Dynamics of the orientation tuning of postsynaptic potentials in the cat visual cortex

Perception and decision mechanisms involved in average estimation of spatiotemporal ensembles

A number of studies on texture and ensemble perception have shown that humans can immediately estimate the average of spatially distributed visual information. The present study characterized mechanisms involved in estimating averages for information distributed over both space and time. Observers viewed a rapid sequence of texture patterns in which elements' orientation were determined by dynamic Gaussian noise with variable spatial and temporal standard deviations (SDs). We found that discrimination thresholds increased beyond a certain spatial SD if temporal SD was small, but if temporal SD was large, thresholds remained nearly constant regardless of spatial SD. These data are at odds with predictions that threshold is uniquely determined by spatiotemporal SD. Moreover, a reverse correlation analysis revealed that observers judged the spatiotemporal average orientation largely depending on the spatial average orientation over the last few frames of the texture sequence - a recency effect widely observed in studies of perceptual decision making. Results are consistent with the notion that the visual system rapidly computes spatial ensembles and adaptively accumulates information over time to make a decision on spatiotemporal average. A simple computational model based on this notion successfully replicated observed data.

An Integrated Neuronal Model of Claustral Function in Timing the Synchrony Between Cortical Areas

It has been suggested that the function of the claustrum (CL) may be to orchestrate and integrate the activity of the different cortical areas that are involved in a particular function by boosting the synchronized oscillations that occur between these areas. We propose here a model of how this may be done, thanks to the unique synaptic morphology of the CL and its excitatory and inhibitory connections with most cortical areas. Using serial visual search as an example, we describe how the functional anatomy of the claustral connections can potentially execute the sequential activation of the representations of objects that are being processed serially. We also propose that cross-frequency coupling (CFC) between low frequency signals from CL and higher frequency oscillations in the cortical areas will be an efficient means of CL modulating neural activity across multiple brain regions in synchrony. This model is applicable to the wide range of functions one performs, from simple object recognition to reading and writing, listening to or performing music, etc.

Neocortical Rebound Depolarization Enhances Visual Perception

Animals are constantly exposed to the time-varying visual world. Because visual perception is modulated by immediately prior visual experience, visual cortical neurons may register recent visual history into a specific form of offline activity and link it to later visual input. To examine how preceding visual inputs interact with upcoming information at the single neuron level, we designed a simple stimulation protocol in which a brief, orientated flashing stimulus was subsequently coupled to visual stimuli with identical or different features. Using in vivo whole-cell patch-clamp recording and functional two-photon calcium imaging from the primary visual cortex (V1) of awake mice, we discovered that a flash of sinusoidal grating per se induces an early, transient activation as well as a long-delayed reactivation in V1 neurons. This late response, which started hundreds of milliseconds after the flash and persisted for approximately 2 s, was also observed in human V1 electroencephalogram. When another drifting grating stimulus arrived during the late response, the V1 neurons exhibited a sublinear, but apparently increased response, especially to the same grating orientation. In behavioral tests of mice and humans, the flashing stimulation enhanced the detection power of the identically orientated visual stimulation only when the second stimulation was presented during the time window of the late response. Therefore, V1 late responses likely provide a neural basis for admixing temporally separated stimuli and extracting identical features in time-varying visual environments.

A study of mice and humans shows that prior activity in the visual cortex induces a long-delayed depolarization that enhances perception of subsequent visual stimuli if these are identical to the previous one, thereby extracting invariant visual features from the constantly changing visual world.

Animals are constantly exposed to a visual world that varies over time. To examine how the visual cortex integrates visual information that is temporally spaced, we monitored neuronal activity of the primary visual cortex (V1) using single- and multicell recording techniques. We discovered that a brief visual stimulus induced an early, transient activation as well as a delayed reactivation of V1 neurons in mice and humans. Notably, this reactivation of visual cortex conveyed information about stimulus orientation: presentation of a second visual stimulus during this reactivation enhanced the V1 response specifically when the orientations of the two stimuli were identical. Behavioral tests in mice and humans revealed that the ability to detect visual stimuli was also enhanced when the second stimulus was presented during the time window of V1 reactivation. Because animals extract visual information from an environment in constant change, the modulation of visual responses through cortical reactivation might be a strategy commonly used in the visual system.

