High Contrast Levels Research Articles

Can red flowers be conspicuous to bees?Bombus dahlbomiiand South American temperate forest flowers as a case in point

It has been argued that trichromatic bees with photoreceptor spectral sensitivity peaks in the ultraviolet (UV), blue and green areas of the spectrum are blind to long wavelengths (red to humans). South American temperate forests (SATF) contain a large number of human red-looking flowers that are reported to be visited by the bumblebee Bombus dahlbomii. In the present study, B. dahlbomii's spectral sensitivity was measured through electroretinogram (ERG) recordings. No extended sensitivity to long wavelengths was found in B. dahlbomii. The spectral reflectance curves from eight plant species with red flowers were measured. The color loci occupied by these flowers in the bee color space was evaluated using the receptor noise-limited model. Four of the plant species have pure red flowers with low levels of chromatic contrast but high levels of negative L-receptor contrast. Finally, training experiments were performed in order to assess the role of achromatic cues in the detection and discrimination of red targets by B. dahlbomii. The results of the training experiments suggest that the bumblebee relies on achromatic contrast provided by the L-receptor to detect and discriminate red targets. These findings are discussed in the context of the evolutionary background under which the relationship between SATF species and their flower visitors may have evolved.

Characterization of Carbon Nano-Gap for Surface Conduction Electron Emitters

Field-emission displays (FEDs) retain the advantages of cathode-ray tubes (CRTs), namely, high color reproducibility, fast response, wide viewing angle, and high contrast level. The emitters used in FEDs are based on the principle that electrons are emitted by a strong electric field. It is necessary to obtain an electric field that is sufficiently strong to induce a tunnel current, even at a low voltage, by forming nanometer-sized gaps (nano-gaps) in the emitters. Previously, we formed a carbon nano-gap by our original method and reported that electrons can be stably emitted at a low voltage. In this study, we succeeded in forming a nano-gap with a uniform width by controlling the externally applied activation voltage by the same method. In addition, the strength of the electric field caused by the external voltage (Vf), β×Vf, was found to be constant. In this paper, we discuss the mechanism underlying the formation of a carbon nano-gap and report electrical characteristics, temperature measurement data, and the results of observing cross-sectional images of the carbon nano-gaps obtained by scanning electron microscopy (SEM). In addition, the mechanism underlying the formation of the carbon nano-gap is modeled.

First Direct Observation of Elementary Steps on the Surfaces of Glucose Isomerase Crystals under High Pressure

We have succeeded in the first direct observation under high pressure of the elementary steps (7.2 nm high) on {011} faces of glucose isomerase (GI) crystals, by using a laser confocal microscope combined with a differential interference contrast microscope (LCM-DIM) and a specially designed high-pressure vessel with a sapphire window of 1 mm thickness. The images of elementary steps taken under 50 MPa exhibited a sufficiently high contrast level for subsequent studies of crystal growth. By in situ observations, we directly confirmed that, irrespective of pressure, {011} faces of GI crystals grew by two-dimensional (2D) nucleation growth of the polynucleation type, and that pressure did not affect growth morphology. We measured the solubility of GI crystals under high pressure by observing in situ the growth and dissolution of elementary steps and ridges on the crystals. The resulting solubility curve exhibited much higher precision than those determined by interferometry, and revealed a significant decrease in solubility with increasing pressure. We also measured the 2D nucleation rates of 2D islands and the velocities of elementary steps under high pressure.

Strain Measurement by Digital Image Correlation

Strain Measurement by Digital Image Correlation

Straylight effects in diffractive multifocal IOL compared to monofocal IOL

Abstract Purpose To measure levels of intraocular straylight following cataract surgery and implantation of either the ReSTOR SA60D3 IOL or the SA60AT monofocal IOL. Methods A newly developed straylight meter was used to objectively measure straylight in a prospective open observational case series at 6 months postoperatively in 66 eyes with the diffractive ReSTOR SA60D3 IOL (Alcon Laboratories) and in a control group of 40 eyes with the monofocal SA60AT IOL. A comparison with straylight levels in an age‐matched population without cataract was performed. Results The straylight levels in the ReSTOR SA60D3 IOL and in the control group with the monofocal SA60AT IOL were 1.20±0.16 logunits and 1.10±0.19 logunits. When the difference in mean level of straylight was adjusted for age by multivariate analysis of the combined pseudophakic groups, mean straylight levels were 0.078 logunits lower for the monofocal SA60AT IOL group as compared to the multifocal ReSTOR SA60D3 IOL group (p=0.026). Straylight levels in both pseudophakic groups were lower than those of the normal age‐matched control group without cataract (p<0.0001). Conclusion Levels of intraocular straylight log(s) were significantly lower (p<0.0001) for both types of IOL as compared to age‐matched subjects from the normal population, with expected reduction of complaints of glare and halos and higher levels of contrast sensitivity.The mean level of intraocular straylight 6 months postoperatively was higher for subjects with a ReSTOR SA60D3 IOL as compared to subjects with a monofocal SA60AT IOL, with expected less gain in contrast sensitivity and less reduction of glare and halo complaints.

