum of the two postsynaptic effects taken separately. Coactivation of excitatory and inhibitory SF inputs to a motor neuron results in a partial cancellation of their postsynaptic effects on the motor neuron's firing rate. The antagonistic interactions protect the system from perturbations by stray light, visual adaptation, and variations in the central excited state. The ensemble information code, at the SF level of the optomotor pathway, is a set of differentially weighted mean firing rates. The weightings reflect differences in the strengths of the several SF-to- motor neuron interactions. One consequence of these differences is a selective weighting of the effects of illumination (in different regions of visual space) on the compensatory eye reflex.     

Corticogeniculate neurons, corticotectal neurons, and suspected interneurons in visual cortex of awake rabbits: receptive-field properties, axonal properties, and effects of EEG arousal

The intrinsic stability of the rabbit eye was exploited to enable receptive-field analysis of antidromically identified corticotectal (CT) neurons (n = 101) and corticogeniculate (CG) neurons (n = 124) in visual area I of awake rabbits. Eye position was monitored to within 1/5 degrees. We also studied the receptive-field properties of neurons synaptically activated via electrical stimulation of the dorsal lateral geniculate nucleus (LGNd). Whereas most CT neurons had either complex (59%) or motion/uniform (15%) receptive fields, we also found CT neurons with simple (9%) and concentric (4%) receptive fields. Most complex CT cells were broadly tuned to both stimulus orientation and velocity, but only 41% of these cells were directionally selective. We could elicit no visual responses from 6% of CT cells, and these cells had significantly lower conduction velocities than visually responsive CT cells. The median spontaneous firing rates for all classes of CT neurons were 4-8 spikes/s. CG neurons had primarily simple (60%) and concentric (9%) receptive fields, and none of these cells had complex receptive fields. CG simple cells were more narrowly tuned to both stimulus orientation and velocity than were complex CT cells, and most (85%) were directionally selective. Axonal conduction velocities of CG neurons (mean = 1.2 m/s) were much lower than those of CT neurons (mean = 6.4 m/s), and CG neurons that were visually unresponsive (23%) had lower axonal conduction velocities than did visually responsive CG neurons. Some visually unresponsive CG neurons (14%) responded with saccadic eye movements. The median spontaneous firing rates for all classes of CG neurons were less than 1 spike/s. All neurons synaptically activated via LGNd stimulation at latencies of less than 2.0 ms had receptive fields that were not orientation selective (89% motion/uniform, 11% concentric), whereas most cells with orientation-selective receptive fields had considerably longer synaptic latencies. Most short-latency motion/uniform neurons responded to electrical stimulation of the LGNd (and visual area II) with a high-frequency burst (500-900 Hz) of three or more spikes. Action potentials of these neurons were of short duration, thresholds of synaptic activation were low, and spontaneous firing rates were the highest seen in rabbit visual cortex. These properties are similar to those reported for interneurons in several regions in mammalian central nervous system. Nonvisual sensory stimuli that resulted in electroencephalographic arousal (hippocampal theta activity) had a profound effect on the visual responses of many visual cortical neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Spatial attention enhances the selective integration of activity from area MT

Distinguishing which of the many proposed neural mechanisms of spatial attention actually underlies behavioral improvements in visually guided tasks has been difficult. One attractive hypothesis is that attention allows downstream neural circuits to selectively integrate responses from the most informative sensory neurons. This would allow behavioral performance to be based on the highest-quality signals available in visual cortex. We examined this hypothesis by asking how spatial attention affects both the stimulus sensitivity of middle temporal (MT) neurons and their corresponding correlation with behavior. Analyzing a data set pooled from two experiments involving four monkeys, we found that spatial attention did not appreciably affect either the stimulus sensitivity of the neurons or the correlation between their activity and behavior. However, for those sessions in which there was a robust behavioral effect of attention, focusing attention inside the neuron's receptive field significantly increased the correlation between these two metrics, an indication of selective integration. These results suggest that, similar to mechanisms proposed for the neural basis of perceptual learning, the behavioral benefits of focusing spatial attention are attributable to selective integration of neural activity from visual cortical areas by their downstream targets.     

