Surround suppression by high spatial frequency stimuli in the cat primary visual cortex

Surround suppression is a phenomenon whereby stimulation of the extraclassical receptive field suppressively modulates the visual responses of neurons in the primary visual cortex (V1) (also known as area 17). It is known that surround suppression tunes to spatial frequencies (SFs) that are much lower and broader than the frequencies to which the classical receptive field tunes. In this study, we tested the effects of varying SFs on surround suppression by using a circular sinusoidal grating patch that covered both the classical receptive field and the extraclassical receptive field. Using area-summation tuning curves, we found high-SF-tuned surround suppression in the cat V1. This high-SF-tuned surround suppression causes the SF tuning to shift to low SF for large stimuli. By simulating a model neuron lacking a suppressive surround mechanism, we confirmed that these preferred SF shifts do not occur in the absence of surround suppression. We surmise that the high-SF-tuned suppression, which shifts the preferred SF according to size, functionally contributes to the scale-invariant processing of visual images in V1.     

Contrast independence of cardinal preference: stable oblique effect in orientation maps of ferret visual cortex

The oblique effect was first described as enhanced detection and discrimination of cardinal orientations compared with oblique orientations. Such biases in visual processing are believed to originate from a functional adaptation to environmental statistics dominated by cardinal contours. At the neuronal level, the oblique orientation effect corresponds to the numerical overrepresentation and narrower tuning bandwidths of cortical neurons representing the cardinal axes. The anisotropic distribution of orientation preferences over large cortical regions was revealed with optical imaging, providing further evidence for the cortical oblique effect in several mammalian species. Our present study explores whether the dominant representation of cardinal contours persists at different stimulus contrasts. Performing intrinsic optical imaging in the ferret visual cortex and presenting drifting gratings at various orientations and contrasts (100%, 30% and 10%), we found that the overrepresentation of vertical and horizontal contours was invariant across stimulus contrasts. In addition, the responses to cardinal orientations were also more robust and evoked larger modulation depths than responses to oblique orientations. We conclude that orientation maps remain constant across the full range of contrast levels down to detection thresholds. Thus, a stable layout of the functional architecture dedicated to processing oriented edges seems to reflect a fundamental coding strategy of the early visual cortex.     

Pattern motion is present in V1 of awake but not anaesthetized monkeys

We compared responses of neurons, recorded in striate cortex (area V1) of awake, fixating monkeys, to a single drifting grating with those to a 'plaid' pattern comprised of two superimposed drifting gratings separated in orientation by 90 degrees. Five out of 54 (9%) of V1 direction selective neurons responded to the direction of motion of the whole pattern [pattern motion (PM) selectivity]. Tuning curves for plaid stimuli were similar in both optimum direction and width of tuning to those for single gratings. Twenty nine out of 54 (54%) responded simply to the motion of individual orientated gratings within the pattern [component motion (CM) selectivity]. The remaining 37% (20/54) neurons were unclassified. In control experiments, 39 direction selective neurons were recorded in area V1 of anaesthetized monkey and cats. Unlike area V1 in behaving monkeys, none of these neurons exhibited PM selectivity to the drifting plaids. Twenty eight out of 39 (72%) of them responded to the direction of the component gratings and were classified as CM selectivity. Our results indicate that although most V1 neurons are CM selective, as described in anaesthetized animals, a subpopulation is clearly PM selective in behaving monkeys, reflecting integration of locally derived motion signals. Neurons in V1 therefore carry signals that may contribute to pattern motion processing and perception. This perceptual interpretation in V1 might depend much more critically on information integration mechanisms that only function properly in awake, perceiving animals.     

How do functional maps in primary visual cortex vary with eccentricity?

It is important to understand whether functional maps of primary visual cortex (V1) are organized differently at the representation of different eccentricities. By using optical imaging of intrinsic signals, we compared the maps of orientation and spatial frequency (SF) preference between central (0-3 degrees ) and paracentral (4-8 degrees ) V1 in the prosimian bush baby (Otolemur garnetti). No differences related to eccentricity were found for orientation selectivity or magnitude between central and paracentral V1. We found, however, that cardinal orientations were overrepresented in central but not in paracentral V1 and that isoorientation domain size tended to be smaller in the central representation. We demonstrated that spatial frequency was represented continuously across V1, and that the map of SF preference exhibited eccentricity-dependent variations, with more territory devoted to higher SFs in central than in paracentral V1. Although there were no spatial relationships between orientation domains and cytochrome oxidase (CO) blobs or interblobs, CO blobs tended to prefer lower SFs than interblobs. Taken together with previous research, our data indicate that functional domains in V1 show eccentricity-related differences in organization and also support the idea that different maps (with or without specific geometrical relationships) are organized for adequate coverage of each feature in visual space.     