Parvocellular and Magnocellular Contributions to the Initial Generators of the Visual Evoked Potential: High-Density Electrical Mapping of the “C1” Component

The C1 component of the VEP is considered to index initial afference of retinotopic regions of human visual cortex (V1 and V2). C1 onsets over central parieto-occipital scalp between 45 and 60 ms, peaks between 70 and 100 ms, and then resolves into the following P1 component. By exploiting isoluminant and low-contrast luminance stimuli, we assessed the relative contributions of the Magnocellular (M) and Parvocellular (P) pathways to generation of C1. C1 was maximal at 88 ms in a 100% luminance contrast condition (which stimulates both P and M pathways) and at 115 ms in an isoluminant chromatic condition (which isolates contributions of the P pathway). However, in a 4% luminance contrast condition (which isolates the M pathway), where the stimuli were still clearly perceived, C1 was completely absent. Absence of C1 in this low contrast condition is unlikely to be attributable to lack of stimulus energy since a robust P1-N1 complex was evoked. These data therefore imply that C1 may be primarily parvocellular in origin. The data do not, however, rule out some contribution from the M system at higher contrast levels. Nonetheless, that the amplitude of C1 to P-isolating isoluminant chromatic stimuli is equivalent to that evoked by 100% contrast stimuli suggests that even at high contrast levels, the P system is the largest contributor. These data are related to intracranial recordings in macaque monkeys that have also suggested that the initial current sink in layer IV may not propagate effectively to the scalp surface when M-biased stimuli are used. We also discuss how this finding has implications for a long tradition of attention research that has used C1 as a metric of initial V1 afference in humans. C1 has been repeatedly interrogated for potential selective attentional modulations, particularly in spatial attentional designs, under the premise that modulation of this component, or lack thereof, would be evidence for or against selection at the initial inputs to visual cortex. Given the findings here, we would urge that in interpreting C1 effects, a consideration of the dominant cellular contributions will be necessary. For example, it is plausible that spatial attention mechanisms could operate primarily through the M system and that as such C1 may not always represent an adequate dependent measure in such studies.

Objective assessment of chromatic and achromatic pattern adaptation reveals the temporal response properties of different visual pathways

Abstract Purpose To investigate the temporal response properties of magnocellular, parvocellular and koniocellular pathways using contrast increment/decrement visual evoked potentials (VEPs). Methods Static achromatic and isoluminant chromatic gratings were generated on a monitor (Michelson contrast 0.05 to 0.6). Chromatic gratings were modulated along R/G or subject‐specific tritanopic confusion axes, established using a minimum distinct border criterion. Isoluminance was determined using minimum flicker photometry for a 1‐degree field. Achromatic and chromatic VEPs were recorded to contrast increments and decrements of 0.1 superimposed on the static gratings. Field size for Tritan gratings was restricted to 3‐degrees to minimize the effects of luteal pigment. Results Achromatic increment/decrement VEPs were positive in polarity and largely unaffected by high levels of static contrast at low spatial frequencies. Responses to finer achromatic gratings showed marked attenuation as static contrast was increased. Chromatic VEPs to R/G or Tritan chromatic contrast increments were of negative polarity and showed progressive attenuation as static contrast was increased. Chromatic contrast decrement VEPs were of positive polarity and less sensitive to pattern adaptation. Conclusion The relative contribution of sustained and transient response mechanisms to achromatic processing is spatial‐frequency dependent. Chromatic contrast increment VEPs are highly sensitive to pattern adaptation consistent with high stimulus selectivity and the sustained temporal response properties of the parvocellular and koniocellular pathways.

