Cognition, ocular accommodation, and cardiovascular function in emmetropes and late-onset myopes

PURPOSE. To investigate objectively and noninvasively the role of cognitive demand on autonomic control of systemic cardiovascular and ocular accommodative responses in emmetropes and myopes of late-onset. METHODS. Sixteen subjects (10 men, 6 women) aged between 18 and 34 years (mean ± SD: 22.6 ± 4.4 years), eight emmetropes (EMMs; mean spherical equivalent [MSE] refractive error ± SD: 0.05 ± 0.24 D) and eight with late-onset myopia (LOMs; MSE ± SD: -3.66 ± 2.31 D) participated in the study. Subjects viewed stationary numerical digits monocularly within a Badal optical system (at both 0.0 and -3.0 D) while performing a two-alternative, forced-choice paradigm that matched cognitive loading across subjects. Five individually matched cognitive levels of increasing difficulty were used in random order for each subject. Five 20-second, continuous-objective recordings of the accommodative response measured with an open-view infrared autorefractor were obtained for each cognitive level, whereas simultaneous measurement of heart rate was continuously recorded with a finger-mounted piezoelectric pulse transducer for 5 minutes. Fast Fourier transformation of cardiovascular function allowed the relative power of the autonomic components to be assessed in the frequency domain, whereas heart period gave an indication of the time-domain response. RESULTS. Increasing the cognitive demand led to a significant reduction in the accommodative response in all subjects (0.0 D: by -0.35 ± 0.33 D; -3.0 D: by -0.31 ± 0.40 D, P < 0.001). The greater lag of LOMs compared with EMMs was not significant (P = 0.07) at both distance (0.38 ± 0.35 D) and near (0.14 ± 0.42 D). Mean heart period reduced with increasing levels of workload (P < 0.0005). LOMs exhibited a relative elevation in sympathetic system activity compared to EMMs. Within refractive groups, however, accommodative shifts with increasing cognition correlated with parasympathetic activity (r = 0.99, P < 0.001), more than with sympathetic activity (r = 0.62, P > 0.05). CONCLUSIONS. In an equivalent workload paradigm, increasing cognitive demand caused a reduction in accommodative response that was attributable principally to a concurrent reduction in the relative power of the parasympathetic component of the autonomic nervous system (ANS). The disparity in accommodative response between EMMs and LOMs, however, appears to be augmented by changes in the sympathetic nervous component of the systemic ANS. Copyright © Association for Research in Vision and Ophthalmology.

Autonomic correlates of ocular accommodation and cardiovascular function

PURPOSE: To evaluate the hypothesis that objective measures of open- and closed-loop ocular accommodation are related to systemic cardiovascular function, and ipso facto autonomic nervous system activity. METHODS: Sixty subjects (29 male; 31 female) varying in age from 18 to 33 years (average: 20.3 +/- 2.9 years) with a range of refractive errors [mean spherical equivalent (MSE): -7.12 to +1.82 D] participated in the study. Five 20-s continuous objective recordings of the accommodative response, measured with an open-view IR autorefractor (Shin-Nippon SRW-5000), were obtained for a variety of open- and closed-loop accommodative demands while simultaneous continuous measurement of heart rate was recorded with a finger-mounted piezo-electric pulse transducer for 5 min. Fast Fourier Transformation of cardiovascular function allowed the absolute and relative power of the autonomic components to be assessed in the frequency-domain, whereas heart period gave an indication of the time-domain response. RESULTS: Increasing closed-loop accommodative demand led to a concurrent increase in heart rate of approximately 2 beats/min for a 4.0 D increase in accommodative demand. The increase was attributable to a reduction in the absolute (p < 0.05) and normalised (p < 0.001) input of the systemic parasympathetic nervous system, and was unaffected by refractive group. The interaction with refractive group failed to reach significance. CONCLUSIONS: For sustained accommodation effort, the data demonstrate covariation between the oculomotor and cardiovascular systems which implies that a near visual task can significantly influence cardiovascular behaviour. Accommodative effort alone, however, is not a sufficient stimulus to induce autonomic differences between refractive groups. The data suggest that both the oculomotor and cardiovascular systems are predominantly attributable to changes in the systemic parasympathetic nervous system.

