A model for encoding multiple object motions and self-motion in area MST of primate visual cortex

Many cells in the dorsal part of the medial superior temporal (MST) region of visual cortex respond selectively to specific combinations of expansion/contraction, translation, and rotation motions. Previous investigators have suggested that these cells may respond selectively to the flow fields generated by self-motion of an observer. These patterns can also be generated by the relative motion between an observer and a particular object. We explored a neurally constrained model based on the hypothesis that neurons in MST partially segment the motion fields generated by several independently moving objects. Inputs to the model were generated from sequences of ray-traced images that simulated realistic motion situations, combining observer motion, eye movements, and independent object motions. The input representation was based on the response properties of neurons in the middle temporal area (MT), which provides the primary input to area MST. After applying an unsupervised optimization technique, the units became tuned to patterns signaling coherent motion, matching many of the known properties of MST cells. The results of this model are consistent with recent studies indicating that MST cells primarily encode information concerning the relative three-dimensional motion between objects and the observer.     

Selective and invariant sensitivity to crosses and corners in cat striate neurons

Many neurons (56/174, or 32.2%) studied in the cat striate cortex (area 17) increased significantly (by 3.3 times on average) their responses under stimulation by cruciform or corner figures of specific or non-specific shape and orientation flashing in receptive field as compared with single light bar of preferred orientation. Most of these neurons (71.4%) were found to be highly selective to both the shape (the angle between the figure's lines) and orientation of these figures. In the neuronal selection studied we have also found all possible types of invariance of the cross and corner tuning to orientation and/or shape of these figures. We found neurons with selectivity to the form of the figures and invariance to their orientation and, on the contrary, units invariant to shape but selective to orientation. Some cells were found invariant to both the form and orientation of the cruciform or corner figure but highly sensitive to appearance of any such figure in the receptive field. Two main hypotheses about the mechanisms of selective sensitivity to crosses and angles can be considered. They are as follows: an excitatory convergence of two units with different preferred orientations, and intracortical inhibitory interactions. The cells with double orientation tuning for a single bar are found relatively rarely (about 20%), thus making the first suggestion the most unlikely. This circumstance is of special importance since it provides evidence against the hierarchic formation of the higher-order cortical units from a set of lower-order cells that is still under discussion. The units with high sensitivity to cross or corner seem to be ideally suitable for their selection, rather than to serve as classical orientation detectors only.     

Collinear stimuli regulate visual responses depending on cell's contrast threshold

Neurons in the primary visual cortex are selective for the size, orientation and direction of motion of patterns falling within a restricted region of visual space known as the receptive field. The response to stimuli presented within the receptive field can be facilitated or suppressed by other stimuli falling outside the receptive field which, when presented in isolation, fail to activate the cell. Whether this interaction is facilitative or suppressive depends on the relative orientation of pattern elements inside and outside the receptive field. Here we show that neuronal facilitation preferentially occurs when a near-threshold stimulus inside the receptive field is flanked by higher-contrast, collinear elements located in surrounding regions of visual space. Collinear flanks and orthogonally oriented flanks, however, both act to reduce the response to high-contrast stimuli presented within the receptive field. The observed pattern of facilitation and suppression may be the cellular basis for the observation in humans that the detectability of an oriented pattern is enhanced by collinear flanking elements. Modulation of neuronal responses by stimuli falling outside their receptive fields may thus represent an early neural mechanism for encoding objects and enhancing their perceptual saliency.     

