Temporally advanced dynamic change of receptive field of lateral geniculate neurons during brief visual stimulation: Effects of brainstem peribrachial stimulation

Processing of visual information in the brain seems to proceed from initial fast but coarse to subsequent detailed processing. Such coarse-to-fine changes appear also in the response of single neurons in the visual pathway. In the dorsal lateral geniculate nucleus (dLGN), there is a dynamic change in the receptive field (RF) properties of neurons during visual stimulation. During a stimulus flash centered on the RF, the width of the RF-center, presumably related to spatial resolution, changes rapidly from large to small in an initial transient response component. In a subsequent sustained component, the RF-center width is rather stable apart from an initial slight widening. Several brainstem nuclei modulate the geniculocortical transmission in a state-dependent manner. Thus, modulatory input from cholinergic neurons in the peribrachial brainstem region (PBR) enhances the geniculocortical transmission during arousal. We studied whether such input also influences the dynamic RF-changes during visual stimulation. We compared dynamic changes of RF-center width of dLGN neurons during brief stimulus presentation in a control condition, with changes during combined presentation of the visual stimulus and electrical PBR-stimulation. The major finding was that PBR-stimulation gave an advancement of the dynamic change of the RF-center width such that the different response components occurred earlier. Consistent with previous studies, we also found that PBR-stimulation increased the gain of firing rate during the sustained response component. However, this increase of gain was particularly strong in the transition from the transient to the sustained component at the time when the center width was minimal. The results suggest that increased modulatory PBR-input not only increase the gain of the geniculocortical transmission, but also contributes to faster dynamics of transmission. We discuss implications for possible effects on visual spatial resolution.

Terminals of the major thalamic input to visual cortex are devoid of synapsin proteins

Synapsins are nerve-terminal proteins that are linked to synaptic transmission and key factors in several forms of synaptic plasticity. While synapsins are generally assumed to be ubiquitous in synaptic terminals, whether they are excluded from certain types of terminals is of interest. In the visual pathway, synapsins are lacking in photoreceptor and bipolar cell terminals as well as in retinogeniculate synapses. These are the terminals of the first three feedforward synapses in the visual pathway, implying that lack of synapsins may be a common property of terminals that provide the primary driver activity onto their postsynaptic neurons. To further investigate this idea, we studied the fourth driver synapse, thalamocortical synapses in visual cortex, using glutamatergic terminal antibody markers anti-VGluT1 and VGluT2, anti-Synapsin I and II, and confocal microscopy to analyze co-localization of these proteins in terminals. We also used pre-embedding immunocytochemical labeling followed by electron microscopy to investigate morphological similarities or differences between terminals containing synapsins or VGluT2. In visual cortex, synapsin coincided extensively with non-TC-neuron marker, VGluT1, while thalamocortical terminal marker VGluT2 and synapsin overlap was sparse. Morphologically, synapsin-stained terminals were smaller than non-stained, while VGluT2-positive thalamocortical terminals constituted the largest terminals in cortex. The size discrepancy between synapsin- and VGluT2-positive terminals, together with the complementary staining patterns, indicates that thalamocortical synapses are devoid of synapsins, and support the hypothesis that afferent sensory information is consistently transmitted without the involvement of synapsins. Furthermore, VGluT2 and synapsins were colocalized in other brain structures, suggesting that lack of synapsins is not a property of VGluT2-containing terminals, but a property of primary driver terminals in the visual system.

Changes in firing pattern of lateral geniculate neurons caused by membrane potential dependent modulation of retinal input through NMDA receptors

An optimal visual stimulus flashed on the receptive field of a retinal ganglion cell typically evokes a strong transient response followed by weaker sustained firing. Thalamocortical (TC) neurons in the dorsal lateral geniculate nucleus, which receive their sensory input from retina, respond similarly except that the gain, in particular of the sustained component, changes with level of arousal. Several lines of evidence suggest that retinal input to TC neurons through NMDA receptors plays a key role in generation of the sustained response, but the mechanisms for the state-dependent variation in this component are unclear. We used a slice preparation to study responses of TC neurons evoked by trains of electrical pulses to the retinal afferents at frequencies in the range of visual responses in vivo. Despite synaptic depression, the pharmacologically isolated NMDA component gave a pronounced build-up of depolarization through temporal summation of the NMDA receptor mediated EPSPs. This depolarization could provide sustained firing, the frequency of which depended on the holding potential. We suggest that the variation of sustained response in vivo is caused mainly by the state-dependent modulation of the membrane potential of TC neurons which shifts the NMDA receptor mediated depolarization closer to or further away from the firing threshold. The pharmacologically isolated AMPA receptor EPSPs were rather ineffective in spike generation. However, together with the depolarization evoked by the NMDA component, the AMPA component contributed significantly to spike generation, and was necessary for the precise timing of the generated spikes.

Dynamics of spatial resolution of single units in the lateral geniculate nucleus of cat during brief visual stimulation

Sharpness of vision depends on the resolution of details conveyed by individual neurons in the visual pathway. In the dorsal lateral geniculate nucleus (LGN), the neurons have receptive fields with center-surround organization, and spatial resolution may be measured as the inverse of center size. We studied dynamics of receptive field center size of single LGN neurons during the response to briefly (400-500 ms) presented static light or dark spots. Center size was estimated from a series of spatial summation curves made for successive 5-ms intervals during the stimulation period. The center was wide at the start of the response, but shrank rapidly over 50-100 ms after stimulus onset, whereupon it widened slightly. Thereby, the spatial resolution changed from coarse-to-fine with average peak resolution occurring approximately 70 ms after stimulus onset. The changes in spatial resolution did not follow changes of firing rate; peak firing appeared earlier than the maximal spatial resolution. We suggest that the response initially conveys a strong but spatially coarse message that might have a detection and tune-in function, followed by transient transmission of spatially precise information about the stimulus. Experiments with spots presented inside the maximum but outside the minimum center width suggested a dynamic reduction in number of responding neurons during the stimulation; from many responding neurons initially when the field centers are large to fewer responding neurons as the centers shrink. Thereby, there is a change from coarse-to-fine also in the recruitment of responding neurons during brief static stimulation.

Brainstem modulation of visual response properties of single cells in the dorsal lateral geniculate nucleus of cat

The dorsal lateral geniculate nucleus (dLGN) transmits visual signals from the retina to the cortex. In the dLGN the antagonism between the centre and the surround of the receptive fields is increased through intrageniculate inhibitory mechanisms. Furthermore, the transmission of signals through the dLGN is modulated in a state-dependent manner by input from various brainstem nuclei including an area in the parabrachial region (PBR) containing cholinergic cells involved in the regulation of arousal and sleep. Here, we studied the effects of increased PBR input on the spatial receptive field properties of cells in the dLGN. We made simultaneous single-unit recordings of the input to the cells from the retina (S-potentials) and the output of the cells to the cortex (action potentials) to determine spatial receptive field modifications generated in the dLGN. State-dependent modulation of the spatial receptive field properties was studied by electrical stimulation of the PBR. The results showed that PBR stimulation had only a minor effect on the modifications of the spatial receptive field properties generated in the dLGN. The PBR-evoked effects could be described mainly as increased response gain. This suggested that the spatial modifications of the receptive field occurred at an earlier stage of processing in the dLGN than the PBR-controlled gain regulation, such that the PBR input modulates the gain of the spatially modified signals. We propose that the spatial receptive field modifications occur at the input to relay cells through the synaptic triades between retinal afferents, inhibitory interneurone dendrites, and relay cell dendrites and that the gain regulation is related to postsynaptic cholinergic effects on the relay cells.

