Mechanistic modeling of vertebrate spatial contrast sensitivity and acuity at low luminance.

The validity of the Barten theoretical model for describing the vertebrate spatial contrast sensitivity function (CSF) and acuity at scotopic light levels has been examined. Although this model (which has its basis in signal modulation transfer theory) can successfully describe vertebrate CSF, and its relation to underlying visual neurophysiology at photopic light levels, significant discrepancies between theory and experimental data have been found at scotopic levels. It is shown that in order to describe scotopic CSF, the theory must be modified to account for important mechanistic changes, which occur as cone vision switches to rod vision. These changes are divided into photon management factors [changes in optical performance (for a dilated pupil), quantum efficiency, receptor sampling] and neural factors (changes in spatial integration area, neural noise, and lateral inhibition in the retina). Predictions of both scotopic CSF and acuity obtained from the modified theory were found to be in good agreement with experimental values obtained from the human, macaque, cat, and owl monkey. The last two species have rod densities particularly suited for scotopic conditions.

Stimulus luminance and the spatial acuity of domestic fowl (Gallus g. domesticus).

The luminance dependence of spatial acuity in domestic fowl was measured directly over stimulus luminances ranging from 0.06 to 57.35 cd m(-2). At the highest luminance, acuity was around 6.5 c deg(-1), in agreement with previous studies in this species. As stimulus luminance decreased, acuity fell with increasing rate to 3.2 c deg(-1) at 0.06 cd m(-2), following the same shape as acuity functions for other mammalian and avian species. These findings suggest that the rod-cone transition for domestic fowl is between 0.45 and 1.79 cd m(-2). Over the photopic range from 1.79 to 57.35 cd m(-2) the change of acuity for fowl was 1%, compared with 32% for humans. For domestic fowl, the Rovamo-Barten MTF model of contrast sensitivity accounted for the behaviour of acuity as a function of luminance down to mesopic levels.

Measuring and modelling the spatial contrast sensitivity of the chicken (Gallus g. domesticus).

The spatial contrast sensitivity (CSF) of the chicken has been measured using a behavioural technique. The results obtained show that spatial vision in this species is relatively poor compared with the human observer. For a visual stimulus luminance of 16 c dm(-2), the upper frequency limit of spatial vision in the chicken (acuity) was found to be about 7.0 c deg(-1), with peak spatial vision occurring at around 1.0 c deg(-1). Under equivalent stimulus conditions, the acuity of the human is around 50 c deg(-1) with a peak in spatial vision at about 3.0 c deg(-1). Peak spatial contrast sensitivity in the chicken was also found to be only about 2% that for the human. At a lower stimulus luminance of 0.1 c dm(-2), the chicken CSF reduced in overall magnitude and indicated an acuity level of about 5.0 c deg(-1). These experimental results were successfully modelled using modulation transfer (MTF) theory. This theoretical treatment enabled important neural mechanisms underlying spatial vision in the chicken to be revealed. The role played by spatial vision in the chicken's ability to recognise detailed shapes in its visual environment was also examined by deploying the CSF as a visual weighting function with the Fourier series of a chicken comb.

A mechanistic inter-species comparison of spatial contrast sensitivity.

The validity of the Rovamo-Barten modulation transfer function model for describing spatial contrast sensitivity in vertebrates was examined using published data for the human, macaque, cat, goldfish, pigeon and rat. Under photopic conditions, the model adequately described overall contrast sensitivity for changes in both stimulus luminance and stimulus size for each member of this diverse range of species. From this examination, optical, retinal and post-retinal neural processes subserving contrast sensitivity were quantified. An important retinal process is lateral inhibition and values of its associated point spread function (PSF) were obtained for each species. Some auxiliary contrast sensitivity data obtained from the owl monkey were included for these calculations. Modeled values of the lateral inhibition PSF were found to correlate well with ganglion cell receptive field surround size measurements obtained directly from electrophysiology. The range of vertebrates studied was then further extended to include the squirrel monkey, tree shrew, rabbit, chicken and eagle. To a first approximation, modeled estimates of lateral inhibition PSF width were found to be inversely proportional to the square root of ganglion cell density. This finding is consistent with a receptive field surround diameter that changes in direct proportion to the distance between ganglion cells for central vision. For the main species examined, contrast sensitivity is considerably less than that for the human. Although this is due in part to a reduction in the performance of both optical and retinal mechanisms, the model indicates that poor cortical detection efficiency plays a significant role.

The value of routine histopathological examination of appendicectomy specimens.

BACKGROUND: Appendicectomy specimens removed from patients with suspected acute appendicitis often appear macroscopically normal but histopathological analysis of these cases may reveal a more sinister underlying pathology. We evaluated histopathological reports of 1225 appendicectomy specimens at the Norfolk and Norwich University Hospital (NNUH) over the past three years. METHODS: Histopathology reports for all appendices analysed at the NNUH between March 2003 and March 2006 were reviewed by examination of the case notes. The analysis focussed on the confirmation of acute appendicitis, incidental unexpected incidental findings other than inflammation, whether these abnormalities were suspected on gross examination at the time of surgery, and the effect on patient management and prognosis. RESULTS: The histopathology reports disclosed a variety of abnormal incidental lesions. Of the 1225 specimens, 46 (3.75%) revealed abnormal diagnoses other than inflammatory changes. Twenty-four (1.96%) of these were clinically significant and affected further patient management. Only two of these (0.16%) were suspected on macroscopic examination intra-operatively. CONCLUSION: Twenty-four of the 1225 specimens (1.96%) had an impact on patient management or outcome and were not suspected on macroscopic examination at the time of surgery. These would have been missed had the specimens not been examined microscopically. The intra-operative diagnosis of the surgeon is therefore unreliable in detecting abnormalities of the appendix. This study supports the sending of all appendicectomy specimens for routine histopathological examination.