Nuclear magnetic resonance spectroscopy and ultrasound

The work described in this thesis is directed to the examination of the hypothesis that ultrasound may be used to perturb molecular motion in the liquid phase. These changes can then be detected by nuclear magnetic resonance (NMR) in spin-lattice and spin-spin relaxation times. The objective being to develop a method capable of reducing the pulsed NMR acquisition times of slowly relaxing nuclei. The thesis describes the theoretical principles underlying both NMR spectroscopy and ultrasonics with particular attention being paid to factors that impinge on testing the above hypothesis. Apparatus has been constructed to enable ultrasound at frequencies between 1 and 10 mega-hertz with a variable power up to 100W/cm-2 to be introduced in the NMR sample. A broadband high frequency generator is used to drive PZT piezo-electric transducer via various transducer to liquid coupling arrangements. A commercial instrument of 20 kilo-hertz has also been employed to test the above hypothesis and also to demonstrate the usefulness of ultrasound in sonochemistry. The latter objective being, detection of radical formation in monomer and polymer ultrasonic degradation. The principle features of the results obtained are: Ultrasonic perturbation of T1 is far smaller for pure liquids than is for mixtures. The effects appear to be greater on protons (1H) than on carbon-13 nuclei (13C) relaxation times. The observed effect of ultrasonics is not due to temperature changes in the sample. As the power applied to the transducer is progressively increased T1 decreases to a minimum and then increases. The T1's of the same nuclei in different functional groups are influenced to different extents by ultrasound. Studies of the 14N resonances from an equimolar mixture of N, N-dimethylformamide and deuterated chloroform with ultrasonic frequencies at 1.115, 6, 6.42 and 10 MHz show that as the frequency is increased the NMR signal to noise ratio decreases to zero at the Larmor frequency of 6.42 MHz and then again rises. This reveals the surprising indication that an effect corresponding to nuclear acoustic saturation in the liquid may be observable. Ultrasonic irradiation of acidified ammonium chloride solution at and around 6.42 MHz appears to cause distinctive changes in the proton-nitrogen J coupling resonance at 89.56 MHz. Ultrasonic irradiation of N, N-dimethylacetamide at 2 KHz using the lowest stable power revealed the onset of coalescence in the proton spectrum. The corresponding effect achieved by direct heating required a temperature rise of approximately 30oC. The effects of low frequency (20 KHz) on relaxation times appear to be nil. Detection of radical formation proved difficult but is still regarded as the principle route for monomer and polymer degradation. The initial hypothesis is considered proven with the results showing significant changes in the mega-hertz region and none at 20 KHz.

The environmental control of reproduction in the female dace, Leuciscus leuciscus

An investigation was made into the nature and control of the annual reproductive cycle of the dace, Leuciscus leuciscus. It includes 1) a study of the natural reproductive cycle, 2) the use of Carp Pituitary Extract (CPE) to induce final maturation and ovulation in captive fish, 3) the effect of artificial light treatments on ovarian development and 4) the measurement of serum melatonin levels under different photoperiod regimes. Ovarian development was monitored by endocrinological data, notably serum cycles of 17-oestradiol (E2), testosterone (T), and calcium (as an index of vitellogenin), oocyte diameter, the gonadosomatic index and histological studies of the ovary. Under natural conditions, ovarian development can broadly be divided into 4 stages: 1) oogenesis which occurs immediately after spawning; 2) a primary growth phase (previtellogenic growth) prevalent between spawning and June; 3) a secondary growth phase (yolk vesicle plus vitellogenic growth) occurring between June and December and 4) final maturation and ovulation which occurs in mid-March. During the annual ovarian cycle, the sex steroids E2 and T showed two clear elevations. The first occurred initially in April followed by a rise in serum calcium levels. This subsequently initiated the appearance of yolk granules in the oocytes in June. The second rise occurred in September and levels were maintained until December, after which there was a decline in serum E2 levels. It is proposed that in the dace, high serum E2 levels between September and December were required to maintain vitellogenin production and therefore its uptake into the developing oocytes which occurred during this time, albeit at a slower rate than in the summer months. After December, prior to final maturation, whereas serum E2 and calcium levels declined, serum T levels remained elevated. In captivity, final maturation beyond the germinal vesicle migration stage failed to occur suggesting that the stimuli required for these events were absent. However ovulation could be induced by a single injection of CPE, which induced ovulation between 6 and 14 hours after treatment. Endocrine events associated with the artificial induction of spawning included a rise in serum levels of E2, T and the maturation inducing steroid 1720-dihydroxy progesterone. Photoperiodic manipulation demonstrated that whereas short or increasing daylengths were stimulatory to ovarian development, long days delayed development. Changes from long to short and constant short daylengths early in the reproductive cycle advanced maturation (up to 5 months), suggesting that the stimulus for ovarian development and maturation was a short day. However, experiments conducted later in the reproductive cycle demonstrated that only a simulated ambient photoperiod could induce final maturation. It is proposed therefore that under natural conditions the environmental stimulus for ovarian development and final maturation are short and increasing daylengths respectively. Further support that photoperiod is the dominant timing cue in this species was provided by the pattern of serum melatonin levels. Under different photoperiod treatments, serum melatonin, which is believed to be the chemical transducer of photoperiodic information (similar to other photoperiodic species) was elevated for the duration of the dark phase, indicating that the dace at least has the ability to `measure' changes in daylength.

