Off-center Neurons Research Articles

From Receptive to Perceptive Fields: Size-Dependent Asymmetries in Both Negative Afterimages and Subcortical On and Off Post-Stimulus Responses.

Negative afterimages are perceptual phenomena that occur after physical stimuli disappear from sight. Their origin is linked to transient post-stimulus responses of visual neurons. The receptive fields (RFs) of these subcortical ON- and OFF-center neurons exhibit antagonistic interactions between central and surrounding visual space, resulting in selectivity for stimulus polarity and size. These two features are closely intertwined, yet their relationship to negative afterimage perception remains unknown. Here we tested whether size differentially affects the perception of bright and dark negative afterimages in humans of both sexes, and how this correlates with neural mechanisms in subcortical ON and OFF cells. Psychophysically, we found a size-dependent asymmetry whereby dark disks produce stronger and longer-lasting negative afterimages than bright disks of equal contrast at sizes >0.8°. Neurophysiological recordings from retinal and relay cells in female cat dorsal lateral geniculate nucleus showed that subcortical ON cells exhibited stronger sustained post-stimulus responses to dark disks, than OFF cells to bright disks, at sizes >1°. These sizes agree with the emergence of center-surround antagonism, revealing stronger suppression to opposite-polarity stimuli for OFF versus ON cells, particularly in dorsal lateral geniculate nucleus. Using a network-based retino-geniculate model, we confirmed stronger antagonism and temporal transience for OFF-cell post-stimulus rebound responses. A V1 population model demonstrated that both strength and duration asymmetries can be propagated to downstream cortical areas. Our results demonstrate how size-dependent antagonism impacts both the neuronal post-stimulus response and the resulting afterimage percepts, thereby supporting the idea of perceptual RFs reflecting the underlying neuronal RF organization of single cells.SIGNIFICANCE STATEMENT Visual illusions occur when sensory inputs and perceptual outcomes do not match, and provide a valuable tool to understand transformations from neural to perceptual responses. A classic example are negative afterimages that remain visible after a stimulus is removed from view. Such perceptions are linked to responses in early visual neurons, yet the details remain poorly understood. Combining human psychophysics, neurophysiological recordings in cats and retino-thalamo-cortical computational modeling, our study reveals how stimulus size and the receptive-field structure of subcortical ON and OFF cells contributes to the parallel asymmetries between neural and perceptual responses to bright versus dark afterimages. Thus, this work provides a deeper link from the underlying neural mechanisms to the resultant perceptual outcomes.

Stimulus Contrast Affects Spatial Integration in the Lateral Geniculate Nucleus of Macaque Monkeys.

Gain-control mechanisms adjust neuronal responses to accommodate the wide range of stimulus conditions in the natural environment. Contrast gain control and extraclassical surround suppression are two manifestations of gain control that govern the responses of neurons in the early visual system. Understanding how these two forms of gain control interact has important implications for the detection and discrimination of stimuli across a range of contrast conditions. Here, we report that stimulus contrast affects spatial integration in the lateral geniculate nucleus of alert macaque monkeys (male and female), whereby neurons exhibit a reduction in the strength of extraclassical surround suppression and an expansion in the preferred stimulus size with low-contrast stimuli compared with high-contrast stimuli. Effects were greater for magnocellular neurons than for parvocellular neurons, indicating stream-specific interactions between stimulus contrast and stimulus size. Within the magnocellular pathway, contrast-dependent effects were comparable for ON-center and OFF-center neurons, despite ON neurons having larger receptive fields, less pronounced surround suppression, and more pronounced contrast gain control than OFF neurons. Together, these findings suggest that the parallel streams delivering visual information from retina to primary visual cortex, serve not only to broaden the range of signals delivered to cortex, but also to provide a substrate for differential interactions between stimulus contrast and stimulus size that may serve to improve stimulus detection and stimulus discrimination under pathway-specific lower and higher contrast conditions, respectively.SIGNIFICANCE STATEMENT Stimulus contrast is a salient feature of visual scenes. Here we examine the influence of stimulus contrast on spatial integration in the lateral geniculate nucleus (LGN). Our results demonstrate that increases in contrast generally increase extraclassical suppression and decrease the size of optimal stimuli, indicating a reduction in the extent of visual space from which LGN neurons integrate signals. Differences between magnocellular and parvocellular neurons are noteworthy and further demonstrate that the feedforward parallel pathways to cortex increase the range of information conveyed for downstream cortical processing, a range broadened by diversity in the ON and OFF pathways. These results have important implications for more complex visual processing that underly the detection and discrimination of stimuli under varying natural conditions.

