Direction-selective Units Research Articles

Differences in auditory temporal processing in the left and right auditory cortices of the rat

In the present study, we examined hemispheric differences in the representation and processing of temporally structured auditory stimuli. Neuronal responses evoked by sinusoidally frequency modulated (FM) tones, frequency sweeps, amplitude modulated (AM) tones and noise, click trains with constant inter-click intervals and natural vocalizations were recorded from the left (LAC) and right (RAC) auditory cortices in adult (4–6 months old) anaesthetized F344 rats. Using vector strength, modulation-transfer functions, van Rossum distances, or direction-selectivity index, representation and processing of structured auditory stimuli were compared in the LAC and the RAC. The RAC generally tended to exhibit a higher ability to synchronize with the stimulus, a higher reproducibility of responses, and a higher proportion of direction-selective units. The LAC, on the other hand, mostly had higher relative response magnitudes in the modulation transfer functions. Importantly, the hemispheric differences were dependent on the type of the stimulus and there was also a significant inter-individual variability. Our findings indicate that neural coding in the RAC is based more on timing of action potentials (temporal code), while the LAC uses more the response magnitudes (rate code). It is thus necessary to distinguish between the type of the neural code and the stimulus feature it encodes and reconsider the simple opinion about dominance of the LAC for temporal processing, as it may not hold in general for all types of temporally structured stimuli.

Direction selectivity of the retinotectal system of fish: Findings based on microelectrode extracellular recordings of the tectum opticum

Vision in fish plays an important role in different forms of visually guided behavior. The visual system of fish is available for research by different methods; it is a convenient experimental model for studying and understanding the mechanisms of vision in general. Responses of retinal direction-selective (DS) ganglion cells (GCs) are recorded extracellularly from their axon terminals in the superficial layers of the tectum opticum (TO). They can be divided into three distinct groups according to the preferred directions of stimulus movement: caudorostral, dorsoventral and ventrodorsal. Each of these groups comprises both ON and OFF units in equal proportions. Relatively small receptive fields (3-8?) and fine spatial resolution characterize retinal DS units as local motion detectors. Conversely, the responses of direction-selective tectal neurons (DS TNs) are recorded at two different tectal levels, deeper than the zone of retinal DS afferents. They are characterized by large receptive fields (up to 60?) and are indifferent to any sign of contrast, i.e., they can be considered as ON-OFF-type units. Four types of ON-OFF DS TNs preferring different directions of motion have been recorded. The preferred directions of three types of DS TNs match the preferred directions of three types of DS GCs. Matching the three preferred directions of ON and OFF DS GCs and ON-OFF DS TNs has allowed us to hypothesize that the GCs with caudorostral, ventrodorsal and dorsoventral preferences are input neurons for the corresponding types of DS TNs. On the other hand, the rostrocaudal preference in the fourth type of DS TNs, recorded exclusively in the deep tectal zone, is an emergent property of the TO. In this review, our findings are compared with the results of other authors examining direction selectivity in the fish retinotectal system.

A Computational Mechanism for Seeing Dynamic Deformation.

Human observers perceptually discriminate the dynamic deformation of materials in the real world. However, the psychophysical and neural mechanisms responsible for the perception of dynamic deformation have not been fully elucidated. By using a deforming bar as the stimulus, we showed that the spatial frequency of deformation was a critical determinant of deformation perception. Simulating the response of direction-selective units (i.e., MT pattern motion cells) to stimuli, we found that the perception of dynamic deformation was well explained by assuming a higher-order mechanism monitoring the spatial pattern of direction responses. Our model with the higher-order mechanism also successfully explained the appearance of a visual illusion wherein a static bar apparently deforms against a tilted drifting grating. In particular, it was the lower spatial frequencies in this pattern that strongly contributed to the deformation perception. Finally, by manipulating the luminance of the static bar, we observed that the mechanism for the illusory deformation was more sensitive to luminance than contrast cues.

