Activity-dependent matching of excitatory and inhibitory inputs during refinement of visual receptive fields

The receptive field (RF) of single visual neurons undergoes progressive refinement during development. It remains largely unknown how the excitatory and inhibitory inputs on single developing neurons are refined in a coordinated manner to allow the formation of functionally correct circuits. Using whole-cell voltage-clamp recording from Xenopus tectal neurons, we found that RFs determined by excitatory and inhibitory inputs in more mature tectal neurons are spatially matched, with each spot stimulus evoking balanced synaptic excitation and inhibition. This emerges during development through a gradual reduction in the RF size and a transition from disparate to matched topography of excitatory and inhibitory inputs to the tectal neurons. Altering normal spiking activity of tectal neurons by either blocking or elevating GABA(A) receptor activity significantly impeded the developmental reduction and topographic matching of RFs. Thus, appropriate inhibitory activity is essential for the coordinated refinement of excitatory and inhibitory connections.     

Formation and functional significance of the dark receptive fields of the cat visual cortex

Receptive fields (RFs) of single units in the 17th field of the visual cortex of immobilized cat were investigated under dark adaptation. The mean RF size was equal to 67 degrees and varied from 3 degrees up to 120 degrees. The RFs with centres located near gaze were from 3 degrees up to 120 degrees in dia, but with growth of excentricity the number of small RFs decreased, and in the region of 70 to 100 degrees from gaze only RFs with diameters equal to 100 degrees were found. The shape of "dark" RFs was either ellipsoidal (in most cases) or round. Detector properties (orientational, directional, size and velocity selectivity) of the "dark" RFs were significantly less manifest or absent. Under photopic light adaptation the same units reorganized their RFs to well known sizes and configuration. The hypothesis is discussed of the formation of local detector RF in the visual cortex in light adaptation by selective cortical inhibition which is activated in darkness only slightly. This view is an alternative to the commonly-accepted scheme of local cortical RF formation by the hierarchical and selective excitatory convergence.     

Striate receptive fields mapped with single and bipartite stimuli

In 22 acute experiments with anesthetized and immobilized adult cats, 364 maps of receptive fields (RF) of 47 striate neurons were obtained by means of single local stimuli flashed at different parts of the visual field, or with additional asynchronous activation of the RF excitatory center with oscillating bar of the optimal orientation. Under bipartite stimulation, considerable and significant decrease in the square and weight of the central excitatory RF zone was revealed in more then 75% of the studied cells. Additional excitatory zones appeared in 54% of cases, or the square and weight of the excitatory zones substantially increased, and inhibitory zones developed in 90% of cases. These effects were correlated with the degree of increase in the background firing during transition from the mode of mapping with single stimulation to that with bipartite stimulation. The mechanism and possible functional role of cooperative excitatory and inhibitory intracortical interactions in organization of receptive fields and detection of features of a visual image are discussed.     

Spatio-temporal organization of receptive fields of the cat striate cortex

The responses of cortical cells to gratings and bars were compared. The excitatory and inhibitory on-and off-zones of a simple cell are composed of on- and off-subfields of CGL. Any zone is formed by an opponent pair of subfields one of which gives an excitatory effect, the other — inhibitory. Such organization assumes the linear properties of a simple field. The deviations from linearity are due to spatial dis-placements of the subfields, heterogeneity of subfields, or the absence of one subfield in the opponent pair. Subfields may be both phasic and tonic, even in the same RF. Analysis of the most common type of a complex cell with modulated responses against unmodulated background shows that a mask eliminating stimulation of any half of the RF causes (according to the theory of filtres) increasing the bandwidth due to the increase or the appearance of responses to side low and high frequencies. The modulated components of the responses from both halves of the RF are out of phase. Analysis of this fact and the responses to thin bars suggests that a complex field is formed by linear and nonlinear subsystems converging onto output neuron. Other types of complex fields are organized by different combinations of subsystems. Limited in area by masking the RF responds to much higher spatial frequencies than the whole RF. The optimal frequency in two-dimensional spatial frequency characteristics of the RF does not change with orientation. Simple RFs and a part of complex RF calculate the amplitude and the phase of the stimulus, the other part of complex RFs (with unmodulated response) calculate only amplitude. Given all this, the RFs are grating filters of spatial frequency.     