A study of load support and other criteria appropriate to the selection of industrial conveyor belts

A study of conveying practice demonstrates that belt conveyors provide a versatile and. much-used method of transporting bulk materials, but a review of belting manufacturers' design procedures shows that belt design and selection rules are often based on experience with all-cotton belts no longer in common use, and are net completely relevant to modern synthetic constructions. In particular, provision of the property "load support", which was not critical with cotton belts, is shown to determine the outcome of most belt selection exercises and lead to gross over specification of other design properties in many cases. The results of an original experimental investigation into this property, carried out to determine the belt and conveyor parameters that affect it, how the major role that belt stiffness plays in its provision; the basis for a belt stiffness test relevant to service conditions is given. A proposal for a more rational method of specifying load support data results from the work, but correlation of the test results with service performance is necessary before the absolute toad support capability required from a belt for given working conditions can be quantified. A study to attain this correlation is the major proposal for future work resulting from the present investigation, but a full review of the literature on conveyor design and a study of present practice within the belting industry demonstrate other, less critical, factors that could profitably be investigated. It is suggested that the most suitable method of studying these would be a rational data collection system to provide information on various facets of belt service behaviour; a basis for such a system is proposed. In addition to the work above, proposals for simplifying the present belt selection methods are made and a strain transducer suitable for use in future experimental investigations is developed.

Mechanism of protein catobolism in skeletal muscle during cancer cachexia

Cancer cachexia is characterised by selective depletion of skeletal muscle protein reserves. The ubiquitin-proteasome proteolytic pathway has been shown to be responsible for muscle wasting in a range of cachectic conditions including cancer cachexia. To establish the importance of this pathway in muscle wasting during cancer (and sepsis), a quantitative competitive RT-PCR (QcRT-PCR) method was developed to measure the mRNA levels of the proteasome sub units C2a and C5ß and the ubiquitin-conjugating enzyme E214k. Western blotting was also used to measure the 20S proteasome and E214k protein expression. In vivo studies in mice bearing a cachexia inducing murine colon adenocarcinoma (MAC16) demonstrated the effect of progressive weight loss on the mRNA and protein expression for 20S proteasome subunits, as well as the ubiquitin-conjugating enzyme, E214k, in gastrocnemius and pectoral muscles. QcRT-PCR measurements showed a good correlation between expression of the proteasome subunits (C2 and CS) and the E214k enzyme mRNA and weight loss in gastrocnemius muscle, where expression increased with increasing weight loss followed by a decrease in expression at higher weight losses (25-27%). Similar results were obtained in pectoral muscles, but with the expression being several fold lower in comparison to that in gastrocnemius muscle, reflecting the different degrees of protein degradation in the two muscles during the process of cancer cachexia. Western blot analysis of 20S and E214k protein expression followed a similar pattern with respect to weight loss as that found with mRNA. In addition, mRNA and protein expression of the 20S proteasome subunits and E214k enzyme was measured in biopsies from cachectic cancer patients, which also showed a good correlation between weight loss and proteasome expression, demonstrating a progressive increase in expression of the proteasome subunits and E214k mRNA and protein in cachectic patients with progressively increasing weight loss.The effect of the cachexia-inducing tumour product PIF (proteolysis inducing factor) and 15-hydroxyeicosatetraenoic acid (15-HETE), the arachidoinic acid metabolite (thought to be the intracellular transducer of PIF action) has also been determined. Using a surrogate model system for skeletal muscle, C2C12 myotubes in vitro, it was shown that both PIF and 15-HETE increased proteasome subunit expression (C2a and C5ß) as well as the E214k enzyme. This increase gene expression was attenuated by preincubation with EPA or the 15-lipoxygenase inhibitor CV-6504; immunoblotting also confirmed these findings. Similarly, in sepsis-induced cachexia in NMRI mice there was increased mRNA and protein expression of the 20S proteasome subunits and the E214k enzyme, which was inhibited by EPA treatment. These results suggest that 15-HETE is the intracellular mediator for PIF induced protein degradation in skeletal muscle, and that elevated muscle catabolism is accomplished through upregulation of the ubiquitin-proteasome-proteolytic pathway. Furthermore, both EPA and CV -6504 have shown anti-cachectic properties, which could be used in the future for the treatment of cancer cachexia and other similar catabolic conditions.

