Maintained Discharge Rates Research Articles

Volitional control of single cortical neurons in a brain–machine interface

Volitional control of cortical activity is relevant for optimizing control signals for neuroprosthetic devices. We explored the control of firing rates of single cortical cells in two Macaca nemestrina monkeys by providing visual feedback of neural activity and rewarding changes in cell rates. During ‘brain-control’ sessions, the monkeys modulated the activity of each of 246 cells to acquire targets requiring high or low discharge rates. Cell control improved more than two-fold from the beginning of practice to peak performance. Cell activity was modulated substantially more during brain control than during wrist movements. When recording stability permitted, the monkeys practiced controlling activity of the same cells across multiple days. The performance improved substantially for 27 of 36 cells when practicing brain control across days. The monkeys maintained discharge rates within each target for 1 s, but could maintain rates for up to 3 s for some cells, and performed the brain-control task equally well using cells recorded from the pre-central cortex compared to cells in the post-central cortex, and independently of any directional tuning. These findings demonstrate that arbitrary single cortical neurons, regardless of the strength of directional tuning, are capable of controlling cursor movements in a one-dimensional brain–machine interface. It is possible that direct conversion of activity from single cortical cells to control signals for neuroprosthetic devices may be a useful complementary strategy to population decoding of the intended movement direction.

An analysis of the cross-correlation between ganglion cells in the retina of goldfish.

Neighboring ganglion cells in the retinae of vertebrates show cross-correlation between their impulse trains. Cross-correlation is found both in maintained discharges and during responses to visual stimulation. There has been speculation about the function of this statistical dependence, but little is known about its genesis. This study examines the statistics of the interimpulse intervals preceding and those following impulses that coincide with an impulse in the other train. Short intervals are rarer than expected preceding a coincidence, regardless of the form of the cross-correlation. Short intervals are more common than expected following a coincidence when the cross-correlation is positive, but rarer than expected following coincidences during negative cross-correlation. These results contradict the extant models for cross-correlation, but may be explained by the multiplicative combination of a variable common input and the variability within each cell. In addition, the lag (relative timing of coincident impulses in the two cells) is found to be related to the maintained discharge rates of the cells, implying that the lags may be explained without invoking specific delay circuits.

Laminar organization of receptive field properties in the dorsal lateral geniculate nucleus of the tree shrew (Tupaiaglis belangeri)

A laminar analysis of the receptive field properties of relay cells in the binocular region of the tree shrew dorsal lateral geniculate nucleus (LGN) found three main subdivisions. Lamina 1 (receiving ipsilateral eye input) and lamina 2 (contralateral) comprise a pair of layers that contain only ON-center neurons. Laminae 4 (contralateral) and 5 (ipsilateral) comprise a pair of layers with mostly OFF-center cells (86%). Laminae 3 and 6 (both contralaterally innervated) also form a distinct pair, although lamina 3 contains a mixture of cells with ON-centers (43%) or OFF-centers (57%), and lamina 6 contains mostly cells with ON-OFF centers and suppressive surrounds (81%). Cells located in the interlaminar zones resembled neurons in laminae 3 and 6. In comparison with the cells in the OFF-center laminae 4 and 5, the ON-center cells in laminae 1 and 2 had smaller, more elliptical receptive field centers with stronger responses to flashed visual stimuli. In addition, cells in the ipsilateral eye laminae 1 and 5 showed a greater change in center diameter, with eccentricity from the area centralis, than cells in the contralateral eye laminae 2 and 4. Principal components analysis using six receptive field properties (latency to optic chiasm stimulation, receptive field center diameter, maintained discharge rate, response onset latency, peak spike density, and phasic-tonic index) suggested that the cells in laminae 3 and 6 and the interlaminar zones are W-like. Principal components analysis of the same receptive field properties in laminae 1, 2, 4, and 5 did not reveal differences clearly related to X-like (parvocellular) and Y-like (magnocellular) categories. Ninety-seven percent of the cells tested for linearity of spatial summation in laminae 1, 2, 4, and 5 were linear. We conclude that the dominant organizational features of the tree shrew LGN are the ON-center, OFF-center, and W pairs of layers that project to different regions within the striate cortex.

