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D C Van Essen

Publications and source records attributed to D C Van Essen.

At least 19 recordsLinked to original sources
PubMed1992

Information processing in the primate visual system: an integrated systems perspective.

The primate visual system contains dozens of distinct areas in the cerebral cortex and several major subcortical structures. These subdivisions are extensively interconnected in a distributed hierarchical network that contains several intertwined processing streams. A number of strategies are used for efficient information processing within this hierarchy. These include linear and nonlinear filtering, passage through information bottlenecks, and coordinated use of multiple types of information. In addition, dynamic regulation of information flow within and between visual areas may provide the computational flexibility needed for the visual system to perform a broad spectrum of tasks accurately and at high resolution.

Animals↗
PubMed1992

Visual cortex: cartography, connectivity, and concurrent processing.

The mammalian visual cortex contains a complex mosaic of areas that are richly connected with one another. Recent progress has advanced our understanding of both macroscopic and microscopic aspects of cortical organization, and of information flow within and between functionally specialized processing streams.

Animals↗
PubMed1990

Blur into focus.

Explore the source record for details and available documents.

Humans↗
PubMed1990

Synaptic dynamics at the neuromuscular junction: mechanisms and models.

During development, the neuromuscular junction passes through a stage of extensive polyinnervation followed by a period of wholesale synapse elimination. In this report we discuss mechanisms and interactions that could mediate many of the key aspects of these important developmental events. Our emphasis is on (1) establishing an overall conceptual framework within which the role of many distinct cellular interactions and molecular factors can be evaluated, and (2) generating computer simulations that systematically test the adequacy of different models in accounting for a wide range of biological data. Our analysis indicates that several relatively simple mechanisms are each capable of explaining a variety of experimental observations. On the other hand, no one mechanism can account for the full spectrum of experimental results. Thus, it is important to consider models that are based on interactions among multiple mechanisms. A potentially powerful combination is one based on (1) a scaffold within the basal lamina or in the postsynaptic membrane which is induced by nerve terminals and which serves to stabilize terminals by a positive feedback mechanism; (2) a sprouting factor whose release by muscle fibers is down-regulated by activity and perhaps other factors; and (3) an intrinsic tendency of motor neurons to withdraw some connections while allowing others to grow.

Animals↗
PubMed1990

Antibody labeling of functional subdivisions in visual cortex: Cat-301 immunoreactivity in striate and extrastriate cortex of the macaque monkey.

We have examined the distribution of immunoreactivity for the monoclonal antibody Cat-301 in visual cortex of the macaque monkey. Remarkably, those portions of striate cortex (V1) and extrastriate cortex that are most immunoreactive for Cat-301 are anatomically interconnected and are dominated by inputs arising from the magnocellular layers of the LGN (which are themselves highly immunoreactive). In particular, we found that a band of Cat-301 labeled neurons known to exist in layer 4 of V1 is centered on the boundary between layers 4C alpha and 4B and thus includes portions of both the primary target of the magnocellular LGN and its subsequent relay through layer 4B. We also demonstrated consistently strong Cat-301 immunoreactivity in all three extrastriate targets of layer 4B: areas V3, MT, and the cytochrome-oxidase (CO) enriched thick stripes of V2. In V2, there was a close correlation between Cat-301 labeling and clusters of cells projecting to MT but not to V4. This was true even in regions where the CO pattern was equivocal or irregular, indicating that Cat-301 is a more reliable marker than CO for the thick-stripe subregions of V2. Finally, we found strong Cat-301 immunoreactivity in at least parts of areas V3A, the MST complex, and the posterior parietal complex, but not in area V4 or inferotemporal cortex. The molecular specificity revealed by this single marker thus correlates with functionally specific subdivisions at each hierarchical level over nearly the entire known extent of the visual pathway in macaques. This supports the notion that these subdivisions form an anatomically, physiologically, and now molecularly distinct pathway known as the M-stream.

Animals↗
PubMed1989

Slowing of synapse elimination by alpha-bungarotoxin superfusion of the neonatal rabbit soleus muscle.