Use of ultrasonic agitation in iron group alloy electrodepositions

The effects of ultrasonic agitation on deposition from two iron group alloy plating solutions, nickel-cobalt and bright nickel-iron, have been studied. Comparison has been made with deposits plated from the same solutions using controlled air agitation. The ultrasonic equipment employed had a fixed frequency of 13 KHz but the power output from each transducer was variable up to a maximum of 350 watts. The effects of air and ultrasonic agitation on hardness, ductility, tensile strength, composition, structure, surface topography, limiting current density, cathode current efficiency and macro-throwing power were determined. Transmission and scanning electron microscopy, electron-probe microanalysis and atomic absorption spectrophotometry have been employed to study the nickel alloy deposits produced. The results obtained show that the use of Ultrasonics increased significantly the hardness of both alloy deposits and altered their composition by decreasing the cobalt and iron contents from nickel-cobalt and nickeliron solutions respectively. The ductility of coatings improved but the tensile strength did not change very much. Ultrasonic agitation gave larger grained deposits than air and they seemed to have a lower stress. Dull cobalt-nickel deposits had a similar pyramidal surface topography regardless of the type of agitation but the bright appearance of the nickel-iron was destroyed by ultrasonic agitation; an unusual ribbed pattern was produced. The use of ultrasonic agitation permitted approximately a twofold increase in the plating current density at which sound deposits could be achieved but there was only a slight increase in cathode current efficiency. Macro-throwing power of the solutions was increased slightly by the use of ultrasonic agitation. ultrasonic agitation is an expensive means of agitating plating Solutions and would be worthwhile only if significant improvements in properties could be achieved. The simultaneous improvement in hardness and ductility is a novel feature that should have useful engineering applications.

Behavioural correlates of ocular accomodation and the autonomic nervous system

The binding issue of th is thesis was the examination of workload, induced by relinotopic and spatiotopic stimuli, on both the ocu lomotor and cardiovascular systems together with investigating the covariation between the two systems - the 'eye-heart' link. Further, the influence of refractive error on ocular accommodation and cardiovascular function was assessed. A clinical evaluation was undertaken to assess the newly available open-view infrared Shin-Nippon NVision-K 5001 optometer, its benefit being the capability to measure through pupils = 2.3 mm. Measurements of refractive error taken with the NVision-K were found to be both accurate (Difference in Mean Spherical Equivalent: 0.14 ± 0.35 D; p = 0.67) and repeatable when compared to non-cycloplegic subjective refraction. Due to technical difficulties, however, the NVision-K could not be used for the purpose of the thesis, as such, measures of accommodation were taken using the continuously recording Shin-Nippon SRW-5000 openview infrared optometer, coupled with a piezo-electric finger pulse transducer to measure pulse. Heart rate variability (HRV) was spectrally analysed to determine the systemic sympathetic and parasympathetic components of the autonomic nervous system (ANS). A large sample (n = 60), cross-sectional study showed late-onset myopes (LOMs) display less accurate responses when compared to other refractive groups at high accommodative demand levels (3 .0 0 and 4.0D). Tonic accommodation (TA) was highest in the hypermetropes, fo llowed by emmetropes and early-onset myopes while the LOM subjects demonstrated statistically significant lower levels of TA. The root-meansquare (RMS) value of the accommodative response was shown to amplify with increased levels of accommodative demand. Changes in refractive error only became significant between groups at higher demand levels (3.0 D and 4.0 D) with the LOMs showing the largest magnification in oscilIations. Examination of the stimulus-response cross-over point with the unit ratio line and TA showed a correlation between the two (r = 0.45, p = 0.001), where TA is approximately twice the dioptric value of the stimulus-response cross-over point. Investigation of the relationship between ocular accommodation and systemic ANS function demonstrated covariation between the systems. Subjects with a faster heart rate (lower heart period) tended to have a higher TA value (r = -0.27, p < 0.05). Further, an increase in accommodative demand accompanies a faster heart rate. The influence of refractive error on the cardiovascular response to changes in accommodative demand, however, was equivocal. Examination of the microfluctuations ofacconunodation demonstrated a correlation between the temporal frequency location of the accommodative high Frequency component (HFC) and the arterial pulse frequency. The correlation was present at a range of accommodative demands from 0.0 D to 4.0 D and in all four refractive groups, suggesting that the HFC was augmented by physiological factors. Examination of the effect of visual cognition on ocular accommodation and the ANS confirmed that increasing levels of cognition affect the accommodative mechanism. The accommodative response shifted away from the subject at both near and far. This shift in accommodative response accompanied a decay in the systemic parasympathetic innervation to the heart. Differences between refractive groups also existed with LOMs showing less accurate responses compared to emmetropes. This disparity, however, appeared to be augmented by the systemic sympathetic nervous system. The investigations discussed explored Ihe role of oculomotor and cardiovascular fu nction in workload enviromnents, providing evidence for a behavioural link between the cardiovascular and oculomotor systems.

