Postsynaptic potentials in the cat's visual cortex following electrical stimulation of afferent pathways

ERAASR: an algorithm for removing electrical stimulation artifacts from multielectrode array recordings

OBJECTIVE

Electrical stimulation is a widely used and effective tool in systems neuroscience, neural prosthetics, and clinical neurostimulation. However, electrical artifacts evoked by stimulation prevent the detection of spiking activity on nearby recording electrodes, which obscures the neural population response evoked by stimulation. We sought to develop a method to clean artifact-corrupted electrode signals recorded on multielectrode arrays in order to recover the underlying neural spiking activity.

APPROACH

We created an algorithm, which performs estimation and removal of array artifacts via sequential principal components regression (ERAASR). This approach leverages the similar structure of artifact transients, but not spiking activity, across simultaneously recorded channels on the array, across pulses within a train, and across trials. The ERAASR algorithm requires no special hardware, imposes no requirements on the shape of the artifact or the multielectrode array geometry, and comprises sequential application of straightforward linear methods with intuitive parameters. The approach should be readily applicable to most datasets where stimulation does not saturate the recording amplifier.

MAIN RESULTS

The effectiveness of the algorithm is demonstrated in macaque dorsal premotor cortex using acute linear multielectrode array recordings and single electrode stimulation. Large electrical artifacts appeared on all channels during stimulation. After application of ERAASR, the cleaned signals were quiescent on channels with no spontaneous spiking activity, whereas spontaneously active channels exhibited evoked spikes which closely resembled spontaneously occurring spiking waveforms.

SIGNIFICANCE

We hope that enabling simultaneous electrical stimulation and multielectrode array recording will help elucidate the causal links between neural activity and cognition and facilitate naturalistic sensory protheses.

Area- and band-specific representations of hand movements by local field potentials in caudal cingulate motor area and supplementary motor area of monkeys

The caudal cingulate motor area (CMAc) and the supplementary motor area (SMA) play important roles in movement execution. The present study examined the neural mechanisms underlying these roles by investigating local field potentials (LFPs) from these areas while monkeys pressed buttons with either their left or right hand. During hand movement, power increases in the high-gamma (80-120 Hz) and theta (3-8 Hz) bands and a power decrease in the beta (12-30 Hz) band were observed in both the CMAc and SMA. High-gamma and beta activity in the SMA predominantly represented contralateral hand movements, whereas activity in the CMAc preferentially represented movement of either hand. Theta activity in both brain regions most frequently reflected movement of either hand, but a contralateral hand bias was more evident in the SMA than in the CMAc. An analysis of the relationships of the laterality representations between the high-gamma and theta bands at each recording site revealed that, irrespective of the hand preference for the theta band, the high-gamma band in the SMA preferentially represented contralateral hand movement, whereas the high-gamma band in the CMAc represented movement of either hand. These findings suggest that the input-output relationships for ipsilateral and contralateral hand movements in the CMAc and SMA differ in terms of their functionality. The CMAc may transform the input signals representing general aspects of movement into commands to perform movements with either hand, whereas the SMA may transform the input signals into commands to perform movement with the contralateral hand.

Coupling of mesoscopic brain oscillations: recent advances in analytical and theoretical perspectives

Oscillatory brain activities have been traditionally studied in the context of how oscillations at a single frequency recorded from a single area could reveal functional insights. Recent advances in methodology used in signal analysis have revealed that cross-frequency coupling, within or between functional related areas, is more informative in determining the possible roles played by brain oscillations. In this review, we begin by describing the cellular basis of oscillatory field potentials and its theorized as well as demonstrated role in brain function. The recent development of mathematical tools that allow the investigation of cross-frequency and cross-area oscillation coupling will be presented and discussed in the context of recent advances in oscillation research based on animal data. Particularly, some pitfalls and caveats of methods currently available are discussed. Data generated from the application of examined techniques are integrated back into the theoretical framework regarding the functional role of brain oscillations. We suggest that the coupling of oscillatory activities at different frequencies between brain regions is crucial for understanding the brain from a functional ensemble perspective. Effort should be directed to elucidate how cross-frequency and area coupling are modulated and controlled. To achieve this, only the correct application of analytical tools may shed light on the intricacies of information representation, generation, binding, encoding, storage and retrieval in the brain.

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation

To combine insights obtained from electric field potentials (LFPs) and neuronal spiking activity (MUA) we need a better understanding of the relative spatial summation of these indices of neuronal activity. Compared to MUA, the LFP has greater spatial coherence, resulting in lower spatial specificity and lower stimulus selectivity. A differential propagation of low- and high-frequency electric signals supposedly underlies this phenomenon, which could result from cortical tissue specifically attenuating higher frequencies, i.e., from a frequency-dependent impedance spectrum. Here we directly measure the cortical impedance spectrum in vivo in monkey primary visual cortex. Our results show that impedance is independent of frequency, is homogeneous and tangentially isotropic within gray matter, and can be theoretically predicted assuming a pure-resistive conductor. We propose that the spatial summation of LFP and MUA is determined by the size of these signals' generators and the nature of neural events underlying them, rather than by biophysical properties of gray matter.

