Spatial and temporal frequency tuning and contrast sensitivity of single neurons in area 21a of the cat

Pulvinar Modulates Contrast Responses in the Visual Cortex as a Function of Cortical Hierarchy

The pulvinar is the largest extrageniculate visual nucleus in mammals. Given its extensive reciprocal connectivity with the visual cortex, it allows the cortico-thalamocortical transfer of visual information. Nonetheless, knowledge of the nature of the pulvinar inputs to the cortex remains elusive. We investigated the impact of silencing the pulvinar on the contrast response function of neurons in 2 distinct hierarchical cortical areas in the cat (areas 17 and 21a). Pulvinar inactivation altered the response gain in both areas, but with larger changes observed in area 21a. A theoretical model was proposed, simulating the pulvinar contribution to cortical contrast responses by modifying the excitation-inhibition balanced state of neurons across the cortical hierarchy. Our experimental and theoretical data showed that the pulvinar exerts a greater modulatory influence on neuronal activity in area 21a than in the primary visual cortex, indicating that the pulvinar impact on cortical visual neurons varies along the cortical hierarchy.

Pulvinar Modulates Synchrony across Visual Cortical Areas

The cortical visual hierarchy communicates in different oscillatory ranges. While gamma waves influence the feedforward processing, alpha oscillations travel in the feedback direction. Little is known how this oscillatory cortical communication depends on an alternative route that involves the pulvinar nucleus of the thalamus. We investigated whether the oscillatory coupling between the primary visual cortex (area 17) and area 21a depends on the transthalamic pathway involving the pulvinar in cats. To that end, visual evoked responses were recorded in areas 17 and 21a before, during and after inactivation of the pulvinar. Local field potentials were analyzed with Wavelet and Granger causality tools to determine the oscillatory coupling between layers. The results indicate that cortical oscillatory activity was enhanced during pulvinar inactivation, in particular for area 21a. In area 17, alpha band responses were represented in layers II/III. In area 21a, gamma oscillations, except for layer I, were significantly increased, especially in layer IV. Granger causality showed that the pulvinar modulated the oscillatory information between areas 17 and 21a in gamma and alpha bands for the feedforward and feedback processing, respectively. Together, these findings indicate that the pulvinar is involved in the mechanisms underlying oscillatory communication along the visual cortex.

Understanding neuronal systems in movement control using Wiener/Volterra kernels: a dominant feature analysis

Although Volterra kernels have been extensively applied in modelling and analysis of biological systems, the relationship between the kernel characteristics and physiologically important features under study is still not revealed clearly. In this study, the link between Volterra kernels and dynamic response of neural systems which control animal movements was investigated and demonstrated using a dominant feature analysis. The new results show an effective but simplified method to use Volterra or Wiener kernels to understand and classify the neural systems which are responsible for the fundamental movements such as flexion and extension of animal limbs, and importantly demonstrate how the neuron pathways in locusts control joint activities of low and high frequency and perform fundamental joint movements such as position, velocity and acceleration. These results provide a useful insight into the nonlinear characteristics of neural systems in movement control and show a useful approach to the analysis of physiological systems using Volterra/Wiener kernels.

Drifting grating stimulation reveals particular activation properties of visual neurons in the caudate nucleus

The role of the caudate nucleus (CN) in motor control has been widely studied. Less attention has been paid to the dynamics of visual feedback in motor actions, which is a relevant function of the basal ganglia during the control of eye and body movements. We therefore set out to analyse the visual information processing of neurons in the feline CN. Extracellular single-unit recordings were performed in the CN, where the neuronal responses to drifting gratings of various spatial and temporal frequencies were recorded. The responses of the CN neurons were modulated by the temporal frequency of the grating. The CN units responded optimally to gratings of low spatial frequencies and exhibited low spatial resolution and fine spatial frequency tuning. By contrast, the CN neurons preferred high temporal frequencies, and exhibited high temporal resolution and fine temporal frequency tuning. The spatial and temporal visual properties of the CN neurons enable them to act as spatiotemporal filters. These properties are similar to those observed in certain feline extrageniculate visual structures, i.e. in the superior colliculus, the suprageniculate nucleus and the anterior ectosylvian cortex, but differ strongly from those of the primary visual cortex and the lateral geniculate nucleus. Accordingly, our results suggest a functional relationship of the CN to the extrageniculate tecto-thalamo-cortical system. This system of the mammalian brain may be involved in motion detection, especially in velocity analysis of moving objects, facilitating the detection of changes during the animal's movement.