Neural mechanisms of coarse-to-fine discrimination in the visual cortex

Vision is a dynamic process that refines the spatial scale of analysis over time, as evidenced by a progressive improvement in the ability to detect and discriminate finer details. To understand coarse-to-fine discrimination, we studied the dynamics of spatial frequency (SF) response using reverse correlation in the primary visual cortex (V1) of the primate. In a majority of V1 cells studied, preferred SF either increased monotonically with time (group 1) or changed nonmonotonically, with an initial increase followed by a decrease (group 2). Monotonic shift in preferred SF occurred with or without an early suppression at low SFs. Late suppression at high SFs always accompanied nonmonotonic SF dynamics. Bayesian analysis showed that SF discrimination performance and best discriminable SF frequencies changed with time in different ways in the two groups of neurons. In group 1 neurons, SF discrimination performance peaked on both left and right flanks of the SF tuning curve at about the same time. In group 2 neurons, peak discrimination occurred on the right flank (high SFs) later than on the left flank (low SFs). Group 2 neurons were also better discriminators of high SFs. We examined the relationship between the time at which SF discrimination performance peaked on either flank of the SF tuning curve and the corresponding best discriminable SFs in both neuronal groups. This analysis showed that the population best discriminable SF increased with time in V1. These results suggest neural mechanisms for coarse-to-fine discrimination behavior and that this process originates in V1 or earlier.

Role of feedforward geniculate inputs in the generation of orientation selectivity in the cat's primary visual cortex

Neurones of the mammalian primary visual cortex have the remarkable property of being selective for the orientation of visual contours. It has been controversial whether the selectivity arises from intracortical mechanisms, from the pattern of afferent connectivity from lateral geniculate nucleus (LGN) to cortical cells or from the sharpening of a bias that is already present in the responses of many geniculate cells. To investigate this, we employed a variation of an electrical stimulation protocol in the LGN that has been claimed to suppress intra cortical inputs and isolate the raw geniculocortical input to a striate cortical cell. Such stimulation led to a sharpening of the orientation sensitivity of geniculate cells themselves and some broadening of cortical orientation selectivity. These findings are consistent with the idea that non-specific inhibition of the signals from LGN cells which exhibit an orientation bias can generate the sharp orientation selectivity of primary visual cortical cells. This obviates the need for an excitatory convergence from geniculate cells whose receptive fields are arranged along a row in visual space as in the classical model and provides a framework for orientation sensitivity originating in the retina and getting sharpened through inhibition at higher levels of the visual pathway.

Spatial and temporal features of synaptic to discharge receptive field transformation in cat area 17

The aim of the present study was to characterize the spatial and temporal features of synaptic and discharge receptive fields (RFs), and to quantify their relationships, in cat area 17. For this purpose, neurons were recorded intracellularly while high-frequency flashing bars were used to generate RFs maps for synaptic and spiking responses. Comparison of the maps shows that some features of the discharge RFs depended strongly on those of the synaptic RFs, whereas others were less dependent. Spiking RF duration depended poorly and spiking RF amplitude depended moderately on those of the underlying synaptic RFs. At the other extreme, the optimal spatial frequency and phase of the discharge RFs in simple cells were almost entirely inherited from those of the synaptic RFs. Subfield width, in both simple and complex cells, was less for spiking responses compared with synaptic responses, but synaptic to discharge width ratio was relatively variable from cell to cell. When considering the whole RF of simple cells, additional variability in width ratio resulted from the presence of additional synaptic subfields that remained subthreshold. Due to these additional, subthreshold subfields, spatial frequency tuning predicted from synaptic RFs appears sharper than that predicted from spiking RFs. Excitatory subfield overlap in spiking RFs was well predicted by subfield overlap at the synaptic level. When examined in different regions of the RF, latencies appeared to be quite variable, but this variability showed negligible dependence on distance from the RF center. Nevertheless, spiking response latency faithfully reflected synaptic response latency.

Coding of stimulus sequences by population responses in visual cortex

Neuronal populations in sensory cortex represent the time-changing sensory input through a spatiotemporal code. What are the rules that govern this code? We measured membrane potentials and spikes from neuronal populations in cat visual cortex (V1), through voltage-sensitive dyes and electrode arrays. We first characterized the population response to a single orientation. As response amplitude grew, population tuning width remained constant for membrane potential responses and became progressively sharper for spike responses. We then asked how these single-orientation responses combine to code for successive orientations. We found that they combine through simple linear summation. Linearity, however, is violated after stimulus offset, when responses exhibit an unexplained persistence. Thanks to linearity, the interactions between responses to successive stimuli are minimal. We demonstrate that higher cortical areas may reconstruct the stimulus sequence from V1 population responses through a simple instantaneous decoder. In area V1, therefore, spatial and temporal coding operate largely independently.

Theoretical analysis of reverse-time correlation for idealized orientation tuning dynamics

A theoretical analysis is presented of a reverse-time correlation method used in experimentally investigating orientation tuning dynamics of neurons in the primary visual cortex. An exact mathematical characterization of the method is developed, and its connection with the Volterra-Wiener nonlinear systems theory is described. Various mathematical consequences and possible physiological implications of this analysis are illustrated using exactly solvable idealized models of orientation tuning.