Spatiotemporal frequency and speed tuning in the owl visual wulst

The avian visual wulst is hodologically equivalent to the mammalian primary visual cortex (V1). In contrast to most birds, owls have a massive visual wulst, which shares striking functional similarities with V1. To provide a better understanding of how motion is processed within this area, we used sinusoidal gratings to characterize the spatiotemporal frequency and speed tuning profiles of 131 neurones recorded from awake burrowing owls. Cells were found to be clearly tuned to both spatial and temporal frequencies, and in a way that is similar to what has been reported in the striate cortex of primates and carnivores. Our results also suggest the presence of spatial frequency tuning domains in the wulst. Speed tuning was assessed by several methods devised to measure the degree of dependence between spatial and temporal frequency tuning. Although many neurones were found to be independently tuned, a significant proportion of cells showed at least some degree of dependence, compatible with the idea that some kind of initial transformation towards an explicit representation of speed is being carried out by the owl wulst. Interestingly, under certain constraints, a higher incidence of spatial frequency-invariant speed tuned profiles was obtained by combining our experimentally measured responses using a recent cortical model of speed tuning. Overall, our findings reinforce the notion that, like V1, the owl wulst is an important initial stage for motion processing, a function that is usually attributed to areas of the tectofugal pathway in lateral-eyed birds.     

Temporal properties of spatial frequency tuning of surround suppression in the primary visual cortex and the lateral geniculate nucleus of the cat

In primary visual cortex (V1) neurons, a stimulus placed in the extraclassical receptive field suppresses the response to a stimulus within the classical receptive field (CRF), a phenomenon referred to as surround suppression. The aim of the present study was to elucidate the mechanisms of surround suppression in V1. Using stationary‐flashed sinusoidal grating as stimuli, we observed temporal changes of surround suppression in V1 and the lateral geniculate nucleus (LGN) and of the response to CRF stimulation in V1. The spatial frequency (SF) tuning of surround suppression in V1 neurons changed over time after the stimulus onset. In the early phase (< 50 ms), the SF tuning was low‐pass, but later became band‐pass that tuned to the optimal SF in response to CRF stimulation. On the other hand, the SF tuning of CRF responses in V1 was band‐pass throughout the response time whereas the SF peak shifted slightly toward high SF. Thus, SF tuning properties of the CRF response dissociated from that of surround suppression in V1 only in the early phase. We also confirmed that the temporal changes of the SF tuning of surround suppression in the LGN occurred in the same direction as surround suppression in V1, but the shift from low‐pass to band‐pass SF tuning started later than that in V1. From these results, we suggest that subcortical mechanisms contribute to early surround suppression in V1, whereas cortical mechanisms contribute to late surround suppression.     

Functional Differentiation of Mouse Visual Cortical Areas Depends upon Early Binocular Experience

The mammalian visual cortex contains multiple retinotopically defined areas that process distinct features of the visual scene. Little is known about what guides the functional differentiation of visual cortical areas during development. Recent studies in mice have revealed that visual input from the two eyes provides spatiotemporally distinct signals to primary visual cortex (V1), such that contralateral eye-dominated V1 neurons respond to higher spatial frequencies than ipsilateral eye-dominated neurons. To test whether binocular visual input drives the differentiation of visual cortical areas, we used two-photon calcium imaging to characterize the effects of juvenile monocular deprivation (MD) on the responses of neurons in V1 and two higher visual areas, LM (lateromedial) and PM (posteromedial). In adult mice of either sex, we find that MD prevents the emergence of distinct spatiotemporal tuning in V1, LM, and PM. We also find that, within each of these areas, MD reorganizes the distinct spatiotemporal tuning properties driven by the two eyes. Moreover, we find a relationship between speed tuning and ocular dominance in all three areas that MD preferentially disrupts in V1, but not in LM or PM. Together, these results reveal that balanced binocular vision during development is essential for driving the functional differentiation of visual cortical areas. The higher visual areas of mouse visual cortex may provide a useful platform for investigating the experience-dependent mechanisms that set up the specialized processing within neocortical areas during postnatal development.SIGNIFICANCE STATEMENT Little is known about the factors guiding the emergence of functionally distinct areas in the brain. Using in vivo Ca²⁺ imaging, we recorded visually evoked activity from cells in V1 and higher visual areas LM (lateromedial) and PM (posteromedial) of mice. Neurons in these areas normally display distinct spatiotemporal tuning properties. We found that depriving one eye of normal input during development prevents the functional differentiation of visual areas. Deprivation did not disrupt the degree of speed tuning, a property thought to emerge in higher visual areas. Thus, some properties of visual cortical neurons are shaped by binocular experience, while others are resistant. Our study uncovers the fundamental role of binocular experience in the formation of distinct areas in visual cortex.     

