Spectrally opponent inputs to the human luminance pathway: slow +L and −M cone inputs revealed by low to moderate long-wavelength adaptation

The luminance pathway has slow (s), spectrally opponent cone inputs in addition to the expected fast (f), non-opponent inputs. The nature of these inputs to luminance flicker perception was further explored psychophysically by measuring middle- (M-) and long-wavelength-sensitive (L-) cone modulation sensitivities, M- and L-cone phase delays, and flicker spectral sensitivities under three conditions of low to moderate long-wavelength adaptation. Under these conditions we find that the luminance channel has fast M- and L-cone input signals (+fM and +fL), and slow, spectrally opponent cone input signals (+sL and −sM). The slow signals found under these conditions are therefore of the opposite polarity to those (+sM and −sL) found under more intense long-wavelength adaptation. At these less intense levels, fast and slow M-cone signals of opposite polarity (−sM and +fM) cancel at low frequencies, but then constructively interfere at intermediate frequencies ( ca 12.5–22.5 Hz, depending on adapting level) because of the delay between them. In contrast, fast and slow L-cone signals of the same polarity (+sL and +fL) sum at low frequencies, but then destructively interfere at intermediate frequencies. Importantly, the spectrally opponent signals (+sL and −sM) contribute to flicker nulls without producing visible colour variation. Although its output generates an achromatic percept, the luminance channel has slow spectrally opponent as well as fast non-opponent inputs.     

Cone inputs in macaque primary visual cortex

To understand the role of primary visual cortex (V1) in color vision, we measured directly the input from the 3 cone types in macaque V1 neurons. Cells were classified as luminance-preferring, color-luminance, or color-preferring from the ratio of the peak amplitudes of spatial frequency responses to red/green equiluminant and to black/white (luminance) grating patterns, respectively. In this study we used L-, M-, and S-cone-isolating gratings to measure spatial frequency response functions for each cone type separately. From peak responses to cone-isolating stimuli we estimated relative cone weights and whether cone inputs were the same or opposite sign. For most V1 cells the relative S-cone weight was <0.1. All color-preferring cells were cone opponent and their L/M cone weight ratio was clustered around a value of -1, which is roughly equal and opposite L and M cone signals. Almost all cells (88%) classified as luminance cells were cone nonopponent, with a broad distribution of cone weights. Most cells (73%) classified as color-luminance cells were cone opponent. This result supports our conclusion that V1 color-luminance cells are double-opponent. Such neurons are more sensitive to color boundaries than to areas of color and thereby could play an important role in color perception. The color-luminance population had a broad distribution of L/M cone weight ratios, implying a broad distribution of preferred colors for the double-opponent cells.     

Concealed colour opponency in ganglion cells of the rhesus monkey retina.

Criteria for distinguishing colour-opponent from spectrally non-opponent cells and identifying colour-opponent subtypes on the basis of the cone inputs they receive, have been examined in ganglion cells of the macaque retina using threshold and suprathreshold stimuli, with and without chromatic adaptation. 2. Criteria based on suprathreshold responses were found to be insufficient for distinguishing between opponent and non-opponent cells in one-third of the sample. Criteria based on a 560 nm neutral point were found to be insufficient for distinguishing between colour-opponent subtypes in one-half of the remaining cells. 3. The neutral point of colour-opponent ganglion cells varies with the geometry and intensity of the stimulus, as well as with the amount of centre-surround interaction and the receptive-field location of a cell. As a result, the neutral point is often an ambiguous criterion for identifying colour-opponent subtypes on the basis of their cone inputs. 4. About one third of the colour-opponent ganglion cells did not show colour opponency in the presence of neutral backgrounds, and only revealed this behaviour in the presence of chromatic adaptation (concealed colour opponency). 5. The proportion of these concealed colour-opponent cells increased towards the peripheral areas of the retina.     

