Visual sensitivity of ground squirrels to spatial and temporal luminance variations

Low achromatic contrast sensitivity in birds: a common attribute shared by many phylogenetic orders

Vision is an important sensory modality in birds, which can outperform other vertebrates in some visual abilities. However, sensitivity to achromatic contrasts - the ability to discern luminance difference between two objects or an object and its background - has been shown to be lower in birds compared with other vertebrates. We conducted a comparative study to evaluate the achromatic contrast sensitivity of 32 bird species from 12 orders using the optocollic reflex technique. We then performed an analysis to test for potential variability in contrast sensitivity depending on the corneal diameter to the axial length ratio, a proxy of the retinal image brightness. To account for potential influences of evolutionary relatedness, we included phylogeny in our analyses. We found a low achromatic contrast sensitivity for all avian species studied compared with other vertebrates (except small mammals), with high variability between species. This variability is partly related to phylogeny but appears to be independent of image brightness.

Mechanisms of simultaneous linear and nonlinear computations at the mammalian cone photoreceptor synapse

Neurons enhance their computational power by combining linear and nonlinear transformations in extended dendritic trees. Rich, spatially distributed processing is rarely associated with individual synapses, but the cone photoreceptor synapse may be an exception. Graded voltages temporally modulate vesicle fusion at a cone's ~20 ribbon active zones. Transmitter then flows into a common, glia-free volume where bipolar cell dendrites are organized by type in successive tiers. Using super-resolution microscopy and tracking vesicle fusion and postsynaptic responses at the quantal level in the thirteen-lined ground squirrel, Ictidomys tridecemlineatus, we show that certain bipolar cell types respond to individual fusion events in the vesicle stream while other types respond to degrees of locally coincident events, creating a gradient across tiers that are increasingly nonlinear. Nonlinearities emerge from a combination of factors specific to each bipolar cell type including diffusion distance, contact number, receptor affinity, and proximity to glutamate transporters. Complex computations related to feature detection begin within the first visual synapse.

The contrast sensitivity function of a small cryptobenthic marine fish

Spatial resolution is a key property of eyes when it comes to understanding how animals' visual signals are perceived. This property can be robustly estimated by measuring the contrast sensitivity as a function of different spatial frequencies, defined as the number of achromatic vertical bright and dark stripe pairs within one degree of visual angle. This contrast sensitivity function (CSF) has been estimated for different animal groups, but data on fish are limited to two free-swimming, freshwater species (i.e., goldfish and bluegill sunfish). In this study, we describe the CSF of a small marine cryptobenthic fish (Tripterygion delaisi) using an optokinetic reflex approach. Tripterygion delaisi features a contrast sensitivity that is as excellent as other fish species, up to 125 (reciprocal of Michelson contrast) at the optimal spatial frequency of 0.375 c/°. The maximum spatial resolution is instead relatively coarse, around 2.125 c/°. By comparing our results with acuity values derived from anatomical estimates of ganglion cells' density, we conclude that the optokinetic reflex seems to be adapted to process low spatial frequency information from stimuli in the peripheral visual field and show that small marine fish can feature excellent contrast sensitivity at optimal spatial frequency.

Retinal temporal resolution and contrast sensitivity in the parasitic lamprey Mordacia mordax and its non-parasitic derivative Mordacia praecox

Lampreys and hagfishes are the sole extant representatives of the early agnathan (jawless) vertebrates. We compared retinal function of fully metamorphosed, immature Mordacia mordax (which are about to commence parasitic feeding) with those of sexually mature individuals of its non-parasitic derivative Mpraecox We focused on elucidating the retinal adaptations to dim-light environments in these nocturnally active lampreys, using electroretinography to determine the temporal resolution (flicker fusion frequency, FFF) and temporal contrast sensitivity of enucleated eyecups at different temperatures and light intensities. FFF was significantly affected by temperature and light intensity. Critical flicker fusion frequency (cFFF, the highest FFF recorded) of M. praecox and M. mordax increased from 15.1 and 21.8 Hz at 9°C to 31.1 and 36.9 Hz at 24°C, respectively. Contrast sensitivity of both species increased by an order of magnitude between 9 and 24°C, but remained comparatively constant across all light intensities. Although FFF values for Mordacia spp. are relatively low, retinal responses showed a particularly high contrast sensitivity of 625 in M. praecox and 710 in M. mordax at 24°C. This suggests selective pressures favour low temporal resolution and high contrast sensitivity in both species, thereby enhancing the capture of photons and increasing sensitivity in their light-limited environments. FFF indicated all retinal photoreceptors exhibit the same temporal response. Although the slow response kinetics (i.e. low FFF) and saturation of the response at bright light intensities characterise the photoreceptors of both species as rod-like, it is unusual for such a photoreceptor to be functional under scotopic and photopic conditions.