To examine the role of postsynaptic activity in regulating the rate of neuromuscular synapse elimination, contractile activity of neonatal rabbit soleus muscles was decreased by chronic superfusion of alpha-bungarotoxin (alpha-BGT) over their surfaces. Superfusion was begun at 6 days postnatal and continued for a variable duration (2 to 5 days) before muscles were analyzed. The percentage of polyinnervated fibers was assessed both physiologically and anatomically for alpha-BGT-treated muscles and their contralateral muscles, in addition to normal and control muscles of the same age. Within muscles exposed to alpha-BGT, polyinnervation was significantly greater than that for muscles from each of the control groups. The anatomical assay further revealed that the retention of polyinnervation in alpha-BGT-treated muscles was most pronounced near the muscle's surface, although end plates at the center were also affected. This finding, coupled with evidence that only a small percentage of the muscle fibers were completely inactivated, suggests that the activity block was also most pronounced near the surface and relatively low at the muscle's center. The percentage of end plates at which synapse elimination was delayed was greater than the estimated percentage whose activity was completely blocked, suggesting that synapse loss was slowed even in muscle fibers retaining some postsynaptic activity. These observations indicate that the rate of synapse elimination depends on the levels of functional acetylcholine receptors. This process could be mediated in a graded fashion by changes in postsynaptic activity (subthreshold or suprathreshold) or by a nonelectrical effect of blocking postsynaptic receptors.

Aging↗
PubMed1989

Lack of fiber type selectivity during reinnervation of neonatal rabbit soleus muscle.

Fast and slow contracting fibers in neonatal mammalian skeletal muscle are each innervated in a highly specific manner by motor neurons of the corresponding type, even at an age when polyinnervation is widespread. Chemospecific recognition is a possible mechanism by which this pattern of innervation could be established. We have investigated this possibility by studying the degree of specificity during reinnervation of rabbit soleus muscle following nerve crush on Postnatal Day 1 or 4. We assayed fiber type composition by measuring the twitch rise times of motor units within 2 days of the onset of functional reinnervation (5-6 days after nerve crush). In contrast to the broad, bimodal distribution of single motor unit twitch rise times seen in normal muscles, motor units in reinnervated muscles yielded a narrower, unimodal distribution of rise times. Rise times of reinnervated units were intermediate to those of normal fast and slow units, suggesting that reinnervated units were composed of a mixture of fast and slow contracting fibers. An alternative possibility, that specific reinnervation was masked by contractile dedifferentiation of muscle fibers, was examined by maintaining a transmission blockade induced by botulinum toxin poisoning for an equivalent interval. Twitch rise times of treated motor units exhibited the distinctly bimodal distribution characteristic of normal muscles, suggesting that muscle fibers can retain contractile diversity during a transient period of denervation. We carried out computer simulations to estimate the amount of rise time diversity induced by varying degrees of specificity during reinnervation. Based on this analysis, we conclude that there is little if any selective reinnervation of muscle fiber types at the ages studied.

Animals↗
PubMed1989

Differential loss of neuromuscular connections according to activity level and spinal position of neonatal rabbit soleus motor neurons.

We have tested whether the ability of synapses to compete for occupancy of endplates during neuromuscular synapse elimination is affected by differences in the spinal position or in the activity level of the parent motor neuron. To test the role of spinal position, the relative sizes of motor units for motor neurons from middle and extreme (rostral/caudal) positions in the rabbit soleus motor pool were determined at 3 postnatal ages: 4-5 d ("early" ages, when the soleus is heavily polyinnervated), 8-9 d ("intermediate"), and 11-15 d ("late," when the soleus has just reached singly innervated state). Average motor unit sizes from extreme ventral roots were similar to those from middle ventral roots in early-aged soleus muscles but were significantly smaller (by 18-27%) for both intermediate and late muscles. Thus, motor neurons from extreme positions evidently compete less effectively for retention of synapses than those from middle positions. To test the role of differential activity, inactive and active synapses were pitted directly against one another by implanting Silastic plugs laden with tetrodotoxin (TTX) into one of the spinal nerves containing a minority of the soleus motor axons. Differential activity was maintained during a period of extensive synapse loss, from the time of the implant at day 4 or 5 until the intermediate age (day 8-9). Motor unit twitch tensions were subsequently measured to determine the relative number of synapses retained by individual active and inactive motor neurons. The inactivated motor units were on average significantly larger (by more than 50%) than the corresponding group from normal and control-implanted animals. The abnormally large size of inactivated motor units persisted in animals allowed to recover from the TTX block and examined after multiple innervation had disappeared. Hence, the effect of the TTX block cannot be attributed to a simple slowing of synapse elimination specifically among the inactive motor neurons. We conclude that complete presynaptic inactivity improves the chances of survival relative to that for normal activity during synapse elimination in the neonatal rabbit soleus muscle. This difference in competitive ability may contribute to the development of an important characteristic of adult muscles, the correlation between motor unit size and recruitment threshold.

Animals↗
PubMed1987

Topographic organization of the middle temporal visual area in the macaque monkey: representational biases and the relationship to callosal connections and myeloarchitectonic boundaries.

