Increment-threshold functions for different rodent species

The contribution of cone responses to rat electroretinograms

The contribution of rods and cones to the scotopic electroretinogram (ERG) of small animals is unclear, with a recent report suggesting that the mouse has no cone a-wave. The present study considered the contribution of cones to the ERG of the rat. Dark-adapted Long Evans rats (n = 4) had ERG signals collected following a single flash, which stimulated rods and cones (mixed response), or a twin-fash paradigm (short interstimulus interval, 1 s), which isolated cone responses. Rod signals were derived by digital subtraction of the cone signal from the mixed rod/cone ERG. The rat a-wave was found to be dominated by rod responses but cone responses contributed substantially (45%) to post-receptoral waveforms (b-wave) at higher light levels.

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.

Receptive field properties of single neurons in rat primary visual cortex

The rat is used widely to study various aspects of vision including developmental events and numerous pathologies, but surprisingly little is known about the functional properties of single neurons in the rat primary visual cortex (V1). These were investigated in the anesthetized (Hypnorm-Hypnovel), paralyzed animal by presenting gratings of different orientations, spatial and temporal frequencies, dimensions, and contrasts. Stimulus presentation and data collection were automated. Most neurons (190/205) showed sharply tuned (</=30 degrees bandwidth at half height) orientation selectivity with a bias for horizontal stimuli (31%). Analysis of response modulation of oriented cells showed a bimodal distribution consistent with the distinction between simple and complex cell types. Orientation specific interactions occurred between the center and the periphery of receptive fields, usually resulting in strong inhibition to center stimulation when both stimuli had the same orientation. There was no evidence for orientation columns nor for orderly change in optimal orientation with tangential tracks through V1. Responses were elicited by spatial frequencies ranging from zero (no grating) to 1.2 cycle/degree (c/ degrees ), peaking at 0.1 c/ degrees, and with a modal cutoff of 0.6 c/ degrees. Half of the neurons responded optimally to drifting gratings rather than flashing uniform field stimuli. Directional preference was seen for 59% of oriented units at all depths in the cortex. Optimal stimuli velocities varied from 10 to 250 degrees /s. Some units, mainly confined to layer 4, responded to velocities as high as 700 degrees /s. Response versus contrast curves (best fit with Naka-Rushton) varied from nearly linear to extremely steep (mean contrast semisaturation 50% and threshold 6%). There was a trend for cells from superficial layers to be more selective to different stimulus parameters than deeper layers cells. We conclude that neurons in rat V1 have complex and diverse visual properties, necessary for precise visual form perception with low spatial resolution.

Increment thresholds of the three spectral mechanisms in the retina of the California ground squirrel (Spermophilus beecheyi)

Increment threshold (IT) functions were obtained for the electroretinogram (ERG) and for single units in the optic nerve of the California ground squirrel. The three spectral mechanisms providing input to ganglion cells in this retina were isolated and their increment thresholds to large field, long-duration stimuli were examined. Mean IT functions for the 519 nm mechanism (irrespective of unit class), the 500 nm mechanism, and the ERG all showed shallow log-log slopes between 0.52 and 0.63. The slope of the mean IT function for the 440 nm mechanism depended on the spectral character of the adapting light. When the dominant wavelength of this light was close to the peak sensitivity of the 440 nm mechanism, the mean IT function was steep (0.92), but when the dominant wavelength of the adapting light was at the cross-point of the opponent cells, the slope of the function was shallow (0.49). The difference in IT slope under these two conditions may be attributed to an additional sensitivity loss occuring at a spectrally-opponent site.

Visual discrimination impairments after lesions in zona incerta or lateral terminal nucleus of accessory optic tract

Possible visual functions of zona incerta (ZI) and lateral terminal nucleus of the accessory optic tract (LTN) in rats were investigated by comparing the effects of localized lesions on a series of discrimination learning tasks. In Experiment 1 animals with ZI lesions were impaired on a simultaneous black versus white (BW) discrimination but not on a simultaneous horizontal versus vertical (HV) task. Animals with LTN lesions were impaired on HV but not on BW. Rats with ZI lesions were impaired on a successive bright versus dim but not a successive HV discrimination when subsequently tested in Experiment 2. Animals with LTN lesions were impaired on both tasks. The results are related to the possibility that visual intensity information gains access to the motor system through a pathway running from the ventral lateral geniculate nucleus to the basal ganglia via ZI. Since LTN lesions impaired acquisition of a HV discrimination, a task highly susceptible to the effects of geniculostriate ablation, it is suggested that such lesions impair the use of visual information without distroying the sensory channels mediating the discrimination.

Spectral sensitivity and colour vision in the ground-dwelling sciurids: results from golden mantled ground squirrels and comparisons for five species

Behavioural measurements of several indices of visual sensitivity and colour vision for the golden-mantled ground squirrel (Spermophilus lateralis) are reported. The spectral sensitivity functions for this animal show a change from having a single peak at about 520 nm to having a strongly bipartite appearance with peaks at about 540 and 460 nm as the level of light adaptation is increased from low to higher levels. Discrimination tests reveal that these squirrels have dichromatic colour vision with spectral neutral points centered at 507.5 nm. Comparisons are made of the visual capacities of five species of ground-dwelling sciurids, all of which have been tested in the same situations. These comparisons indicate that: (1) the retinas of all of these species contain a small number of functional rods in addition to a much more obvious cone population; (2) the spectral sensitivity functions of these species do not show systematic variations; (3) the cone pigments found in the retinas of these animals have sensitivity peaks at about 440 and 525 nm; (4) all of these species have dichromatic colour vision; (5) there is no evidence for systematic differences in visual sensitivity or colour vision among these five species.

Effects of constant illumination on vision in the albino rat

The characteristics of photically-induced blinding in the albino rat were investigated with physiological (the electroretinogram--ERG) and behavioral (luminance discrimination) techniques. These indices were obtained both before and during exposure to constant illumination (2600 lux). In trast to previous results, the findings of this study indicate that both the amplitude of the ERG and the ability to discriminate test lights decline rapidly and at approximately the same rate following the onset of constant illumination. These results, in conjunction with others already in the literature, lead to the suggestion that the degree of correspondence between behavioral and other measures of photically-induced blinding is dependent on the rate of retinal degeneration.

Scotopic and photopic vision in the California ground squirrel: physiological and anatomical evidence

The California ground squirrel is a highly diurnal species previously thought to have an all-cone retina. This issue was re-examined in physiological and anatomical experiments. The electroretinogram (ERG) was used to measure the spectral sensitivity of the eye under different conditions of adaptation. The occurrence of a Purkinje shift could be demonstrated, although there was some indication that not all members of this species show such a shift. Spectral sensitivity of the dark-adapted eye of this squirrel is close to that predicted by a typical mammalian rhodopsin. Light adaptation produces a shift in spectral sensitivity to a peak location of about 525 nm. It was shown that two mechanisms having different spectral sensitivities contribute to the photopically recorded ERG. The degree to which these two mechanisms contribute to the ERG was found to be strikingly different from the degree to which the two contribute to visual behavior. Our anatomical results indicate that the retina of the California ground squirrel has two structurally distinct photoreceptors which, on the basis of various criteria, can be classified as cone and rod-like. The rod-like receptors comprise about 6-7% of the total. The two photoreceptor types differ in placement of their inner segments, size of their outer segments, outer segment ultrastructure, and terminal structure and organization.