Organizational Evolution with Fuzzy Technological Boundaries: Tape Drive Producers in the World Market, 1951-1998

We study how tape drive producers respond to the almost continuous emergence of new drive formats across the technology's history. The analysis characterizes the technological formats of tape drives according to their degree of contrast (distinctiveness and visibility) from other formats. High contrast implies strong boundaries, which prevent information leakage and appear to provide solid strategic footing because they are not easy to adopt and replicate; yet formats with strong boundaries may appear risky to potential customers who may prefer formats more readily substitutable. We develop and test arguments about how different types of tape drive manufacturers add and drop the production of formats as a function of the contrast of formats. A key distinction we make is between single-format producers whose format has high contrast (yielding a clear firm level identity) and multi-format producers whose various formats blur the firm's identity when they are high in contrast. In the empirical analysis, we find that single-format firms producing formats with high contrast experience a lower rate of mortality, while average high contrast in the technology portfolio of multi-format producers lowers their survival chances. We also find that single-format firms with technology formats characterized by high levels of contrast are: (1) more likely to add newly emerging formats; and (2) less likely to drop existing formats. By contrast, multi-format manufacturers show mixed patterns.

Magnocellular Pathway Impairment in Schizophrenia: Evidence from Functional Magnetic Resonance Imaging

Sensory processing deficits in schizophrenia have been documented for several decades, but their underlying neurophysiological substrates are still poorly understood. In the visual system, the pattern of pathophysiology reported in several studies is suggestive of dysfunction within the magnocellular visual pathway beginning in early sensory cortex or even subcortically. The present study used functional magnetic resonance imaging to investigate further the neurophysiological bases of visual processing deficits in schizophrenia and in particular the potential role of magnocellular stream dysfunction. Sinusoidal gratings systematically varying in spatial frequency content were presented to subjects at low and high levels of contrast to differentially bias activity in magnocellular and parvocellular pathways based on well established differences in neuronal response profiles. Hemodynamic responses elicited by different spatial frequencies were mapped over the occipital lobe and then over the entire brain. Retinotopic mapping was used to localize the occipital activations with respect to the boundaries of visual areas V1 and V2, which were demarcated in each subject. Relative to control subjects, schizophrenia patients showed markedly reduced activations to low, but not high, spatial frequencies in multiple regions of the occipital, parietal, and temporal lobes. These findings support the hypothesis that schizophrenia is associated with impaired functioning of the magnocellular visual pathway and further suggest that these sensory processing deficits may contribute to higher-order cognitive deficits in working memory, executive functioning, and attention.

Cuttlefish camouflage: The effects of substrate contrast and size in evoking uniform, mottle or disruptive body patterns

Cuttlefish are cephalopod molluscs that achieve dynamic camouflage by rapidly extracting visual information from the background and neurally implementing an appropriate skin (or body) pattern. We investigated how cuttlefish body patterning responses are influenced by contrast and spatial scale by varying the contrast and the size of checkerboard backgrounds. We found that: (1) at high contrast levels, cuttlefish body patterning depended on check size; (2) for low contrast levels, body patterning was independent of “check” size; and (3) on the same check size, cuttlefish fine-tuned the contrast and fine structure of their body patterns, in response to small contrast changes in the background. Furthermore, we developed an objective, automated method of assessing cuttlefish camouflage patterns that quantitatively differentiated the three body patterns of uniform/stipple, mottle and disruptive. This study draws attention to the key roles played by background contrast and particle size in determining an effective camouflage pattern.

The quantification of fingerprint quality using a relative contrast index

Research into fingermark enhancement techniques has traditionally used visual comparisons and qualitative methods to assess their effectiveness based on the quality of the developed fingermark. However, with increasing research into the optimisation of these techniques the need for a quantitative evaluative method has arisen. Parameters for acceptable fingerprint quality are not well defined and generally encompass clear, sharp edges and high levels of contrast between the fingermark ridges and background material. Using these current parameters, a conclusive measurement of fingerprint quality and thus the effectiveness of development techniques cannot be achieved. This study presents a model through which an aspect of fingerprint quality can be objectively and impartially measured based on a relative contrast index, constructed through measuring the reflective intensity of the fingermark ridges against the background material. Using a fibre-optic spectrophotometer attached to a microscope with axial illumination, the intensity counts of the ridge detail and background material were measured and a logarithmic contrast index constructed. The microscope and spectrophotometer parameters were experimentally tested using a standard colour resolution chart with known reflective properties. The protocol was successfully applied to four sample groups: black inked fingerprints on white paper; latent fingermarks on white paper developed separately with ninhydrin and physical developer; and fingermarks in blood deposited on white tiles and enhanced with amido black. The contrast indices obtained quantitatively reflect the level of contrast and provide an indication of fingerprint quality through a numerical representation rather than previous qualitative methods. It has been suggested that the proposed method of fingerprint quantification may be viable for application in the forensic research arena as it allows the definitive measurement of contrast to aid the evaluation of fingermark detection and enhancement techniques.

