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Unlabelled: The main differences between the geniculate and the ascending tectal/extrageniculate visual systems are the presence of multisensory units and the absence of the strict retinotopical organization in the tectal system., Aim: To investigate the coding of the visual and multisensory information in the tectal system, and to clarify the direction of visual information flow between the extrastriate cortex along the anterior ectosylvian sulcus and the lateral medialis-suprageniculate nucleus of the posterior thalamus., Method: Extracellular single-cell recordings were carried out in anesthetized, immobilized cats to visual and multisensory stimulation., Result: The thalamo-cortical route seems to have a dominant role in the bidirectional information flow between the LM-Sg and the AES cortex. The panoramic neurons in the AES cortex and the LM-Sg may code in their discharge rate the site of the spatial visual and multisensory information and exhibited multisensory cross-modal integration., Conclusion: Our results demonstrate the existence of a distributed population code of spatial multisensory information in the tectal visual system.

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.

Sensorimotor co-ordination in mammals is achieved predominantly via the activity of the basal ganglia. To investigate the underlying multisensory information processing, we recorded the neuronal responses in the caudate nucleus (CN) and substantia nigra (SN) of anaesthetized cats to visual, auditory or somatosensory stimulation alone and also to their combinations, i.e. multisensory stimuli. The main goal of the study was to ascertain whether multisensory information provides more information to the neurons than do the individual sensory components. A majority of the investigated SN and CN multisensory units exhibited significant cross-modal interactions. The multisensory response enhancements were either additive or superadditive; multisensory response depressions were also detected. CN and SN cells with facilitatory and inhibitory interactions were found in each multisensory combination. The strengths of the multisensory interactions did not differ in the two structures. A significant inverse correlation was found between the strengths of the best unimodal responses and the magnitudes of the multisensory response enhancements, i.e. the neurons with the weakest net unimodal responses exhibited the strongest enhancement effects. The onset latencies of the responses of the integrative CN and SN neurons to the multisensory stimuli were significantly shorter than those to the unimodal stimuli. These results provide evidence that the multisensory CN and SN neurons, similarly to those in the superior colliculus and related structures, have the ability to integrate multisensory information. Multisensory integration may help in the effective processing of sensory events and the changes in the environment during motor actions controlled by the basal ganglia.

This study describes the visual information coding ability of single neurons in the suprageniculate nucleus (Sg), and provides new data concerning the visual information flow in the suprageniculate/anterior ectosylvian pathways of the feline brain. The visual receptive fields of the Sg neurons have an internal structure rather similar to that described earlier in the anterior ectosylvian visual area (AEV). The majority of the Sg units can provide information via their discharge rate at the site of the visual stimulus within their large receptive fields. This suggests that they may serve as panoramic localizers. The sites of maximum responsivity of the Sg neurons are distributed over the whole investigated part of the visual field. There is no significant difference between the distributions of spatial location of maximum sensitivity of the AEV and the Sg neurons. The mean visual response latency of the Sg units was found to be significantly shorter than the mean latency of the AEV neurons, but there was no difference between the shortest latency values of the thalamic and the cortical single-units. This suggests that the visual information flows predominantly from the Sg to the AEV, though the cortico-thalamic route is also active. The Sg seems to represent a thalamic nucleus rather similar in function to both the first-order relays and the higher-order thalamic nuclei. These results, together with the fact that the superior colliculus provides the common ascending source of information to the suprageniculate/anterior ectosylvian pathway, suggest a unique function of the AEV and the Sg in sensorimotor integration.

This study describes a possible mechanism of coding of multisensory information in the anterior ectosylvian visual area of the feline cortex. Extracellular microelectrode recordings on 168 cells were carried out in the anterior ectosylvian sulcal region of halothane-anaesthetized, immobilized, artificially ventilated cats. Ninety-five neurons were found to respond to visual stimuli, 96 responded to auditory stimuli and 45 were bimodal, reacting to both visual and auditory modalities. A large proportion of the neurons exhibited significantly different responses to stimuli appearing in different regions of their huge receptive field. These neurons have the ability to provide information via their discharge rate on the site of the stimulus within their receptive field. This suggests that they may serve as panoramic localizers. The ability of the bimodal neurons to localize bimodal stimulus sources is better than any of the unimodal localizing functions. Further, the sites of maximal responsivity of the visual, auditory and bimodal neurons are distributed over the whole extent of the large receptive fields. Thus, a large population of such panoramic visual, auditory and multisensory neurons could accurately code the locations of the sensory stimuli. Our findings support the notion that there is a distributed population code of multisensory information in the feline associative cortex.

The pedunculopontine tegmental nucleus (PPT) projects its cholinergic fibers to both the lateralis medialis-suprageniculate nucleus (LM-Sg) and the superior colliculus (SC). For the purpose of verification of whether a single neuron in the PPT projects to both the LM-Sg and the SC, we injected dextran tetramethylrhodamine (DR) into the LM-Sg and dextran fluorescein (DF) into the ipsilateral SC. The DR-positive neurons labeled retrogradely in the PPT are small (mean: 27.13+/-1.22 micro m) and distributed in the rostral two-thirds of this nucleus, whereas the DF-positive neurons are small (mean: 27.54+/-1.16 micro m) or medium-sized (mean: 40.18+/-1.43 micro m), and are located throughout the PPT. Thirty-five percent of all labeled neurons are double-labeled and small. The present study indicates that the PPT projection to the LM-Sg in part involves neurons bifurcating to the SC.

The spatial and temporal visual sensitivity to drifting sinusoidal gratings was studied in 75 neurons of the feline anterior ectosylvian visual area (AEV). Extracellular single-unit recordings were performed in halothane-anesthetized (0.6%), immobilized, artificially ventilated cats. Most cells were strongly sensitive to the direction of drifting gratings. The mean value of the direction tuning widths was approximately 90 deg. Most of the cells (69 of the 75 cases) displayed rather narrowly tuned band-pass characteristics in the low spatial frequency range, with a mean optimal spatial frequency of 0.2 cycles/degree (c/deg). The mean spatial bandwidth was 1.4 octaves. The remainder of the units was low-pass tuned. A majority of the units responded optimally to high temporal frequencies (mean 6.3 Hz), although some cells did exhibit preferences for every examined temporal frequency between 0.6 Hz and 10.8 Hz. The temporal frequency-tuning functions mostly revealed a band-pass character with a mean temporal bandwidth of 1.1 octaves. Our results demonstrate that the neurons along the anterior ectosylvian sulcus display particular spatial and temporal characteristics. The AEV neurons, with their preference for low spatial frequencies and with their fine spatial and temporal tuning properties, seem to be candidates for special tasks in motion perception.

Visual single-unit activity was recorded in the caudate nucleus of halothane-anaesthetized, immobilized, artificially respirated cats. Visually sensitive neurons were found in the dorsolateral part of the caudate body. A majority of the units responded optimally to small spot-like stimuli moving with velocities between 30 and 120 degrees /s. The receptive field of these units is large: it covers a major part of both the contra- and ipsilateral visual hemifields. No signs of retinotopy were observed. Most of the neurons display directional selectivity and are narrowly tuned to the direction of the moving stimulus. These physiological properties are consistent with recent morphological results that reveal multiple connections of the caudate nucleus with the superior colliculus through tecto-extrageniculo-thalamic pathways in the mammalian brain.