Search

Visual perception is formed over time through the formation process and visual pathway. Exercise improves visual perception, but it is unclear whether exercise modulates nonspecifically or specifically the formation process and pathway of visual perception. Healthy young men performed the visual detection task in a backward masking paradigm before and during cycling exercise at a mild intensity or rest (control). The task presented gratings of a circular patch (target) and annulus (mask) arranged concentrically as a visual stimulus and asked if the presence and striped pattern (feature) of the target were detected. The relationship between the orientations of the gratings of the target and the mask included iso‐orientation and orthogonal orientation to investigate the orientation selectivity of the masking effect. The masking effect was evaluated by perceptual suppressive index (PSI). Exercise improved feature detection (∆PSI; Exercise: −20.6%, Control: 1.7%) but not presence detection (∆PSI; Exercise: 8.9%, Control: 29.6%) compared to the control condition, and the improving effect resulted from the attenuation of the non‐orientation‐selective (∆PSI; Exercise: −29.0%, Control: 16.8%) but not orientation‐selective masking effect (∆PSI; Exercise: −3.1%, Control: 11.7%). These results suggest that exercise affects the formation process of the perceptual feature of the target stimulus by suppressively modulating the neural networks responsible for the non‐orientation‐selective surround interaction in the subcortical visual pathways, whose effects are inherited by the cortical visual pathways necessary for perceptual image formation. In conclusion, our findings suggest that acute exercise improves visual perception transiently through the modulation of a specific formation process of visual processing. [ABSTRACT FROM AUTHOR]

Feedback regulation from the higher association areas is thought to control the primary sensory cortex, contribute to the cortical processing of sensory information, and work for higher cognitive functions such as multimodal integration and attentional control. However, little is known about the underlying neural mechanisms. Here, we show that the posterior parietal cortex (PPC) persistently inhibits the activity of the primary visual cortex (V1) in mice. Activation of the PPC causes the suppression of visual responses in V1 and induces the short‐term depression, which is specific to visual stimuli. In contrast, pharmacological inactivation of the PPC or disconnection of cortical pathways from the PPC to V1 results in an effect of transient enhancement of visual responses in V1. Two‐photon calcium imaging demonstrated that the cortical disconnection caused V1 excitatory neurons an enhancement of visual responses and a reduction of orientation selectivity index (OSI). These results show that the PPC regulates the response properties of V1 excitatory neurons. Our findings reveal one of the functions of the PPC, which may contribute to higher brain functions in mice. [ABSTRACT FROM AUTHOR]

Responses of most neurons in the primary visual cortex of mammals are markedly selective for stimulus orientation and their orientation tuning does not vary with changes in stimulus contrast. The basis of such contrast invariance of orientation tuning has been shown to be the higher variability in the response for low-contrast stimuli. Neurons in the lateral geniculate nucleus ( LGN), which provides the major visual input to the cortex, have also been shown to have higher variability in their response to low-contrast stimuli. Parallel studies have also long established mild degrees of orientation selectivity in LGN and retinal cells. In our study, we show that contrast invariance of orientation tuning is already present in the LGN. In addition, we show that the variability of spike responses of LGN neurons increases at lower stimulus contrasts, especially for non-preferred orientations. We suggest that such contrast- and orientation-sensitive variability not only explains the contrast invariance observed in the LGN but can also underlie the contrast-invariant orientation tuning seen at the level of the primary visual cortex. [ABSTRACT FROM AUTHOR]

Neurons in the visual cortex exhibit heterogeneity in feature selectivity and the tendency to generate action potentials synchronously with other nearby neurons. By examining visual responses from cat area 17 we found that, during gamma oscillations, there was a positive correlation between each unit's sharpness of orientation tuning, strength of oscillations, and propensity towards synchronisation with other units. Using a computational model, we demonstrated that heterogeneity in the strength of rhythmic inhibitory inputs can account for the correlations between these three properties. Neurons subject to strong inhibition tend to oscillate strongly in response to both optimal and suboptimal stimuli and synchronise promiscuously with other neurons, even if they have different orientation preferences. Moreover, these strongly inhibited neurons can exhibit sharp orientation selectivity provided that the inhibition they receive is broadly tuned relative to their excitatory inputs. These results predict that the strength and orientation tuning of synaptic inhibition are heterogeneous across area 17 neurons, which could have important implications for these neurons' sensory processing capabilities. Furthermore, although our experimental recordings were conducted in the visual cortex, our model and simulation results can apply more generally to any brain region with analogous neuron types in which heterogeneity in the strength of rhythmic inhibition can arise during gamma oscillations. [ABSTRACT FROM AUTHOR]

