The retinas of placental mammals appear to lack the large number and morphological diversity of cone subtypes found in diurnal reptiles. We have now studied the photoreceptor layer of a South American marsupial (Didelphis marsupialis aurita) by peanut agglutinin labeling of the cone sheath and by labeling of cone outer segments with monoclonal anti-visual pigment antibodies that have been proven to consistently label middle-to-long wavelength (COS-1) and short-wavelength (OS-2) cone subpopulations in placental mammals. Besides a dominant rod population (max. = 400,000/mm2) four subtypes of cones (max. = 3000/mm2) were identified. The outer segments of three cone subtypes were labeled by COS-1: a double cone with a principal cone containing a colorless oil droplet, a single cone with oil droplet, and another single cone. A second group of single cones lacking oil droplets was labeled by OS-2 antibody. The topography of these cone subtypes showed striking anisotropies. The COS-1 labeled single cones without oil droplets were found all over the retina and constituted the dominant population in the area centralis located in the temporal quadrant of the upper, tapetal hemisphere. The population of OS-2 labeled cones was also ubiquitous although slightly higher in the upper hemisphere (200/mm2). The COS-1 labeled cones bearing an oil droplet, including the principal member of double cones, were concentrated (800/mm2) in the inferior, non-tapetal half of the retina. The two spectral types of single cones resemble those of dichromatic photopic systems in most placental mammals. The additional set of COS-1 labeled cones is a distinct marsupial feature. The presence of oil droplets in this cone subpopulation, its absence in the area centralis, and the correlation with the non-tapetal inferior hemisphere suggest a functional specialization, possibly for mesopic conditions. Thus, sauropsid features have been retained but probably with a modified function.

This paper examines how the responses of cells in area 17 of the cat vary as a function of the vernier offset between a bright and a dark bar. The study was prompted by the finding that human vernier acuity is reduced for bars or edges of opposite contrast sign (Mather & Morgan, 1986; O'Shea & Mitchell, 1990). Both simple and complex cells showed V-shaped tuning curves for reverse contrast stimuli: i.e. response was minimum at alignment, and increased with increasing vernier offset. For vernier bars with the same contrast sign, γ-shaped tuning curves were found, as reported earlier (Swindale & Cynader, 1986). Sensitivity to offset was inversely correlated in the two paradigms. However, complex cells with high sensitivity to offsets in a normal vernier stimulus were significantly less sensitive to offsets in reverse contrast stimuli. A cell's response to a vernier stimulus in which both bars are bright can be predicted by the shape of its orientation tuning curve, if the vernier stimulus is approximated by a single bar with an orientation equal to that of a line joining the midpoints of the two component bars (Swindale & Cynader, 1986). This approximation did not hold for the reverse contrast condition: orientation tuning curves for compound barswere broad and shallow, rather than bimodal, with peaks up to 40 deg from the preferred orientation. Results from simple cells were compared with predictions made by a linear model of the receptive field. The model predicted the V-shaped tuning curves found for reverse contrast stimuli. It also predicted that absolute values of tuning slopes for vernier offsets in reverse contrast stimuli might sometimes be higher than with normal stimuli. This was observed in some simple cells. The model was unable to explain the shape of orientation tuning curves for compound bars, nor could it explain the breakdown of the equivalent orientation approximation.

Rotundal neurons in pigeons (Columba livia) were examined for the effects of glutamate and its agonists NMDA and AMPA, antagonists CPP and CNQX, as well as of GABA and its antagonist bicuculline, on visual and tectal stimulation-evoked responses. Glutamate applied by iontophoresis excited all 48 rotundal cells tested, and this excitation was blocked by CNQX but not by CPP in 98% of cases, with 2% of cells being blocked by either CNQX or CPP. Out of 21 cells excited by AMPA, 20 were also excited by NMDA, indicating that AMPA and NMDA receptors may coexist in most rotundal cells. Action potentials were evoked in 36 additional cells by electrical stimulation applied to the tectum and they were also blocked by CNQX but not CPP. Visual responses recorded from a further eight luminance units and 21 motion-sensitive units were also blocked by CNQX and not CPP. On the other hand, GABA inhibited visual responses as well as responses evoked by tectal stimulation. An inhibitory period following tectal stimulation was eliminated by bicuculline. Taken together, these results indicate that glutamate may be an excitatory transmitter acting predominantly through non-NMDA receptors (AMPA receptors) in tectorotundal transmission. Meanwhile, GABA may be an inhibitory transmitter in the pigeon nucleus rotundus.

