Uniform Stimuli Research Articles

Surround suppression in primate V1.

We investigated the spatial organization of surround suppression in primate primary visual cortex (V1). We utilized drifting stimuli, configured to extend either from within the classical receptive field (CRF) to surrounding visual space, or from surrounding visual space into the CRF or subdivided to generate direction contrast, to make a detailed examination of the strength, spatial organization, direction dependence, mechanisms, and laminar distribution of surround suppression. Most cells (99/105, 94%) through all cortical layers, exhibited suppression (mean reduction 67%) to uniform stimuli exceeding the CRF, and 43% exhibited a more than 70% reduction. Testing with an annulus revealed two different patterns of surround influence. Some cells (37% of cells), classical surround suppression (CSS) cells exhibited responses to an annulus encroaching on the CRF that were less than the plateau in the spatial summation curve. The majority (63%), center-gated surround suppression (CGSS) cells, showed responses to annuli that equaled or exceeded the plateau in the spatial summation curve. Analysis suggested the CSS mechanism was implemented in all cells while the CGSS mechanism was implemented in varying strength across the sample with the extreme reflected in cells that gave larger responses to annuli than to a center stimulus. Reversing the direction of motion of the portion of the stimulus surrounding the CRF revealed four different patterns of effect: no reduction in the degree of suppression (22% of cells), a reduction in surround suppression (41%), a facilitation of the response above the level to the inner stimulus alone (37%), and a facilitation of the response above that to the inner stimulus alone that also exceeded the values associated with an optimal inner stimulus. The facilitatory effects were only seen for reverse direction interfaces between the central and surrounding stimulus at diameters equal to or more than the CRF size. The zones driving the suppressive influences and the direction contrast facilitation were often spatially heterogeneous and for a number of cells bore strong comparison with the class of behavior reported for surround mechanisms in MT. This suggests a potential role, for example, in extracting information about motion contrast in the representation of the three dimensional structure of moving objects.

Lightness dependencies and the effect of texture on suprathreshold lightness tolerances

A psychophysical experiment was performed to determine the effects of lightness dependency on suprathreshold lightness tolerances. Using a pass/fail method of constant stimuli, lightness tolerance thresholds were measured using achromatic stimuli centered at CIELAB L* = 10, 20, 40, 60, 80, and 90 using 44 observers. In addition to measuring tolerance thresholds for uniform samples, lightness tolerances were measured using stimuli with a simulated texture of thread wound on a card. A texture intermediate between the wound thread and the uniform stimuli was also used. A computer-controlled CRT was used to perform the experiments. Lightness tolerances were found to increase with increasing lightness of the test stimuli. For the uniform stimuli this effect was only evident at the higher lightnesses. For the textured stimuli, this trend was more evident throughout the whole lightness range. Texture had an effect of increasing the tolerance thresholds by a factor of almost 2 as compared to the uniform stimuli. The intermediate texture had tolerance thresholds that were between those of the uniform and full-textured stimuli. Transforming the results into a plot of threshold vs. intensity produced results that were more uniform across the three conditions. This may indicate that CIELAB is not the best space in which to model these effects. © 2000 John Wiley & Sons, Inc. Col Res Appl, 25, 241–249, 2000

Revising and validating the random search model for competitive search.

A random search model was fit to a total of 2592 visual search times on a single-target detection task. By using a competing homogeneous background and uniform stimulus material, specifying viewing distance, controlling the presentation of search task material, and eliminating some options for extreme search strategies, very high correlation coefficients were found when a random search model was fit to both the individual data and to pooled data. A response time parameter was incorporated into the traditional random-search model and very good predictions of search performance were obtained.

