Self-organization of orientation sensitive cells in the striate cortex

A nerve net model for the visual cortex of higher vertebrates is presented. A simple learning procedure is shown to be sufficient for the organization of some essential functional properties of single units. The rather special assumptions usually made in the literature regarding preorganization of the visual cortex are thereby avoided. The model consists of 338 neurones forming a sheet analogous to the cortex. The neurones are connected randomly to a retina of 19 cells. Nine different stimuli in the form of light bars were applied. The afferent connections were modified according to a mechanism of synaptic training. After twenty presentations of all the stimuli individual cortical neurones became sensitive to only one orientation. Neurones with the same or similar orientation sensitivity tended to appear in clusters, which are analogous to cortical columns. The system was shown to be insensitive to a background of disturbing input excitations during learning. After learning it was able to repair small defects introduced into the wiring and was relatively insensitive to stimuli not used during training.     

A geometric model of orientation tuning dynamics in visual neurons]

A geometrical model for the dynamics of orientation tuning of visual neurones was proposed, which makes it possible to study the dynamics of configuration, localization, and weight of excitatory and inhibitory subzones of the receptive field. The model reproduces typical patterns of orientation tuning dynamics, observed in neurophysiological experiments on cat visual cortex neurones. The parameters of the model (size and mutual position of excitatory and inhibitory zones of the receptive field, their weight, and dynamics type) were estimated that correspond to the main types of orientation tuning dynamics in natural conditions. It is shown that selective and acute tuning of neurones can be formed and/or sharpened by intracotrical inhibition, while the dynamics of preferred orientation is due to changes in the geometry of the inhibitory subzone of the receptive field.     

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.     

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.     

Conjugation of unit activity in the visual and sensomotor areas of the rabbit neocortex during a conditioned reflex to sound and light

Conjugation of unit activity in the visual and sensorimotor neocortical areas was studied by means of histograms of cross- and autocorrelation in rabbits with conditioned reflex to light (1st group) and sound (2nd group). Relative number of neurones pairs acting in correlation in the areas remote from each other, in intersignal intervals both before and after stimuli did not differ in the 1st and 2nd groups. At the same time delays in neuronal discharges in one area after the other were different. In the 1st group animals there was a predominance of the number of visual area neurones discharging after sensorimotor with a delay up to 125 ms, in comparison with the number of sensorimotor area neurones discharging after the visual one. In the 2nd group rabbits the number of visual area neurones with such a delay of discharges after sensorimotor was less and, on the contrary, a predominance of sensorimotor area neurones was observed discharging after the visual one. The obtained results allow to suggest that neurones of the visual and sensorimotor neocortex areas form a single functional system in cases when conditioned and unconditioned stimuli are addressed to these areas and when only one of the studied areas is the projection zone for the combined stimuli. Organization of the neurones activity in systems in these two cases is different.     

Trace reactions following combination of a sensory stimulus with polarization of the visual region of the cortex]

Spontaneous activity and responses to photic flashes and tones of 133 neurones were recorded in the visual cortex during polarization of the same area (1.5 to 10 muA, 5 to 30 min) and after it (one to 52 min). Responses of cells to two unimodal stimuli of different parameters were analysed, of which one was presented repeatedly during the polarization ("positive"), and the other one to three times ("negative"). Depending on the previous "learning", 47.4% of the units responded after the polarization to "positive" photic stimulus and 37,8%--to acoustic stimulus. The trace effects of the stimuli pairings are reproduced in polarization after-effect by the action of the sensory signal alone. The recorded differences in the nature and duration of the reproduction of trace processes formed to an adequate and inadequate stimuli, are due to the dissimilar action of polarizing currents on neurones of the cortex cross-section and to different effectivity of the visual and non-visual influences related to it.     

Activity of callosal neurons of the visual cortex in the cat

Activity of 28 identified neurones of the visual cortex was recorded in cats immobilized by d-tubocurarine. Stimulation of the callosal body with a single stimulus or high-frequency train elicited a short-latency antidromic reaction of neurones in the visual cortex whose axons constitute the main part of the large cerebral commissure. Some commissural neurones responded to a single callosal stimulation by two action potentials the first one being antidromic, the second one being of long-latency postsynaptic origin. The second action potential was generated as a result of activation of axonal collaterals of the same neurone or the neighboring callosal neurones. More than a half of callosal neurones responded to a single stimulation of the lateral geniculate body by short-latency antidromic discharges and by long-latency postsynaptic reactions. These data indicate the existence of the systems of two-way neuronal connections, i.e. calloso-geniculate and geniculo-callosal ones.     

