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The relation between size and performance is central for understanding the evolution of sensory systems, and much interest has been focused on mammalian eyes and ears. However, we know very little about olfactory organ size (OOS), as data for a representative set of mammals are lacking. Here, we present a cranial endocast method for estimating OOS by measuring an easily accessible part of the system, the perforated part of the ethmoid bone, through which the primary olfactory axons reach the olfactory bulb. In 16 species, for which relevant data are available, the area of the perforated ethmoid bone is directly proportional to the area of the olfactory epithelium. Thus, the ethmoid bone is a useful indicator enabling us to analyse 150 species, and describe the distribution of OOS within the class Mammalia. In the future, a method using skull material may be applied to fossil skulls. In relation to skull size, humans, apes and monkeys have small olfactory organs, while prosimians have OOSs typical for mammals of their size. Large ungulates have impressive olfactory organs. Relating anatomy to published thresholds, we find that sensitivity increases with increasing absolute organ size.

Responses to light were recorded from rods, horizontal cells, and ganglion cells in dark-adapted toad eyecups. Sensitivity was defined as response amplitude per isomerization per rod for dim flashes covering the excitatory receptive field centers. Both sensitivity and spatial summation were found to increase by one order of magnitude between rods and horizontal cells, and by two orders of magnitude between rods and ganglion cells. Recordings from two hyperpolarizing bipolar cells showed a 20 times response increase between rods and bipolars. At absolute threshold for ganglion cells (Copenhagen, D.R., K. Donner, and T.Reuter. 1987. J. Physiol. 393:667-680) the dim flashes produce 10-50-#V responses in the rods. The cumulative gain exhibited at each subsequent synaptic transfer from the rods to the ganglion cells serves to boost these small amplitude signals to the level required for initiation of action potentials in the ganglion cells. The convergence of rod signals through increasing spatial summation serves to decrease the variation of responses to dim flashes, thereby increasing the signalto-noise ratio. Thus, at absolute threshold for ganglion cells, the convergence typically increases the maximal signal-to-noise ratio from 0.6 in rods to 4.6 in ganglion cells.

We have modelled the effect of microsaccades on retinal responses to achromatic borders and lines using physiologically realistic parameters. Typical microsaccade movement sequences were applied to the retinal image of stationary spatial contrast patterns as projected on the foveal cone mosaic after being passed through the optical transfer function of the eye. The resulting temporal contrast modulation over a cone receptive field was convolved with an analytical expression for the response waveform of primate cones (photocurrent: [Schnapf, J. L., Nunn, B. J., Meister, M. & Baylor, D. A. (1990). Visual transduction in cones of the monkey Macaca fascicularis. Journal of Physiology, 427, 681-713]; photovoltage: [Schneeweis, D. M. & Schnapf, J. L. (1999). The photovoltage of macaque cone photoreceptors: Adaptation, noise, and kinetics. Journal of Neuroscience, 19, 1203-1216]). The input to the ganglion cell was derived from the cone responses by the difference-of-Gaussians receptive field model of Donner and Hemilä [Donner, K. & Hemilä, S. (1996). Modelling the spatio-temporal modulation response of ganglion cells with difference-of-Gaussians receptive fields: Relation to photoreceptor response kinetics. Visual Neuroscience, 13, 173-186]. The modelled response waveforms suggest that microsaccades may significantly enhance sensitivity to edges, "re-sharpen" the image and, most interestingly, improve resolution of two closely spaced lines. The reason is that fine spatial structure of the retinal image when moving at suitable velocities is translated into a correlated temporal structure of responses of single cones and ganglion cells. The information content of the signal is not strongly dependent on positional accuracy and the effect is thus distinct from the presumed retinal basis of vernier acuity. Other eye movements (drift) with velocity distributions similar to that of the microsaccade's slow return phase might be similarly useful, although the microsaccade has some distinguishing features that could be functionally significant, e.g., the neural motor control and the biphasic movement pattern.

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ERG mass photoreceptor responses were recorded across the isolated, aspartate-perfused retina of the frog, Rana temporaria, in order to determine spectral sensitivities of cones. Cone responses were distinguished from rod responses by their faster kinetics, and responses from different cone types were isolated by selective background adaptation. Our main finding is that of a novel short-wavelength sensitive cone population peaking at about 431 nm. Further, we find that the sensitivity spectrum of the dominant long-wavelength sensitive cone population fully accounts for the most common type of photopic ganglion cell spectrum. Both can be described by a nomogram with lambda max = 562 nm. This resolves a long-standing apparent conflict between cone absorbance spectra and ganglion cell sensitivities. Including the 502 nm cones previously described by microspectrophotometry, the frog possesses a collection of cones that could support trichromatic photopic vision.