Dynamics of orientation tuning in cat v1 neurons depend on location within layers and orientation maps

Analysis of the timecourse of the orientation tuning of responses in primary visual cortex (V1) can provide insight into the circuitry underlying tuning. Several studies have examined the temporal evolution of orientation selectivity in V1 neurons, but there is no consensus regarding the stability of orientation tuning properties over the timecourse of the response. We have used reverse-correlation analysis of the responses to dynamic grating stimuli to re-examine this issue in cat V1 neurons. We find that the preferred orientation and tuning curve shape are stable in the majority of neurons; however, more than forty percent of cells show a significant change in either preferred orientation or tuning width between early and late portions of the response. To examine the influence of the local cortical circuit connectivity, we analyzed the timecourse of responses as a function of receptive field type, laminar position, and orientation map position. Simple cells are more selective, and reach peak selectivity earlier, than complex cells. There are pronounced laminar differences in the timing of responses: middle layer cells respond faster, deep layer cells have prolonged response decay, and superficial cells are intermediate in timing. The average timing of neurons near and far from pinwheel centers is similar, but there is more variability in the timecourse of responses near pinwheel centers. This result was reproduced in an established network model of V1 operating in a regime of balanced excitatory and inhibitory recurrent connections, confirming previous results. Thus, response dynamics of cortical neurons reflect circuitry based on both vertical and horizontal location within cortical networks.

Standing waves and traveling waves distinguish two circuits in visual cortex

The visual cortex represents stimuli through the activity of neuronal populations. We measured the evolution of this activity in space and time by imaging voltage-sensitive dyes in cat area V1. Contrast-reversing stimuli elicit responses that oscillate at twice the stimulus frequency, indicating that signals originate mostly in complex cells. These responses stand clear of the noise, whose amplitude decreases as 1/frequency, and yield high-resolution maps of orientation preference and retinotopy. We first show how these maps are combined to yield the responses to focal, oriented stimuli. We then study the evolution of the oscillating activity in space and time. In the orientation domain, it is a standing wave. In the spatial domain, it is a traveling wave propagating at 0.2-0.5 m/s. These different dynamics indicate a fundamental distinction in the circuits underlying selectivity for position and orientation, two key stimulus attributes.

Dynamic properties of orientation discrimination assessed by using classification images

Recent physiological studies indicate that the tuning properties of neurons under acute preparation in primary visual cortex can change over time. We used a psychophysical reverse correlation paradigm to examine the potential repercussions of this neuronal property for human observers' ability to discriminate the orientation of targets over time. Observers were required to identify the orientation of a Gabor target presented within dynamic white noise. Frames from the noise movies were pooled to compute dynamic classification images (CIs) associated with the observers' discrimination performance, which then were fit with a weighted difference-of-Gabor function. Best-fitting templates were temporally bandpass, tuned to more oblique orientations than the stimulus but, crucially, did not change over time. The results suggest that the template for orientation discrimination is selected within the first 50 ms of stimulus onset and that, unlike the response of single cells, there is no measurable dynamic component to either orientation or spatial frequency tuning of human orientation discrimination.

Dynamic changes in the tuning of striate neurons to the shapes of cross-shaped figures

Time slice analysis was used to study the dynamics of tuning to the shapes of cross-shaped figures flashing in the receptive fields of 83 neurons in the primary visual cortex (field 17) of the cat brain. Tuning was assessed in terms of the numbers of spikes in the overall response and its sequential 20-msec fragments. Only 11.7% of neurons produced reproducibly developing spike responses to a given shape (defined as the angle between the lines), i.e., had a preferred cross-shaped figure. In the remaining cases (88.3%), tuning of neurons to the shape of the cross showed dynamic changes. In 7.2% of cases, changes in the preferred shape of the cross occurred monophasically; changes were biphasic in 27.0% of cases, while in the remaining 54.1% of cases, the dynamics in changes in the preferred cross shape were undulatory. The tuning of receptive field zones is assessed as the cause of these effects and their difference from the previously observed dynamics of preferred orientations of single bars and cross-shaped figures; the functional significance of these effects is also discussed.

Encoding and decoding target locations with waves in the turtle visual cortex

Visual stimuli elicit waves of activity that propagate across the visual cortex of turtles. An earlier study showed that these waves encode information about the positions of stimuli in visual space. This paper addresses the question of how this information can be decoded from the waves. Windowing techniques were used to temporally localize information contained in the wave. Sliding encoding windows were used to represent waves of activity as low dimensional temporal strands in an appropriate space. Expanding detection window (EDW) or sliding detection window (SDW) techniques were combined with statistical hypothesis testing to discriminate input stimuli. Detection based on an EDW was more reliable than detection based on a SDW. Detection performance improved at a very early stage of the cortical response as the length of the detection window is increased. The property of intrinsic noise was explicitly considered. Assuming that the noise is colored provided a more reliable estimate than did the assumption of a white noise in the cortical output.

