Processing of form and motion in area 21a of cat visual cortex

1993 ◽  
Vol 10 (1) ◽  
pp. 93-115 ◽  
Author(s):  
B. Dreher ◽  
A. Michalski ◽  
R. H. T. Ho ◽  
C. W. F. Lee ◽  
W. Burke
Keyword(s):  
Spatial Organization ◽  
Receptive Fields ◽  
Direction Selectivity ◽  
Orientation Tuning ◽  
Area 17 ◽  
Horizontal Strip ◽  
Tuning Curves ◽  
Mean Width ◽  
Area 21A ◽  
The Mean

AbstractExtracellular recordings from single neurons have been made from presumed area 21a of the cerebral cortex of the cat, anesthetized with N2O/O2/sodium pentobarbitone mixture. Area 21a contains mainly a representation of a central horizontal strip of contralateral visual field about 5 deg above and below the horizontal meridian.Excitatory discharge fields of area 21a neurons were substantially (or slightly but significantly) larger than those of neurons at corresponding eccentricities in areas 17, 19, or 18, respectively. About 95% of area 21a neurons could be activated through either eye and the input from the ipsilateral eye was commonly dominant. Over 90% and less than 10% of neurons had, respectively, C-type and S-type receptive-field organization. Virtually all neurons were orientation-selective and the mean width at half-height of the orientation tuning curves at 52.9 deg was not significantly different from that of neurons in areas 17 and 18. About 30% of area 21a neurons had preferred orientations within 15 deg of the vertical.The mean direction-selectivity index (32.8%) of area 21a neurons was substantially lower than the indices for neurons in areas 17 or 18. Only a few neurons exhibited moderately strong end-zone inhibition. Area 21a neurons responded poorly to fast-moving stimuli and the mean preferred velocity at about 12.5 deg/s was not significantly different from that for area 17 neurons.Selective pressure block of Y fibers in contralateral optic nerve resulted in a small but significant reduction in the preferred velocities of neurons activated via the Y-blocked eye. By contrast, removal of the Y input did not produce significant changes in the spatial organization of receptive fields (S or C type), the size of the discharge fields, the width of orientation tuning curves, or direction-selectivity indices.Our results are consistent with the idea that area 21a receives its principal excitatory input from area 17 and is involved mainly in form rather than motion analysis.

Response properties of area 17 neurons in cats reared in stroboscopic illumination

1987 ◽  
Vol 57 (5) ◽  
pp. 1511-1535 ◽  
Author(s):  
J. Cremieux ◽  
G. A. Orban ◽  
J. Duysens ◽  
B. Amblard
Keyword(s):  
Peripheral Vision ◽  
Receptive Fields ◽  
Response Strength ◽  
Direction Selectivity ◽  
Orientation Tuning ◽  
Area 17 ◽  
Central Vision ◽  
Response Properties ◽  
Adult Cat ◽  
Areas 17 And 18

