Continuous flash suppression of neural responses and population orientation coding in macaque V1.

Computational models of the primary visual cortex (V1) have suggested that V1 neurons behave like Gabor filters followed by simple nonlinearities. However, recent work employing convolutional neural network (CNN) models has suggested that V1 relies on far more nonlinear computations than previously thought. Specifically, unit responses in an intermediate layer of VGG-19 were found to best predict macaque V1 responses to thousands of natural and synthetic images. Here, we evaluated the hypothesis that the poor performance of lower layer units in VGG-19 might be attributable to their small receptive field size rather than to their lack of complexity per se. We compared VGG-19 with AlexNet, which has much larger receptive fields in its lower layers. Whereas the best-performing layer of VGG-19 occurred after seven nonlinear steps, the first convolutional layer of AlexNet best predicted V1 responses. Although the predictive accuracy of VGG-19 was somewhat better than that of standard AlexNet, we found that a modified version of AlexNet could match the performance of VGG-19 after only a few nonlinear computations. Control analyses revealed that decreasing the size of the input images caused the best-performing layer of VGG-19 to shift to a lower layer, consistent with the hypothesis that the relationship between image size and receptive field size can strongly affect model performance. We conducted additional analyses using a Gabor pyramid model to test for nonlinear contributions of normalization and contrast saturation. Overall, our findings suggest that the feedforward responses of V1 neurons can be well explained by assuming only a few nonlinear processing stages.

RECEPTIVE FIELDS OF OPTIC TRACT AXONS AND LATERAL GENICULATE CELLS: PERIPHERAL EXTENT AND BARBITURATE SENSITIVITY.

With the help of the current experiment, we wanted to learn more about the impact of visually demanding vs. cognitively demanding secondary tasks on the attention allocation of older pedestrians during the phase of traffic perception within the process of road crossing. For this purpose, we used two different road crossing tasks as well as two different secondary tasks. The road crossing “stop task” was a signal detection task, where an approaching car had to be detected. The road crossing “go task” was a dynamic visual search task, where the resolution of a busy road situation had to be identified. The visual secondary task was a static visual search task and the cognitive secondary task was a 1-back (memory) task. One younger group (≤ 30 years) and one older group (≥ 65 years) of participants completed the tasks as single vs. dual-tasks in all possible combinations. Performance was measured through errors and response time; in addition, the subjective workload was assessed via NASA-TLX. Analyses show that the visual secondary task reduces performance in the road crossing more strongly than the cognitive task, while the visual task itself is less impaired by the road crossing tasks than is the cognitive task. Overall, performance diminishes from single to dual-task completion. Results further indicate age effects in terms of increased errors and response time for older compared to younger participants. In addition to these age effects, age-specific dual-task effects emerge for response time in the go task along with the visual task as well as for response time in the cognitive task along with the go task. Subjective workload is higher in the dual-task conditions than in the single tasks. Findings are discussed with regard to theoretical and practical implications.

Surround modulation is predominantly orientation-unspecific in macaque V1.

The sensory neocortex is a highly connected associative network that integrates information from multiple senses, even at the level of the primary sensory areas. Although a growing body of empirical evidence supports this view, the neural mechanisms of cross-modal integration in primary sensory areas, such as the primary visual cortex (V1), are still largely unknown. Using two-photon calcium imaging in awake mice, we show that the encoding of audiovisual stimuli in V1 neuronal populations is highly dependent on the features of the stimulus constituents. When the visual and auditory stimulus features were modulated at the same rate (i.e., temporally congruent), neurons responded with either an enhancement or suppression compared with unisensory visual stimuli, and their prevalence was balanced. Temporally incongruent tones or white-noise bursts included in audiovisual stimulus pairs resulted in predominant response suppression across the neuronal population. Visual contrast did not influence multisensory processing when the audiovisual stimulus pairs were congruent; however, when white-noise bursts were used, neurons generally showed response suppression when the visual stimulus contrast was high whereas this effect was absent when the visual contrast was low. Furthermore, a small fraction of V1 neurons, predominantly those located near the lateral border of V1, responded to sound alone. These results show that V1 is involved in the encoding of cross-modal interactions in a more versatile way than previously thought.SIGNIFICANCE STATEMENT The neural substrate of cross-modal integration is not limited to specialized cortical association areas but extends to primary sensory areas. Using two-photon imaging of large groups of neurons, we show that multisensory modulation of V1 populations is strongly determined by the individual and shared features of cross-modal stimulus constituents, such as contrast, frequency, congruency, and temporal structure. Congruent audiovisual stimulation resulted in a balanced pattern of response enhancement and suppression compared with unisensory visual stimuli, whereas incongruent or dissimilar stimuli at full contrast gave rise to a population dominated by response-suppressing neurons. Our results indicate that V1 dynamically integrates nonvisual sources of information while still attributing most of its resources to coding visual information.