Precuing the Target Location in Visual Searching by a Chimpanzee (Pan troglodytes): Effects of Precue Validity

A chimpanzee (Pan troglodytes) performed a visual search task using a modified matching‐to‐sample procedure in which a sample stimulus was followed by the search display, which contained one stimulus identical to the sample (target) and several uniform stimuli different from the sample (distractors). On cued trials, while the subject was observing the sample, a white square (precue) appeared at the location where the target was to be presented (valid trials), or elsewhere (invalid trials). The validity of the precue (correspondence between the cued and the target locations) was changed from 0% to 100% across conditions. Cost‐benefit analyses were performed on the difference between valid and noncued trials (benefit) and between invalid and noncued trials (cost). Under the high‐validity conditions, the response times were shorter when the cued location corresponded to the target location than when the precue did not appear. When the cued location did not correspond to the target location, on the other hand, the subject took longer to select the target than on noncued trials. When the validity of the precue was relatively low, however, cost of the invalid trials disappeared, while benefit of the valid trials remained. These results confirmed the two‐process (automatic and attentional) theory of priming in human information processing; the advance information had the same effects on a chimpanzee's visual search performance as on humans'.

Cortical Coding of Extended Coloured Figures

We have studied the neural coding of extended coloured objects. Single cells were recorded from parafoveal regions of V1 and V2 in alert, fixating monkeys. In V1 most of our recordings were from the upper layers. A square figure (usually 4 deg × 4 deg, that is, much larger than the response field) of uniform colour on a gray background was placed at various positions relative to the receptive field of the cell. Orientation and colour were optimised for each cell. The vast majority of cells (75/89 in V1, 43/47 in V2) responded only when the receptive field was positioned on or near an edge, but not inside the figure (‘edge-only response’). Of the few cells that did respond with the receptive field inside the figure (‘surface response’), only some were colour selective (V1: 6/14, V2: 2/4); that is, the cell response clearly differentiated colours of similar luminance. This lack of surface responses is surprising, since most LGN cells, namely type I and type II cells respond to large uniform stimuli of some colour. On the other hand, many of the ‘edge-only’ cells were colour selective by the same criterion, some responding exclusively within a small volume of colour space. Most ‘edge-only’ cells were selective for orientation, and also many for edge polarity. V1 and V2 may encode colour of extended figures by edge signals rather than an isomorphic representation. In this case, colour filling-in would result from decay of the colour-specific edge signals rather than spreading of the colour-specific surround activity into the figure.

Pulsatile GnRH stimulates normal cyclic ovarian function in amenorrheic lactating postpartum women.

The postpartum period is characterized hormonally by elevated levels of PRL and low levels of gonadotropins and sex steroids. In breast feeding, this state of postpartum amenorrhea can persist for an extended period, even though PRL levels decrease slowly. Although the action of PRL on multiple target sites has frequently been suggested as the cause of this ovarian quiescence, a suckling-induced alteration in hypothalamic gonadotropin-releasing hormone (GnRH) production has also been hypothesized. To test this latter hypothesis, we provided a uniform pulsatile GnRH stimulus to eight exclusively breast-feeding women for an 8-week duration beginning at 4 weeks postpartum. Five women with functional hypothalamic amenorrhea served as a comparison group. All women received GnRH administered at a dose of 200 ng/kg every 90 min sc via a portable infusion pump. Serial blood sampling for LH, FSH, and PRL was performed weekly for 5 h at 10-min intervals beginning immediately before initiation of GnRH, during the period of GnRH, and 1 week after the cessation of GnRH. The women collected daily urine aliquots for estrone-3-glucuronide, pregnanediol-3-glucuronide, and LH determinations. Serial transvaginal sonography was used to monitor follicular development. Before GnRH treatment the urinary steroid and serum gonadotropin levels of the two groups were low and similar. As expected, PRL levels were higher in the postpartum women (87 micrograms/mL vs. 4.25 micrograms/L, P < 0.05). After initiation of pulsatile GnRH, LH values increased and FSH values decreased in both groups. The LH increase with GnRH was significantly greater in the breast-feeding group than in the hypothalamic amenorrhea group (19.75 mIU/mL vs. 12.34 mIU/mL, P < 0.05). Analysis of pulse frequency and amplitude revealed a nearly complete 1:1 induction of LH pulses by the exogenous GnRH in both groups, with the breast-feeding group showing a greater amplitude (12.26 mIU/mL vs. 5.34 mIU/mL, P < 0.05). The cycle lengths, urinary steroids, and vaginal ultrasonography demonstrated a more rapid initial ovarian responsiveness in the breast-feeding group, as determined by the length of the first follicular phase. The breast-feeding group also showed a brisker ovarian response, as evidenced by a greater number of follicles that were 12 mm or greater (2.3 vs. 1.2, P < 0.05), and a greater luteal phase peak and integrated pregnanediol excretion, respectively (3.02 micrograms/L creatinine and 39.87 micrograms/L creatinine/cycle vs. 1.89 micrograms/L creatinine and 7.69 micrograms/L creatinine/cycle, P < 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Colour reversal films in multichannel optical correlators for polychromatic image recognition

