Posteromedial Lateral Suprasylvian Area Research Articles

Delayed signal transmission in area 17, area 18 and the posteromedial lateral suprasylvian area of aged cats

To investigate the effect of senescence on signal transmission, we have compared the visual response latency and spontaneous activity of cells in the lateral geniculate nucleus (LGN), area 17, area 18 and posteromedial lateral suprasylvian area (PMLS) of young and old cats. We found that LGN cells in old cats exhibit largely normal visual response latency. In contrast, all the other three areas exhibited significant aging-related delays in the visual response latency. On average, PMLS showed most pronounced delays among these three areas. Area 18 slowed more than area 17, but this was not significant. The degradation of signal timing in the visual cortex might provide insight into neuronal response mechanism underlying perception slowing during aging.

Cross‐modal reorganization of cortical afferents to dorsal auditory cortex following early‐ and late‐onset deafness

Cat auditory cortex is known to undergo cross-modal reorganization following deafness, such that behavioral advantages in visual motion detection are abolished when a specific region of deaf auditory cortex, the dorsal zone (DZ), is deactivated. The purpose of the present investigation was to examine the connectional adaptations that might subserve this plasticity. We deposited biotinylated dextran amine (BDA; 3,000 MW), a retrograde tracer, unilaterally into the posterior portion of the suprasylvian fringe, corresponding to area DZ of hearing, early-deafened (onset <1 month), and late-deafened (onset >3 months) cats to reveal cortical afferent projections. Overall, the pattern of cortical projections to DZ was similar in both hearing and deafened animals. However, there was a progressive increase in projection strength among hearing and late- and early-deafened cats from an extrastriate visual cortical region known to be involved in the processing of visual motion, the posterolateral lateral suprasylvian area (PLLS). Additionally, although no such change was documented for the posteromedial lateral suprasylvian area (PMLS), labeled neurons were present within a subregion of PMLS devoted to foveal vision in both late- and early-deafened animals but not in hearing controls. PMLS is also an extrastriate visual motion processing area and is widely considered to be the homolog of primate middle temporal area. No changes in auditory cortical connectivity were observed among groups. These observations suggest that amplified cortical projections from extrastriate visual areas involved in visual motion processing to DZ may contribute to the cross-modal reorganization that functionally manifests as superior visual motion detection ability in the deaf animal.

Neurons in V1, V2, and PMLS of Cat Cortex Are Speed Tuned But Not Acceleration Tuned: The Influence of Motion Adaptation

We studied neurons in areas V1, V2, and posteromedial lateral suprasylvian area (PMLS) of anesthetized cats, assessing their speed tuning using steps to constant speeds and acceleration and deceleration tuning using speed ramps. The results show that the speed tuning of neurons in all three cortical areas is highly dependent on prior motion history, with early responses during speed steps tuned to higher speeds than later responses. The responses to speed ramps are profoundly influenced by speed-dependent response latencies and ongoing changes in neuronal speed tuning due to adaptation. Acceleration evokes larger transient and sustained responses than subsequent deceleration of the same rate with this disparity increasing with ramp rate. Consequently, there was little correlation between preferred speeds measured using speed steps, acceleration or deceleration. From 146 recorded cells, the proportion of cells that were clearly speed tuned ranged from 69 to 100% across the three brain areas. However, only 13 cells showed good skewed Gaussian fits and systematic variation in their responses to a range of accelerations. Although suggestive of acceleration coding, this apparent tuning was attributable to a cell's speed tuning and the different stimulus durations at each acceleration rate. Thus while the majority of cells showed speed tuning, none unequivocally showed acceleration tuning. The results are largely consistent with an existing model that predicts responses to accelerating stimuli developed for macaque MT, which showed that the responses to acceleration can be decoded if adaptation is taken into account. However, the present results suggest future models should include stimulus-specific adaptation and speed-dependent response latencies.

