Dependence of unit responses of the catlateral geniculate body on photic stimulus contrast

Unit responses in the lateral geniculate body of cats to photic stimuli of different contrast were investigated. The number of spikes in the initial phase of the responses (the first 30–45 msec) was found not to change at first, but then to decrease with an increase in the intensity of background illumination. The background intensity starting from which the response diminishes was shown to increase with an increase in the intensity of the test stimulus. The unit response is a linear function of the logarithm of stimulus contrast if the contrast is changed through variation of the intensity of the test stimulus. If contrast increases on account of a decrease in the intensity of background illumination the responses first increase and then remain unchanged. The range of contrasts within which the response is a linear function is narrowed if the intensity of the test stimulus is reduced. Counting the number of spikes in different time intervals of the response (t) showed that the greater the value of t (within the first 70–90 msec of the response) the steeper the curve of the number of spikes as a function of contrast. The Weber-Fechner law applies in the receptive field of the lateral geniculate body. The results are compared with those of some psychophysiological experiments.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 9, No. 3, pp. 267–274, May–June, 1977.     

The ultrastructure of the uterine epithelium of the pig during the estrous cycle and early pregnancy

Summary In the compound eye of the moth Antheraea polyphemus, three types of visual pigments were found in extracts from the retina and by microspectrophotometry in situ. The absorption maxima of the receptor pigment P and the metarhodopsin M are at (1) P 520–530 nm, M 480–490 nm; (2) P 460–480 nm, M 530–540 nm; (3) P 330–340 nm, M 460–470 nm. Their localization was investigated by electron microscopy on eyes illuminated with different monochromatic lights. Within the tiered rhabdom, constituted of the rhabdomeres of nine visual cells, the basal cell contains a blue-and the six medial cells have a greenabsorbing pigment. The two distal cells of most ommatidia also have the blue pigment; only in the dorsal region of the eye, these cells contain a UV-absorbing pigment, which constitutes a portion of only 5% of the visual pigment content within the entire retina. The functional significance of this distribution is discussed.     

A new visual area on the inferior wall of the cat cruciate sulcus

Properties of 187 neurons in the inferior wall of the cruciate sulcus, in an area where electrical stimulation evoked unidirectional saccadic eye movements, were investigated in waking cats. Of the total number 172 responded to visual stimulation. Neurons in the surface layers of the cortex responded to simple visual stimuli: light or dark spots or bars, both stationary and moving at speeds of around 30 deg/sec. These neurons showed no selectivity as regards stimulus orientation but sometimes behaved selectively toward the direction of their movements. In the intermediate layers the maximal neuronal response was obtained to a model of a bird flaping its wings. Neuronal responses in the depth of the cortex were characterized by selectivity to movement of stimuli toward or away from the animal in a certain part of the visual field, irrespective of whether a light stimulus was presented against a dark background or a dark stimulus against the light background. Responses to visual stimulation were exhibited only if the animal was in a state of activation, when the EEG showed desynchronization, and they were absent in a state of quite wakefulness. No responses were obtained to auditory or somatic stimulation. Responses to visual stimulation were not found in neurons of the medial wall of the brain beneath the cruciate sulcus, but responses were recorded to eye movements of definite size or orientation. It is postulated that at least two contiguous retinotopically organized zones exist in this part of the brain. Activity of one of them is connected with visual function, that of the other with eye movements.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 16, No. 6, pp. 766–773, November–December, 1984.     