Short exposure to an enriched environment accelerates plasticity in the barrel cortex of adult rats

Cortical sensory neurons adapt their response properties to use and disuse of peripheral receptors in their receptive field. Changes in synaptic strength can be generated in cortex by simply altering the balance of input activity, so that a persistent bias in activity levels modifies cortical receptive field properties. Such activity-dependent plasticity in cortical cell responses occurs in rat cortex when all but two whiskers are trimmed for a period of time at any age. The up-regulation of evoked responses to the intact whiskers is first seen within 24 h in the supragranular layers [Laminar comparison of somatosensory cortical plasticity. Science 265(5180):1885-1888] and continues until a new stable state is achieved [Experience-dependent plasticity in adult rat barrel cortex. Proc Natl Acad Sci U S A 90(5):2082-2086; Armstrong-James M, Diamond ME, Ebner FF (1994) An innocuous bias in whisker use in adult rat modifies receptive fields of barrel cortex neurons. J Neurosci 14:6978-6991]. These and many other results suggest that activity-dependent changes in cortical cell responses have an accumulation threshold that can be achieved more quickly by increasing the spike rate arising from the active region of the receptive field. Here we test the hypothesis that the rate of neuronal response change can be accelerated by placing the animals in a high activity environment after whisker trimming. Test stimuli reveal an highly significant receptive field bias in response to intact and trimmed whiskers in layer IV as well as in layers II-III neurons in only 15 h after whisker trimming. Layer IV barrel cells fail to show plasticity after 15-24 h in a standard cage environment, but produce a response bias when activity is elevated by the enriched environment. We conclude that elevated activity achieves the threshold for response modification more quickly, and this, in turn, accelerates the rate of receptive field plasticity.     

Population receptive fields of ON and OFF thalamic inputs to an orientation column in visual cortex

The primary visual cortex of primates and carnivores is organized into columns of neurons with similar preferences for stimulus orientation, but the developmental origin and function of this organization are still matters of debate. We found that the orientation preference of a cortical column is closely related to the population receptive field of its ON and OFF thalamic inputs. The receptive field scatter from the thalamic inputs was more limited than previously thought and matched the average receptive field size of neurons at the input layers of cortex. Moreover, the thalamic population receptive field (calculated as ON - OFF average) had separate ON and OFF subregions, similar to cortical neurons in layer 4, and provided an accurate prediction of the preferred orientation of the column. These results support developmental models of orientation maps that are based on the receptive field arrangement of ON and OFF visual inputs to cortex.     

Neuronal assembly dynamics in the rat auditory cortex during reorganization induced by intracortical microstimulation

Single neurons, acting alone, cannot account for the complex and rapid computations that are routinely accomplished by the behaving nervous system. Recent studies with separable multineuron recordings are showing that neuronal assemblies can indeed be detected and that their organization is very dynamic, depending on variables such as time, physical stimulus, and context. Here we explore both single-neuron and assembly properties in the rat's auditory cortex. Acoustic stimuli are used as a normal, physiological input, and weak electrical intracortical microstimulation (ICMS) as a perturbation that forces a rapid cortical reorganization. In this setting, various aspects of neuronal interactions are changed by the ICMS. We found that cortical neurons exhibited highly synchronous oscillatory firing patterns that were enhanced by ICMS. Cross-correlation studies between two spike trains showed that statistically significant correlations depended on the anatomical distance between the two neurons. ICMS changed the strength and the local number of such correlations. Joint petristimulus analysis and gravity analysis showed that the correlation between neuronal activities varied dynamically at several time scales. We have identified neuronal assemblies in two ways, defined through similarity of receptive field properties and defined through correlated firing. Close anatomical spacing between neurons was conducive to, but not sufficient for membership in, the same assembly with either definition. ICMS changed cortical organization by altering assembly membership. Our data show that neuronal assemblies in the rat auditory cortex can be established transiently in time and that their membership is dynamic.     

Organization of receptive fields in the forebrain ofEmys orbicularis

Interaction between circumscribed areas of the receptive field of visual cortical neurons was investigated in the turtle forebrain. The neurons of the superficial cortical strata summate excitation arriving from different points of the receptive field. Interaction between local areas of the receptive field of deep cortical neurons depends essentially on the distance between the interacting points.Pharmacological blocking (KC1, GABA) of circumscribed areas of the forebrain cortex suppresses the circumscribed areas of tne receptive field of deep cortical neurons. Thus it is deduced that the cortex of the turtle's forebrain contains representations of the local areas of the retina, even though these neurons have extensive receptive fields. This deduction is confirmed by the fact that among the optic fibers entering the cortex, there are single fibers with receptive fields of between 2 and 5.     