Putative targets of direction-selective retinal ganglion cells in the tectum opticum of cyprinid fish

Responses of direction selective (DS) units of retinal and tectal origin were recorded extracellularly from the tectum opticum (TO) of immobilized fish. The data were collected from three cyprinid species – goldfish, carp and roach. Responses of the retinal DS ganglion cells (GCs) were recorded from their axon terminals in the superficial layers of TO. According to their preferred directions DS GCs, characterized by small receptive fields (3−8°), can be divided in three distinct groups, each group containing ON and OFF subtypes approximately in equal quantity. Conversely, direction-selective tectal neurons (DS TNs), recorded at two different tectal levels deeper than the zone of retinal DS afferents, are characterized by large receptive fields (up to 60°) and are indifferent to any sign of contrast i.e. can be considered as ON-OFF type units. Fish DS TNs unlike the retinal DS GCs, select four preferred directions. Three types of tectal DS units prefer practically the same directions as those already selected on the retinal level – caudo-rostral, dorso-ventral and ventro-dorsal. The fact that three preferred directions of DS GCs and DS TNs coincide allows us to assume that three types of DS GCs are input neurons for corresponding types of DS TNs. The fourth group of DS TNs has the emergent rostro-caudal preference not explicitly present in any of the DS GC inputs. These units are recorded in deep TO layers exclusively. Receptive fields of these DS neurons could be entirely formed on the tectal level. Possible interrelations between retinal and tectal DS units are discussed.

Direction-selective units in the frog's basal optic root nucleus.

Electrophysiological responses from 91 direction-selective (DS) units located in the basal optic root (nBOR) and the adjacent dorsomedial area have been recorded extracellularly in the frog (Rana temporaria L.). Based on characteristics of the recorded responses, 23 of the units are considered to be retinal ganglion cells' (RGC; axon terminals). The majority of the remaining 68 units were considered to be DS neurons of the nBOR. The results of our study suggest that in the nBOR area there might be four subtypes of the tegmental and retinal DS units that respond selectively to stimuli moving in the dorso-ventral, ventro-dorsal, caudo-rostral and rostro-caudal directions. The receptive field (RF) sizes of the nBOR DS neurons were estimated to be about 30-[Formula: see text], while those for the retinal units were significantly smaller- just 6-[Formula: see text]. In response to abrupt darkening within the units' RFs, both the nBOR DS neurons and the RGC DS units respond by a weak discharge. Our results indicate that the frog nBOR DS neurons integrate the inputs from the retinal OFF-type DS units over relatively large segments of the visual field.

A neuronal network model for context-dependence of pitch change perception.

Many natural stimuli have perceptual ambiguities that can be cognitively resolved by the surrounding context. In audition, preceding context can bias the perception of speech and non-speech stimuli. Here, we develop a neuronal network model that can account for how context affects the perception of pitch change between a pair of successive complex tones. We focus especially on an ambiguous comparison—listeners experience opposite percepts (either ascending or descending) for an ambiguous tone pair depending on the spectral location of preceding context tones. We developed a recurrent, firing-rate network model, which detects frequency-change-direction of successively played stimuli and successfully accounts for the context-dependent perception demonstrated in behavioral experiments. The model consists of two tonotopically organized, excitatory populations, Eup and Edown, that respond preferentially to ascending or descending stimuli in pitch, respectively. These preferences are generated by an inhibitory population that provides inhibition asymmetric in frequency to the two populations; context dependence arises from slow facilitation of inhibition. We show that contextual influence depends on the spectral distribution of preceding tones and the tuning width of inhibitory neurons. Further, we demonstrate, using phase-space analysis, how the facilitated inhibition from previous stimuli and the waning inhibition from the just-preceding tone shape the competition between the Eup and Edown populations. In sum, our model accounts for contextual influences on the pitch change perception of an ambiguous tone pair by introducing a novel decoding strategy based on direction-selective units. The model's network architecture and slow facilitating inhibition emerge as predictions of neuronal mechanisms for these perceptual dynamics. Since the model structure does not depend on the specific stimuli, we show that it generalizes to other contextual effects and stimulus types.

