Calculating direction maps from intrinsic signals revealed by optical imaging

Mutual consistency of multiple visual feature maps constrains combined map topology

The primary visual cortex (V1) is the first cortical area that processes visual information relayed from the thalamus. The topologies permitted in joint ocular dominance (OD), orientation preference (OP), and direction preference (DP) maps in V1 are considered, with the aim of finding a maximally symmetric periodic case that can serve as the basis for perturbations toward natural realizations. It is shown that mutual consistency of the maps selects just two possible such lattice structures, and that one of these is much closer to experiment than the other. This comprises a hexagonal lattice of alternating positive and negative OP singularities, with each unit cell or hypercolumn containing four such singularities, each of which radiates three DP discontinuities that follow OP contours and end at OP singularities of opposite sign. Other DP discontinuities emanate at 90 degrees to the midpoints of the ones that link OP singularities, and cross OP contours perpendicularly. These features explain experimentally observed relationships between DP discontinuities and OP contours, including sudden approximately 90-degree changes of direction in the former.

Immediate Impact of Acute Elevation of Intraocular Pressure on Cortical Visual Motion Processing

Purpose

To physiologically examine the impairment of cortical sensitivity to visual motion during acute elevation of intraocular pressure (IOP).

Methods

Motion processing in the cat brain is well characterized, its X and Y cell visual pathways being functionally analogous to parvocellular and magnocellular pathways in primates. Using this model, we performed ocular anterior chamber perfusion to reversibly elevate IOP over a range from 30 to 90 mm Hg while monitoring cortical activity with intrinsic signal optical imaging. Drifting random-dot fields and gratings were used to characterize cortical population responses to motion direction and orientation in early visual areas 17 and 18.

Results

We found that acute IOP elevations at 50 mm Hg and above, which is often observed in acute glaucoma, suppressed cortical motion direction responses. This suppression was more profound in area 17 than in area 18, and more profound in central than peripheral visual field (eccentricities 0°–4° vs. 4°–8°) within area 17. In addition, orientation responses were more suppressed than motion direction responses for the same IOP modulation.

Conclusions

In contrast to human chronic glaucoma that may cause greater dysfunction in large-cell magnocellular than in small-cell parvocellular visual pathways, our direct measurement of cortical processing networks implies that the small X-cell pathway shows greater vulnerability to acute IOP elevation than the large Y-cell pathway in visual motion processing. The results demonstrate that fine discrimination mechanisms for motion in the central visual field are particularly impacted by acute IOP attacks, suggesting a neural basis for immediate visual deficits in the fine motion perception of acute glaucoma patients.

Beyond Rehabilitation of Acuity, Ocular Alignment, and Binocularity in Infantile Strabismus

Infantile strabismus impairs the perception of all attributes of the visual scene. High spatial frequency components are no longer visible, leading to amblyopia. Binocularity is altered, leading to the loss of stereopsis. Spatial perception is impaired as well as detection of vertical orientation, the fastest movements, directions of movement, the highest contrasts and colors. Infantile strabismus also affects other vision-dependent processes such as control of postural stability. But presently, rehabilitative therapies for infantile strabismus by ophthalmologists, orthoptists and optometrists are restricted to preventing or curing amblyopia of the deviated eye, aligning the eyes and, whenever possible, preserving or restoring binocular vision during the critical period of development, i.e., before ~10 years of age. All the other impairments are thus ignored; whether they may recover after strabismus treatment even remains unknown. We argue here that medical and paramedical professionals may extend their present treatments of the perceptual losses associated with infantile strabismus. This hypothesis is based on findings from fundamental research on visual system organization of higher mammals in particular at the cortical level. In strabismic subjects (as in normal-seeing ones), information about all of the visual attributes converge, interact and are thus inter-dependent at multiple levels of encoding ranging from the single neuron to neuronal assemblies in visual cortex. Thus if the perception of one attribute is restored this may help to rehabilitate the perception of other attributes. Concomitantly, vision-dependent processes may also improve. This could occur spontaneously, but still should be assessed and validated. If not, medical and paramedical staff, in collaboration with neuroscientists, will have to break new ground in the field of therapies to help reorganize brain circuitry and promote more comprehensive functional recovery. Findings from fundamental research studies in both young and adult patients already support our hypothesis and are reviewed here. For example, presenting different contrasts to each eye of a strabismic patient during training sessions facilitates recovery of acuity in the amblyopic eye as well as of 3D perception. Recent data also demonstrate that visual recoveries in strabismic subjects improve postural stability. These findings form the basis for a roadmap for future research and clinical development to extend presently applied rehabilitative therapies for infantile strabismus.