No doubt about offset latency

Neuronal response latency usually refers to the time between the presentation of a visual stimulus and the elevation in firing rate that follows. Expanding on this idea, the concept of response offset latency refers to the time between the removal of a stimulus (or its replacement with one that is less effective) and the resulting decline in firing rate. The initial observation that offset latency is usually shorter than onset latency (Bair et al., 2002) has been called into question on the basis of the pulsatile nature of visual stimuli presented on a CRT (Gawne & Woods, 2003). Here, a counter argument is presented in support of the results of Bair et al., 2002.

Temporal Dynamics of Binocular Disparity Processing in the Central Visual Pathway

To solve the stereo correspondence problem (i.e., find the matching features of a visual scene in both eyes), it is advantageous to combine information across spatial scales. The details of how this is accomplished are not clear. Psychophysical studies and mathematical models have suggested various types of interactions across spatial scale, including coarse to fine, fine to coarse, averaging, and population coding. In this study, we investigate dynamic changes in disparity tuning of simple and complex cells in the cat's striate cortex over a short time span. We find that disparity frequency increases and disparity ranges decrease while optimal disparity remains constant, and this conforms to a coarse-to-fine mechanism. We explore the origin of this mechanism by examining the frequency and size dynamics exhibited by binocular simple cells and neurons in the lateral geniculate nucleus (LGN). The results suggest a strong role for a feed-forward mechanism, which could originate in the retina. However, we find that the dynamic changes seen in the disparity range of simple cells cannot be predicted from their left and right eye monocular receptive field (RF) size changes. This discrepancy suggests the possibility of a dynamic nonlinearity or disparity specific feedback that alters tuning or a combination of both mechanisms.

Dynamics of orientation tuning in macaque V1: the role of global and tuned suppression

The temporal development of neural selectivity to physical attributes of a visual stimulus, such as its orientation and spatial frequency, can provide important clues about mechanisms of cortical tuning. We measured the dynamics of orientation tuning in macaque primary visual cortex (V1) and found several dynamical features in the data: changes in global enhancement and suppression, narrowing of orientation bandwidth, small but significant shifts in preferred orientation, and "Mexican-hat" tuning curves. The dynamics data were analyzed with a model that sums two fixed, tuned components (enhancement and suppression) and one global (untuned) component. The analysis suggests that there is early global enhancement followed by global and tuned suppression. Tuned suppression accounts for the dynamical reduction of orientation bandwidth and for the generation of Mexican-hat tuning profiles. Our findings imply that global and tuned suppression are important factors that determine the selectivity and dynamics of V1 responses to orientation.

Cooperation between area 17 neuron pairs enhances fine discrimination of orientation

We examined 66 complex cells in area 17 of cats that were paralyzed and anesthetized with propofol and N2O. We studied changes in ensemble responses for small (<10 degrees ) and large (>10 degrees ) differences in orientation. Examination of temporal resolution and discharge history revealed advantages in discrimination from both dependent (e.g., synchronization) and independent (e.g., bursting) interspike interval properties. For 27 pairs of neurons, we found that the average cooperation (the advantage gained from the joint activity) was 57.6% for fine discrimination of orientation but <5% for gross discrimination. Dependency (probabilistic quantification of the interaction between the cells) was measured between 29 pairs of neurons while varying orientation. On average, the dependency tuning for orientation was 35.5% narrower than the average firing rate tuning. The changes in dependency around the peak orientation (at which the firing rate remains relatively constant) lead to substantial cooperation that can improve discrimination in this region. The narrow tuning of dependency and the cooperation provide evidence to support a population-encoding scheme that is based on biologically plausible mechanisms and that could account for hyperacuities.

Synaptic integration by V1 neurons depends on location within the orientation map

Neurons in the primary visual cortex (V1) are organized into an orientation map consisting of orientation domains arranged radially around "pinwheel centers" at which the representations of all orientations converge. We have combined optical imaging of intrinsic signals with intracellular recordings to estimate the subthreshold inputs and spike outputs of neurons located near pinwheel centers or in orientation domains. We find that neurons near pinwheel centers have subthreshold responses to all stimulus orientations but spike responses to only a narrow range of orientations. Across the map, the selectivity of inputs covaries with the selectivity of orientations in the local cortical network, while the selectivity of spike outputs does not. Thus, the input-output transformation performed by V1 neurons is powerfully influenced by the local structure of the orientation map.