The use of colour reversal film applied to polychromatic pattern recognition is studied. Colour slides contain the input information of the target and the scene for the generation of the matched filters and for the multichannel optical correlation. The authors test the effects on the discrimination capability of the recognition system due to the differences between the reflectance of real diffuse objects and the transmittance of their recorded images in the slide. The response of several commercial films is analysed: firstly, through experimental transmittance curves of slides which reproduce uniform colour stimuli with different exposures; secondly, through computer results obtained for the recognition of polychromatic images. The suitability of each film for colour pattern recognition purposes is discussed.

Immobilized anti-CD3-induced T cell growth: Comparison of the frequency of responding cells within various T cell subsets

Human T cells can be divided into subsets based on the expression of CD29, CD45RA, CD45RO, LFA-3, or CD11a. It has been suggested that the subset of CD4+ T cells that expresses high densities of CD29, CD11a, CD45RO, and LFA-3 contains “memory” T cells, whereas the subset of cells that expresses CD45RA contains “naive” T cells. In order to obtain a more complete picture of the functional capacities of human naive and memory CD4+ and CD8+ T cell subsets, highly purified T cells were activated with a uniform stimulus and responses were examined in bulk cultures and under limiting dilution conditions. T cell activation was achieved with an immobilized mAb to the CD3 molecular complex, 64.1. In bulk cultures, immobilized 64.1 stimulated a vigorous response. Moreover, the number of cells entering the cell cycle, the magnitude of the [ 3H]thymidine incorporation, and the growth of the cells over 6 days in culture by naive and memory CD4+ T cells was comparable. To delineate the frequency of responsive cells in each subset more precisely, cells were cultured with immobilized 64.1 at limiting dilution and the precursor frequency of responding cells was assessed by examining wells microscopically for visible growth. Immobilized 64.1 was able to induce some T cells from each subset to grow in the complete absence of AC, when exogenous IL2 was present. The number of responding CD4+ and CD8+ cells was comparable. The percentage of naive cells responding in each population was approximately three times greater than the frequency of memory cells. IL4 could also support the growth of immobilized 64.1-activated CD4+ T cells, but the frequency of responding cells was much lower than that supported by IL2. The vast majority of the IL-4 responsive CD4+ cells resided within the naive cell subset. The data indicate that the response of CD4+ and CD8+ naive and memory T cell subsets to immobilized anti-CD3 depends on the density of responding cells. Naive T cells have an enhanced capacity to grow when cultured in the absence of other T cells or accessory cells. This ability may facilitate their expansion during primary immune responses.

Cortical modulation of visual contrast

The mechanisms that produce simultaneous contrast have been thought to depend on retinal gain control and the retina is supposed to send signals to the brain only in terms of local-border contrast (Shapley, 1986). However, it was found that, when an object on a uniform background and border-concealing stimuli are presented to different eyes, the brightness of the object is greatly influenced if the border-concealing stimuli are perceptually superimposed on the border of the object. The change in the object's brightness in this condition is almost identical to that observed when both the object and the border-concealing stimuli are presented to the same eye, suggesting that the brain can compute brightness by using luminosity information when contrast information is disrupted.