The hemodynamic signal as a first-order low-pass temporal filter: Evidence and implications for neuroimaging studies

Neuronal activation triggers local changes in blood flow and hemoglobin oxygenation. These hemodynamic signals can be recorded through functional magnetic resonance imaging or intrinsic optical imaging, and allows inferring neural activity in response to stimuli. These techniques are widely used to uncover functional brain architectures. However, their accuracy suffers from distortions inherent to hemodynamic responses and noise. The analysis of these signals currently relies on models of impulse hemodynamic responses to brief stimuli. Here, in order to infer precise functional architectures, we focused on integrated signals associated to the dynamic response of functional maps. To this end, we recorded orientation and direction maps in cat primary visual cortex and compared two protocols: the conventional episodic stimulation technique and a continuous, periodic stimulation paradigm. Conventional methods show that the dynamics of activation and deactivation of the functional maps follows a linear first-order differential equation representing a low-pass filter. Comparison with the periodic stimulation methods confirmed this observation: the phase shifts and magnitude attenuations extracted at various frequencies were consistent with a low-pass filter with a 5s time constant. This dynamics presumably reflects the variations in deoxyhemoglobin mediated by arterial dilations. This dynamics open new avenues in the analysis of neuroimaging data that differs from common methods based on the hemodynamic response function. In particular, we demonstrate that inverting this first-order low-pass filter minimized the distortions of the signal and enabled a much faster and accurate reconstruction of functional maps.

The mechanism for processing random-dot motion at various speeds in early visual cortices

All moving objects generate sequential retinotopic activations representing a series of discrete locations in space and time (motion trajectory). How direction-selective neurons in mammalian early visual cortices process motion trajectory remains to be clarified. Using single-cell recording and optical imaging of intrinsic signals along with mathematical simulation, we studied response properties of cat visual areas 17 and 18 to random dots moving at various speeds. We found that, the motion trajectory at low speed was encoded primarily as a direction signal by groups of neurons preferring that motion direction. Above certain transition speeds, the motion trajectory is perceived as a spatial orientation representing the motion axis of the moving dots. In both areas studied, above these speeds, other groups of direction-selective neurons with perpendicular direction preferences were activated to encode the motion trajectory as motion-axis information. This applied to both simple and complex neurons. The average transition speed for switching between encoding motion direction and axis was about 31°/s in area 18 and 15°/s in area 17. A spatio-temporal energy model predicted the transition speeds accurately in both areas, but not the direction-selective indexes to random-dot stimuli in area 18. In addition, above transition speeds, the change of direction preferences of population responses recorded by optical imaging can be revealed using vector maximum but not vector summation method. Together, this combined processing of motion direction and axis by neurons with orthogonal direction preferences associated with speed may serve as a common principle of early visual motion processing.

Stable learning of functional maps in self-organizing spiking neural networks with continuous synaptic plasticity

This study describes a spiking model that self-organizes for stable formation and maintenance of orientation and ocular dominance maps in the visual cortex (V1). This self-organization process simulates three development phases: an early experience-independent phase, a late experience-independent phase and a subsequent refinement phase during which experience acts to shape the map properties. The ocular dominance maps that emerge accommodate the two sets of monocular inputs that arise from the lateral geniculate nucleus (LGN) to layer 4 of V1. The orientation selectivity maps that emerge feature well-developed iso-orientation domains and fractures. During the last two phases of development the orientation preferences at some locations appear to rotate continuously through ±180° along circular paths and referred to as pinwheel-like patterns but without any corresponding point discontinuities in the orientation gradient maps. The formation of these functional maps is driven by balanced excitatory and inhibitory currents that are established via synaptic plasticity based on spike timing for both excitatory and inhibitory synapses. The stability and maintenance of the formed maps with continuous synaptic plasticity is enabled by homeostasis caused by inhibitory plasticity. However, a prolonged exposure to repeated stimuli does alter the formed maps over time due to plasticity. The results from this study suggest that continuous synaptic plasticity in both excitatory neurons and interneurons could play a critical role in the formation, stability, and maintenance of functional maps in the cortex.