Mechanism of High-Frequency Signaling at a Depressing Ribbon Synapse

Ribbon synapses mediate continuous release in neurons that have graded voltage responses. While mammalian retinas can signal visual flicker at 80-100 Hz, the time constant, τ, for the refilling of a depleted vesicle release pool at cone photoreceptor ribbons is 0.7-1.1 s. Due to this prolonged depression, the mechanism for encoding high temporal frequencies is unclear. To determine the mechanism of high-frequency signaling, we focused on an "Off" cone bipolar cell type in the ground squirrel, the cb2, whose transient postsynaptic responses recovered following presynaptic depletion with a τ of ∼0.1 s, or 7- to 10-fold faster than the τ for presynaptic pool refilling. The difference in recovery time course is caused by AMPA receptor saturation, where partial refilling of the presynaptic pool is sufficient for a full postsynaptic response. By limiting the dynamic range of the synapse, receptor saturation counteracts ribbon depression to produce rapid recovery and facilitate high-frequency signaling.

The squirrel as a rodent model of the human visual system

Over the last 50 years, studies of receptive fields in the early mammalian visual system have identified many classes of response properties in brain areas such as retina, lateral geniculate nucleus (LGN), and primary visual cortex (V1). Recently, there has been significant interest in understanding the cellular and network mechanisms that underlie these visual responses and their functional architecture. Small mammals like rodents offer many advantages for such studies, because they are appropriate for a wide variety of experimental techniques. However, the traditional rodent models, mice and rats, do not rely heavily on vision and have small visual brain areas. Squirrels are highly visual rodents that may be excellent model preparations for understanding mechanisms of function and disease in the human visual system. They use vision for navigating in their environment, predator avoidance, and foraging for food. Visual brain areas such as LGN, V1, superior colliculus, and pulvinar are particularly large and well elaborated in the squirrel, and the squirrel has several extrastriate cortical areas lateral to V1. Unlike many mammals, most squirrel species are diurnal with cone-dominated retinas, similar to the primate fovea, and have excellent dichromatic color vision that is mediated by green and blue cones. Owing to their larger size, squirrels are physiologically more robust than mice and rats under anesthesia, and some hibernating species are particularly tolerant of hypoxia that occurs during procedures such as brain slicing. Finally, many basic anatomical and physiological properties in the early visual system of squirrel have now been described, permitting investigations of cellular mechanisms. In this article, we review four decades of anatomical, behavioral, and physiological studies in squirrel and make comparisons with other species.

Spatial contrast sensitivity of birds

Contrast sensitivity (CS) is the ability of the observer to discriminate between adjacent stimuli on the basis of their differences in relative luminosity (contrast) rather than their absolute luminances. In previous studies, using a narrow range of species, birds have been reported to have low contrast detection thresholds relative to mammals and fishes. This was an unexpected finding because birds had been traditionally reported to have excellent visual acuity and color vision. This study reports CS in six species of birds that represent a range of visual adaptations to varying environments. The species studied were American kestrels (Falco sparverius), barn owls (Tyto alba), Japanese quail (Coturnix coturnix japonica), white Carneaux pigeons (Columba livia), starlings (Sturnus vulgaris), and red-bellied woodpeckers (Melanerpes carolinus). Contrast sensitivity functions (CSFs) were obtained from these birds using the pattern electroretinogram and compared with CSFs from the literature when possible. All of these species exhibited low CS relative to humans and most mammals, which suggests that low CS is a general characteristic of birds. Their low maximum CS may represent a trade-off of contrast detection for some other ecologically vital capacity such as UV detection or other aspects of their unique color vision.

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.

Psychophysical measurement of temporal modulation sensitivity in the tree shrew (Tupaia belangeri)

Temporal modulation sensitivity functions (MSFs) were measured behaviorally in three adult tree shrews (Tupaia belangeri). Shrews were trained to detect temporal sinusoidally-modulated full-field luminance variations in one of three stimuli, the two alternatives being static stimuli of equal size and time-averaged luminance (34 cd/m2). Modulation depth was varied trial-by-trial using a modified staircase technique under ambient illumination of 16 lux. Threshold modulation depths were determined for five temporal frequencies ranging from 3.7 to 47 Hz. Results revealed temporal MSFs that peaked at 15 Hz with a low-frequency roll-off and an extrapolated high-frequency cut-off beyond 50 Hz. These findings confirm the comparatively good temporal vision of Tupaia predicted by behavioral observations.