Neuronal responses to edges defined by luminance vs. temporal texture in macaque area V1

We examined the responsivity, orientation selectivity, and direction selectivity of a sample of neurons in cortical area V1 of the macaque using visual stimuli consisting of drifting oriented contours defined by each of two very different figural cues: luminance contrast and temporal texture. Comparisons of orientation and direction tuning elicited by the different cues were made in order to test the hypothesis that the neuronal representations of these parameters are form-cue invariant. The majority of the sampled cells responded to both stimulus types, although responses to temporal texture stimuli were generally weaker than those elicited by luminance-defined stimuli. Of those units exhibiting orientation selectivity when tested with the luminance-defined stimuli, more than half were also selective for the orientation of the temporal texture stimuli. There was close correspondence between the preferred orientations and tuning bandwidths revealed with the two stimulus types. Of those units exhibiting directional selectivity when tested with the luminance-defined stimuli, about two-thirds were also selective for the direction of the temporal texture stimuli. There was close correspondence between the preferred directions revealed with the two stimulus types, although bidirectional responses were somewhat more common when temporal texture stimuli were used. These results indicate that many V1 neurons encode orientation and direction of motion of retinal image features in a manner that is largely independent of whether the feature is defined by luminance or temporal texture contrast. These neurons may contribute to perceptual phenomena in which figural cue identity is disregarded.     

Generation of dendritic cells from fresh and frozen cord blood CD34+ cells

Extracellular recordings obtained from the extrastriate cortex of the California ground squirrel, a diurnal sciurid, show that large receptive fields and a strong direction selectivity are present in the middle lateral area (ML) and the lateral area (L), located laterally to V2 and V3. Direction selectivity was tested by presenting stimuli of varying dimensions, shapes and speeds at different locations in the visual field. Most cells in ML and L (84%) were direction selective, with a preference for fast speeds, indicating that these areas share a role in motion processing. Areas ML and L may be homologous to area MT or may represent a case of homoplasia. A directional anisotropy for motion towards the vertical meridian was found in ML and L cells, suggesting that these areas may be involved in detecting predators and other moving objects coming from the periphery, rather than in processing flow fields caused by forward locomotion, for which a centrifugal bias might be expected.     

A model for the neuronal implementation of selective visual attention based on temporal correlation among neurons

We propose a model for the neuronal implementation of selective visual attention based on temporal correlation among groups of neurons. Neurons in primary visual cortex respond to visual stimuli with a Poisson distributed spike train with an appropriate, stimulus-dependent mean firing rate. The spike trains of neurons whose receptive fields do not overlap with the "focus of attention" are distributed according to homogeneous (time-independent) Poisson process with no correlation between action potentials of different neurons. In contrast, spike trains of neurons with receptive fields within the focus of attention are distributed according to non-homogeneous (time-dependent) Poisson processes. Since the short-term average spike rates of all neurons with receptive fields in the focus of attention covary, correlations between these spike trains are introduced which are detected by inhibitory interneurons in V4. These cells, modeled as modified integrate-and-fire neurons, function as coincidence detectors and suppress the response of V4 cells associated with non-attended visual stimuli. The model reproduces quantitatively experimental data obtained in cortical area V4 of monkey by Moran and Desimone (1985).     

Visual motion processing in the anterior ectosylvian sulcus of the cat

1. Neurons that are selectively sensitive to the direction of motion of elongated contours have been found in several cortical areas in many species. However, in the striate cortex of the cat and monkey, and the extrastriate posteromedial lateral suprasylvian visual area of the cat, such cells are generally component motion selective, signaling only the direction of movement orthogonal to the preferred orientation; a direction that is not necessarily the same as the motion of the entire pattern or texture of which the cell's preferred contour is part. The primate extrastriate middle temporal area is the only cortical region currently known to contain a substantial population of pattern-motion-selective cells that respond to the shared vector of motion of mixtures of contours. 2. From analyzing published data on the connectivity of the cat's cortex, we predicted that the anterior ectosylvian visual area (AEV), situated within the anterior ectosylvian sulcus, might be a higher-order motion processing area and thus likely to contain pattern-motion-selective neurons. This paper presents the results of a study on neuronal responses in AEV. 3. Ninety percent of AEV cells that responded strongly to drifting grating and/or plaid stimuli were directionally selective (directionality index > 0.5). For this group, the mean directionality index was 0.75. Moreover, 55% of these cells were unequivocally classified as pattern motion selective and only one neuron was classified as definitely component motion selective. Thus high-level pattern motion coding occurs in the cat extrastriate cortex and is not limited to the primate middle temporal area. 4. AEV contains a heterogeneous population of directionally selective cells. There was no clear relation between the degree of directional selectivity for plaids or gratings and the degree of selectivity for pattern motion or component motion. Nevertheless, 28% of the highly responsive cells were both more strongly modulated by plaids than gratings and more pattern motion selective than component motion selective. Such cells could correspond to a population of "selection units" signaling the salience of local motion information. 5. AEV lacks global retinotopic order but the preferred direction of motion of neurons (rather than axis of motion, as in the middle temporal area and the posteromedial lateral suprasylvian visual area) is mapped systematically across the cortex. Our data are compatible with AEV being a nonretinotopic, feature-mapped area in which cells representing similar parts of "motion space" are brought together on the cortical sheet.     