Long-term voltage-sensitive dye imaging reveals cortical dynamics in behaving monkeys

A novel method of chronic optical imaging based on new voltage-sensitive dyes (VSDs) was developed to facilitate the explorations of the spatial and temporal patterns underlying higher cognitive functions in the neocortex of behaving monkeys. Using this system, we were able to explore cortical dynamics, with high spatial and temporal resolution, over period of <or=1 yr from the same patch of cortex. The visual cortices of trained macaques were stained one to three times a week, and immediately after each staining session, the monkey started to perform the behavioral task, while the primary and secondary visual areas (V1 and V2) were imaged with a fast optical imaging system. Long-term repeated VSD imaging (VSDI) from the same cortical area did not disrupt the normal cortical architecture as confirmed repeatedly by optical imaging based on intrinsic signals. The spatial patterns of functional maps obtained by VSDI were essentially identical to those obtained from the same patch of cortex by imaging based on intrinsic signals. On comparing the relative amplitudes of the evoked signals and differential map obtained using these two different imaging methodologies, we found that VSDI emphasizes subthreshold activity more than imaging based on intrinsic signals, that emphasized more spiking activity. The latency of the VSD-evoked response in V1 ranged from 46 to 68 ms in the different monkeys. The amplitude of the V2 response was only 20-60% of that in V1. As expected from the anatomy, the retinotopic responses to local visual stimuli spread laterally across the cortical surface at a spreading velocity of 0.15-0.19 m/s over a larger area than that expected by the classical magnification factor, reaching its maximal anisotropic spatial extent within approximately 40 ms. We correlated the observed dynamics of cortical activation patterns with the monkey's saccadic eye movements and found that due to the slow offset of the cortical response relative to its onset, there was a short period of simultaneous activation of two distinct patches of cortex following a saccade to the visual stimulus. We also found that a saccade to a small stimulus was followed by direct transient activation of a cortical region in areas of V1 and V2, located retinotopically within the saccadic trajectory.

Surfing a spike wave down the ventral stream

Numerous theories of neural processing, often motivated by experimental observations, have explored the computational properties of neural codes based on the absolute or relative timing of spikes in spike trains. Spiking neuron models and theories however, as well as their experimental counterparts, have generally been limited to the simulation or observation of isolated neurons, isolated spike trains, or reduced neural populations. Such theories would therefore seem inappropriate to capture the properties of a neural code relying on temporal spike patterns distributed across large neuronal populations. Here we report a range of computer simulations and theoretical considerations that were designed to explore the possibilities of one such code and its relevance for visual processing. In a unified framework where the relation between stimulus saliency and spike relative timing plays the central role, we describe how the ventral stream of the visual system could process natural input scenes and extract meaningful information, both rapidly and reliably. The first wave of spikes generated in the retina in response to a visual stimulation carries information explicitly in its spatio-temporal structure: the most salient information is represented by the first spikes over the population. This spike wave, propagating through a hierarchy of visual areas, is regenerated at each processing stage, where its temporal structure can be modified by (i). the selectivity of the cortical neurons, (ii). lateral interactions and (iii). top-down attentional influences from higher order cortical areas. The resulting model could account for the remarkable efficiency and rapidity of processing observed in the primate visual system.

Latency effects in orientation popout

A target that differs in orientation from neighboring lines and "pops out" has been found to evoke larger responses in cortical V1 cells than lines in the uniform texture surround which do not popout (e.g., Journal of Neurophysiology 67 (1992) 961). If this is more than a coincidence of observations, physiological properties of contextual modulation should be reflected in the perception of salience. In particular, as the differential suppression from texture surround has been reported to be delayed, target salience may be affected by the history of surrounding lines, i.e. by their orientation before the target was presented. This was tested using a feature flicker paradigm in which target and background lines changed their orientations (Experiment 2). All subjects (N = 4) indicated a benefit in target detection when target orientation was not previously present in the surround. A control experiment showed that this effect was not caused by the purely temporal aspects of asynchronous stimulus presentation (Experiment 3). To distinguish this effect from other sources of delayed processing, Experiment 1 compared the performance in target detection and target identification tasks, for single-lines and popout targets. All subjects required longer stimulus presentation time to identify the orientation of a single line than to detect the line itself, indicating that orientation coding needs longer processing than encoding stimulus onset. However, most subjects needed even longer presentations to detect popout, suggesting that the processing of orientation contrast adds to this delay. In an appendix, putative response variations of V1 cells to asynchronous flicker are computed.

The timing of response onset and offset in macaque visual neurons

We used fast, pseudorandom temporal sequences of preferred and antipreferred stimuli to drive neuronal firing rates rapidly between minimal and maximal across the visual system. Stimuli were tailored to the preferences of cells recorded in the lateral geniculate nucleus (magnocellular and parvocellular), primary visual cortex (simple and complex), and the extrastriate motion area MT. We found that cells took longer to turn on (to increase their firing rate) than to turn off (to reduce their rate). The latency difference (onset minus offset) varied from several to tens of milliseconds across cell type and stimulus class and was correlated with spontaneous or driven firing rates for most cell classes. The delay for response onset depended on the nature of the stimulus present before the preferred stimulus appeared, and may result from persistent inhibition caused by antipreferred stimuli or from suppression that followed the offset of the preferred stimulus. The onset delay showed three distinct types of dependence on the temporal sequence of stimuli across classes of cells, implying that suppression may accumulate or wear off with time. Onset latency is generally longer, can be more variable, and has marked stimulus dependence compared with offset latency. This suggests an important role for offset latency in assessing the speed of information transmission in the visual system and raises the possibility that signal offsets provide a timing reference for visual processing. We discuss the origin of the delay in onset latency compared with offset latency and consider how it may limit the utility of certain feedforward circuits.