The functional asymmetry of ON and OFF channels in the perception of contrast.

To fully understand the relationship between perception and single neural responses, one should take into consideration the early stages of sensory processing. Few studies, however, have directly examined the neural underpinning of visual perception in the lateral geniculate nucleus (LGN), only one synapse away from the retina. In this study we recorded from LGN parvocellular (P) ON-center and OFF-center neurons while monkeys either passively viewed or actively detected a full range of contrasts. We found that OFF neurons were more sensitive in detecting negative contrasts than ON neurons were in detecting positive contrasts. Also, OFF neurons had higher spontaneous activities, higher peak response amplitudes, and were more sustained than ON neurons in their contrast responses. Puzzlingly, OFF neurons failed to show any significant correlations with the monkeys' perceptual choices, despite their greater contrast sensitivities. If, however, choice probabilities were calculated from interspike intervals instead of spike counts (thus taking into account the higher firing rates of OFF neurons), OFF neurons but not ON neurons were significantly correlated with behavioral choices. Taken together, these results demonstrate in awake, behaving animals that: 1) the ON and OFF pathways do not simply mirror each other in their functionality but instead carry qualitatively different types of information, and 2) the responses of ON and OFF neurons can be correlated with perceptual choices even in the absence of physical stimuli and interneuronal correlations.

The Functional Asymmetry of ON and OFF Channels in the Lateral Geniculate Nucleus (LGN) during a Perceptual Decision Task

Early behavioral studies indicated that separate psychophysical channels underlie the perception of light (i.e. contrast increments) and dark (i.e. contrast decrements) (Anstis 1967; De Valois 1977; Krauskopf 1980; Cavanaugh and Anstis 1986; Chichilnisky and Wandell 1996; Westheimer 2007), and that human observers are faster and more accurate in perceiving contrast decrements rather than increments (Blackwell 1946; Short 1966; Bowen et al. 1989, 1992; Chan and Tyler 1992; Kontsevich and Tyler 1999; but see Komban et al. 2011). In the early visual system, the segregation of ON and OFF channels starts at bipolar cells (Nomura et al. 1994; Masu et al. 1995; Vardi and Morigawa 1997; Wassle 2004) and is preserved throughout the lateral geniculate nucleus (LGN) (Casagrande and Norton 1991; Westheimer 2007; Schiller 2010). These ON and OFF pathways were initially thought of as independent, parallel systems that mirror each other in their light/dark responses (Schiller 1992, 2010; Kremers et al. 1993; Benardete and Kaplan 1997, 1999). Several physiological studies, however, have revealed significant functional asymmetries between the two systems. On the one hand, it was reported that the ON pathway could signal both contrast increments (i.e. via increase in spikes) and decrements (via decrease in spikes) at low contrast, whereas the OFF pathway could only signal contrast decrements (Chichilnisky and Kalmar 2002). On the other hand, it was reported that over a great range of luminance levels, the contrast responses of OFF neurons were stronger and more linear than those of ON neurons (Kremkow et al. 2014), and, under scotopic conditions, OFF neurons responded to much higher temporal frequencies than ON neurons (ArmstrongGold and Rieke 2003; Pandarinath et al. 2010). Additionally, projections from the OFF pathway were reported to dominate the cortical representation of central vision (Jin et al. 2008, 2011), and visual responses to dark stimuli were reported to dominate the superficial layers of V1 (Yeh et al. 2009; Xing et al. 2010). How do the ON and OFF pathways contribute to the perception of light and dark? How can the physiological asymmetry of the ON/OFF pathways be linked to the psychophysical asymmetry of light and dark perception? To answer these questions, we recorded from LGN Parvocellular (P) ON-Center and OFF-Center neurons in awake monkeys while they passively viewed (i.e. simple fixation) and actively detected (i.e. two-alternative, forced-choice detection) contrast increment and decrement changes. Neurometric Sensitivity