Receptive field properties and sensitivity to edges defined by motion in the postero-lateral lateral suprasylvian (PLLS) area of the cat

The present study investigated the spatial properties of cells in the postero-lateral lateral suprasylvian (PLLS) area of the cat and assessed their sensitivity to edges defined by motion. A total of one hundred and seventeen (117) single units were isolated. First, drifting sinusoidal gratings were used to assess the spatial properties of the cells' receptive fields and to determine their spatial frequency tuning functions. Second, random-dot kinematograms were used to create illusory edges by drifting textured stimuli (i.e. a horizontal bar) against a similarly textured but static background. Almost all the cells recorded in PLLS (96.0%) were binocular, and a substantial majority of receptive fields (79.2%) were end-stopped. Most units (81.0%) had band-pass spatial frequency tuning functions and responded optimally to low spatial frequencies (mean spatial frequency: 0.08 c./degree). The remaining units (19.0%) were low-pass. All the recorded cells responded vigorously to edges defined by motion. The vast majority (96.0%) of cells responded optimally to large texture elements; approximately half the cells (57.3%) also responded to finer texture elements. Moreover, 38.5% of the cells were selective to the width of the bar (i.e., the distance between the leading and the trailing edges). Finally, some (9.0%) cells responded in a transient fashion to leading and to trailing edges. In conclusion, cells in the PLLS area are low spatial frequency analyzers that are sensitive to texture and to the distance between edges defined by motion.

Spectral receptive field properties of neurons in the feline superior colliculus

The spatio-temporal frequency response profiles of 73 neurons located in the superficial, retino-recipient layers of the feline superior colliculus (SC) were investigated. The majority of the SC cells responded optimally to very low spatial frequencies with a mean of 0.1 cycles/degree (c/deg). The spatial resolution was also low with a mean of 0.31 c/deg. The spatial frequency tuning functions were either low-pass or band-pass with a mean spatial frequency bandwidth of 1.84 octaves. The cells responded optimally to a range of temporal frequencies between 0.74 cycles/s (c/s) and 26.41 c/s with a mean of 6.84 c/s. The majority (68%) of the SC cells showed band-pass temporal frequency tuning with a mean temporal frequency bandwidth of 2.4 octaves, while smaller proportions of the SC units displayed high-pass (19%), low-pass (8%) or broad-band (5%) temporal tuning. Most of the SC units exhibited simple spectral tuning with a single maximum in the spatio-temporal frequency domain, while some neurons were tuned for spatial or temporal frequencies or speed tuned. Further, we found cells excited by gratings moving at high temporal and low spatial frequencies and cells whose activity was suppressed by high velocity movement. The spatio-temporal filter properties of the SC neurons show close similarities to those of their retinal Y and W inputs as well as those of their inputs from the cortical visual motion detector areas, suggesting their common role in motion analysis and related behavioral actions.

Frequency-Following and Connectivity of Different Visual Areas in Response to Contrast-Reversal Stimulation

The sensitivity of visual areas to different temporal frequencies, as well as the functional connections between these areas, was examined using magnetoencephalography (MEG). Alternating circular sinusoids (0, 3.1, 8.7 and 14 Hz) were presented to foveal and peripheral locations in the visual field to target ventral and dorsal stream structures, respectively. It was hypothesized that higher temporal frequencies would preferentially activate dorsal stream structures. To determine the effect of frequency on the cortical response we analyzed the late time interval (220-770 ms) using a multi-dipole spatio-temporal analysis approach to provide source locations and timecourses for each condition. As an exploratory aspect, we performed cross-correlation analysis on the source timecourses to determine which sources responded similarly within conditions. Contrary to predictions, dorsal stream areas were not activated more frequently during high temporal frequency stimulation. However, across cortical sources the frequency-following response showed a difference, with significantly higher power at the second harmonic for the 3.1 and 8.7 Hz stimulation and at the first and second harmonics for the 14 Hz stimulation with this pattern seen robustly in area V1. Cross-correlations of the source timecourses showed that both low- and high-order visual areas, including dorsal and ventral stream areas, were significantly correlated in the late time interval. The results imply that frequency information is transferred to higher-order visual areas without translation. Despite the less complex waveforms seen in the late interval of time, the cross-correlation results show that visual, temporal and parietal cortical areas are intricately involved in late-interval visual processing.