Adaptability of the receptive fields of neurons of the posterior temporal cortex and their sensitivity to parameters of photic stimulation in the cat

Study of receptive fields (RFs) of neurones in the postero-temporal cortex (field 21) of alert cat at three levels of visual adaptation: light photopic, light mesopic and practically dark or extremely low scotopic adaptations--revealed invariance of the most part of the studied RFs to the level of visual adaptation. Reorganization of RFs, connected with change of background luminosity were observed only in 12% of visually activated neurones. Significant reduction of responses to optic stimulation is shown at increase of the level of luminosity in 75% of neurones, revealing adaptive reorganizations. It is suggested that these reorganizations may take place in analogy with neurones of the field 17 on account of different involvement of intracortical inhibitory mechanisms (and, probably, not only in the postero-temporal cortex, but also in structures which precede it in visual hierarchy). Study of neurones sensitivity in the field 21 to parameters of optic stimulation revealed their considerable invariance to the length and orientation of the optic stimulus moving through the RF (60% of cases). Testing of RF by a rhombic optic stimulus did not change neuronal reactions, the form and dimensions of RF did not significantly change.     

Reorganization of Receptive Fields of Cortical Field 21a Neurons and Formation of Responses to Moving Visual Stimuli

Using extracellular recording of spike activity from single neurons of field 21a of the cat neocortex, we examined in detail the spatial organization of receptive fields (RFs) of such cells after conditions of presentation of an immobile blinking light spot (a static RF) and moving visual stimuli (dynamic RFs). As was shown, the excitability of different RF subfields of a group of neurons possessing homogeneous on–off organization of the static RF changes significantly depended on the contrast, shape, dimension, orientation, and direction of movement of the applied mobile visual stimulus. This is manifested in changes in the number of discharge centers and shifts of their spatial localization. A hypothesis on the possible role of synchronous activation of the neurons neighboring the cell under study in the formation of an additional neuronal mechanism providing specialization of neuronal responses is proposed.     

Laminar differences in bicuculline methiodide's effects on cortical neurons in the rat whisker/barrel system

Extracellular unit recordings were made at various depths within SmI barrel cortex of immobilized, sedated rats, in the presence and absence of titrated amounts of the GABAA receptor antagonist bicuculline methiodide (BMI). Principal and adjacent whiskers were moved singly, or in paired combination in a condition-test paradigm, to assess excitatory and inhibitory receptive field (RF) characteristics. Neurons were classified as regular- or fast-spike units, and divided into three laminar groups: supragranular, granular (barrel), and infragranular. BMI increased response magnitude and duration, but did not affect response latencies. The excitatory RFs of barrel units, which are the most tightly focused on the principal whisker, were the most greatly defocused by BMI; infragranular units were least affected. All three layers had approximately equal amounts of adjacent whisker-evoked, surround inhibition, but BMI counteracted this inhibition substantially in barrel units and less so in infragranular units. The effects of BMI were most consistent in the barrel; more heterogeneity was found in the non-granular layers. These lamina-dependent effects of BMI are consistent with the idea that between-whisker inhibition is generated mostly within individual layer IV barrels as a result of the rapid engagement of strong, local inhibitory circuitry, and is subsequently embedded in layer IV's output to non-layer IV neurons. The latter's surround inhibition is thus relatively resistant to antagonism by locally applied BMI. The greater heterogeneity of non-granular units in terms of RF properties and the effects of BMI is consistent with other findings demonstrating that neighboring neurons in these layers may participate in different local circuits.     

Characteristics of the receptive fields of neurons of the posterior temporal area of the cortex of the cat

In records of 219 single units in the posterotemporal cortical area (field 21) of nonanaesthetized cats, 51% of cells reacted to visual stimulation. The neurones had receptive fields (RFs) with central (0-10 degrees) or peripheral (10-52 degrees) localization in the visual field, their size increasing with eccentricity. Carting of RFs by a light bar scanning the visual field revealed a considerable variability of RFs shape, size and orientation in different cells. RFs sizes of the majority of recorded cells (100-1000 grad) were very large and exceeded the size of large RFs of neurones in the primary projection zone of the visual cortex.     