We investigated the recovery of light-adaptive spinule formation following dopamine depletion with intraocular injection of 6-hydroxydopamine (6-OHDA) and subsequent neogeneration of dopamine interplexiform cells (DA-IPC) at the marginal zone. DA-IPCs were gone by 2 weeks postinjection and appeared at the marginal zone by 6 weeks postinjection, at which time DA-IPC neurites grew toward the central retina, reaching within 0.5 mm of the central retina by 1 year. Retinas from day time, light-adapted fish at 2 weeks, 4 weeks, 3 months, and 1 year postinjection with 6-OHDA were processed for pre-embedding tyrosine hydroxylase immunoreactivity (TOH-IR) and compared to sham-injected and control retinas at the electron-microscopical (EM) level. Only 6-OHDA fish that tilted markedly toward the injected eye were used for these experiments. The tilt mimics the dorsal light reaction, indicating a 2–2.5 log unit increase in the photopic sensitivity of the 6-OHDA eye. Spinule formation was reduced by about 60% in the 2- and 4-week 6-OHDA retinas, but returned to control levels throughout the entire retina of 3-month and 1 year 6-OHDA retinas even though the central region of these retinas contained no detectable TOH-IR. Intraocular injection with 10 μM SCH 23390 (a Dl antagonist) reduced light-adaptive spinule formation by 50% both in control eyes as well as those eyes that were 3 months post 6-OHDA injected. The full return of spinule formation with only partial reinnervation of the retina with DA-IPC processes and their subsequent inhibition by SCH 23390 indicates that dopamine diffused large distances within the retina to regulate this synaptic plasticity (i.e. displayed volume transmission). Also, since all 6-OHDA injected fish displayed an increased photopic sensitivity in the injected eye when sacrificed, we suggest that horizontal cell spinules are not required for photopic luminosity coding in the outer retina.

The electroretinogram (ERG) of the dark-adapted cat eye in response to brief ganzfeld flashes of a wide range of intensities was recorded after intravitreal injection of n-methyl dl aspartate (NMdlA, cumulative intravitreal concentration of 1.3–3.9 mM) to suppress inner-retinal components, and after intravitreal dl or L-2-amino-4-phosphonobutyric acid (dl-APB, 1–3 mM; l-APB, 1.2 mM) and 6-cyano-7-nitroquinoxaline-2, 3 dione (CNQX, 40–60 µM), to suppress all post-receptoral neuronal responses. Rod PII, the ERG component arising from rod bipolar cells, was derived by subtracting records obtained after APB and CNQX from post-NMDLA records. When we measured the derived response at fixed times after the stimulus, we found that PII initially increased in proportion to stimulus intensity without any sign of a threshold. The leading edge of PII at early times after the stimulus, when the response was still small, was well described by V(t) = kI(t −td)5 where k is a constant, I is the intensity of the stimulus, and td is a brief delay of about 3 ms. Correspondingly, the time for the response to rise to an arbitrary small criterion voltage Vcrit was adequately fitted by tcrit = td + (Vcrit/kI)1/5. The time course of the leading edge of the PII response can be interpreted to indicate that the mechanism generating PII introduces three stages of temporal integration in addition to the three stages that are provided by the mechanism of the rod photoreceptors. This finding is consistent with the operation within the rod bipolar cell of a G-protein cascade similar to that in the rods.

The Aii amacrine cell of mammalian retina collects signals from several hundred rods and is hypothesized to transmit quantal “single-photon” signals at scotopic (starlight) intensities. One problem for this theory is that the quantal signal from one rod when summed with noise from neighboring rods would be lost if some mechanism did not exist for removing the noise. Several features of the Aii might together accomplish such a noise removal operation: The Aii is interconnected into a syncytial network by gap junctions, suggesting a noise-averaging function, and a quantal signal from one rod appears in five Aii cells due to anatomical divergence. Furthermore, the Aii contains voltage-gated Na+ and K+ channels and fires slow action potentials in vitro, suggesting that it could selectively amplify quantal photon signals embedded in uncorrelated noise. To test this hypothesis, we simulated a square array of AII somas (Rm = 25,000 Ohm-cm2) interconnected by gap junctions using a compartmental model. Simulated noisy inputs to the Aii produced noise (3.5 mV) uncorrelated between adjacent cells, and a gap junction conductance of 200 pS reduced the noise by a factor of 2.5, consistent with theory. Voltage-gated Na+ and K+ channels (Na+: 4 nS, K+: 0.4 nS) produced slow action potentials similar to those found in vitro in the presence of noise. For a narrow range of Na+ and coupling conductance, quantal photon events (-5–10 mV) were amplified nonlinearly by subthreshold regenerative events in the presence of noise. A lower coupling conductance produced spurious action potentials, and a greater conductance reduced amplification. Since the presence of noise in the weakly coupled circuit readily initiates action potentials that tend to spread throughout the AII network, we speculate that this tendency might be controlled in a negative feedback loop by up-modulating coupling or other synaptic conductances in response to spiking activity.