Non-dominant suppression in the dorsal lateral geniculate nucleus of the cat: laminar differences and class specificity.

Binocular non-dominant suppression (NDS) in the dorsal lateral geniculate nucleus (LGNd) of the cat was studied by recording from single neurons in the LGNd of anaesthetized, paralysed cats while stimulating the non-dominant eye with a moving light bar. The maintained discharge rate of LGNd neurons was varied by stimulating the dominant eye in various ways: by varying the size or contrast of a flashed spot, by varying the inner diameter of a flashed annulus of large outer diameter, by varying the velocity of a moving light bar, and by covering the eye. Non-dominant suppression was quantified either as the decrease in the maintained discharge rate (the "dip"), expressed as spikes per second, or as the ratio of the dip to the maintained discharge rate (the "dip ratio"). At low maintained discharge rates the dip, although low in value, frequently approached the maintained rate, i.e. the dip ratio approached unity. As the maintained discharge rate increased the dip value also increased, but more slowly than the maintained discharge rate, i.e. the dip ratio decreased. At maintained discharge rates above about 30 spikes/s, in many neurons the dip appeared to be approaching a constant value. This strong dependence of NDS on the maintained discharge rate of the LGNd neuron suggests that the inhibitory input to the cell arises from a region of the brain that receives an input both from the non-dominant eye and from the LGNd cell. Reasons are given for thinking that this region is the perigeniculate nucleus. Because of the strong dependence of dip and dip ratio on the maintained discharge rate, it was necessary to adopt stringent criteria when comparing NDS in two different sets of neurons or of the same set of neurons in different conditions. We recognized a significant difference in NDS between two classes of neurons or between two states only if: (1) there was no significant difference between the maintained discharge rates, and (2) there was a significant difference for both dip and dip ratio between the two classes or states. Using these criteria we found: (1) no difference between non-lagged X (XNL) and non-lagged Y (YNL) cells, (2) no difference between on-centre and off-centre cells for either XNL or YNL cells, (3) no difference between XNL cells and lagged X (XL) cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Neurophysiological Correlates of Colour Induction on White Surfaces.

Coloured light surrounding a white surface of about equal luminance makes the white surface appear illuminated with an unsaturated light of the complementary colour. In an attempt to discover the neurophysiological basis of such colour induction, we recorded from spectrally opponent cells of the parvocellular layers of the lateral geniculate nucleus (P-LGN) of anaesthetized macaques. Only cells with wide-band (W) spectral sensitivity in the short (S) or long wavelength (L) part of the spectrum (WS, WL) are excited by white spots of light centred on their receptive field. Cells with narrow-band (N) spectral sensitivity (NS, NL) and light-inhibited (LI) cells are inhibited by white light. Therefore, it is likely that the code for white is contained in a balanced excitation of the W cells. The effects of continuous illumination of remote surrounds with different wavelengths on the responses to achromatic light stimuli were investigated. Responses [on minus maintained discharge rate (MDR) or on-minus-off] were determined for white spots (1 - 3 degrees diameter) flashed on the receptive field centre, presented either alone or in the presence of an annular surround of equal luminance (inner diameter 5 degrees; outer diameter 20 degrees ). During red surround illumination the responses of WL cells to white spots tended to be reduced as were those of WS cells during blue surround illumination. Surround illumination with the opponent colour had more variable effects, neither WS nor WL cells showing a significant alteration of their mean response to white during surround illumination with opponent light. Response alterations were to a large extent due to changes in MDR, which increased in WS cells during blue surround illumination and in WL cells during red surround illumination. It is argued that the surround effects on centre responses are due to intraocular stray light rather than lateral connections in the retina. The surround effects also depended to some extent on the size of the test spot. LI cells and the very rare parvocellular panchromatic on-cells showed no chromatic response changes during coloured surround illumination. Inasmuch as the excitation of WS cells, either alone or in combination with NS cell activation, is involved in coding for green and blue, and that of WL cells, in combination with NL cell activation, is involved in coding for red and yellow in perception, the shift of excitation towards one or the other W cell group indicates relatively more red or green signals in the white response, consistent with and in the same direction as colour induction. In addition, the summed population response of WS and WL cells is decreased during surround illumination with any colour including white. This is related to brightness decrease during surround illumination in perception.

