Dynamics of Stability of Orientation Maps Recorded with Optical Imaging.

Orientation selectivity is an important feature of visual cortical neurons. Optical imaging of the visual cortex allows for the generation of maps of orientation selectivity that reflect the activity of large populations of neurons. To estimate the statistical significance of effects of experimental manipulations, evaluation of the stability of cortical maps over time is required. Here, we performed optical imaging recordings of the visual cortex of anesthetized adult cats. Monocular stimulation with moving clockwise square-wave gratings that continuously changed orientation and direction was used as the mapping stimulus. Recordings were repeated at various time intervals, from 15 min to 16 h. Quantification of map stability was performed on a pixel-by-pixel basis using several techniques. Map reproducibility showed clear dynamics over time. The highest degree of stability was seen in maps recorded 15-45 min apart. Averaging across all time intervals and all stimulus orientations revealed a mean shift of 2.2 ±â€¯0.1°. There was a significant tendency for larger shifts to occur at longer time intervals. Shifts between 2.8° (mean ±â€¯2SD) and 5° were observed more frequently at oblique orientations, while shifts greater than 5° appeared more frequently at cardinal orientations. Shifts greater than 5° occurred rarely overall (5.4% of cases) and never exceeded 11°. Shifts of 10-10.6° (0.7%) were seen occasionally at time intervals of more than 4 h. Our findings should be considered when evaluating the potential effect of experimental manipulations on orientation selectivity mapping studies.

[The Pulvinar of the Cat: Structural and Functional Organization and the Role in Cognitive Processes].

The present review is devoted to modern knowledge about a structure and function of the cat's lateral posterior-pulvinar complex of the thalamus (LP-P). The LP-P is a subcortical structure belonging to visual system. This complex appears in phylogenesis simultaneously with lateral geniculate body and visual cortex, develops structurally and in human, occupies about 1/3 of the thalamus. The LP-P is a so-called associative nucleus of the thalamus and it is anatomically and functionally complex structure. The complex has reciprocal connections with many cortical areas and may participate in the regulation of a flow of visual information to the cortex modulating cortical processes. The function of the LP-P is still not fully understood. Experimental data allows to believe that this complex participate in such cognitive processes as attention and orienting to visual stimuli, in visually-guided behavior and spatial coding of visual stimuli as well as in binding of particular features of visual objects in the whole percept and maybe in the processes of short-term memory during analysis of visual stimuli.

Repetitive adaptation induces plasticity of spatial frequency tuning in cat primary visual cortex.

Sensory neurons display transient changes in their response properties following prolonged exposure to an appropriate stimulus (adaptation). In adult cat primary visual cortex, spatial frequency-selective neurons shift their preferred spatial frequency (SF) after being adapted to a non-preferred SF. In anesthetized cats prepared for electrophysiological recordings in the visual cortex, we applied a non-preferred spatial frequency for two successive periods of adaptation (a recovery and interval of ∼90 min separated both phases of adaptation) in order to determine if a first adaptation retained an influence on a second adaptation. The first application of a non-preferred SF shifted the tuning curve of the cell mainly in the direction of the imposed SF. The results showed that attractive shifts occurred more frequently (68%) than repulsive (12%) changes in cortical cells. The increase of responsivity was band-limited and occurred around the imposed SF, while flanked responses remained unmodified in all conditions. After a recovery period allowing neurons to restore their original SF tuning curves, we carried out a second adaptation which produced four major results: (1) a higher proportion of repulsive shifts (31%) compared to attractive shifts (49%), (2) an increase of the magnitude of the attractive shifts, (3) an additional enhancement of the evoked firing rate for the newly acquired SF, and (4) for the acquired SF the variability coefficient decreased following the second adaptation. The supplementary response changes suggest that neurons in area 17 keep a "memory" trace of the previous stimulus properties. It also highlights the dynamic nature of basic neuronal properties in adult cortex since repeated adaptations modified both the spatial frequency tuning selectivity and the response strength to the preferred spatial frequency. These enhanced neuronal responses suggest that the range of adaptation-induced plasticity available to the visual system is broader than anticipated.

Long adaptation reveals mostly attractive shifts of orientation tuning in cat primary visual cortex.

