The topographical relations of the visual field with the superior colliculus of the rat was investigated using constant small visual stimuli and recording the electrical response of aggregate unit activity with fine tip metal microelectrodes. A precise essentially linear projection onto the contralateral colliculus was demonstrated after appropriate corrections for brain curvature were performed. The general pattern and arrangement of the retinotopic projection is similar to that found in infra-mammalian vertebrates. An ipsilateral projection appears to be absent. The phasic properties and rhythmic discharge patterns of single units in the colliculus were studied with stationary and moving luminous stimuli.Several recent studies indicate a n orderly projection of the retina onto the optic tectum i n fishes (Jacobson and Gaze, '64; Schwassmann and Kruger, '65a j, amphibians (Gaze, '58), reptiles (Heric and Kruger, '65) and birds (Hamdi and Whitteridge, '54), and an essentially similar spatial pattern has been described in all forms studied. In mammals, the superior colliculus, the presumed homologue of the optic tectum in other vertebrates, does not constitute the principal visual projection, although retinal fibers are known to terminate in this region in an orderly fashion (Bodian, '37; Lashley, '34; Barris et al., '35). Electrophysiological studies of mammals have provided some detailed information concerning the retinotopic pattern of the superior colliculus of cat (Apter, '45), rabbit and goat (Cooper et al., '53; Hamdi and Whitteridge, ' 5 3 ) , but to date no mammal has been mapped with sufficient precision for comparison with the results obtained in sub-mammalian forms.Specification of the detailed organization of the superior colliculus and its specialized properties should be useful in evaluating the persistent view that spatial analysis is solely dependent upon the thalamo-cortical visual pathway, and that the superior colliculus merely subserves total luminous flux discrimination after removal of the principal projection to the cerebral cortex (Lashley, '34; Kluver, '41 j . The present study was initiated in order to J. COMP. NEUn., 127: 435-444. map the retinotopic projection to the superior colliculus of a mammal with a relatively unspecialized visual system as a preliminary to a more extensive analysis of receptive field organization and properties of visual neurons in this region. The rat, by virtue of possessing an almost completely crossed optic pathway and a rod retina, should provide a suitable basis for evaluation of the retinal specialization and phyletic status of other forms. A preliminary account of the present findings has been presented (Siminoff et al., '65). MATERIALS AND METHODSSuccessful experiments were performed in 29 pigmented rats of the LongEvans strain. Several attempts to map the retinal projection to the superior colliculus in albino rats proved unsatisfactory because of difficulty in delimiting receptive fields adequately. Animals were anesthetized with urethane, ...

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Effects of chromatic adaptation on C-type bipolar cells (BC) in human retinal fovea are studied. Adaptation of the r-g channel is linear for both central fovea and parafovea. Adaptation of the parafovea bl-y channel, on the other hand, is nonlinear, which is accounted for by the slower adaptation rate of blue-sensitive cones with white light intensity as compared to rates of red- and green-sensitive cones. Achromatic adaptation of red- and green-center BCs produces uniform response decreases but without unique yellow loci shifts. Achromatic adaptation of blue-center BCs, on the other hand, does cause shifts of the unique green locus. Shifts of the crossover points for the BC response spectra occur with chromatic adaptation; the unique yellow loci shifts to shorter wavelengths with adapting wavelengths shorter than 550 nm and longer wave-lengths with longer adapting wavelengths than 550 nm. Chromatic adaptation is sufficient to explain the Bezold-Brüke effects; but to fully account for these shifts a novel hypothesis is proposed. For the green and red spectrum regions Bezold-Brücke shifts are due to r-g channel chromatic adaptation, while for the blue spectrum region bl-y channel chromatic adaptation accounts for Bezold-Brücke shifts. The two channels function independently in an either/or manner. The bl-y channel, besides having a unique green locus at 517.7 nm, has a crossover point at about 670 nm. Chromatic adaptation of the bl-y channel produces shifts of the unique red locus, which may account for extraspectral hue shifts.

Unit responses to visual stimuli in the pretectal region of the rat were investigated with steel microelectrodes. Most units could be classified into two broad categories; "phasic" (brief on and off discharges) and "tonic-on" (sustained discharge during the visual stimulus). The responsive region appears to be coextensive with the n. praetectalis anterior (Pta) which can be divided into a dorsal portion containing "phasic" units and a ventral portion containing "tonicon" units. The projection of the contralateral visual field is a mirror reversal of the field projection onto the superior colliculus. "Tonic-on" units may subserve total luminous flux detection necessary f o r the pupillary reflex. The topographical organization of units within Pta, as well as their diverse properties may account for residual visual function in animals with extensive lesions of the main visual pathways.The pretectal region has often been implicated in vision by virtue of its proximity to the lateral geniculate body and the overlying superior colliculus, evidence of optic tract fibers terminating in the principal pretectal nucleus (Polyak, '57, Hayhow et al., '60; Giolli, '65) and experiments suggesting this region is involved in the pupillary light reflex (Magoun, '35; Magoun and Ranson, '35). The present study is a continuation of a study of the organization of the visual field projection onto the superior colliculus of the rat (Siminoff et al., '66a) in which a distinct topography and some unusual unit properties were encountered below the anterior portion of the colliculus. Evidence is presented indicating that some portions of the pretectal nuclear group of the thalamus display distinctive unit properties in response to visual stimuli. A preliminary account of some of these findings has been presented earlier (Siminoff et al., '66b). METHODSFourteen pigmented Long-Evans rats were employed in the present experiments.The animals were anesthetized by intraperitoneal injection of 1.3 g/kg urethane, followed by one supplementary dose of 0.4 g/kg. The caudal portion of the right cerebral hemisphere was exposed and aspirated, thus uncovering the dorsal J. COMP. NEUR., 130: 329-342.surface of the superior colliculus as well as the caudal diencephalon.The skull was fixed to a metal plate which rigidly suspended the animal from above without obscuring the left visual field, and centering of the eye and location of the optic disc was accomplished in the manner described in the previous paper (Siminoff et al., '66a).Luminous stimuli were applied at a hemispheric surface, and several methods were employed for mapping the receptive fields of units. In most cases a flexible fiber optics bundle was used with a circular aperture subtending one degree of visual angle. All animals were dark adapted. The intensity of this "spot" of light was controlled by a neutral density wedge and monitored by a photocell. A number of units failed to respond to this small stimulus. In these cases (these units invariably had relatively large receptive fie...

A model of the cone-L-HC circuit for the catfish retina is presented with the following features: the outer segment consists of a compression factor and 7 low-pass filters in tandem; the cone pedicle consists of an internal negative feedback circuit in series with a low-pass filter; and the L-HC consists of a low-pass filter and forms a negative feedback circuit with the cone pedicle. By proper adjustment of the various time constants of the low-pass filters and the gain factors, the impulse responses for cones and L-HCs of the catfish retina (and turtle) can be duplicated. The negative feedback gain increases with increasing levels of mean illuminance which causes the monophasic impulse responses to become faster, biphasic and decrease in amplitude, i.e. in gain. This is an expression of the Weber-Fechner law.