Visuospatial properties of ventral premotor cortex

In macaque ventral premotor cortex, we recorded the activity of neurons that responded to both visual and tactile stimuli. For these bimodal cells, the visual receptive field extended from the tactile receptive field into the adjacent space. Their tactile receptive fields were organized topographically, with the arms represented medially, the face represented in the middle, and the inside of the mouth represented laterally. For many neurons, both the visual and tactile responses were directionally selective, although many neurons also responded to stationary stimuli. In the awake monkeys, for 70% of bimodal neurons with a tactile response on the arm, the visual receptive field moved when the arm was moved. In contrast, for 0% the visual receptive field moved when the eye or head moved. Thus the visual receptive fields of most "arm + visual" cells were anchored to the arm, not to the eye or head. In the anesthetized monkey, the effect of arm position was similar. For 95% of bimodal neurons with a tactile response on the face, the visual receptive field moved as the head was rotated. In contrast, for 15% the visual receptive field moved with the eye and for 0% it moved with the arm. Thus the visual receptive fields of most "face + visual" cells were anchored to the head, not to the eye or arm. To construct a visual receptive field anchored to the arm, it is necessary to integrate the position of the arm, head, and eye. For arm + visual cells, the spontaneous activity, the magnitude of the visual response, and sometimes both were modulated by the position of the arm (37%), the head (75%), and the eye (58%). In contrast, to construct a visual receptive field that is anchored to the head, it is necessary to use the position of the eye, but not of the head or the arm. For face + visual cells, the spontaneous activity and/or response magnitude was modulated by the position of the eyes (88%), but not of the head or the arm (0%). Visual receptive fields anchored to the arm can encode stimulus location in "arm-centered" coordinates, and would be useful for guiding arm movements. Visual receptive fields anchored to the head can likewise encode stimuli in "head-centered" coordinates, useful for guiding head movements. Sixty-three percent of face + visual neurons responded during voluntary movements of the head. We suggest that "body-part-centered" coordinates provide a general solution to a problem of sensory-motor integration: sensory stimuli are located in a coordinate system anchored to a particular body part.     

Strobe rearing reduces direction selectivity in area 17 by altering spatiotemporal receptive-field structure

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.     

Two visual areas located in the middle suprasylvian gyrus (cytoarchitectonic field 7) of the cat's cortex

Neuronal properties and topographic organization of the middle suprasylvian gyrus (cortical cytoarchitectonic field 7) were studied in three behaving cats with painlessly fixed heads. Two main neuronal types were found within this field. Type 1 neurons occupied the lateral part of the field and bordered representation of directionally selective neurons of the lateral suprasylvian visual area by vertical retinal meridian. Type 1 neurons had elongated and radially oriented receptive fields located in the lower part of contralateral visual field. Type 1 neurons preferred stimuli moving out or to the centre of gaze at a low or moderate speed, and many of them were depth selective. The responses were enhanced by attention, oriented to the presented stimulus. Medial part of the field 7 along the border with the area V3 was occupied by neurons with not elongated receptive fields (type 2). These neurons preferred moderate and high speeds of motion, and gratings of proper spatial frequency and orientation were effective stimuli for them. Border between representations of type 2 and type 1 neurons coincided with projection of horizontal retinal meridian. At the rostral and caudal borders of the field 7 abrupt changes of neuronal properties took place. Neurons which abutted field 7 anteriorly and posteriorly resembled hypercomplex cells and their small receptive fields were located in the central part of the visual field. Topographical considerations and receptive field properties allowed us to conclude that the medial part of the field 7 (included type 2 neurons) is functionally equivalent to the area V4 in the cortex of primates, while the lateral part (type 1 neurons) may correspond to the area V4T.     