Rapid Plasticity of Visual Responses in the Adult Lateral Geniculate Nucleus

Compared to the developing visual system, where neuronal plasticity has been well characterized at multiple levels, little is known about plasticity in the adult, particularly within subcortical structures. We made intraocular injections of 2-amino-4-phosphonobutyric acid (APB) in adult cats to block visual responses in On-center retinal ganglion cells and examined the consequences on visual responses in the lateral geniculate nucleus (LGN) of the thalamus. In contrast to current views of retinogeniculate organization, which hold that On-center LGN neurons should become silent with APB, we find that ∼50% of On-center neurons rapidly develop Off-center responses. The time course of these emergent responses and the actions of APB in the retina indicate the plasticity occurs within the LGN. These results suggest there is greater divergence of retinogeniculate connections than previously recognized and that functionally silent, nonspecific retinal inputs can serve as a substrate for rapid plasticity in the adult.

The effect of dark adaptation on the responses of cat retinal ganglion cells to eyeball deformation

Eyeball deformation in total darkness leads to an activation of on-center ganglion cells and an inhibition of off-center ganglion cells. After “deformation off” most on-center ganglion cell activity decreased slowly to the normal spontaneous dark level, while in off-center ganglion cells some returned according to an exponential function to normal dark activity, while others had a transient postinhibitory activation period. In general, the response type of latency class I and latency class II neurons was the same. Dark adaptation of 30–45 min duration only changed this neuronal response pattern slightly, if at all. A detailed statistical analysis is provided for the four classes of retinal ganglion cells recorded: latency class I on-center and off-center neurons and latency class II on-center and off-center neurons. The missing effects of dark adaptation on neuronal responses evoked by eyeball deformation are explained by three possible models. The more plausible one assumes that horizontal cells are depolarized by retinal stretch. Their interaction with cone on-bipolars or cone off-bipolars is fairly independent of photoreceptor adaptation or transmitter release at the cone pedicles and is still effective when all molecular receptor sites at cone/bipolar cell synapses are occupied during scotopic states of dark adaptation. In psychophysical experiments (two subjects), as in the neuronal responses, we also could not find any indication that the “pressure phosphenes” evoked by lateral eyeball indentation are altered during dark adaptation.

Segregation of on- and off-center afferents in mink visual cortex.

In the lateral geniculate nucleus of the mink, on-center and off-center neurons occupy separate layers [LeVay, S. & McConnell, S.K. (1982) Nature (London) 300, 350-351]. To study the mode of termination of geniculate afferents in area 17, we recorded from their terminal arborizations in layer IV after the destruction of cortical neurons by injection of kainic acid. At the majority of recording sites, multifiber responses were entirely or predominantly of one type: on-center or off-center. Responses obtained during perpendicular penetrations showed the same preferred sign of contrast throughout the thickness of layer IV. During tangential penetrations through the layer, we encountered sequences of on- and off-center activity separated by stretches of mixed responses. We conclude that on- and off-center afferents terminate in separate, alternating patches that occupy the full thickness of layer IV. These coexist with another set of patches in which the same afferents are segregated by eye of origin.

Temporal frequency characteristics of spatial interaction in human vision.