Distortions of Perceived Orientation

SEVERAL visual discriminations are best for vertical and horizontal test objects and worst for objects oriented at 45° to the main axes^1–3. For resolution acuity it is quite clear that the differences are not due to any optical defects¹ and it seems probable that the same is true for tasks involving the discrimination of differences in orientation^2,3. A likely source of these variations is the orientationally sensitive neurones of the type found in the cat and monkey visual cortex^4,5. Psychophysical evidence^1,3,6 suggests that similar neurones exist in the human visual system and so we might expect some aspect of the sensitivity of these units to vary as a function of the orientation to which they are maximally sensitive.     

A comparison of the spike train activity of the cortical neurons and of the spatial synchronization of the EEG

In chronic experiments EEG coherence and conjugation of impulse activity were compared of neurones of the visual and sensorimotor areas of rabbits neocortex simultaneously recorded with the same electrodes. Connection was revealed between the presence and properties of conjugated neurones activity and EEG coherence at various frequencies. At correlated neurones activity a greater EEG coherence was observed on frequencies of 3-4,5 Hz than at the independent activity. At the highest level of the EEG coherence the neurones discharged with less delay of one after the other in pairs, and in their synchronization a common source participated more often than at the lowest level of the EEG coherence.     

Encoding of plant odour information in insects: peripheral and central mechanisms

Insects are suitable model organisms for studying mechanisms underlying olfactory coding and olfactory learning, by their unique adaptation to host plants in which the chemical senses are essential. Recent molecular biological studies have shown that a large number of genes in insects and other organisms are coding for olfactory receptor proteins. In general, one receptor type seems to be expressed in each neurone. The functional characterisations of olfactory receptor neurones have been extensive in certain insect species, demonstrating a fine-tuning of single neurones to biologically relevant odourants; both insect and plant produced volatiles. Stained neurones of the same functional type have been shown to project in one and the same glomerular unit in the primary olfactory centre, the antennal lobe. This corresponds to molecular biological studies, showing projections in one glomerulus by neurones expressing the same receptor type. Comparison of these findings with physiological and morphological characterisations of antennal lobe neurones has indicated correspondence between input and output of the glomerular units. Examples are presented from studies of heliothine moths. From the antennal lobe, the olfactory information is further conveyed to the mushroom bodies, particularly important for learning, and the lateral protocerebrum, a premotoric area. The three brain areas are regions of synaptic plasticity important in learning of odours, which is well studied in the honeybee but also in species of moths.     

Innervation pattern of suboesophageal ventral unpaired median neurones in the honeybee brain

In honeybees (Apis mellifera), the biogenic amine octopamine has been shown to play a role in associative and non-associative learning and in the division of labour in the hive. Immunohistochemical studies indicate that the ventral unpaired median (VUM) neurones in the suboesophageal ganglion (SOG) are putatively octopaminergic and therefore might be involved in the octopaminergic modulation of behaviour. In contrast to our knowledge about the behavioural effects of octopamine, only one neurone (VUMmx1) has been related to a behavioural effect (the reward function during olfactory learning). In this study, we have investigated suboesophageal VUM neurones with fluorescent dye-tracing techniques and intracellular recordings combined with intracellular staining. Ten different VUM neurones have been found including six VUM neurones innervating neuropile regions of the brain and the SOG exclusively (central VUM neurones) and four VUM neurones with axons in peripheral nerves (peripheral VUM neurones). The central VUM neurones innervate the antennal lobes, the protocerebral lobes (including the lateral horn) and the mushroom body calyces. Of these, a novel mandibular VUM neurone, VUMmd1, exhibits the same branching pattern in the brain as VUMmx1 and responds to sucrose and odours in a similar way. The peripheral VUM neurones innervate the antennal and the mandibular nerves. In addition, we describe one labial unpaired median neurone with a dorsal cell body, DUMlb1. The possible homology between the honeybee VUM neurones and the unpaired median neurones in other insects is discussed. This work was supported by the DFG ME 365/24-2.     

Neurophysiologic and biochemical aspects of visual system development in the rabbit under conditions of photic deprivation]

Changes in the functional state of the visual cortex were studied by behavioral and electrophysiological cues and the chemism of its neurones at the cellular and subcellular levels in rabbits raised for one to two months in the dark. It has been shown that visual deprivation leads to retarded dynamics of elaboration and consolidation of the conditioned defensive reflex to light and to changes of opposite signs of the visual cortex surface EPs to specific and non-specific stimuli. Typical of the EPs to photic stimuli is a considerable decrease in amplitude and longer latency as compared with normal, and enhanced amplitude and shorter latency to acoustic stimuli. It has been cytochemically established that about half of the pyramidal neurones of the visual cortex layer V of the experimental animals display features of biochemical underdevelopment (of the size of the cytoplasmatic mass, the protein reserve). Under the same conditions activity depression was revealed in cytochromoxydase, Na, P-ATPhase and ACHE, expressed to a different degree in separate subcellular fractions of the visual cortex. The MAO activity selectively augments in the subfraction of cholinergic synaptosomes. It has been assumed that functional and biochemical changes in different groups of the visual cortex neurones due to deprivation are linked with both the properties of the synaptic structures in regard to perception of impulses of different modalities and the peculiarities of their chemism.     