Orientation selective or not? - Measuring significance of tuning to a circular parameter

Orientation and direction tuning are among the most studied features of the visual system and are routinely measured during experiments to estimate the quality of neuronal responses. However, standard approaches to report orientation selectivity are only narrowly quantitative and strongly depend on the signal quality, while the more sophisticated ones are computationally exhaustive, making them difficult to use during ongoing experiments. We propose a fast and efficient method for reporting the reliability of coding applicable to any circular parameter. Similar to standard deviation in the linear statistics, reproducibility measures trial-to-trial variability of a circular response parameter. Reproducibility is a normalized measure easily transformed to p-values, which provide explicit information about significance of the estimated orientation preference. The proposed approach is applicable to a wide range of signal types. Here, we discuss examples from optical imaging and electrophysiological recordings, and provide a more thorough examination based on tuning curves modeled in silico.

Three-dimensional visual feature representation in the primary visual cortex

In the cat primary visual cortex, it is accepted that neurons optimally responding to similar stimulus orientations are clustered in a column extending from the superficial to deep layers. The cerebral cortex is, however, folded inside a skull, which makes gyri and fundi. The primary visual area of cats, area 17, is located on the fold of the cortex called the lateral gyrus. These facts raise the question of how to reconcile the tangential arrangement of the orientation columns with the curvature of the gyrus. In the present study, we show a possible configuration of feature representation in the visual cortex using a three-dimensional (3D) self-organization model. We took into account preferred orientation, preferred direction, ocular dominance and retinotopy, assuming isotropic interaction. We performed computer simulation only in the middle layer at the beginning and expanded the range of simulation gradually to other layers, which was found to be a unique method in the present model for obtaining orientation columns spanning all the layers in the flat cortex. Vertical columns of preferred orientations were found in the flat parts of the model cortex. On the other hand, in the curved parts, preferred orientations were represented in wedge-like columns rather than straight columns, and preferred directions were frequently reversed in the deeper layers. Singularities associated with orientation representation appeared as warped lines in the 3D model cortex. Direction reversal appeared on the sheets that were delimited by orientation-singularity lines. These structures emerged from the balance between periodic arrangements of preferred orientations and vertical alignment of the same orientations. Our theoretical predictions about orientation representation were confirmed by multi-slice, high-resolution functional MRI in the cat visual cortex. We obtained a close agreement between theoretical predictions and experimental observations. The present study throws a doubt about the conventional columnar view of orientation representation, although more experimental data are needed.

A motion direction map in macaque V2

In mammals, the perception of motion starts with direction-selective neurons in the visual cortex. Despite numerous studies in monkey primary and second visual cortex (V1 and V2), there has been no evidence of direction maps in these areas. In the present study, we used optical imaging methods to study the organization of motion response in macaque V1 and V2. In contrast to the findings in other mammals (e.g., cats and ferrets), we found no direction maps in macaque V1. Robust direction maps, however, were found in V2 thick/pale stripes and avoided thin stripes. In many cases direction maps were located within thick stripes and exhibited pinwheel or linear organizations. The presence of motion maps in V2 points to a newfound prominence of V2 in motion processing, for contributing to motion perception in the dorsal pathway and/or for motion cue-dependent form perception in the ventral pathway.

Orientation and Direction-of-Motion Response in the Middle Temporal Visual Area (MT) of New World Owl Monkeys as Revealed by Intrinsic-Signal Optical Imaging

Intrinsic-signal optical imaging was used to evaluate relationships of domains of neurons in middle temporal visual area (MT) selective for stimulus orientation and direction-of-motion. Maps of activation were elicited in MT of owl monkeys by gratings drifting back-and-forth, flashed stationary gratings and unidirectionally drifting fields of random dots. Drifting gratings, typically used to reveal orientation preference domains, contain a motion component that may be represented in MT. Consequently, this stimulus could activate groups of cells responsive to the motion of the grating, its orientation or a combination of both. Domains elicited from either moving or static gratings were remarkably similar, indicating that these groups of cells are responding to orientation, although they may also encode information about motion. To assess the relationship between domains defined by drifting oriented gratings and those responsive to direction-of-motion, the response to drifting fields of random dots was measured within domains defined from thresholded maps of activation elicited by the drifting gratings. The optical response elicited by drifting fields of random dots was maximal in a direction orthogonal to the map of orientation preference. Thus, neurons in domains selective for stimulus orientation are also selective for motion orthogonal to the preferred stimulus orientation.