Contrast-invariant orientation tuning in cat visual cortex: thalamocortical input tuning and correlation-based intracortical connectivity

The origin of orientation selectivity in visual cortical responses is a central problem for understanding cerebral cortical circuitry. In cats, many experiments suggest that orientation selectivity arises from the arrangement of lateral geniculate nucleus (LGN) afferents to layer 4 simple cells. However, this explanation is not sufficient to account for the contrast invariance of orientation tuning. To understand contrast invariance, we first characterize the input to cat simple cells generated by the oriented arrangement of LGN afferents. We demonstrate that it has two components: a spatial-phase-specific component (i.e., one that depends on receptive field spatial phase), which is tuned for orientation, and a phase-nonspecific component, which is untuned. Both components grow with contrast. Second, we show that a correlation-based intracortical circuit, in which connectivity between cell pairs is determined by the correlation of their LGN inputs, is sufficient to achieve well tuned, contrast-invariant orientation tuning. This circuit generates both spatially opponent, "antiphase" inhibition ("push-pull"), and spatially matched, "same-phase" excitation. The inhibition, if sufficiently strong, suppresses the untuned input component and sharpens responses to the tuned component at all contrasts. The excitation amplifies tuned responses. This circuit agrees with experimental evidence showing spatial opponency between, and similar orientation tuning of, the excitatory and inhibitory inputs received by a simple cell. Orientation tuning is primarily input driven, accounting for the observed invariance of tuning width after removal of intracortical synaptic input, as well as for the dependence of orientation tuning on stimulus spatial frequency. The model differs from previous push-pull models in requiring dominant rather than balanced inhibition and in predicting that a population of layer 4 inhibitory neurons should respond in a contrast-dependent manner to stimuli of all orientations, although their tuning width may be similar to that of excitatory neurons. The model demonstrates that fundamental response properties of cortical layer 4 can be explained by circuitry expected to develop under correlation-based rules of synaptic plasticity, and shows how such circuitry allows the cortex to distinguish stimulus intensity from stimulus form.     

Visual interneurons, sensitive to the size of objects and the direction of their movement in dragonflies of the genus Sympetrum]

Experiments have been made on 4 dragonfly species -- Sympetrum vulgatum, S. flaveolum, S sanguineum, S. danae. A pair of neurons was found in the thoracic ganglia and connectives, which has symmetrical contralateral receptive fields. These neurons are selectively sensitive to swift upward motion of a target of 3--10 degrees in size. This type of response was originally described by Zenkin and Pigarev [1, 2]. The receptive field, 120X25 degrees in size, is oriented horizontally from the medial rim of the eye. The center of sensitivity has the following polar coordinates: 15 degrees laterally from the medial plane and 20 degrees above the equatorial one. The relation of detecting properties of the observed neurones to key stimuli which trigger hunting behaviour is discussed. It is suggested that filtration of single and small optic stimuli by specialized detector neurons results not from the processes in the own receptive field of the neuron, but from the interaction with other neurons which are sensitive to motion of large objects and complex patterns.     