Optimizing geochemical threshold selection while evaluating exploration techniques using a minimum hypergeometric probability method

Hypergeometric statistics have recently been used to establish a quantitative measure of performance for geochemical exploration techniques over known mineral occurrences. Using this method, the effectiveness of new exploration techniques can be compared objectively with conventional approaches. Sample site classification is the basis on which survey results are compared. This performance measure requires prior knowledge of the location of mineralization and a model for element dispersion from the primary into the secondary environment. This information allows assignment of sample sites that should give ‘anomalous’ (e.g. those overlying mineralized zones) or ‘background’ results to be identified prior to the orientation survey. Previous application of hypergeometric statistics requires that a high contrast exists between ‘anomalous’ and ‘background’ subpopulations in the geochemical orientation data so that there is no uncertainty in the classification of samples. In this paper, a refinement is developed that allows consideration of geochemical variables that do not exhibit this required high level of geochemical contrast, that is, where ‘anomalous’ and ‘background’ subpopulations exhibit significant overlap. This refinement involves determining the hypergeometric probabilities of obtaining the same result at random ( P ( x )) for a range of geochemical thresholds, instead of only one threshold (i.e. the one used to classify anomalous and background samples in cases with high geochemical contrast). At each threshold, different numbers of samples will be classified as ‘anomalous’ and different numbers of ‘anomalous’ samples will occur at ‘anomalous’ sites. As a consequence, the resulting random hypergeometric probabilities will change with threshold level. Using a range of thresholds to classify the geochemical orientation survey results allows identification of the minimum hypergeometric probability (MHP) for the dataset. Using this threshold, the classification of anomalies will bear the least resemblance to what would be expected if the survey results were generated at random. Employing this refined MHP approach, one can simultaneously evaluate the effectiveness of an exploration method, and select the threshold that optimally classifies anomalous and background samples.

Fusion Viewer: A New Tool for Fusion and Visualization of Multimodal Medical Data Sets

A new application, Fusion Viewer, available for free, has been designed and implemented with a modular object-oriented design. The viewer provides both traditional and novel tools to fuse 3D data sets such as CT (computed tomography), MRI (magnetic resonance imaging), PET (positron emission tomography), and SPECT (single photon emission tomography) of the same subject, to create maximum intensity projections (MIP) and to adjust dynamic range. In many situations, it is desirable and advantageous to acquire biomedical images in more than one modality. For example, PET can be used to acquire functional data, whereas MRI can be used to acquire morphological data. In some situations, a side-by-side comparison of the images provides enough information, but in most of the cases it may be necessary to have the exact spatial relationship between the modalities presented to the observer. To accomplish this task, the images need to first be registered and then combined (fused) to create a single image. In this paper, we discuss the options for performing such fusion in the context of multimodal breast imaging. Additionally, a novel spline-based dynamic range technique is presented in detail. It has the advantage of obtaining a high level of contrast in the intensity range of interest without discarding the intensity information outside of this range while maintaining a user interface similar to the standard window/level windowing procedure.

High-Resolution Neurometabolic Coupling in the Lateral Geniculate Nucleus

The relationships between neural and metabolic processes in activated brain regions are central to the interpretation of noninvasive imaging. To examine this relationship, we have used a specialized sensor to measure simultaneously tissue oxygen changes and neural activity in colocalized regions of the cat's lateral geniculate nucleus (LGN). Previous work with this sensor has shown that a decrease or increase in tissue oxygen can be elicited by selective control of the location and extent of neural activation in the LGN. In the current study, to evaluate the temporal integration and homogeneity of neurometabolic coupling, we have determined the relationship between multiunit extracellular neural activity and tissue oxygen responses to visual stimuli of various durations and contrasts. Our results show that the negative but not the positive oxygen response changes in an approximately linear manner with stimulus duration. The relationship between the negative oxygen response and neural activity is relatively constant with stimulus duration. Moreover, both negative and positive oxygen responses saturate at high stimulus contrast levels. Coupling between neural activity and negative oxygen responses is well described by a power law function. These results help elucidate differences between the initial negative and subsequent positive metabolic responses and may be directly relevant to questions concerning brain mapping with functional magnetic resonance imaging.