Pattern adaptation and cross-orientation interactions in the primary visual cortex

The mechanosensory role of TRPA1 and its contribution to mechanical hypersensitivity in sensory neurons remains enigmatic. We elucidated this role by recording mechanically activated currents in conjunction with TRPA1 over- and under-expression and selective pharmacology. First, we established that TRPA1 transcript, protein and functional expression are more abundant in smaller-diameter neurons than larger-diameter neurons, allowing comparison of two different neuronal populations. Utilising whole cell patch clamping, we applied calibrated displacements to neurites of dorsal root ganglion (DRG) neurons in short-term culture and recorded mechanically activated currents termed intermediately (IAMCs), rapidly (RAMCs) or slowly adapting (SAMCs). Trpa1 deletion (–/–) significantly reduced maximum IAMC amplitude by 43% in small-diameter neurons compared with wild-type (+/+) neurons. All other mechanically activated currents in small- and large-diameter Trpa1−/− neurons were unaltered. Seventy-three per cent of Trpa1+/+ small-diameter neurons responding to the TRPA1 agonist allyl-isothiocyanate (AITC) displayed IAMCs to neurite displacement, which were significantly enhanced after AITC addition. The TRPA1 antagonist HC-030031 significantly decreased Trpa1+/+ IAMC amplitudes, but only in AITC responsive neurons. Using a transfection system we also showed TRPA1 over-expression in Trpa1+/+ small-diameter neurons increases IAMC amplitude, an effect reversed by HC-030031. Furthermore, TRPA1 introduction into Trpa1−/− small-diameter neurons restored IAMC amplitudes to Trpa1+/+ levels, which was subsequently reversed by HC-030031. In summary our data demonstrate TRPA1 makes a contribution to normal mechanosensation in a specific subset of DRG neurons. Furthermore, they also provide new evidence illustrating mechanisms by which sensitisation or over-expression of TRPA1 enhances nociceptor mechanosensitivity. Overall, these findings suggest TRPA1 has the capacity to tune neuronal mechanosensitivity depending on its degree of activation or expression.

Developmental Switch in Neuropeptide Y and Melanocortin Effects in the Paraventricular Nucleus of the Hypothalamus

Metabotropic glutamate receptors (mGluRs) couple the actions of glutamate to intracellular second messenger systems through G-proteins. The mGluRs play an important role in the regulation of basal ganglia function. Ligand binding studies have revealed that the basal ganglia contain at least two pharmacological types of metabotropic binding sites. Agonists of mGluRs can affect both in vitro electrophysiologic responses of striatal neurons and motor behavior in vivo. Recently, cDNAs encoding five mGluRs have been cloned, each with distinct structural and pharmacological properties. In order to elucidate the function of these receptors in the biology of the extrapyramidal motor system, we have used in situ hybridization to examine the regional and cellular expression patterns of mGluR1-mGluR5 in the adult rat basal ganglia. In the striatum, all of these mGluRs were present in widely varying relative densities and cellular patterns. MGluR5 was particularly prominent, and exhibited a heterogeneous cellular distribution, with labeled and unlabeled populations of neurons. MGluR2 was expressed in a small population of large polygonal striatal neurons. The subthalamic nucleus was the only other basal ganglia structure that expressed mGluR2. Distinct cellular distributions of mGluR expression were also observed within the nucleus accumbens, globus pallidus, ventral pallidum, and substantia nigra pars reticulata. MGluR3 was expressed in glia in all basal ganglia structures, but was observed in neurons only in the striatum, substantia nigra pars reticulata, and very weakly in the subthalamic nucleus. Comparison of the restricted mGluR2 and mGluR3 mRNA distributions with that of metabotropic ligand binding sites supports a possible presynaptic location for these receptors in the basal ganglia. MGluR1 was the only mGluR message prominently expressed in the dopaminergic neurons of the substantia nigra pars compacta, suggesting the involvement of this receptor in the regulation of dopamine release from nigrostriatal terminals.

This study used a dual task design to investigate the effects of two different types of cognitive tasks on stability (as measured by center of pressure displacement) in young vs older adults with and without a history of falls. Two secondary cognitive tasks, a sentence completion and a visual perceptual matching task, were used to produce changes in attention during quiet stance under flat vs compliant surface conditions in 20 healthy young adults, 20 healthy older adults, and 20 older adults with a history of imbalance and falls. Postural stability was quantified using forceplate measures of center of pressure (COP). Speed and accuracy of verbal response on the cognitive tasks were also quantified. During the simultaneous performance of a cognitive and postural task, decrements in performance were found in the postural stability measures rather than the cognitive measures for all three groups. While no differences were found between the young adults and the older healthy adults on the firm surface, no task condition, when task complexity was increased (either through the introduction of a secondary cognitive task, or a more challenging postural condition such as standing on the compliant surface), significant differences in postural stability between the two groups became apparent. In contrast to the young and healthy older adults, postural stability in older adults with a history of falls was significantly affected by both cognitive tasks. Results suggest that when postural stability is impaired, even relatively simple cognitive tasks can further impact balance. Results further suggest that the allocation of attention during the performance of concurrent tasks is complex; depending on many factors including the nature of both the cognitive and postural task, the goal of the subject and the instructions.