Frequent Video Game Players Resist Perceptual Interference

Playing certain types of video games for a long time can improve a wide range of mental processes, from visual acuity to cognitive control. Frequent gamers have also displayed generalized improvements in perceptual learning. In the Texture Discrimination Task (TDT), a widely used perceptual learning paradigm, participants report the orientation of a target embedded in a field of lines and demonstrate robust over-night improvement. However, changing the orientation of the background lines midway through TDT training interferes with overnight improvements in overall performance on TDT. Interestingly, prior research has suggested that this effect will not occur if a one-hour break is allowed in between the changes. These results have suggested that after training is over, it may take some time for learning to become stabilized and resilient against interference. Here, we tested whether frequent gamers have faster stabilization of perceptual learning compared to non-gamers and examined the effect of daily video game playing on interference of training of TDT with one background orientation on perceptual learning of TDT with a different background orientation. As a result, we found that non-gamers showed overnight performance improvement only on one background orientation, replicating previous results with the interference in TDT. In contrast, frequent gamers demonstrated overnight improvements in performance with both background orientations, suggesting that they are better able to overcome interference in perceptual learning. This resistance to interference suggests that video game playing not only enhances the amplitude and speed of perceptual learning but also leads to faster and/or more robust stabilization of perceptual learning.     

Disinhibition as a mechanism of tuning of cross-like figures in the visual cortex neurons

A discrete neural net was used for simulation of cross-sensitivity in 40% of neurones of the cat visual cortex' area 17th. It is based on disinhibition of the end-stopping inhibition in receptive field from the side-disinhibitory zone. Highly selective or invariant sensitivity of the simulated neurone in respect to shape and orientation of a cross-like figure was observed under changes of location, size and weight of the receptive field zones. The disinhibitory mechanism seems to be critically involved in the selection of the second-order features of the images in the primary visual cortex.     

Neuronal specialization of the motor cortex in normal rabbits and following destruction of the visual cortex

The activity of neurones of the anterolateral part of the motor cortex in food-acquisition behaviour was compared in two control rabbits and in three rabbits after the operation of bilateral ablation of the striatal cortex. In two of three operated rabbits the pattern of behavioural specialization lost considerably the specificity peculiar to the motor cortex (predominance of G-neurones activated in grasping of food), approaching (but not becoming identical) the pattern of specialization of the visual cortex neurones: the number of G-neurones decreased in a half, and the number of L-neurones (activated in connection with the acts of instrumental food-acquisition behaviour which animals were trained to in the experimental cage) was doubled. Changes of the activity were significantly less expressed in the third operated rabbit. The number of the neurones activated in food-acquisition behaviour in operated rabbits in comparison with the control ones was reduced in the upper layers of the cortex and increased in the lower layers. The resemblance is discussed of the basic processes of animals learning and behaviour recovery.     

Activity-dependent regulation of receptive field properties of cat area 17 by supervised Hebbian learning.

Most algorithms currently used to model synaptic plasticity in self-organizing cortical networks suppose that the change in synaptic efficacy is governed by the same structuring factor, i.e., the temporal correlation of activity between pre- and postsynaptic neurons. Functional predictions generated by such algorithms have been tested electrophysiologically in the visual cortex of anesthetized and paralyzed cats. Supervised learning procedures were applied at the cellular level to change receptive field (RF) properties during the time of recording of an individual functionally identified cell. The protocols were devised as cellular analogs of the plasticity of RF properties, which is normally expressed during a critical period of postnatal development. We summarize here evidence demonstrating that changes in covariance between afferent input and postsynaptic response imposed during extracellular and intracellular conditioning can acutely induce selective long-lasting up- and down-regulations of visual responses. The functional properties that could be modified in 40% of cells submitted to differential pairing protocols include ocular dominance, orientation selectivity and orientation preference, interocular orientation disparity, and the relative dominance of ON and OFF responses. Since changes in RF properties can be induced in the adult as well, our findings also suggest that similar activity-dependent processes may occur during development and during active phases of learning under the supervision of behavioral attention or contextual signals. Such potential for plasticity in primary visual cortical neurons suggests the existence of a hidden connectivity expressing a wider functional competence than the one revealed at the spiking level. In particular, in the spatial domain the sensory synaptic integration field is larger than the classical discharge field. It can be shaped by supervised learning and its subthreshold extent can be unmasked by the pharmacological blockade of intracortical inhibition.     