Receptive field dynamics of neurons in the cat visual cortex and lateral geniculate body

Spike responses of single neurons in the primary visual cortex and lateral geniculate body to random presentation of local photic stimuli in different parts of the receptive field of the cell were studied in acute experiments on curarized cats. Series of maps of receptive fields with time interval of 20 msec obtained by computer enabled the dynamics of the excitatory and inhibitory zones of the field to be assessed during development of on- and off-responses to flashes. Receptive fields of all cortical and lateral geniculate body neurons tested were found to undergo regular dynamic reorganization both after the beginning and after the end of action of the photic stimulus. During the latent period of the response no receptive field was found in the part of the visual field tested, but later a small zone of weak responses appeared only in the center of the field. Gradually (most commonly toward 60–100 msec after application of the stimulus) the zone of the responses widened to its limit, after which the recorded field began to shrink, ending with complete disappearance or disintegration into separate fragments. If two bursts of spikes were generated in response to stimulation, during the second burst the receptive field of the neuron changed in the same way. The effects described were clearly exhibited if the level of background illumination, the intensity of the test bars, their contrast with the background, duration, angles subtended, and orientation were varied, although the rate and degree of reorganization of the receptive field in this case changed significantly. The functional importance of the effect for coding of information about the features of a signal by visual cortical neurons is discussed.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 14, No. 6, pp. 622–630, November–December, 1982.     

Selectivity of visual cortical neurons of rabbits to magnitude of moving stimuli

Unit responses of the rabbit visual cortex were investigated in relation to size of visual stimuli moving in their receptive field. With an increase in size of the stimulus in a direction perpendicular to the direction of movement ("width" of the stimulus) an initial increase in the intensity of the unit response through spatial summation of excitory effects is followed by a decrease through lateral inhibition. This inhibition is observed between zones of the receptive field which behave as activating when tested by a stimulus of small size. Each neuron has its own "preferred" size of stimuli evoking its maximal activation. No direct correlation is found between the "preferred" stimulus size and the size of the receptive field. With a change in stimulus size in the direction of movement ("length" of the stimulus) the responses to stimuli of optimal size may be potentiated through mutual facilitation of the effects evoked by the leading and trailing edges of the stimulus and weakened in response to stimuli of large size. The selective behavior of the neurons with respect to stimulus size is intensified in the case of coordinated changes in their length and width. It is postulated that the series of neurons responding to stimuli of different "preferred" dimensions may constitute a system classifying stimuli by their size.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 636–644, November–December, 1972.     

The response properties of retinula cells in the flyCalliphora erythrocephala as a function of the wavelength and polarization properties of visible and ultraviolet light

Detailed functional response properties of the class (1–6) retinula cells in the insectCalliphora erythrocephela have been determined as a function of the wavelength of monochromatic light and polarization conditions over a six log unit range of light adaptation levels. These stimuli were produced as a small parallel beam that could combine various wavelengths in both the ultraviolet and visible regions whose polarization planes could be individually controlled. The wild type and white-eyed mutant were studied and shown to have no significant differences in their response properties. It was shown that previous tests using transient stimuli, large compared to the adaptation levels produced responses of high orders of nonlinearity and rapidly changing transient properties. In contrast, the use of white-noise tests did not cause changes in the basic cell properties as established by a given light adaptation condition. These tests accurately model the visual conditions of a wide and representative range of normal behavioral conditions and the model response data thus obtained can be directly correlated to important physiological tests for the same stabilized adaptation conditions. The simultaneous application of both visible and ultraviolet monochromatic light revealed a complex response interaction which changed the self response contribution of the ultraviolet light from that due only to ultraviolet light but had no effect on the visible light response. It also produced a mutual interaction. This effect was found to be the same for all members of the (1–6) class of cells when the light was unpolarized but varied for different members of the class for polarized light depending upon the relative angles of theE-vectors for the two types of light which in turn vary for different members of the six cell classes.     