NMDA antagonists in the superior colliculus prevent developmental plasticity but not visual transmission or map compression

Partial ablation of the superior colliculus (SC) at birth in hamsters compresses the retinocollicular map, increasing the amount of visual field represented at each SC location. Receptive field sizes of single SC neurons are maintained, however, preserving receptive field properties in the prelesion condition. The mechanism that allows single SC neurons to restrict the number of convergent retinal inputs and thus compensate for induced brain damage is unknown. In this study, we examined the role of N-methyl-D-aspartate (NMDA) receptors in controlling retinocollicular convergence. We found that chronic 2-amino-5-phosphonovaleric acid (APV) blockade of NMDA receptors from birth in normal hamsters resulted in enlarged single-unit receptive fields in SC neurons from normal maps and further enlargement in lesioned animals with compressed maps. The effect was linearly related to lesion size. These results suggest that NMDA receptors are necessary to control afferent/target convergence in the normal SC and to compensate for excess retinal afferents in lesioned animals. Despite the alteration in receptive field size in the APV-treated animals, a complete visual map was present in both normal and lesioned hamsters. Visual responsiveness in the treated SC was normal; thus the loss of compensatory plasticity was not due to reduced visual responsiveness. Our results argue that NMDA receptors are essential for map refinement, construction of receptive fields, and compensation for damage but not overall map compression. The results are consistent with a role for the NMDA receptor as a coincidence detector with a threshold, providing visual neurons with the ability to calculate the amount of visual space represented by competing retinal inputs through the absolute amount of coincidence in their firing patterns. This mechanism of population matching is likely to be of general importance during nervous system development.     

Blockade of intracortical inhibition in kitten striate cortex: Effects on receptive field properties and associated loss of ocular dominance plasticity

Summary We have investigated the importance of GABAergic inhibition for the receptive field properties and plasticity of cells in the visual cortex of kittens. Osmotic minipumps were used to continuously infuse the GABA-antagonist, bicuculline methiodide (BIC), into striate cortex. Extracellular recordings were made during BIC infusion to assess neuronal response properties during the blockade of inhibition. Recordings were also made from other kittens after concurrent monocular deprivation and BIC infusion to investigate the importance of response selectivity for ocular dominance plasticity. The minipump delivery technique was used to produce a large volume of cortex presumably free of GABA-ergic inhibition. Compared to recordings in saline-infused control hemispheres, about half of the cells in bicuculline-infused hemispheres had abnormally low orientation selectivity. The low selectivity was generally accompanied by marked anomalies in several other receptive field properties. Particularly striking was the large size of the receptive fields. At eccentricities less than 10 deg many receptive fields subtended from 10 to over 30 deg of arc. The less selective neurons also had abnormal responses to flashed stimuli, giving strong transient responses to the onset and offset of large stationary stimuli which filled their receptive fields. These results imply that intracortical inhibition normally suppresses responses to stimuli within a large excitatory zone beyond the classical receptive field. Inhibition is necessary for the normal orientation selectivity of many cells, although the selectivity may be partially established by the cell's excitatory input. Additionally, intracortical inhibition appears to be necessary for the antagonism and segregation of ON and OFF receptive field subregions. In our study of plasticity, we exploited the fact that BIC treatment greatly increases the range of stimuli that activate cortical neurons. Kittens were monocularly deprived for 7 days concurrently with cortical infusion of BIC. After cessation of the drug treatment, physiological recordings were made. Response properties had returned to normal but neurons in BIC-infused hemispheres had a significantly reduced ocular dominance shift compared to neurons in control hemispheres. This is probably related to the reduced selectivity of cells during BIC infusion. The suggestion here is that there is diminished ocular dominance plasticity in BIC-infused hemispheres because of an increased probability of correlated activity between spontaneous discharge from the closed eye and the cortical activity evoked by the open eye afferents.     