Spatial and temporal visual properties of single neurons in the suprageniculate nucleus of the thalamus

The spatial and temporal visual sensitivity to drifting sinusoidal gratings was studied in 105 neurons of the suprageniculate nucleus of the feline thalamus. Extracellular single-unit recordings were performed in halothane-anesthetized, immobilized, artificially ventilated cats. Most suprageniculate nucleus cells were strongly sensitive to the direction of drifting gratings. The suprageniculate nucleus units had a clear preference for very low spatial frequencies with a mean of 0.05 cycle/deg. The spatial resolution was also very low with a mean of 0.16 cycle/deg. Most of the cells displayed low-pass spatial tuning characteristics, while the remainder of the units were band-pass tuned. The suprageniculate nucleus units were extremely narrowly tuned, to spatial frequencies with a mean spatial bandwidth of 1.07 octaves. A majority of the units responded optimally to high temporal frequencies, with a mean of 8.53 Hz. The temporal frequency tuning functions predominantly revealed a band-pass character, with a mean temporal bandwidth of 1.66 octaves. These results demonstrate that the neurons in the suprageniculate nucleus display particular spatial and temporal characteristics. The spatial and temporal tuning properties of the suprageniculate nucleus neurons are very similar to those of the superior colliculus and the anterior ectosylvian cortex, structures that provide the main visual afferentation toward the suprageniculate nucleus. This suggests their common function in motion perception, and especially in the recording of movements of the visual environment relative to the body, and the related behavioral action.

Binocular fusion/suppression to spatial frequency differences at the border of areas 17/18 of the cat

As shown by various human psychophysical studies, interocular spatial frequency disparities can yield a variety of percepts. In order to examine how binocular fusion is affected by spatial frequency differences, we have recorded cells in the border region of areas 17/18 of anesthetized cats. The optic axes of the eyes were deviated onto cathode-ray screens, and the optimal spatial frequency of each eye was assessed by monocular stimulations using drifting sinusoidal gratings. The optimal relative phase using identical spatial frequencies in both eyes was first determined. Spatial frequency differences were then introduced by keeping the optimal spatial frequency constant in one eye and varying the spatial frequency in the other. Results indicate that cells (39%) responded with an increased firing rate (facilitation) to similar spatial frequencies in each eye and with a gradual attenuation (occlusion or suppression) when spatial frequency differences were increased. However, binocular facilitation did not always occur to the presentation of identical stimuli. For 16% of the cells, maximal responses were observed when lower spatial frequencies than the optimal one were presented in one eye while higher spatial frequencies produced suppression. The opposite pattern was observed only for two cells. These findings are discussed in terms of binocular fusion and suppression.

Origin of negative blood oxygenation level-dependent fMRI signals

Functional magnetic resonance imaging (fMRI) techniques are based on the assumption that changes in spike activity are accompanied by modulation in the blood oxygenation level-dependent (BOLD) signal. In addition to conventional increases in BOLD signals, sustained negative BOLD signal changes are occasionally observed and are thought to reflect a decrease in neural activity. In this study, the source of the negative BOLD signal was investigated using T2*-weighted BOLD and cerebral blood volume (CBV) techniques in isoflurane-anesthetized cats. A positive BOLD signal change was observed in the primary visual cortex (area 18) during visual stimulation, while a prolonged negative BOLD change was detected in the adjacent suprasylvian gyrus containing higher-order visual areas. However, in both regions neurons are known to increase spike activity during visual stimulation. The positive and negative BOLD amplitudes obtained at six spatial-frequency stimuli were highly correlated, and negative BOLD percent changes were approximately one third of the positive changes. Area 18 with positive BOLD signals experienced an increase in CBV, while regions exhibiting the prolonged negative BOLD signal underwent a decrease in CBV. The CBV changes in area 18 were faster than the BOLD signals from the same corresponding region and the CBV changes in the suprasylvian gyrus. The results support the notion that reallocation of cortical blood resources could overcome a local demand for increased cerebral blood flow induced by increased neural activity. The findings of this study imply that caution should be taken when interpreting the negative BOLD signals as a decrease in neuronal activity.