AMPA receptor properties at the synapse between retinal afferents and thalamocortical cells in the dorsal lateral geniculate nucleus of the rat

The thalamocortical (TC) cells in dorsal lateral geniculate nucleus transfer signals from retinal afferents to the primary visual cortex. The excitatory retinal input to the TC cells is mediated by ionotropic receptors of the N-methyl-D-aspartate (NMDA) and non-NMDA type. In the present study the excitatory postsynaptic current (EPSC) mediated by non-NMDA receptors in this synapse was characterised by means of voltage-clamp recordings from TC neurons in rat thalamic slices. The specific alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist GYKI-53655 fully blocked the non-NMDA mediated EPSC, evoked by optic tract stimulation. The EPSC peak amplitudes were linearly related to the command voltage, suggesting that the receptor complex includes the GluR2 subunit. The EPSC amplitude and decay time increased during application of the desensitisation blocker, cyclothiazide, showing that the EPSC was partly controlled by fast desensitisation.

Muscarinic regulation of dendritic and axonal outputs of rat thalamic interneurons: a new cellular mechanism for uncoupling distal dendrites

Inhibition is crucial for sharpening the sensory information relayed through the thalamus. To understand how the interneuron-mediated inhibition in the thalamus is regulated, we studied the muscarinic effects on interneurons in the lateral posterior nucleus and lateral geniculate nucleus of the thalamus. Here, we report that activation of muscarinic receptors switched the firing pattern in thalamic interneurons from bursting to tonic. Although neuromodulators switch the firing mode in several other types of neurons by altering their membrane potential, we found that activation of muscarinic subtype 2 receptors switched the fire mode in thalamic interneurons by selectively decreasing their input resistance. This is attributable to the muscarinic enhancement of a hyperpolarizing potassium conductance and two depolarizing cation conductances. The decrease in input resistance appeared to electrotonically uncouple the distal dendrites of thalamic interneurons, which effectively changed the inhibition pattern in thalamocortical cells. These results suggest a novel cellular mechanism for the cholinergic transformation of long-range, slow dendrite- and axon-originated inhibition into short-range, fast dendrite-originated inhibition in the thalamus observed in vivo. It is concluded that the electrotonic properties of the dendritic compartments of thalamic interneurons can be dynamically regulated by muscarinic activity.

Spatial summation and center-surround antagonism in the receptive field of single units in the dorsal lateral geniculate nucleus of cat: comparison with retinal input

Spatial summation and degree of center-surround antagonism were examined in the receptive field of nonlagged cells in the dorsal lateral geniculate nucleus (dLGN). We recorded responses to stationary light or dark circular spots that were stepwise varied in width. The spots were centered on the receptive field. For a sample of nonlagged X-cells, we made simultaneous recordings of action potentials and S-potentials, and could thereby compare spatial summation in the dLGN cell and in the retinal input to the cell. Plots of response versus spot diameter showed that the response for a dLGN cell was consistently below the response in the retinal input at all spot sizes. There was a marked increase of antagonism at the retinogeniculate relay. The difference between the retinal input and dLGN cell response suggested that the direct retinal input to a relay cell is counteracted in dLGN by an inhibitory field that has an antagonistic center-surround organization. The inhibitory field seems to have the same center sign (ON- or OFF-center), but a wider receptive-field center than the direct retinal input to the relay cell. The broader center of the inhibitory field can explain the increased center-surround antagonism at the retinogeniculate relay. The ratio between the response of a dLGN cell and its retinal input (transfer ratio) varied with spot width. This variation did not necessarily reflect a nonlinearity at the retinogeniculate relay. Plots of dLGN cell response against retinal input were piecewise linear, suggesting that both excitatory and inhibitory transmission in dLGN are close to linear. The variation in transfer ratio could be explained by sustained suppression evoked by the background stimulation, because such suppression has relatively stronger effect on the response to a spot evoking weak response than to a spot evoking a strong response. A simple model for the spatial receptive-field organization of nonlagged X-cells, that is consistent with our findings, is presented.

Mathematical models for the spatial receptive-field organization of nonlagged X-cells in dorsal lateral geniculate nucleus of cat

Spatial receptive fields of relay cells in dorsal lateral geniculate nucleus (dLGN) have commonly been modeled as a difference of two Gaussian functions. We present alternative models for dLGN cells which take known physiological couplings between retina and dLGN and within dLGN into account. The models include excitatory input from a single retinal ganglion cell and feedforward inhibition via intrageniculate interneurons. Mathematical formulas describing the receptive field and response to circular spot stimuli are found both for models with a finite and an infinite number of ganglion-cell inputs to dLGN neurons. The advantage of these models compared to the common difference-of-Gaussians model is that they, in addition to providing mathematical descriptions of the receptive fields of dLGN neurons, also make explicit contributions from the geniculate circuit. Moreover, the model parameters have direct physiological relevance and can be manipulated and measured experimentally. The discrete model is applied to recently published data (Ruksenas et al., 2000) on response versus spot-diameter curves for dLGN cells and for the retinal input to the cell (S-potentials). The models are found to account well for the results for the X-cells in these experiments. Moreover, predictions from the discrete model regarding receptive-field sizes of interneurons, the amount of center-surround antagonism for interneurons compared to relay cells, and distance between neighboring retinal ganglion cells providing input to interneurons, are all compatible with data available in the literature.

Roles of N-methyl-D-aspartate receptors in ocular dominance plasticity in developing visual cortex: re-evaluation

We have re-examined whether N-methyl-D-aspartate receptors play a specific role in experience-dependent plasticity in kitten visual cortex. A specific antagonist of this glutamate receptor subtype, D,L-2-amino-5-phosphonovaleric acid, was directly and continuously infused into kitten striate cortex for one week concurrently with monocular lid suture. In the hemisphere infused with 50 mM antagonist, we found the usual shift in ocular dominance toward the open eye with only a few binocular cells remaining. The changes were accompanied by an extremely high incidence (38%) of abnormal cells lacking orientation selectivity across different ocular dominance groups. In kitten cortex infused with 10 mM antagonist concurrently with monocular deprivation for a week, recording from a drug-affected region near the infusion centre, we again found the U-shaped ocular dominance distribution with the high incidence of non-selective cells. In antagonist-infused, otherwise normal striate cortex of adult cats, we found that the proportion of binocular cells decreased by one-half in two cellular populations: one recorded during the continuous infusion of 10 mM antagonist under general anaesthesia and paralysis, and the other about two days after stopping the infusion. We also established that in vivo concentrations of chronically infused 10 mM antagonist decreased, not near-exponentially, but linearly with increasing distance from the infusion site. Thus, the effects of a directly and continuously infused, concentrated antagonist of N-methyl-D-aspartate receptors on receptive-field properties of visuocortical cells are complex. The present findings strongly suggest that the antagonist effects in the developing cortex may be due primarily to blockade of normal synaptic transmission rather than specific disruption of an experience-dependent mechanism underlying ocular dominance plasticity.