The neurophysiological validation of the hyperpolarization theory of internal inhibition

The experiments in conscious non-immobilized rabbits showed that cessation of the reactions without reinforcement (elaboration of the internal inhibition) is accompanied by an enhanced phasic state, by alternation of activation and inhibition of neuron firing, and by the corresponding slow potential oscillation (SPO). These changes can be either localized, predominantly in the structures of conditioned stimulus, or, under enhancement of the inhibitory state, generalized in the brain structures. On the basis of our experience and published data, it is concluded that the above event results from relative enhancement of the inhibitory hyperpolarizing processes due to increase in reactivity of the inhibitory systems to stimulus, which acquires inhibitory properties during learning. Changes in the excitability and reactivity of neuron populations appearing during enhancement of the hyperpolarizing inhibition, and differing in the various brain structures, play an active role in the execution of the main function of the internal inhibition: limitation of excitation transmission to the effectors. An inhibitory mediator gamma aminobutyric acid (GABA) is of great importance in inhibiting the excitation in response to the stimulus which lost its biological significance. These experimental data and their interpretation in the light of published data give the basis for the development of the hyperpolarization theory of internal inhibition.

Strength and orientation tuning of the thalamic input to simple cells revealed by electrically evoked cortical suppression

Is thalamic input to the visual cortex strong and well tuned for orientation, as predicted by Hubel and Wiesel's (1962) model of orientation selectivity in simple cells? We directly measured the size of the thalamic input to single simple cells intracellularly by combining electrical stimulation of the cortex with a briefly flashed visual stimulus. In nearby cells, the electrical stimulation evoked a long-lasting inhibition that prevented them from firing in response to the visual stimulus. The visually evoked excitatory postsynaptic potentials (EPSPs) recorded during the period of cortical suppression, therefore, reflected largely the thalamic input. In 16 neurons that received monosynaptic input from the thalamus, cortical suppression left 46% of normal visual response on average (12%-86% in range). In those cells tested, this remaining visual response was as well tuned for orientation as the normal response to the visual stimulus alone. We conclude that the thalamic input to cortical simple cells with monosynaptic input from the thalamus is strong and well tuned in orientation, and that the intracortical input does not appear to sharpen orientation tuning in these cells.

Dynamics of orientation coding in area V1 of the awake primate

To investigate the importance of feedback loops in visual information processing, we have analyzed the dynamic aspects of neuronal responses to oriented gratings in cortical area V1 of the awake primate. If recurrent feedback is important in generating orientation selectivity, the initial part of the neuronal response should be relatively poorly selective, and full orientation selectivity should only appear after a delay. Thus, by examining the dynamics of the neuronal responses it should be possible to assess the importance of feedback processes in the development of orientation selectivity. The results were base on a sample of 259 cells recorded in two monkeys, of which 89% were visually responsive. Of these, approximately two-thirds were orientation selective. Response latency varied considerably between neurons, ranging from a minimum of 41 ms to over 150 ms, although most had latencies of 50-70 ms. Orientation tuning (defined as the bandwidth at half-height) ranged from 16 deg to over 90 deg, with a mean value of around 55 deg. By examining the selectivity of these different neurons by 10-ms time slices, starting at the onset of the neuronal response, we found that the orientation selectivity of virtually every neuron was fully developed at the very start of the neuronal response. Indeed, many neurons showed a marked tendency to respond at somewhat longer latencies to stimuli that were nonoptimally oriented, with the result that orientation selectivity was highest at the very start of the neuronal response. Furthermore, there was no evidence that the neurons with the shortest onset latencies were less selective. Such evidence is inconsistent with the hypothesis that recurrent intracortical feedback plays an important role in the generation of orientation selectivity. Instead, we suggest that orientation selectivity is primarily generated using feedforward mechanisms, including feedforward inhibition. Such a strategy has the advantage of allowing orientation to be computed rapidly, and avoids the initially poorly selective neuronal responses that characterize processing involving recurrent loops.

Effect of optic nerve stimulation on neurons in pericruciate cortex of cats

The distribution of optic chiasm input to different types of neurons in pericruciate cortex of cats agreed with previous work using light flashes. Neuron response times served to differentiate the input pathways to pericruciate cortex, and the types of neurons they influence. Input from the optic chiasm arrived in three distinct surges: the first via the superior colliculus, the second via an unidentified pathway, and the third via the visual cortex. A fourth, diffuse surge arrived in the postcruciate cortex via some unidentified pathway. Stimulation of the contralateral side of the optic chiasm had a weaker effect than stimulation of the ipsilateral side; it evoked activity at a higher threshold, with fewer spikes per response, and at a longer latency. The difference in response latency between the two sides was largest on neurons responding to the first surge, decreasing in later surges, and being least on those neurons responding to the last surge. About 2.3% of the postcruciate and 15% of the precruciate neurons responded only to optic chiasm stimulation; they were isolated in the granular layers, and their responses could not be influenced by prior cutaneous input. It is suggested that much of the visual input to pericruciate cortex serves to modulate on-going cortical output and, thereby, the behavior of the animal.