The spectral sensitivity of eye movements in response to light flashes inDaphnia magna

Summary The crustaceanDaphnia magna responds to a flash of light with a ventral rotation of its compound eye; this behavior is termed eye flick. We determined the spectral sensitivity for the threshold of eye flick in response to light flashes having three different spatial characteristics: (1) full-field, extending 180° from dorsal to ventral in the animal's field of view; (2) dorsal, 30° wide and located in the dorsal region of the visual field; (3) ventral, same as dorsal but located ventrally. All three stimuli extended 30° to the right and to the left of the animal's midplane. We found that spectral sensitivity varies with the spatial characteristics of the stimulus. For full-field illumination, the relative sensitivity was maximal at 527 nm and between 365 nm and 400 nm, with a significant local minimum at 420 nm. For the dorsal stimulus, the relative sensitivity was greatest at 400 nm, but also showed local maxima at 440 nm and 517 nm. For the ventral stimulus, the relative sensitivity maxima occurred at the same wavelengths as those for the full-field stimulus. At wavelengths of 570 nm and longer, the responses to both dorsal and ventral stimuli showed lower relative sensitivity than the full-field stimulus. No circadian or other periodic changes in threshold spectral sensitivity were observed under our experimental conditions. Animals which had their nauplius eyes removed by means of laser microsurgery had the same spectral sensitivity to full-field illumination as normal animals. Our results are discussed in terms of our current knowledge of the spectral classes of photoreceptors found in theDaphnia compound eye.     

Distinct responses of cones and melanopsin-expressing retinal ganglion cells in the human electroretinogram

ABSTRACT: BACKGROUND: The discovery of the novel photoreceptor, melanopsin-expressing retinal ganglion cells (mRGCs), has raised researchers' interest in photoreceptive tasks performed by the mRGC, especially in non-image-forming visual functions. In a prior study, we investigated the mRGC response to light stimuli independent of rods and cones with the four-primary illumination system, which modulates stimulus levels to the mRGC and cones independently, and mRGC baseline responses were recorded in the electroretinogram (ERG). METHODS: In the present study, we used the same illumination system to compare independent responses of the mRGC and cones in five subjects (mean +/- SD age, 23.0 +/- 1.7 years). The ERG waveforms were examined as direct measurements of responses of the mRGCs and cones to stimulation (250 msec). Implicit times (the time taken to peaks) and peak values from 30 stimuli given to each subject were analyzed. RESULTS: Two distinct positive peaks appeared in the mRGC response, approximately 80 msec after the onset of the stimuli and 30 msec after their offset, while no such peaks appeared in the cone response. The response to the mRGC stimulus was significantly higher than that to the cone stimulus at ~80 msec (p < 0.05) and tended to be higher than the cone stimulus at ~280 msec (p = 0.08). CONCLUSIONS: Implicit time of the first peak was much longer than that to the b-wave and this delay might reflect mRGC's sluggish responses. This is the first report of amplitudes and implicit time in the ERG from the response of the mRGC that is independent of rods and cones and obtained using the four-primary illumination system.     

Excitation signal processing times in Halobacterium halobium phototaxis.

Phototaxis responses of Halobacterium halobium were monitored with a computerized cell-tracking system coupled to an electronic shutter controlling delivery of photostimuli. Automated analysis of rates of change in direction and linear speeds provided detection of swimming reversals with 67 ms resolution, permitting measurement of distinct phases of the responses to attractant and repellent stimuli. After stimulation, there was a latency period in which the population reversal frequency was unchanged, followed by an excitation phase in which reversal frequency increased, and a slower adaptation phase in which reversal frequency returned to its prestimulus value. A step-decrease in illumination of the attractant receptor slow-cycling or sensory rhodopsin (SR) (lambda max, 587 nm) was interpreted by the cells as an unfavorable stimulus and, after a minimum latency of 0.70 +/- 0.14 s, induced swimming reversals with the peak response occurring 1.34 +/- 0.07 s after onset of the stimulus. Two distinct repellent responses in the near UV/blue were observed. One was a reversal response to 400 nm light, which was dependent on orange-red background illumination as expected for the photointermediate repellent form of SR (lambda max, 373 nm). The minimum latency of this response was approximately the same as that of the SR attractant system. The second was a reversal response with shorter minimum latency (0.40 +/- 0.07 s) to light of longer wavelength (450 nm) than absorbed by the known SR repellent form. This result confirms recent findings of an additional repellent photosystem in this spectral range. Further, the longer wavelength repellent response is independent of orange-red background illumination, indicating that the photoreceptor mediating this response is not a photointermediate of SR.     