The response properties of 196 area 17 cells were studied qualitatively in seven cats reared from birth in a stroboscopically illuminated environment (frequency, 2/s; duration, 200 microseconds). Quantitative testing with the multihistogram technique was carried out in 115 cells. As control population, 453 neurons recorded in area 17 of the normal adult cat and tested qualitatively (of which 301 neurons were tested quantitatively) were available. In area 17 of strobe-reared cats, a number of spatial characteristics of receptive fields investigated with hand-held stimuli were found to be abnormal. There was a strong reduction in the encounter frequency both of end-stopped cells and of binocularly driven cells in the strobe-reared cats. Central receptive fields in strobe-reared cats were wider than in normal cats, but the increase in receptive-field width with eccentricity was still observed. More cells than in normal cats showed either no selectivity or only a weak bias for stimulus orientation, but the orientation tuning of orientation-selective cells was similar in strobe-reared and normal cats. Quantitative testing revealed that the velocity preference of cells in area 17 subserving central vision was different in strobe-reared cats from that of normal cats, due to a reduction in the encounter frequency of cells showing a preference for low velocities. There was no difference in velocity preference between strobe-reared and normal cats in the parts of area 17 that subserve peripheral vision, the proportion of neurons responding to fast velocities showing a similar increase in both groups of animals. Fewer cells were direction selective in strobe-reared cats than in normal cats. Most of the remaining direction-selective cells had peripheral receptive fields and the synergism between leaving an OFF subregion and entering an ON subregion contributed to their direction selectivity. Latency of neurons in area 17 of strobe-reared cats was slightly higher than in normal cats, but the response strength of neurons was the same in the two groups. The proportion of cells failing to respond to briefly flashed stationary stimuli was significantly lower in strobe-reared than in normal animals. Qualitative and quantitative testing showed that strobe rearing has a stronger effect on the parts of area 17 that subserve central vision than on those that subserve peripheral vision. Comparing the present results with those of Kennedy and Orban (37) shows that strobe rearing has less effect on area 17 than on area 18 and that the functional differences between areas 17 and 18 in strobe-reared cats are smaller than in normal cats.

Strobe Rearing Prevents the Convergence of Inputs With Different Response Timings Onto Area 17 Simple Cells

1998 ◽  
Vol 80 (6) ◽  
pp. 3005-3020 ◽  
Author(s):  
Allen L. Humphrey ◽  
Alan B. Saul ◽  
Jordan C. Feidler
Keyword(s):  
Single Cells ◽  
Receptive Fields ◽  
Direction Selectivity ◽  
Area 17 ◽  
Simple Cells ◽  
Absolute Phase ◽  
Wide Range ◽  
The Mean ◽  
Different Response ◽  
Response Timing

Humphrey, Allen L., Alan B. Saul, and Jordan C. Feidler. Strobe rearing prevents the convergence of inputs with different response timings onto area 17 simple cells. J. Neurophysiol. 80: 3005–3020, 1998. The preceding paper showed that the loss of direction selectivity in simple cells induced by strobe rearing reflects the elimination of spatially ordered response timing differences across the receptive field that underlie spatiotemporal (S-T) inseparability. Here we addressed whether these changes reflected an elimination of certain timings or an alteration in how timings were associated in single cells. Timing in receptive fields was measured using stationary bars undergoing sinusoidal luminance modulation at different temporal frequencies (0.5–6 Hz). For each bar position, response phase versus temporal frequency data were fit by a line to obtain two measures: absolute phase and latency. In normal cats, many individual simple cells display a wide range of timings; in layer 4, the mean range for absolute phase and latency was 0.21 cycles and 39 ms, respectively. Strobe rearing compressed the mean timing ranges in single cells, to 0.08 cycles and 31 ms, respectively, and this compression accounted for the loss of inseparability. A similar compression was measured in layer 6 cells. In contrast, the range of timing values across the simple-cell population was relatively normal. Single cells merely sampled narrower than normal regions of the timing space. We sought to understand these cortical changes in terms of how inputs from the lateral geniculate nucleus (LGN) may have been affected by strobe rearing. In normal cats, a wide range of absolute phase and latency values exists among lagged and nonlagged LGN cells, and these thalamic timings account for most of the cortical timings. Also, S-T inseparability in many simple cells can be attributed to the convergence of lagged and/or nonlagged inputs. Strobe rearing did not change the sampling of lagged and nonlagged cells, and the geniculate timings continued to account for most of the cortical timings. However, strobe rearing virtually eliminated cortical receptive fields with mixed lagged and nonlagged timing, and it compressed the timing range in cells dominated by one or the other geniculate type. Thus strobe rearing did not eliminate certain timings in LGN or cortex, but prevented the convergence of different timings on single cells. To account for these results, we propose a developmental model in which strobe stimulation alters the correlational structure of inputs based on their response timing. Only inputs with similar timing become associated on single cortical cells, and this produces S-T separable receptive fields that lack the ability to confer a preferred direction of motion.

Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in area 18 of the visual cortex of the cat

1992 ◽  
Vol 9 (1) ◽  
pp. 65-78 ◽  
Author(s):  
B. Dreher ◽  
A. Michalski ◽  
B. G. Cleland ◽  
W. Burke
Keyword(s):  
Optic Nerve ◽  
Receptive Field ◽  
Direction Selectivity ◽  
Orientation Tuning ◽  
Area 17 ◽  
Area 18 ◽  
Eye Dominance ◽  
Receptive Field Properties ◽  
Velocity Sensitivity ◽  
The Mean

AbstractRecordings were made from single neurons in area 18 of anesthetized cats (N2O/O2 mixture supplemented by continuous intravenous infusion of barbiturate) in which one optic nerve had been pressure blocked to selectively block conduction in the largest (Y-type) fibers. Cortical neurons were stimulated visually via the normal eye or via the eye with the pressure-blocked optic nerve (“Y-blocked eye”). Several properties of the receptive fields such as their spatial organization (S or C cells), orientation tuning, and the presence and strength of end-zone inhibition appear to be unaffected by removal of the Y input. By contrast, the removal of the Y input resulted in a small but significant reduction in the size of the discharge field and in the direction-selectivity index. In three respects, peak response discharge rate, eye dominance, and velocity sensitivity, removal of the Y input had strong and highly significant effects. Thus, the mean peak discharge frequency of responses evoked by the stimulation of binocular neurons via the Y-blocked eye was significantly lower than that of responses evoked by the stimulation via the normal eye. Accordingly, the eye-dominance histogram was shifted markedly towards the normal eye (more so than in the homologous experiment conducted on area 17 — Burke et al., 1992). Finally, the mean preferred velocity of responses of cells activated via the normal eye was in the vicinity of 145 deg/s, whereas for cells activated via the Y-blocked eye the value was about 35 deg/s. Overall, the results of the present study imply that (1) apart from Y-type excitatory input there are significant excitatory non-Y-inputs to area 18; these inputs at least partially consist of indirect X-type input relayed via area 17; (2) in neurons of area 18 that receive both Y-type and non-Y-type excitatory inputs, the Y-type input has a major influence on strength of the response and velocity sensitivity and a lesser influence on the direction selectivity and size of the discharge fields; and (3) area 18 contains mechanisms determining such receptive-field properties as S- or C-type organization, orientation tuning, and direction selectivity which can be accessed either by the Y input or by non-Y input.

Are neurons in cat posteromedial lateral suprasylvian visual cortex orientation sensitive? Tests with bars and gratings

1995 ◽  
Vol 12 (1) ◽  
pp. 141-151 ◽  
Author(s):  
Yuri Danilov ◽  
Rodney J. Moore ◽  
Von R. King ◽  
Peter D. Spear
Keyword(s):  
Visual Cortex ◽  
Receptive Fields ◽  
Orientation Tuning ◽  
Orientation Sensitivity ◽  
Tuning Curves ◽  
Preferred Direction ◽  
Response Measures ◽  
Direction Of Movement ◽  
The Difference ◽  
Quantitative Response

AbstractThere is controversy in the literature concerning whether or not neurons in the cat's posteromedial lateral suprasylvian (PMLS) visual cortex are orientation selective. Previous studies that have tested cells with simple bar stimuli have found that few, if any, PMLS cells are orientation selective. Conversely, studies that have used repetitive stimuli such as gratings have found that most or all PMLS cells are orientation selective. It is not known whether this difference in results is due to the stimuli used or the laboratories using them. The present experiments were designed to answer this question by testing individual PMLS neurons for orientation sensitivity with both bar and grating stimuli. Using quantitative response measures, we found that most PMLS neurons respond well enough to stationary flashed stimuli to use such stimuli to test for orientation sensitivity. On the basis of these tests, we found that about 85% of the cells with well-defined receptive fields are orientation sensitive to flashed gratings, and a similar percentage are orientation sensitive to flashed bars. About 80% of the cells were orientation sensitive to both types of stimuli. The preferred orientations typically were similar for the two tests, and they were orthogonal to the preferred direction of movement. The strength of the orientation sensitivity (measured as the ratio of discharge to the preferred and nonpreferred orientations) was similar to both types of stimuli. However, the width of the orientation tuning curves was systematically broader to bars than to gratings. Several hypotheses are considered as to why previous studies using bars failed to find evidence for orientation sensitivity. In addition, a mechanism for the difference in orientation tuning to bars and gratings is suggested.