Colour and brightness signals of parvocellular lateral geniculate neurons

Summary We recorded from single neurons in the parvocellular layers of the lateral geniculate body of anesthetized monkeys. Spectral response curves of parvocellular neurons depended on the luminance ratio between the chromatic stimuli and achromatic background. From response/intensity curves, we determined the relative luminance between a coloured and an achromatic (white) light at which a given cell became non-responsive (critical luminance ratio, CLR). The spectral dependence of the CLRs of narrow (N) and wide band (W) cells with opponent receptor input showed characteristic differences. The activity of W-cells increased with luminance increase of a white light and of a coloured light in the specific spectral region of the cell (yellow-red for the long wave length sensitive WL-, and yellow-green-blue for the short wave length sensitive WS-cells), while N-cells were activated by their specific spectral light (blue for NS-cells, red for NL-cells) and by a luminance decrease of achromatic white. N-cells discriminate best between their characteristic colour and white at luminance ratios below their respective CLR, while W-cells distinguish best between a light of their characteristic colour and white at chromatic/ achromatic luminance ratios above their respective CLR. Yellow sensitive W-cells with a narrow spectral sensitivity peaking around 570 nm and with only a small or no response to white light, could enable distinction between white and yellow of similar luminance. The findings are consistent with the opponency model of spectrally sensitive cells in the LGB. We discuss their implications for colour coding by parvocellular cells. N- and W-cells appear to behave complementary with respect to luminance information (N-cells may be compared to the cat's off-cells, W-cells to on-cells). S- and L-cells are complementary with respect to colour. The yellow sensitive WM-cells are critical for the discrimination of yellow and white, while cells with excitatory cone input from blue and red cones (W-SL-cells) may aid the perception of purple. The fact that, at different relative luminance ratios between a chromatic stimulus and a white background, the whole family of parvocellular cells is involved differently in coding for colour, may explain the different appearance of colours against a white background at different luminance ratios and the perception of induced colours.This work was supported by a NATO collaborative research grant to Dr. Arne Valberg (650/83)     

Properties of concentrically organized X and Y ganglion cells of macaque retina.

1. Macaque retinal ganglion cells having concentrically organized receptive fields were classified as X- or Y-cells on the basis of the linearity or nonlinearity of their spatial summation to a "null" test of alternating contrast and drifting gratings. 2. When an alternating-phase, bipartite field positioned at the middle of the receptive field was used as a stimulus, X-cells had a null position, whereas Y-cells showed a doubling of the response frequency. When drifting sine-wave gratings of low contrast were used as a stimulus, X-cells showed a periodic modulation of their discharge having the same mean value for different spatial frequencies, whereas Y-cells showed a large increase in the mean value of their discharges. 3. X-cells had opponent-color responses that received cone-specific signals, i.e., center and surround responses were mediated by input from spectrally different types of cone, whereas Y-cells had broad-band spectral responses receiving mixed-cone signals, i.e., center and surround responses were totally or partly mediated by input from the same type(s) of cone. In most Y-cells, the spatially opponent responses from the center and the surround were mediated by the same types of cone and were thus spectrally nonopponent; other Y-cells showed spectral opponency, since one of the types of cone mediating responses of one region of the receptive field (e.g., center) was absent in the responses of the other region (e.g., surround). 4. X- and Y-cells projected to the lateral geniculate body. Opponent-color X- and Y-cells did not project to the superior colliculus, whereas a fraction of spectrally non-opponent Y-cells projected to this structure. 5. X-cells tended to have longer conduction latencies, less transient responses to small stimuli, and a more central retinal distribution than Y-cells; these differences, however, represented tendencies and not invariant properties. 6. The results show that the X/Y dichotomy of ganglion cells is present in the retina of macaques and indicate that the degree of the linearity of spatial summation of incoming cone signals to the cells is related to the degree of cone specificity of spectral inputs to the receptive-field mechanisms.     

Spectral properties of dark-adapted retinal ganglion cells in the plaice (Pleuronectes platessa, L.)

1. Spectral, spatial and temporal properties of receptive fields of dark-adapted, on-off retinal ganglion cells in the intact eye of the plaice, were analysed by recording from their axon terminals in the superficial layers of the optic tectum with indium micro-electrodes.2. Two cell-types were identified. The first gave fast-adapting, spectrally opponent on-off responses without centre-surround subdivisions of the receptive field. On and off response-components were mutually antagonistic. The second type gave slow-adapting on-off or off responses for different stimulus positions within the receptive field, with centre-surround or adjacent field configurations. Only on-off centre cells, showing mutual antagonism between field centre and surround, or off centre cells with inhibitory centres, were found. These cells had weak opponent or non-opponent properties.3. Most cells of each type received inputs both from cones and rods. At stimulus intensities suprathreshold for cones, response-components gave spectral peaks which have been classified into one of four wave-length ranges; blue, 440-460 nm; blue-green, 470-490 nm; green, 510-540 nm; and orange, 560-590 nm. No cells analysed gave sensitivity maxima in the red. At low stimulus intensities all cells with rod input gave a single spectral peak between 510 and 530 nm.     