Phase- and position-disparity coding in the posteromedial lateral suprasylvian area of the cat

The posteromedial lateral suprasylvian area of the cat is known to be involved in the analysis of motion and motion in depth. However, it remains unclear whether binocular cells in the posteromedial lateral suprasylvian area rely upon phase or positional offsets between their receptive fields in order to code binocular disparity. The present study aims at clarifying more precisely the neural mechanisms underlying stereoperception with two objectives in mind. First, to determine whether cells in the posteromedial lateral suprasylvian area code phase disparities. Secondly, to examine whether the cells sensitive to phase disparity are the same as those which code for position disparities or whether each group represent a different sub-population of disparity-sensitive neurons. We investigated this by testing both types of disparities on single neurons in this area. The results show that the vast majority of cells (74%), in the posteromedial lateral suprasylvian area, are sensitive to relative interocular phase disparities. These cells showed mostly facilitation (95%) and a few (5%) summation interactions. Moreover, most cells (81%) were sensitive to both position and phase disparities. The results of this study show that most binocular cells in the posteromedial lateral suprasylvian area are sensitive to both positional and phase offsets which demonstrate the importance of this area in stereopsis.

Spatial properties and direction selectivity of single neurons in area 21b of the cat

The receptive field properties of single units were assessed in area 21b of the cat visual cortex. Visual cells in this area were binocular and showed relatively large receptive fields. Most cells were strongly sensitive to the direction of drifting gratings. The mean value of the half-widths of the direction tuning curves (32 degrees ) suggests broader direction tunings than are typically found in other visual areas. The spatial frequency tuning functions were either band-pass or low-pass. Cells responded optimally to low spatial frequencies (mean =0.08c/deg) and also showed low spatial resolution (mean =0.29c/deg.). The estimated values of spatial bandwidths (mean=2.2 octaves) suggest that area 21b cells act as relatively good spatial filters. Although some cells exhibited a low contrast threshold, most cells began to respond at intermediate or high contrast values (mean threshold =15.5%). Temporal frequency tuning functions were mostly band-pass and usually broad (mean temporal bandwidth=3.3 octaves). Cells were found that responded optimally to various temporal frequencies (mean optimal temporal frequency=3.2Hz), although the majority preferred a temporal frequency below 4Hz.These results suggest that visual properties (receptive fields sizes, spatial resolution and orientation/direction selectivity) of cells in area 21b differ from those of cells previously observed in the adjoining area 21a. These differences provide evidence in support of functional distinction between these two visual areas.

Binocular phase interactions in area 21a of the cat

1. Binocular interactions related to retinal disparity were investigated in single neurons in area 21a of extrastriate cortex in the anaesthetized cat using sinusoidal luminance gratings. 2. The responses of approximately two-thirds of neurons were profoundly modulated by a relative phase difference between identical drifting gratings presented to each eye. This modulation included both facilitatory and inhibitory interocular interactions. The selectivity for binocular disparity was about twice as sharp as the selectivity for monocular spatial position. 3. Significant phase modulation was retained in many neurons at interocular orientation differences exceeding 45 deg. The response suppression associated with stimulation at a phase shift 180 deg from the optimum was stronger than the response suppression to an interocular orientation difference of 90 deg. 4. The proportion of phase modulated neurons and the potency of modulation in area 21a neurons exceed that reported for phase-selective complex cells in area 17. Neurons in area 21a show sharp disparity tuning that is relatively insensitive to changes in orientation and monocular position, which suggests that this extrastriate region has a role in stereoscopic depth perception.