Short-wavelength light sensitivity of circadian, pupillary, and visual awareness in humans lacking an outer retina

As the ear has dual functions for audition and balance, the eye has a dual role in detecting light for a wide range of behavioral and physiological functions separate from sight. These responses are driven primarily by stimulation of photosensitive retinal ganglion cells (pRGCs) that are most sensitive to short-wavelength ( approximately 480 nm) blue light and remain functional in the absence of rods and cones. We examined the spectral sensitivity of non-image-forming responses in two profoundly blind subjects lacking functional rods and cones (one male, 56 yr old; one female, 87 yr old). In the male subject, we found that short-wavelength light preferentially suppressed melatonin, reset the circadian pacemaker, and directly enhanced alertness compared to 555 nm exposure, which is the peak sensitivity of the photopic visual system. In an action spectrum for pupillary constriction, the female subject exhibited a peak spectral sensitivity (lambda(max)) of 480 nm, matching that of the pRGCs but not that of the rods and cones. This subject was also able to correctly report a threshold short-wavelength stimulus ( approximately 480 nm) but not other wavelengths. Collectively these data show that pRGCs contribute to both circadian physiology and rudimentary visual awareness in humans and challenge the assumption that rod- and cone-based photoreception mediate all "visual" responses to light.     

The roles ofDrosophila ocelli and compound eyes in phototaxis

Summary Drosophila have three types of photoreceptors in their compound eyes: R1–6, R7, and R8. In addition they have simple eyes, ocelli, with another type of photoreceptor. The role of each type of receptor and the possible interaction of their inputs were examined in an innate visual preference task, fast walking phototaxis. Flies were found to be attracted to light, i.e., positively phototactic. We compared the strength of the photopositive response and the spectral preference of normal fly strains and mutant fly strains lacking functional ocelli, R1–6, or R7, singly or in combination. Electroretinographic measures were used to confirm the specificity of deficits in visual mutant strains and the normal functioning of intact receptors.The strength of the photopositive response was strong, as indicated by the high correlation between increases in the intensity of the variable stimulus and increasing numbers of flies attracted toward it. Nearly all strains with or without intact receptor types showed high correlations whether the constant intensity stimulus offered as the alternative choice was bright 467 nm light (Figs. 1 and 2) or dim 572 nm light (Figs. 3 and 4). These constant stimuli were selected so that data in relevant intensity ranges of receptor function would be obtained. An important exception to the high correlations in the intensityresponse functions occurred with flies lacking function in all receptor types except R8; their positive phototaxis was extremely weak in dim light (Fig. 3).Analyses of the phototactic spectral sensitivities (Figs. 5 and 6), as well as comparisons with known electrophysiological spectral sensitivities, were used to determine the inputs from compound eye receptors and to demonstrate central interaction of these inputs with ocellar input. Several experiments with converging evidence suggest that R7 (when present) and R8 dominate fast phototaxis in the conditions of our experiment. R1–6 is the predominant compound eye receptor type in ERG measures; however, its behavioral input is clearly demonstrated only as enhancing R8 dominance of phototaxis in experiments using a dim constant stimulus and as enhancing R7 dominance of phototaxis in experiments using a bright constant stimulus. Similarly, the presence of ocellar receptors also facilitates R8 input in dim light and R7 input in bright light. The data substantiating these respective conclusions are: (1) a lack of dim light phototaxis in a mutant strain with only R8 functional (Fig. 3); and (2) a lack of an ultraviolet (UV) maximum from R7 in bright light phototaxis in a mutant strain with only R7 and R8 functional (Fig. 5c).Generally, absence of the ocelli and R1–6 had remarkably little effect on fast phototactic behavior except for the interaction with R7 and R8 inputs. This interaction is consistent with a theory that ocelli serve to modulate compound eye sensitivity.Abbreviations ERG electroretinogram - PDA prolonged depolarizing afterpotential - R (1–6, 7, 8) retinular cell(s) - UV ultraviolet We thank K. Frayer, F. Garfinkel, K. Hansen, M. Johnson, R. Srygley, and G. Sullivan for technical assistance; K. Hansen was instrumental in running the experiments at extremely dim conditions. Supported by grants NSF-BNS-76-11921 and NIH-1-RO1-EY-02487-01A1 (to W.S.S.). Experiments reported in this paper were included in a dissertation (Karin G. Hu) submitted in partial fulfillment of the requirements of the Ph.D. degree to the Department of Psychology, The Johns Hopkins University, Baltimore, Maryland 21218. We thank members of the Graduate Board Dissertation Examining Committee for their comments: Drs. E. Blass, R. DeVoe, K. Muller and W. Sofer.     

