Light adaptation in retinal rods of the rabbit and two other nonprimate mammals

The responses of rabbit rods to light were studied by drawing a single rod outer segment projecting from a small piece of retina into a glass pipette to record membrane current. The bath solution around the cells was maintained at near 40 degrees C. Light flashes evoked transient outward currents that saturated at up to approximately 20 pA. One absorbed photon produced a response of approximately 0.8 pA at peak. At the rising phase of the flash response, the relation between response amplitude and flash intensity (IF) had the exponential form 1-e-kappa FIF (where kappa F is a constant denoting sensitivity) expected from the absence of light adaptation. At the response peak, however, the amplitude-intensity relation fell slightly below the exponential form. At times after the response peak, the deviation was progressively more substantial. Light steps evoked responses that rose to a transient peak and rapidly relaxed to a lower plateau level. The response-intensity relation again indicated that light adaptation was insignificant at the early rising phase of the response, but became progressively more prominent at the transient peak and the steady plateau of the response. Incremental flashes superposed on a steady light of increasing intensity evoked responses that had a progressively shorter time-to-peak and faster relaxation, another sign of light adaptation. The flash sensitivity changed according to the Weber-Fechner relation (i.e., inversely) with background light intensity. We conclude that rabbit rods adapt to light in a manner similar to rods in cold-blooded vertebrates. Similar observations were made on cattle and rat rods.     

Quantitative Beziehungen zwischen Lichtreiz und Kontraktion des Musculus sphincter pupillae vom Scheibenzüngler (Discoglossus pictus)

Summary The Musculus sphincter pupillae of the toad Discoglossus pictus contracts when exposed to light, even if the iris with the muscle is cut out of the eye. Normally the size of the pupil is controlled by this photosensitivity and in addition by a nervous control mechanism. —Experiments are discribed in which the M. sphincter pupillae of the isolated iris was stimulated by various light programs. The contractile force was measured isometrically with a compensation system. It is shown that the response of the sphincter-muscle to changing light stimuli depends on the mean light intensity; i.e. the sphincter-muscle is an adapting system. The amplitude and phaseshift of the reactions to sinusoidal light stimuli were measured. When stimulus and response are both considered as functions of time, the transformation of stimulus in response by the muscle is linear, provided that the frequency of the sinusoidal light is high and its intensity and modulation degree are low. From this result the hypothesis is derived, that the transformation of stimulus into response is linear only when the level of adaptation does not change during contraction. From the response to sinusoidal light program it was possible to predict the reaction to square wave light programs. The time-course of the reaction to step up light stimuli are similar to the time-course of receptor potentials of phasic-tonic sense cells, showing a maximum, a minimum and a plateau. Within certain limits the amplitude of the reaction depends linearly on the tension of the muscle. Since in isometric experiments the reaction consists of an increase of muscle tension, the muscle becomes more sensitive during reaction and the contractile force increases for a long time, even when light intensity is held constant.     

Effects of intracellular injection of calcium buffers on light adaptation in Limulus ventral photoreceptors

The calcium sequestering agent, EGTA, was injected into Limulus ventral photoreceptors. Before injection, the inward membrane current induced by a long stimulus had a large initial transient which declined to a smaller plateau. Iontophoretic injection of EGTA tended to prevent the decline from transient to plateau. Before injection the plateau response was a nonlinear function of light intensity. After EGTA injection the response-intensity curves tended to become linear. Before injection, bright lights lowered the sensitivity as determined with subsequent test flashes. EGTA injection decreased the light-induced changes in sensitivity. Ca-EGTA buffers having different levels of free calcium were pressure-injected into ventral photoreceptors; the higher the level of free calcium, the lower the sensitivity measured after injection. The effects of inotophoretic injection of EGTA were not mimicked by injection or similar amounts of sulfate and the effects of pressure injection of EGTA buffer solutions were not mimicked by injection of similar volumes of pH buffer or mannitol. The data are consistent with the hypothesis that light adaptation is mediated by a rise of the intracellular free calcium concentration.     