Mechanisms of inhibition in cat visual cortex

1. Neurones from layers 2-6 of the cat primary visual cortex were studied using extracellular and intracellular recordings made in vivo. The aim was to identify inhibitory events and determine whether they were associated with small or large (shunting) changes in the input conductance of the neurones. 2. Visual stimulation of subfields of simple receptive fields produced depolarizing or hyperpolarizing potentials that were associated with increased or decreased firing rates respectively. Hyperpolarizing potentials were small, 5 mV or less. In the same neurones, brief electrical stimulation of cortical afferents produced a characteristic sequence of a brief depolarization followed by a long-lasting (200-400 ms) hyperpolarization. 3. During the response to a stationary flashed bar, the synaptic activation increased the input conductance of the neurone by about 5-20%. Conductance changes of similar magnitude were obtained by electrically stimulating the neurone. Neurones stimulated with non-optimal orientations or directions of motion showed little change in input conductance. 4. These data indicate that while visually or electrically induced inhibition can be readily demonstrated in visual cortex, the inhibition is not associated with large sustained conductance changes. Thus a shunting or multiplicative inhibitory mechanism is not the principal mechanism of inhibition.

A functional microcircuit for cat visual cortex

1. We have studied in vivo the intracellular responses of neurones in cat visual cortex to electrical pulse stimulation of the cortical afferents and have developed a microcircuit that simulates much of the experimental data. 2. Inhibition and excitation are not separable events, because individual neurones are embedded in microcircuits that contribute strong population effects. Synchronous electrical activation of the cortex inevitably set in motion a sequence of excitation and inhibition in every neurone we recorded. The temporal form of this response depends on the cortical layer in which the neurone is located. Superficial layer (layers 2+3) pyramidal neurones show a more marked polysynaptic excitatory phase than the pyramids of the deep layers (layers 5+6). 3. Excitatory effects on pyramidal neurones, particularly the superficial layer pyramids, are in general not due to monosynaptic input from thalamus, but polysynaptic input from cortical pyramids. Since the thalamic input is transient it does not provide the major, sustained excitation arriving at any cortical neurone. Instead the intracortical excitatory connections provide the major component of the excitation. 4. The polysynaptic excitatory response would be sustained well after the stimulus, were it not for the suppressive effect of intracortical inhibition induced by the pulse stimulation. 5. Intracellular recording combined with ionophoresis of gamma-aminobutyric acid (GABA) agonists and antagonists showed that intracortical inhibition is mediated by GABAA and GABAB receptors. The GABAA component occurs in the early phase of the impulse response. It is reflected in the strong hyperpolarization that follows the excitatory response and lasts about 50 ms. The GABAB component occurs in the late phase of the response, and is reflected in a sustained hyperpolarization that lasts some 200-300 ms. Both components are seen in all cortical pyramidal neurones. However, the GABAA component appears more powerful in deep layer pyramids than superficial layer pyramids. 6. The microcircuit simulates with good fidelity the above data from experiments in vivo and provides a novel explantation for the apparent lack of significant inhibition during visual stimulation. The basic circuit may be common to all cortical areas studied and thus the microcircuit may be a 'canonical' microcircuit for neocortex.

Increased regularity of activity of cortical neurons in learning due to disinhibitory effect of reinforcement

This paper reviews the author's studies on neurophysiologic mechanisms of conditioned reflex learning. Electroencephalograms, evoked potentials, activity of neocortical and hippocampal neurons and the rabbits' behavior in the course of elaboration of defensive and inhibitory conditioned reflexes to light flashes have been recorded. Electric shock (ECS) applied to the paw served as reinforcement. The study demonstrated three types of reinforcement effect on the activity of cortical neurons: activating, disinhibitory, and inhibitory. EEG activation due to reinforcement is accompanied by a change in phasic cortical neuronal activity from chaotic or irregular, typical of rest or inhibition, to regular tonic discharges (in neocortex and hippocampus) and group discharges in the stress rhythm, 5-7 Hz in the hippocampus. Following a number of conditioning trials, the effect of reinforcement is simulated by the effect of a conditioned stimulus. With EEG activation and increased regularity in impulses, facilitation of motor reactions is observed.

Serial waking-sleep EEGs and evolution of somatosensory potentials in Creutzfeldt-Jakob disease

We have analysed the serial sleep and waking EEGs of 3 patients with Creutzfeldt-Jakob disease (CJD), demonstrated by pathology studies. The presence of periodic complexes (PCs) was a constant finding in all the records, including those made on admission (1-3 months after the onset of symptoms) although at this stage the PCs were not as persistent of generalized as in the more advanced stages. The physiological states of sleep were replaced by cyclic changes which initially consisted of phases with PCs alternating with theta-delta activity. As the disease progressed, brief periods of suppression of background rhythm were added, followed by slower PCs. In case 3, the SEPs obtained on stimulating the median nerves were normal at the first test. However, 17 days later a further test revealed two types of abnormality. On the one hand, using a non-cephalic reference, the SEPs were of lesser amplitude and had become disorganized. On the other hand, with a cephalic reference, a potential of enormous amplitude was obtained unilaterally. On the contrary, the SEPs obtained with a cephalic reference on stimulating the posterior tibial nerves were of small amplitude and disorganized. These findings, in combination with the EEG anomalies, suggest a progressive dysfunction of the cerebral cortex. The recording of a giant potential only at C'3-Fz on stimulating the median nerve could be related to a pathological facilitation of long-loop transcortical stretch reflexes of the hand.

Geniculate orientation biases seen with moving sine wave gratings: implications for a model of simple cell afferent connectivity

Orientation bias of cat dorsal lateral geniculate (LGN) neurones varied with the spatial frequency of a moving sine wave grating. At low spatial frequencies there was little orientation bias, whereas near the high-frequency limit, the dependence on orientation was marked. It is proposed that, if such cells were to drive the cortical inhibitory interneurones responsible for the orientation sensitivity of striate simple cells, it would explain many distinguishing features of cortical cells besides their orientation sensitivity.