We have used physiological and anatomical techniques to address three general issues concerning the topographic organization of the middle temporal visual area (MT) of the macaque monkey. First, we carried out a quantitative analysis of irregularities and asymmetries in the visual representation in MT. This analysis revealed a striking overemphasis on a restricted portion of the visual field that runs obliquely through the inferior contralateral quadrant and largely avoids both the horizontal meridian and the inferior vertical meridian. This corresponds to the portion of the visual field that would be maximally stimulated during visually guided hand movements. Second, the physiologically determined topographic organization of MT was compared to the pattern of callosal inputs in the same hemisphere, which are known to be distributed irregularly within MT. Callosal inputs tended to be densest near the representation of the vertical meridian, but there were numerous exceptions to this trend. Thus, topographic irregularities account for only part of the irregularities in callosal inputs to MT. Finally, comparison of these data with previous reports shows a strong correlation between body weight and the average size of MT. The representation in myeloarchitectonically defined MT was found to include much of the visual periphery, although it is unclear from our data whether this representation is invariably complete.

Animals↗
PubMed1987

Synapse elimination by fiber type and maturational state in rabbit soleus muscle.

We have compared the development of fast and slow motor innervation in the neonatal rabbit soleus, a muscle which contains two distinct motor unit types during the early period of polyneuronal innervation. The innervation state of individual muscle fibers was ascertained using an intracellular electrode; a fluorescent dye was then injected into particular fibers to permit subsequent identification of histochemical type. We found no significant difference in the time course of synapse elimination for fast and slow motor units as judged by the percentage of fibers remaining polyneuronally innervated at two ages: 7-8 days, when most fibers are multiply innervated, and 10-11 days, when the level of polyinnervation is low. In a second experiment, we examined a phenomenon in which compound end-plate potentials were occasionally seen in muscle fibers at an age (17-23 days) well past the major episode of synapse elimination. We present evidence that this apparent polyinnervation in fact derives from an electrode-induced electrical coupling artifact and that genuinely polyinnervated fibers are very rare at this stage, if present at all.

Aging↗
PubMed1987

Shifter circuits: a computational strategy for dynamic aspects of visual processing.

We propose a general strategy for dynamic control of information flow between arrays of neurons at different levels of the visual pathway, starting in the lateral geniculate nucleus and the geniculorecipient layers of cortical area V1. This strategy can be used for resolving computational problems arising in the domains of stereopsis, directed visual attention, and the perception of moving images. In each of these situations, some means of dynamically controlling how retinal outputs map onto higher-level targets is desirable--in order to achieve binocular fusion, to allow shifts of the focus of attention, and to prevent blurring of moving images. The proposed solution involves what we term "shifter circuits," which allow for dynamic shifts in the relative alignment of input and output arrays without loss of local spatial relationships. The shifts are produced in increments along a succession of relay stages that are linked by diverging excitatory inputs. The direction of shift is controlled at each stage by inhibitory neurons that selectively suppress appropriate sets of ascending inputs. The shifter hypothesis is consistent with available anatomical and physiological evidence on the organization of the primate visual pathway, and it offers a sensible explanation for a variety of otherwise puzzling facts, such as the plethora of cells in the geniculorecipient layers of V1.

Animals↗
PubMed1987

Receptive field properties of neurons in area V3 of macaque monkey extrastriate cortex.

Receptive field properties of 147 neurons histologically verified to be located in area V3 were investigated during semichronic recording from paralyzed anesthetized macaque monkeys. Quantitative analyses were made of neuron selectivities for direction, orientation, speed, binocular disparity, and color. The majority of neurons in V3 (76%) were strongly orientation selective; 40% demonstrated strong direction selectivity. Most cells were tuned for stimulus speed and almost half showed optimum responses at 16 degrees/s. The distribution of optimum speeds ranged primarily from 4 to 32 degrees/s. Several cells in V3 displayed multi-peaked orientation- and/or direction-tuning curves. These cells had two or more narrowly tuned peaks that were not co-axial. In some ways, they resemble higher-order hypercomplex cells of cat area 19 and may subserve a higher level of form or motion analysis than is seen at antecedent visual areas. Roughly half (45%) of the cells were selective for binocular disparity. Approximately half of these were tuned excitatory in that they showed weak responses when tested through either eye alone, but showed strong binocular facilitation centered on the fixation plane. The other disparity-selective cells were tuned inhibitory or asymmetric in their responses in front and behind the fixation plane. Contrary to previous reports, approximately 20% of the neurons in V3 were color selective in terms of showing a severalfold greater response to the best monochromatic wavelength compared with the worst. Color-tuning curves of the subset of color selective cells had, on average, a full bandwidth at half maximum response of 80-100 nm. A comparison of the receptive field properties of neurons in V3 to those in other areas of visual cortex suggests that V3, like MT, is well suited for the analysis of several aspects of stimulus motion. V3 may also be involved in some aspects of form analysis, particularly at low contrast levels. Comparison with area VP, a thin strip of cortex anterior to ventral V2, which was previously considered part of V3, indicates that direction selectivity is much more prevalent in V3 than in VP. Conversely, color-selective cells are the majority in VP but a minority in V3. This suggests that visual information is processed differently in the upper and lower visual fields.