Time course of neural responses discriminating different views of the face and head.

1. Measurements of the magnitude and time course of response were made from 44 cells responsive to static head views at different levels of stimulus effectiveness. In this way responses to complex stimulus patterns evoking good, poor, and midrange responses could be compared across the cell population. 2. Cells exhibiting both good and poor initial discrimination between head views were found at short and long latencies; there was no correlation of any of the temporal response parameters measured with cell response latency. 3. The time course of the population response to the most effective stimuli showed a rapid increase to a peak firing rate (onset to peak, rise time, 58 ms) that was on average 115 spikes/s above spontaneous activity (S/A), followed by slower decay (decay time, 93 ms) to a maintained discharge rate (15% of the peak rate above S/A). 4. Discrimination between responses to different head views exhibited by the population showed a sharp rise and reached highly significant levels within 25 ms after the population's response onset. 5. On average, activity in a single neuron (the Average Cell) rises to 44% of its peak response rate within 5 ms of the response onset. 6. The Average Cell also showed exceptionally fast discrimination between views, significant within 5 ms of response onset. 7. It is argued that the fast rise in firing rate, followed by a decay to a lower rate and the very fast emergence of discrimination are features of pattern processing present in real neural systems that are lacking in many processing models based on artificial networks of neuronlike elements, particularly those where discrimination relies on top-down and/or lateral competitive inhibition. 8. It is concluded that the only way to account for the rapid discrimination is to consider a coding system in which the first spike from multiple sources is used to transmit information between stages of processing.

The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons

The colour of an object is changed by surround colours so that the perceived colour is shifted in a direction complementary to the surround colour. To investigate the physiological mechanism underlying this phenomenon, we recorded from 260 neurons in the parvo-cellular lateral geniculate nucleus (P-LGN) of anaesthetized monkeys (Macaca fascicularis), and measured their responses to 1.0-2.0 degrees diameter spots of equiluminant light of various spectral composition, centered over their receptive field (spectral response function, SRF). Five classes of colour opponent neurons and two groups of light inhibited cells were distinguished following the classification proposed by Creutzfeldt et al. (1979). In each cell we repeated the SRF measurement while an outer surround (inner diameter 5 degrees, outer diameter 20 degrees) was continuously illuminated with blue (452 nm) or red (664 nm) light of the same luminance as the center spots. The 1.0-1.5 degree gap between the center and the surround was illuminated with a dim white background light (0.5-1cd/m2). During blue surround illumination, neurons with an excitatory input from S- or M-cones (narrow- and wide-band/short-wavelength sensitive cells, NS- and WS-cells, respectively) showed a strong attenuation of responses to blue and green center spots, while their maintained discharge rate (MDR) increased. During red surround illumination the on-minus-off-responses of NS- and WS-cells showed a clear increment. L-cone excited WL-cells (wide-band/long-wavelength sensitive) showed a decrement of on-responses to red, yellow and green center spots during red surround illumination and, in the majority, also an increment of MDR. The response attenuation of narrow-band/long-wave-length sensitive (NL)-cells was more variable, but their on-minus-off-responses were also clearly reduced in the average during red surrounds. Blue surround illumination affected WL-cell responses little and less consistently than those of NL-cells, but often broadened the SRF also in the WL-cells towards shorter wavelengths. The M-cone excited and S-cone suppressed WM-cells were strongly suppressed by blue but only little affected by red surround illumination. The changes of spectral responsiveness came out clearly in the group averages of the different cell classes, but showed some variation between individual cells in each group. The zero-crossing wavelengths derived from on-minus-off-responses were also characteristically shifted towards wavelengths complementary to those of the surround.(ABSTRACT TRUNCATED AT 400 WORDS)