Anterior-Posterior Direction Opponency in the Superficial Mouse Lateral Geniculate Nucleus

We show functional-anatomical organization of motion direction in mouse dorsal lateral geniculate nucleus (dLGN) using two-photon calcium imaging of dense populations in thalamus. Surprisingly, thesuperficial 75μm region contains anterior and posterior direction-selective neurons (DSLGNs) intermingled with nondirection-selective neurons, while upward- and downward-selective neurons are nearlyabsent. Unexpectedly, the remaining neurons encode both anterior and posterior directions, forming horizontal motion-axis selectivity. A model of random wiring consistent with these results makes quantitative predictions about the connectivity of direction-selective retinal ganglion cell (DSRGC) inputs to the superficial dLGN. DSLGNs are more sharply tuned than DSRGCs. These results suggest that dLGN maintains and sharpens retinal direction selectivity and integrates opposing DSRGC subtypes in a functional-anatomical region, perhaps forming a feature representation for horizontal-axis motion, contrary to dLGN being a simple relay. Furthermore, they support recent conjecture that cortical direction and orientation selectivity emerge in part from a previously undescribed motion-selective retinogeniculate pathway.

Presynaptic and postsynaptic single-unit responses in the goldfish tectum as revealed by a reversible synaptic transmission blocker

A variety of visually evoked responses are recorded in the fish optic tectum using single-cell recording technique. Based on indirect criteria (frequency power spectrum of spikes, spike waveform, receptive field size), they may be divided into two groups: those presumably recorded from axon terminals of retinal ganglion cells projecting to the tectum (precynaptic recording), and those recorded from tectal neurons (postsynaptic recording). In the present study, we used cobalt, a reversible blocker of synaptic transmission, as a more crucial criterion to identify the source of these responses. After cobalt application, some units (such as ON- and OFF-types of direction-selective units, orientation-selective and spontaneously active units) were visually responsive, while others (including ON-OFF direction-selective units with large receptive fields) ceased firing. Discrimination of the units by the use of cobalt has been found to coincide with that by the indirect physiological criteria. Thus, the differences in frequency power spectrum of spikes, spike waveform, and receptive field size may be used for efficient and reliable discrimination between pre- and post-synaptic recordings in the fish tectum.

RECEPTIVE FIELD SIZES OF DIRECTION-SELECTIVE UNITS IN THE FISH TECTUM

Responses of direction-selective (DS) ganglion cells (GCs) were recorded extracellularly from their axon terminals in the superficial layer of tectum opticum (TO) of immobilized cyprinid fish Carassius gibelio (Bloch, 1782). Excitatory receptive field (ERF) sizes of six types of DS GCs (ON and OFF cells, each of three distinct preferred directions) were evaluated on the basis of four different methods. In Method 1, the ERF width was calculated as a product of duration of spike train, generated in response to contrast edge moving across the ERF in preferred direction, and the velocity of the stimulus movement. The duration of spike train was estimated either as an interval between the first and the last spikes, or on the basis of the width of bell-shaped post-stimulus histogram of spike response according to its standard deviation. More precise size and position of the ERF can be outlined with edges moving in many different directions. So, in Method 2 diameter of the ERF was calculated on the basis of a mean distance of position of spike appearance from the center of ERF. Method 3 - ERF tracing by small contrast spot moving on several parallel tracks allowed estimation of the ERF width by number of spikes along each track and the ERF length by the duration of spike train. When tracing in two mutually orthogonal projections, the method also permitted calculation of the value of the temporal delay in the network from the same experiment. Canonical method (Method 4) used the ERF mapping with contrast spots flickering sequentially in different places of stimulation area. The length, width and orientation of the ERF were evaluated according to the two-dimensional equivalent of the standard deviation for this data set. All applied methods gave consistent estimates of ERF sizes - mean values of ERF sizes for all four procedures ranged between 4 degrees and 4.8 degrees . These angle values corresponded to retinal area of approximately 300 mum. Small ERFs of the fish DS GCs measured in the current study, indicate that the fish DS units should be classified as "fast" DS units, and are most likely involved in the detection of small objects moving in the surrounding environment.