Central mechanisms of tactile shape perception

Studies show that while the cortical mechanisms of two-dimensional (2D) form and motion processing are similar in touch and vision, the mechanisms of three-dimensional (3D) shape processing are different. 2D form and motion are processed in areas 3b and 1 of SI cortex by neurons with receptive fields (RFs) composed of excitatory and inhibitory subregions. 3D shape is processed in area 2 and SII and relies on the integration of cutaneous and proprioceptive inputs. The RFs of SII neurons vary in size and shape with heterogeneous structures consisting of orientation-tuned fingerpads mixed with untuned excitatory or inhibitory fingerpads. Furthermore, the sensitivity of the neurons to cutaneous inputs changes with hand conformation. We hypothesize that these RFs are the kernels underlying tactile object recognition.     

Assembly of the dorsal horn somatotopic map

We hypothesize: (a) peripheral innervation densities determine map scales in dorsal horn, (b) dorsal horn cell (DHC) receptive field (RF) geometries are determined by map scales, and (c) morphologies of primary afferents (PAs) and DHCs reflect their developmental history. We suggest the following sequence: (A) PAs project in a somatotopic mediolateral sequence. (B) DHCs assemble prototype RFs by sampling presynaptic neuropil with their dendrites. (C) PAs then project to all levels where their RFs are contained within prototype RFs of DHCs. (D) A competitive mechanism produces the adult form of DHC RFs. (E) Adult distributions of PA terminals and DHC dendrites reflect this developmental history. (F) Mediolateral somatotopic gradients are determined by RF densities of axons entering at the same levels. (G) Map scales at different rostrocaudal levels are determined by somatotopic gradients. (H) Geometries of DHC RFs are determined by constant convergence and divergence of monosynaptic connections. (I) Secondary processes further modify geometries of DHC RFs. (J) Residual self-organizing capacity supports maintenance and plastic mechanisms. We adduce the following evidence: (1) agreement between monosynaptically coupled inputs and cells' excitatory low threshold mechanoreceptive fields; (2) the temporal sequence of events during penetration of the gray matter by PAs; (3) variation of PA terminal and DHC dendritic domains as a function of map scale; (4) somatotopic gradients and geometries of DHC RFs in adult dorsal horn; (5) calculations of peripheral innervation densities and dorsal horn map scales; and (6) constant divergence and convergence between PAs and DHCs.     

Functional features of the receptive fields of neurons of the posterior temporal cortex of the cat

Dynamic characteristics of the visual receptive fields (RF) of neurones in the postero-temporal cortical area (field 21) of alert cat were studied in prolonged repeated testings of RFs in the absence of influence controlled in the experiment. Significant fluctuations of RFs were found in the studied neurones--in changes of RFs form, their size and orientation and also in RF fragmentation, i.e. simultaneous and/or alternating appearance of separate zones (discharge centers) of RFs in the process of testing. It is suggested that plasticity and variability of neurones' RFs in time in this study are mainly due to modulating effects of extravisual structures, uncontrolled in the test,--to reticular inputs in particular.     

Responses of neurons of the extrastriate cortex of the cat brain to moving stimuli of different forms

We examined responses of neurons of the field 21b of the cat brain cortex to presentation of moving visual stimuli of different forms. Characteristics of the responses of about 54% of the studied neurons showed that in these cases configurations of the contours of moving stimuli were to a certain extent discriminated. Most neurons selectively reacting to changes in the form of the stimulus were dark-sensitive units (they generated optimum responses to presentation of dark visual stimuli on the light background). Detailed examination of the spatial infrastructure of receptive fields (RFs) of the neurons and comparison of this structure with the selectivity of neuronal responses showed that there is no significant correlation between static organization of the RF and responses of the neuron to the movements of stimuli of different forms. We hypothesize that the dynamic infrastructure of the RF and the combined activity of functional groups of neurons, whose RFs spatially overlap the RF of the neuron under study, play a definite role in the mechanisms responsible for neuronal discrimination of the form of the visual stimulus. Neirofiziologiya/Neurophysiology, Vol. 38, No. 1, pp. 61–71, January–February, 2006.     