In psychophyscial experiments the bright-dark contrast effects observed in a steady test-field were measured as a function of the temporal frequency of an inducing-field modulated symmetrically about the test-field luminance. The frequency-contrast functions obtained from these measurements were interpreted as reflecting the temporal frequency characteristics of the lateral pathways within the B and D systems (the on-center and off-center neurons) in human vision. Psychophysical evidence is further presented that the lateral neural pathways have lower temporal cut-off frequencies than the "straight-through" pathways. The results are discussed in terms of the frequency characteristics of the center and surround of the receptive fields of on-center and off-center neurons. It is doubtful, however, whether the psychophysical results can be fully explained by the properties of the single-unit receptive field mechanisms.

The shift-effect in retinal ganglion cells of the rhesus monkey.

McIlwain's periphery effect [7, 1, 5, 6] and the shift-effect of Krfiger and Fischer [2, 4, 6] are elicited by moving patterns, projected outside and at a long distance from the receptive field of retinal ganglion cells. The shift effects studied in the cat's retinal ganglion cells [4, 6] and lateral geniculate neurons [2] point to a visual information processing which is different from that performed by the classical receptive field mechanism and requires long range neuronal connections in the retina. So far, these effects were reported for cats only. This research report gives the first description of the shift effect in the monkey's retinal neurons. In the present experiments the monkeys faced a projection screen. Artificial pupils 5 mm in diameter and contact lenses checked by retinoscopy were used. During the recordings both eyes were open. Extracellular spikes from axons of retinal ganghon cells have been recorded in the optic tract using the same surgical preparation, anesthetics, muscle relaxation and optical stimulation as in cats (for detailed description see [4]). 79 optic tract fibres have been recorded from two adult rhesus monkeys and in 63 of these the receptive fields could be located and classified as 40 on-center and 23 off-center neurons: In all these 63 neurons we observed phasic discharges following grating shifts in the periphery. These responses were elicited by eae displacement of a grating well outside the receptive field. An example is shown in Fig. la. The receptive field of this on-center neuron was adequately illuminated by a steady bright spot of 1.8 deg. in diameter separated by a large dark surround from the moving grating as indicated at the right of Fig. la. (Stationary area 40 deg. in diameter; amplitude of displacement, 2 deg. ; velocity of displacement, 400 deg./see). Under these conditions the response latencies were about 50 ms. In most neurons an adequate steady visual stimulus was necessary to obtain the shift-effect, i.e7 bright spots of suitably chosen size against a dark surround for on-center neurons, dark spots against a bright surround for off-center neurons, provided a steady background activation on which the shift response appears. In a homogeneous field of brightness or darkness the shift-effect was almost absent or at least weak. Reversing brightness and darkness in the stationary field illumination ("inadequate illumination") abolished the shift-effect in all units tested and in two neurons we observed a weak inhibitory shift-effect. Thirteen neurons whose receptive fields could not be located and which therefore could not be activated showed no shift-effect under homogeneous illumination. This does not prove, however, that they had no shift-effect.

The effect of area and intensity on the response of cat retinal ganglion cells to brief light flashes.