Graded classes of cortical connections: quantitative analyses of laminar projections to motion areas of cat extrastriate cortex

Current hierarchical models of the cerebral cortex are mainly based on qualitative connection studies. From wheatgerm-agglutinin-horseradish peroxidase injections, we examined the laminar patterns of projections to and between the three major subdivisions of the motion-processing lateral suprasylvian (LS) complex [areas posteromedial lateral suprasylvian area (PMLS), anteromedial lateral suprasylvian (AMLS), posterolateral lateral suprasylvian area (PLLS)] of cat extrastriate cortex and of an adjoining form-processing area, 21a. We counted approximately 145,000 labelled projection cells in 20 cortical areas in 11 cats, and applied various analyses to the data, expressed as the percent supragranular layer (%SG) origin of each connection. We report two main results. (i) A wide range of %SG values was obtained, both from each individual cat and across the 163 projections examined. Nonetheless, both hierarchical and non-parametric cluster analyses of the pooled connection origins suggested the presence of three distinct laminar projection classes, constrained by graded %SG values of 0-33%, 39-69% and 76-97%. These conformed, respectively, to feedback, lateral and feedforward laminar patterns seen qualitatively in our material. (ii) Hierarchical connectivity analyses suggested that PMLS, AMLS and PLLS are ordered in a hierarchical sequence. Macaque motion areas V5/MT, MST and FST are arranged in a similar sequence, and areas at equivalent levels of the two motion hierarchies have some analogous functional specializations. Our findings provide the first objective support for the three laminar projection classes that underpin existing theoretical models of hierarchical cortical organization, and they suggest that the implementation of higher-order motion processing evolved along similar lines in the cat and monkey visual cortex.

Ocular dominance in extrastriate cortex of strabismic amblyopic cats

Ocular dominance in extrastriate visual cortex of cats with behaviorally defined strabismic amblyopia was studied using extracellular recording techniques. In area 18, the amblyopic eye drove about as many cells as the normal one. In area posteromedial lateral suprasylvian area (PMLS), about 60% of the cells responded exclusively to stimulation of the normal eye and 30% to stimulation of the amblyopic eye. In area 21a more than 75% of the cells were monocularly driven by the non-amblyopic eye while only 5% were monocularly driven by the amblyopic eye. These findings suggest that ventral pathways (area 21a) are more affected in amblyopia than dorsal pathways (area PMLS).

Adaptation of PMLS neurons to prolonged optic flow stimuli.

Changes in neuronal responses during and after adaptation to prolonged optic flow stimulation were investigated by extracellular single-unit recording in the posteromedial lateral suprasylvian area (PMLS) of the cat. In comparison with translation stimuli, the complex optic flow patterns (radiation and rotation) produced more pronounced adaptation and after-effects by inducing larger response reduction, and altered the direction selectivity of many neurons obviously as well. Generally, the adaptation effects were direction-specific for radiation/rotation, but independent of the direction of test stimulus for translation. These results suggest that PMLS may play an important role in the perception of motion after-effects to complex optic flow fields, while the adaptation to simple translation might be generated at a relatively earlier level of the visual system.

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.

Pattern and component motion selectivity in cortical area PMLS of the cat.

Visual motion perception is one of the most prominent functions performed by the mammalian cerebral cortex. The moving images are commonly considered to be processed in two stages. The first-stage neurons are sensitive to the motion of one-dimensional orientated components, and their outputs are combined at the second stage to perceive the global motion of the whole pattern. Alternatively, the pattern motion may be signalled by monitoring a distinctive feature of the image, such as a line-end or a corner. In the present study, a series of 'random-line' patterns were used to measure the direction-tuning responses of 138 neurons in the posteromedial lateral suprasylvian area of the cat. The novel stimuli comprised identical thin line segments, with a length : width ratio no less than 10 : 1, which were moved perpendicularly or obliquely to their common orientation during the recordings. When the component lines were much shorter than the size of receptive field, the majority of cells were selective to the direction of pattern motion while only a small subset was sensitive to the direction of component motion. However, the response profiles of most cells became more component-motion selective with the increment of orientation element in stimulus by elongating the component lines in the patterns. These findings imply that the two-stage theory might be incomplete for modelling the visual motion analysis. Even at relatively low levels of the visual system, some kind of nonorientation-based processing may coexist with the orientation-sensitive processing in a dynamic competition, where one rises as the other falls depending upon the strength of the orientation element in the stimulus, so that under some circumstances it becomes possible to signal the veridical direction of pattern motion.