In the adult brain, sensory cortical neurons undergo transient changes of their response properties following prolonged exposure to an appropriate stimulus (adaptation). In cat V1, orientation-selective cells shift their preferred orientation after being adapted to a non-preferred orientation. There are conflicting reports as to the direction of those shifts, towards (attractive) or away (repulsive) from the adapter. Moreover, the mechanisms underlying attractive shifts remain unexplained. In the present investigation we show that attractive shifts are the most frequent outcome of a 12 min adaptation. Overall, cells displaying selectivity for oblique orientations exhibit significantly larger shifts than cells tuned to cardinal orientations. In addition, cells selective to cardinal orientations had larger shift amplitudes when the absolute difference between the original preferred orientation and the adapting orientation increased. Conversely, cells tuned to oblique orientations exhibited larger shift amplitudes when this absolute orientation difference was narrower. Hence, neurons tuned to oblique contours appear to show more plasticity in response to small perturbations. Two different mechanisms appear to produce attractive and repulsive orientation shifts. Attractive shifts result from concurrent response depression on the non-adapted flank and selective response facilitation on the adapted flank of the orientation tuning curve. In contrast, repulsive shifts are caused solely by response depression on the adapted flank. We suggest that an early mechanism leads to repulsive shifts while attractive shifts engage a subsequent late facilitation. A potential role for attractive shifts may be improved stimulus discrimination around the adapting orientation.

Adaptation changes the spatial frequency tuning of adult cat visual cortex neurons.

The modular layout of striate cortex is arguably a hallmark of all cortical organization. Neurons of a given module or domain respond optimally to very few specific properties, such as orientation or direction. However, it is possible, under appropriate conditions, to compel a neuron to respond preferentially to a different optimal property. In anesthetized cats, prepared for electrophysiological recordings in the visual cortex, we applied a spatial frequency (SF) that differs (by 0.25-3.0 octaves) from the optimal one for 7-13 min without interruption. This application shifted the tuning curve of the cell mainly in the direction of the imposed SF. Indeed, results indicate an attractive push occurring more frequently (50%) than a repulsive (30%) shift in cortical cells. The increase of responsivity is band-limited and is around the imposed SF, while flanked responses remained unmodified in all conditions. We hypothesize that the observed reversible plasticity is obtained by a modulation of the balance between the strengths of the respective synaptic inputs. These changes in preferred original optimal spatial frequencies may allow a dynamic reaction of cortex to a new environment and particularly to ''zoom'' cellular activity toward persistent stimuli in spite of the tuning inherited from genetic programming of response properties and environmental conditions during critical periods in new born animals.

Effects of excitation and inactivation in area 17 on paired cells in area 18.

This investigation examines how neighboring neurons of area 18 react when area 17 inputs are excited or depressed. In anesthetized cats, area 18 responses to a sine-wave grating in the receptive field were analyzed, while a second grating was positioned in its periphery and responses were recorded in area 17. This latter site was also inactivated with GABA. A waveform template process sorted out at least two individual, neighboring cells with similar orientation preferences in area 18. These cells frequently displayed opposite reactions to stimulation and inactivation in area 17. Experiments suggest that nearby neurons belonging to the same functional domain in the visual cortex may simultaneously carry disparate information.

Contextual modulation of synchronization to random dots in the cat visual cortex.

Synchronization of neuronal activity has been proposed as a binding mechanism for integration of image properties into one coherent percept. In the present study, we investigated the contextual modulation of synchronization to random dot patterns. Coherent motion of random dots evoked well synchronized responses in area 17 of anaesthetized cats when the stimulus was presented in the compound receptive field of recorded sites. Gradually changing the directional coherence of random dots in the surround while maintaining fully coherent motion of the stimulus in the receptive field significantly suppressed synchronization of neuronal activity for some stimulus conditions. However, usually one or two peaks of increased synchronization were found in the surround coherence tuning curves with low (8-12%) and/or moderate (25-50%) coherence in the surround. At the population level, synchronization was significantly depressed with incoherent motion in the receptive field and when both the surround and the receptive field were jointly stimulated with 0% coherence. The intriguing finding was the discovery of two distinct groups of cells with opposite synchronization changes dependent on the presence or absence of significant synchronization in their spontaneous activity. The latter group of neurons showed peaks of increased synchronization with lower surround coherence, thus probably being more sensitive to the direction of the surround motion. Overall, our findings support the notion that binding of stimulus properties can be achieved by synchronized activity of cortical cells. However, our findings go further than the original hypothesis of feature binding by synchrony to show that synchronization of cortical activity may be directly related to the decision making processes, which in turn are related to the threshold of perception of coherent motion.