1. The action potentials of single optic tract axons were recorded in cats anaesthetized by pentobarbital. The receptive fields were stimulated with brief light flashes (20 or 25 msec) of variable area (0.18 less than or equal to A less than or equal to 11.8 degrees) and stimulus intensity (0.36 less than or equal to I less than or equal to 360 cd-m-2). 2. Retinal on-center and off-center neurons responded in one of two ways to brief light flashes: oscillatory responses (O-response) to certain stimulus combinations or non-oscillatory responses (N-response) for all stimulus combinations. 3. The O-respnse of on-center neurons was characterized in its PST-histogram by up to five peaks. Neurons exhibiting an O-response at medium and/or strong stimulus intensities responded at low AxI-values with an N-response. Other neurons, however, exhibited for all stimulus combinations an N-response, in which the instantaneous neuronal impulse rate decreased after an initial peak approximately expronentially with time. 4. The O-response of off-center neurons exhibited with medium AxI-values up to four peaks in the PST-histograms. N-type off-center neurons had at low and medium AxI-values a short primary inhibition period which was followed by a fast rising and slowly decaying activity period. 5. Ricco's law was only valid for small AxI-values not more than one 10log unit above threshold. For higher stimulus values (especially in the O-response) an increment of the stimulus intensity was usually more effective than an equal increment of the stimulus area. 6. On-center and off-center neurons located in about the same region of the retina had a strong tendency to discharge alternately with each other. 7. A superposition model is proposed as an explanation of the experimental data. 8. Possible correlations between the neurophysiological findings and the fast oscillatory after-images seen in corresponding psychophysical experiments are discussed.

Symmetry between the visual B- and D-systems and equivalence of center and surround: studies of light increment and decrement in retinal and geniculate neurons of the cat.

The dual center surround organization of retinal and geniculate neurons in two antagonistic subsystems B and D, having on-center or off-center receptive fields and signalling brightness or darkness respectively, has been studied by local light increments and decrements. Intensity response functions obtained by the introduction and withdrawal of small center spots either brighter or darker than a common homogeneous field are similar in a given neuron, but the phasic responses are stronger in on-center neurons than in off-center neurons. Center size increments and decrements, however, lead to equal excitations in the B- and D-system, respectively, provided that both luminance steps start from the same level and are of equal size on a linear scale. Decrementing and incrementing the surrounding luminance of the same optimal center spots lead to equal surround responses in the two subsystems if the two luminance steps terminate at the same level. This lateral activation is elicited by light decrement in the B-system and by light increment in the D-system. Center and surround responses within a given subsystem are of comparable amplitude, but generally slightly stronger responses are elicited by optimal center increments (decrements) than by the equivalent surround decrements (increments) which lead to the same spatial contrast for B-(D-) neurons. The symmetry relations between the B- and D-system and the equivalence relations between center and surround in each subsystem hold for retinal and geniculate neurons. The difference between center and surround response latencies is about 9 ms in both subsystems at the retinal and 14 ms at the geniculate level. Stimulus response functions of on- and off-center neurons are unified on the basis of linear relative luminance scales.

Brightness and darkness enhancement during flicker: Perceptual correlates of neuronal B- and D-systems in human vision

In psychophysical experiments the brightness and darkness judgements made by human observers to the light and dark phases of suprathreshold square-wave flicker were measured by means of a matching technique and compared across variations in flicker frequency, mean luminance and the depth of luminance modulation of the flicker field. The results are discussed within the context of Jung's theory of the B and D systems, and compared to the results from electrophysiological flicker experiments performed on single on-center and off-center units in the visual system of the cat. In general, where parallel experiments make comparisons possible, good agreement was found between neuronal recordings and perceptual data. A notable exception, however, was that in man the darkness perception corresponding to responses of the neuronal D system (the off-center neurons) seem more strongly affected by variations in flicker frequency than brightness perception corresponding to the neuronal B system (the on-center neurons) while neuronal data from the cat indicate a tendency in the opposite direction. Possible sources of this discrepancy are discussed.

Quantitative aspects of the shift-effect in cat retinal ganglion cells

A sudden displacement of a pattern, even when far away from the classical border of a concentrically organized receptive field, elicits a transient excitatory response in on- and off-center retinal ganglion cells (shift-effect). Peak response and latency of the shift-effect have been studied under various stimulus conditions in on- and off-center ganglion cells. Increasing the retinal distance between the site of stimulation and receptive field yields responses of increasing latency corresponding to an intraretinal conduction velocity of about 0.35 m/sec. Beyond a distance of 20° response amplitudes decrease steadily in on- and off-center neurons. Shift amplitude and contrast of the shifting grating have almost no influence above a threshold of 30 min of arc and 10–20% modulation respectively (all-or-none behavior). Shifts of decreasing velocity (600-10°/sec) increase the latency. The response magnitude decreases only at very slow movements. Other properties of the shift-effect are briefly reported. Possible functional significances in relation to eye movements and brightness perception and the retinal pathway of the shift-effect are discussed.