Spatial disparity sensitivity in area PMLS of the Siamese cat

Previous studies of the visual system of Siamese cats have shown that binocular cells are scarce in areas 17, 18 and 19, yet significantly more abundant in suprasylvian areas such as the postero-medial lateral suprasylvian area (PMLS). The present study aims at evaluating the sensitivity to spatial disparity of PMLS binocular cells in paralyzed and anesthetized Siamese cats. Centrally located receptive fields were mapped, separated using prisms and then stimulated simultaneously using two luminous bars optimally adjusted to the size of the excitatory receptive fields. Delays were introduced in the arrival of the luminous bars in the receptive fields so as to create the desired spatial disparities. Results indicate that approximately a third of PMLS units are binocular and that these binocular cells can detect spatial disparity cues. Indeed, although the sample was relatively small, cells of the tuned excitatory (14/34), tuned inhibitory (2/34), near (6/34) and far (1/34) types were identified. The spatial selectivity, as measured by the width at half height of the tuning curves of the excitatory and inhibitory cells and the slopes of the near and far cells, was similar to that obtained in PMLS of normal cats but not as precise as that found for primary visual areas in these animals. This suggests that these cells might serve as a substrate for coarse stereopsis.

Neurons in the posteromedial lateral suprasylvian area of the cat are sensitive to binocular positional depth cues.

Single units in the posteromedial lateral suprasylvian area of the cat are known to be very sensitive to movement. A proportion of these cells can encode movement in depth, but it is unclear whether posteromedial lateral suprasylvian cells only rely upon motion cues to evaluate stimulus depth or whether they can also code for spatial cues. The present study aims at assessing the sensitivity to spatial disparity of binocular cells, in the postero-medial lateral suprasylvian area, in order to determine whether these units are tuned to positional depth cues. A total of 126 single cells located in the posteromedial lateral suprasylvian area of anesthetized, paralyzed cats were examined. As recordings were performed in the central visual field representation, receptive fields were small. A third of the receptive fields were surrounded by an inhibitory region and almost three-quarters of the cells were direction-selective. Most cells (110/114) were binocular, and a large proportion of single neurons responded to stimuli appearing on the fixation plane by increasing (tuned excitatory cells, 43%) or decreasing (tuned inhibitory cells, 14%) their response rate. A smaller proportion of cells increased their firing rate in response to crossed (near cells, 10%) or uncrossed (far cells, 6%) spatial disparities, hence demonstrating respective preference for stimuli presumably appearing in front of or behind the fixation plane. As compared to primary visual cortex, the proportion of disparity-sensitive cells in posteromedial lateral suprasylvian area is similar, but selectivity is significantly coarser. As the posteromedial lateral suprasylvian area can code for both spatial and temporal aspects of stimuli, this area might be involved in the spatiotemporal integration of depth cues, a process that may also participate in the control of accommodation and vergence.

Area-specific laminar distribution of cortical feedback neurons projecting to cat area 17: quantitative analysis in the adult and during ontogeny.

Corticocortical pathways can be classified as feedback and feedforward, in part according to the laminar distribution of the parent cell bodies. Here, we have developed exhaustive sampling procedures to determine unambiguously this laminar distribution. This shows that individual extrastriate areas in the adult cat have highly stereotyped proportions of supragranular layer neurons with respect to the total population of neurons back-projecting to area 17. During development, these adult laminar patterns emerge from an initially uniform radial distribution through a process of selective reorganization, which is highly specific to each area. Injections of fluorescent retrograde tracers were made in area 17. In areas 19, 20, posteromedial lateral suprasylvian area, and anteromedial lateral suprasylvian area, we defined a projection zone as the region containing retrogradely labeled neurons. In the neonate, counts of labeled neurons throughout the projection zones show constant percentages of 40% in the supragranular layers. During development, there is an area-specific reduction in the percentage of supragranular labeled neurons generating the laminar distributions characteristic of each area. Numbers of labeled neurons were estimated at different eccentricities of the projection zone. This finding indicates that during development there is a relative decrease in the numbers of labeled neurons of the periphery of the projection zone in the supragranular layers but not in the infragranular layers. This decrease is accompanied by a relative decrease in the dimensions of the supragranular projection zone with respect to the infragranular projection zone. These findings suggest that each extrastriate area precisely adjusts the proportions of supragranular layer neurons back-projecting to striate cortex in part by developmental changes in the divergence-convergence values of individual neurons. This shaping of corticocortical connectivity occurs relatively late in postnatal development and could, therefore, be under epigenetic control.