Photoreceptor inputs to cat lateral geniculate nucleus cells

Summary The spectral sensitivities of single on- and off-centre, brisk sustained and brisk transient cells recorded from the A laminae of the lateral geniculate nucleus (LGNd) in cats anaesthetised with nitrous oxide and oxygen supplemented with halothane were measured under photopic and mesopic achromatic adapting conditions. All cells possessed spectral sensitivity functions with a single peak at about 556 nm under photopic conditions. Intense chromatic adapting fields superimposed on the photopic background affected neither the shape of the spectral sensitivity functions nor the position of the peak wavelength. Under mesopic adapting conditions cells possessed spectral sensitivities with two peaks, one at 507 nm and one at 556 nm. These results are interpreted as suggesting that the cells of the A laminae of the LGNd receive inputs from rods with maximum sensitivity at 507 nm and a single class of cones maximally sensitive at 556 nm.     

Spatial relations of flicker signals in the two rod pathways in man.

1. Flicker signals originating from the human rod photoreceptors seem to have access to two retinal pathways: one slow and sensitive, the other fast and insensitive. The phase lag between signals in the two pathways grows monotonically with frequency, reaching 180 deg near 15 Hz. 2. At 15 Hz, destructive interference between the slow and the fast signals can cause two related phenomena: (i) a suprathreshold intensity region--the perceptual null--within which the perception of flicker vanishes, and (ii) a double branching of the 15 Hz rod-detected flicker threshold versus intensity (TVI) curve. 3. Here we investigate the effect of changing target size on these phenomena in normal human observers. We find that the double-branched flicker TVI curve and the perceptual null are found for all targets larger than 2 deg in diameter. For smaller diameter targets, however, neither the lower branch of the double-branched flicker TVI curve nor the null are found. 4. While this might suggest that the slow rod signals are selectively disadvantaged by the use of small targets, phase measurements relative to a cone standard reveal that the slow signals are always present. For targets < or = 2 deg in diameter, however, they remain below detection threshold because of destructive interference with the fast rod signals. Thus, for small targets, the perceptual null is not absent, but has merged with (and therefore obliterated) the lower branch of the double-branched flicker TVI function. 5. This situation could arise if decreasing the target size causes a parallel reduction in the sensitivities of both pathways, rather than a selective reduction in the sensitivity of either one. Our findings are therefore consistent with a model in which the large-scale spatial organization of the two rod pathways is roughly similar.     

Interactions between luminance and colour channels in visual search and their relationship to parallel neural channels in vision

We have compared visual search under conditions that tend to isolate the magnocellular, parvocellular and koniocellular channels of the human visual system. We used isoluminant red–green stimuli that do not modulate short-wavelength sensitive (SWS) cones to isolate the parvocellular pathway, isoluminant SWS-cone isolating stimuli to stimulate only the koniocellular system and addition of small luminance contrasts to selectively activate the magnocellular pathway. We found that in the case of conjunction search, where attentional resources were required, the red–green (parvocellular) system can use accompanying small luminance (magnocellular) signals to improve visual search. On the other hand, when using SWS-cone isolating stimuli to selectively stimulate the blue–yellow (koniocellular) system, addition of similar luminance signals did not increase the efficiency of the serial visual search. The results indicate that S-cone signals may be processed in a separate pathway that does not get converging inputs from the magnocellular pathway. This is unlike the case with the red–green opponent system, which functions more synergistically with the magnocellular system.     