Lateral geniculate neuron responses to drifting sine-wave gratings in rabbits

Neurons of the lateral geniculate body in rabbits were excited with drifting sine-wave gratings. Rabbits were anesthetized and paralyzed under conventional methods to record action potentials of single cells using tungsten in glass microelectrodes. All classes of geniculate cells responded in a modulatory pattern. It appears that the unmodulatory pattern typical of complex cell types of the cortex is extremely infrequent or absent. In the spatial domain most cells are low pass and bandpass. Only one unit was high pass. In the temporal domain lowpass and bandpass cells were the most frequently recorded. Four geniculate cells were high pass. It appears, therefore, that neurons of rabbits' geniculate are tuned over spatial and temporal frequencies of sine-wave gratings. The comparison with cortical recordings revealed that geniculate cells are more broadly tuned than cortical neurons. This study suggests that the rabbit's visual system is sensitive to gratings. However cells respond optimally to lower values, e.g., broader gratings, than neurons of frontalized eye animals.     

Multiple adaptable mechanisms early in the primate visual pathway

We describe experiments that isolate and characterize multiple adaptable mechanisms that influence responses of orientation-selective neurons in primary visual cortex (V1) of anesthetized macaque (Macaca fascicularis). The results suggest that three adaptable stages of machinery shape neural responses in V1: a broadly tuned early stage and a spatio-temporally tuned later stage, both of which provide excitatory input, and a normalization pool that is also broadly tuned. The early stage and the normalization pool are revealed by adapting gratings that themselves fail to evoke a response from the neuron: either low temporal frequency gratings at the null orientation or gratings of any orientation drifting at high temporal frequencies. When effective, adapting stimuli that altered the sensitivity of these two mechanisms caused reductions of contrast gain and often brought about a paradoxical increase in response gain due to a relatively greater desensitization of the normalization pool. The tuned mechanism is desensitized only by stimuli well matched to a neuron's receptive field. We could thus infer desensitization of the tuned mechanism by comparing effects obtained with adapting gratings of preferred and null orientation modulated at low temporal frequencies.     

Drifting grating stimulation reveals particular activation properties of visual neurons in the caudate nucleus

The role of the caudate nucleus (CN) in motor control has been widely studied. Less attention has been paid to the dynamics of visual feedback in motor actions, which is a relevant function of the basal ganglia during the control of eye and body movements. We therefore set out to analyse the visual information processing of neurons in the feline CN. Extracellular single-unit recordings were performed in the CN, where the neuronal responses to drifting gratings of various spatial and temporal frequencies were recorded. The responses of the CN neurons were modulated by the temporal frequency of the grating. The CN units responded optimally to gratings of low spatial frequencies and exhibited low spatial resolution and fine spatial frequency tuning. By contrast, the CN neurons preferred high temporal frequencies, and exhibited high temporal resolution and fine temporal frequency tuning. The spatial and temporal visual properties of the CN neurons enable them to act as spatiotemporal filters. These properties are similar to those observed in certain feline extrageniculate visual structures, i.e. in the superior colliculus, the suprageniculate nucleus and the anterior ectosylvian cortex, but differ strongly from those of the primary visual cortex and the lateral geniculate nucleus. Accordingly, our results suggest a functional relationship of the CN to the extrageniculate tecto-thalamo-cortical system. This system of the mammalian brain may be involved in motion detection, especially in velocity analysis of moving objects, facilitating the detection of changes during the animal's movement.     