Dynamics and constancy in cortical spatiotemporal patterns of orientation processing

How does the high selectivity to stimulus orientation emerge in the visual cortex? Thalamic feedforward-dominated models of orientation selectivity predict constant selectivity during the visual response, whereas intracortical recurrent models predict dynamic improvement in selectivity. We imaged the cat visual cortex with voltage-sensitive dyes to measure orientation-tuning dynamics of a large neuronal population. Tuning-curve width did not narrow after response onset, whereas the difference between preferred and orthogonal responses (modulation depth) first increased, then declined. We identified a suppression of the evoked responses, referred to as the evoked deceleration-acceleration (DA) notch, which was larger for the orthogonal response. Furthermore, peak selectivity of the tuning curves was contemporaneous with the evoked DA notch. These findings suggest that in the cat brain, sustained visual cortical processing does not narrow orientation tuning; rather, intracortical interactions may amplify modulation depth and suppress the orthogonal response relatively more than the preferred. Thus, feedforward models and recurrent models of orientation selectivity must be combined.

Dynamics of the orientation-tuned membrane potential response in cat primary visual cortex

Neurons in the primary visual cortex are highly selective for stimulus orientation, whereas their thalamic inputs are not. Much controversy has been focused on the mechanism by which cortical orientation selectivity arises. Although an increasing amount of evidence supports a linear model in which orientation selectivity is conferred upon visual cortical cells by the alignment of the receptive fields of their thalamic inputs, the controversy has recently been rekindled with the suggestion that late cortical input--delayed by multiple synapses--could lead to sharpening of orientation selectivity over time. Here we used intracellular recordings in vivo to examine temporal properties of the orientation-selective response to flashed gratings. Bayesian parameter estimation demonstrated that both preferred orientation and tuning width were stable throughout the response to a single stimulus.

Processing time of contour integration: the role of colour, contrast, and curvature

We investigated the temporal properties of the red - green, blue-yellow, and luminance mechanisms in a contour-integration task which required the linking of orientation across space to detect a 'path'. Reaction times were obtained for simple detection of the stimulus regardless of the presence of a path, and for path detection measured by a yes/no procedure with path and no-path stimuli randomly presented. Additional processing times for contour integration were calculated as the difference between reaction times for simple stimulus detection and path detection, and were measured as a function of stimulus contrast for straight and curved paths. We found that processing time shows effects not apparent in choice reaction-time measurements. (i) Processing time for curved paths is longer than for straight paths. (ii) For straight paths, the achromatic mechanism is faster than the two chromatic ones, with no difference between the red-green and blue-yellow mechanisms. For curved paths there is no difference in processing time between mechanisms. (iii) The extra processing time required to detect curved compared to straight paths is longest for the achromatic mechanism, and similar for the red - green and blue-yellow mechanisms. (iv) Detection of the absence of a path requires at least 50 ms of additional time independently of chromaticity, contrast, and path curvature. The significance of these differences and similarities between postreceptoral mechanisms is discussed.

Temporal resolution of orientation-based texture segregation

We analysed the temporal-frequency characteristics of two functional processes involved in orientation-based texture segregation: local orientation coding and subsequent orientation-contrast coding. Two texture images, in which each micropattern was rotated by 90 degrees, were alternated at various temporal frequencies. A micropattern was a second-derivative (D2) of a Gaussian that loses orientation information when temporally fused with the orthogonal D2 pattern. We measured the upper temporal-frequency limits for localising the target region whose mean orientation differed from the background by 90 degrees or by 45 degrees. If the temporal limit of the texture perception is determined by the most sluggish processing stage, the temporal limit for the 90 degrees texture should be determined by local orientation coding or by orientation-contrast coding, depending on which stage has the lower temporal precision. On the other hand, the 45 degrees texture should always be segregated below the temporal limit of local orientation coding regardless of the temporal limit of orientation-contrast coding. We found that the temporal limit for the 90 degrees texture was slightly higher than that for the 45 degrees texture under spatial conditions appropriate for texture segregation. Moreover, an orientation-noise analysis of segregation performance for a wide range of temporal frequencies revealed that the temporal-frequency sensitivities for the two textures were nearly identical. These results imply that the temporal limit for orientation-based texture segregation depends only on that of local orientation coding. This conclusion further suggests that the potential temporal resolution of orientation-contrast coding is not lower than that of local orientation coding, which would imply that the orientation-contrast coding is unlikely to be mediated by sluggish neural processes.