Responses of neurons of reticular and ventral anterior nuclei of the cat thalamus to afferent stimuli of different modalities

Responses of 146 spontaneously active neurons of the reticular nucleus (R) and of 98 neurons of the ventral anterior (VA) nucleus of the thalamus to electrical stimulation of the skin of the footpads, to flashes, and to clicks were studied in experiments on cats immobilized with D-tubocurarine or myorelaxin. Stimulation of the contralateral forelimb was the most effective: 24.9% of R neurons and 31.3% of VA neurons responded to this stimulation. A response to clicks was observed in only 4.4% of R neurons and 2.4% of VA neurons. Nearly all responding neurons did so by phasic (one spike or a group of spikes) or tonic excitation. Depression of spontaneous activity was observed only in response to electrical stimulation of the skin. Depending on the site of stimulation, it was observed in 2.6–4.3% of R neurons and 1.7–2.1% of VA neurons tested. The latent period of the phasic responses of most neurons was 6–64 msec to electrical stimulation of the contralateral forelimb, 11–43 msec in response to stimulation of the hindlimb on the same side, 10–60 msec to photic and 8–60 msec to acoustic stimulation. Depending on the character of stimulation, 75.1–95.6% of R neurons and 68.7–97.6% of VA cells did not respond at all to the stimuli used. Of the total number of cells tested against the whole range of stimuli, 25% of R neurons and 47% of VA neurons responded to stimulation of different limbs, whereas 16% of R neurons and 22% of VA cells responded to stimuli of different sensory modalities. The functional role of the convergence revealed in these experiments is to inhibit (or, less frequently, to facilitate) the response of a neuron to a testing stimulus during the 40–70 msec after conditioning stimulation.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 7, No. 6, pp. 563–571, November–December, 1975.     

Neural correlates of novelty detection in pulse-type weakly electric fish

Evoked potentials (EPs) and single unit recordings from various electrosensory-processing regions of several pulse-type gymnotiform species were made to investigate neural activity patterns that could be associated with novelty detection. Whereas the electrosensory afferents and cells in the ELL exhibited only minor changes in response size as stimuli were presented less frequently (novel stimuli), most units studied in the torus semicircularis (TS) showed very strong, increased responsiveness to stimuli presented less frequently relative to stimuli presented persistently (at every EOD event. The responses of the TS were graded with respect to stimulus frequency. The discrimination between novel and persistent stimuli by the TS occurred with stimuli presented transversely or longitudinally with respect to the fish's long axis, and regardless of the timing of the stimulus with respect to the fish's pacemaker-related signal (PS). When electrosensory novelties were presented persistently the responses of the TS rapidly habituated. This may indicate that activity in this region of the TS is novelty related. This novelty-related activity in the TS can be correlated with certain aspects of the fish's behavior, i.e., EOD interval length during a behavioral novelty response. However, TS activity may continue to indicate the occurrence of electrosensory novelties after the behavior has habituated. It is suggested that the novelty-related activity of the TS of these fish is necessary, but not sufficient, for the production of electrosensory novelty-induced behavioral responses. Lesions of the region of the TS containing the rapidly-habituating neurons abolished the electrosensory novelty response, but not that resulting from visual and auditory stimulation.     