Evidence that the Glucose-Tolerance Test Does Not Provide a Uniform Stimulus to Pancreatic Islets in Pregnancy

Evidence that the Glucose-Tolerance Test Does Not Provide a Uniform Stimulus to Pancreatic Islets in Pregnancy

OPTIC NEURITIS: VARIATIONS IN TEMPORAL MODULATION SENSITIVITY WITH RETINAL ECCENTRICITY

Temporal modulation sensitivity functions were measured centrally and at eccentricities of 2.5 degrees, 5 degrees and 10 degrees in the temporal visual field of 12 patients with recovered optic neuritis and in a group of matched normal controls. A circular, spatially uniform stimulus of 1 degree angular subtense was presented with sinusoidal modulation at 5, 8, 14 and 23 Hz. The general pattern of results in patients was a loss of sensitivity relative to normal controls at all temporal frequencies at 0 degree and 2.5 degrees eccentricity, with rather greater losses occurring at the medium-to-lower temporal frequencies. At 5 degrees eccentricity, the losses were confined to medium temporal frequencies only, and at 10 degrees eccentricity there was no significant loss at any temporal frequency. These findings may be explained by a greater vulnerability of optic nerve fibers of small diameters to the effects of demyelinating disease.

Evidence that the oral glucose tolerance test does not provide a uniform stimulus to the pancreas in pregnancy

Fifty consecutive pregnant patients referred for a glucose tolerance test were classified into two groups on the basis of rising or falling hematocrit following the oral administration of a 75g glucose load. Twenty eight patients showed a drop in hematocrit following the load while twenty showed a rise in hematocrit, a response previously seen in patients with the dumping syndrome. Patients with rising hematocrit showed significantly flatter rises in glucose level post load. The mean fall in plasma phosphate levels (measured as an index of glucose uptake by cells) was significantly less (P=0.05) two hours post load in the group with flatter glucose responses suggesting that the flat response is due to poor glucose absorption rather than an exaggerated insulin response. These results indicate that the oral glucose tolerance test is a variable stress to the pancreatic islets in pregnant subjects due to variability in gastrointestinal handling of the glucose load. This could lead to a normal result in patients who might otherwise become hyperglycemic following normal meals. It is suggested that alternative screens be investigated to assess patients postprandially.

Evidence that the oral glucose-tolerance test does not provide a uniform stimulus to pancreatic islets in pregnancy.

Fifty consecutive pregnant patients referred for a glucose-tolerance test were classified on the basis of increasing (n = 20) or decreasing (n = 28) hematocrit after an oral 75-g glucose load. (The hematocrit did not change in the other two patients.) Patients with increasing hematocrit, a response previously seen in patients with the dumping syndrome, showed significantly flatter increases in glucose concentrations in plasma after the load. The mean decrease in the concentration of phosphate in plasma, measured as an index of glucose uptake by cells, was significantly less (P less than 0.05) 2 h after the load in the group with flatter glucose responses, suggesting that the flat response is ascribable to poor glucose absorption rather than to an exaggerated insulin response. These results indicate that the oral glucose-tolerance test stresses the pancreatic islets differently in different pregnant subjects, owing to individual variations in the gastrointestinal handling of the glucose load. Consequently, patients may give a "normal" result who might otherwise become hyperglycemic after normal meals. We suggest that alternative screening procedures be investigated to assess pregnant patients postprandially.

All or none cell responses of Ca2+-dependent K channels elicited by calcium or lead in human red cells can be explained by heterogeneity of agonist distribution

We have studied the all or none cell response of Ca2+-dependent K+ channels to added Ca in human red cells depleted of ATP by incubation with iodoacetate and inosine. A procedure was used which allows separation and differential analysis of responding and nonresponding cells. Responding (H for heavy) cells incubated in medium containing 5 mM K lose KCl and water and increase their density to the point of sinking on diethylphthalate (specific gravity = 1.12) on centrifugation. Nonresponding (L for light) cells do not lose KCl at all. There is no intermediate behavior. Increasing the Ca concentration in the medium increases the fraction of cells which become H. No differences in the sensitivity to Ca2+ of the individual K+ channels were detected in inside-out vesicles prepared either from H or from L cells. The Ca content of H cells was higher than that of L cells. Cells depleted of ATP by incubation with iodoacetate and inosine sustain pump-leak Ca fluxes of about 15 mumol/liter cells per hour. ATP seems to be resynthesized in these cells at the expense of cell 2,3-diphosphoglycerate stores at a rate of about 150 mumol/liter cells per hour. Inhibition of 2,3-diphosphoglycerate phosphatase by tetrathionate increased 6-8 times the measured rate of uptake of external 45Ca. This was accompanied by an increase in the fraction of H cells. All or none cell responses of Ca2+-dependent K channels have also been evidenced in intact human red cells on addition of Pb. They have the same characteristics as those in responding and nonresponding cells. The detailed study of the kinetics of Pb-induced shrinkage of red cells suspended in medium containing 5 mM K showed that changes of Pb concentration changed not only the fraction of H cells but also the rate of shrinkage of responding cells. H cells generated by Pb treatment contained significantly more lead than L cells. The above results suggest that the two all or none cell responses studied here can be explained by heterogeneity of agonist distribution among cells. Since pump-leak fluxes exist in both cases, differences of agonist distribution could be generated by heterogeneity of pumping among cells. This interpretation turns interest from K channels to Ca pumps to explain the heterogeneous behavior of red cells in response to a uniform stimulus.