Animals↗
PubMed1987

Retinotopic organization of human visual cortex mapped with positron-emission tomography.

The retinotopic organization of primary visual cortex was mapped in normal human volunteers. Positron-emission tomographic measurements of regional cerebral blood flow were employed to detect focal functional brain activation. Oxygen-15-labeled water, delivered by intravenous bolus, was used as the blood flow tracer to allow multiple stimulated-state (n = 5) and control-state (n = 3) measurements to be acquired for each of 7 subjects. Responses were identified by applying a maximum-detection algorithm to subtraction-format images of the stimulus-induced change in cerebral blood flow. Response locales were described using a standardized system of stereotactic coordinates. Changes in stimulus location (macular, perimacular, peripheral, upper-field, lower-field) caused systematic, highly significant changes in response locale within visual cortex. Discrete extrastriate visual responses were also observed.

Adult↗
PubMed1986

Ventral posterior visual area of the macaque: visual topography and areal boundaries.

Using both physiological and anatomical techniques, we have studied the topographic organization of extrastriate visual cortex on the ventral surface of the occipital lobe in macaque monkeys. Our results show that a topographically organized representation of the superior contralateral quadrant of the visual field lies immediately anterior to the ventral half of V2. This area is organized in a mirror symmetric fashion to ventral V2: it shares a horizontal meridian representation with V2 and a representation of the superior vertical meridian forms its anterior border. A well-defined strip of callosal inputs runs along the vertical meridian representation, thereby providing a reliable anatomical marker for areal boundaries in ventral extrastriate cortex. We refer to this area as the ventral posterior area (VP) because it is, in all these respects, notably similar to VP in the owl monkey. Ventral V2 has strong reciprocal connections with VP, and the topography of the V2 projection agrees closely with the topography revealed in our physiological mapping experiments. The visual field representation in VP is strikingly anisotropic, with linear magnification factor being much larger along contours of constant polar angle than along contours of constant eccentricity.

Animals↗
PubMed1986

Anatomical and physiological asymmetries related to visual areas V3 and VP in macaque extrastriate cortex.

This report provides an overview of the functional organization of cortex immediately anterior to area V2 in extrastriate visual cortex of the macaque monkey. Contrary to previous suggestions that a single area, V3, lies anterior to V2, we have obtained evidence that this strip of cortex includes two separate areas, V3 and the ventral posterior area, VP. The evidence supporting this conclusion is based on dorso-ventral asymmetries in cortico-cortical connections, myeloarchitecture, and single-unit physiological properties relating to the processing of information about color and motion.

Action Potentials↗
PubMed1986

Processing of color, form and disparity information in visual areas VP and V2 of ventral extrastriate cortex in the macaque monkey.

The responses of single cells to light bars of different orientation, direction of motion, speed, binocular disparity, and wavelength were systematically analyzed in areas V2 and VP of ventral extrastriate visual cortex in the macaque monkey. Selectivity for each of these parameters was assessed quantitatively using computer-controlled procedures. In both VP and V2 (both representing the superior contralateral quadrant), more than half of the cells studied were selective for stimulus color and more than half for stimulus orientation. In contrast, only a small minority of the VP and V2 cells were selective for the direction of stimulus motion. Comparison with reports of single-unit properties in dorsal extrastriate cortex suggests there are no major differences in the incidence of orientation, direction, and color selectivity between ventral and dorsal subdivisions of V2. Between V3 and VP, though, there are marked differences: Color-selective cells are much less common in V3 than VP, whereas direction-selective cells are more common in V3. This dorsoventral difference in the distribution of neuronal response properties suggests a significant asymmetry in the way visual information is processed in upper and lower parts of the visual field. The properties of cells in VP suggest that it plays an important role in both form and color vision, similar to that attributed to area V4.

Animals↗