The bresiliid shrimp, Rimicaris exoculata, lives in large masses on the sides of hydrothermal vent chimneys at two sites on the Mid-Atlantic Ridge. Although essentially no daylight penetrates to depths of 3500 m, very dim light is emitted from the hydrothermal vents themselves. To exploit this light, R. exoculata has evolved a modified compound eye on its dorsal surface that occupies about 0.5% of the animal's body volume. The eye's morphology suggests that it is extremely sensitive to light. The cornea of the dorsal eye is smooth with no dioptric apparatus. The retina consists of two wing-shaped lobes that are fused across the midline anteriorly. The rhabdomeral segments of the 7000 ommatidia form a compact layer of photosensitive membrane with an entrance aperture of more than 26 mm2. Within this layer, the volume density of rhabdom is more than 70%. Below the rhabdomeral segments, a thick layer of white diffusing cells scatters light upward into the photoreceptors. The arhabdomeral segments of the five to seven photoreceptors of each ommatidium are mere strands of cytoplasm that expand to accommodate the photoreceptor nuclei. The rhabdom is comprised of well-organized arrays of microvilli, each with a cytoskeletal core. The rhabdomeral segment cytoplasm contains mitochondria, but little else. The perikaryon contains a band of mitochondria, but has only small amounts of endoplasmic reticulum. There is no ultrastructural indication of photosensitive membrane cycling in these photoreceptors. Vestigial screening pigment cells and screening pigment granules within the photoreceptors are both restricted to the inner surface of the layer of the white diffusing cells. Below the retina, photoreceptor axons converge in a fan-shaped array to enter the dorsal surface of the brain. The eye's size and structure are consistent with a role for vision in shrimp living at abyssal hydrothermal vents.

The influence of center-surround antagonism on light adaptation in cone photoreceptors was investigated by intracellular recording from red-sensitive cones in the retina of the turtle, Pseudemys scripta elegans. Test flashes of 0.15-mm diameter were applied at the center of background fields of 0.25-mm or 2.2-mm diameter. Immediately upon expanding the background from 0.25 to 2.2 mm, the membrane potential depolarized by about 1–4 mV. The test flash response was enhanced if the depolarization was primarily due to synaptic feedback from horizontal cells, whereas the response was attenuated if the prolonged depolarization, an intrinsic response of the cone, was the dominant source of the depolarization. After several seconds, however, only the synaptic depolarization was maintained so maintained illumination of the large background field produced an enhancement of the cone's incremental sensitivity. The enhancement was examined in detail in steady-state conditions by obtaining amplitude-intensity measurements for centered test flashes on steady background fields over a large range of intensity. The effect of the large background field at any fixed intensity was fairly well described as a vertical (upward) shift of the amplitude-intensity curve obtained on the small field. This operation constitutes a quasi-subtractive mechanism of light adaptation and might provide a basis for the sort of subtractive mechanisms inferred from psychophysical studies of human vision. The enhancement was quantified by measuring the incremental sensitivity over four decades of background illumination. The magnitude of the enhancement increased with background intensity and then tended to stabilize at higher background intensities. The maximum difference in incremental sensitivity obtained on the large vs. small background field averaged 0.46 log unit (±0.12 s.d.). At higher background intensities, incremental sensitivity conformed to Weber's Law behavior about equally well for flashes applied on either small or large background fields. In sum, the present results provide evidence for an additional mechanism of light adaptation in cone photoreceptors by showing that the incremental light sensitivity, initially set by mechanisms in the outer segment, can be modulated some three-fold by synaptic feedback at the inner segment of the cone.