Quantal properties of spontaneous EPSCs in neurones of the guinea-pig dorsal lateral geniculate nucleus

1. Spontaneous non-NMDA glutamate receptor-mediated EPSCs were recorded with the whole-cell patch-clamp technique from twenty-six neurones in the dorsal lateral geniculate nucleus in thalamic slices from guinea-pig. 2. Amplitude distributions of the EPSCs were skewed towards larger values. The skewness could be accounted for by multiquantal properties. The multiquantal properties were most clearly demonstrated in four cells that had prominent peaks in the amplitude distribution, and peak separation approximately corresponding to the modal value. The amplitude distribution for all cells could be adequately fitted by a quantal model consisting of a sum of Gaussians with means equal to integer multiples of a quantal unit. The variance of each Gaussian was equal to the sum of the noise variance of the recordings and an additional non-negative variance which increased linearly with the number of the Gaussian in the series. The estimated mean quantal size was 152 +/- 37 pS. The estimated mean quantal coefficient of variation was 15%. Addition of tetrodotoxin did not significantly change any of the quantal parameters (n = 5). 3. The waveform of the EPSCs was similar for small and large events, and similar to that of events evoked by stimulation of retinal input fibres. There was a positive correlation between peak amplitude and rise time. This is the opposite of that expected if differences in electrotonic distances were to explain differences in amplitude. 4. The spontaneous EPSCs occurred randomly at an average frequency of 3.1 Hz. The events with amplitudes approximately equal to multiples of the quantal size were, in most cells, more numerous than could be accounted for by coincidence of randomly occurring events of quantal size. 5. The results indicate that spontaneous EPSCs can reflect more than a single quantum, and suggest that quantal events may be coupled due to action potential-independent near-synchronous multiquantal release of transmitter.

Brainstem modulation of signal transmission through the cat dorsal lateral geniculate nucleus

We studied changes in retinogeniculate transmission that occur during variation of modulatory brainstem input and during variation of stimulus contrast. Responses of single cells in the dorsal lateral geniculate nucleus (dLGN) to a stationary flashing light spot of varying contrast were measured with and without electrical stimulation of the peribrachial region (PBR) of the brainstem. PBR stimulation increased the contrast gain (slope of response versus contrast curve) and the dynamic response range (range between spontaneous activity and maximal firing). Lagged and nonlagged X-cells reached the midpoint of the dynamic response range at lower contrasts during PBR stimulation than in the controls. No comparable change was seen for Y-cells. Only minor changes of threshold contrast were seen. The characteristics of the retinogeniculate transmission were directly studied by comparing the response of dLGN cells with their retinal input (slow potentials, S-potentials). With increasing contrast there was a marked increase in the transfer ratio (proportion of impulses in the input that generates action potentials in the dLGN cell). The transfer ratio seemed to be primarily determined by the firing rate of the retinal input. The transfer ratio increased with increasing input rates from low values near threshold to values that could approach 1 at high-input firing rates. PBR stimulation increased the transfer ratio, particularly at moderate input firing rates. The increased transfer ratio, caused by increasing input firing rates, enhanced the response versus contrast characteristics through an increase in contrast gain and dynamic response range. The modulatory input from the PBR further enhanced these characteristics.

The quantal size at retinogeniculate synapses determined from spontaneous and evoked EPSCs in guinea-pig thalamic slices

1. To determine the quantal size at retinogeniculate synapses, spontaneous and evoked excitatory postsynaptic currents (EPSCs) were recorded in twelve neurones of the dorsal lateral geniculate nucleus in guinea-pig thalamic slices using the whole-cell patch-clamp technique. We limited our study to the fast non-N-methyl-D-aspartate (NMDA) component of the EPSCs by adding the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid to the perfusion medium. 2. Spontaneous EPSCs occurred at a frequency between 0.5 and 6.6 Hz (mean 2.5 Hz). The modal value of the peak conductance change of spontaneous excitatory events varied between cells from 102 to 179 pS. 3. EPSCs were evoked by electrical stimulation in the optic tract. The peak conductance change of EPSCs evoked by stimulation of a putative single input fibre ranged from 0.6 to 3.4 nS (mean 1.7 nS). 4. To resolve the quantal components of evoked EPSCs the external Ca2+ concentration was reduced and the external Mg2+ concentration increased for four cells. In this condition failures occurred and the amplitude histograms were multimodal with approximately equidistant peaks. 5. These multimodal histograms could be fitted by a sum of Gaussian functions with mean values corresponding to integer multiples of the modal value of the spontaneous EPSCs for the same cell. Thus, the quantal size of evoked EPSCs was the same as the modal value of spontaneous EPSCs. The mean of the apparent quantal conductance change was 138 pS. The estimated number of quanta released by stimulating a putative single input fibre in the control condition ranged from 4 to 27 (mean 13).(ABSTRACT TRUNCATED AT 250 WORDS)

Response variability of single cells in the dorsal lateral geniculate nucleus of the cat. Comparison with retinal input and effect of brain stem stimulation

1. We studied the degree and source of response variability in different classes of cell in the dorsal lateral geniculate nucleus (dLGN). The response of single cells to a series of contrasts of a stationary flashing light spot was measured. The variability analyses were based on the mean and SD of the response to a number of repeated stimulus presentations. The relative variability was expressed by the coefficient of variation (Cv; SD/mean). 2. At a given contrast, the Cv for lagged cells was larger than for nonlagged cells. No difference was found between the Cv of X and Y cells. The magnitude of the Cv was about the same as previously found for cells in striate cortex. Accordingly, little variability is added at the cortical level. The Cv decreased with increasing contrast showing that the reliability of response and the signal-to-noise ratio was improved with increasing contrast. 3. For some cells, the retinal input was determined by recording S potentials in addition to action potentials. The Cv of the retinal input was smaller than the Cv of the dLGN cells at a given contrast. Thus in the paralyzed and anesthetized preparation, variability was added at the geniculate relay. 4. The additional variability was related to modulatory input from the brain stem. This was shown by comparing Cv versus contrast curves for the dLGN cells obtained during electrical stimulation of the peribrachial region of the brain stem (PBR) with corresponding curves obtained without PBR stimulation. During PBR stimulation, which presumably mimics the effects of arousal on the dLGN cell, the Cv at a given contrast was reduced toward the value for the retinal input to the cell. Furthermore PBR stimulation increased the signal-to-noise-ratio of the cell to the level of the retinal input. 5. When Cv was plotted against response rather than against contrast, approximately the same function was found for the various dLGN cell classes. This indicated that the variability basically depended on firing rate rather than on stimulus contrast. No difference of Cv was seen between lagged and nonlagged cells at a given level of response. The difference found at a given level of contrast reflected differences in firing rate of the two cell classes. During PBR stimulation, there was no clear difference between the Cvs of the dLGN cell and its retinal input at a given level of response.(ABSTRACT TRUNCATED AT 400 WORDS)