What is the minimum field of view required for efficient navigation?

Critical points were computed to determine the minimum field of view (FOV) size required for efficient navigation. Navigation performance in 20 normally sighted subjects was assessed using an immersive virtual environment. Subjects were instructed to walk through a virtual forest to a target tree as quickly as possible without hitting any obstacles (trees, boulders, and holes). The navigation task was performed in three FOV and image contrast conditions under binocular, monocular, chromatic and achromatic viewing conditions. FOV was constricted to 10°, 20° and 40° diameter and average image contrast was nominally high (11%), medium (6%) and low (3%). Navigation performance was scored as latency in walk initiation, walk time to reach goal and the number of obstacle contacts. The results revealed a linear relationship between log FOV and the two time measures, log latency and log walk time. The slopes of the linear regressions for log latency and log walk time ranged between –0.11 and –0.41. Critical points were computed from the non-linear relationships found between the number of obstacle contacts and FOV. The critical points for efficient navigation were FOVs of 32.1°, 18.4° and 10.9° (diam.) for low, medium and high image contrast levels, respectively, highlighting the importance of contrast on the size of the FOV required for efficient navigation. Neither binocularity nor image chromaticity significantly affected navigation performance. The findings of this study have important implications in the design and prescription of head mounted displays intended to augment navigation performance.

Refractive Error and Visual Acuity Changes in Orthokeratology Patients

To evaluate the refractive error and visual acuity (VA) at various contrast levels in the two eyes of overnight orthokeratology (ortho-k) subjects, and to compare their postortho-k VA with the best corrected VA of spectacle-wearing control subjects matched for age, gender, and initial refractive error. Distance postortho-k uncorrected and best corrected logMAR VA at four different contrast levels of 31 ortho-k (test) subjects (aged 7-35 years old) and the best corrected VA of 31 spectacle-wearing (control) subjects were measured and compared using the Waterloo Four-Contrast LogMAR VA Chart, which incorporated four sets of letters at different contrast levels: 90%, 48%, 21%, and 7%. Noncycloplegic manifest refractive error was measured in both eyes. The mean +/- SD percentage reductions in spherical equivalent achieved in the current study were 92% +/- 11% in the better eye and 84% +/- 14% in the worse eye of the test subjects. Postortho-k uncorrected VAs were significantly correlated with the residual overall blurring strength (length of the vector representing the residual refractive error) in both eyes at all contrast levels. The mean postortho-k uncorrected VA in the better eye were 0.00 +/- 0.11, 0.08 +/- 0.11, 0.21 +/- 0.12, and 0.46 +/- 0.13 with the 90%, 48%, 21%, and 7% contrast charts, respectively. These were comparable to the best corrected VA of the better eye of the control group with the 90% (-0.03 +/- 0.07) and 48% contrast charts (0.03 +/- 0.09), but worse than those of the control group with the 21% (0.13 +/- 0.10) and 7% (0.35 +/- 0.13) contrast charts. Postortho-k VA, with the four different contrast charts, improved by 0.07 to 0.12 log units in the better eye and 0.15 to 0.18 log units in the worse eye after correction of the residual refractive error; the improved VA was comparable to the best corrected VA of the control group. Postortho-k visual outcomes were compromised primarily due to the presence of residual refractive error. Although the uncorrected postortho-k VA was comparable to the best corrected VA of the spectacle wearers at high-contrast levels, it was worse at low-contrast levels and caused a significant between-eye difference at all contrast levels. Therefore, we suggested that monocular VA at high- and low-contrast levels should be evaluated for ortho-k patients.

Iterative design of multiplane holograms: experiments and applications

We present an experimental confirmation of optical properties of multiplane holograms designed with our novel iterative method. The method allows encoding many input intensity distributions into a single phase-only hologram. The object planes can be placed at variable distances, and their content is fully customizable. The reconstructed three-dimensional (3D) scenes exhibit high contrast and low noise level in all designed image planes. The results of numerical simulations are compared with those of a reconstruction in an optical setup. Holograms for optical reconstructions were manufactured using two methods: photographic and electron beam lithography (EBL). Experimental results achieved with both methods are compared. We present our research on a new class of iterative holograms, containing up to eleven object planes, designed in close distance to each other. The elements exhibit unusual light focusing possibilities and extraordinary imaging properties, thus introducing a number of possible practical applications, which are discussed.