It has been proposed that one area of extrastriate cortex in the cat, the lateral suprasylvian area (LS), plays an important role in visual analysis during locomotion (Rauschecker et al., 1987). Cells in LS reportedly tend to prefer directions along a trajectory originating at the center of gaze, and passing outward through the receptive-field center. Such directions coincide with the directions of image motion in an optic flow field, the pattern seen by locomoting observers when they fixate the point towards which they are heading (Gibson, 1950). We re-examined this issue for cells in LS with receptive fields in the lower visual field. Cells recorded posterior to Horsley-Clarke A2 showed a clear correlation between preferred direction and receptive-field location, but not that predicted: preferred directions were generally orthogonal to “optic flow” directions. Since these cells were all located posterior to those in studies showing a bias for “optic flow” directions, we hypothesized that there are two cell populations within LS, an anterior population that tends to prefer radial-outward directions, and a posterior population that tends to prefer directions orthogonal to radial. Data from earlier mapping experiments (Sherk & Mulligan, 1993) supported this idea.

In zebrafish, the first class of cone photoreceptor to become morphologically distinct is the ultraviolet-sensitive short single cone, at 4 days postfertilization, whereas the last class, the red- and green-sensitive double cone, becomes distinct at 10 days postfertilization. We have examined the time course of visual pigment gene expression in zebrafish using whole-mount in situ hybridization. Within the retina, opsins may be detected as early as 40 h postfertilization with the ultraviolet and rod visual pigments being expressed before the blue- (48 h) and red- (60 h) sensitive pigments. In the pineal, red-sensitive opsin is expressed at 48 h postfertilization. Visual pigment expression provides a useful tool for investigations of early cell fate in zebrafish.

Stimulation of neuronal cells by the excitatory amino acid, glutamate, often leads to a rise in cytosolic free calcium concentration ([Ca2+]$sub/sub$), which can affect cell survival and differentiation. The early appearance of endogenous glutamate in the embryonic rabbit retina suggests that it may be involved in intercellular signalling during development. Thus, the effect of glutamate on the [Ca2+]$sub/sub$ of cells in the fetal and neonatal rabbit retina was examined using Ca2+ imaging techniques, which enabled the responses of large numbers of morphologically identified classes of cell to be compared directly. Ganglion cells and amacrine cells, the first retinal neurons to differentiate, showed a rise in [Ca2+]$sub/sub$ in the presence of glutamate from the earliest age studied (embryonic day 20; E20). These responses were mediated by non-NMDA (non-N-methyl-D-aspartate) receptors. NMDA stimulated ganglion cells and amacrine cells only several days later, at about E24. Moreover, whilst most, if not all, putative ganglion cells responded to NMDA, only a subset of putative amacrine cells were sensitive to NMDA throughout development. Photoreceptors, bipolar cells, horizontal cells, and Müller cells differentiate later than the ganglion cells and amacrine cells. Between E20 and birth, cells in the ventricular zone are largely the precursors of these cell types. During this period, 50–60% of ventricular cells responded to glutamate with an increase in [Ca2+]$sub/sub$, upon activation of ionotropic non-NMDA receptors. At no age studied were these ventricular cells, or their differentiated counterparts, stimulated by NMDA. After birth, most cells in the inner nuclear layer were sensitive to non-NMDA receptor agonists, but photoreceptors showed no response. Taken together, the results suggests that NMDA and non-NMDA receptors may adopt separate roles during retinal development, and that non-NMDA receptors, rather than NMDA receptors, may be involved in developmental processes in the ventricular zone.

Synaptic connections of amacrine cells with substance P-like or neuropeptide Y-like immunoreactivity (SP-LI or NPY-LI) in the retina of the cane toad, Bufo marinus, were investigated using ultrastructural immunocytochemistry. The perikarya of SP-LI or NPY-LI amacrine cells were located in the innermost row of the inner nuclear layer. The synapses associated with SP-LI amacrine cells were distributed mainly in sublaminae 3 and 4 with about 10% in sublamina 1 of the inner plexiform layer. The synapses formed by NPY-LI amacrine cells were found in sublaminae 1, 2, and 4 with approximately equal frequency. Of a total of 175 SP-LI profiles, 56% were in presynaptic positions and 44% in postsynaptic positions. The synaptic inputs to SP-LI profiles predominantly derived from other unlabeled amacrine cell dendrites, and to a lesser extent, from bipolar cell terminals. The majority of synaptic outputs from SP-LI amacrine cell dendrites were directed onto unlabeled amacrine cell processes. The SP-LI profiles also made synapses onto bipolar cell terminals and formed synapses onto presumed ganglion cell dendrites. Of a total of 200 NPY-LI profiles, 48% were in presynaptic positions and 52% in postsynaptic positions. The profiles of NPY-LI amacrine cells mainly received their synaptic inputs from other unlabeled amacrine cell processes, and to a lesser extent, from bipolar cell terminals. The majority of NPY-LI amacrine cell profiles gave their synaptic outputs onto unlabeled amacrine cell dendrites, and others formed synapses onto presumed ganglion cell processes. These results suggest that these two populations of neuropeptide-containing amacrine cells in the Bufo retina are involved in different synaptic circuits.