Luminance flicker sensitivity in positive- and negative-symptom schizophrenia

The aim of the present research was to investigate magnocellular and parvocellular channel disorders using luminance-flicker sensitivity in normal observers and a group with schizophrenia. The threshold sensitivity for a sine wave-modulated patch of achromatic flickering light in a gaussian envelope was measured as a function of its temporal frequency (1.0 Hz, 4.0 Hz, 8.0 Hz, 16.0 Hz, 32.0 Hz) and three space average luminance levels (mesopic 3.0 cd/m2, photopic 33.0 cd/m2 and 66.0 cd/m2). The Andreasen scales for the assessment of positive and negative symptoms in schizophrenia were used to classify subjects into subgroups with predominantly positive and negative symptoms. The results showed that there were no significant differences between the control and positive-symptom group in flicker sensitivity as a function of temporal frequency and luminance level, and there were no differences in flicker sensitivity between the three groups at 1.0 Hz at each of the three luminance levels. At 3.0 cd/m2 the negative-symptom group showed significant reductions in flicker sensitivity at 4.0 Hz, 8.0 Hz, 16.0 Hz and 32.0 Hz in comparison with the control and positive-symptom group. At 33.0 cd/m2 the negative-symptom group showed significant reductions in flicker sensitivity at 4.0 Hz and 32.0 Hz, and at 66.0 cd/m2 they showed significant reductions in flicker sensitivity at 4.0 Hz, 8.0 Hz and 32.0 Hz only in comparison with the control. It was concluded that the non-significant differences in flicker sensitivity in the positive-symptom group showed that the processing of temporal information in parvo- and magnocellular channels was unimpaired. Furthermore, the non-significant differences in flicker sensitivity at 1.0 Hz at each of the three luminance levels in the three groups provided evidence that functioning in parvocellular channels was unimpaired in the positive- and negative-symptom group. Finally, it was concluded that the significant reductions in flicker sensitivity at medium and high temporal frequencies in the negative-symptom group provided evidence for an impairment in magnocellular channels.     

Cone inputs to simple and complex cells in V1 of awake macaque

Rules by which V1 neurons combine signals originating in the cone photoreceptors are poorly understood. We measured cone inputs to V1 neurons in awake, fixating monkeys with white-noise analysis techniques that reveal properties of light responses not revealed by purely linear models used in previous studies. Simple cells were studied by spike-triggered averaging that is robust to static nonlinearities in spike generation. This analysis revealed, among heterogeneously tuned neurons, two relatively discrete categories: one with opponent L- and M-cone weights and another with nonopponent cone weights. Complex cells were studied by spike-triggered covariance, which identifies features in the stimulus sequence that trigger spikes in neurons with receptive fields containing multiple linear subunits that combine nonlinearly. All complex cells responded to nonopponent stimulus modulations. Although some complex cells responded to cone-opponent stimulus modulations too, none exhibited the pure opponent sensitivity observed in many simple cells. These results extend the findings on distinctions between simple and complex cell chromatic tuning observed in previous studies in anesthetized monkeys.     

Response variability of marmoset parvocellular neurons

This study concerns the properties of neurons carrying signals for colour vision in primates. We investigated the variability of responses of individual parvocellular lateral geniculate neurons of dichromatic and trichromatic marmosets to drifting sinusoidal luminance and chromatic gratings. Response variability was quantified by the cycle-to-cycle variation in Fourier components of the response. Averaged across the population, the variability at low contrasts was greater than predicted by a Poisson process, and at high contrasts the responses were approximately 40% more variable than responses at low contrasts. The contrast-dependent increase in variability was nevertheless below that expected from the increase in firing rate. Variability falls below the Poisson prediction at high contrast, and intrinsic variability of the spike train decreases as contrast increases. Thus, while deeply modulated responses in parvocellular cells have a larger absolute variability than weakly modulated ones, they have a more favourable signal: noise ratio than predicted by a Poisson process. Similar results were obtained from a small sample of magnocellular and koniocellular ('blue-on') neurons. For parvocellular neurons with pronounced colour opponency, chromatic responses were, on average, less variable (10–15%, p < 0.01) than luminance responses of equal magnitude. Conversely, non-opponent parvocellular neurons showed the opposite tendency. This is consistent with a supra-additive noise source prior to combination of cone signals. In summary, though variability of parvocellular neurons is largely independent of the way in which they combine cone signals, the noise characteristics of retinal circuitry may augment specialization of parvocellular neurons to signal luminance or chromatic contrast.     