Mechanism of directional selectivity of neurons with complex receptive fields in the cat visual cortex

Spatio-temporal interactions within complex receptive fields in the cat visual cortex were investigated by sequential presentation of two stationary stimuli. When two stimuli were presented in phase (on-on or off-off) in the order corresponding to preferred direction of movement, facilitation or weak inhibition of the response to the second stimulus was observed, whereas if it corresponded to zero direction of movement, the response was strongly inhibited. In the case of stimulation out of phase (on-off or off-on), in the order corresponding to the preferred direction of movement, considerable inhibition of the response to the second stimulus was observed, whereas in the opposite order, facilitation or weak inhibition was observed. The strength of interaction between different parts of the field depended on the distance between them and the duration of the interval between stimuli. Directional selectivity of "complex" neurons is thus ensured by asymmetry of spatio-temporal interactions between receptive field inputs of the same type. Interactions between inputs of different types, arising when a multiedge stimulus (bar, grating) can be used by the visual system to distinguish an object from the background and to assess changes in size of objects and the relative velocity of their movement.V. Kapsukas State University, Vilnius. Translated from Neirofiziologiya, Vol. 16, No. 4, pp. 505–512, July–August, 1984.     

Unit responses of the sensomotor cortex to paired electrical stimuli

Unit responses of the sensomotor cortex to paired electrical stimulation and visual cortex, applied either simultaneously or after various delays (from 0 to 200 msec) depend on the order of application of the stimuli and on the interval between them. If stimulation of the sensomotor cortex was used in a conditioning role the response continued unchanged when the intervals between stimuli were increased to 200 msec. If, however, stimulation of the sensomotor cortex had a testing role interaction was observed between the stimuli so that responses to both first and second stimuli were blocked; this was exhibited most clearly for intervals of 40–80 msec between stimuli. The blocking effect persisted on some neurons with delays of up to 200 msec between stimuli, while the response of others to both the first and the second stimulus was restored.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 5, No. 6, pp. 628–635, November–December, 1973.     

Responses and properties of receptive fields of neurons in the visual projection zone of the pigeon hyperstriatum

Unit responses in the hyperstriatal region of the pigeon forebrain to the action of various visual stimuli were investigated. Particular attention was paid to the discovery of retinotopic projection in the Wulst region. It was shown that as the electrode was advanced in the caudal direction in the zone of visual projection of the hyperstriatum the receptive fields of the neurons recorded shifted in the opposite direction in the visual field. The receptive fields of neurons of the ventral and dorsal hyperstriatum lie higher in the visual field and are larger in diameter than those of neurons of the accessory hyperstriatum. Unit responses in the visual projection zone of the Wulst depend on the intensity of illumination, size, and speed and direction of movement of the test objects across the receptive field. The functional role of the retino-thalamo-telencephalic system in visual interpretation in birds is discussed and it is suggested that the Wulst region is comparable with the striatal and also with the frontal regions of the mammalian cortex.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 230–236, May–June, 1976.     

Characteristics of dynamic reorganization of receptive fields of cortical and geniculate visual neurons of cat in response to changes in photic stimulation parameters

The effect of parameters of local photic stimulation of different points of the receptive field on the characteristics of dynamic reorganization of receptive fields of cortical and geniculate visual neurons within microintervals of time observed previously was studied in computer-controlled experiments on unanesthetized, curarized cats. Dependence on the degree of widening of the receptive field and the temporal characteristics of this process on the background illumination level, intensity, contrast, area, duration, energy, and orientation of a local rectangular or circular photic stimulus flashing in random order at 100 points of the tested part of the visual field was studied. It was concluded that the background illumination level and the intensity, size, duration, and orientation of the stimulus have a specific effect on dynamic reorganization of the receptive field. The effects of all the parameters studied on the dynamics of the receptive field were shown to be nonlinear functions with optimal values that differed for different cells. The possible functional role of this effect and also the probability that it may participate in information coding in the visual system are discussed.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 15, No. 4, pp. 339–346, July–August, 1983.     