Color properties of movement detectors in the Carassius gibelio (Bloch, 1782) tectum opticum studied by selective stimulation of different cone types

and takes part in orienting responses revealed in rapid eye, head, and/or body movemements. This function can proceed without taking into account information about color, and the superior colliculus in mammals is color-blind (Michael, 1973). In the frog, the system carrying chromatic information also originates as a channel separate from the tectal pathway, providing the main information needed for the frog’s reactions to moving objects (Maximov et al., 1985). Color blindness of movement detectors projecting to the fish tectum was proved in color-matching experiments for both orientation-selective (Maximova, 1999) and direction-selective (Maximova et al., 2005) units. Recent experiments using selective stimulation of different cone types revealed new features of these projections. The color vision of Prussian carp (Carassius gibelio Bloch, 1782) adults is determined by three types of cones: long-wavelength (L), middle-wavelength (M), and short-wavelength (�), con taining three A2-based visual pigments that absorb maximally at about 622, 545, and 434 nm, respectively (Maximova et al., 2005). Color opponent cells were presented at different levels of the fish visual system, and single-unit extracellular recordings from the C. gibelio tectum opticum were made in order to examine the possible role of action potential timing in coding chromatic stimuli.

Direction Selectivity in the Goldfish Tectum Revisited

Responses of direction-selective (DS) ganglion cells (GCs) were recorded extracellularly from their axon terminals in the superficial layer of the tectum opticum (TO) of immobilized goldfish, Carassius auratus gibelio (Bloch). Directional tuning curves were measured with contrast edges moving in 12 or more different directions across the receptive field (RF). All directional tuning curves had cardioid-like appearance, their acceptance angles amounted to somewhat more than 180 degrees . According to their preferred directions DS GCs proved to comprise three distinct groups, each group containing DS GCs of ON and OFF subtypes approximately in equal quantity. Thus, this gives six physiological types of DS GCs in total. The preferred direction of a DS GC does not depend to some extent on a value of contrast, speed, size, and form of the stimuli. Coincidence in number of preferred directions with number of semicircular canals implies that DS GCs projecting to tectum are involved in some multimodal sensory integration in postural, locomotor, and oculomotor control in the three-dimensional aquatic world. DS neurons of the TO itself respond independently of the sign of stimulus contrast, have enormous receptive fields, and seem likely to collect signals from the retinal DS units of both ON and OFF subtypes with the same preferred direction.

Cooperative and competitive spatial interactions in motion integration.

Recovering the velocity of objects moving in the visual field requires both the integration and segmentation of local neuronal responses elicited by moving stimuli in primary visual cortex. Herein, we investigate the effects of the contrast, density, spatial proximity, spatial frequency, and spatial configuration of component motions on these complementary processes. Measuring the ability of human observers to discriminate the global direction of motion displays composed of spatially distributed patches of drifting gratings whose motion is locally ambiguous, we provide psychophysical evidence that linking component motion across space is facilitated at low contrast and high patch density. Furthermore, direction discrimination depends on the spatial frequency of component gratings and is more accurate for spatial configurations that contain "virtual" L junctions as compared to configurations composed of "virtual" T junctions. We suggest that the conditions yielding global motion coherence can be accounted for by the existence of anisotropic cooperative/competitive, contrast-dependent, long-range interactions among oriented direction-selective units. In addition, we bring evidence that motion segmentation processes rely upon the processing of moving local spatial discontinuities. The results are discussed in the light of recent psychophysical and physiological evidence that long-range excitatory and inhibitory interactions within primary visual cortex modulate perceptual linking.

Response properties of pretectal omnidirectional pause neurons in the behaving primate.