Modulation of the synchronization between cells in visual cortex by contextual targets.

It has been suggested that synchronization of action potentials encodes diverse features of a single image. However, properties of the synchronization, which occurs on a time scale of approximately 1-5 ms, are still poorly understood. We have tested the modulation of synchronization by manipulating the contextual targets introduced in the surround of the receptive field. Experiments were carried out on anaesthetized cats prepared for multiunit and single-cell recordings in area 17. Initially, a patch of sine-wave drifting grating was positioned over the overlapping receptive fields of several neurons. If this coherent motion produced a significant synchronization in cross-correlograms, contextual targets were added. The first contextual stimuli were two sine-wave patches placed above and below the central compound receptive field. Only the contrast of contextual targets changed. Results show that the larger the differential contrast the higher the synchronization. The second contextual stimulus was a lateral shift of a sine-wave patch. Data show that the wider the distance between the central and peripheral patches the better the synchronization. Furthermore, results suggest that the synchrony pattern computed by cross correlating multiunit recordings from two sites differs when the cross correlation is carried out between individual units belonging to each multiunit recording. Together with our previous results it appears that synchronization is stimulus dependent and its strength increases with larger disparities included in the whole stimulating image.

Comparative computations of spike synchronization in visual cortex of cats.

In recent years it has been proposed that synchronous activity between neurons is a putative mechanism to bind together various trigger features of an image. Thus the measure of synchronization becomes an important issue since it may be an electrophysiological sign of visual perception. This paper describes and compares six techniques of computing synchronization strength, that is, the central peak of a cross-correlogram. Data were obtained in anesthetized cats prepared for electrophysiological recordings in a conventional fashion. Results indicate that: (1) eye fits are misleading. Visual inspection of cross-correlograms, may be interesting if one needs to estimate approximately synchronization strength and the presence of oscillations in the cross-correlograms, however it may be misleading if one wants to compare different cross-correlograms; (2) regression analysis to compare one method against the others yields a relatively poor correlation suggesting that methods are not directly comparable; (3) the sensitivity of each computational method is unequal. The results may indicate that some functional connections are either under- or over-evaluated depending upon the strategy employed to measure synchronization.

Stimuli outside the classical receptive field modulate the synchronization of action potentials between cells in visual cortex of cats.

It is proposed that various attributes of an image are bound neuronally when responsive units fire in synchrony. Our investigations describe the influences of the contextual stimuli upon the occurrence of synchronization, in anaesthetized cats. Once a significant synchronization was recorded in the cross-correlogram (XCRG) between evoked action potentials of two groups of neurons in response to a drifting sine-wave grating, additional gratings were positioned outside the compound receptive field. The synchronization strength was then measured in relation to the difference between the orientations of the central and peripheral gratings. In the majority of cases results indicate that the synchronization is facilitated with larger orientation disparities. Thus, our data support the notion that contrasting features of images facilitate synchrony of activity between neurons.

Relationships between image structure and gamma oscillations and synchronization in visual cortex of cats.

The relationships between visual object configurations and interneuronal spike synchronization and gamma oscillations are examined in the present investigation. Cells were initially stimulated with moving, optimally oriented, single 20 degrees -long bars of light, centred on the compound receptive field of a pool of cortical neurons. When this kind of stimulus evoked intrinsic gamma oscillations and/or synchronization, we gradually fractured the original target. In addition, colinearity was ruptured by forming L- and T-shaped configurations. All fractures and discontinuities were introduced well outside the excitatory receptive field. Multiunit activity in the visual cortex (areas 17 and 18) was recorded in anaesthetized cats. Recording sites were separated by 0.4-1.2 mm. The data analysis indicates that gamma oscillations follow a rule by which unfractured bars yielded the highest S/N ratios. Synchronization strength, as revealed by the central peak in cross-correlograms, also seemed to depend upon stimulus configuration. However, the magnitude of the central peak failed to follow a consistent trend. For instance, the greatest magnitude of the central peak occurred for both colinear and orthogonal types of target. Our results support the notion that both gamma oscillations and neuronal synchronization are stimulus-dependent.