Plasticity of neuronal response properties in adult cat striate cortex

1998 ◽  
Vol 15 (1) ◽  
pp. 177-196 ◽  
Author(s):  
J. MCLEAN ◽  
L.A. PALMER
Keyword(s):  
Visual Cortex ◽  
Striate Cortex ◽  
Direction Selectivity ◽  
Orientation Tuning ◽  
Tuning Curves ◽  
Spatial Phase ◽  
Associative Conditioning ◽  
Phase Tuning ◽  
Adult Cat ◽  
Cat Striate Cortex

We have utilized an associative conditioning paradigm to induce changes in the receptive field (RF) properties of neurons in the adult cat striate cortex. During conditioning, the presentation of particular visual stimuli were repeatedly paired with the iontophoretic application of either GABA or glutamate to control postsynaptic firing rates. Similar paradigms have been used in kitten visual cortex to alter RF properties (Fregnac et al., 1988, 1992; Greuel et al., 1988; Shulz & Fregnac, 1992). Roughly half of the cells that were subjected to conditioning with stimuli differing in orientation were found to have orientation tuning curves that were significantly altered. In general, the modification in orientation tuning was not accompanied by a shift in preferred orientation, but rather, responsiveness to stimuli at or near the positively reinforced orientation was increased relative to controls, and responsiveness to stimuli at or near the negatively reinforced orientation was decreased relative to controls. A similar proportion of cells that were subjected to conditioning with stimuli differing in spatial phase were found to have spatial-phase tuning curves that were significantly modified. Conditioning stimuli typically differed by 90 deg in spatial phase, but modifications in spatial-phase angle were generally 30–40 deg. An interesting phenomenon we encountered was that during conditioning, cells often developed a modulated response to counterphased grating stimuli presented at the null spatial phase. We present an example of a simple cell for which the shift in preferred spatial phase measured with counterphased grating stimuli was comparable to the shift in spatial phase computed from a one-dimensional Gabor fit of the space-time RF profile. One of ten cells tested had a significant change in direction selectivity following associative conditioning. The specific and predictable modifications of RF properties induced by our associative conditioning procedure demonstrate the ability of mature visual cortical neurons to alter their integrative properties. Our results lend further support to models of synaptic plasticity where temporal correlations between presynaptic and postsynaptic activity levels control the efficiency of transmission at existing synapses, and to the idea that the mature visual cortex is, in some sense, dynamically organized.

scholarly journals Projection of orthogonal tiling from the retina to the visual cortex

2020 ◽  
Author(s):  
Jaeson Jang ◽  
Min Song ◽  
Gwangsu Kim ◽  
Se-Bum Paik
Keyword(s):  
Visual Cortex ◽  
Spatial Frequency ◽  
Spatial Organization ◽  
Ocular Dominance ◽  
Receptive Fields ◽  
Orientation Tuning ◽  
List Type ◽  
Strongly Correlated ◽  
Electrode Recording ◽  
Alignment Angle