Auditory motion induces directionally dependent receptive field shifts in inferior colliculus neurons

This research focused on the response of neurons in the inferior colliculus of the unanesthetized mustached bat, Pteronotus parnelli, to apparent auditory motion. We produced the apparent motion stimulus by broadcasting pure-tone bursts sequentially from an array of loudspeakers along horizontal, vertical, or oblique trajectories in the frontal hemifield. Motion direction had an effect on the response of 65% of the units sampled. In these cells, motion in opposite directions produced shifts in receptive field locations, differences in response magnitude, or a combination of the two effects. Receptive fields typically were shifted opposite the direction of motion (i.e., units showed a greater response to moving sounds entering the receptive field than exiting) and shifts were obtained to horizontal, vertical, and oblique motion orientations. Response latency also shifted as a function of motion direction, and stimulus locations eliciting greater spike counts also exhibited the shortest neural latency. Motion crossing the receptive field boundaries appeared to be both necessary and sufficient to produce receptive field shifts. Decreasing the silent interval between successive stimuli in the apparent motion sequence increased both the probability of obtaining a directional effect and the magnitude of receptive field shifts. We suggest that the observed directional effects might be explained by "spatial masking," where the response of auditory neurons after stimulation from particularly effective locations in space would be diminished. The shift in auditory receptive fields would be expected to shift the perceived location of a moving sound and may explain shifts in localization of moving sources observed in psychophysical studies. Shifts in perceived target location caused by auditory motion might be exploited by auditory predators such as Pteronotus in a predictive tracking strategy to capture moving insect prey.     

Responses of MT and MST neurons to one and two moving objects in the receptive field

To test the effects of complex visual motion stimuli on the responses of single neurons in the middle temporal visual area (MT) and the medial superior temporal area (MST) of the macaque monkey, we compared the response elicited by one object in motion through the receptive field with the response of two simultaneously presented objects moving in different directions through the receptive field. There was an increased response to a stimulus moving in a direction other than the best direction when it was paired with a stimulus moving in the best direction. This increase was significant for all directions of motion of the non-best stimulus and the magnitude of the difference increased as the difference in the directions of the two stimuli increased. Similarly, there was a decreased response to a stimulus moving in a non-null direction when it was paired with a stimulus moving in the null direction. This decreased response in MT did not reach significance unless the second stimulus added to the null direction moved in the best direction, whereas in MST the decrease was significant when the second stimulus direction differed from the null by 90 degrees or more. Further analysis showed that the two-object responses were better predicted by taking the averaged response of the neuron to the two single-object stimuli than by summation, multiplication, or vector addition of the responses to each of the two single-object stimuli. Neurons in MST showed larger modulations than did neurons in MT with stimuli moving in both the best direction and in the null direction and the average better predicted the two-object response in area MST than in area MT. This indicates that areas MT and MST probably use a similar integrative mechanisms to create their responses to complex moving visual stimuli, but that this mechanism is further refined in MST. These experiments show that neurons in both MT and MST integrate the motion of all directions in their responses to complex moving stimuli. These results with the motion of objects were in sound agreement with those previously reported with the use of random dot patterns for the study of transparent motion in MT and suggest that these neurons use similar computational mechanisms in the processing of object and global motion stimuli.     

Antisaccade performance predicted by neuronal activity in the supplementary eye field

The voluntary control of gaze implies the ability to make saccadic eye movements specified by abstract instructions, as well as the ability to repress unwanted orientating to sudden stimuli. Both of these abilities are challenged in the antisaccade task, because it requires subjects to look at an unmarked location opposite to a flashed stimulus, without glancing at it. Performance on this task depends on the frontal/prefrontal cortex and related structures, but the neuronal operations underlying antisaccades are not understood. It is not known, for example, how excited visual neurons that normally trigger a saccade to a target (a prosaccade) can activate oculomotor neurons directing gaze in the opposite direction. Visual neurons might, perhaps, alter their receptive fields depending on whether they receive a pro- or antisaccade instruction. If the receptive field is not altered, the antisaccade goal must be computed and imposed from the top down to the appropriate oculomotor neurons. Here we show, using recordings from the supplementary eye field (a frontal cortex oculomotor centre) in monkeys, that visual and movement neurons retain the same spatial selectivity across randomly mixed pro- and antisaccade trials. However, these neurons consistently fire more before antisaccades than prosaccades with the same trajectories, suggesting a mechanism through which voluntary antisaccade commands can override reflexive glances.     

