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.