Macaque primary visual cortex (V1) exhibits exquisite columnar organization, while midlevel area V4 does not. Here we investigated the functional organization and representational bases of intervening area V2 in three macaques (one male, two females) with high-density Neuropixels recordings and a variety of visual stimuli-shape, texture, drifting grating, and translational motion patches. We observed dense clusters of similarly tuned neurons often spanning ∼500 µm, consistent with a columnar structure. In terms of representational bases, V2 responses were largely explained by stimulus features based on local image statistics: shape tuning is well modeled by a linear combination of orientation filters, and direction selectivity is stronger with surface compared with object motion, in striking contrast to V4. Overall, our results support the progression from columns to sparse clusters as neuronal representations transform from encoding local features and feature conjunctions in V1/V2 to a high-dimensional object-based code in V4.

Voltage-gated potassium channels that are composed of Kv3 subunits exhibit distinct electrophysiological properties: activation at more depolarized potentials than other voltage-gated K+ channels and fast kinetics. These channels have been shown to contribute to the high-frequency firing of fast-spiking (FS) GABAergic interneurons in the rat and mouse brain. In the rodent neocortex there are distinct patterns of expression for the Kv3.1b and Kv3.2 channel subunits and of coexpression of these subunits with neurochemical markers, such as the calcium-binding proteins parvalbumin (PV) and calbindin D-28K (CB). The distribution of Kv3 channels and interrelationship with calcium-binding protein expression has not been investigated in primate cortex. We used immunoperoxidase and immunofluorescent labeling and stereological counting techniques to characterize the laminar and cell-type distributions of Kv3-immunoreactive (ir) neurons in macaque V1. We found that across the cortical layers approximately 25% of both Kv3.1b- and Kv3.2-ir neurons are non-GABAergic. In contrast, all Kv3-ir neurons in rodent cortex are GABAergic (Chow et al. [1999] J Neurosci. 19:9332-9345). The putatively excitatory Kv3-ir neurons were mostly located in layers 2, 3, and 4b. Further, the proportion of Kv3-ir neurons that express PV or CB also differs between macaque V1 and rodent cortex. These data indicate that, within the population of cortical neurons, a broader population of neurons, encompassing cells of a wider range of morphological classes may be capable of sustaining high-frequency firing in macaque V1.

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

We perceive objects as permanent and stable despite frequent occlusions and eye movements, but their representation in the visual cortex is neither permanent nor stable. Feature selective cells respond only as long as objects are visible, and their responses depend on eye position. We explored the hypothesis that the system maintains object pointers that provide permanence and stability. Pointers should send facilitatory signals to the feature cells of an object, and these signals should persist across temporary occlusions and remap to compensate for image displacements caused by saccades. Here, we searched for such signals in monkey areas V2 and V4 (Macaca mulatta). We developed a new paradigm in which a monkey freely inspects an array of objects in search for reward while some of the objects are being occluded temporarily by opaque drifting strips. Two types of objects were used to manipulate attention. The results were as follows. 1) Eye movements indicated a robust representation of location and type of the occluded objects; 2) in neurons of V4, but not V2, occluded objects produced elevated activity relative to blank condition; 3) the elevation of activity was reduced for objects that had been fixated immediately before the current fixation ('inhibition of return'); and 4) when attended, or when the target of a saccade, visible objects produced enhanced responses in V4, but occluded objects produced no modulation. Although results 1-3 confirm the hypothesis, the absence of modulation under occlusion is not consistent. Further experiments are needed to resolve this discrepancy.NEW & NOTEWORTHY The way we perceive objects as permanent contrasts with the short-lived responses of visual cortical neurons. A theory postulates pointers that give objects continuity, predicting a class of neurons that respond not only to visual objects but also when an occluded object moves into their receptive field. Here, we tested this theory with a novel paradigm in which a monkey freely scans an array of objects while some of them are transiently occluded.

Information in the brain is encoded by large populations of neurons – neural ensembles. The place cells in the hippocampal CA1 field have become an experimental model for the study of neural ensembles of the brain due to the convenience of research. This review is devoted to the latest studies of place cells in the CA1 field. We consider the principles of encoding space by place cells, mechanisms for controlling the activity of place cells, anatomical and physiological features of place cells in different parts of the CA1 field. Key points: 1. There are rate and phase coding; 2. Dense local connections between pyramidal neurons can provide information processing; 3. Interneurons are involved in the formation of both the rate and phase code of place cells; 4. Pyramidal neurons are anatomically and functionally divided into deep and superficial; 5. Along the dorsoventral axis, the spatial and non-spatial component of information is generalized. The CA1 field has extensive capabilities for signal processing and can implement a computationally complex operation in the cognitive processes of the brain.