Spatiotemporal frequency responses of cat retinal ganglion cells.

Spatiotemporal frequency responses were measured at different levels of light adaptation for cat X and Y retinal ganglion cells. Stationary sinusoidal luminance gratings whose contrast was modulated sinusoidally in time or drifting gratings were used as stimuli. Under photopic illumination, when the spatial frequency was held constant at or above its optimum value, an X cell's responsivity was essentially constant as the temporal frequency was changed from 1.5 to 30 Hz. At lower temporal frequencies, responsivity rolled off gradually, and at higher ones it rolled off rapidly. In contrast, when the spatial frequency was held constant at a low value, an X cell's responsivity increased continuously with temporal frequency from a very low value at 0.1 Hz to substantial values at temporal frequencies higher than 30 Hz, from which responsivity rolled off again. Thus, 0 cycles X deg-1 became the optimal spatial frequency above 30 Hz. For Y cells under photopic illumination, the spatiotemporal interaction was even more complex. When the spatial frequency was held constant at or above its optimal value, the temporal frequency range over which responsivity was constant was shorter than that of X cells. At lower spatial frequencies, this range was not appreciably different. As for X cells, 0 cycles X deg-1 was the optimal spatial frequency above 30 Hz. Temporal resolution (defined as the high temporal frequency at which responsivity had fallen to 10 impulses X s-1) for a uniform field was approximately 95 Hz for X cells and approximately 120 Hz for Y cells under photopic illumination. Temporal resolution was lower at lower adaptation levels. The results were interpreted in terms of a Gaussian center-surround model. For X cells, the surround and center strengths were nearly equal at low and moderate temporal frequencies, but the surround strength exceeded the center strength above 30 Hz. Thus, the response to a spatially uniform stimulus at high temporal frequencies was dominated by the surround. In addition, at temporal frequencies above 30 Hz, the center radius increased.

Regulation of salivary proteins.

Previous studies have shown that several factors--such as alloxan-induced diabetes, adrenalectomy, or removal of the thyroid-parathyroid gland complex--can influence the flow rate, protein concentration, and protein composition of rat parotid saliva. The present study was undertaken to explore further the influence of glucocorticoids and thyroxine on rat parotid saliva in hormonally intact animals. As compared with untreated animals, adult male rats treated with 10 micrograms dexamethasone per 100 g body weight for eight days demonstrated a 75% reduction in volume of parotid saliva secreted in response to a uniform stimulus. The protein concentration of the saliva was increased three-fold. Sodium dodecyl sulfate polyacrylamide gel electrophoresis showed relative decreases in acidic and basic proline-rich proteins and in a protein identified as Fraction V, while amylase was increased. The electron microscopic appearance of the granules was markedly different from that of the control, in that the granules exhibited an electron-dense periphery and core, with the remainder of the granule having an electronlucent appearance. In contrast, rats treated for eight days with 20 micrograms thyroxine per 100 g body weight exhibited a 50% increase in volume of saliva collected in response to a secretory stimulus. Although the concentration of protein was not different from that of the control, gel electrophoresis showed relative increases in acidic and basic proline-rich proteins and a decrease in Fraction V. Amylase was unchanged. The secretory granules of thyroxine-treated rats were electronlucent and amorphous. The granules appeared to coalesce within the cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Myosin polymorphism in single fibers of chronically stimulated rabbit fast-twitch muscle.