Strong periphery effect in cat retinal ganglion cells. Excitatory responses in ON- and OFF- center neurones to single grid displacements.

Strong periphery effect in cat retinal ganglion cells. Excitatory responses in ON- and OFF- center neurones to single grid displacements.

Dependence of surround effects on receptive field center illumination in cat retinal ganglion cells.

1. 57 on- and off-center units in the cat's optic tract were studied after 30 min of dark adaptation by stimulating the receptive field periphery with flashing annuli. 49 reacted with center-like responses, 4 did not respond and 4 offcenter cells showed weak antagonistic surround responses. 54 showed antagonistic surround responses when a steady adapting spot of suitably chosen luminance was directed on to the center of the fields. 2. With equal stimulation of the surround the total flux of the steady center spot determined the condition of reversal from center to surround responses. 3. Under the condition of reversal the on-going firing rate was also affected. Reversal of responses was also observed with short stimulus durations (≥ 50 msec). 4. The recovery time from reversed responses to center-like responses was less than I min after darkening the center again. 5. With increasing center spot luminance, the latency of the center-type excitatory responses from on- and off-center neurones remained approximately constant, whereas the latency of the surround excitatory response was decreased. 6. Neuronal mechanisms are considered which give rise to excitatory responses in total darkness and inhibitory responses if neighbouring parts of the retina are light adapted. The equivalence of light adaptation and lateral inhibition as well as the effects of stray light are discussed.

Spatial summation of excitation and inhibition in receptive fields of neurons in the lateral geniculate body of the cat and the influence of visual deprivation.

1. The spatial summation in receptive fields of neurons in the lateral geniculate body (LGB) of the cat was measured with continuously changing round light stimuli which were modulated sinusoidally. The mean impulse frequencies and the position of the phasic discharge in relation to the stimuli were investigated as a function of the size of stimuli. 2. The on-center neurons showed small receptive field centers and marked inhibition by the surrounding at frequencies from 1.5 to 20 cps. In the off-center neurons the on-excitation from the periphery exceeded the on-inhibition from the center when the entire receptive field was stimulated. Characteristic shifting of the discharges was ascertained. 3. Neurons fed by the visually deprived eye showed reduced effects of the surrounding. 4. The mechanisms of inhibition that can be derived from the results and the changes in the LGB during visual deprivation are discussed.

Maintained activity in the lateral geniculate body of the cat and the effects of visual deprivation.

Experiments were made with normal and visually deprived cats. The neuronal activity was measured under constant stimulus conditions with micro-electrodes from single neurons in the optic radiation and the lateral geniculate body (LGB). Interval-histograms, joint-interval-histograms and transition probabilities were analysed and characteristic statistical values were computed. The results are first presented for normal neurons. For off-center neurons and on-center neurons under equal stimulus conditions differences between neurons from the normal eye and neurons fed by the visually deprived eye could be established: for the latter the mean intervals were shorter, the fluctuations smaller, and the shape of histograms showed a decreased slope of decay. The serial dependences did not display diverse results. The findings allow cautious statements concerning the functional changes in the LGB of visually deprived cats.