Brain stem modulation of spatial receptive field properties of single cells in the dorsal lateral geniculate nucleus of the cat

1. We studied the effect of electrical stimulation of the peribrachial region (PBR) in the brain stem on the visual response of single cells in the dorsal lateral geniculate nucleus (dLGN) to a light slit presented in a series of positions across the receptive field. The response was plotted against slit position, giving a spatial receptive field profile. 2. PBR stimulation markedly increased the visual response. In the middle of the receptive field center, the absolute response increase was considerably larger than in the peripheral parts of the receptive field or than the increase of spontaneous activity. The PBR stimulation also led to a small increase of the diameter of the receptive field center. 3. The maximum steepness of the receptive field profile for the dLGN cells increased by PBR stimulation. We suggest that the visual resolution in the dLGN cell is directly related to this maximal slope of the receptive field profile rather than to the width of the receptive field center. This would mean that increased input from the PBR, as presumably occurs during arousal, increases the visual resolution of the dLGN cells. 4. For some of the cells we could record S-potentials (slow potentials) in addition to action potentials. This allowed us to directly compared the receptive field center size of a dLGN cell with that of its retinal input. For these cells, the center size was considerably reduced by the geniculate relay. During PBR stimulation, the center size of these cells also increased slightly, but even in this condition it was reduced compared with the retinal input. The maximal slope of the receptive field profile in the dLGN cell during PBR stimulation was larger than for the retinal input. 5. We also examined the effect of ionophoretical application of acetylcholine (ACh) and bicuculline methchloride (BMC) on the spatial receptive field properties of dLGN cells. The effects of ACh were similar to those of PBR stimulation. Application of BMC, on the other hand, made the receptive field profile more similar to that of retinal ganglion cells.

Simultaneous luminance contrast with chromatic colors

The variations in the color of a test field of constant luminance during changes in the luminance of a contiguous inducing field was measured psychophysically. The fields had the same hue (red, green, or blue). The colors induced in the test field could be specified by the strength of a chromatic quality, and by the strength of the opponent qualities luminous/black. The psychophysical relationship between the two kinds of perceptive variables and test and inducing luminance followed distinctly different functions. The luminous/black variable varied linearly with contrast over a large range, as previously found for achromatic colors. The contrast gain of the luminous/black variable for the red and green colors was the same as for achromatic colors. The gain for the blue colors was twice the gain for the other colors. The chromatic variables were primarily related to the local luminance. For a given test luminance they were maximal near zero contrast. They followed the same function as the white component of achromatic colors. It is suggested that the luminous/black variable is related to spectrally broadband cells with a strong center/surround antagonism, while the chromatic variables and white are related to cells that lack a spectrally broadband surround in their receptive field.

The effect of acetylcholine on the visual response of lagged cells in the cat dorsal lateral geniculate nucleus

We examined the influence of acetylcholine (ACh) on the visual response properties of lagged cells in the dorsal lateral geniculate nucleus of anaesthetised cats. By means of electrophysiological techniques, the response of single cells was recorded before, during and after ionophoretic application of ACh. ACh evoked a clear enhancement of the visual response. The initial suppression that a visual stimulus evokes in lagged cells was resistant to the effects of ACh. The characteristic anomalous response component of lagged cells was also present during application of ACh. The difference in latency to half-rise and to half-fall of the visual response that is found between lagged and non-lagged cells was maintained during application of ACh. Taken together, the results support previous evidence from experiments with brain stem stimulation that the fundamental visual response characteristics of lagged cells are state independent.

Brain-stem influence on visual response of lagged and nonlagged cells in the cat lateral geniculate nucleus

This study examined the influence of the pontomesencephalic peribrachial region (PBR) on the visual response properties of cells in the dorsal lateral geniculate nucleus (LGN). The response of single cells to a stationary flashing light spot was recorded with accompanying electrical stimulation of the PBR. The major objectives were to compare the effects of PBR stimulation on lagged and nonlagged cells, to examine how the visual response pattern of lagged cells could be modified by PBR stimulation and to examine whether the physiological criteria used to classify lagged and nonlagged cells are applicable during increased PBR input to the LGN. During PBR stimulation, the visual response was enhanced to a similar degree for lagged and nonlagged cells and the latency to half-rise of the visual response was reduced, particularly for the lagged X cells. The latency to half-fall of the visual response of lagged cells was not changed by PBR stimulation. Accordingly, the division of LGN cells into lagged and nonlagged cells based on visual response latencies was maintained during PBR stimulation. The initial suppression that a visual stimulus evokes in lagged cells was resistant to the effects of PBR stimulation. For the lagged cells, the largest response increase occurred for the initial part of the visual response. For the nonlagged cells, the largest increase occurred for the tonic part of the response. The results support the hypothesis that the differences in temporal response properties between lagged and nonlagged cells belong to the basic distinctions between these cell classes.

The effect of contrast on the visual response of lagged and nonlagged cells in the cat lateral geniculate nucleus

The response vs. contrast characteristics of different cell classes in the dorsal lateral geniculate nucleus (LGN) were compared. The luminance of a stationary flashing light spot was varied stepwise while the background luminance was constant. Lagged X cells had lower slope of the response vs. contrast curve (contrast gain), and they reached the midpoint of the response range over which the cells' response varied (dynamic response range) at higher contrast than nonlagged X cells. These results indicated that nonlagged cells are well suited for detection of small contrasts, whereas lagged cells may discriminate between contrasts over a larger range. The contrast gain and the contrast corresponding to the midpoint of the dynamic response range were similar for X and Y cells. The latency to onset and to half-rise of the visual response decreased with increasing contrast, most pronounced for lagged cells. Even at the highest contrasts, the latency of lagged cells remained longer than for nonlagged cells. For many lagged cells, the latency to half-fall decreased with increasing contrast. It is shown that the differences in the response vs. contrast characteristics between lagged and nonlagged X cells in the cat are similar to the differences between the parvocellular and magnocellular neurones in the monkey.

A bidimensional theory of achromatic color vision

The theory is based on a perceptive color system where the achromatic colors are specified by their degree of similarity to the three qualities white, black and luminous. Black and luminous are treated as opponent variables. It is assumed that white and luminous/black are determined by different kinds of visual processes termed the w- and the b-process. The relationship between these processes and luminance parameters in a simple disc/ring configuration is derived from available data. The b-process is related to stimulus contrast in a simple manner. It is assumed to involve cells with antagonistic center/surround organization of the receptive field. The w-process is primarily determined by the local luminance, and it is assumed to involve cells that lack a center/surround organization of the receptive field. The w-process has properties similar to the processes involved in chromatic color vision. The theory can account for different kinds of psychophysical data on achromatic colors like data on simultaneous contrast, color scaling, and color constancy.