Network‐selectivity and stimulus‐discrimination in the primary visual cortex: cell‐assembly dynamics

Visual neurons coordinate their responses in relation to the stimulus; however, the complex interplay between a stimulus and the functional dynamics of an assembly still eludes neuroscientists. To this aim, we recorded cell assemblies from multi‐electrodes in the primary visual cortex of anaesthetized cats in response to randomly presented sine‐wave drifting gratings whose orientation tilted in 22.5° steps. Cross‐correlograms revealed the functional connections at all the tested orientations. We show that a cell‐assembly discriminates between orientations by recruiting a ‘salient’ functional network at every presented orientation, wherein the connections and their strengths (peak‐probabilities in the cross‐correlogram) change from one orientation to another. Within these assemblies, closely tuned neurons exhibited increased connectivity and connection‐strengths compared with differently tuned neurons. Minimal connectivity between untuned neurons suggests the significance of neuronal selectivity in assemblies. This study reflects upon the dynamics of functional connectivity, and brings to the fore the importance of a ‘signature’ functional network in an assembly that is strictly related to a specific stimulus. It appears that an assembly is the major ‘functional unit’ of information processing in cortical circuits, rather than the individual neurons.     

Neuronal Activities in the Mouse Visual Cortex Predict Patterns of Sensory Stimuli

Visual cortex forms the basis of visual processing and plays important roles in visual encoding. By using the recently published Allen Brain Observatory dataset consisting of large-scale calcium imaging of mouse V1 activities under visual stimuli, we were able to obtain high-quality data capturing simultaneous neuronal activities at multiple sub-areas and cortical depths of V1. Using prediction models, we analyzed the activity profiles related to static and drifting grating stimuli. We conducted a comprehensive survey of the coding ability of multiple cortical locations toward different stimulus attributes. Specifically, we focused on orientations and spatial frequencies (for static stimuli), as well as moving directions and speed (for drifting stimuli). By using results produced from a prediction model, we quantified the decoding performance profile at different sub-areas and layers of V1. In addition, we analyzed the interactions and interference between different stimulus attributes. The insights obtained from these discoveries would contribute to more precise and quantitative understanding of V1 coding mechanisms.     

Differential Effects of Endogenous and Exogenous Attention on Sensory Tuning

Covert spatial attention (without concurrent eye movements) improves performance in many visual tasks (e.g., orientation discrimination and visual search). However, both covert attention systems—endogenous (voluntary) and exogenous (involuntary)—exhibit differential effects on performance in tasks mediated by spatial and temporal resolution suggesting an underlying mechanistic difference. We investigated whether these differences manifest in sensory tuning by assessing whether and how endogenous and exogenous attention differentially alter the representation of two basic visual dimensions—orientation and spatial frequency (SF). The same human observers detected a grating embedded in noise in two separate experiments (with endogenous or exogenous attention cues). Reverse correlation was used to infer the underlying neural representation from behavioral responses, and we linked our results to established neural computations via a normalization model of attention. Both endogenous and exogenous attention similarly improved performance at the attended location by enhancing the gain of all orientations without changing tuning width. In the SF dimension, endogenous attention enhanced the gain of SFs above and below the target SF, whereas exogenous attention only enhanced those above. Additionally, exogenous attention shifted peak sensitivity to SFs above the target SF, whereas endogenous attention did not. Both covert attention systems modulated sensory tuning via the same computation (gain changes). However, there were differences in the strength of the gain. Compared with endogenous attention, exogenous attention had a stronger orientation gain enhancement but a weaker overall SF gain enhancement. These differences in sensory tuning may underlie differential effects of endogenous and exogenous attention on performance.SIGNIFICANCE STATEMENT Covert spatial attention is a fundamental aspect of cognition and perception that allows us to selectively process and prioritize incoming visual information at a given location. There are two types: endogenous (voluntary) and exogenous (involuntary). Both typically improve visual perception, but there are instances where endogenous improves perception but exogenous hinders perception. Whether and how such differences extend to sensory representations is unknown. Here we show that both endogenous and exogenous attention mediate perception via the same neural computation—gain changes—but the strength of the orientation gain and the range of enhanced spatial frequencies depends on the type of attention being deployed. These findings reveal that both attention systems differentially reshape the tuning of features coded in striate cortex.     