Membrane potential and firing rate in cat primary visual cortex

We have investigated the relationship between membrane potential and firing rate in cat visual cortex and found that the spike threshold contributes substantially to the sharpness of orientation tuning. The half-width at half-height of the tuning of the spike responses was 23 +/- 8 degrees, compared with 38 +/- 15 degrees for the membrane potential responses. Direction selectivity was also greater in spike responses (direction index, 0.61 +/- 0.35) than in membrane potential responses (0.28 +/- 0.21). Threshold also increased the distinction between simple and complex cells, which is commonly based on the linearity of the spike responses to drifting sinusoidal gratings. In many simple cells, such stimuli evoked substantial elevations in the mean potential, which are nonlinear. Being subthreshold, these elevations would be hard to detect in the firing rate responses. Moreover, just as simple cells displayed various degrees of nonlinearity, complex cells displayed various degrees of linearity. We fitted the firing rates with a classic rectification model in which firing rate is zero at potentials below a threshold and grows linearly with the potential above threshold. When the model was applied to a low-pass-filtered version of the membrane potential (with spikes removed), the estimated values of threshold (-54.4 +/- 1.4 mV) and linear gain (7.2 +/- 0.6 spikes. sec(-1). mV(-1)) were similar across the population. The predicted firing rates matched the observed firing rates well and accounted for the sharpening of orientation tuning of the spike responses relative to that of the membrane potential. As it was for stimulus orientation, threshold was also independent of stimulus contrast. The rectification model accounted for the dependence of spike responses on contrast and, because of a stimulus-induced tonic hyperpolarization, for the response adaptation induced by prolonged stimulation. Because gain and threshold are unaffected by visual stimulation and by adaptation, we suggest that they are constant under all conditions.

Comparison of the selectivity of postsynaptic potentials and spike responses in cat visual cortex

Intracellular recordings were made from neurons in the cat visual cortex (area 17) to compare the orientation and direction selectivities of the output of a cell with those of the input the cell receives. The input to a cell was estimated from the PSPs (postsynaptic potentials) evoked by visual stimulation, and the output estimated from the number of spikes generated during the same responses. For the whole sample, selectivity of the output of cells was significantly higher than selectivity of their input. Upon PSP to spike transformation, the selectivity index was, on average, doubled. However, the degree of the selectivity improvement in individual cells was very different, varying from cases in which highly selective output was created from a poorly selective input and thus selectivity was greatly improved, to little or no improvement in other neurons. The improvement of selectivity was not correlated with resting membrane potential, threshold for action potential generation, background discharge rate or amplitude of optimal PSP response. Further, no systematic difference was found between simple and complex cells in the input-output relations, indicating that the 'tip of the iceberg' effect on shaping the response selectivity was cell specific, but not cell type specific. This supports the notion that multiple mechanisms are responsible for generation of the response selectivity, and that the contribution of any particular mechanism may vary from one cell to the other. The heterogeneity of the input-output relations in visual cortical cells could indicate different functions of cells in the cortical network; some cells are creating selectivity de novo, the function of other neurons probably being repetition and amplification of the selected signal and arrangement of the output of a whole column.

Spatial phase and the temporal structure of the response to gratings in V1

We recorded single-unit activity of 25 units in the parafoveal representation of macaque V1 to transient appearance of sinusoidal gratings. Gratings were systematically varied in spatial phase and in one or two of the following: contrast, spatial frequency, and orientation. Individual responses were compared based on spike counts, and also according to metrics sensitive to spike timing. For each metric, the extent of stimulus-dependent clustering of individual responses was assessed via the transmitted information, H. In nearly all data sets, stimulus-dependent clustering was maximal for metrics sensitive to the temporal pattern of spikes, typically with a precision of 25-50 ms. To focus on the interaction of spatial phase with other stimulus attributes, each data set was analyzed in two ways. In the "pooled phases" approach, the phase of the stimulus was ignored in the assessment of clustering, to yield an index Hpooled. In the "individual phases" approach, clustering was calculated separately for each spatial phase and then averaged across spatial phases to yield an index Hindiv. Hpooled expresses the extent to which a spike train represents contrast, spatial frequency, or orientation in a manner which is not confounded by spatial phase (phase-independent representation), whereas Hindiv expresses the extent to which a spike train represents one of these attributes, provided spatial phase is fixed (phase-dependent representation). Here, representation means that a stimulus attribute has a reproducible and systematic influence on individual responses, not a neural mechanism for decoding this influence. During the initial 100 ms of the response, contrast was represented in a phase-dependent manner by simple cells but primarily in a phase-independent manner by complex cells. As the response evolved, simple cell responses acquired phase-independent contrast information, whereas complex cells acquired phase-dependent contrast information. Simple cells represented orientation and spatial frequency in a primarily phase-dependent manner, but also they contained some phase-independent information in their initial response segment. Complex cells showed primarily phase-independent representation of orientation but primarily phase-dependent representation of spatial frequency. Joint representation of two attributes (contrast and spatial frequency, contrast and orientation, spatial frequency and orientation) was primarily phase dependent for simple cells, and primarily phase independent for complex cells. In simple and complex cells, the variability in the number of spikes elicited on each response was substantially greater than the expectations of a Poisson process. Although some of this variation could be attributed to the dependence of the response on the spatial phase of the grating, variability was still markedly greater than Poisson when the contribution of spatial phase to response variance was removed.