Spatial and temporal matching of receptive field properties of binocular cells in area 19 of the cat

The spatial and temporal properties of single neurons were investigated in area 19 of the cat. We evaluated the matching of binocular receptive field properties with regard to the respective strength of the ipsilateral and contralateral inputs. Results indicate that most cells in area 19 are well tuned to spatial and temporal frequencies and exhibit relatively low contrast threshold (mean=6.8%) when assessed using optimal parameters and tested through the dominant eye. Spatial resolution (mean=0.75 c/degree), optimal spatial frequencies (mean=0.16 c/degree) were relatively low and spatial bandwidths (mean=2.1 octaves) were broader as compared to those of cells in area 17 but comparable to those of cells in other extrastriate areas. On the other hand temporal resolution (mean=10.7 Hz), optimal temporal frequency (mean=4.5 Hz) and temporal bandwidths (mean=2.9 octaves) were higher and broader than in primary visual cortex. A significant relationship exists between most of the cell's properties assessed through either eye. For some parameters, such as spatial and temporal resolution, ocular dominance was shown to be significantly related to the extent of matching between the two eyes. For these parameters, binocular cells that exhibited a balanced ocular dominance were generally well matched with regard to the receptive field properties of each eye whereas the largest mismatches were found in cells that were more strongly dominated by one eye. These results suggest that visual input contributes to the activation of cells in area 19 in a redundant manner, possibly attesting to the multiplicity of parallel pathways to this area in the cat.

Spatial resolution and contrast sensitivity of single neurons in area 19 of split-chiasm cats: a comparison with primary visual cortex

Electrophysiological recordings were carried out in the callosal recipient zone of area 19 in normal and split-chiasm cats and, for comparison purposes, at the border of areas 17 and 18 of split-chiasm cats. The influences of retinothalamic and callosal inputs on a single cortical neurons were thereby evaluated. Extracellular recordings of single cells were made in anaesthetized and paralysed cats in the zone representing the central visual field. Receptive field properties were assessed using sine wave gratings drifting in optimal directions. Results showed that in area 19 and areas 17/18 one-third of the cells were binocularly driven after section of the optic chiasm. In area 19, the spatial resolution and contrast sensitivity of cells driven via the dominant eye were similar in the normal and split-chiasm groups. In areas 17/18 and area 19 of split-chiasm cats, binocular cells showed significant interocular matching of their receptive field properties (spatial resolution and contrast threshold), although small differences were observed. These small interocular differences were related to the cell's ocular dominance rather than to the signal transmission route (thalamic or callosal).

Spatial and temporal frequency selectivity of cells in area 21a of the cat

1. The spatial and temporal response properties of single cells in area 21a of the anaesthetized cat were assessed using drifting sinusoidal gratings presented at the optimum orientation for each cell. 2. Responses to sinusoidal gratings were dominated by an elevation of the mean discharge, with a relatively small modulated component at the temporal frequency of grating drift. The relative modulation ratio for the majority of cells was less than 1, similar to complex cells in the striate cortex. 3. Of those cells responsive to stimulation with sinusoidal gratings, 94% displayed spatial bandpass characteristics. Values derived from spatial frequency tuning curves were: mean optimum spatial frequency, 0.26 cycles deg-1; mean spatial resolution, 0.86 cycles deg-1; mean spatial bandwidth, 1.8 octaves; and mean normalized bandwidth, 1.3. Two cells (6%) displayed spatial low-pass characteristics. 4. Approximately half our sample of cells (44%) displayed temporal low-pass tuning, while 35% displayed temporal bandpass characteristics. The mean optimum temporal frequency of bandpass cells was 3.3 Hz and the mean temporal bandwidth 1.9 octaves. The remaining cells were classified as temporal broadband (17%) and temporal high-pass (4%). 5. We conclude that the dominant functional input to cells with relatively high spatial frequency selectivity and/or temporal low-pass response properties most probably arises from area 17. The responses of the remaining cells may be explained by input from area 17 or 18.