Neural correlates of knowledge: stable representation of stimulus associations across variations in behavioral performance

Behavioral responses to a sensory stimulus are often guided by associative memories. These associations remain intact even when other factors determine behavior. The substrates of associative memory should therefore be identifiable by neuronal responses that are independent of behavioral choices. We tested this hypothesis using a paired-associates task in which monkeys learned arbitrary associations between pairs of visual stimuli. We examined the activity of neurons in inferior temporal cortex as the animals prepared to choose a remembered stimulus from a visual display. The activity of some neurons (22%) depended on the monkey's behavioral choice; but for a novel class of neurons (54%), activity reflected the stimulus that the monkey was instructed to choose, regardless of the behavioral response. These neurons appear to represent memorized stimulus associations that are stable across variations in behavioral performance. In addition, many neurons (74%) were modulated by the spatial arrangement of the stimuli in the display.     

Varying response of caudate neurons to visual stimuli in awake cats

Activity in 62 caudate nucleus neurons produced during presentation of visual stimuli was recorded during experiments on awake cats. Response of a sensory pattern, associated with a photic stimulus falling within a certain section of the visual field was observed in 52% of the neurons tested as against only 11% manifesting motor response related to eye movement guided towards a target. About a quarter of the cells responded to biologically significant stimuli, producing a nonspecific response, i.e., not specifically related to the nature of the visual stimuli presented. Several different response patterns could be recorded from a single unit. A hypothesis that more than one parallel pathway for afferent visual inferences on the caudate nucleus may exist is presented on the basis of findings from this research.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 3, May–June, pp. 372–378, 1989.     

Dynamics of skate horizontal cells

The all-rod retina of the skate (Raja erinacea or R. oscellata) is known to have the remarkable capability of responding to incremental flashes superimposed on background intensities that initially block all light-evoked responses and are well above the level at which rods saturate in mixed rod/cone retinas. To examine further the unusual properties of the skate visual system, we have analyzed responses of their horizontal cells to intensity-modulated step, sinusoidal, and white-noise stimuli. We found that during exposures to mean intensities bright enough to block responses to incremental stimuli, decremental stimuli were also initially blocked. Thereafter, the horizontal cells underwent a slow recovery phase during which there was marked nonlinearity in their response properties. The cell first (within 2-3 min) responded to decrements in intensity and later (after greater than 10 min) became responsive to incremental stimuli. After adaptation to a steady state, however, the responses to intensity modulation were nearly linear over a broad range of modulation depths even at the brightest mean levels of illumination. Indeed, examination of the steady-state responses over a 5-log-unit range of mean intensities revealed that the amplitude of the white noise-evoked responses depended solely on contrast, and was independent of the retinal irradiance as the latter was increased from 0.02 to 20 muW/cm2; i.e., contrast sensitivity remained unchanged over this 1,000-fold increase in mean irradiance. A decrement from the mean as brief as 2 s, however, disturbed the steady state. Another unexpected finding in this all-rod retina concerns surround-enhancement, a phenomenon observed previously for cone-mediated responses of horizontal cells in the retinas of turtle and catfish. While exposure to annular illumination induced response compression and a pronounced sensitivity loss in response to incremental light flashes delivered to the dark central region, the cell's sensitivity showed a significant increase when tested with a white noise or sinusoidally modulated central spot. Unlike horizontal cells in other retinas studied thus far, however, response dynamics remained unchanged. Responses evoked either by a small spot (0.25-mm diam) or by a large field light covering the entire retina were almost identical in time course. This is in contrast with past findings from cone-driven horizontal cells whose response waveform (dynamics) was dependent upon the size of the retinal area stimulated.