Geometrical illusions and the response of neurones in the cat's visual cortex to angle patterns

1. This report describes the responses of thirty-six single neurones in the primary visual area of the cat's neurologically isolated and unanaesthetized forebrain, to movements of thin white lines across the visual field. The experiments were designed to record the effects upon the response to a single test line of an added line, which was either parallel to the test line or joined it, making an angle-pattern of 30 degrees . Unit responses were measured in terms of the peak probability of firing derived from a post-stimulus histogram.2. All of the cortical neurones tested exhibited a preferred orientation for stimulation by the test line, i.e. an orientation of the line which produced a maximal response when the line passed through the centre of the unit's receptive field.3. There was no evidence that the orientation of a single test line preferred by cortical neurones was different from that preferred by the same cell when excited by an angle pattern, one arm of which was the original test line.4. The position of a test line (with preferred orientation) in the visual field that produced a maximal response from cortical neurones, was not always the same as the position for maximal response, when a second line was added to make either an angle pattern or to make a pattern of two parallel lines.5. Where the two lines of these patterns were close together and separated by less than the radius of the receptive field, the position for maximal response to the test line was shifted towards the added line. Where the two lines were further apart than this but separated by less than a receptive field diameter, the optimal position for the test line was displaced away from the added line.6. Some evidence was found of a lateral inhibition in the visual system, sufficient to account for the displacements described in paragraphs 4 and 5 above.7. It is concluded that the tip of an angle pattern of 30 degrees produces a distorted cortical image within the primary visual area.8. This neural distortion of sensory information seems adequate to explain the well known illusions of orientation that are associated with human perception of patterns containing acute angles.     

Chronic electrical stimulation of afferents from one eye changes ocular dominance of visual cortical neurons in kittens

Binocular visual responsiveness of neurons in visual cortex of the cat can be changed by monocular visual deprivation in the critical period of postnatal development. It is hypothesized that afferents from each eye compete with one another for synaptic connections with cortical neurons so that less active afferents from the deprived eye fail to maintain the connections. This hypothesis predicts that an increase in inputs from one eye instead of decrease due to deprivation should also change binocular responsiveness of cortical neurons. However, the hypothesis has not successfully been tested with experimental activation of afferents from one eye. In the present study, we activated one of the optic nerves by chronic electrical stimulation of theta-burst type in behaving kittens for 2 days. After such a monocular activation, visual cortical neurons showed a significant ocular dominance shift in favor of the electrically activated eye, although neurons in the activated and nonactivated layers of the dorsal lateral geniculate nucleus had no biased visual responses. Also, we found no detectable difference between activated and nonactivated eye responses of cortical neurons in other response properties such as orientation selectivity. These results support the hypothesis that the balance between activities of both afferents is critical for formation or consolidation of each eye-specific pathway.     

Correlated discharges among putative pyramidal neurons and interneurons in the primate prefrontal cortex

Neurophysiological recordings have revealed that the discharges of nearby cortical cells are positively correlated in time scales that range from millisecond synchronization of action potentials to much slower firing rate co-variations, evident in rates averaged over hundreds of milliseconds. The presence of correlated firing can offer insights into the patterns of connectivity between neurons; however, few models of population coding have taken account of the neuronal diversity present in cerebral cortex, notably a distinction between inhibitory and excitatory cells. We addressed this question in the monkey dorsolateral prefrontal cortex by recording neuronal activity from multiple micro-electrodes, typically spaced 0.2-0.3 mm apart. Putative excitatory and inhibitory neurons were distinguished based on their action potential waveform and baseline discharge rate. We tested each pair of simultaneously recorded neurons for presence of significant cross-correlation peaks and measured the correlation of their averaged firing rates in successive trials. When observed, cross-correlation peaks were centered at time 0, indicating synchronous firing consistent with two neurons receiving common input. Discharges in pairs of putative inhibitory interneurons were found to be significantly more strongly correlated than in pairs of putative excitatory cells. The degree of correlated firing was also higher for neurons with similar spatial receptive fields and neurons active in the same epochs of the behavioral task. These factors were important in predicting the strength of both short time scale (<5 ms) correlations and of trial-to-trial discharge rate covariations. Correlated firing was only marginally accounted for by motor and behavioral variations between trials. Our findings suggest that nearby inhibitory neurons are more tightly synchronized than excitatory ones and account for much of the correlated discharges commonly observed in undifferentiated cortical networks. In contrast, the discharge of pyramidal neurons, the sole projection cells of the cerebral cortex, appears largely independent, suggesting that correlated firing may be a property confined within local circuits and only to a lesser degree propagated to distant cortical areas and modules.