ELECTROPHYSIOLOGY OF EXTRAOCULAR PHOTORECEPTORS IN THE SQUID LOLIGO FORBESI(CEPHALOPODA: LOLIGINIDAE)

The electrophysiology of extraocular photoreception in the myopsidsquid, Loligo forbesi Steenstrup 1856 has been examined. Extracellulargenerator potentials were evoked by white light flashes. Intracellularrecordings from extraocular photoreceptor cells in the parolfactorybodies of the squid demonstrated that they had resting potentialsaround –40 mV, and were depolarised by flashes of white,but not red light (>650 nm). The evoked depolarisation consistedof a transient component, followed by a steady plateau component.The amplitude of depolarisation increased with the logarithmof the light intensity and was maintained for the duration ofthe light stimulus. Action potentials were seen in some recordingsand these increased in frequency during light flash stimulation. (Received 11 February 1997; accepted 10 May 1997)     

Two light-dependent conductances in Lima rhabdomeric photoreceptors

Light-dependent membrane currents were recorded from solitary Lima photoreceptors with the whole-cell clamp technique. Light stimulation from a holding voltage near the cell's resting potential evokes a transient inward current graded with light intensity, accompanied by an increase in membrane conductance. While the photocurrent elicited by dim flashes decays smoothly, at higher stimulus intensities two kinetically distinct components become visible. Superfusion with TEA or intracellular perfusion with Cs do not eliminate this phenomenon, indicating that it is not due to the activation of the Ca-sensitive K channels that are present in these cells. The relative amplitude of the late component vs. the early peak of the light response is significantly more pronounced at -60 mV than at -40 mV. At low light intensities the reversal potential of the photocurrent is around 0 mV, but with brighter lights no single reversal potential is found; rather, a biphasic response with an inward and an outward component can be seen within a certain range of membrane voltages. Light adaptation through repetitive stimulation with bright flashes diminishes the amplitude of the early but not the late phase of the photocurrent. These observations can be accounted for by postulating two separate light-dependent conductances with different ionic selectivity, kinetics, and light sensitivity. The light response is also shown to interact with some of the voltage-sensitive conductances: activation of the Ca current by a brief conditioning prepulse is capable of attenuating the photocurrent evoked by a subsequent test flash. Thus, Ca channels in these cells may not only shape the photoresponse, but also participate in the process of light adaptation.     

Microvillar components of light adaptation in blowflies

The process of light adaptation in blowfly photoreceptors was analyzed using intracellular recording techniques and double and triple flash stimuli. Adapting flashes of increasing intensity caused a progressive reduction in the excitability of the photoreceptors, which became temporarily suppressed when 3 x 10(6) quanta were absorbed by the cell. This suppression was confirmed by subsequently applying an intense test flash that photoactivated a considerable fraction of the 10(8) visual pigment molecules in the cell. The period of temporary desensitization is referred to as the refractory period. The stimulus intensity to render the receptor cell refractory was found to be independent of the extracellular calcium concentration over a range of 10(-4) and 10(-2) M. During the refractory period (30-40 ms after the adapting flash) the cell appears to be "protected" against further light adaptation since light absorption during this period did not affect the recovery of the cell's excitability. Calculations showed that the number of quantum absorptions necessary to induce receptor refractoriness is just sufficient to photoactivate every microvillus of the rhabdomere. This coincidence led to the hypothesis that the refractoriness of the receptor cells is due to the refractoriness of the individual microvilli. The sensitivity of the receptor cells after relatively weak adapting flashes was reduced considerably more than could be accounted for by the microvilli becoming refractory. A quantitative analysis of these results suggests that a photoactivated microvillus induces a local adaptation over a relatively small area of the rhabdomere around it, which includes several tens of microvilli. After light adaptation with an intense flash, photoactivation of every microvillus by the absorption of a few quanta produced only a small receptor response whereas photoactivation of every rhodopsin molecule in every microvillus produced the maximum response. The excitatory efficiency of the microvilli therefore increases with the number of quanta that are absorbed simultaneously.     