Excitatory synapses arising from local neurons in the cat visual cortex are much more numerous than the thalamocortical synapses, which provide the primary sensory input. Many of these local circuit synapses are involved in the connections between cortical layers, but lateral connections within layers provide a major component of the local circuit synapses. We tested the influence of these lateral connections in the primary visual cortex of cats by inactivating small patches of cortex about 450 μm lateral from the recording pipette. By use of the neurotransmitter γ-aminobutyric acid (GABA), small patches of cortex were inhibited and released from inhibition in seconds. Orientation tuning curves derived from responses to oriented drifting gratings were obtained during short control periods interleaved with periods of GABA inactivation. About 30% of the cells (18/62, recorded in all layers) changed their orientation tuning when a small portion of their lateral input was silenced. There was no broadening of the orientation tuning curve during lateral inactivation. Instead, the recorded cells shifted their preferred orientation towards the orientation of the inactivated site. One explanation is that the GABA inactivation alters the balance of excitatory and inhibitory inputs to a cell, which results in a shift of the cell’s preferred orientation. [ABSTRACT FROM AUTHOR]

We are able to judge the direction of movement and orientation of objects because they have contrast-defined edges. However, we are also able to perceive the orientation and direction of movement of stereobars made of random dot stereograms in the absence of contrast-defined edges. We recorded 207 disparity-sensitive cells from visual areas V1 and V2 of two Macaca mulatta monkeys while performing an attentive fixation task. Luminance defined bars and random-dot stereo-defined bars were used to assess direction and orientation selectivity of these cells. Orientation and direction preference for luminance bars and for stereobars showed a statistically significant relationship ( r = 0.83, P < 0.01 for direction; r = 0.63, P < 0.01 for orientation). However, disparity-sensitive cells from these areas seem to be more sensitive to luminance than to stereobars regarding orientation and direction of movement. Similar results were obtained when the two areas were considered separately. Our results show that cells in areas V1 and V2 of the monkey visual cortex are able to detect the orientation and direction of movement of stereobars in a manner similar to those of luminance-defined bars. This finding is relevant because to detect the direction and orientation of stereobars a comparison between left and right eye inputs is required. [ABSTRACT FROM AUTHOR]

Form perception from coherent motion is an important aspect of vision. Representations of one-, two- and three-dimensional forms have been found at various stages of cortical processing using random-dot stimuli, whereas representations of biological objects like a walking human being concentrate at higher stages of processing. The perception of biological objects can be induced by sparse dot stimuli that consist of a few dots that mark the joints of the human body[G. Johansson (1973)Percept. Psychophys.,14, 201–211]. In the present study, we aimed to investigate whether neurons in early visual areas that respond to bars and edges defined by luminance contrast also signal bar-like objects from sparse dot stimuli. We studied single neurons with rows of 3–24 dots that were either collinear or scattered within a rectangular form. These dots were moved coherently on a uniform or dotted background, and human observers perceived them as rigid rods or other bar-like objects. We found neurons in the visual cortex of the awake, behaving monkey that responded to these stimuli and were sensitive to the orientation of these objects as for conventional bars or edges. Stimulus conditions that failed to induce these percepts in human observers also evoked weaker responses or none in these neurons. We found these neurons with increasing frequency in areas V1, V2 and V3/V3A. The results suggest that the visual cortex not only detects biological objects, but also lines and other bar-like objects from sparse dot stimuli, and that this function evolves at an early stage of processing. [ABSTRACT FROM AUTHOR]