We have identified a region in the pretectum of rhesus monkeys (Macaca mulatta) that contains units that evince a complete cessation in firing immediately after saccades. The pause occurs for saccades to target steps and catch up saccades during smooth pursuit, spontaneously in complete darkness or after quick phases of nystagmus. Because the pause in unit firing always follows saccade onset, we call these neurons following omnidirectional pause neurons (FOPNs). Because the pause also occurs with saccades in the dark, it is related to the saccade per se and is not a visually contingent response. The duration of the pause in firing exceeded the duration of all saccades up to 40 deg. For targeting saccades, the start of the pause was locked rather tightly to the beginning of the saccade but began an average of 51 ms after the saccade did. The end of the pause was linked only loosely to either the beginning or end of the saccade. About half (54%) of our 59 FOPNs also discharged a distinct burst of firing that preceded the pause. In different units, the burst preceded saccade onset by from 0 to 20 ms with an average of 11 ms and therefore could signal the occurrence of an impending saccade. The presaccadic burst was not correlated with any parameter of the saccade. Most FOPNs were found 278 microns, on average, dorsal to the direction-selective units characteristic of the pretectal nucleus of the optic tract (NOT) and occasionally slightly beyond the anterior-posterior and medial-lateral borders of the NOT. The FOPN region does not coincide with any known anatomically or functionally delineated pretectal nucleus. Because the characteristics of the FOPN pause are not reflected in the characteristics of the saccade and the FOPN pause occurs well after the saccade is over, it is unlikely that the pause in pretectal FOPNs is involved with saccade generation. On the other hand, the leading burst exhibited by the majority of FOPNs reliably signals that a saccade is occurring but neither its size nor direction. Perhaps this signal indicating the occurrence of all saccades is routed to visual relay neurons to effect saccadic modification of visual pathways. The substantial efferent connections of the FOPN/NOT region to the pregeniculate nucleus and the saccadic discharge or pregeniculate cells are discussed in the context of this suggestion.

Single-unit responses to whole-field visual stimulation in the pretectum of Rana pipiens

Visually responsive single units were recorded from the pretectal region that includes the large-celled nucleus lentiformis mesencephali (nLM) in the leopard frog, Rana pipiens. During monocular stimulation of the contralateral eye, 60 single units responding to movement of a large-field., random-dot pattern were quantitatively analyzed using horizontal and vertical directions at each of four pattern velocities (0.4–40°/s). All units were spontaneously active, motion sensitive, and the majority showed ‘on’-‘off’ responses. Several different response profiles were observed, including velocity-sensitive units with peak response at 10°/s, most of which showed directional selectivity, and speed-sensitive units that showed increasing spike frequencies as pattern velocity increased, but little or no directional selectivity. About one-third of all units analyzed were direction-selective, and 55% of those responded optimally to the temporal-to-nasal (T-N) direction of motion. T-N units were recorded primarily from an area that lies dorsolaterally between nLM and the optic tectum, in the ‘peri-nLM’ region. The pronounced monocular optokinetic nystagmus (OKN) response asymmetry that occurs in anurans appears to be reflected in the response profiles of the T-N direction selective units.

The role of intervening patterns in the storage of the movement aftereffect.

Wohlgemuth, having measured the duration of the motion aftereffect (MAE), instructed subjects to close their eyes immediately after adaptation for a period of time longer than the MAE. Upon opening their eyes the subjects reported a residual effect, albeit somewhat shorter than the original effect. Thus the decay of the aftereffect appeared to have been retarded by the period of darkness. This effect is known as 'storage' and poses a problem for any model of the MAE based on the fatiguing of direction-selective units in the visual pathway. A reexamination is made of storage of the MAE, again concentrating on the intervening stimulation between movement adaptation and aftereffect test. The results suggest that the nature of the intervening pattern between adaptation and test conditions is remarkably unimportant. A total of 11 different storage patterns were examined after adaptation to high-contrast drifting horizontal sinewave gratings. For 10 of these patterns large and robust storage effects were found. The exception occurred when the spatial pattern of the storage stimulus was identical to the adaptation and test stimuli. It is proposed that storage cannot be understood in terms of a simple fatigue model of the MAE and that one component of the effect may share similarities with contingent aftereffects.