MDCT of the abdomen

Multi-detector row computed tomography (MDCT) enables fast and thin acquisition of the abdominal anatomy. This allows multi-pass multi-planar studies that can be obtained during defined circulatory phases. When bolus timing is adequate, arterial phases with high contrast levels provide "free lunch" CT angiographies eliminating the need for diagnostic angiographies in most cases. In addition to established clinical indications for abdominal CT such as preoperative MDCT of the liver or pancreas, MDCT of the abdomen is especially gaining ground in the work up for acute abdominal pain and abdominal trauma and is opening new indications for MDCT of the gastrointestinal tract. Indications for gastrointestinal MDCT include tumors, bleeding and ischemia of the small and large bowel as well as diverticulitis. The question of whether to use positive or negative contrast material for bowel distention for MDCT of the gastrointestinal tract is still a controversial issue. In selected cases, modifying the protocol to perform a "CT enteroclysis" might improve sensitivity and specificity in depicting small bowel tumors or inflammatory changes such as in Crohn's disease. The most common gastrointestinal mesenchymal tumor is the gastrointestinal stromal tumor (GIST). MDCT may show hypervascular submucosal masses. Acute gastrointestinal (GI) bleeding is common with patients presenting with melena, hematemesis or hematochezia. In addition to the established initial work-up MDCT is beginning to establish itself for this indication. It may be especially helpful in the work up of obscure bleeding. Another relatively rare but important cause for acute abdominal pain is mesenteric ischemia. It may be caused by many conditions and may mimic various intestinal diseases. Bowel ischemia severity ranges from transient superficial changes of the intestinal mucosa to life-threatening transmural bowel wall necrosis. CT can demonstrate changes in ischemic bowel segments accurately, is often helpful in determining the primary cause of ischemia, and can demonstrate important coexistent findings or complications.

STEM Imaging Optimized for Biological Specimen

Within STEM mode the signal of both the inelastic and elastic scattered electrons contribute to the contrast of the image. This is an advantage compared to zero-loss filtered TEM imaging, where a significant amount of electrons are not contributing to the image formation, especially in thicker samples, due to multiple scattering events. In combination with an ultra sensitive HAADF-detector the scattering angle can be varied to obtain the best contrast, respectively signal to noise ratio, whatever gives more flexibility to optimize the signal compared to CCD camera acquisition. With CCD cameras these parameters are fixed. Especially, with radiation sensitive specimens it is advantageous to reduce a possible temperature increase by using a small area of illumination [1]. Thus, for a ten-fold increase in current density the temperature rise is much smaller for a focused probe than with flood-beam illumination. This is known as part of the “beam induced movement”. Minimizing this effect, directly increases the dose stability of the sample in STEM compared to TEM imaging. Furthermore, as a beneficial side effect, it improves the signal-to-noise ratio (SNR) in the images [2]. The sum of these effects clearly improves the image quality of negative stained viruses and sectioned biological material, which is shown in the comparison of STEM and TEM images of biological samples from the same area (Fig. 1, 2 and 3). It is striking to see the high contrast level of biological material in the STEM images. The lower frequencies are better preserved and though this will improve and facilitate, e.g. the alignment of individual particles for single particle analysis or for the alignment of tomographic tilt series. Finally, in modern microscopes, STEM in combination with low dose acquisition schemes is already implemented [3,4], easy to use and comparable to the convenient handling of a TEM in regular bright field mode. Thus, it increases the flexibility not only in tuning and adjusting of the microscope, but also in the possibility to choose the appropriate illumination mode and camera conditions enabling the user to get the best results from investigations of biological specimens.

Temporal summation and reaction times for detecting gratings

The effects of the duration of presentation of stimuli on the reaction time for detection of gratings of different spatial frequencies were studied. The product of contrast and duration (the contrast "energy") was kept constant for each duration. These experiments showed that at near-threshold "energies," reaction times did not change with increases in the duration to 15 and 30 msec at low and high spatial frequencies respectively. At high contrast "energies," this critical duration was identical (15 msec) for both low and high spatial frequencies. The effects of the duration of a linearly increasing front for grating presentation on detection reaction times were also studied. Increases in the duration of the front (to 60 msec) significantly lengthened the reaction time at low spatial frequencies at all contrast levels studied. Conversely, at high spatial frequencies, a significant increase in the reaction time was seen only at high contrast levels. These results suggest that reaction time depends on two types of mechanisms (phasic and tonic) at near-threshold contrast and one type (phasic) at high contrast.