Lesion studies suggest that primary auditory cortex (A1) is required for accurate sound localization by carnivores and primates. In order to elucidate further its role in spatial hearing, we examined the behavioural consequences of reversibly inactivating ferret A1 over long periods, using Elvax implants releasing the GABAA receptor agonist muscimol. Sub-dural polymer placements were shown to deliver relatively constant levels of muscimol to underlying cortex for >5 months. The measured diffusion of muscimol beneath and around the implant was limited to 1 mm. Cortical silencing was assessed electrophysiologically in both auditory and visual cortices. This exhibited rapid onset and was reversed within a few hours of implant removal. Inactivation of cortical neurons extended to all layers for implants lasting up to 6 weeks and throughout at least layers I–IV for longer placements, whereas thalamic activity in layer IV appeared to be unaffected. Blockade of cortical neurons in the deeper layers was restricted to ≤ 500 µm from the edge of the implant, but was usually more widespread in the superficial layers. In contrast, drug-free Elvax implants had little discernible effect on the responses of the underlying cortical neurons. Bilateral implants of muscimol–Elvax over A1 produced significant deficits in the localization of brief sounds in horizontal space and particularly a reduced ability to discriminate between anterior and posterior sound sources. The performance of these ferrets gradually improved over the period in which the Elvax was in place and attained that of control animals following its removal. Although similar in nature, these deficits were less pronounced than those caused by cortical lesions and suggest a specific role for A1 in resolving the spatial ambiguities inherent in auditory localization cues. [ABSTRACT FROM AUTHOR]

Abstract Exposure to a high-contrast visual stimulus causes adaptation, a psychophysical phenomenon that is quite selective for stimulus orientation. Its mechanism is largely cortical but the underlying circuitry is still not unambiguously resolved. It has been suggested that adaptation could be the result of integration of inputs from cells within a large local pool, effectively scaling their outputs with respect to local contrast. In this case, orientation selectivity of neuronal adaptation should depend on the location of neurons within the cortical map of orientation preference. We tested this hypothesis by quantifying adaptation to optimally oriented and to orthogonal-to-optimum gratings among neurons recorded either from iso-orientation domains or orientation pinwheel centres, as identified by optical imaging of cat visual cortex. We did not find a significant difference in adaptation characteristics for these two populations of cells, implying that these characteristics do not depend on the local functional architecture. Surprisingly, however, we additionally observed that under isoflurane (but not halothane) anaesthesia, most neurons exhibited adaptation by cross-oriented gratings, regardless of their location within the orientation map. It seems likely that, under isoflurane, inputs became visible that were masked by the commonly used, deeper halothane anaesthesia. For individual cells, the presence of these inputs was independent of their location within the cortical orientation map. [ABSTRACT FROM AUTHOR]

Abstract Individuals suffering from classical migraine report an astonishing diversity of migraine auras. A frequently reported symptom is a visual hallucination known as fortification illusion (FI). Here we demonstrate that the typical zig-zag pattern of the FI can be reproduced using experimental data of orientation maps of the primary visual cortex (V1) assuming that a continuous excitation front propagates across V1. We put forward a model in which the cortical neurons within this excitation wave are activated sufficiently to contribute to conscious perception. It is shown that the discontinuous repetitive nature of the zig-zag pattern of the FI can reflect the specific layout of visual cortical orientation maps. Additionally, dynamic features of the FI are predicted based on our model. [ABSTRACT FROM AUTHOR]

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

Abstract The functional specificity of corticocortical connections with respect to the topography of orientation selectivity was studied by optical imaging of intrinsic signals and bulk injections of fluorescent latex beads (green and red) and biocytin into layer 4. The distributions of retrogradely labelled cells and anterogradely labelled axon terminals were histologically reconstructed from all cortical laminae, and the resulting anatomical maps compared with the optically imaged functional maps. Layer 4 injections produced extensive horizontal labelling up to 2–3 mm from the injection centres albeit without the clear patchy pattern described after layer 2/3 injections (Gilbert & Wiesel 1989, J. Neurosci., 9, 2432–2442; Kisvárday et al. 1997, Cerebral Cortex, 7, 605–618). The functional (orientation) distribution of the labelled projections was analysed according to laminar location and lateral spread. With regard to the former, no major difference in the orientation topography between supragranular- (upper tier), granular- (middle tier) and infragranular (lower tier) layers was seen. Laterally, proximal and distal projections were distinguished and further dissected into three orientation categories, iso- (± 30°), oblique- (± 30–60°) and cross-orientations (± 60–90°) with respect to the orientation preference at the injection sites. The majority of distal connections (retrograde and anterograde) was equally distributed across orientations (35.4% iso-, 33.7% oblique-, and 30.9% cross-orientations) that are equivalent with a preponderance to dissimilar orientations (oblique- and cross-orientations, 64.6%). In one case, distal excitatory and inhibitory connections could be morphologically distinguished. For both categories, a marked bias to dissimilar orientations was found (excitatory, 63.7%; inhibitory, 86.6%). Taken together, these results suggest that the long-range layer 4 circuitry has a different functional role from that of the iso-orientation biased (52.9%, Kisvárday et al. 1997, Cerebral Cortex, 7, 605–618) layer 2/3 circuitry, and is perhaps involved in feature difference-based mechanisms, e.g. figure ground segregation. [ABSTRACT FROM AUTHOR]