Neurotransmitter receptors mediating excitatory input to cells in the cat lateral geniculate nucleus. II. Nonlagged cells

1. We studied the type of receptor for excitatory amino acids (EAA) that mediates visual responses of nonlagged cells in the dorsal lateral geniculate nucleus (LGN) by recording the visual response of single cells to a stationary flashing spot before, during, and after iontophoretical application of antagonists and agonists to EAA receptors. 2. The visual response of the nonlagged cells was strongly suppressed, in a dose-dependent manner, by the specific quisqualate/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). The average degree of suppression was 74%. Quisqualate enhanced the visual response. 3. Specific antagonists to the N-methyl-D-aspartate (NMDA) receptor had a weak suppressing effect on most nonlagged cells. The average degree of suppression was 22%. Measurement of such weak effects was complicated by the considerable spontaneous fluctuations of responsivity in the LGN cells. In the majority of cells where the visual response was suppressed by NMDA antagonists, the tonic response component was more strongly suppressed than the initial transient response component. The visual response was enhanced by NMDA, and this enhancement was antagonized by NMDA antagonists. 4. These findings suggest that the excitatory input from the retina to nonlagged LGN cells is mainly mediated by non-NMDA receptors. The non-NMDA receptors mediate fast EPSPs, and this can explain the fast onset and offset of the visual response of the nonlagged cells. 5. The generally small contribution from NMDA receptors to the visual response of the nonlagged cells might reflect a minor involvement of these receptors in the retinal input, or it could be related to the excitatory input to LGN from the visual cortex. 6. To study whether the expression of NMDA receptors was related to modulatory brain stem input to LGN, we examined the effects of the NMDA antagonists when the visual response was enhanced with gamma-aminobutyric acid (GABA) antagonists or acetylcholine (ACh). Neither of these pharmacologic manipulations consistently increased the relative contribution of NMDA receptors to the visual response. 7. No pharmacologic difference was found between nonlagged X- and Y-cells, or between ON- and OFF-center cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurotransmitter receptors mediating excitatory input to cells in the cat lateral geniculate nucleus. I. Lagged cells

1. Synaptic mechanisms that might explain the functional properties of the recently discovered class of lagged cells in the dorsal lateral geniculate nucleus (LGN) were analyzed with electrophysiological and pharmacologic techniques. To study the type of excitatory amino acid (EAA) receptor that mediates visual responses of lagged cells, we recorded the response of single cells to a stationary flashing light spot before, during, and after microiontophoretic application of antagonists and agonists to EAA receptors. 2. The visual response of the lagged cells could be almost completely blocked by an antagonist to the N-methyl-D-aspartate (NMDA) receptor. The degree of suppression was dose dependent, and the average maximum degree of suppression for all the cells was 94%. NMDA enhanced the response, and this enhancement was antagonized by NMDA antagonists. A quisqualate/kainate receptor antagonist had no significant effect on the lagged cells. 3. These findings indicate that the visual response in lagged cells is dependent upon activation of NMDA receptors, which may directly result from activation of retinal inputs. 4. No pharmacologic difference was seen between lagged X- and Y-cells, or between lagged ON- and OFF-center cells. 5. gamma-Aminobutyric acid-A (GABA-A) receptor antagonists were used to study whether the characteristic lag of the visual response and the suppression of the initial transient response component of the lagged cells are dependent on geniculate inhibition. Beside enhancement of the visual response, the GABA antagonists strongly reduced the lag of the visual response, and an initial transient response component occurred instead of the initial suppression. The lag remained slightly longer than for nonlagged cells, and the peak firing rate of the transient was below values typical for nonlagged cells, indicating that the lagged cell properties are dependent on other factors beside GABA-A receptor-mediated inhibition. 6. The enhanced visual response during iontophoresis of GABA antagonists could be completely blocked by simultaneous iontophoresis of an NMDA-receptor antagonist. This gives further support to the hypothesis that the retinal input to these cells is mediated by NMDA receptors. 7. The NMDA-receptor/ionophore complex mediates excitatory postsynaptic potentials (EPSPs) characterized by slow rise and decay times and long duration. The ionophore is characterized by a voltage-dependent blockade that makes these receptors particularly sensitive to inhibitory input. The temporal interplay between the slow NMDA receptor-mediated EPSPs and the fast GABA receptor-mediated inhibitory postsynaptic potentials (IPSPs) may explain the characteristic response properties of the lagged cells.

Postnatal development of glutamatergic, GABAergic, and cholinergic neurotransmitter phenotypes in the visual cortex, lateral geniculate nucleus, pulvinar, and superior colliculus in cats

We have analyzed the postnatal development of glutamatergic/aspartergic, GABAergic, and cholinergic neurotransmitter systems in the visual cortical Areas 17 and 18, lateral geniculate nucleus (LGN), pulvinar, and the visual and non-visual parts of superior colliculus (SC) in kittens. High-affinity uptake of D-aspartate (HA D-Asp), glutamate decarboxylase (GAD), and choline acetyltransferase (ChAT) activities were measured as a means of probing the development of the respective transmitter systems. HA D-Asp exceeded the adult level several-fold in all areas during the postnatal period which corresponded with the period of maximal dendritic/axonal branching patterns and synapse densities in the respective regions. GAD exhibited a gradual increase towards adult levels during the first month. The adult level was reached during postnatal week (PNW) 5-6 in Areas 17 and 18, during PNW3 within LGN, pulvinar, and the visual part of SC. In the nonvisual part of SC, the adult GAD level was reached as early as PNW2. ChAT exhibited biphasic developmental profiles in Areas 17 and 18. An initial peak of near adultlike activity in PNW2 was followed by a decline and subsequently by a slow increase towards adult levels during PNW5-17. ChAT developed very slowly in LGN and pulvinar, and in the latter structure only approximately 70% of the adult activity had been attained by PNW17. In both subdivisions of SC, ChAT had reached adult levels during PNW3-5. Dark-rearing from birth until PNW6 moderately attenuated GAD development in all areas and increased ChAT activity in Areas 17 and 18 but did not affect development of HA D-Asp in any part of the kitten visual system. Our neurochemical findings in the developing cat visual system are consistent with available evidence regarding localization of neurotransmitter systems, as well as postnatal changes in terms of cytoarchitectonics, synaptogenesis, functional development, and susceptibility to neonatal dark-rearing in visual pathways.

Response to rates of luminance change of sustained and transient cells in the cat lateral geniculate nucleus and optic tract

We recorded the response of sustained (X) and transient (Y) cells in the cat lateral geniculate nucleus (LGN) and optic tract to a stationary spot while the spot luminance was increased and decreased with a constant rate (linear luminance functions), or modulated sinusoidally. The spot filled the receptive field center, and was surrounded by an annulus of fixed luminance. The LGN X cells seemed to perform a differentiation-like operation in the time domain at slow temporal modulations, giving information about rate of luminance change. To the linear luminance functions the cells responded with a constant firing rate. The on-center cells were activated during increasing luminance, the off-center cells during decreasing luminance. This firing rate increased monotonically with rate of luminance change. To low-frequency sinusoidal modulations the cells had a marked negative phase shift. The response of the LGN Y cells had a transient component shortly after the luminance started to increase (on-center cells) or decrease (off-center cells), followed by a secondary, gradually changing component. The peak of the transient component occurred on average when the response of the X cells increased most rapidly. To low-frequency sinusoidal modulation the average negative phase shift of this peak was twice the average of the X cells. The Y system could accordingly provide information about rate of change in the response of the X system. In the optic tract the X fiber response resembled the LGN X cell response in most respects. The Y fibers had only a weak transient response component, so this component was accentuated in the thalamic relay. Also the sensitivity for rate of luminance change was increased in LGN.