Visual cortical mechanisms responsible for direction selectivity

We measured the response of some 171 directionally selective (DS) neurons in visual area 17 of the cat. Sequences of long duration (400 msec) stimulus onsets and offsets were employed to analyze the underlying mechanisms. We corroborate the existence of sub-regions of localized, anisotropic inhibition as the basic mechanism of directional tuning. At extremely small and at rather large asynchronies, DS neurons inhibit virtually non-selectively. Optimal directional selectivity occurs when the second response peak of S1 coincides with the first response peak of S2 (usually at an asynchrony of approximately 75 msec). The mechanism appears to involve the linear temporal convolution of impulse response functions.

Neurophysiologic mechanisms of internal inhibition

The experimental results obtained in the authors' laboratory as a result of multiple recording of slow biopotentials, the recording of neuronal activity and of mathematical modeling, are reviewed. The authors conclude that the elaboration of internal inhibition is followed, and determined to a great extent, by the restriction in conduction of excitations due to the increase of inhibitory hyperpolarization and discordance in the periodicity of slow potentials, reflecting oscillations in excitability of neuronal populations in the cortex and other brain structures.

Laminar differences in development of afferent innervation to striate cortex neurones in kittens

We studied 587 cells in the striate cortex of 47 kittens, ranging in age postnatally from the 8th day to the 20th week, to explore differences in rates of functional maturation of cells and afferents among cortical layers. For all cells studied we determined spontaneous activity level, visual responsiveness, responsiveness to electrical stimulation of the afferent pathways and histologically reconstructed their laminar localization. At the 2nd-3rd week postnatally, single shocks applied to the dorsal lateral geniculate nucleus (LGN) elicited relatively inconsistent responses with long latencies in most of the cells in all layers, except in layer V where responses were consistent. In layers IVab, V, and VIu (the upper half of layer VI) the LGN-latency decreased very rapidly in the 4th week, while in layer IVc such a decrease occurred in the 5th week. In layers II+III and VIl (the lower half of layer VI) a less substantial decrease in latency occurred during the 4th and 5th week. At 2 weeks, nearly half of layer V cells had orientation-specific visual responses or spontaneous activity, but such cells were rare in other layers. The proportions of the specific or spontaneously active cells matured successively in the order of layers V leads to IVab and VIu leads to IVc leads to II+III and VIl. We conclude that the maturation of functional properties of cortical cells may occur successively in the above-mentioned laminar order, and that cells size and connectivity with afferents may be factors influencing the rate of functional maturation of cortical cells. The intracortical synaptic delay was estimated for each of the cells driven mono-synaptically from the LGN. The average delay decreased most rapidly during the 4th and 5th week. Conduction velocities of afferents innervating the mono-synaptic cells in layers IVab and IVc were calculated. The velocity of the former afferents increased very quickly and reached a value suggesting myelination at 4 weeks, while that of the latter afferents at 5 weeks. Since each type of LGN cell is known to project separately to layers IVab and IVc, respectively, this suggests that Y-cells of LGN may mature earlier than X-cells.

Spatial sampling by dendritic trees in visual cortex

Kittens were reared in vertically or horizontally striped cylinders. After rearing exposures of 400-500 h, responses of single neurons were determined as a function of orientation of a square wave grating stimulus. These data suggest that the rearing environment did alter orientation preference in some of the kittens. The visual cortices of the stripe-reared kittens and of control kittens were impregnated according to a Golgi-Cox method. Dendrites of layer IV stellate cells were tracked and analyzed in three dimensions by a computer-microscope. Four methods of analyzing the spatial distribution of dendrites are described and discussed. Two methods previously described in the literature were not sufficiently sensitive to detect any differences among kittens exposed to vertical or horizontal stripes or to a control environment. Two newly developed methods were able to provide initial evidence for rearing effects on dendritic trees in visual cortex. The more detailed of these new methods describes the angular location of dendritic segments, with respect to standard brain axes, as a function of distance from the cell body. Data obtained by means of this method of dendritic angular distribution (DAD) plots suggests a number of conclusions. Rearing animals in a striped environment may influence the way in which dendrites of layer IV stellate cells of visual cortex distribute themselves in the neuropil. The effect of selective rearing on dendritic distribution does not appear to extend back to those portions of the dendritic tree closest to the cell body. This influence of rearing in a selective environment may be explained by hypothesizing that during development dendrites distribute themselves in ways that tend to maximize the effects of spatiotemporal summation for the postsynaptic neurons.

Selective synaptic antagonism by atropine and alpha-aminoadipate of pulvinar and cortical afferents to the suprasylvian visual area (Clare-Bishop area)

The synaptic excitations of cells of the Clare-Bishop cortical region produced by electrical stimulation of the pulvinar and ipsilateral cortex, have been shown to be differentially antagonized by iontophoretically applied antagonists. Atropine attenuated the responses evoked by pulvinar stimulation without having an appreciable effect against either iontophoretically applied aspartate or cortically evoked responses. alpha-Aminoadipate antagonized aspartate elicited excitations and those obtained with cortical stimulation while leaving unaffected acetylcholine and pulvinar evoked responses. The results are supportive of the view that acetylcholine and aspartate, or a similar excitatory amino acid, act as synaptic transmitters of some afferents from the pulvinar and ipsilateral cerebral cortex, respectively.