Although the basis of our knowledge of experience-dependent plasticity comes from studies on carnivores and primates, studies examining the physiological and molecular mechanisms that underlie development and plasticity have increasingly employed mice. We have used several common rearing paradigms, such as dark-rearing and monocular deprivation (MD), to examine the timing of the physiological and molecular changes to altered experience in the cat primary visual cortex. Dark-rearing from birth or for 1 week starting at 4 weeks of age produced a similar reduction in the amplitude of responses measured through intrinsic signal imaging and a reduction in orientation selectivity. One week of visual experience following dark-rearing until 4 weeks of age yielded normal responses in both amplitude and orientation selectivity. The depression of deprived-eye responses was similar in magnitude after 2 and 7 days of MD. In contrast, non-deprived-eye responses almost doubled in magnitude after 7 days compared with 2 days of MD. These changes in the functional properties of primary visual cortex neurons were mirrored by specific changes in synaptic protein expression. Changes in proteins such as the NR2A and NR2B subunits of the N-methyl-D-aspartate receptor, postsynaptic density protein 95, alpha-CA2+/calmodulin-dependent protein kinase II (αCaMKII), and GABAAα1a indicated that the levels of sensory activity regulated mechanisms associated with both excitatory (NR2A and NR2B) and inhibitory (GABAAα1a) transmission so as to maintain response homeostasis. Additionally, we found that MD regulated the AMPA receptor glutamate (GluR1) subunit as well as signalling molecules (αCaMKII and synaptic Ras GTPase activating protein, SynGAP) downstream of N-methyl-D-aspartate receptors. Proteins in a common signalling pathway appeared to have similar developmental expression profiles that were broadly similar between cats and rodents. [ABSTRACT FROM AUTHOR]

Abstract Circumscribed retinal lesions in adult cats result in a reorganization of circuitry in area 17 such that neurons in the lesion projection zone (LPZ) can now be activated, not from their original receptive fields (RFs) but from regions of normal retina adjacent to the lesion (‘ectopic’ RFs). We have studied this phenomenon further by making circumscribed monocular retinal lesions in 8-week-old kittens and recording responses to visual stimuli of neurons in the LPZ of area 17 when these cats reached adulthood. These responses have been compared with those in adult-lesioned cats either of relatively short postlesion survival (2–24 weeks) or long postlesion survival (3.5–4.5 years). In both kitten-lesioned and adult-lesioned animals most LPZ neurons recorded from the supragranular layers (II and III) not only exhibited new ectopic RFs when stimuli were presented via the lesioned eye but the RF properties (e.g. the sizes of excitatory RFs, orientation and direction selectivities, velocity preferences and upper cut-off velocities) were often indistinguishable from those seen when stimuli were presented via the nonlesioned eye. Similarly, in both kitten-lesioned and adult-lesioned animals, most LPZ neurons recorded from the granular and infragranular layers (IV, V, VI), like those recorded from the supragranular layers, were binocular. However, in adult-lesioned but not in kitten-lesioned animals, the responses and the upper cut-off velocities of LPZ cells recorded from the granular and infragranular layers to stimuli presented via ectopic RFs tended to be, respectively, substantially weaker and lower than those for stimuli presented via the nonlesioned eye. The age-related laminar differences in reorganizational plasticity of cat striate cortex correlate with the lamino-temporal pattern of distribution of N -methyl-d-aspartate glutamate receptors in striate cortex. [ABSTRACT FROM AUTHOR]