A countermanding paradigm was utilized to investigate the regulation of saccade initiation. Two rhesus monkeys were instructed to generate a saccade to a peripheral target; however, on a fraction of trials after a delay, the monkeys were signaled to inhibit saccade initiation. With short delays between the presentation of the target and the signal to inhibit saccade generation, monkeys withheld saccades to the peripheral target. As the delay of the stop signal increased, monkeys increasingly failed to withhold the saccade. The hypothesis that the generation of the saccade is determined by a race between a go and a stop process provides three explicit means of estimating the covert latency of response to the stop signal. This latency, known as stop signal reaction time, was estimated to be on average 82 ms for both monkeys. Because the stop signal latency represents the time required to exert inhibitory control over saccade production, the countermanding paradigm will be useful for studying neural mechanisms that regulate saccade initiation.

The endogenous opioid peptide [Met5]-enkephalin is a tonically active opioid growth factor (OGF) with an inhibitory action on DNA synthesis in the developing rat retina. In this study, the ontogeny of the spatial and temporal expression of OGF and its binding activity was examined. OGF-like immunoreactivity was detected in the retina at gestation day (E) 20, but not at E18, and was localized to ganglion cell and neuroblast layers; immunochemical reaction was no longer seen in the retina by postnatal day 6. Native OGF was further identified and characterized by high-performance liquid chromatography (HPLC) studies and immunodot assays, which revealed that [Met5]-enkephalin was present in the neonatal, but not adult, rat retina. OGF binding activity was detected as early as E18 using [125I]-[Met5]-enkephalin and in vitro receptor autoradiography. Little OGF binding activity was noted for prenatal retinas, but appreciable activity was observed from birth to postnatal day 4; no OGF binding could be detected after postnatal day 5 or in the adult. These results reveal the transient appearance of the OGF, [Met5]-enkephalin, and its receptor binding activity in the developing mammalian retina, and show that their ontogeny coincides with the timetable of DNA synthesis of retinal neuroblasts.

In the human rod visual system, self-cancellation of flicker signals is observed at high rod intensity levels near 15 Hz, both perceptually and in the electroretinogram (ERG). This and other evidence suggests that two rod signals are transmitted through the human retina with different speeds of transmission. Here we report a series of flicker ERG recordings from a normal observer and an observer who lacks cone vision. From these results, we propose a quantitative model of the two rod signals, which assumes (1) that the amplitude of the slow signal grows linearly with log intensity but then saturates at ~1 scot, td; (2) that the amplitude of the fast signal grows linearly with intensity; (3) that there is a difference in time delay of ~33 ms between two rod signals of the same polarity (or of ~67 ms if the signals are of inverted polarity); and (4) that the time delay of both signals declines linearly with log intensity (by ~10 ms per log scot. td). These simple assumptions provide a remarkably good account of the experimental data. Our results and model are relevant to current anatomical theories of the mammalian rod visual system. We speculate that the slower signal in the human ERG may reflect the transmission of the rod response via the rod bipolars and the An amacrine cells, while the faster signal may reflect its transmission via the rod-cone gap junctions and the cone bipolars. There are, however, several objections to this simple correspondence.