Complementary roles of two excitatory pathways in retinal directional selectivity

The two major excitatory synapses onto ON-OFF directionally selective (DS) ganglion cells of the rabbit retina appear to be nicotinic cholinergic and NMDA glutamatergic. Blockade of either of these synapses with antagonists does not eliminate directional selectivity. This suggests that these synapses may have complementary roles in the computation of the direction of motion. To test this hypothesis, quantitative features of the DS cell excitatory pathways were determined by collecting responses, under nicotinic and/or NMDA blockade, to a sweeping bar, hyperacute apparent motions, or a drifting sinusoidal grating. Sweeping bar responses were reduced, but directional selectivity not eliminated, by blockade of either excitatory path, as previously shown (Cohen & Miller, 1995; Kittila & Massey, 1997). However, residual responses under combined blockades were not statistically significantly DS. NMDA blockade reduced responses more than nicotinic blockade for each protocol, and shifted hyperacute motion thresholds to higher values. This supported the notion that glutamate provides the main excitatory drive to DS cells, that is, the one responsible for contrast sensitivity. In turn, nicotinic, but not NMDA blockade eliminated directional selectivity to a drifting low spatial-frequency sinusoidal grating in these cells. This suggested that acetylcholine (ACh) is the main excitatory input with regards to directional selectivity for some textured stimuli, that is, those with multiple peaks in their spatial luminance profile. Moreover, nicotinic blockade raised the low temporal-frequency cutoff of the grating responses, consistent with the proposal that preferred-direction facilitation, which is temporally sustained, is dependent on the cholinergic input. These different properties of the NMDA and nicotinic pathways are consistent with a recently proposed two-asymmetric-pathways model of directional selectivity.     

Intracortical arborizations and receptive fields of identified ventrobasal thalamocortical afferents to the primary somatic sensory cortex in the cat

The intracortical arborizations of neurons from the ventroposterolateral thalamic nucleus (VPL) in the cat were studied by intraaxonal injections of horseradish peroxidase (HRP) following identification of their receptive fields. In the primary somatic sensory cortex (SI) VPL cells terminated in different cytoarchitectonic areas according to their receptive field modality. Fibers excited by deep tissue or joint rotation arborized preferentially in area 3a. Those responding tonically to cutaneous stimuli were located in the anterior part of area 3b; hairdriven cells terminated in area 3b and in the rostral pole of area 1. All fibers had a similar laminar distribution within SI. Axons terminated mostly in layers VI, iV, and the lower part of layer III. None terminated in layers I and II. Most terminal arbors were oriented along the mediolateral axis of the brain. The main arborization of a single VPL cell formed a bush of about 500 micrometers in diameter. some fibers generated two such bushes with an uninvaded region of about 300 micrometer between them. It is proposed that this patchy organization underlies in part the columnar organization of areas SI. Many VPL cells had secondary projection sites in SI. These were issued from smaller-sized collaterals and were located in a different cytoarchitectonic area than that of the main terminal plexuses. A significant number of these collaterals projected to area 4, Insufficient filling of the collaterals by HRP prevented a more complete characterization of the secondary arbors.     

Spinal NMDA receptor involvement in expansion of dorsal horn neuronal receptive field area produced by intracutaneous histamine

Histamine elicits the sensation of itch at the site of skin application as well as alloknesis (itch elicited by innocuous mechanical stimuli) in a surrounding area in humans and expansion of the low-threshold mechanosensitive receptive field area of spinal wide dynamic range (WDR)-type dorsal horn neurons in rats. We presently tested if the histamine-evoked expansion of neuronal receptive field area depends on a spinal N-methyl-D-aspartate (NMDA) receptor-mediated process. In pentobarbital sodium-anesthetized rats, mechanical receptive field areas of single WDR-type dorsal horn neurons were mapped with graded von Frey filaments before and 10 min after intracutaneous (ic) microinjection of histamine (1 microl; 1, 3, or 10%) at a low-threshold site within the receptive field. Intracutaneous microinjection of histamine evoked dose-related increases in firing rate, as well as a dose-dependent expansion in mean receptive field area 10 min after 3 and 10%, but not 1%, histamine doses. When a noncompetitive or competitive NMDA receptor antagonist dizocilpine [MK-801; D(-)-2-amino-5-phosphonovalerate (APV), respectively; 1 microM] was first applied topically to the surface of the spinal cord, there was no significant change in mean receptive field area after ic microinjection of 10% histamine. The mean neuronal response to histamine in the presence of spinal MK-801 or APV was not significantly different from the mean response to histamine in the absence of these drugs. These results suggest that spinal NMDA receptors are involved in histamine-induced expansion of mechanical receptive field area, a neural event possibly involved in the development of alloknesis.     