Bimodal modulation and continuous stimulation in optical imaging to map direction selectivity

In the visual system, neurons with similar functional properties such as orientation and direction selectivity are clustered together to form modules. Optical imaging recordings in combination with episodic paradigms have been previously used to estimate direction selectivity, a fundamental property of visual neurons. The major drawback of the episodic approach is that the extraction of the signal from various forms of physiological noise is difficult, leading to a poor estimation of direction. Recent work, based on periodic stimulation and Fourier decomposition improved the extraction of periodic stimulus responses from noise and thus, reduced the recording time considerably. Given the success of this new paradigm in mapping orientation, the present study evaluated its reliability to measure direction selectivity in the visual cortex of anesthetized cats. Here, a model that exploits the harmonics of the Fourier decomposition is proposed where the first harmonic is related to direction responses, and the second to orientation. As expected, the first harmonic was absent when a static stimulus was presented. Contrarily, the first harmonic was present when moving stimuli were presented and the amplitude was greater with random dots kinematograms than with drifting gratings. The phase of the first harmonic showed a good agreement with direction preference measured by episodic paradigm. The ratio of the first/the second harmonic amplitude, related to a direction index, was weaker in fracture. It was also weaker in areas of the ventral pathway (areas 17 and 21a) where direction selectivity is known to be reduced. These results indicate that a periodic paradigm can be easily used to measure specific parameters in optical signals, particularly in situations when short acquisition periods are needed.

Vision and cortical map development

Functional maps arise in developing visual cortex as response selectivities become organized into columnar patterns of population activity. Recent studies of developing orientation and direction maps indicate that both are sensitive to visual experience, but not to the same degree or duration. Direction maps have a greater dependence on early vision, while orientation maps remain sensitive to experience for a longer period of cortical maturation. There is also a darker side to experience: abnormal vision through closed lids produces severe impairments in neuronal selectivity, rendering these maps nearly undetectable. Thus, the rules that govern their formation and the construction of the underlying neural circuits are modulated-for better or worse-by early vision. Direction maps, and possibly maps of other properties that are dependent upon precise conjunctions of spatial and temporal signals, are most susceptible to the potential benefits and maladaptive consequences of early sensory experience.

Pinwheel patterns give rise to the direction selectivity of complex cells in the primary visual cortex

Understanding the relation between the 'pinwheel orientation map' and the 'direction map' in the cat primary visual cortex (V1) is a long-standing problem. Several relations between the direction and orientation maps have been previously noted; for example, one iso-orientation region is usually subdivided into two subregions preferring opposite directions. However, the reasons for these relations remain unknown. Here, we demonstrate that the organization of the cortical map for direction selectivity (DS) is intimately related to, and can be predicted from, the organization of the cortical orientation map. We have found that the magnitude of the direction preference of a complex cell is proportional to the change in the number of simple cells driven by a stimulus line moving across an iso-orientation region; the preferred direction of the complex cell points to the place where the number of simple cells driven by the visual stimulus is maximal. According to this rule, the direction map can be uniquely determined from the pinwheel orientation map, and the phenomena relating the two maps can be explained.

Visual resolution with retinal implants estimated from recordings in cat visual cortex

We investigated cortical responses to electrical stimulation of the retina using epi- and sub-retinal electrodes of 20-100 microm diameter. Temporal and spatial resolutions were assessed by recordings from the visual cortex with arrays of microelectrodes and optical imaging. The estimated resolutions were approximately 40 ms and approximately 1 degrees of visual angle. This temporal resolution of 25 frames per second and spatial resolution of about 0.8 cm at about 1m and correspondingly 8 cm at 10 m distance seems sufficient for useful object recognition and visuo-motor behavior in many in- and out-door situations of daily life.