Reduced binocularity in the noradrenaline-infused striate cortex of acutely anesthetized and paralyzed, otherwise normal cats

In anesthetized and paralyzed cats, the normal alignment of the visual axes is disturbed by paralysis of the eye muscles. Thus, the separation between paired receptive fields of binocular cells in visual cortex is increased (paralysis squint). This increased separation is normally tolerated by the majority of visuocortical cells, about 80% of them being binocularly driven (Hubel and Wiesel 1962). It was shown previously that neuronal plasticity in visual cortex can be enhanced in both normal adult cats (Kasamatsu et al. 1979) and kittens (Kuppermann and Kasamatsu 1984) by intracortical microinfusion of noradrenaline (NA). In the present study we tested whether the usual range of disparity produced by the paralysis squint is sufficient to induce ocular dominance changes in visual cortex of adult cats when the neuronal plasticity is enhanced by NA. NA was continuously infused into visual cortex throughout the experiments. The period of the paralysis squint varied from experiment to experiment between 9 and 47 h. We found: (1) These short periods were sufficient to produce a marked reduction in the proportion of binocular cells. (2) The proportion decreased linearly with increasing the duration of the squint period at a rate of 0.17 per 10 h up to about 22 h. (3) At longer durations the average binocularity remained at about 0.30 and could not be further reduced in the present paradigm. (4) The binocularity seemed to decrease with increasing separation of paired receptive fields. (5) Binocularity increased again toward the normal value after optical correction of the squint. (6) The amount of increased binocularity was linearly correlated with the duration of the period after the squint correction. (7) The binocularity increased at a rate of 0.18 per 10 h, reaching the normal value in less than 30 h. We thus concluded that if visuocortical plasticity is maintained at a high level through the continuous infusion of NA it is possible to change the ocular dominance distribution in the mature visual cortex by manipulations of the alignment of the visual axes even in the acutely anesthetized and paralyzed condition.

Quantitative studies of enhancement and suppression zones in the receptive field of simple cells in cat striate cortex

The configuration and extension of enhancement and suppression zones were compared with the configuration and extension of on- and off-response zones across the receptive field of simple cells in cat striate cortex. The enhancement and suppression zones were determined by a dual-stimulus technique where a stationary flashing light slit produced activity against which activation profiles across the receptive field were plotted with a parallel stationary test slit. The activation profiles showed less variation in receptive field configuration than plots of on- and off-discharge zones. Whereas the number of on- and off-zones varied between one and five, the activation profiles showed at least three distinct subregions in the receptive field, i.e. a central zone with an adjacent oppositely responding zone on each side. The responsivity was clearly stronger in one of these proximal flank zones. An additional zone occurred distal to the strong proximal flank zone in 53% of the cells, and in 10% such a distal zone occurred distal to both proximal flank zones. There was good correspondence between the location of on- and off-discharge zones and the location of the enhancement and suppression regions, although some subregions seen in the activation profiles did not appear in plots made with a single slit. The maximum discharge and the maximum enhancement and suppression effects in a subregion were found at the same receptive field location. The width of a subregion was measured as the width of the eventual on- or off-discharge zone determined with a single slit, as the width of the enhancement zone, and as the width of the suppression zone determined with the dual-slit technique. The enhancement zone was narrower, and the suppression zone wider than the discharge zone. The strong proximal flank zone had the same width as the central zone, but was wider than the weak proximal flank zone. For most cells the distances between successive extreme points across the activation profiles were constant, and this may explain the selectivity of the cells for spatial frequency of periodic stimuli. The strongest flank suppression occurred for most cells in that of the two proximal flank zones which had the strongest discharge to the single slit. Nevertheless, there was no correlation between the degree of discharge and the degree of suppression produced by opposite light cycles.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative studies of the discharge fields of single cells in cat striate cortex

The configuration and width of on- and off-response zones in the discharge field of single cells in cat striate cortex was analysed by quantitative methods. The responses across on- and off-zones were plotted for 321 cells with a stationary optimum oriented light slit. The cells fell into two completely distinct subgroups with respect to the degree of overlap between adjacent on- and off-zones. The simple cells had a mean overlap of 16.8%, the complex cells 94.5%. For simple cells the ratio between the maximum off- and maximum on-response in the discharge field was bimodal, showing that two distinct subgroups termed on- and off-dominant cells could be distinguished. For the complex cells the corresponding frequency distribution was unimodal. The maximum response on the two regions adjacent to the most responsive discharge zone (the dominant zone) differed markedly for most simple cells, and only a very few cells had discharge fields approximating an ideal even symmetric field. The frequency distribution of the ratio between the maximum response in the two regions was unimodal showing that odd and even symmetric fields did not form distinct subgroups of simple cells. The number of different discharge zones in simple cells varied from one to five. The zones were arranged as alternating on- and off-zones across the discharge field. The maximum response in the subzones decreased with increasing sequential distance from the dominant zone, so the response pattern across each side of the discharge field resembled a damped wave-form pattern. All the complex cells had one on- and off-zone which overlapped. The mean width of the subregions in the simple cell discharge field and the mean distance between the response maxima in the subzones increased in the same proportion with increasing eccentricity. The paracentral fields were therefore like magnified central fields. The average width of the whole discharge field was not significantly different for the simple and the complex cells at the various eccentricities.

Plasticity in cat visual cortex restored by electrical stimulation of the locus coeruleus

It has been proposed that the presence of noradrenaline (NA)-containing terminals and NA-related receptors within the visual cortex is necessary to maintain the high level of neuronal plasticity in the immature visual cortex of kittens. In the present study we wanted to show whether electrical stimulation of the locus coeruleus (LC), which contains the somata of these cortical NA fibers, can restore neuronal plasticity to the normally aplastic visual cortex of juvenile and adult cats. We consistently found a significant loss of binocular cells in the visual cortex of mature animals which had monocular vision for only 12 h dispersed over 6 days (2 h a day, otherwise kept in the dark) in combination with concurrent LC stimulation. This result was interpreted as indicating that endogenous NA released from NA terminals restored susceptibility to monocular vision in the mature visual cortex. We next examined how long the restored plasticity lasts in the same animals after the LC stimulation was ended. The animals revived from the first recording session were either returned to the same daily schedule of brief monocular exposure (light/dark = 2/22 h) as before, or subjected to the usual monocular lid suture and kept in a cat colony environment (light/dark = 16/8 h). The LC electrodes had been removed and no more electrical stimulation was delivered at this stage. In the animals subjected to reiteration of brief monocular exposure, the state of reduced binocularity gradually returned to normal over a period of 2-3 weeks after stopping LC stimulation. We calculated that the revived plasticity disappeared at an average rate of a 22% loss every 7 days. This result sharply contrasted with the result obtained in the animals subjected to usual monocular lid suture. In this test the state of reduced binocularity continued for at least the next 3 weeks, suggesting that the restored plasticity was sustained throughout a period of 3 weeks (longest term tested). The different results obtained in the two paradigms may be explained by the different strength of binocular imbalance in the two tests imposed on the visual cortex in which neuronal plasticity was restored partially.