GABAergic inhibitory control of the transient and sustained components of orientation selectivity in a model microcolumn in layer 4 of cat visual cortex

Recently proposed models of orientation tuning in layer 4 of cat primary visual cortex (Somers, Nelson, & Sur, 1995; Douglas, Koch, Mahowald, Martin, & Suarez, 1995) rely on widespread inhibitory intracortical connections to suppress the nonoptimal component of a broadly tuned thalamic input, while local excitatory intracortical connections amplify the optimal component. However, new experimental data (Ferster, Chung, & Wheat, 1996) and theoretical analyses (Ferster, 1987; Krukowski, Priebe, & Miller, 1996) show that the temporally modulated component of thalamic input is well tuned and that the cortical circuitry must simply subtract an unmodulated DC component at nonoptimal orientations to obtain sharp tuning. In addition, within a single hypercolumn in layer 4, inhibitory and excitatory layer 4 neurons have approximately equal-sized axonal fields, making the most of their synapses within their own dendritic field (Kisvarday, Martin, Whitteridge, & Somogyi, 1985; Martin & Whitteridge, 1984). We have constructed a model of a single microcolumn in which GABA inhibition subtracts the DC and controls the sustained response, while GABA inhibition controls the response to transient and suprathreshold inputs. The model fits experimental data based on stimulation with drifting sine-wave gratings as well as flashed bars, explains a counterintuitive property of the GABA conductance, and at suboptimal orientations and submaximal contrasts produces an exponential distribution of firing frequencies.

The contribution of intracortical inhibition to dynamics of orientation tuning in cat striate cortex neurons

Orientation tuning of some neurons in cat visual cortex (area 17) revealed successive shifts of the preferred orientation and widening of tuning in time during the first 150 ms after onset of a flashing light bar. The mechanisms of these dynamics and the possible role of intracortical inhibition are still under discussion. In this study we analysed the dynamics using the time slice method before and during blockade of GABAergic inhibition by microiontophoretic application of bicuculline and observed two main types of neuronal behaviour. The first group of neurons (39 of 68 units or 57.4%) with relatively sharp tuning and absence or relatively small shifts of preferred orientation under control conditions increased or developed this shift during bicuculline application. Changes in tuning were observed between 30 and 150 ms after stimulus onset when inhibition was blocked. Neurons of the second group (29 units or 42.6% of cases) displayed pronounced shifts of preferred orientation under control conditions which was typically diminished or lost during blockade of inhibition. The results indicate different contributions of intracortical inhibition to different neurons distinguishing by stability or time dependence of their orientation preference during normal response generation. In one group of striate cells orientation tuning was kept narrow and constant in time by intracortical inhibition, while in another group orientation tuning dynamics are induced by inhibitory mechanisms.

Sensory coding in cortical neurons. Recent results and speculations

We described a novel approach to the study of how spike trains encode sensory information. This approach emphasizes the idea that spike trains are sequences of discrete events, rather than approximations to continuous signals. Aided by some simple heuristics, such as a caricature of neurons as coincidence detectors, we constructed candidate notions of "distances" between spike trains, considered as points in an abstract space. Each candidate distance was evaluated for relevance to biological encoding by determining whether it led to systematic, stimulus-dependent, clustering of the neural responses. We showed here that these distance can also be used to construct a "response space" for the neuron. The response space, which is typically not Euclidean, can represent two or three stimulus attributes. We also introduced the notion of a "consensus spike train," defined as the spike train with minimum average distance from a set of observed responses. For the distances we considered, the consensus spike train (for a particular stimulus) contained only those spikes that were present at consistent times across the observed responses to that stimulus, and thus contained fewer spikes than the typical observed responses. Nevertheless, these consensus spike trains provided an equivalent (or even superior) representation of the stimulus array.

Multiple mechanisms underlying the orientation selectivity of visual cortical neurones

For over three decades, the mechanism of orientation selectivity of visual cortical neurones has been hotly debated. While intracortical inhibition has been implicated as playing a vital role, it has been difficult to observe it clearly. On the basis of recent findings, we propose a model in which the visual cortex brings together a number of different mechanisms for generating orientation-selective responses. Orientation biases in the thalamo-cortical input fibres provide an initial weak selectivity either directly in the excitatory input or by acting via cortical interneurones. This weak selectivity of postsynaptic potentials is then amplified by voltage-sensitive conductances of the cell membrane and excitatory and inhibitory intracortical circuitry, resulting in the sharp tuning seen in the spike discharges of visual cortical cells.