Cellular effects of electromagnetic fields

Studies at the cellular level are needed to reveal the cellular and molecular biological mechanisms underlying the biological effects and possible health implications of non-ionising radiation, such as extremely low frequency (ELF) magnetic fields (MFs) and radiofrequency (RF) fields. Our research group has studied the effects of 50 Hz ELF MFs (caused by power lines and electric devices) and 872 MHz or 900 MHz RFs (emitted by mobile phones and their base stations) on cellular ornithine decarboxylase activity, cell cycle kinetics, cell proliferation, and necrotic or apoptotic cell death. For RFs, pulse-modulated (217 Hz modulation frequency corresponding a global system for mobile communication-type signal) or continuous wave (unmodulated) signals were used. To expose the cell cultures to MFs or RFs, specially developed exposure systems were used, where levels of electromagnetic field exposure and the conditions of cell culture could be precisely controlled. A coexposure approach was used in many studies, i.e. the cell cultures were exposed to other stressors in addition to MFs or RFs. Ultraviolet radiation, serum deprivation, or fresh medium addition, were used as co-exposures. The results presented in this short review show that the effects of mere MFs or RF on cell culture models are quite minor, but that various co-exposure approaches warrant additional study.     

Effects of intraoral mechanoreceptor stimulation on reticular formation neurones, in the rabbit

The effects of intraoral mechanoreceptor stimulation on the firing rate of single neurones of the brain stem reticular formation (RF) were investigated in rabbits. 30% of RF neurones responded to periodontal mechanoreceptor stimulation; 16% to mucosal mechanoreceptor stimulation and 6% to both types of stimuli. Periodontal stimulation induced mainly inhibitory effects localized within the mesencephalic and rostral pontine RF. Among periodontal afferents incisors were the most widely represented. The effects of mucosal mechanoreceptor stimulation were predominant in the medullary and pontine RF and they were mainly excitatory. The present results support the hypothesis that brain stem RF neurones can be recruited into regulating mastication and biting also by stimulation of intraoral mechanoreceptors.     

Receptive Fields of Visually Sensitive Neurons of the Extrastriate Associative Area 21b of the Cat Cerebral Cortex

Organization of the receptive fields (RFs) of neurons of the extrastriate associative region 21b of the cerebral cortex was studied in cats. Most neurons under study (63%) were “monocular,” while 37% of the cells were “binocular” units. Among 178 neurons examined in detail, heterogeneous RF functional organization was typical of about 76% of the units; point-to-point testing of the entire RF area by stationary stimuli resulted in the generation of various types of responses (on, off, or on-off). The rest of the neurons (24%) generated homogeneous responses. The dimension, form, and functional organization of RFs of the neurons under study depended to a certain extent on the parameters of visual stimuli used for the measurements. Examination of the influence of the visual space, which surrounded the RF, on responses of the neurons evoked by stimulation of the RF per se showed that darkening of the visual space adjacent to the RF inhibited neuronal responses to moving stimuli; in some cases the responses were totally suppressed. Analysis of spatial overlapping of the RF sequentially recorded in the course of each insertion of the electrode showed that the density of distribution of the overlapping RF areas of neighboring neurons with the RF of the examined neuron is irregular, and that the RF is of a mosaic nature. We hypothesize that the visual space surrounding the RF plays a significant role in the formation of responses of visually sensitive neurons to presentation of moving stimuli. Neirofiziologiya/Neurophysiology, Vol. 37, No. 3, pp. 223–234, May–June, 2005.     