Rabbit tibialis anterior (TA) muscles were indirectly stimulated (10 Hz, 24 h/d) for 30 d and 60 d and single fibers were analysed using a combined histochemical and biochemical technique (Staron and Pette 1986, 1987a, b). After 30 d of chronic stimulation there was a pronounced increase in the normally rare (0.5%) C fiber population (i.e., fibers containing slow- and fast-myosins in varying ratios). At this time, C fibers amounted to almost 60% of the total population. In the 60 d stimulated muscles, the major population (98%) consisted of an atypical type It fiber. This fiber type which was not detectable in normal TA muscle, differed histochemically and biochemically from type I fibers. It contained the slow-myosin light chains LC1s and LC2s, the heavy chain HCI, and, in addition, high concentrations of the fast-myosin alkali light chain LC1f and possibly traces of a heavy chain with an electrophoretic mobility comparable with that of the fast-myosin heavy chain HCIIa. These It fibers were occasionally observed in the unstimulated, contralateral TA muscles which also contained an increased population of C fibers (1.3-6.3%). Although the transformation even after 60 d of chronic stimulation was incomplete, these changes demonstrate the ability of muscle fibers to adapt in a specific manner to altered functional demands brought about by an altered stimulus pattern. In addition, the pronounced heterogeneity of the fiber population undergoing transformation indicates a nonuniform response to a uniform stimulus pattern.

Visual lobe dimensions and search performance for targets on a competing homogeneous background.

The relationship between visual lobe area and search performance has been demonstrated by previous researchers under a variety of experimental conditions and using different measures of lobe area. The relationship has not been as highly significant as might be expected, given the obvious importance of lobe area for search performance. Using uniform stimulus material for lobe size and search performance tests, specifying viewing distance, controlling the presentation of search task material, and eliminating some extreme search strategy options, we obtained a very highly significant correlation between lobe area and search performance for 54 Cantonese Chinese subjects. This result was obtained in spite of the subjects* homogeneity with regard to age and near foveal acuity. An unexpected result was that, relative to the horizontal lobe dimension, the vertical dimension was highly significantly related to lobe area and to search performance.

The Octavo-Lateralis Percepton in Lower Aquatic Vertebrates: Clustered Versus Dispersed Sensor Configurations

This paper discusses the biological relevance of the topology of octavo-lateralis sense organs in lower aquatic vertebrates. The sense organs of the octavo-lateralis system are arranged either as a dense cluster or as a dispersed set. The dense cluster is situated deep in the head, near the hind brain, and is mainly involved in the detection of uniform stimulus fields. It collects information about distant stimulus sources. The dispersed set is spread over the integument and meets the demands for the detection and interpretation of non-uniform stimulus fields, usually emitted by nearby stimulus sources. It is argued that both the stimulation of the semicircular canals by rotation and the stimulation of the dispersed ampullary electroreceptors in uniform stimulus fields present paradoxes that nevertheless fit this general view.

Representation of spatial details in textured patterns by cells of the cat striate cortex.

The ability of single cells to represent the spatial details of textured stimuli was investigated. Two complementary aspects of cell response were considered, the ability to discriminate fine stimulus details and the property of integration over wider areas of a structure to encode differences in mean luminance. Responses of simple and complex cells were distinct in some respects. Spatial discrimination: Simple cells would encode orientation of line arrays as long as individual line elements could be spatially resolved. By contrast, complex cells were able to distinguish the orientation of texture areas even when the individual lines of the stimulus were not resolved in their response. Threshold sensitivity for texture orientation was of the same order in both cell classes despite differences in receptive field size. Spatial integration: Complex cells responded to texture luminance differences of much coarser patterns than did simple cells. These responses, however, were not biased for contour orientation unless finer patterns were used. Only with very fine textures did responses become indistinguishable from those to uniform stimuli for both simple and complex cells. For complex cells, there was a smooth transition from resolution to fusion of spatial details with increasing structural density. Simple cells were insensitive to both detailed and global properties of a stimulus pattern over a wide range of texture density. Implications for alternative measures of visual acuity of single cells are discussed.