Development of spatial receptive-field organization and orientation selectivity in kitten striate cortex

The functional organization of the receptive field of neurons in striate cortex of kittens from 8 days to 3 mo of age was studied by extracellular recordings. A quantitative dual-stimulus technique was used, which allowed for analysis of both enhancement and suppression zones in the receptive field. Furthermore the development of orientation selectivity was studied quantitatively in the same cells. Already in the youngest kittens the receptive fields were spatially organized like adult fields, with a central zone and adjacent flanks that responded in opposite manner to the light stimulus. The relative suppression in the subzones was as strong as in adult cells. Both simple and complex cells were found from 8 days. The receptive fields were like magnified adult fields. The width of the dominant discharge-field zone and the distance between the positions giving maximum discharge and maximum suppression decreased with age in the same proportions. The decrease could be explained by a corresponding decrease of the receptive-field-center size of retinal ganglion cells. Forty percent of the cells were orientation selective before 2 wk, and the fraction increased to 94% at 4 wk. Cells whose responses could be attenuated to at least half of the maximal response by changes of slit orientation were termed orientation selective. The half-width of the orientation-tuning curves narrowed during the first 5 wk, and this change was most marked in simple cells. The ability of the cells to discriminate between orientations in statistical terms was weak in the youngest kittens due to a large response variability, and showed a more pronounced development than the half-width did. The orientation-tuning curves were fitted by an exponential function, which showed the shape to be adultlike in all age groups. Two kittens were dark reared until recording at 1 mo of age. The spatial receptive-field organization and the orientation selectivity in these kittens were similar to normal-reared kittens at 1 mo. The responsivity of the cells of the dark-reared kittens was lower, and the latency before firing was longer than in the normal-reared kittens of the same age, and these response properties were more similar to those in 1- to 2-wk-old normal kittens. Our results indicate that the spatial organization of the receptive field is innate in most cells and that visual experience is unnecessary for the organization to be maintained and for the receptive-field width to mature during the first month postnatally.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of area 17 ablation on neurotransmitter parameters in efferents to area 18, the lateral geniculate body, pulvinar and superior colliculus in the cat

The result of unilateral ablation of visual cortical area 17 in adult cats was consistent with glutamate-aspartate being the neurotransmitter in efferents to the lateral geniculate body, the pulvinar and the visual part of superior colliculus but not in efferents to area 18 and the non-visual strata of superior colliculus. Furthermore, the distribution of glutamatergic, GABAergic and cholinergic markers within the various subdivisions of the cat visual system complied well with observations made previously with biochemical, neurophysiological, histochemical and immunohistochemical methods, in this and other mammalian species.

Eye-opening in kittens: effects of light and some biological factors

Individual variation in the age of eye-opening was studied in 158 kittens. The abiotic and the biotic environments affected eye-opening. Eyes opened earlier in total darkness than in light, and earlier in kittens of young mothers than those of older mothers. Female kittens tended to open their eyes earlier than males, and a paternity effect appeared.

Spatial summation in subregions of simple-cell receptive fields in cat striate cortex as a function of slit length

Spatial summation along the optimum stimulus orientation in subregions of simple-cell receptive fields in cat striate cortex was studied quantitatively by measuring the response to stationary light slits of variable length. Before summation analysis, the cell's discharge field was mapped by flashing a test slit on and off in a sequence of positions through the receptive field. A static activation procedure was used to determine the extension of subregions of the receptive field where light stimulation increased (enhancement) or decreased (suppression) the firing rate. An activation slit in the optimum orientation was positioned in the most responsive position of the discharge field and the effects of a parallel test slit, in a series of broadside positions, were assessed from the changes induced in the discharge elicited by the activation slit. Length-response curves for on and off responses were made by positioning a test slit in the respective subregions of the discharge field. The activation procedure was used to make length-response curves for suppression. A test slit of variable length was positioned in a suppression region defined by the activation profiles and an activation slit of fixed length was centred in the most responsive discharge field position. Length summation was found for all cells, both with respect to on and off responses, and suppression. The curves for on and off responses had a maximum value beyond which the response declined or levelled off, but some cells had a secondary, more shallow increase beyond an initial, steeply rising part. Similar properties were found for summation of suppression except that the effects were opposite in sign. Curves made for both on and off regions in the same cell often differed in shape. Such differences were also found when length-response curves made in different suppression regions of the same cell were compared. The various length-summation functions were explained by a model presuming that simple-cell receptive fields consist of partially overlapping non-concentric excitatory and inhibitory fields. This arrangement would also explain why length-response curves in various subregions often had different shapes.

Direction asymmetry by moving stimuli and static receptive field plots for simple cells in cat striate cortex

It has been suggested that direction asymmetry of simple cells by moving stimuli is due to asymmetries in flanking response regions in the receptive field, and that the stronger response in the preferred direction is caused by synchronized On- and Off-responses. The hypothesis was tested by comparing the response of direction asymmetric cells to moving and static stimuli. The hypothesis had a weak but statistically significant predictive power. However, both for cells where the predicted preferred direction was correct and where it was wrong, there was clear suppression of the response in the nonpreferred movement direction, indicating that direction asymmetry is due to inhibitory processes rather than to synchronized On and Off-responses. It is suggested that the asymmetries in flanking regions of static receptive field plots are caused by the same inhibition which produces direction asymmetry, thus explaining why the static plots have some predictive power on direction asymmetry.

Orientation selectivity and the spatial distribution of enhancement and suppression in receptive fields of cat striate cortex cells

The relationship between orientation selectivity and spatial receptive field organization was analyzed. Receptive field maps were made with a dual stimulus technique where an optimally oriented activation slit was presented in the most responsive region to produce activity against which the effect of a test spot in various positions was determined. Both simple and complex cells had receptive fields which were subdivided into adjacent elongated and antagonistic subregions. When the two stimuli were presented in phase (both ON or OFF simultaneously) the fields had a central enhancement region with a strong suppression flank on one or both sides. Optimal slit orientation was related to the location of the suppression flank relative to the location of the central enhancement region, and the degree of orientation selectivity to the shape of the subregions and the distance between them. Estimated orientation tuning curves calculated from the receptive field maps gave satisfactory first approximations to experimental curves. The relative contribution of enhancement and suppression to orientation selectivity was studied by presenting a test slit in different orientations in phase with an optimally oriented activation slit. The orientation selectivity was produced almost exclusively by the flank suppression indicating that orientation selectivity is produced by inhibitory input. The flank suppression lacked any specific orientation selectivity, and it occurred only when both the central region and the flanks were activated in phase. Orientation selectivity in both simple and complex cells is explained by a receptive field organization where the cells have input from partially overlapping excitatory and inhibitory fields which have their centers slightly displaced from each other.