The application of robotics to the turbine blade encapsulation process

A survey of the existing state-of-the-art of turbine blade manufacture highlights two operations that have not been automated namely that of loading of a turbine blade into an encapsulation die, and that of removing a machined blade from the encapsulation block. The automation of blade decapsulation has not been pursued. In order to develop a system to automate the loading of an encapsulation die a prototype mechanical handling robot has been designed together with a computer controlled encapsulation die. The robot has been designed as a mechanical handling robot of cylindrical geometry, suitable for use in a circular work cell. It is the prototype for a production model to be called `The Cybermate'. The prototype robot is mechanically complete but due to unforeseen circumstances the robot control system is not available (the development of the control system did not form a part of this project), hence it has not been possible to fully test and assess the robot mechanical design. Robot loading of the encapsulation die has thus been simulated. The research work with regard to the encapsulation die has focused on the development of computer controlled, hydraulically actuated, location pins. Such pins compensate for the inherent positional inaccuracy of the loading robot and reproduce the dexterity of the human operator. Each pin comprises a miniature hydraulic cylinder, controlled by a standard bidirectional flow control valve. The precision positional control is obtained through pulsing of the valves under software control, with positional feedback from an 8-bit transducer. A test-rig comprising one hydraulic location pin together with an opposing spring loaded pin has demonstrated that such a pin arrangement can be controlled with a repeatability of +/-.00045'. In addition this test-rig has demonstrated that such a pin arrangement can be used to gauge and compensate for the dimensional error of the component held between the pins, by offsetting the pin datum positions to allow for the component error. A gauging repeatability of +/- 0.00015' was demonstrated. This work has led to the design and manufacture of an encapsulation die comprising ten such pins and the associated computer software. All aspects of the control software except blade gauging and positional data storage have been demonstrated. Work is now required to achieve the accuracy of control demonstrated by the single pin test-rig, with each of the ten pins in the encapsulation die. This would allow trials of the complete loading cycle to take place.

The application of parallel processing methods to vibrational analysis

The trend in modal extraction algorithms is to use all the available frequency response functions data to obtain a global estimate of the natural frequencies, damping ratio and mode shapes. Improvements in transducer and signal processing technology allow the simultaneous measurement of many hundreds of channels of response data. The quantity of data available and the complexity of the extraction algorithms make considerable demands on the available computer power and require a powerful computer or dedicated workstation to perform satisfactorily. An alternative to waiting for faster sequential processors is to implement the algorithm in parallel, for example on a network of Transputers. Parallel architectures are a cost effective means of increasing computational power, and a larger number of response channels would simply require more processors. This thesis considers how two typical modal extraction algorithms, the Rational Fraction Polynomial method and the Ibrahim Time Domain method, may be implemented on a network of transputers. The Rational Fraction Polynomial Method is a well known and robust frequency domain 'curve fitting' algorithm. The Ibrahim Time Domain method is an efficient algorithm that 'curve fits' in the time domain. This thesis reviews the algorithms, considers the problems involved in a parallel implementation, and shows how they were implemented on a real Transputer network.