The rhythm of cortical excitability

Evidence has been gained which suggests that the spectral properties of the excitability cycle are other than those first proposed or assumed to exist in a number of experiments and theories. Instead of appearing as a sinusoidal cycle at the alpha frequency, the excitability cycle seems to be irregular and slower on the average than the EEG alpha rhythm. The rhythm appears to be continuously operative. It is not very sensitive in its spectral composition to the conditions of illumination or the level of behavioral activity. The rhythm operates synchronously in the right and left visual cortex. Methods similar to those described here could be used to map the extent to which synchrony is maintained in structures throughout the brain. They also suggest new approaches for psycho-physical studies which are aimed at establishing relationships between behavioral or perceptual phenomena and measures of neural activity.

Hierarchical and parallel mechanisms in the organization of visual cortex

We argue that it seems fruitful to regard the retino-geniculate-cortical pathway, and perhaps the visual pathways in general, as comprising distinct neuronal channels which begin with the major groupings of ganglion cells, and subserve distinct functions within the overall operation of the visual system. One problem for future work is to determine the extent and, equally importantly, the limitations of the idea of independently functioning neuronal channels operating within the visual system. Some evidence of those limitations is already available. Kulikowski and Tolhurst have provided evidence suggesting that pattern detection is mediated by the X-like system at high spatial frequencies and by the Y-like system at low frequencies, but that at intermediate frequencies, both systems are likely to contribute to this function. Again, there is already physiological and psychophysical evidence of inhibitory interaction between X- and Y-cell systems, which may contribute to their functioning. That is, although there is little evidence of excitatory interaction between W-, X- and Y-cell systems, at least up to the first cortical synapse, the functioning of, say, the X-cell system may depend on the inhibitory influences impinging on it from Y-cell activity. Further, it may prove to be the case that one cell 'system' may be involved in several distinct functions and considerable work may be required to establish whether or not these functions can be considered constituent parts of an overall function, such as 'ambient' or 'foveal' vision. In the following section we suggest a classification and terminology for visual neurones which may provide a framework for future work on these lines.

A comparison of the organization of the projections of the dorsal lateral geniculate nucleus, the inferior pulvinar and adjacent lateral pulvinar to primary visual cortex (area 17) in the macaque monkey

Both anterograde and retrograde transport tracing methods were used to study the organization of the projections of the dorsal lateral geniculate (DLG), the inferior pulvinar and subdivisions of the lateral pulvinar to primary visual cortex (striate cortex or area 17). The DLG projects only to striate cortex. These projections are retinotopically organized, and do not extend to any cortical layers above layer IVA. In contrast the inferior pulvinar (PI) and the immediately adjacent portion of the lateral pulvinar (PL alpha 48) project to both striate and prestriate cortex. The projections from these two thalamic areas to the striate cortex are also retinotopically organized and exist in parallel with those from the DLG. In contrast to the DLG, the projections from PI and PL alpha terminate above layer IVA in striate cortex, i.e. layers I, II and III. In prestriate cortex the layers of termination include layers IV, III and I. The pulvinar terminations in layers II and III of area 17 occur in segregated patches as do the geniculate terminations in layers IVC and IVA. On the other hand the pulvinar terminations in layer I which overlie those in layers II and III of area 17 appeared to be continuous. Control studies show that the remainder of the lateral pulvinar overlying PL alpha does not project to striate cortex. It is concluded that there are 3 visuotopically organized inputs from the lateral thalamus to primary visual cortex and that each of these inputs have different layers of termination. The inputs from PI and DLG can convey direct retinal inputs while those from PI and PL alpha can also be involved in intrinsic cortico-thalamocortical connection with prestriate cortex. It remains, then that it cannot be tacitly assumed that the ascending inputs which influence the response properties of the primary cortical neurons arise solely from the dorsal lateral geniculate nucleus. It is also argued that these inputs to the supragranular layers may be excitatory as those from the DLG to the IVth layer.

Inhibitory and excitatory binocular convergence to visual cortical neurons of the cat

Responses of 182 visual cortical (VC) neurons to electrical stimulation of both optic nerves (ON) were recorded intra- and extracellularly, and their eye dominance determined with visual stimuli. Many VC neurons could not be excited by ON stimulation without simultaneous activation by visual stimulation of the eye of the other side. The ON-excited units (25 simple and 71 complex cells) had essentially the same response latency from both ONs. In almost all VC neurons including those not driven from the ON, ON stimulation elicited inhibition which was shown to be post-synaptic in all neurons recorded intra- or quasi-intracellularly. The onset latency of IPSPs not preceded by an action potential or EPSP was 4.2+/-1.0 msec, suggesting that intracortical inhibition was initiated by afferent impulses mediated through fast conducting fibers. In visually monocular neurons, ON stimulation of the non-driving side also elicited primary inhibition. Visually binocular, but monocular-dominant neurons responded more reliably to ON stimulation of the dominant side than to the other. In most binocular neurons with equal visual responsiveness to both eyes, inhibition and excitation evoked from both ONs had about the same latency and magnitude.