Response component analysis of simple and complex cells of area 18 during depression of area 17.

Simple and complex cells of visual areas of cats may be reliably classified according to the modulatory index (MI) of their responses. This investigation is aimed at analysing the MI in area 18 when a small region (about 200-400 microm in diameter) of area 17 was inactivated with a microinjection of GABA, in anesthetized cats. Cells were stimulated with sine-wave gratings whose orientation, spatial, and temporal frequencies were optimal for the studied unit. The AC and DC response components, and the MI were computed along with fast Fourier transforms of evoked discharges recorded as peristimulus time histograms. Results showed that these response components were relatively unaffected in simple cells, whereas complex cells exhibited large changes when area 17 was silenced. In particular, a large proportion of complex cells showed a MI greater than 1, thereby adopting a response pattern resembling simple cells. It is suggested that this subpopulation of complex cells receives a direct input from geniculate X cells.

Pulvinar participates in synchronizing neural assemblies in the visual cortex, in cats.

It has been proposed that the perception of a coherent image necessitates two processes, that is, an ensemble of neurons which synchronizes discharges of individual cells and stimulus-specific gamma-band (gamma) neuronal oscillations which may serve as carrier signals for a temporal code. We tested the hypothesis that cortical gamma-oscillations and synchronization depend upon the interactions between the lateral posterior-pulvinar complex of the thalamus (LP-P) and visual cortex. Local reversible inactivation of the LP-P was achieved by pressure injections of gamma-aminobutyric acid (GABA). In the majority of cases the LP-P depression decreased the strength of the synchronization and oscillations. Also, the results demonstrate that the occurrence of stimulus-dependent oscillations and the synchronization of neuronal responses are two distinct processes and consequently they may occur or disappear independently of each other.

Maturation of visual receptive field properties in the rat superior colliculus.

Visually responsive neurons were recorded in the superficial layers of rat superior colliculus from postnatal day 12 to 28. Receptive field properties such as size, type (ON, OFF, ON-OFF and motion sensitive) and direction selectivity were analyzed to disclose changes during maturation. Although some aspects of sensory properties are modified during development (latency, receptive field sizes, and proportions of receptive field types), a high level of sophistication is also present in young animals even before eyelid opening. For instance, direction selective and direction biased cells, which require complex synaptic relations, are already observed when the first light evoked responses emerge in the superior colliculus (P13), strongly suggesting that this property develops without visual experience. Furthermore, direction selectivity is present in the colliculus prior to the appearance of visually evoked activity in the cortex. This indicates that direction selectivity can not be attributable to incoming cortical afferents. This study provides the first direct evidence that, unlike the cat, the rat's cortico-tectal pathway is only weakly involved in the establishment of direction selectivity in collicular neurons.

Stimulus-dependent oscillations in the cat visual cortex: differences between bar and grating stimuli.

We have investigated the dependence of cortical oscillations on the type of visual stimulus. Single unit recordings were performed in areas 17 and 18 of the cat visual cortex. Among 217 cortical neurons oscillations in the frequency range of 22-102 Hz were found in 29 cells (13%). The proportion of oscillating cells was higher (16%) if both bar and grating stimuli were used to stimulate cortical neurons. It was found that gratings are more effective than bars in triggering oscillatory patterns in cortical cells. Among 21 oscillating cells which were stimulated with both bar and grating stimuli, oscillations evoked with gratings were found in 17 neurons (81%) while oscillations evoked with bar stimuli were triggered in 7 cells (33%). The distributions of oscillation frequencies were statistically different for oscillations evoked with bars and gratings. Frequencies of oscillations evoked with bars were in the lower and higher range than frequencies of oscillations evoked with gratings. In 3 cells (14%), rhythmic patterns could be evoked with both bar and grating stimuli. However, the oscillations were of different frequencies. No significant correlation was found between the strength of oscillations and firing rate of cortical neurons. Both simple and complex cells manifested the same dependence on stimulus type. However, complex cells mostly exhibited oscillations in the lower frequency range while simple cells did so when neurons were stimulated with bars. The results suggest that various classes of visual stimuli can be coded by a temporal pattern of cortical responses.