Estimating receptive fields from responses to natural stimuli with asymmetric intensity distributions

The reasons for using natural stimuli to study sensory function are quickly mounting, as recent studies have revealed important differences in neural responses to natural and artificial stimuli. However, natural stimuli typically contain strong correlations and are spherically asymmetric (i.e. stimulus intensities are not symmetrically distributed around the mean), and these statistical complexities can bias receptive field (RF) estimates when standard techniques such as spike-triggered averaging or reverse correlation are used. While a number of approaches have been developed to explicitly correct the bias due to stimulus correlations, there is no complementary technique to correct the bias due to stimulus asymmetries. Here, we develop a method for RF estimation that corrects reverse correlation RF estimates for the spherical asymmetries present in natural stimuli. Using simulated neural responses, we demonstrate how stimulus asymmetries can bias reverse-correlation RF estimates (even for uncorrelated stimuli) and illustrate how this bias can be removed by explicit correction. We demonstrate the utility of the asymmetry correction method under experimental conditions by estimating RFs from the responses of retinal ganglion cells to natural stimuli and using these RFs to predict responses to novel stimuli.     

Representing three-dimensional objects by sets of activities of receptive fields

Idealized models of receptive fields (RFs) can be used as building blocks for the creation of powerful distributed computation systems. The present report concentrates on investigating the utility of collections of RFs in representing three-dimensional objects under changing viewing conditions. The main requirement in this task is that the pattern of activity of RFs vary as little as possible when the object and the camera move relative to each other. I propose a method for representing objects by RF activities, based on the observation that, in the case of rotation around a fixed axis, differences of activities of RFs that are properly situated with respect to that axis remain invariant. Results of computational experiments suggest that a representation scheme based on this algorithm for the choice of stable pairs of RFs would perform consistently better than a scheme involving random sets of RFs. The proposed scheme may be useful under object or camera rotation, both for ideal lambertian objects, and for real-world objects such as human faces.     

A cooperation and competition based simple cell receptive field model and study of feed-forward linear and nonlinear contributions to orientation selectivity

We present a model for development of orientation selectivity in layer IV simple cells. Receptive field (RF) development in the model, is determined by diffusive cooperation and resource limited competition guided axonal growth and retraction in geniculocortical pathway. The simulated cortical RFs resemble experimental RFs. The receptive field model is incorporated in a three-layer visual pathway model consisting of retina, LGN and cortex. We have studied the effect of activity dependent synaptic scaling on orientation tuning of cortical cells. The mean value of hwhh (half width at half the height of maximum response) in simulated cortical cells is 58° when we consider only the linear excitatory contribution from LGN. We observe a mean improvement of 22.8° in tuning response due to the non-linear spiking mechanisms that include effects of threshold voltage and synaptic scaling factor.     

Stimulus size selectivity and receptive field organization of ectostriatal neurons in the pigeon

The avian ectostriatum is the telencephalic recipient zone of the tectofugal pathway, which may be homologous to the colliculo-pulvinar-cortical pathway in mammals. The present paper studies the visual response properties and receptive field organization of ectostriatal cells in pigeons with extracellular recording and computer mapping techniques. The results indicate that 90% of ectostriatal cells prefer forward, downward and upward motion of visual stimuli at velocities in the range of 16-128 degrees s(-1). They respond optimally to small stimuli (1-4 degrees visual angle), mostly preferring a target of 2 degrees. Most cells (78.8%) have one excitatory receptive field that usually possesses one or two hotspots, some cells (13.5%) have two excitatory receptive fields each having one or two hotspots, and a few cells (7.7%) have one excitatory receptive field with an unresponsive region in the center. An inhibitory receptive field is not found surrounding the excitatory receptive field in the ectostriatal cells examined. These response properties and receptive field organization may reflect the possible roles of the ectostriatum in stimulus discrimination and visual cognition.     

Flexibility of spatial averaging in visual perception

The classical receptive field (RF) concept-the idea that a visual neuron responds to fixed parts and properties of a stimulus-has been challenged by a series of recent physiological results. Here, we extend these findings to human vision, demonstrating that the extent of spatial averaging in contrast perception is also flexible, depending strongly on stimulus contrast and uniformity. At low contrast, spatial averaging is greatest (about 11 min of arc) within uniform regions such as edges, as expected if the relevant neurons have orientation-selective RFs. At high contrast, spatial averaging is minimal. These results can be understood if the visual system is balancing a trade-off between noise reduction, which favours large areas of averaging, and detail preservation, which favours minimal averaging. Two distinct populations of neurons with hard-wired RFs could account for our results, as could the more intriguing possibility of dynamic, contrast-dependent RFs.