The distribution of degenerating axons after small lesions in the intact and isolated visual cortex of the cat

The extent of the spread of axonal degeneration was investigated in the visual cortex of the cat after making small lesions restricted to the grey matter. Two series of experiments were undertaken. In the first, normal adult cats were used, and in the second, the cortex of the postlateral gyrus was isolated from its extrinsic afferents by surgical undercutting 3 months before making the lesions. The results were similar in the two series in most respects. 1. Horizontal fibres extended in considerable numbers for some 500 micrometer from the lesion, mainly in layers I, III/IV and V, a few reaching 2/3 mm. These fibres were better seen in the intact than in the isolated cortex. Their spread was usually asymmetrical, being greater posteromedially than anterolaterally. 2. Oblique axons ran downwards from the middle layers into layers V and VI, or upwards into layers I and II. 3. Axons arising from layers II to VI descended vertically into the white matter. Degeneration patterns after lesions in areas 17 and 18 were compared.

Dose-dependent dual effect of morphine on electrophysiologic correlates of positive reinforcement (reward contingent positive variation: RCPV) in the cat

In cats trained to press a lever for milk reward, the postreinforcement EEG synchronization (PRS) and the associated epicortical steady potential shift, known as "Reward Contingent Positive Variation (RCPV) restricted to the occipital cortex, were studied prior to and after administration of morphine sulfate. Contrary to what has ordinarily been described as a typical feline response to morphine, such as restlessness, aggressiveness, rage, and exaggerated startle reaction to environmental stimuli, associated with an increased tonus of the brainstem-hypothalamic arousal system and desynchronized EEG patterns, doses of 0.1-0.4 mg/kg, IM, caused a strong monophasic enhancement of the PRS-RCPV phenomenon. Doses of 0.6-1.0 mg/kg, IM, had clearly a biphasic action: the initial enhancement of the PRS-RCPV responses was followed by their strong suppression. Chlorpromazine (0.1-1.6 mg/kg, i.e.) promptly restored the EEG responses. During the peak effect of the enhancing doses of morphine, the reward-related EEG phenomena also occurred prior to or after the nonrewarded bar press when the animals licked the empty cup. This dissociation of the PRS-RCPV from consumption was much more conspicuous in animals whose control frequency of the PRS oscillations was higher, and after morphine showed more significant slowing. Despite the strong facilitation of the PRS-RCPV in the presence of light, morphine, in contrast to LSD-25, was not able to restore the reward-induced phenomena in the dark.

Reticular suppression of flash-evoked IPSPs in visual cortex neurons

In a majority of visual cortex (VC) neurons recorded intracellularly in chronically implanted rabbits, light flashes evoked responses consisting of protracted IPSPs (duration 160-200 msec) followed in some cases by a rebound. Inhibition of spontaneous activity through high frequency reticular stimulation (MRF) or through presentations of non-visual arousing stimuli, was not associated with strong hyperpolarization of the cell membrane. Reticular stimulation evoking both activation and inhibition of spontaneous neuronal activity elicited attenuation or complete elimination elimination (disinhibition) of flash-evoked protracted IPSPs and of the postinhibitory rebounds. Rhythmic neuronal discharges elicited by stimulation of the visual pathway was also reduced during reticular activation, due to attenuation of the IPSP-rebound sequence.

Activation and inhibition of the lateral hypothalamic neurones elicited by medial forebrain bundle stimulation

1. Single neurone discharges were recorded from the lateral hypothalamus (LH) at the level of the ventromedial nucleus. Single stimulation of the medial forebrain bundle (MFB) at the level of the mesodiencephalic junction elicited a strong inhibition lasting for 100-300 msec in these neurones. 2. However, by the use of multiple stimulating electrodes, it was possible to find a circumscribed locus in the MFB which gave rise to an initial activation of firing discharges with a latency of 1-15 msec (in different neurones) preceding the period of inhibition. 3. One class of the LH neurones was antidromically activated, and was thought to give off axons into the descending MFB. The other was activated orthodromically through a highly efficient synaptic connexion, and was viewed as 'relay' neurones of the ascending MFB. 4. Although the former could also be activated orthodromically, the efficacy of driving was distinctly lower than that observed in the latter. Presumably they integrate the messages from the visceral centres of the medulla and pons, thus participating in the viscero-motor outflow from the hypothalamus. 5. Intracellularly recorded spike potentials of these neurones deteriorated rapidly leaving only hyperpolarization which corresponded in its time course to the suppression of extracellular spike discharges following MFB stimulation. It was suggested that the inhibition was elicited via recurrent routes localized within the hypothalamus.

The contribution of inhibitory mechanisms to the receptive field properties of neurones in the striate cortex of the cat