Behavioral and electrophysiological sensitivity to temporally modulated visual stimuli in the ground squirrel

Behavioral and electrophysiological methods were used to measure sensitivity to flickering lights in a dichromatic species, the California ground squirrel (Spermophilus beecheyi). Discrimination tests were used to determine spectral sensitivity at stimulus frequencies from 5-50 Hz and increment threshold spectral sensitivity. The contributions of retinal mechanisms to these capacities were assessed by recording the responses of optic nerve fibers to temporally modulated monochromatic lights. In the ground squirrel, as in the human, the shape of the spectral-sensitivity function depends on the temporal frequency of the stimulus. Results from single-unit recording show that all of the classes of optic nerve fibers in the ground squirrel are highly phase-locked to the stimulus for modulation rates as high as 50 Hz. Neither the responses of photoreceptors nor any class of optic nerve fiber can singly account for the behavioral results. The electrophysiological results are also counter to models which propose that temporally dependent changes in the spectral sensitivity of spectrally opponent fibers account for the behavior. The temporal resolution of the optic nerve fibers exceeds that of the behaving animal suggesting that retinal mechanisms do not limit behavioral temporal resolution.

Contrast sensitivity and visual acuity of the pigmented rat determined electrophysiologically

The contrast sensitivity function of the rat was assessed by investigating the relationship between the amplitude of visually evoked cortical potentials (VECP) and the spatial frequency and contrast of grating stimuli. Pattern reversal VECPs were recorded in Area 17 in the region of representation of the central binocular visual field. Transient responses were obtained with a 1 Hz contrast reversal. The mean contralateral monocular CSF peaked around 0.1 c/deg, with a threshold sensitivity of 20-25, low frequency attenuation and a high frequency cut-off of 1.18 c/deg. The mean binocular CSF showed a cut-off of 1.20 c/deg, which matches several behavioural measurements of visual acuity. The greater binocular sensitivity in the low frequency range (0.04 c/deg) could be tentatively attributed to the greater influence of the population of large ganglion cells that reaches its maximal density in the ipsilateral projection.

Refractive state, depth of focus and accommodation of the eye of the California ground squirrel (Spermophilus beecheyi)

Retinoscopy and electrophysiological refraction were performed on 55 and 24 far-accommodated eyes of California ground squirrels (Spermophilus beecheyi), respectively. These two indices were highly correlated, revealing the eye of this animal to be roughly emmetropic (-0.25 to -0.13 D). Depth of focus was assessed by measuring the effect which defocusing produced on the spatial resolving power of 32 optic nerve fibers. Depth of focus of the ground squirrel eye for a pupil diameter of 2.5 mm is estimated to be +/- 1.6 D, but will increase rapidly for smaller pupils. Accommodation in eleven ground squirrels ranged from 2 to 6 D, with a mean value of 3.9 D.

Spatial contrast sensitivity of the tree shrew

Spatial contrast sensitivity functions (CSFs) were measured for 3 tree shrews. Our two-alternative forced-choice discrimination paradigm required the animals to discriminate a vertical sine-wave luminance grating from a homogeneous field of the same size (16 degrees) and mean luminance (35 cd/m2). Spatial frequencies tested ranged from 0.1 to 2.0 c/deg and grating contrast was varied trial-by-trial using a modified staircase technique. Small between-subject variations in the shape of the CSFs appeared to be correlated with our estimates of refractive error for each animal. In general, the CSFs were of the typical band-pass type with peak sensitivity occurring at approximately 0.7 c/deg. Estimates of grating acuity derived from the CSFs ranged from 1.2 to 2.4 c/deg and are within the limitations set by the eye size and retinal anatomy of the tree shrew.

Contrast sensitivity function and visual acuity of the opossum

The Modulation Transfer Function (MTF) of the visual system of the opossum, D. marsupialis aurita, was determined using the amplitude of Visually Evoked Cortical Potentials (VECP) as response indicator. Stimuli consisted of a 180 degrees phase reversal of sinusoidally modulated gratings with an average luminance of 2.4 cd/m2. Contrast sensitivity was determined for various spatial frequencies and the MTF was calculated by the least square fit of an exponential function. The average acuity value obtained was 1.25 c/deg. The Fourier transform of the MTF was considered an approximation of the Line Spread Function of the visual system. The lowest value observed was 14 min of arc. The visual acuity observed in the mesopic range was not altered when stimulus intensity was raised to photopic levels.