Faculty Bibliography

Motion perception is the process through which one gathers information on the dynamic visual world, in terms of the speed and movement direction of its elements. Motion sensation takes place from the retinal light sensitive elements, through the visual thalamus, the primary and higher visual cortices. In the present review we aim to focus on the extrageniculo-extrastriate cortical and subcortical visual structures of the feline and macaque brain and discuss their functional role in visual motion perception. Special attention is paid to the ascending tectofugal system that may serve for detection of the visual environment during self-motion.

Although the visual perception depends on the integration of spatial and temporal information, no knowledge is available concerning the responsiveness of neurons in the intermediate layers of the superior colliculus (SCi) to extended visual grating stimuli. Accordingly, we set out to investigate the responsiveness of these neurons in halothane-anesthetized cats to drifting sinewave gratings at various spatial and temporal frequencies. The SCi units responded optimally to gratings of low spatial frequencies (none of the analyzed SCi units exhibited maximal activity to spatial frequencies higher than 0.3c/deg) and exhibited low spatial resolution and narrow spatial frequency tuning. On the other hand, the SCi neurons preferred high temporal frequencies and exhibited high temporal resolution. Thus, the SCi neurons seem to be good spatio-temporal filters of visual information in the low spatial and high temporal frequency domain. Based upon the above summarized results we suggest that the SCi units can detect large contours moving at high velocities well, but are unable to distinguish small details. This is in line with the generally held view that the SCi could possess visuomotor function, such as organizing the complex, sensory-guided oculomotor and skeletomotor responses during the self-motion of the animal.

Single unit activity (SUA) was extensively studied in the lateral geniculate nucleus (LGN) but less attention was paid to the analysis of the local field potentials (LFP). In the present study, we investigate how and to what extent LFP and SUA correlate with visual stimulus eccentricity. SUAs and LFPs recorded extracellularly from 52 electrode positions were analyzed. Both LFP and SUA recordings contained well defined time-segments, which correlated with stimulus eccentricity. The spectral analysis of the LFPs indicated that in addition to the phasic, short latency activity of the 20 Hz frequency band, a tonic, 2-10 Hz, elongated component was also present. The time-domain analysis of the phasic and tonic LFP segments revealed a non-linear decrease of the mean LFP amplitude. The frequency-domain investigation made it obvious that the low and high frequency components exhibit a spatially localized increase of the response, in contrast to the time-domain curve. Our results confirm that the local field potentials as a measure of the mesoscopic level neuronal activity provide additional information concerning the activity of neuronal populations, thus enhancing our present knowledge about the functional circuitry as the foundation of various neuronal processes.

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.

Neuronal response onset latency provides important data on the information processing within the central nervous system. In order to enhance the quality of the onset latency estimation, we have developed a 'double sliding-window' technique, which combines the advantages of mathematical methods with the reliability of standard statistical processes. This method is based on repetitive series of statistical probes between two virtual time windows. The layout of the significance curve reveals the starting points of changes in neuronal activity in the form of break-points between linear segments. A second-order difference function is applied to determine the position of maximum slope change, which corresponds to the onset of the response. In comparison with Poisson spike-train analysis, the cumulative sum technique and the method of Falzett et al., this 'double sliding-window, technique seems to be a more accurate automated procedure to calculate the response onset latency of a broad range of neuronal response characteristics.

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.

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.