1. The iontophoretic application of the GABA antagonist bicuculline to simple and complex cells in the striate cortex of the cat produced extensive modifications of receptive field properties. These modifications appear to relate to a block or reduction of GABA-mediated intracortical inhibitory influences acting on the cells examined. 2. For simple cells the effects of bicuculline on receptive field properties involved a loss of the subdivision of the receptive field into antagonistic "on" and "off" regions, a reduction in orientation specificity and a reduction or elimination of directional specificity. 3. The effect on the "on" and "off" subdivisions of the simple cell receptive field was such that all stationary flashing stimuli, whether covering the whole receptive field, or located within the receptive field over a previously determined "on" or "off" region, resulted in an "on and off" response. 4. The orientation specificity of complex cells was reduced during the application of bicuculline such that in many cases the original specificity of the cell was virtually lost with the response to the orientation at 90 degrees to the optimal being of similar magnitude to the optimal. The directional specificity of complex cells was generally less affected than that of simple cells. Often when large changes in orientation specificity were observed the directional specificity was relatively unaffected. 5. For some cells apparently showing to all visual stimuli only inhibitory responses, the application of bicuculline resulted in the appearance of excitatory responses. 6. In all cases receptive field properties reverted to the original state after termination of the bicuculline application. It was not generally possible to duplicate the effects of bicuculline by raising neuronal excitability with iontophoretically applied glutamate. 7. On the basis of these results it is suggested that the normal subdivision of the simple cell receptive field into separate "on" and "off" regions and its directional specificity are dependent on intracortical inhibitory processes that are blocked by bicuculline. The orientational tuning of simple cells conversely appears to be largely determined by the excitatory input but normally enhanced by lateral type inhibitory processes acting in the orientation domain. 8. It also appears that the excitatory input to some complex cells is not orientation specific. This suggests that for these cells it is extremely unlikely that they receive an orientation specific excitatory input from simple cells.

The velocity tuning of single units in cat striate cortex

1. The activity of single units was recorded from the striate cortex (area 17) of anaesthetized, paralysed cats. Responses to stimuli moving at different velocities were examined. 2. Peak evoked firing frequency, rather than fotal evoked spikes, is used throughout as a measure of response. The former mea-ure gives curves of response vs. velocity that correlate well with curves of contrast sensitivity vs. velocity, wheras the latter does not. 3. Cortical receptive fields were classified according to the criteria of Hubel & Wiesel. Simple cells were found to prefer lower velocities (mean 2-2 deg sec-1) than complex cells( mean 18-8 deg sec-1). The response of simple cells to stimuli moving faster than 20 deg sec-1 is generally poor; complex cells usually discharge briskly to these speeds. 4. Cells classified as hypercomplex by the end-inhibition criterion were further chara-terized as type I or type II, according to the suggestion of Dreher (1972). Type I units are indistinguishable from simple cells in their velocity tuning, and type II units equally clearly resemble complex cells. These results are therefor consistent with Dreher's sbudivision. 5. Teh selectivity of cells for velocity is variable but can be quite marked. The average selectivities of simple and complex cells are not significantly different. There is an inverse correlation between preferred velocity and the sharpness of velocity selectivity for simple cells; no trend is apparent for other cell types. 6. No clear correlation is observed between the velocity preferances of units and their degree of direction selectivity, or receptive field arrangement. Simple cells with 'sustainef' temporal responses to flashed stimuli tend to prefer slower rates of movement than 'transient' ones, and to be less selective for velocity. 7. The results for different cortical cell-types are compared with the velocity tuning of X- and Y-cells in the lateral geniculate nucleus.

A study of inhibitory antagonism in cat visual cortex

Since there seems to be good evidence that GABA may act as an inhibitory neurotransmitter in the mammalian cortex, we tested the effects of an antagonist of GABA, namely the alkaloid bicuculine, on the response properties of visual cortex neurons, using a computer-controlled stimulus presentation system to assess quantitatively the changes in receptive field organization after the drug. Complex cells were most affected, increasing both evoked and spontaneous activity and losing some of their specificities for stimulus parameters such as orientation and direction. Hyper-complex cells lost their inhibitory flanks, responding equally well to long and short bars after the drug. Simple cells were the least affected, usually becoming somewhat depressed after the drug. Preliminary tests with another inhibitory amino acid antagonist, strychnine, showed that it excited simple cells, indicating that possibly more than one inhibitory transmitter is at work in the cortex. The results are discussed with relation to the synaptic anatomy of the cortex, and it is concluded that a class of stellate cells, using GABA, is a likely candidate for the transmitter of some intracortical inhibition.

Innate and environmental factors in the development of the kitten's visual cortex

1. This is a study of the receptive fields of 771 cells recorded in the visual cortex of twenty-five kittens reared normally or subjected to various kinds of visual deprivation or environmental manipulation. 2. Kittens deprived of patterned visual experience, by dark rearing or diffuse occlusion of the eyes, have a majority of cirtical neurones with little or no specificity for the orientation or axis of movement of visual stimuli. However, in such deprived animals, especially those younger than 3 weeks, there are a number of genuinely orientation selective cells. They are broadly "turned" (by adult standards), they are almost always of the simple type, are heavily dominated by one eye, and are found mainly in the deeper layers of the cortex, especially layer IV. 3...

Identification of cone mechanisms in graded responses of foveal striate cortex

1. The earliest electrical response detectable in foveal striate cortex of anaesthetized Rhesus monkeys following light stimulation is a graded potential which is positive at the cortical surface and negative in the grey matter and has a peak latency of about 60 msec. The response is similar at both the on- and the off-phase of a light stimulus.2. The relationship of this graded potential to the depth of the recording electrode, to the latency of extracellular impulses and to post-synaptic potentials suggests that it is generated by the depolarization of cortical cells.3. Action spectra obtained in the presence of strong selective chromatic adaptation indicate the participation of all three cone mechanisms in this response.4. Each cone mechanism contributes a similar potential to the response but antagonism between cone mechanisms is apparent. The proportion in which a cone mechanism contributes to the response varies from one area to another implying topographical differences in the representation of cone mechanisms in striate cortex.