On the calculation of optical performance factors from vertebrate spatial contrast sensitivity.

A novel technique for calculating the visual optical modulation transfer function (OMTF) is described. The technique involves application of the Rovamo-Barten model of spatial vision to measured contrast sensitivity data. [For details of the basic model see; Rovamo, J., Mustonen, J., & Nasanen, R. (1994). Modelling contrast sensitivity as a function of retinal illuminance and grating area. Vision Research, 34, 1301-1314 and Barten, P. J. G. (1999). Contrast sensitivity of the human eye and its effects on image quality. Washington: SPIE Optical Engineering Press.] In order to obtain OMTF, the model was simplified for use in the high spatial frequency range and also modified to include a transfer function term relating to attenuation by the retinal receptor sampling process. Calculations of OMTF were initially obtained from published contrast sensitivity for the human, cat, rat and chicken. The results were found to correlate well with OMTF values directly obtained through a double-pass optical measuring technique applied to all four species. It was assumed, following this initial test, that the modified Rovamo-Barten model could be used to extract OMTF from vertebrate contrast sensitivity data in general. Using published behavioural contrast sensitivity, further OMTF values were calculated from the model for the pigeon, goldfish, owl monkey, and tree shrew. The results obtained were used to provide a direct inter-species comparison of optical performance for a matched stimulus luminance. This study also confirms that, in many cases, vertebrate optical and receptor sampling processes are well matched in their attenuation properties.

A mechanistic inter-species comparison of flicker sensitivity.

The general validity of both the Rovamo [Vision Res. 39 (1999) 533] and Barten (Contrast sensitivity of the human eye, SPIE Optical Engineering Press, 1999), modulation transfer function models for describing flicker sensitivity in vertebrates was examined using published data for goldfish, chickens, tree shrews, ground squirrels, cats, pigeons and humans. Both models adequately described the flicker response in each species at frequencies greater than approximately 1 Hz. At lower frequencies, response predictions differed between the two models and this was due, in part, to dissimilar definitions of the role played by lateral inhibition in the retina. Modelled flicker sensitivity for a matched retinal illuminance condition enabled a direct inter-species comparison of signal processing response times at the photoreceptor level. The modelled results also quantified differences between species in post-retinal signal processing capability. Finally, the relationship between flicker frequency response curves and the perception of temporal signals in real visual scenes was examined for each species. It is proposed that the area under the flicker sensitivity function may offer a single "figure of merit" for specifying overall sensitivity to time signals in a species' environment.

Measuring and modelling the photopic flicker sensitivity of the chicken (Gallus g. domesticus).

The photopic flicker sensitivity of the chicken was determined using an operant conditioning psychophysical technique. The results show both high- and low-frequency fall-off in the sensitivity response, which peaked around 15 Hz. Flicker sensitivity was determined for a range of stimulus luminance levels, and directly compared to human flicker response measured under similar stimulus conditions. At five luminance levels (10, 100, 200, 500 and 1000 cd/m(2)), the overall chicken flicker sensitivity was found to be considerably lower than for humans, except at high frequencies. A greater degree of frequency tuning was also found in the chicken response. The critical flicker fusion values were either similar or slightly higher for chickens compared to humans (40.8, 50.4, 53.3, 58.2 and 57.4 Hz vs 39.2, 54.0, 54.0, 57.4 and 71.5 Hz respectively for humans and chickens for increasing stimulus luminance level). A recently proposed model for flicker sensitivity [Vision Research 39 (1999) 533], which incorporates low- and high-pass temporal filters in cascade, was found to be applicable to the chicken response. From this model, deductions were made concerning mechanisms controlling the transfer of temporal information.

A comparative study of stimulus-specific pupil responses in the domestic fowl (Gallus gallus domesticus) and the human.

Pupil responses triggered by specific stimulus attributes such as spatial structure, colour and light flux changes were measured in eight domestic fowl. Comparative experiments were also carried out in human subjects. The results were unexpected in that large increments in light flux caused only small constrictions of the pupil. A red stimulus, on the other hand, caused a relatively large pupil response, but a green stimulus was less effective. This finding suggests that the size of the pupil, apart from being controlled by well-described pretectal pathways that mediate luminance responses, is also subject to other inputs. The pupil response in the domestic fowl may therefore make an effective quantitative indicator of things of significance to the animal. In some ways these observations are similar to other findings in primates in that the processing of stimulus attributes such as colour and structure that are not normally associated with the light reflex pathway can cause a pupil response. The fowl pupil does however respond very fast when large light flux changes or red stimuli are involved. Results obtained with sinusoidally modulated light flux changes reveal a short response latency of 105 ms (SD=8.3). In contrast, human responses measured for similar stimulus conditions reveal a latency of 434 ms (SD=36). The speed of pupil response in the fowl is significantly higher than in humans, but the response amplitude is usually small. Another interesting observation is the lack of sustained response to changes in ambient illumination. These findings suggest that the input to the pupilloconstrictor neurones in the fowl consists largely of transient neurones with little sustained component.