Excitatory convergence of Y and non-Y information channels on single neurons in the PMLS area, a motion area of the cat visual cortex.

We analysed the receptive field properties of neurons in the posteromedial lateral suprasylvian (PMLS) visual cortical area of anaesthetized cats in which there was selective conduction block of the largest (Y-type) fibres in one optic nerve. As in normal cats, in cats with selective block of one optic nerve the great majority of PMLS cells could be activated by photic stimulation through either eye. However, the responses evoked by stimulation via the eye with the selectively pressure-blocked optic nerve ('Y-blocked eye') were significantly weaker than those of the same cells evoked by the stimulation via the normal eye. Accordingly, eye dominance histograms were shifted markedly in favour of the normal eye. Furthermore, there was a significant shift towards lower preferred velocities when PMLS cells were photically stimulated via the Y-blocked eye. Finally, when stimulated via the Y-blocked eye, PMLS cells responded poorly or not at all to high stimulus velocities (> or = 100 degrees/s). On the other hand, a number of receptive field properties, such as the spatial organization of receptive fields, the size of the discharge fields, orientation tuning and direction selectivity indices, were not significantly affected by the removal of the Y input. We conclude that virtually all neurons in the PMLS area of the cat receive excitatory input from both Y and non-Y information channels, although the Y channel provides the dominant input and makes the principal contribution to the detection of high-velocity motion.

Limits of parallel processing: excitatory convergence of different information channels on single neurons in striate and extrastriate visual cortices.

1. It has been postulated that the distinct parallel retino-geniculo-cortical information channels characterizing visual pathways of virtually all mammals are selectively linked to parallel motion, colour and/or form information processing 'streams' distinguishable within the primary visual cortices, extrastriate cortical areas of occipital lobes and the temporal and parietal visual cortices. 2. Using selective pressure-blocking of the large-fibre channel (the so-called Y-channel) in the optic nerve of the cat, we have experimentally examined the 'selective excitatory parallel links' hypothesis. We conclude that the majority of neurons in the primary visual cortices (areas 17, 18) as well as in the two 'higher order' visual areas, area 21a and posteromedial lateral suprasylvian (PMLS) area, constituting, respectively, part of the 'form' and part of the 'motion' processing streams, receive their excitatory inputs from both Y- and non-Y-information channels. In areas 17, 18 and 21a (but not in PMLS area), there are, however, subpopulations of cells that apparently receive excitatory inputs from only one information channel. 3. Review of the relevant work on the macaque monkey suggests that the situation is similar in the primate: that is, there is a substantial degree of excitatory convergence of different retino-geniculo-cortical information channels on single neurons in the primary visual cortices and the extrastriate cortices constituting parts of the form/colour or the motion processing streams. 4. Despite this high degree of excitatory convergence of different information channels, the large-fibre channels (the Y-channel in the cat and the magnocellular or Y-like channel in macaque), are in both carnivores and primates the principal contributors to the motion processing cortical streams.

Lateral suprasylvian visual cortex is activated earlier than or synchronously with primary visual cortex in the cat

To explore functional connectivity between area 17 and posteromedial lateral suprasylvian (PMLS) area of the cat cerebral cortex, we carried out cross-correlation analysis of spike trains of neurons recorded simultaneously from both areas of anesthetized, paralyzed cat. Most of correlated pairs had peaks which indicate that PMLS cells were activated earlier than or simultaneously with their partners in area 17, suggesting that ‘backward’ as well as synchronous activation operates between these cortical areas.

Areas PMLS and 21a of cat visual cortex: two functionally distinct areas.

We have compared the receptive field properties of neurons recorded from visuotopically corresponding regions of area 21a and the posteromedial lateral suprasylvian area (PMLS) of cat visual cortex. In both areas, the great majority of neurons were orientation-selective and binocular, and their responses to moving contours were modulated by simultaneous in-phase or anti-phase motion of large textured background stimuli ('visual noise'). However, despite the great hodological similarity between the two areas, PMLS neurons had on average significantly higher peak discharge rates, exhibited substantially greater direction selectivity indices, and preferred substantially higher stimulus velocities than area 21a neurons. Furthermore, the majority of binocular neurons in the PMLS area and in area 21a were dominated respectively by contralateral and ipsilateral eyes. Finally, while 46% of PMLS neurons were excited by movement of visual noise per se, only 25% of area 21a neurons could be excited by such stimuli. We argue that the PMLS area, like its presumed primate homologue the middle-temporal (MT) area, is mainly involved in motion analysis. By contrast, area 21a appears to be involved in pattern analysis rather than motion analysis. It is likely that phylogenetically area 21a derives from the PMLS area.