AbstractIn higher mammals, the primary visual cortex (V1) is organized into diverse tuning maps of visual features such as orientation, spatial frequency and ocular dominance. The topography of these maps is observed to intersect orthogonally, implying that a developmental principle for efficient tiling of sensory modules may exist. However, it remains unclear how such a systematic relationship among cortical tuning maps could develop. Here, we show that the orthogonal organization of tuning modules already exist in retinal ganglion cell (RGC) mosaics, and that this provides a blueprint of the orthogonal organization in V1. Firstly, from the analysis of multi-electrode recording data in V1, we found that the ON-OFF subregion distance of receptive fields varies periodically across the cortical surface, strongly correlated to ocular dominance and spatial frequency in the area. Further, the ON-OFF alignment angle, that is orthogonal to the ON-OFF distance, appears to correlate with orientation tuning. These suggest that the orthogonal organization in V1 may originate from the spatial organization of the ON-OFF receptive fields in the bottom-up projections, and this scenario was tested from analysis of the RGC mosaics data in monkeys and cats. We found that the ON-OFF RGC distance and ON-OFF angle of neighbouring RGCs are organized into a topographic tiling across mosaics, analogous to the orthogonal intersection of cortical tuning maps. These findings suggest that the regularly structured ON-OFF patterns mirrored from a retina may initiate efficient tiling of functional domains in V1.HighlightsOrthogonal organization of visual tuning maps are observed in both V1 and the retinaCortical tuning maps are correlated with the profile of ON-OFF feedforward projectionsThe profile of ON-OFF receptive fields varies periodically across the V1 surfaceRegularly structured RGC patterns initiate the orthogonal tiling of sensory modules in V1

scholarly journals Sub-Optimality of the Early Visual System Explained Through Biologically Plausible Plasticity

2021 ◽  
Vol 15 ◽  
Author(s):  
Tushar Chauhan ◽  
Timothée Masquelier ◽  
Benoit R. Cottereau
Keyword(s):  
Visual Cortex ◽  
Primary Visual Cortex ◽  
Receptive Fields ◽  
Spike Timing ◽  
Orientation Tuning ◽  
Biologically Relevant ◽  
Tuning Curves ◽  
Stdp Rule ◽  
Early Visual Cortex ◽  
Minimisation Problem

The early visual cortex is the site of crucial pre-processing for more complex, biologically relevant computations that drive perception and, ultimately, behaviour. This pre-processing is often studied under the assumption that neural populations are optimised for the most efficient (in terms of energy, information, spikes, etc.) representation of natural statistics. Normative models such as Independent Component Analysis (ICA) and Sparse Coding (SC) consider the phenomenon as a generative, minimisation problem which they assume the early cortical populations have evolved to solve. However, measurements in monkey and cat suggest that receptive fields (RFs) in the primary visual cortex are often noisy, blobby, and symmetrical, making them sub-optimal for operations such as edge-detection. We propose that this suboptimality occurs because the RFs do not emerge through a global minimisation of generative error, but through locally operating biological mechanisms such as spike-timing dependent plasticity (STDP). Using a network endowed with an abstract, rank-based STDP rule, we show that the shape and orientation tuning of the converged units are remarkably close to single-cell measurements in the macaque primary visual cortex. We quantify this similarity using physiological parameters (frequency-normalised spread vectors), information theoretic measures [Kullback–Leibler (KL) divergence and Gini index], as well as simulations of a typical electrophysiology experiment designed to estimate orientation tuning curves. Taken together, our results suggest that compared to purely generative schemes, process-based biophysical models may offer a better description of the suboptimality observed in the early visual cortex.

scholarly journals Strobe Rearing Reduces Direction Selectivity in Area 17 by Altering Spatiotemporal Receptive-Field Structure

1998 ◽  
Vol 80 (6) ◽  
pp. 2991-3004 ◽  
Author(s):  
Allen L. Humphrey ◽  
Alan B. Saul
Keyword(s):  
Receptive Field ◽  
Frequency Tuning ◽  
Temporal Frequency ◽  
Receptive Fields ◽  
Direction Selectivity ◽  
Field Structure ◽  
Area 17 ◽  
Directional Tuning ◽  
Simple Cells ◽  
Spatiotemporal Receptive Field

