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Since the advent of gene manipulating techniques, it has become increasingly important to study the neural functional properties of the mouse. The bcl2 gene has a powerful inhibitory action on naturally occurring cell death. As a consequence the brain of bcl2 overexpressing mouse is 1.5 times bigger than the brain of a wild type animal and the retina has more than twice the ganglion cells than normal (Martinou, Dubois-Dauphin, Staple, Rodriguez, Frankowski, Missotten, Albertini, Talabot, Catsicas, Pietra, & Huarte (1994). Neuron, 13: 1017–1030). Since in most mammals the upper limit of behavioural visual acuity is imposed by ganglion cells density, the visual acuity should be higher in bcl2 mice than in wild type mice. We measured behavioural visual acuity in wild type and transgenic mice and, contrary to the expectation, we found it to be of the same order (0.5–0.6 c/deg) in the two groups of animals, indicating that an increase in ganglion cells density is not effective in improving visual resolution.

Multicellular organisms face the necessity of removing superfluous or injured cells during normal development, tissue turn-over and in response to damaging conditions. These finalised killings occur throughout a process, commonly called programmed cell death (PCD), which is placed under strict cellular control. PCD is regulated by the products of the expression of a number of genes. This fact raises the intriguing possibility of inhibiting such degenerative processes by operating on some of the controlling genes. Central neurons of transgenic mice overexpressing bcl-2, a powerful inhibitor of PCD, are remarkably resistant to degeneration induced by noxious stimuli. We have explored the fate of retinal ganglion cells and of their axons, when such transgenic animals have been challenged by a lesion of the optic nerve. These results have direct bearing on the possibility of attaining functional restoration of the injured pathway.

Postnatal development of rat visual cortical functions was studied by recording extracellularly from the primary visual cortex of 22 animals ranging in age from postnatal day 17 (PIT) to P45. We found that in the youngest animals (P17-P19) all visual cortical functions tested were immature. Selectivity for orientation and movement direction of visual stimuli was almost absent, most cells received binocular input and their mean receptive field size was 5–6 times the adult size. Visual acuity was half its adult value. These functional properties developed gradually during the following weeks and by P45 they were all adult-like. This functional development is affected by manipulations of the visual input such as dark rearing (DR) and monocular deprivation (MD). DR prevented the normal postnatal maturation of visual cortical functions: in P60 rats, dark reared from birth, their visual cortical functions resembled those of P19–P21 rats. MD from P15 to P45 resulted in a dramatic shift of the ocular dominance distribution (ODD) in favour of the open eye and in a loss of visual acuity for the deprived eye. To determine the sensitive period of rat visual cortex to MD (critical period) we evaluated the shift in ODD of visual cortical neurones in rats that were subjected to the progressive delay of the onset of fixed MD period (10 days). Our results show that the critical period begins around the end of the third postnatal week, peaks between the fourth and fifth week and starts to decline from the end of the fifth week.

Visual evoked potentials were recorded from the occipital scalp of two anaesthetized Lynx (Lynx europea) in response to alternating gratings of various spatial frequencies and contrasts. The visual acuity of the Lynx was found to be around 5-6 c/deg, i.e. very close to the visual acuity of the cat and by far inferior to human acuity.

The interaction between two scotopic light stimuli has been investigated in the human peripheral retina. The test stimulus, a 20 min arc disc, was surrounded by a ring of variable diameter and steady luminance. The luminance of the disc could be kept constant or sinusoidally modulated in time. Rings of small diameter facilitate the perception of the steady disc, but inhibit the detection of the time varying component of the modulated signal. The opposite occurs with rings of larger diameter. These results are very similar to those obtained from a similar experiment on the ganglion cells of the cat.

Unitary responses to sinusoidal gratings either moving or alternating in phase have been investigated in the optic tract, lateral geniculate body and visual cortex of the cat as a function of the spatial frequency, position of the grating with respect to the cell receptive field and grating contrast. From the retina to the simple cells of the cortex there is a progressive narrowing of the spatial frequency band at which the cells are sensitive. The response of simple cells changes systematically with the position of an alteranting grating on the cell receptive field. The response of cells of the retina, geniculate body and visual cortex increases monotonically when the contrast of the grating is increased. For the simple cortical cells and for a part of complex cells there is a linear relation between the amplitude of the response and the logarithm of grating contrast.

Binocular depth perception arising from stereoscopic presentation of two gratings of different spatial frequencies is investigated. The angle of tilt of the stereoscopic image from the frontal plane is a function of the perceptual frequency difference as well as of the average frequency of the two gratings. A stereoscopic effect is described: a tilt of the stereoscopic image can be induced by lowering the contrast of one of two gratings having the same spatial frequency. A model is proposed for a mechanism of stereoscopic depth perception based on the comparison of spatial frequency content in the two retinal images.