Low level feature detection in human vision

Influential models of edge detection have generally supposed that an edge is detected at peaks in the 1st derivative of the luminance profile, or at zero-crossings in the 2nd derivative. However, when presented with blurred triangle-wave images, observers consistently marked edges not at these locations, but at peaks in the 3rd derivative. This new phenomenon, termed ‘Mach edges’ persisted when a luminance ramp was added to the blurred triangle-wave. Modelling of these Mach edge detection data required the addition of a physiologically plausible filter, prior to the 3rd derivative computation. A viable alternative model was examined, on the basis of data obtained with short-duration, high spatial-frequency stimuli. Detection and feature-making methods were used to examine the perception of Mach bands in an image set that spanned a range of Mach band detectabilities. A scale-space model that computed edge and bar features in parallel provided a better fit to the data than 4 competing models that combined information across scale in a different manner, or computed edge or bar features at a single scale. The perception of luminance bars was examined in 2 experiments. Data for one image-set suggested a simple rule for perception of a small Gaussian bar on a larger inverted Gaussian bar background. In previous research, discriminability (d’) has typically been reported to be a power function of contrast, where the exponent (p) is 2 to 3. However, using bar, grating, and Gaussian edge stimuli, with several methodologies, values of p were obtained that ranged from 1 to 1.7 across 6 experiments. This novel finding was explained by appealing to low stimulus uncertainty, or a near-linear transducer.

Targeting the glycoprotein 130 receptor subunit to control pain and inflammation

The glycoprotein 130 (gp130) is a shared signal-transducing-membrane-associated receptor for several hematopoietic cytokines. Its activation is implicated in pain and in a variety of diseases via signaling of proinflammatory cytokines. These include interleukin-6 (IL-6) subfamily cytokines, many of which play important roles in the pathogenesis of diseases such as rheumatoid arthritis, Castleman's disease, and Kaposi's sarcoma. Several strategies have been developed to block gp130-receptor-mediated signaling. These include the application of monoclonal antibodies, the creation of mutant form(s) of the gp130 with increased binding affinity for such ligands as IL-6/sIL-6R complex, and the generation of antagonists by selective mutagenesis of the specific cytokine/gp130 receptor binding site(s). Other strategies include targeting gp130-mediated signaling pathways such as that involving signal transducer and activator of transcription-3. This review provides a summary of the latest research pertaining to the role of gp130 in the pathogenesis of inflammatory and other diseases in which the gp130 receptor is implicated. An overview of antagonists targeting the gp130 receptor is included with particular emphasis on their mechanism of action and their limitations and potential for therapeutic application.

The slope of the psychometric function and non-stationarity of thresholds in spatiotemporal contrast vision

The slope of the two-interval, forced-choice psychometric function (e.g. the Weibull parameter, ß) provides valuable information about the relationship between contrast sensitivity and signal strength. However, little is known about how or whether ß varies with stimulus parameters such as spatiotemporal frequency and stimulus size and shape. A second unresolved issue concerns the best way to estimate the slope of the psychometric function. For example, if an observer is non-stationary (e.g. their threshold drifts between experimental sessions), ß will be underestimated if curve fitting is performed after collapsing the data across experimental sessions. We measured psychometric functions for 2 experienced observers for 14 different spatiotemporal configurations of pulsed or flickering grating patches and bars on each of 8 days. We found ß ˜ 3 to be fairly constant across almost all conditions, consistent with a fixed nonlinear contrast transducer and/or a constant level of intrinsic stimulus uncertainty (e.g. a square law transducer and a low level of intrinsic uncertainty). Our analysis showed that estimating a single ß from results averaged over several experimental sessions was slightly more accurate than averaging multiple estimates from several experimental sessions. However, the small levels of non-stationarity (SD ˜ 0.8 dB) meant that the difference between the estimates was, in practice, negligible.

Theory and data for area summation of contrast with and without uncertainty:evidence for a noisy energy model

Contrast sensitivity improves with the area of a sine-wave grating, but why? Here we assess this phenomenon against contemporary models involving spatial summation, probability summation, uncertainty, and stochastic noise. Using a two-interval forced-choice procedure we measured contrast sensitivity for circular patches of sine-wave gratings with various diameters that were blocked or interleaved across trials to produce low and high extrinsic uncertainty, respectively. Summation curves were steep initially, becoming shallower thereafter. For the smaller stimuli, sensitivity was slightly worse for the interleaved design than for the blocked design. Neither area nor blocking affected the slope of the psychometric function. We derived model predictions for noisy mechanisms and extrinsic uncertainty that was either low or high. The contrast transducer was either linear (c1.0) or nonlinear (c2.0), and pooling was either linear or a MAX operation. There was either no intrinsic uncertainty, or it was fixed or proportional to stimulus size. Of these 10 canonical models, only the nonlinear transducer with linear pooling (the noisy energy model) described the main forms of the data for both experimental designs. We also show how a cross-correlator can be modified to fit our results and provide a contemporary presentation of the relation between summation and the slope of the psychometric function.