Biased Orientation and Color Tuning of the Human Visual Gamma Rhythm

Narrowband γ oscillations (NBG: ∼20-60 Hz) in visual cortex reflect rhythmic fluctuations in population activity generated by underlying circuits tuned for stimulus location, orientation, and color. A variety of theories posit a specific role for NBG in encoding and communicating this information within visual cortex. However, recent findings suggest a more nuanced role for NBG, given its dependence on certain stimulus feature configurations, such as coherent-oriented edges and specific hues. Motivated by these factors, we sought to quantify the independent and joint tuning properties of NBG to oriented and color stimuli using intracranial recordings from the human visual cortex (male and female). NBG was shown to display a cardinal orientation bias (horizontal) and also an end- and mid-spectral color bias (red/blue and green). When jointly probed, the cardinal bias for orientation was attenuated and an end-spectral preference for red and blue predominated. This loss of mid-spectral tuning occurred even for recording sites showing large responses to uniform green stimuli. Our results demonstrate the close, yet complex, link between the population dynamics driving NBG oscillations and known feature selectivity biases for orientation and color within visual cortex. Such a bias in stimulus tuning imposes new constraints on the functional significance of the visual γ rhythm. More generally, these biases in population electrophysiology will need to be considered in experiments using orientation or color features to examine the role of visual cortex in other domains, such as working memory and decision-making.SIGNIFICANCE STATEMENT Oscillations in electrophysiological activity occur in visual cortex in response to stimuli that strongly drive the orientation or color selectivity of visual neurons. The significance of this induced “γ rhythm” to brain function remains unclear. Answering this question requires understanding how and why some stimuli can reliably generate oscillatory γ activity while others do not. We examined how different orientations and colors independently and jointly modulate γ oscillations in the human brain. Our data show that γ oscillations are greatest for certain orientations and colors that reflect known response biases in visual cortex. Such findings complicate the functional significance of γ oscillations but open new avenues for linking circuits to population dynamics in visual cortex.     

Activation of Parvalbumin-Positive Neurons in Both Retina and Primary Visual Cortex Improves the Feature-Selectivity of Primary Visual Cortex Neurons

Several recent studies using either viral or transgenic mouse models have shown different results on whether the activation of parvalbumin-positive(PV~+)neurons expressing channelrhodopsin-2(ChR2) in the primary visual cortex(V1) improves the orientation-and direction-selectivity of V1 neurons. Although this discrepancy was thoroughly discussed in a follow-up communication, the issue of using different models to express ChR2 in V1 was not mentioned. We found that ChR2 was expressed in retinal ganglion cells(RGCs) and V1 neurons in ChR2fl/~+; PV-Cre mice. Our results showed that the activation of PV~+RGCs using white drifting gratings alone significantly decreased the firing rates of V1 neurons and improved their direction-and orientation-selectivity. Longduration activation of PV~+interneurons in V1 further enhanced the feature-selectivity of V1 neurons in anesthetized mice, confirming the conclusions from previous findings. These results suggest that the activation of both PV~+RGCs and V1 neurons improves feature-selectivity in mice.     

Periodic-pattern-selective cells in monkey visual cortex.

To study the visual processing of periodic and aperiodic patterns, we have analyzed neuronal responses in areas V1 and V2 of the visual cortex of alert monkeys during behaviorally induced fixation of gaze. Receptive field eccentricities ranged between 0.5 degrees and 4 degrees. We found cells that responded vigorously to gratings, but weakly or not all to bars and edges. In some cells the aperiodic stimuli even reduced the activity below the spontaneous level. The distribution of a bar-grating response index indicated a discrete population of "grating cells" characterized by more than 10-fold superiority of gratings. We estimated that these cells have a frequency of 4% in V1 and 1.6% in V2, and that about 4 million grafting cells of V1 subserve the central 4 degrees of vision. The converse, cells that responded to isolated bars but not to gratings of any periodicity, was also observed. The grating cells of V1 were mostly (23 of 26) found in layers 2, 3, and 4B. They preferred spatial frequencies between 2.6 and 19 cycles/degree (median, 9.3), with tuning widths at half-amplitude between 0.4 and 1.4 octaves (median, 1.0). Their tunings were narrower, and their preferred frequencies higher, than those of other cells on average. Grating cells were also narrowly tuned for orientation. Those of V2 were similarly selective. The responses of grating cells depended critically on the number of cycles of the gratings. With square waves of optimum periodicity responses required a minimum of 2-6 grating cycles and leveled off at 4-14 (median, 7.5). The corresponding receptive field widths were 0.34-2.4 degrees (median, 0.78 degrees) for V1 and 0.72-2.4 degrees (median, 1.4 degrees) for V2. Grating cells typically gave unmodulated responses to drifting gratings, were unselective for direction of motion, and were strongly activated also by stationary gratings. Half of those of V1 were monocular, the others binocular, some showing strong binocular facilitation and disparity sensitivity. Length summation was usually monotonic, but strong end-inhibition was also observed. In contrast to other cells, grating cells were not activated by harmonic components. Spatial-frequency response curves for sine-wave, square-wave, and line gratings were similar. Square-wave gratings of one-third the preferred frequency failed to excite the cells, while the isolated 3f component (f = the fundamental of the square wave) of these gratings evoked strong responses. In spite of the nonlinear features, grating cells had low contrast thresholds.(ABSTRACT TRUNCATED AT 400 WORDS)     