The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons

We report on experiments which were undertaken in an attempt to clarify mechanisms underlying the contrast effects of chromatic surround illumination on spectral responsiveness of cells in the parvocellular layers of the LGN (P-LGN-cells), that had been demonstrated under standard conditions in the preceding companion paper. The experiments were done in anesthetized macaques (Macaca fascicularis). In some neurons, S-potentials were recorded together with the post-synaptic action potentials, and all effects seen in P-LGN-cells were present already in their retinal afferents indicating their retinal origin. The responsiveness of the cells for center stimuli of different wavelengths and during illumination of the receptive field center or the outer surround was determined. Continuous outer surround illumination altered maintained discharge rate (MDR), sensitivity and gain of P-LGN and retinal ganglion cells in the same way and empirically not distinguishable from direct illumination of the receptive field. Responses to surround flashes showed the same dependence on spectral composition as those to center flashes. Adaptation and excitation caused by outer surround illumination (inner diameter 5 degrees, outer diameter 20 degrees) were, in the average, ten times weaker than those exerted by light of the same spectral composition shone directly into the receptive field. Surround effects decreased proportional to r-2. Excitation by outer surround flashes was reduced by adaptation of the receptive field center in the same manner as responses to center flashes. The findings indicate that outer surround light has a direct excitatory and adaptive effect on the excitatory or inhibitory cones feeding into the receptive field. This indicates that straylight from the surround into the center could be responsible for the adaptive and excitatory effects of surround illumination. The straylight fraction from the remote surround into the receptive field must be higher, however, than that estimated from the psychophysically determined point spread function. It comes closer to earlier direct straylight measurements in excised eyes, but may be enhanced by chromatic aberration. If a surround of excitatory colour is flashed simultaneously with an excitatory center stimulus, additivity of center and surround excitation is observed only at low center intensities, while at higher center intensities the gain for center excitation is reduced similar to adaptive gain control. This could be explained by lateral interaction through horizontal connections in the retina, which decays within seconds, while adaptation of the cones feeding into the receptive field center is fully effective only after about 3 s.(ABSTRACT TRUNCATED AT 400 WORDS)

Receptive field properties and latencies of cells in the lateral geniculate nucleus of the North American opossum (Didelphis virginiana).

Relay cells in the lateral geniculate nucleus (LGN) of the North American opossum were classified as types 1, 2, and 3 on the basis of their receptive field properties and afferent latencies to optic nerve (ON) and optic chiasm (OX) stimulation and antidromic latencies to stimulation of cortex. Type 1 and type 3 cells gave transient responses and type 2 cells gave sustained responses to appropriate standing contrast in the receptive field center. The differences in response pattern were quantified with a phasic-tonic index (PTI); the PTI values for type 2 cells (PTI less than 63) did not overlap those for type 1 cells (PTI greater than 68) or type 3 cells (PTI greater than 80). With a homogeneous field (1.3 cd/m2), the maintained discharge rates (spikes/s) of type 3 cells (less than 1-11) were significantly lower than those of type 1 cells (3-23) and of type 2 cells (1-22). For all type 2 cells tested with a counterphased sine-wave grating, a null position of the grating was found and the cells were classified as linear. The type 1 and type 3 cells tested were nonlinear (i.e., exhibited excitatory doubling and did not have a null grating position). The maximum velocity of movement that reliably elicited responses (cut-off velocity) was low for type 3 cells (mean = 29.1 degrees/s) and relatively high for type 1 cells (mean = 70.3 degrees/s). Cut-off velocities for type 2 cells (mean = 61.2 degrees/s) were slightly lower than for type 1 cells. Type 1 cells had relatively short afferent (ON and OX) latencies, fast afferent conduction velocities, and short antidromic (cortex) latencies; type 2 cells had intermediate afferent and antidromic latencies and intermediate afferent conduction velocities; and type 3 cells had relatively long afferent and antidromic latencies and slow afferent conduction velocities. The receptive field center diameters were similar for type 1 (4.2-25.5 degrees; mean = 11.4 degrees) and type 3 cells (2.8-23.7 degrees; mean = 11.0 degrees), whereas the receptive field centers for type 2 cells (2.9-15.1 degrees; mean = 6.9 degrees) were significantly smaller. The majority of type 1 (66.7%) and type 3 cells (63.3%) had on-center receptive fields, whereas the proportion of on-center fields was even greater for type 2 cells (83.0%). Only a few of the cells encountered in the opossum LGN (6%) had on-off receptive fields, and a portion of these could not be shown to be relay cells.(ABSTRACT TRUNCATED AT 400 WORDS)