Inhibition of neuronal activity in the nucleus of the optic tract due to electrical stimulation of the medial terminal nucleus in the rat.

Morphologically, a GABAergic connection between the medial terminal nucleus of the accessory optic system and the nucleus of the optic tract, two primary visual nuclei involved in the optokinetic reflex, has been demonstrated. In this study it was investigated if the medial terminal nucleus forms an inhibitory input to movement direction selective units in the nucleus of the optic tract. Neurons in the nucleus of the optic tract were visually stimulated with moving large random square patterns in their preferred and non-preferred direction, and their activity was recorded extracellularly. Concomitantly, bipolar electrical stimulation was applied to the medial terminal nucleus and its effect was studied on the visual responses of units in the nucleus of the optic tract. Units in the nucleus of the optic tract were strongly inhibited during electrical stimulation of the medial terminal nucleus. The role of GABA in mediating this inhibition was investigated by applying bicuculline, a GABAA receptor antagonist, iontophoretically to the recorded units in the nucleus of the optic tract. However, although average spike rate levels of units in the nucleus of the optic tract increased with bicuculline, bicuculline did not reduce inhibition invoked by electrical stimulation of the medial terminal nucleus. A possible explanation for this observation is that this inhibition is GABAB receptor mediated.

Segregation of direction selective neurons and synaptic organization of inhibitory intranuclear connections in the medial terminal nucleus of the rat: an electrophysiological and immunoelectron microscopical study.

A combined electrophysiological and morphological investigation of the medial terminal nucleus (MTN) in the rat was undertaken, aimed at a better understanding of the relationship between structure and function in this nucleus. The locations of upward and downward direction selective units in the MTN were documented with extracellular electrophysiological recording. By means of tracer experiments, with Phaseolus vulgaris-leucoagglutinin, biocytin, and cholera toxin subunit B-horseradish peroxidase, the internal connections of the MTN, its retinal afferents, and the projection neurons to the inferior olive were visualized. Terminals originating from the retina and from internal connections were characterized at the ultrastructural level. Their termination pattern on cells in the MTN, including identified inferior olive projection neurons, were determined. Additionally, postembedding GABA immunocytochemistry was performed to identify GABAergic elements. From reconstructions of the positions of electrophysiologically recorded units in the MTN, a local segregation between upward and downward direction selective units was revealed. Upward direction selective units were found in the dorsal part and ventromedially, whereas downward direction selective units were found ventral and laterally in the MTN. The MTN receives optic fibers via two separate routes which, based on their trajectory, presumably terminate in different parts of the MTN: the inferior fascicle of the accessory optic tract in the dorsal part, and the posterior fiber bundle of the superior fascicle in the ventral part of the MTN. A correspondence has been found between the segregation of direction selective units and the areas in the MTN where the retinal fibers from the two pathways distribute. It is, therefore, proposed that the inferior fasciculus conveys upward direction selectivity and the posterior fiber bundle downward direction selectivity, and that the two fiber bundles terminate segregated in the MTN. After anterograde tracing from the eye, retinal terminals were found evenly distributed throughout the MTN. They are characterized as GABA negative R-type terminals. After retrograde tracing from the inferior olive, identified MTN-inferior olive projection neurons were found in the dorsal MTN and medially in the ventral MTN. Their location in the MTN suggests that MTN-inferior olive projection neurons are upward direction selective. MTN-inferior olive projection neurons are large non-GABAergic cells, with a variable form. A majority of both F- and R-type terminals were found to make synaptic contacts on the dendrites of MTN cells. MTN-inferior olive projection neurons did not differ from other neurons in this respect.

Synaptic organization of the nucleus of the optic tract in the rabbit: a combined Golgi-electron microscopic study.