We have examined the effects of neonatal monocular enucleation on the volume of the dorsal lateral geniculate nucleus (dLGN), the area of area 17, and the size and numbers of geniculate relay neurons identified by retrograde transport of HRP from cortex. Compared to values for normal animals, the only significant change contralateral to the remaining eye was an increase in relay cell radius. The effects ipsilateral to the remaining eye were more widespread: we found significant reductions in the volume of the dLGN (27% reduction), the area of striate cortex (22%), and the number (16%) and average soma radius (6%) of geniculate relay neurons. The relay neurons were also more densely packed, suggesting that other geniculate cell types were affected similarly, although this was not explicitly examined. These changes were not uniform throughout the nucleus, and as such, reflected the changes in retinal input. The greatest reduction in cell size occurred in the region of the ipsilateral dLGN receiving the most sparse retinal input subsequent to enucleation. Nor was the shrinkage of the dLGN uniform, being most apparent in the coronal plane especially along the axis orthogonal to the pia; there appeared to be little change in the anteroposterior extent. Shrinkage in area 17 ipsilateral to the remaining eye was the same (about 22%) whether it was defined by myelin staining or transneuronal transport of WGA-HRP. These results show that the transneuronal changes seen in the organization of visual cortex after early monocular enucleation in rodents are associated with only a moderate loss of geniculate relay cells.

The large receptive fields of retinal horizontal cells are thought to reflect extensive electrical coupling via gap junctions. It was shown recently that the biotinylated tracers, biocytin and Neurobiotin, provide remarkable images of coupling between many types of retinal neuron, including horizontal cells. Further, these demonstrations of tracer coupling between horizontal cells rivaled the size of their receptive fields, suggesting that the pattern of tracer coupling may provide some index of the extent of electrical coupling. We studied this question by comparing the receptive field and tracer coupling size of dark-adapted horizontal cells recorded in the superfused, isolated retina-eyecup of the rabbit. Both the edge-to-edge receptive field and space constants (λ) were computed for each cell using a long, narrow slit of light displaced across the retinal surface. Cells were subsequently labeled by iontophoretic injection of Neurobiotin. The axonless A-type horizontal cells showed extensive, homologous tracer coupling in groups greater than 1000 covering distances averaging about 2 mm. The axon-bearing B-type horizontal cells were less extensively tracer coupled, showing homologous coupling of the somatic endings in groups of about 100 cells spanning approximately 400 μm and a separate homologous coupling of the axon terminal endings covering only about 275 μm. Moreover, we observed a remarkable, linear relationship between the size of the receptive fields of each of the three horizontal cell endings and the magnitude of their tracer coupling. Our findings suggest that the extent of tracer coupling provides a strong, linear index of the magnitude of electrical current flow, as derived from receptive-field measures, across groups of coupled horizontal cells. These data thus provide the first direct evidence that the receptive-field size of horizontal cells is related to the extent of their coupling via gap junctions.

Recent electrophysiological experiments have shown that retinal pigment epithelium (RPE) cells begin to produce neuronal-type voltage-dependent sodium currents when placed in dissociated cell culture. In this study, the sodium channel types induced in cultured rat RPE cells were identified. Sodium channel mRNAs encoding two distinct alpha subunits were detected in the cultured RPE cells, brain type II/IIA, and a novel rat mRNA which we have termed RET1. These two sodium channel types may correspond to the TTX-sensitive and TTX-insensitive components of sodium current reported previously in cultured rat RPE cells.

In the frog Xenopus laevis, the isthmotectal projection, which relays input from the ipsilateral eye, exhibits anatomical reorganization following surgical eye rotation performed during tadpole stages while the isthmotectal projection in the frog Rana pipiens fails to show reorganization. This plasticity has been shown to be dependent upon activation of the N-methyl-D-aspartate (NMDA) receptor located on tectal cell dendrites. The reorganization process in Xenopus is hypothesized to employ a Hebbian mechanism requiring correlated firing of ipsilateral and contralateral inputs to a given tectal cell; when an ipsilateral axon synapses onto a tectal cell that receives input from a contralateral axon with a matching receptive-field location, the correlation in activity triggers stabilization of the ipsilateral synapse. However, in neither Xenopus nor Rana do ipsilateral and contralateral inputs begin to fire simultaneously in response to a given visual stimulus; the ipsilateral input is delayed because it reaches the tectum indirectly, through a polysynaptic relay via the opposite tectum and nucleus isthmi. The objective of this experiment was to test whether there is a significant difference in this intertectal delay between Xenopus laevis and Rana pipiens in order to determine whether intertectal delay could be a contributing factor in this species-specific ability to exhibit visual plasticity. We have found that intertectal delay is 26.16 ms longer in Rana pipiens (36.53 ms) than in Xenopus laevis (10.37 ms).