Humphrey, Allen L. and Alan B. Saul. Strobe rearing reduces direction selectivity in area 17 by altering spatiotemporal receptive-field structure. J. Neurophysiol. 80: 2991–3004, 1998. Direction selectivity in simple cells of cat area 17 is linked to spatiotemporal (S-T) receptive-field structure. S-T inseparable receptive fields display gradients of response timing across the receptive field that confer a preferred direction of motion. Receptive fields that are not direction selective lack gradients; they are S-T separable, displaying uniform timing across the field. Here we further examine this link using a developmental paradigm that disrupts direction selectivity. Cats were reared from birth to 8 mo of age in 8-Hz stroboscopic illumination. Direction selectivity in simple cells was then measured using gratings drifting at different temporal frequencies (0.25–16 Hz). S-T structure was assessed using stationary bars presented at different receptive-field positions, with bar luminance being modulated sinusoidally at different temporal frequencies. For each cell, plots of response phase versus bar position were fit by lines to characterize S-T inseparability at each temporal frequency. Strobe rearing produced a profound loss of direction selectivity at all temporal frequencies; only 10% of cells were selective compared with 80% in normal cats. The few remaining directional cells were selective over a narrower than normal range of temporal frequencies and exhibited weaker than normal direction selectivity. Importantly, the directional loss was accompanied by a virtual elimination of S-T inseparability. Nearly all cells were S-T separable, like nondirectional cells in normal cats. The loss was clearest in layer 4. Normally, inseparability is greatest there, and it correlates well ( r = 0.77) with direction selectivity; strobe rearing reduced inseparability and direction selectivity to very low values. The few remaining directional cells were inseparable. In layer 6 of normal cats, most direction-selective cells are only weakly inseparable, and there is no consistent relationship between the two measures. However, after strobe rearing, even the weak inseparability was eliminated along with direction selectivity. The correlated changes in S-T structure and direction selectivity were confirmed using conventional linear predictions of directional tuning based on responses to counterphasing bars and white noise stimuli. The developmental changes were permanent, being observed up to 12 yr after strobe rearing. The deficits were remarkably specific; strobe rearing did not affect spatial receptive-field structure, orientation selectivity, spatial or temporal frequency tuning, or general responsiveness to visual stimuli. These results provide further support for a critical role of S-T structure in determining direction selectivity in simple cells. Strobe rearing eliminates directional tuning by altering the timing of responses within the receptive field.

scholarly journals Responses of pigeon vestibulocerebellar neurons to optokinetic stimulation. I. Functional organization of neurons discriminating between translational and rotational visual flow

1993 ◽  
Vol 70 (6) ◽  
pp. 2632-2646 ◽  
Author(s):  
D. R. Wylie ◽  
T. Kripalani ◽  
B. J. Frost
Keyword(s):  
Functional Organization ◽  
Mossy Fiber ◽  
Receptive Fields ◽  
Direction Selectivity ◽  
Maximum Depth ◽  
Large Field ◽  
Directional Tuning ◽  
Tuning Curves ◽  
Downward Motion ◽  
Stimulation Of