跨膜区缺失的流感病毒M2蛋白表达纯化及其抗原性检测

目的 表达并纯化缺失跨膜区的流感病毒M2蛋白，并检测其抗原性。方法 利用RT．PCR方法从流感病毒A／PR／8／34（H1N1）的cDNA中扩增全长M2基因，利用两套引物扩增出缺失跨膜区26—43位氨基酸的sM2基因，并克隆到pET30a载体中表达融合蛋白，用镍柱纯化后的融合蛋白免疫小鼠获得抗血清，免疫荧光检测其抗原性。结果 sM2融合蛋白在大肠埃希菌中可高效表达，纯化后可获得高纯度的重组蛋白。免疫小鼠获得的血清进行免疫荧光检测，显示表达的融合蛋白有免疫原性。结论 跨膜区缺失的流感病毒M2融合蛋白与完整M2蛋白有相同的抗原性。     

Chromatic and spatial properties of parvocellular cells in the lateral geniculate nucleus of the marmoset (Callithrix jacchus)

The parvocellular (PC) division of the afferent visual pathway is considered to carry neuronal signals which underlie the red–green dimension of colour vision as well as high-resolution spatial vision. In order to understand the origin of these signals, and the way in which they are combined, the responses of PC cells in dichromatic ('red–green colour-blind') and trichromatic marmosets were compared. Visual stimuli included coloured and achromatic gratings, and spatially uniform red and green lights presented at varying temporal phases and frequencies.
The sensitivity of PC cells to red–green chromatic modulation was found to depend primarily on the spectral separation between the medium- and long-wavelength-sensitive cone pigments (20 or 7 nm) in the two trichromatic marmoset phenotypes studied. The temporal frequency dependence of chromatic sensitivity was consistent with centre–surround interactions. Some evidence for chromatic selectivity was seen in peripheral PC cells. The receptive field dimensions of parvocellular cells were similar in dichromatic and trichromatic animals, but the achromatic contrast sensitivity of cells was slightly higher (by about 30%) in dichromats than in trichromats. These data support the hypothesis that the primary role of the PC is to transmit high-acuity spatial signals, with red–green opponent signals appearing as an additional response dimension in trichromatic animals.     

Correlated firing of cat retinal ganglion cells. I. Spontaneously active inputs to X- and Y-cells

1. The shared inputs to cat retinal ganglion cells have been investigated by studying correlations in the maintained firing of neighboring ganglion cells. The firing of one cell was recorded from its axon in the optic tract, while that of a neighboring cell was simultaneously recorded with a second electrode in the retina. The recorded cells were of the X- or Y-type and viewed a uniform screen having a luminance of 10 cd/m2. 2. Ganglion cells with overlapping receptive-field centers showed two basic forms of correlated firing: if they had the same center sign (both on-center or both off-center), then they tended to fire at the same time, as shown by a peak in their cross-correlogram; but if they had opposite center signs (an on- and and off-center cell), they tended not to fire at the same time, as shown by a well, or dip, in their cross-correlogram. 3. Both of these tendencies were strongest for cells that were close together and did not appear for cells with nonoverlapping receptive-field centers. The strongest correlations were between neighboring Y-cells, cells with large fields, and the weakest were between X-cells, cells with small fields. In general, the strength of the correlations depended primarily on the area of the overlap between fields. 4. These correlations in maintained firing appear to be principally or entirely caused by shared inputs to the ganglion cells from more distal retinal neurons. The signals from these distal neurons appear to have strong, brief (4-8 ms), well-defined effects on ganglion cells, which are observed even in the absence of a visual stimulus. The inputs responsible for the correlated firing are thus referred to as spontaneously active inputs or simply as active inputs. 5. An analysis of the features in the various types of cross-correlograms supports the following statements about these spontaneously active inputs. a) There are two types of active inputs: inputs excitatory to on-center cells and simultaneously inhibitory to off-center center cells and inputs excitatory to off-center cells and simultaneously inhibitory to on-center cells. b) The active inputs of each type provide excitation to both X- and Y-cells of one center sign and inhibition to both X- and Y-cells of the other center sign. There is no evidence for a special class of more selective inputs providing input only to X-cells or only to Y-cells. c) Active inputs account for the majority (about 80%) of the spikes in the maintained activity of Y-cells but only a small fraction (about 15%) of the spikes in the maintained activity of X-cells. 6. A likely source of the active input signals appears to be spiking amacrine cells with a low rate of spontaneous activity.     