Ventral intraparietal area of the macaque: anatomic location and visual response properties

1. The middle temporal area (MT) projects to the intraparietal sulcus in the macaque monkey. We describe here a discrete area in the depths of the intraparietal sulcus containing neurons with response properties similar to those reported for area MT. We call this area the physiologically defined ventral intraparietal area, or VIP. In the present study we recorded from single neurons in VIP of alert monkeys and studied their visual and oculomotor response properties. 2. Area VIP has a high degree of selectivity for the direction of a moving stimulus. In our sample 72/88 (80%) neurons responded at least twice as well to a stimulus moving in the preferred direction compared with a stimulus moving in the null direction. The average response to stimuli moving in the preferred direction was 9.5 times as strong as the response to stimuli moving in the opposite direction, as compared with 10.9 times as strong for neurons in area MT. 3. Many neurons were also selective for speed of stimulus motion. Quantitative data from 25 neurons indicated that the distribution of preferred speeds ranged from 10 to 320 degrees/s. The degree of speed tuning was on average twice as broad as that reported for area MT. 4. Some neurons (22/41) were selective for the distance at which a stimulus was presented, preferring a stimulus of equivalent visual angle and luminance presented near (within 20 cm) or very near (within 5 cm) the face. These neurons maintained their preference for near stimuli when tested monocularly, suggesting that visual cues other than disparity can support this response. These neurons typically could not be driven by small spots presented on the tangent screen (at 57 cm). 5. Some VIP neurons responded best to a stimulus moving toward the animal. The absolute direction of visual motion was not as important for these cells as the trajectory of the stimulus: the best stimulus was one moving toward a particular point on the face from any direction. 6. VIP neurons were not active in relation to saccadic eye movements. Some neurons (10/17) were active during smooth pursuit of a small target. 7. The predominance of direction and speed selectivity in area VIP suggests that it, like other visual areas in the dorsal stream, may be involved in the analysis of visual motion.     

Potassium-evoked directionally selective responses from rabbit retinal ganglion cells

Previous studies have shown that directionally selective (DS) retinal ganglion cells cannot only discriminate the direction of a moving object but they can also discriminate the sequence of two flashes of light at neighboring locations in the visual field: that is, the cells elicit a DS response to both real and apparent motion. This study examines whether a DS response can be elicited in DS ganglion cells by simply stimulating two neighboring areas of the retina with high external K+. Extracellular recordings were made from ON-OFF DS ganglion cells in superfused rabbit retinas, and the responses of these cells to focal applications of 100 mM KCl to the vitreal surface of the retina were measured. All cells produced a burst of spikes (typically lasting 50-200 ms) when a short pulse (10-50 ms duration) of KCl was ejected from the tip of a micropipette that was placed within the cell's receptive field. When KCl was ejected successively from the tips of two micropipettes that were aligned along the preferred-null axis of a cell, sequence-dependent responses were observed. The response to the second micropipette was suppressed when mimicking motion in the cell's null direction, whereas an enhancement during apparent motion in the opposite direction frequently occurred. Sequence discrimination in these cells was eliminated by the GABA antagonist picrotoxin and by the Ca(2+)-channel blocker omega-conotoxin MVIIC, two drugs that are known to abolish directional selectivity in these ganglion cells. The spatiotemporal properties of the K(+)-evoked sequence-dependent responses are described and compared with previous findings on apparent motion responses of ON-OFF DS ganglion cells.