Spatial summation in the receptive field of simple cells in the cat striate cortex

Spatial summation was studied quantitatively through width response curves made with an optimally oriented test slit of variable width, and by comparing the response to combined presentation of several parallel slits with the response to each slit alone. Prior to summation analysis, the cell's discharge field (DF) was mapped by presenting a test slit ON and OFF across the receptive field. Activation profiles, showing the extension of subregions where light stimulation increased (enhancement) or decreased the firing rate (suppression), were made by presenting an optimally oriented activation slit in the most responsive DF-position. Against this activity the effects of a parallel test slit were determined in a series of broadside positions. Width response curves were made over the subregions of the DF and the activation profiles. Spatial summation was found in all cells, but the width of the summation region was smaller than the width of the subregions in the respective profiles. The width of the summation region was related to the degree of activation rather than to specific locations within the receptive field. The effect produced by several slits presented together deviated from the algebraic sum of the effects produced by each slit alone. Linear summation was rarely found. Accumulated response curves obtained by integration of DF or activation profiles were compared with width response curves to test linearity of summation. Linear summation throughout the whole receptive field was never found. A satisfactory fit was found only over a narrow region showing that summation was linear within a small part of the summation region. Linearity ended near response maxima or minima in the response profiles. The results indicate that the receptive field of simple cells consists of overlapping excitatory and inhibitory fields, and that the exact location and width of enhancement and suppression zones are determined by an activity-dependent balance between excitatory and inhibitory inputs.

Single cell responses in cat visual cortex to visual stimulation during iontophoresis of noradrenaline

We studied how iontophoresis of noradrenaline (NA) changes responsiveness of individual cells in the feline visual cortex when their visual receptive fields are stimulated with the appropriate visual stimulus. We found three populations of cortical cells which either increased, decreased or did not change their visual responsiveness during NA iontophoresis. About equal numbers of cells belonged to each of these three groups. In the majority of such cells that changed visual responsiveness during NA iontophoresis and that had measureable amounts of spontaneous activity, the ratio of visually evoked to spontaneous activity (signal-to-noise ratio) improved during NA iontophoresis. This improvement was independent of the direction of changes in the response magnitude to visual stimulation. There was a differential effect of NA on simple and complex visual cortical cells: Although most simple cells (86%) clearly changed their responsiveness during NA iontophoresis, the effects were seen in only one-third of complex cells. Furthermore, the effects on complex cells were usually weak compared to those typically seen in simple cells. In some cases the effects of NA were more complicated than an overall enhancement of suppression of the cortical cell's responses to visual stimulation. The possible dual role of NA in the visual cortex is briefly discussed.

Receptive field organization of simple cells in cat striate cortex

The receptive field organization of simple cells was studied by analyzing interaction effects between two stationary flashing light stimuli. One stimulus was positioned in the most responsive part of the receptive field to produce activity against which the effects of the other stimulus in various positions of the visual field could be determined. The receptive field was subdivided into an elongated center and elongated antagonistic flanks. The effects on the flanks were always considerably stronger on one side. Powerful flank suppression could be elicited within a region which usually was only slightly wider than the receptive field center. The suppression was just as stimulus specific as the activation of the center and occurred only by light ON or OFF. The cells were classified into ON-dominant and OFF-dominant depending on the kind of response found in the center. In ON-dominant cells the strong flank suppression occurred only by light ON, and light OFF produced enhancement. Correspondingly, the strong flank suppression occurred only by light OFF in OFF-dominant cells. This is consistent with the interpretation that simple cells have excitatory and inhibitory input from the same type of cells in the lateral geniculate nucleus (LGN), i.e., only from ON-center or OFF-center cells. The small size of the area where strong flank suppression occurred shows that inhibition comes fom a few LGN cells rather than from a large pool of cells. A model for simple cell receptive fields presuming overlapping but acentric excitatory and inhibitory fields with input to both fields frm either ON- or OFF-center LGN cells was tested by computer simulation and shown to fit the experimental data.

Response variability and orientation discrimination of single cells in striate cortex of cat

The response of single cells in the striate cortex of cat to a moving light bar of variable orientation was measured by a method providing data on the mean response as well as the standard deviation (SD) at the different stimulus orientations. At the optimal stimulus orientation the SD was about 1/3 of the mean response. Marked differences in this respect were found between simple and complex cells, i.e., the SD for the simple cells was about 1/2 of the mean response and about 1/4 for the complex cells. The variation coefficient (Vc = SD/mean) was minimal at the optimal orientation and increased relatively in the same manner for simple and complex cells as the stimulus orientation was varied away from optimal orientation. The Vc varied with the mean response at optimal orientation in a nonlinear manner. A function is proposed which fits this relationship and which is equally applicable for both simple and complex cells. The mean orientation discrimination (MOD) was defined as that change in orientation angle away from the optimal which produced a response statistically different--on the 1% level--from the response to the optimal orientation. There were differences in MOD between the two sides of the orientation tuning curve: the mean of the smaller of the two values was 13.5 deg and of the larger 19.7 deg. No significant difference in MOD was found between simple and complex cells despite the fact that the halfwidth of the tuning curves for the two cell types was 19.5 deg and 31.6 deg, respectively. The preciseness in localization of the most sensitive part within the receptive field of single cells was calculated from the variability in time of occurrence of the smallest interspike interval. The degree of preciseness was found to be of the order of 1/4 of the receptive field diameter in both simple and complex cells. When nonoptimal stimulus orientations were presented, the preciseness significantly decreased in complex cells whereas it remained unchanged in simple cells. It is suggested that the same type of intracortical wiring produces orientation selectivity in simple and complex cells, and that the differences in tuning width are mainly due to a larger extension of inhibitory fields in the simple cells. Considering the cortical visual cells as elementary units in a network built for orientation detection and discrimination, the tuning width seems of minor importance for that function.

The depth distribution of optimal stimulus orientations for neurones in cat area 17

Neurones recording during penetrations through cat area 17 as near parallel to the radial fibre bundles as possible have been quantitatively tested as to their optimal orientation. Optimal orientation within any one penetration was similar though considerable variability was observed. Histological reconstruction and other considerations showed that this variability could not be attributed to poor penetration angle or limitations of the microelectrode technique. These results confirm that neurones with similar optimal orientations are found in all cortical layers at one cortical locus, but it is difficult to reconcile the variability observed with a mosaic-like distribution of orientation across the cortical surface. The findings were consistent, however, with the assumption of a continuous distribution of orientation sensitivity across the cortical surface with considerable superimposed scatter.

Responses of striate cortical cells to moving edges of different curvatures

The responses of twenty cells to stimuli of varying curvature were measured in cat's striate cortex. All the investigated cells were sensitive to the orientation of lines and not hypercomplex. Fourteen cells showed a systematic change of response with curvature. The optimal curvatures of the cells were distributed over the whole range investigated. Six cells were insensitive to curvature. The responses from all the typical simple cells (8) varied with curvature, whereas all the complex cells (5) were insensitive to curvature changes. The curvature tuning curves were broad and the variability to individual stimuli was high, independent on whether the cell responded best to straight or to curved edges. The findings do not support the view that individual cells of area 17 could detect curvature.

Neural plasticity in visual cortex of adult cats after exposure to visual patterns

Over a period of 2 weeks, adult cats were twice a day exposed for 1 hour to a visual environment consisting only of vertical stripes and for the rest of the time were kept in darkness. Subsequent investigation of the striate cortex showed a decrease in the number of neurons sensitive to orientations around the vertical relative to those sensitive to horizontal orientations. This indicates that plasticity of functional properties of the cortical neuronal network still exists in adult animals.