W-like response properties of interlaminar zone cells in the lateral geniculate nucleus of a primate ( Galago crassicaudatus)

Recent anatomical studies have suggested that the cells located in the interlaminar zones (ILZs) of the primate dorsal lateral geniculate nucleus (LGN) relay visual information from the retina to the striate cortex in a manner similar to that of W-cells in the LGN of cat. In the present study, we examined this idea directly by recording the response properties of single cells localized to the ILZs in the prosimian primate, Galago crassicaudatus. The properties of the cells in the ILZs were found to be physiologically distinct from the X-like and Y-like properties of the parvocellular and magnocellular LGN layers. Moreover, the small cells located in the interlaminar zones were physiologically similar to the W-like cells found in the specialized small-celled koniocellular layers in these primates. As is the case with the koniocellular layer cells, the ILZ cells exhibited a broad range of properties which, as a group, were distinguished by the following characteristics: the ILZ cells had long latencies to stimulation of the optic chiasm (mean, 3.95 ms) and to antidromic stimulation from striate cortex (mean, 3.31 ms) and had relatively large receptive-field centers (mean, 1.79°). They also had low maintained discharge rates (5.5 spikes/s), relatively long response latencies to light (mean onset, 82 ms; peak, 112) and low peak firing rates (59 spikes/s). Few (25%) had standard receptive-field organization (ON-center, OFF-surround, or vice versa). Only 29% responded well to sine-wave gratings and all were influenced by non-visual (auditory and tactile) stimuli. Our results support the conclusion that cells located in the ILZs of primates, and perhaps all mammals, are part of a W-like system.

Linear signal transmission from prepotentials to cells in the macaque lateral geniculate nucleus.

Prepotentials preceding a neuronal action potential were recorded extracellularly in the lateral geniculate nucleus (LGN) of the macaque. Although prepotentials are found less frequently in the macaque than in the cat LGN, their electrical characteristics are similar, suggesting that they represent the arrival of impulses in a retinal afferent, as in the cat. The visual response properties of prepotentials and associated cells were similar under a variety of conditions, indicating that, apart from some response attenuation, little signal processing takes place in macaque LGN. A constant fraction of prepotentials above a threshold frequency gave rise to neuronal action potentials independent of the stimuli used, so that the frequency of cell action potentials was linearly related to the frequency of prepotentials. Since the maintained discharge rates of a cell and its prepotential always fell on the linear relation, the net responses of a cell and its prepotential to visual stimuli were approximately proportional to one another.

Immediate effects of total visual deafferentation on single unit activity in the visual cortex of freely behaving cats. I. Tonic excitability changes.