Chronic absence of baroreceptor inputs prevents training-induced cardiovascular adjustments in normotensive and spontaneously hypertensive rats

We investigate whether arterial baroreceptors mediate the training-induced blood pressure fall and resting bradycardia in hypertensive (SHR) and normotensive rats (WKY). Male SHR and WKY rats, submitted to sino-aortic denervation (SAD) or sham surgery (SHAM group), were allocated to training (T; 55% of maximal exercise capacity) or sedentary (S) protocols for 3 months. Rats were instrumented with arterial and venous catheters for haemodynamic measurements at rest (power spectral analysis) and baroreceptor testing. Kidney and skeletal muscles were processed for morphometric analysis of arterioles. Elevated mean arterial pressure (MAP) and heart rate (HR) in SHAM SHRS were accompanied by increased sympathetic variability and arteriolar wall/lumen ratio [+3.4-fold on low-frequency (LF) power and +70%, respectively, versus WKYS, P < 0.05]. Training caused significant HR (∼9% in WKY and SHR) and MAP reductions (−8% in the SHR), simultaneously with improvement of baroreceptor reflex control of HR (SHR and WKY), LF reduction (with a positive correlation between LF power and MAP levels in the SHR) and normalization of wall/lumen ratio of the skeletal muscle arterioles (SHR only). In contrast, SAD increased pressure variability in both strains of rats, causing reductions in MAP (−13%) and arteriolar wall/lumen ratio (−35%) only in the SHRS. Training effects were completely blocked by SAD in both strains; in addition, after SAD the resting MAP and HR and the wall/lumen ratio of skeletal muscle arterioles were higher in SHRT versus SHRS and similar to those of SHAM SHRS. The lack of training-induced effects in the chronic absence of baroreceptor inputs strongly suggests that baroreceptor signalling plays a decisive role in driving beneficial training-induced cardiovascular adjustments.     

Pupillary response to chromatic flicker

There is significant evidence for higher-level cortical control of pupillary responses to visual stimuli, suggesting that factors other than luminance changes may induce a pupillary response. In the present study, the pupillary responses to equiluminant flickering stimuli in a range of 3-13 Hz were examined. Flicker stimuli included color-black (luminance-modulated) and color-color (hue-modulated) flicker. Equiluminance was determined both by objective luminance measures as well as by subjective, perceptual equiluminance for each subject. For both objectively and subjectively equiluminant flicker, significant, sustained pupillary constrictions were recorded. The magnitude of these responses was sensitive to both color and frequency parameters; red-blue color-paired flicker consistently produced the strongest constrictions. These responses occurred even when the flicker was of a lower luminance, both physically and perceptually, than a preceding nonflickering color, indicating that chromatic rather than luminance-sensitive mechanisms are involved in this response. Interestingly, the color- and frequency-sensitivity of constriction parallels those of flickers which maximally stimulate photosensitive epileptic patients, raising the possibility that chromatic response may be a factor in photosensitivity.     

Substance P depolarizes striatal projection neurons and facilitates their glutamatergic inputs

The striatum is the main basal ganglia input nucleus, receiving extensive glutamatergic inputs from cortex and thalamus. Medium spiny striatal projection neurons (MSNs) are GABAergic, and their axon collaterals synapse on other MSNs. Approximately 50% of MSNs corelease substance P (SP), but how this neurotransmitter controls MSN activity is poorly understood. We used whole-cell recordings to investigate how SP affects MSNs and their glutamatergic inputs. SP elicited slow depolarizations in 47/90 MSNs, which persisted in the presence of tetrodotoxin (TTX). SP responses were mimicked by the NK1 receptor agonist [Sar9,Met(O₂)11]-substance P (SSP), and fully blocked by the NK1 receptor antagonists L-732,138, or extracellular zinc. When intracellular chloride was altered, the polarity of SP responses depended on the sign of the chloride driving force. In voltage-clamp, SP-induced currents reversed around −68 mV and displayed marked inward rectification. These data indicate that SP increased a ClC-2-type chloride conductance in MSNs, acting through NK1 receptors. SP also strongly increased glutamatergic responses in 49/89 MSNs. Facilitation of glutamatergic responses (which was observed both in MSNs directly affected by SP and in non-affected ones) was reduced by application of L-732,138, and fully blocked by coapplication of L-732,138 and SB222200 (an NK3 receptor antagonists), showing that both NK1 and NK3 receptors were involved. SP-induced facilitation of glutamatergic responses was accompanied by a marked decrease in paired-pulse ratio, indicating a presynaptic mechanism of action. These data provide an electrophysiological correlate for the anatomically known connections between SP-positive MSN terminals and the dendrites and somata of other MSNs.     