The lateral posterior-pulvinar complex modulation of stimulus-dependent oscillations in the cat visual cortex.

It has been suggested that binding coherent targets depends on the capacity of excited cortical cells to fire in synchrony at approximately 40 Hz. However, the origin of stimulus-related cortical oscillations is still not clear. We hypothesized that 40 Hz oscillations might propagate to the visual cortex from the lateral posterior-pulvinar complex (LP-P) whose cells send fibers to the visual cortex and have a tendency to exhibit oscillations. To test our hypothesis, we recorded single unit activity in areas 17 and 18 of anaesthetized cats. The activity of neurons which showed oscillations evoked by optimal visual stimuli was analysed before, during and after a reversible inactivation of the LP-P with GABA. Such inactivation was found to markedly modify the strength of oscillatory activity of cortical neurons whose visual responses were affected by LP-P blockade. In contrast, the oscillation frequencies of cortical neurons were not modified by such inactivation. However, in some cells (three of nine), oscillatory activity was found to be completely abolished by injection of GABA into the LP-P. Collectively, these findings demonstrate that inputs from the LP-P play a key role in modulating the oscillatory activity of visual cortex neurons. Assuming that cortical neurons utilize oscillatory activity to encode perceptual aspects of the visual stimulus, our findings underscore the contribution of the LP-P in this process.

Visually-triggered oscillations in the cat lateral posterior-pulvinar complex.

The so-called 40 Hz oscillations are found at almost all stages of visual processing are thought to play a critical role in perception. The goal of this investigation was to look at the presence of stimulus-specific oscillations in the lateral posterior-pulvinar complex of the thalampus (LP-P) for which the oscillations were still not described. Rhythmic patterns in multiunit LP-P activity of anaesthetized cats were revealed in 14% of recording sites. With the exception of one pool of LP-P cells that exhibited stimulus-dependent rhythmic activity approximately 130 Hz, 90% of autocorrelograms were modulated between 18 and 74 Hz with dominant frequencies of 20-33 Hz. Since the LP-P sends efferents to the visual cortex it seems possible that oscillations from the LP-P can propagate to cortical neurones, especially to complex cells, for which similar dominant frequencies were noted by previous investigators.

[Correlations between thalamic projections to different areas of the parietal association cortex of the cat]. / Sootnosheniia talamicheskikh proektsii v razlichnye chasti temennoi assotsiativnoi kory koshek.

Projections of thalamic neurons to parietal association cortex of cat were examined by means of the retrograde axonal transport of fluorescent dyes (primuline and fast blue). It has been demonstrated that a dorsal part of the pulvinar (PL) and a dorsal part of the caudal area of the lateral posterior nucleus (LP) projected mostly to the middle suprasylvian gyrus (MSSG), while a ventral part of PL and a ventral part of the rostral area of LP--to the rostral suprasylvian gyrus (RSSG). Double labelled neurons were found in PL, LP, suprageniculatus, anterior ventral, ventral lateral as well as in the central lateral, paracentral and central medial nuclei after injections of two different dyes into MSSG and RSSG. Topic organization of sources of cortical projections from the PL--LP complex can probably provide a high level of discrimination of visual signals by single cortical neurons. At the same time during integration of information of a different subcortical origin RSSG and MSSG act, probably, in concord to a considerable extent, that suggests insufficient differentiation of RSSG and MSSG corresponding approximately to cortical areas 5 and 7 of cat.

[The divergence of the axonal collaterals of the thalamic neurons to the visual and associative cortexes in cats]. / Divergentsiia aksonnykh kollateralei neironov talamusa k zritel'noi i assotsiativnoi kore koshek.

Projections of the thalamic neurons to the visual (area 17) and the parietal association (area 7) cortices were examined by retrograde axonal transport of fluorescent dyes. It was found that the pulvinar neurons can be divided into three groups with respect to their connections with these cortical areas: 1--projecting to area 7 (the largest cell group); 2--projecting to area 17 (a smaller cell group) and 3--sending their axons to the both cortical areas (only few cells). Neurons belonging only to first two groups were found in the lateral posterior nucleus. Divergence of axonal collaterals of pulvinar neurons can secure the existence of parallel pathways transmitting information to the visual and associative cortices.