Mathematical formulation for stimulus-response relationships of retinal ganglion cells

1. In order to describe stimulus-response relationships for retinal ganglion cells a mathematical formulation is presented on the basis of experimental data and some simple assumptions. This concept involves two mechanisms — an excitatory and an inhibitory one — both extending over the whole receptive field. 2. The following formula $$S\left( I \right) = \frac{{c{\text{ }}{I \mathord{\left/ {\vphantom {I {I_S - 1}}} \right. \kern-\nulldelimiterspace} {I_S - 1}}}}{{1 + a{\text{ }}a{I \mathord{\left/ {\vphantom {I {I_S + 1}}} \right. \kern-\nulldelimiterspace} {I_S + 1}}}}$$ is used for description of the excitatory responses (on-response for on-center neurones, off-response for off-center neurones). The value ofa depends on the area of light stimulation,c is a constant;I/ISdesignates the ratio of stimulus brightness to threshold brightness. 3. Thresholds and suprathreshold response curves for concentric stimuli are described quantitatively by the formula. The experimental data were obtained by computer counts of spike discharges during 200 and 500 ms following the exciting light increment or decrement. 4. Ricco's, Weber-Fechner's and Stevens's laws are included in our conception as approximations of neuronal summations. These relations are limited to certain stimulus ranges; in contrast the above response function saturates at certain maximum discharge rates, as found by the S-shaped experimental curves; thus the formula is valid over the whole range of stimulation from threshold to very high intensities. 5. Thresholds as well as excitatory and inhibitory effects depend on background illumination and stimulus parameters. Therefore, center and surround size depend on these factors. 6. The formulation is set up for stationary responses and consequently does not describe time dependent characteristics. 7. Within these limits on-center and off-center neurones appear to have identical stimulus-response functions for adequate stimuli. Since in our experiments light increments and decrements were not symmetrical with respect to background illumination, a factor of 2 had to be introduced corresponding to the lower discharge rates of off-center-neurones.

Activity in the optic tract and lateral geniculate nucleus of th cat during the first moments of light adaptation in the scotopic region.

1. Short duration background luminances at scotopic levels suppress the spontaneous activity of off-center neurones of the cat's optic tract and activate on-center neurones, up to a maximal intensity limit after which the ongoing discharge in off-center neurones disappears and in on-center neurones is no longer influenced. Response patterns in the lateral geniculate nucleus (LGN) in off-center neurones are similar to the tract; activity in on-center neurones is poorly correlated with background luminance. 2. Transient or phasic response was measured for all types of neurones to a short, constant intensity stimulus given in the dark and at various adaptation luminances. For on-center neurones of both tract and LGN the phasic response was attenuated regularly over several log units of increasing background luminance. Off-center neurones are far weaker in phasic response rate than on-center neurones and are easily suppressed by background light. 3. Curves describing phasic response as a function of contrast between test light and adaptation luminance very probably reflect changes in the retinal “gain control”. Ongoing activity, while showing essential differences between the two antagonistic neuronal subsystems (off-center neurones or D-system and on-center or B-system), is clearly related to neither phasic response nor the retinal gain control. 4. Human subjects could easily identify differences in contrast between test light and background luminance. They are less successful in identifying background alone. 5. The findings are discussed regarding psychophysical results and some theories relating perception and nervous activity, and to the role of activation and inhibition in optic tract and geniculate.

Reciprocal lateral inhibition of on- and off-center neurones in the lateral geniculate body of the cat.

In the lateral geniculate body (LGB), intra- and quasi-intracellular records were done. With small light stimuli shone into different parts of the receptive field, EPSPs and IPSPs could be elicited. Stimulation of the exact center of an on-center cell produced a pure excitatory response, that of an off-center neurone pure inhibition. This response lasted throughout the stimulus. At light off, inhibition was elicited in on-center cells and excitation in off-center cells. A stimulus in the field periphery produced a mixed response with a small and short excitation followed by large inhibition in on-center cells, and a short inhibition followed by postsynaptic depolarization in off-center cells. At light off, on-center cells showed depolarization after a short polarizing phase, and off-center cells a broad polarization which interrupted the initial small excitation. The latencies of both the excitatory and inhibitory center responses at light on and off characteristic for the two types of neurones, were 20–30 msec shorter than the reversed responses elicited by stimulation of the receptive field surround.