Functional glutamatergic and glycinergic inputs to several superior olivary nuclei of the rat revealed by optical imaging

Superior olivary complex (SOC) neurons receive excitatory and inhibitory inputs from both ears. We determined the nature of such inputs to the main SOC nuclei with an optical imaging system. To do so, brainstem slices of postnatal (P) rats (P3-13) were treated with the fast voltage-sensitive dye RH795, and ipsilateral and contralateral SOC inputs were activated electrically. Optical signals, equivalent to membrane potential changes, were detected by a 464-photodiode array. The signals consisted mostly of two components which were identified as pre- and postsynaptic potentials in experiments with Ca2+-free solutions. They correlated with morphological structures, i.e. the presynaptic components were prominent in neuropil regions whereas the postsynaptic components dominated in somata regions. Postsynaptic components were distinguished pharmacologically with the glycine receptor blocker strychnine and the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Concerning the lateral superior olive, we confirmed the known glutamatergic inputs from the ipsilateral side and the glycinergic inputs from the ipsilateral and contralateral sides. Furthermore, we identified a CNQX-sensitive input from the contralateral side. In the medial superior olive, we corroborated the glutamatergic and glycinergic inputs from the ipsilateral and contralateral sides. Both ipsi- and contralaterally, the glutamatergic input was more pronounced than the glycinergic input. In the superior paraolivary nucleus, we also identified ipsilateral and contralateral inputs. Besides the known glycinergic input from the contralateral side, we found a novel glycinergic input from the ipsilateral side and identified CNQX-sensitive inputs from the contralateral and ipsilateral sides. The latter was very weak and appeared only in 30% of the experiments. The data show the feasibility of identifying functional inputs to the SOC with voltage-sensitive dye recordings.     

Background-induced flicker enhancement in cat retinal horizontal cells. I. Temporal and spectral properties

1. Dim backgrounds can enhance small-spot flicker responses of cat retinal horizontal cells by a factor of 2 or more. 2. Intracellular marking with horseradish peroxidase (HRP) reveals that this enhancement effect occurs in--but is not necessarily limited to--the cone-connected, A-type horizontal cell. 3. Flicker amplitudes decrease over a frequency range from 3 to 36 Hz of square-wave photic stimulation. There is little evidence of flicker-response enhancement at 3 Hz. Flicker-response enhancement is typically 2-6 times larger at 35 than at 6 Hz. 4. Inspection of flicker waveforms indicates both a scaling-up of response signals with backgrounds and a distortion composed of 2- to 5-ms-latency decrease, expressed primarily within a quick component of OFF-repolarization. 5. Flicker enhancement first increases as a function of background irradiance and then decreases. The increasing limb has the dynamic range and spectral sensitivity of cat rods (507-nm peak). Enhancement is maintained during rod after-effects. The decreasing limb of the background-versus-intensity function results from light adaptation of cat, long-wavelength (red) cones. 6. The flicker responses themselves peak spectrally at approximately 555 nm and reflect only the activity of cat long-wavelength (red) cones, without evidence of intermixing of other photoreceptor mechanisms. 7. Thus within the first synaptic layer of the cat visual system, rod signals interact with the flicker responses of red cones, both increasing cone-signal amplitudes and modifying cone-signal waveforms. 8. The results are closely analogous to "suppressive rod-cone interaction" (SRCI) as described in human psychophysics. 9. An outer-plexiform-layer circuit involving rods, horizontal cells and cones may mediate rod-induced enhancement of cone flicker. This being the case, notions of horizontal-cell feedback interactions with cones may have to be modified and extended. A specific feedback model is elaborated in the companion paper.