Cortical visual evoked potentials (VEPs) in response to gratings temporally modulated in counterphase were recorded in normal and dark-reared pigmented rats. Temporal modulation was either sinusoidal (0.25–15 Hz, steady state condition) or abrupt (0.5 Hz, transient condition). In normals, the amplitude spectrum of contrast VEPs has two peaks (at about 0.5 and 4 Hz) and a high temporal frequency cut-off of the order of 11 Hz. The VEP phase lags with temporal frequency, showing two different linear slopes for separate frequency ranges (0.25–1 Hz and 1–7 Hz) centred on the peaks of the curve. The different slopes correspond to apparent latencies of 500 and 136 msec, respectively. Dark rearing reduced the cut-off frequency by about 3 Hz and increased apparent latencies by about 42 msec in the low temporal frequency range and 30 msec in the high temporal frequency range. The latency of the first peak of transient VEPs was increased by about 47 msec. Results indicate that the frequency response of rat contrast VEPs is qualitatively similar to that of other mammals (including human), albeit shifted to a lower range of temporal frequencies. Dark rearing significantly alters the VEP temporal characteristics, suggesting that visual experience is necessary for their correct development. Copyright © 1997 Elsevier Science Ltd

Genetically manipulated mice are important tools for studies on plasticity and degeneration/regeneration in the visual system. However, a description of the basic properties of the visual performance of the wild type mouse is still lacking. To characterize the visual physiology of the wild type (C57BL/6J) mouse we recorded Visual Evoked Potentials (VEPs) from the primary visual cortex. As compared to behavioral methods, VEPs may have the advantage that different aspects of vision can be screened readily and simultaneously in the same animals, including those with poor visual behavior due to motor or learning deficits. Local VEP responses to patterned visual stimuli have been recorded from the binocular visual cortex of anesthetized mice. Spatial (visual acuity, contrast threshold) and temporal (temporal function, response latency, motion sensitivity) aspects of VEPs were evaluated. The mouse VEP acuity was 0.6 c/deg, which is comparable to the behavioral visual acuity. The VEP peak contrast threshold was 5% (no behavioral data are available). Cortical representation of visual coordinates and cortical magnification factor corresponded to those previously reported using single cell recordings. Laminar analysis of VEPs indicated a dipole source in the supragranular layers of the visual cortex as a major response generator. VEPs showed contribution from both eyes, although biased strongly towards the eye contralateral to the recorded cortex. Results provide a comprehensive framework for characterizing visual phenotypes of a variety of transgenic mice.

We have recorded the unitary activity of the neurons of area 17 of the cat either in penetrations perpendicular to the surface of the cortex or parallel to it. We have examined the functional architecture of this cortical area with respect to the spatial frequency of the visual stimulus (sinusoidal gratings) at which each neuron is most sensitive. In penetrations perpendicular to the surface of the cortex (columns) the cells of the various layers or even of the same layer have the same preferred orientation but different preferred spatial frequencies. In penetrations parallel to the surface of the cortex the cells of the same layer or sublayer present a variety of preferred orientations, but maintain the same preferred spatial frequency and visual acuity over long microelectrode track distances.

Increment thresholds for a steady light disc and amplitude-modulation thresholds for a time-modulated disc centered at 5 deg from the fovea have been measured in the presence of an annulus of variable diameter concentric to the disc and viewed either by the same eye as the disc or by the other eye. Facilitatory and inhibitory interactions between steady stimuli are found to occur only for homolateral presentation of the stimuli, whereas spatial interactions between time-modulated stimuli occur also with dichoptic stimulus presentation.

Simultaneous contrast has been ascribed so far to spatial interactions in retinal receptive fields. The antagonistic surround of retinal receptive fields disappears after dark adaptation. The present work shows that simultaneous contrast for large fields and border contrast effects are preceived at low scotopic luminances. Moreover, both the spatial and the temporal psychophysical contrast sensitivity functions show a low-frequency attenuation at scotopic mean luminances. These findings support the conclusion drawn from recent electrophysiological results ( Maffei and Fiorentini , 1972 ) that the analysis of contrast is not performed in the retina, but at a higher level of the visual system.

Electroretinogram (ERG) activity recorded from the cat was analysed for various states of dark adaptation and for different background levels in order to determine whether or not an equivalent background illumination, such as that detected with threshold data during dark adaptation, has the same effect in the retina as an actual background illumination. The frequency response characteristics of the ERG potentials were used as a quantitative indicator of the electrophysiological state of the elements of the retina. With this index, it was not possible to match the characteristics of any response occurring during the first 6–8 min of dark adaptation with those of a response obtained with greater than −0.2 Lux background. It was concluded that equivalent electrophysiological states do not exist in the retina in the two adaptation conditions.

We have recorded from simple and complex cells in area 17 of the cat cortex. Around the classical receptive field (regions within which a moving or flashing bar can elicit a response from a cell) there are large regions which dramatically influence the cell's responsiveness. In some cells these regions are facilitatory, in others, inhibitory. Since these regions do not respond in isolation to moving or flashing bars, we called them unresponsive regions of the receptive field. Both the inhibitory and facilitatory unresponsive regions show spatial-frequency selectivity. All the facilitatory unresponsive regions show orientation selectivity, while most of the inhibitory unresponsive regions are not orientation selective. Both the facilitatory and the inhibitory unresponsive regions contribute to the size-selectivity of the cell. The facilitatory unresponsive regions show a great influence on the orientation selectivity of the cell, in that they sharpen the orientation channel of the cell. The inhibitory unresponsive regions, on the contrary, have not such an effect.