Effects of total visual deafferentation (eye enucleation) on maintained and evoked single unit activity in the cat visual cortex (VC) were studied. Three recording states were distinguished: resting arousal (RA), light sleep (LS) and deep sleep (DS). Two characteristic discharge patterns were noted in deafferented units. One was characterized by rhythmic synchronous firing of several units which was accompanied by high-amplitude slow wave bursts in the EEG (6–8/sec) during RA. Another was the abrupt occurrence of long-lasting spike trains which appeared unpredictively irrespective of vigilance levels. The overall discharge rate tended to decrease after deafferentation in both RA and DS while in LS tended to increase slightly. Three types of VC units were distinguished, as in intact ones, on the basis of the correlation function between the mean discharge rate in RA and its changes in LS. The relative incidence for the three types were different in the deafferented VC. The largest group (47.2%) had a sequence of mean discharge rate DS<RA<LS, while in intact animals the largest group (44.3%) had a sequence LS<RA<DS. For deafferented VC units the gradation of unit responsiveness to geniculate stimulation in the three states also paralleled that of the maintained discharge rate as in controls. A significant correlation between the mean discharge rate and the response latency to geniculate stimulation was not apparent following deafferentation due to loss of units with short latency responses and at the same time moderate discharge rates.

Properties of sustained and transient ganglion cells in the cat retina.

1. The functional basis for a sustained/transient classification of cat retinal ganglion cells has been strengthened by quantitative measurements of the sizes of the centre and surround components of receptive fields. Transient cells had larger surrounds than sustained; the distributions were non-overlapping. Although the distributions of centre sizes overlapped, the transient cells had very significantly larger centres on average.2. There were characteristic differences in area-threshold and annulus-threshold curves and shapes of responses to brief and long flashes of light, and relative differences in the nature of surround adaptation and maintained discharge rates.3. The distinction of sustained from transient units survived over a wide range of backgrounds including the two principle reorganizations of function: relative weakening of surround components at low background; Purkinje shift at high.4. Sustained and transient units were differentially distributed in the retina: there was an overwhelming preponderance of sustained units in the area centralis.5. It is proposed that the transient units are the so-called multidendritedeep cells and the sustained units are the variously styled small ganglion cells.

Maintained activity of monkey optic tract fibers and lateral geniculate nucleus cells

The maintained discharge rates (MDRs) of monkey optic tract fibers and LGN cells were recorded to a wide range of different light intensities. “On” center optic tract fibers increase their firing almost linearly over 8 log units of background lights, while “off” center units show the opposite pattern. “On” center LGN cells show increases in rate up to 10 −2 cd/m 2 and decreases thereafter; “off” center units showed decreases in MDR up to 10 −2 cd/m 2 and increases in rate thereafter. The MDRs of tract fibers and LGN cells differ at high luminance levels because of a stronger tonic antagonism from the LGN receptive field surround than from optic tract receptive field surrounds. These differences in surround antagonism can be demonstrated with transient stimuli also. Results further suggest that while optic tract fibers can accurately convey absolute luminance signals, LGN cells cannot. Results are discussed in terms of receptive field organization.

Maintained activity of lateral geniculate nucleus neurons as a function of background luminance

The maintained activity of single lateral geniculate nucleus (LGN) neurons was recorded from anesthetized cats as a function of background luminance. Neuronal activity was recorded at 5 luminance levels ranging from complete darkness to 25-ft L. Both transient and steady-state neuronal activity was recorded at each luminance level. In some LGN neurons increasing the background luminance caused a systematic rise in their firing rate; in other LGN neurons luminance increments caused a systematic depression of firing rate. The firing rate of a few LGN neurons was not affected systematically by successive luminance changes. Generally, immediately after each luminance change there was a pronounced effect on the maintained discharge of most LGN neurons; this effect decreased in magnitude as time lapsed, but was still clearly present 15 min after the luminance change. The above results suggest that at least some LGN neurons signal, to the visual cortex, the absolute level of ambient luminance by changes in their maintained discharge rate.