Receptive fields of simple cells in the cat striate cortex

1. The excitatory and inhibitory components in the receptive fields of unimodal simple cells in the striate cortex of the cat anaesthetized with nitrous oxide have been described using slits of light and single light-dark edges as stimuli.2. There is a small excitatory region (excitatory complex) centrally located in the receptive field that is made up of various combinations and spatial arrangements of subliminal excitatory and discharge subregions or centres.3. The subliminal excitatory centres were revealed by a binocular facilitation technique. The excitability of the cell was raised by repeated stimulation via one eye while the neurone was tested with single edges via the other eye.4. The subliminal excitatory and discharge centres are each specifically activated by only one type of edge, light-dark or dark-light, and then only in one direction of motion. All the subregions in the excitatory complex have the same optimal stimulus orientation.5. Inhibitory components in the receptive field were identified by stimulating the cell with bars of light and single edges against an artificial background discharge produced by repeated stimulation separately applied either to the same eye (monocular conditioning) or to the other eye (binocular conditioning). There are powerful inhibitory sidebands to either side of the excitatory complex and these inhibitory regions merge to include the excitatory complex when stimulus orientation is angled away from the optimal.6. Excitation is highly stimulus specific whereas inhibition is non-specific.7. The organization of the two receptive fields of a binocularly discharged cell can be closely similar.8. The attempt is made to translate the concept of subliminal excitatory and discharge centres into specific neural mechanisms involving both the geniculo-cortical input and various intracortical circuits.9. These new developments call for only minor modifications to the model we have proposed for the organization of the receptive field.

Interaction effects of visual contours on the discharge frequency of simple striate neurones

1. The discharge frequency of simple neurones in the cat striate cortex responding to the two edges of a slit of light moving over their receptive fields was studied as a function of slit width. While one edge of the slit was discharging the cell, the other edge had a modifying influence on that discharge either by way of facilitation or of inhibition.2. The most common form of the curve relating discharge frequency and slit width had a maximal discharge at narrow slit widths (< 0.5 degrees ) and relative inhibition at medium widths (between 0.5 degrees and 2 degrees ). At greater slit widths there was usually a region of facilitation before the effects of the two edges became independent of one another. Three other response patterns to slits of different width are described.3. The curve relating slit width and response amplitude for a particular cell provides an important clue to the various activity profiles for that cell. An activity profile plots the excitability of a cell along a line through the receptive field in the direction of stimulus movement. Each type of edge, light and dark, has its own set of activity profiles which differ depending upon stimulus parameters such as the direction of the movement of the edge.4. Two other methods were used to provide further data concerning the activity profiles and as a check on the evidence provided by the responses to slits of different width. One of these two methods used the test stimulus against the background of an artificially produced maintained discharge and the other involved the interaction of the two receptive fields of binocularly activated cells.5. A model is put forward to explain the receptive field organization of simple striate neurones which takes into account not only the main features of what is known concerning the synaptology of the visual cortex but also the new data provided by the present paper and the one which precedes it.

Responses to visual contours: spatio-temporal aspects of excitation in the receptive fields of simple striate neurones

1. The properties of the receptive fields of simple cells in the cat striate cortex have been studied by preparing average response histograms both to moving slits of light of different width and to single light-dark edges or contours.2. The movement of a narrow (< 0.3 degrees ) slit across the receptive field gives rise to average response histograms that are either unimodal, bimodal or multimodal. A slit of light has leading (light) and trailing (dark) edges. By increasing the width of the slit it was shown that a discharge peak in the histogram coincides with the passage of one or other of the two edges over a particular region (discharge centre) in the receptive field. Each edge has its own discharge centre which is fired when the edge has the correct orientation and direction of movement.3. The discharge centres in forty-three simple cell receptive fields were located by using one or more of the following stimuli for each cell:(i) slits of different width;(ii) single light and dark edges;(iii) a wide (3 degrees ) slit moved over a range of different velocities. The same locations were obtained when all three procedures were used on the same cell.4. Most cells (79%) discharged to both edges though not necessarily in the same direction of movement. The majority (72%) fired in only one direction and most commonly (51%) the cells responded to both edges in this one direction. In only 16% of cells did both types of edge excite in both directions of movement. When the one type of edge, light or dark, was considered, 84% of the cells were direction selective and, for these cells, the other edge fired only in the same direction (51%), in both directions (7%), only in the opposite direction (5%) or not at all (21%).5. Cells responding in one direction with a unimodal average response histogram may be responding to both edges, the two responses being concealed in the one discharge peak. The two discharge centres are then either nearly coincident or, more usually, slightly offset with respect to one another. Most commonly the dark edge centre is slightly in advance of the light edge centre.6. The discharge peaks in the bimodal and multimodal types come from discharge centres that are spatially separate, each centre firing to only one type of edge. In the case of the bimodal type the light edge centre always lies ahead of the dark edge centre.7. When a cell responds to a single edge in both directions of movement, the type of contrast effective in one direction is always the reverse of that in the other. When the cell responded in both directions, whether to one or both edges, most commonly a light edge discharge centre in one direction occupied approximately the same location in space as the dark edge centre in the reverse direction and vice versa for the other edge.8. Temporal aspects of the discharge of simple cells have been examined by recording the responses to moving slits and single edges over a wide range of velocities.