Postnatal development and specification of the cat's visual corticotectal projection: efferent from the posteromedial lateral suprasylvian area

The corticotectal projection in adult cats has a precise topographic and laminar organization. Yet this projection initially grows beyond these adult targets. To begin to understand how the growing cortical axons achieve this precision, the morphological development of axons growing from the posteromedial lateral suprasylvian area (PMLS) to the superior colliculus was studied by injecting the anterograde tracer biocytin into the PMLS of cats between postnatal day (PO) and adulthood. The labeling patterns showed that (1) axons grow independently towards the colliculus and (2) the first axons from the PMLS arrive in the colliculus by P1 and continue to arrive over several days. Labeled growth cones were seen within the colliculus up to P15. (3) After reaching the colliculus, the axons undergo several morphological changes. Initially, they are unbranched and beaded, then short side branches are formed and finally extensive arborizations appear. Comparing the timing of these events with results from electron microscopic 27 and electrophysiological 46 studies that the appearance and increase in labeled axons with short side brances roughly coincides with the appearance and increase in number of synapses in the colliculus, whereas the elaboration of extensive arbors (and hence a corresponding increase in synapses) is well underway before visual influences on the colliculus can be measured. Thick sinuous axons are also labeled during maturation, usually in areas of the colliculus where they would be considered exuberant and may represent degenerating axons. (4) A coarse topography develops as the axons grow into the colliculus and becomes more precise in the following weeks. Initially, some axons extend well beyond their correct terminal zone, growing into the contralateral colliculus, caudally into the inferior colliculus and reaching all laminae of the ipsilateral superior colliculus. Similar targeting ‘errors’ have been reported during the growth of retinotectal axons, suggesting that cortical, retinal and perhaps other sources as well, may use the same extracellular cues to establish an initial coarse topography within the colliculus.

Corticotectal projections in the cat: anterograde transport studies of twenty-five cortical areas.

Retrograde transport studies have shown that widespread areas of the cerebral cortex project upon the superior colliculus. In order to explore the organization of these extensive projections, the anterograde autoradiographic method has been used to reveal the distribution and pattern of corticotectal projections arising from 25 cortical areas. In the majority of experiments, electrophysiological recording methods were used to characterize the visual representation and cortical area prior to injection of the tracer. Our findings reveal that seventeen of the 25 cortical areas project upon some portion of the superficial layers (stratum zonale, stratum griseum superficiale, and stratum opticum, SO). These cortical regions include areas 17, 18, 19, 20a, 20b, 21a, 21b, posterior suprasylvian area (PS), ventral lateral suprasylvian area (VLS), posteromedial lateral suprasylvian area (PMLS), anteromedial lateral suprasylvian area (AMLS), anterolateral lateral suprasylvian area (ALLS), posterolateral lateral suprasylvian area (PLLS), dorsolateral lateral suprasyvian area (DLS), periauditory cortex, cingulate cortex, and the visual portion of the anterior ectosylvian sulcus. While some of these corticotectal projections target all superficial laminae and sublaminae, others are more discretely organized in their laminar-sublaminar distribution. Only the corticotectal projections arising from areas 17 and 18 are exclusively related to the superficial layers. The remaining 15 pathways innervate both the superficial and intermediate and/or deep layers. The large intermediate gray layer (stratum griseum intermedium; SGI) receives projections from almost every cortical area; only areas 17 and 18 do not project ventral to SO. All corticotectal projections to SGI vary in their sublaminar distribution and in their specific pattern of termination. The majority of these projections are periodic, or patchy, and there are elaborate (double tier, bridges, or streamers) modes of distribution. We have attempted to place these findings into a conceptual framework that emphasizes that the SGI consists of sensory and motor domains, both of which contain a mosaic of connectionally distinct afferent compartments (Illing and Graybiel, '85, Neuroscience 14:455-482; Harting and Van Lieshout, '91, J. Comp. Neurol. 305:543-558). Corticotectal projections to the layers ventral to SGI, (stratum album intermediale, stratum griseum profundum, and stratum album profundum) arise from thirteen cortical areas. While an organizational plan of these deeper projections is not readily apparent, the distribution of several corticotectal inputs reveals some connectional parcellation.