Effects of visual deafferentation on mesencephalic reticular activity in freely behaving cats

Effects of bilateral eye enucleation on maintained and evoked unit activity in the mesencephalic reticular formation (MRF) were studied in freely behaving cats. Recording of MRF units was carried out in the same way as before total visual deafferentation. Three recording conditions were classified: resting arousal, light sleep, and deep sleep. The MRF neurons initially decreased greatly, maintained discharge rates following visual deafferentation, and trended to recover their control level 1 month after deafferentation. Visual deafferentation brought about no notable change in the discharge pattern. Phasic enhancement of unit firing in deep sleep in association with bursts of phasic waves in the central visual system was still observable after deafferentation. Discharge rate and neural responsiveness of visually deafferented MRF neurons shifted between the three states in the same fashion as observed in intact neurons, i.e., light sleep → resting arousal → deep sleep in order of increasing sensitivity. After deafferentation there was much less incidence of MRF responses with longer latency although short-latency responses were well preserved. This phenomenon subsided with a lapse of time. The difference in response latency in the three states was detectable in deafferented MRF neurons. Immediately after visual deafferentation the extent of input convergence on MRF neurons tended to decrease in comparison with controls and this reduction sustained through 1 month. Visual deafferentation did not influence the sampling rate of nonresponding units in the MRF. No uniform relation between the location of units in the MRF and their maintained discharge rates was noted after visual deafferentation.

Scotopic and mesopic light adaptation in the cat's retina.

The effects of light adaptation on retinal on-center ganglion cell firing were measured in the scotopic-mesopic range (10−5–1 cd/m2). Using diffuse adapting stimuli, the maintained discharge rate of all on-center units increases with increasing adapting luminance up to 10−3–10−2 cd/m2. Above this luminance it levels off or decreases. The change of the maintained discharge rate with increasing adapting luminances was related to changes in the receptive field organization of the unit and to changes in retinal sensitivity. The sensitivity of the retina at the different adapting luminances was measured by determining the luminance ΔI of a test spot which elicited a constant criterion ganglion cell response. ΔI increased with increasing adapting luminance not proportional toI A (Weber's law) but proportional toI A n (n varying between 0.45 and 0.75). Correspondingly, by changing the adaptation level, the intensity: response curve of a ganglion cell for small center stimuli was displaced on the intensity-axis by less than the adapting luminance step. The intensity response-curves at the different adaptation levels were parallel and S-shaped in the semilogarithmic coordinates with a straight course over 1.5 log units. The intensity-response relation for small center stimuli was best fitted by a log-function at all adaptation levels. It is concluded that the ganglion cell itself is not involved in the adaptation mechanism. Adaptation and spatial summation of signals occur at different levels of the retina.

Continued activity of retinal neurons under the influence of strychnine and picrotoxin

Abstract The activity of 141 single fibers of the optic nerve was studied in seven cats. The units were classified according to their response to short light stimuli. The tonic activity was recorded on tape and from these data nonsequential interval histograms were computed. Three types of interval histograms could be obtained from the spontaneous activity in darkness: 1, Unimodal scew distributions. 2, Histograms with two peaks; the first peak was related to the repeated appearance of bursts in the discharge. 3, Multimodal distributions; the maxima occurring in regular distance of integral multiples of the first peak. During constant diffuse illumination of different intensity (0.88, 15.4 and 220 cd/m 2 ) three possible variations of the maintained activity were observed: 1, The activity was diminished in relation to the intensity of illumination. 2, The maintained discharge rate was not changed by illumination. 3, The activity was increased during illumination. On-center-neurons showed all three possible variations, in off-center-neurons reduction of activity dominated significantly. Therefore, off-center-neurons may be responsible for the overall reduction of retinal activity during continuous illumination. After intravenous injection of strychnine (0.25–0.8 mg/kg) the spontaneous activity was significantly increased. The same types of interval histograms as for the controls could be observed. The tonic and phasic reaction due to light stimulation were similar to those of units not being influenced by strychnine. A low dosage of picrotoxin (0.3 mg/kg) decreased the activity in some neurons; higher dosage (0.6–1.5 mg/kg) accelerated the discharge significantly. With respect to the types of interval histograms and the change of maintained activity during illumination no significant difference to the controls was found. Higher dosage of Picrotoxin increased inhibitory effect of short light stimulation: In some neurons this effect could be so strong, that the response of the unit appeared to be inversed. The inhibition was not extinguished by simultaneous application of strychnine and picrotoxin. According to these results the existence of three different mechanisms of inhibition in the retina may be assumed.