Types of correlative relations of the neuronal impulses of the visual and sensomotor areas of the neocortex in the rabbit

Neuronal activity of the visual and sensorimotor cortical areas was simultaneously recorded in intersignal intervals in rabbits with conditioned reflexes to light and sound. ES-1020 computer built cross-correlation (CCH) and autocorrelation (ACH) histograms of impulse activity of the se neurones. Analysis of CCH form, built with a bin of 2 ms gave no convincing proofs of the presence of synaptic connections between neurone of the visual and sensorimotor cortical areas and of influences from a common source with few switchings on interneurones. Analysis of CCH built with bins of 10 and 30 ms, allowed to single out primary and secondary peaks and troughs. Wide primary peaks with the base covering the beginning of the coordinates, were met in 51.9% of cases; wide peaks, greatly shifted relatively to the coordinates beginning,--in 40.9% and trough--in 7.2% of cases. Secondary peaks did not always reproduce the ACH form of impulses trains of recorded neurones. The CCH analysis allowed to suggest two basic mechanisms, eliciting the correlated discharges--action of common input and influence of the neurones of one area on the other through a number of interneurones. It was not always possible to separate these two cases by means of CCH. In a number of cases, a more complicated character of neurones interaction could be suggested.     

Perceptual Grouping Enhances Visual Plasticity

Visual perceptual learning, a manifestation of neural plasticity, refers to improvements in performance on a visual task achieved by training. Attention is known to play an important role in perceptual learning, given that the observer''s discriminative ability improves only for those stimulus feature that are attended. However, the distribution of attention can be severely constrained by perceptual grouping, a process whereby the visual system organizes the initial retinal input into candidate objects. Taken together, these two pieces of evidence suggest the interesting possibility that perceptual grouping might also affect perceptual learning, either directly or via attentional mechanisms. To address this issue, we conducted two experiments. During the training phase, participants attended to the contrast of the task-relevant stimulus (oriented grating), while two similar task-irrelevant stimuli were presented in the adjacent positions. One of the two flanking stimuli was perceptually grouped with the attended stimulus as a consequence of its similar orientation (Experiment 1) or because it was part of the same perceptual object (Experiment 2). A test phase followed the training phase at each location. Compared to the task-irrelevant no-grouping stimulus, orientation discrimination improved at the attended location. Critically, a perceptual learning effect equivalent to the one observed for the attended location also emerged for the task-irrelevant grouping stimulus, indicating that perceptual grouping induced a transfer of learning to the stimulus (or feature) being perceptually grouped with the task-relevant one. Our findings indicate that no voluntary effort to direct attention to the grouping stimulus or feature is necessary to enhance visual plasticity.     

Neuronal impulse trains of the visual and sensorimotor areas of the rabbit neocortex in calm wakefulness and pseudoconditioning

The conjugation of unit activity in the neocortical visual and sensorimotor areas during calm wakefulness and in intersignal intervals, in two groups of rabbits at pseudoconditioning was studied. The first group was presented in a random order with flashes and electrocutaneous stimuli, the second one--with sounds and electrocutaneous stimuli. The number of neurones pairs working in correlation during calm wakefulness is significantly less (35%) than during pseudoconditioning (49 and 50% in the first and second rabbits groups, respectively). During calm wakefulness and in both groups during pseudoconditioning, the number of pairs with delays of discharges of the visual area neurones after the sensorimotor one, and of the sensorimotor after visual up to 120 ms was equal. Comparison of the data on delayed neuronal discharges during calm wakefulness and pseudoconditioning with those obtained earlier with conditioned reflexes testifies that forestalling of visual area neuronal discharges by sensorimotor discharges is characteristic only for the activity of cortical projections of conditioned and unconditioned stimuli.     

A confined taste area in a lepidopteran brain

Knowledge about the neuronal pathways of the taste system is interesting both for studying taste coding and appetitive learning of odours. We here present the morphology of the sensilla styloconica on the proboscis of the moth Heliothis virescens and the projections of the associated receptor neurones in the central nervous system. The morphology of the sensilla was studied by light microscopy and by scanning- and transmission electron microscopy. Each sensillum contains three or four sensory neurones; one mechanosensory and two or three chemosensory. The receptor neurones were stained with neurobiotin tracer combined with avidin-fluorescein conjugate, and the projections were viewed in a confocal laser-scanning microscope. The stained axons entered the suboesophageal ganglion via the maxillary nerves and were divided into two categories based on their projection pattern. Category one projected exclusively ipsilaterally in the dorsal suboesophageal ganglion/tritocerebrum and category two projected bilaterally and more ventrally in the suboesophageal ganglion confined to the anterior surface of the neuropil. The bilateral projecting neurones had one additional branch terminating ipsilaterally in the dorsal suboesophageal ganglion/tritocerebrum. A possible segregation of the two categories of projections as taste and mechanosensory is discussed.