The mechanism of orientation selectivity in primary visual cortex without a functional map

Neurons in primary visual cortex (V1) display substantial orientation selectivity even in species where V1 lacks an orientation map, such as in mice and rats. The mechanism underlying orientation selectivity in V1 with such a salt-and-pepper organization is unknown; it is unclear whether a connectivity that depends on feature similarity is required, or a random connectivity suffices. Here we argue for the latter. We study the response to a drifting grating of a network model of layer 2/3 with random recurrent connectivity and feedforward input from layer 4 neurons with random preferred orientations. We show that even though the total feedforward and total recurrent excitatory and inhibitory inputs all have a very weak orientation selectivity, strong selectivity emerges in the neuronal spike responses if the network operates in the balanced excitation/inhibition regime. This is because in this regime the (large) untuned components in the excitatory and inhibitory contributions approximately cancel. As a result the untuned part of the input into a neuron as well as its modulation with orientation and time all have a size comparable to the neuronal threshold. However, the tuning of the F0 and F1 components of the input are uncorrelated and the high-frequency fluctuations are not tuned. This is reflected in the subthreshold voltage response. Remarkably, due to the nonlinear voltage-firing rate transfer function, the preferred orientation of the F0 and F1 components of the spike response are highly correlated.     

Spatial and temporal frequency selectivity of neurons in the middle temporal visual area of new world monkeys (Callithrix jacchus)

Information about the responses of neurons to the spatial and temporal frequencies of visual stimuli is important for understanding the types of computations being performed in different visual areas. We characterized the spatiotemporal selectivity of neurons in the middle temporal area (MT), which is deemed central for the processing of direction and speed of motion. Recordings obtained in marmoset monkeys using high-contrast sine-wave gratings as stimuli revealed that the majority of neurons had bandpass spatial and temporal frequency tuning, and that the selectivity for these parameters was largely separable. Only in about one-third of the cells was inseparable spatiotemporal tuning detected, this typically being in the form of an increase in the optimal temporal frequency as a function of increasing grating spatial frequency. However, most of these interactions were weak, and only 10% of neurons showed spatial frequency-invariant representation of speed. Cells with inseparable spatiotemporal tuning were most commonly found in the infragranular layers, raising the possibility that they form part of the feedback from MT to caudal visual areas. While spatial frequency tuning curves were approximately scale-invariant on a logarithmic scale, temporal frequency tuning curves covering different portions of the spectrum showed marked and systematic changes. Thus, MT neurons can be reasonably described as similarly built spatial frequency filters, each covering a different dynamic range. The small proportion of speed-tuned neurons, together with the laminar position of these units, are compatible with the idea that an explicit neural representation of speed emerges from computations performed in MT.     

A retinal source of spatial contrast gain control

Sensory cortex is able to encode a broad range of stimulus features despite a great variation in signal strength. In cat primary visual cortex (V1), for example, neurons are able to extract stimulus features like orientation or spatial configuration over a wide range of stimulus contrasts. The contrast-invariant spatial tuning found in V1 neuron responses has been modeled as a gain control mechanism, but at which stage of the visual pathway it emerges has remained unclear. Here we describe our findings that contrast-invariant spatial tuning occurs not only in the responses of lateral geniculate nucleus (LGN) relay cells but also in their afferent retinal input. Our evidence suggests that a similar contrast-invariant mechanism is found throughout the stages of the early visual pathway, and that the contrast-invariant spatial selectivity is evident in both retinal ganglion cell and LGN cell responses.