1. Extracellular recordings were made from 235 neurons in the vestibulocerebellum (VbC), including the flocculus (lateral VbC), nodulus (folium X), and ventral uvula (ventral folium IXc,d), of the anesthetized pigeon, in response to an optokinetic stimulus. 2. The optokinetic stimuli consisted of two black and white random-dot patterns that were back-projected onto two large tangent screens. The screens were oriented parallel to each other and placed on either side of the bird's head. The resultant stimulus covered the central 100 degrees x 100 degrees of each hemifield. The directional tuning characteristics of each unit were assessed by moving the largefield stimulus in 12 different directions, 30 degrees apart. The directional tuning curves were performed monocularly or binocularly. The binocular directional tuning curves were performed with the direction of motion the same in both eyes (in-phase; e.g., ipsi = upward, contra = upward) or with the direction of motion opposite in either eye (antiphase; e.g., ipsi = upward, contra = downward). 3. Mossy fiber units (n = 17) found throughout folia IXa,b and IXc,d had monocular receptive fields and exhibited direction selectivity in response to stimulation of either the ipsilateral (n = 12) or contralateral (n = 5) eye. None had binocular receptive fields. 4. The complex spike (CS) activity of 218 Purkinje cells in folia IXc,d and X exhibited direction selectivity in response to the large-field visual stimulus moving in one or both visual fields. Ninety-one percent of the cells had binocular receptive fields that could be classified into four groups: descent neurons (n = 112) preferred upward motion in both eyes; ascent neurons (n = 14) preferred downward motion in both eyes; roll neurons (n = 33) preferred upward and downward motion in the ipsilateral and contralateral eyes, respectively; and yaw neurons (n = 40) preferred forward and backward motion in the ipsilateral and contralateral eyes, respectively. Within all groups, most neurons (70%) showed an ipsilateral dominance. 5. For most binocular neurons (91%), the maximum depth of modulation occurred with simultaneous stimulation of both eyes, compared with monocular stimulation of the dominant eye alone. For the translation neurons (descent and ascent), binocular inphase stimulation produced the maximum depth of modulation, whereas for the rotation neurons (roll and yaw), binocular antiphase stimulation produced the maximum depth of modulation. 6. There was a clear functional segregation of the translation and rotation neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The accessory optic system of rabbit. II. Spatial organization of direction selectivity

1988 ◽  
Vol 60 (6) ◽  
pp. 2055-2072 ◽  
Author(s):  
J. I. Simpson ◽  
C. S. Leonard ◽  
R. E. Soodak
Keyword(s):  
Receptive Field ◽  
Spatial Organization ◽  
Inferior Olive ◽  
Receptive Fields ◽  
Vertical Plane ◽  
Direction Selectivity ◽  
Optic System ◽  
Accessory Optic System ◽  
Present Sample ◽  
Ventral Tegmental

1. To compare the spatial organization of the direction selectivity of neurons in the medial terminal nucleus (MTN) of the accessory optic system with that of neurons in the adjacent ventral tegmentum, extracellular single-unit recordings were made in the anesthetized rabbit. The ventral tegmental neurons were located in a region called the visual tegmental relay zone (VTRZ), which is defined by the ventral tegmental terminal field of contralaterally projecting MTN neurons. 2. Some of the present sample of MTN neurons (5 of 34) had monocular receptive fields composed of two parts distinguished by a marked difference in the orientation of their respective direction-selective tuning curves. For one part of the receptive field the preferred excitatory direction was "up," while for the other part it was "down." Such receptive fields for one eye were called bipartite, whereas the more usually encountered MTN receptive fields, which could be characterized by a single direction-selective tuning curve, were called uniform. 3. Of the 16 neurons recorded from the VTRZ, all but one were binocular. For these neurons, both uniform and bipartite receptive fields were found for each eye alone. The only monocular neuron encountered in the VTRZ had a contralateral, bipartite receptive field. 4. The spatial organization of the direction selectivity of bipartite receptive fields strongly suggests that they are suited to represent rotation of the visual field about a horizontal axis located in the vertical plane that divides the receptive field into two parts. 5. The boundary between the two parts of the bipartite receptive fields was found using handheld visual stimuli at one of two azimuthal locations, either close to 45 degrees or between 95 and 125 degrees (the 0 degree reference was rostral in the midsagittal plane). This particular structure of the bipartite receptive fields suggests that their preferred rotation axes have a close spatial relation to the best-response axes of the semicircular canals. 6. Seven VTRZ neurons were antidromically activated by electrical stimulation of the ipsilateral dorsal cap of the inferior olive. Since the receptive fields of VTRZ neurons have many of the structural features characteristic of the receptive fields of rostral dorsal cap neurons we conclude that the spatial organization of the receptive fields of dorsal cap neurons is, for the most part, synthesized prior to the inferior olive.(ABSTRACT TRUNCATED AT 400 WORDS)

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