The organization of the nucleus of the optic tract was investigated with light and electron microscopy in combination with Golgi impregnation. In Golgi material, neurons ranged in size from 10 to 25 microns with three to seven principal dendrites extending predominantly parallel to the fibres of the optic tract, irrespective of their location within the nucleus. In some areas dendrites extended into the neuropil of the adjacent dorsal terminal nucleus of the accessory optic system and the posterior pretectal nucleus. Occasionally spines and appendages were observed. The fine structure of the nuclei, perikarya and the dendritic arborization did not allow a well-defined distinction between interneurons and projection neurons. The synaptic organization of the nucleus of the optic tract showed great resemblance to the neuropil of the lateral geniculate nucleus and the superior colliculus. Similar types of presynaptic terminals were noticed: (i) R-terminals were either large and scalloped or small and regular in outline with spherical vesicles and electron-lucent mitochondria, and showed asymmetric contact zones; (ii) F-terminals with flattened vesicles, opaque mitochondria and symmetric contact zones; (iii) RLD-terminals with spherical vesicles and electron-dense mitochondria and asymmetric contact zones; (iv) P-terminals with pleomorphic vesicles and electron-lucent or opaque mitochondria and asymmetric synaptic thickenings. These different types of terminal were found isolated in the neuropil or in clusters of synapses. The most striking differences between the nucleus of the optic tract and the lateral geniculate nucleus were the relative scarcity of F-terminals in the clusters, the paucity of triadic arrangements and the relatively small size of the R-terminals. The differences in ultrastructure may be related to retinal W-type ganglion cells, which form the main retinal input to the nucleus of the optic tract and could also be related to the physiologically identified direction-selective units within the nucleus of the optic tract.

Is the enthorhinal area (EA) involved in processing of olfactory information?

Visually responsive single units were recorded from the pretectal region that includes the large-celled nucleus lentiformis mesencephali (nLM) in the leopard frog, Rana pipiens. During monocular stimulation of the contralateral eye, 60 single units responding to movement of a large-field., random-dot pattern were quantitatively analyzed using horizontal and vertical directions at each of four pattern velocities (0.4–40°/s). All units were spontaneously active, motion sensitive, and the majority showed ‘on’-‘off’ responses. Several different response profiles were observed, including velocity-sensitive units with peak response at 10°/s, most of which showed directional selectivity, and speed-sensitive units that showed increasing spike frequencies as pattern velocity increased, but little or no directional selectivity. About one-third of all units analyzed were direction-selective, and 55% of those responded optimally to the temporal-to-nasal (T-N) direction of motion. T-N units were recorded primarily from an area that lies dorsolaterally between nLM and the optic tectum, in the ‘peri-nLM’ region. The pronounced monocular optokinetic nystagmus (OKN) response asymmetry that occurs in anurans appears to be reflected in the response profiles of the T-N direction selective units.

Velocity-tuning of motion-sensitive and direction-selective cells in the rabbit striate cortex

The receptive field characteristics of 268 striate cortical cells were classified in 10 Dutch-belted rabbits. From the 66 motion-sensitive and direction-selective units encountered, quantitative data describing the responses of 41 motion-sensitive, and 10 direction-selective cells to moving gratings were obtained. The results showed that all cells responded with increased spike discharges to increased velocities of the moving gratings up to a maximum, then they became less responsive to further increase of the velocities. The patterns of the response curves varied from cell to cell. When the 51 cells were taken as a whole, there were more cells responding to the velocities of 10–30 deg/sec than to higher or lower velocities. The total range over which these cells responded was 0.1–300 deg/sec. The preferred velocities for individual cells ranged from 1.5 to 150 deg/sec, showing a moderate concentration at the 6–40 deg/sec region. The extent of the effective velocities for individual cells formed a continuous distribution. No segregation into groups was seen in either of the two cell types studied or in all cells taken as a group. These data were compared with those reported for the rabbit retinal ganglion cells and the cells in the nucleus of the optic tract, as well as the cells in the cat visual system.