Synchronization of oscillatory neuronal responses between striate and extrastriate visual cortical areas of the cat.

Recent studies have shown that neurons in area 17 of cat visual cortex display oscillatory responses which can synchronize across spatially separate orientation columns. Here, we demonstrate that unit responses recorded from the posteromedial lateral suprasylvian area, a visual association area specialized for the analysis of motion, also exhibit an oscillatory temporal structure. Cross-correlation analysis of unit responses reveals that cells in area 17 and the posteromedial lateral suprasylvian area can oscillate synchronously. Moreover, we find that the interareal synchronization is sensitive to features of the visual stimuli, such as spatial continuity and coherence of motion. These results support the hypothesis that synchronous neuronal oscillations may serve to establish relationships between features processed in different areas of visual cortex.

Functional compensation in the lateral suprasylvian visual area following bilateral visual cortex damage in kittens

Previous studies have shown that functional compensation is present in the cat's posteromedial lateral suprasylvian (PMLS) area of cortex after damage to areas 17, 18, and 19 (visual cortex) early in life but not after damage in adults. These studies all have investigated animals with a unilateral visual cortex lesion, whereas all behavioral studies of compensation for early visual cortex damage have investigated animals with a bilateral lesion. In the present experiment, we investigated whether functional compensation also is present in PMLS cortex after a bilateral visual cortex lesion early in life. We recorded from single neurons in the PMLS cortex of adult cats that had received a bilateral lesion of areas 17, 18, and 19 on the day of birth or at 8 weeks of age. We found that PMLS cells in both groups of cats had functional compensation (normal direction selectivity and ocular dominance) similar to that seen after a unilateral lesion at the same ages. These results are consistent with the hypothesis that PMLS cortex is involved in the behavioral compensation seen after early visual cortex damage. In addition, the results indicate that inputs from contralateral visual cortex are not necessary for the development of functional compensation seen in PMLS cortex.

Organization of the cortico-ponto-cerebellar pathway to the dorsal paraflocculus. An experimental study with anterograde and retrograde transport of WGA-HRP in the cat.

To reveal the organization and relative magnitude of connections from various parts of the cerebral cortex to the dorsal paraflocculus via the pontine nuclei, WGA-HRP was injected in the dorsal paraflocculus in conjunction with injection of the same tracer in various parts of the cerebral cortex in 17 cats. Termination areas of cortical fibres (anterogradely labelled) and pontine neurons projecting to the dorsal paraflocculus (retrogradely labelled) were carefully plotted in serial transverse sections. As an average of countings in ten cats, 90% of the labelled cells were found in the pontine nuclei contralateral to the injection, and the majority (70%) were located in the rostral half of the nuclei. The highest degree of overlap between anterograde and retrograde labelling was found after injections of the parietal association cortex (areas 5 and 7). In an experiment with double anterograde tracing, it was shown that both area 5 and 7 contribute substantially to the cerebral inputs to the dorsal paraflocculus. High degree of overlap also occurred after injections of several visual cortical areas (areas 17, 18, 19, 20 and the posteromedial lateral suprasylvian visual area, PMLS). Cases with injections restricted to individual visual areas indicate that they all contribute to the parafloccular input. Considerably less overlap occurred after injections of the primary sensorimotor region (SI, MI) and second somatosensory area (SII), while the supplementary motor area, the auditory cortex and gyrus cinguli probably have no or very restricted access to the dorsal paraflocculus. It is concluded that the dorsal paraflocculus has its major cortical input from the parietal association cortex and the visual cortical areas. Since all the various cortical regions studied project to largely different parts of the pontine nuclei, and overlap with neurons projecting to the dorsal paraflocculus takes place at numerous places, it follows that the pontine neurons projecting to the dorsal paraflocculus must consist of many subgroups differing with regard to their cortical input.