Posteromedial lateral suprasylvian motion area modulates direction but not orientation preference in area 17 of cats

In visual cortices of cats there are two major, largely parallel, feedforward processing streams which conduct visual information from the primary visual cortices to the parietal and temporal visual cortices, processing motion and form information, respectively. In addition to the feedforward streams, there exist many feedback projections from higher-order visual cortices to lower-order visual cortices. Using the intrinsic signal optical imaging, this study has examined the influence of feedback signals originating from area posteromedial lateral suprasylvian (PMLS), the dominant motion-processing region of the parietal cortex, on responses of neurons, orientational maps, and directional maps in cats' area 17 (striate cortex). The inactivation of area PMLS by local application of GABA resulted in the reduction of the magnitude of responses of area 17 cells though area 17 of the cat is mainly involved in form information processing rather than motion. Furthermore, inactivation of area PMLS abolished the global layout of direction maps in area 17 but did not affect the basic structure of the orientation maps in area 17. Thus, it appears that higher-order cortical areas along one information processing stream may exert cross-stream modulatory effects on fundamental properties of neurons located in the lower-order areas along distinct information processing streams.

The development of direction selectivity in ferret visual cortex requires early visual experience

Development of the selective response properties that define columns in sensory cortex is thought to begin early in cortical maturation, without the need for experience. We investigated the development of direction selectivity in ferret visual cortex using optical imaging and electrophysiological techniques and found an exception to this view. Unlike orientation selectivity and ocular dominance, direction selectivity was not detected at eye opening. Direction selectivity emerged several days later and strengthened to adult levels over the following 2 weeks. Visual experience was essential for this process, as shown by the absence of direction selectivity in dark-reared ferrets. The impairment persisted in dark-reared ferrets that were given experience after this period, despite the recovery of response amplitude, preference and bandwidth for stimulus orientation, spatial and temporal frequency, and contrast. Visual experience in early postnatal life plays a necessary and unique role in the development of cortical direction selectivity.

Orientation and direction selectivity of synaptic inputs in visual cortical neurons: a diversity of combinations produces spike tuning

This intracellular study investigates synaptic mechanisms of orientation and direction selectivity in cat area 17. Visually evoked inhibition was analyzed in 88 cells by detecting spike suppression, hyperpolarization, and reduction of trial-to-trial variability of membrane potential. In 25 of these cells, inhibition visibility was enhanced by depolarization and spike inactivation and by direct measurement of synaptic conductances. We conclude that excitatory and inhibitory inputs share the tuning preference of spiking output in 60% of cases, whereas inhibition is tuned to a different orientation in 40% of cases. For this latter type of cells, conductance measurements showed that excitation shared either the preference of the spiking output or that of the inhibition. This diversity of input combinations may reflect inhomogeneities in functional intracortical connectivity regulated by correlation-based activity-dependent processes.

Axonal topography of cortical basket cells in relation to orientation, direction, and ocular dominance maps

The axonal (bouton) distributions of a layer 4 clutch cell (CC), two layer 3 medium-sized basket cells (MBC), and a layer 3 large basket cell (LBC) to orientation, direction, and ocular dominance maps were studied quantitatively. 1) The CC provided exclusively local projections (<380 microm from the soma) and contacted a narrow "niche" of functional representations. 2) The two MBCs emitted local projections (75% and 79% of all boutons), which were engaged with isoorientations (61% and 48%) and isodirections, and long-range projections (25% and 21%, >313 microm and >418 microm), which encountered cross-orientation sites (14% and 12%) and isoorientation sites (7% and 5%). Their direction preferences were mainly perpendicular to or opposite those of local projections. 3) The LBC provided the majority (60%) of its boutons to long-range distances (>437 microm). Locally, LBC boutons showed a rather balanced contribution to isoorientations (19%) and cross-orientations (12%) and preferred isodirections. Remotely, however, cross-orientation sites were preferred (31% vs. 23%) and the directional output was balanced. 4) Monte Carlo simulations revealed that the differences between the orientation specificity of local and long-range projections cannot be explained by a homogeneous lateral distribution of the boutons. 5) There was a similar eye preference in the local and long-range projection fields of the MBCs. The LBC contacted both contra- and ipsilateral eye domains. 6) The basket axons showed little laminar difference in orientation and direction topography. The results suggest that an individual basket cell can mediate a wide range of effects depending on the size and termination pattern of the axonal field.