Sensitization and habituation of the swimming behavior in ascidian larvae to light

Ascidian larvae of Ciona intestinalis change their photic behavior during the course of development. Newly hatched larvae show no response to a light stimulus at any intensity. At 4 hr after hatching, larvae were induced to start to swimming upon the cessation of illumination, and to stop swimming upon the onset of illumination. At a weaker light intensity (5.0 x 10(-3) J/m (2).s), the larvae showed similar responses to either a single stimulus or repeated stimuli of onset and cessation of light until 10 hr after hatching. At a stronger light intensity (3.2 x 10(-1) J/m(2).s), when the stimulus was repeated, they showed sensitization and habituation of the swimming response. At 3 hr after hatching the larvae failed to show any response to an initial stimulus at any intensity of light, but after several repeated stimuli (sensitization) they showed a swimming response at light intensities above 4.0 x 10(-2) J/m (2).s. At 5 hr and with intensity above 1.0 x 10 (-2) J/m(2).s, the larvae showed photoresponses to the first stimulus, but after several repetitions the larvae failed to stop swimming upon the onset of light (habituation). A repeated series of stimuli at stronger intensities of light caused greater habituation; this habituation was retained for about 1 min. Since the larval central nervous system in Ciona is comprised of only about 100 neurons, learning behavior in ascidian larvae should provide insights for a minimal mechanism of memory in vertebrates.     

High resistance of the rat retina rods to hypoxia

The amplitude of photoreceptor potential was shown to be constant in hypoxia. Under this condition, the energy support of the photoreceptor potential seems to be realized through an anaerobic process rather than the oxidative phosphorylation. The effect of hypoxia was shown to manifest itself in a decrease of the sagged peak part of the photoreceptor potential occurring in response to bright flashes. Such changes of wave form are typical for the case when the rod intracellular potential decreases. The decrease of response amplitude to bright flashes shortens the light scale of the amplitude/light intensity curve.     

Effect of sodium nitrite on the activity of the neocortex neurons during realization of defense and inhibition conditioned reflexes

A decrease in intensity and duration of short-latency reaction components of the sensorimotor and visual cortical neurons to specific stimuli (pain reinforcement and light flashes, respectively) was observed after the administration of NO-generating sodium nitrite (11 mg/kg, subcutaneously). Activation decrease in the visual cortex took place irrespective of biological significance of the light flashes, i.e., in case when this stimulus was a signal of defensive conditioning and in case when these flashes were applied with continuous light (a conditioned inhibitor). Sodium nitrite almost did not change the late activation of sensorimotor and visual neurons in response to pain reinforcement and disinhibitory action of the latter. The results confirm the viewpoint about different neurotransmitters in "specifically modal" and "non-specific" pathways to the neocortex during learning.     

Enhancement and phototransduction in the ventral eye of limulus

Limulus ventral photoreceptors were voltage clamped to the resting (dark) potential and stimulated by a 20-ms test flash and a 1-s conditioning flash. At a constant level of adaptation, we measured the response to the test flash given in the dark (control) and the incremental response produced when the test flash occurred within the duration of the conditioning flash. The incremental response is defined as the response to the conditioning and test flashes minus the response to the conditioning flash given alone. When the test flash was presented within 100 ms after the onset of the conditioning flash we observed that: (a) for dim conditioning flashes the incremental response equaled the control response; (b) for intermediate intensity conditioning flashes the incremental response was greater than the control response (we refer to this as enhancement); (c) for high intensity conditioning flashes the incremental response nearly equaled the control response. Using 10-μm diam spots of illumnination, we stimulated two spatially separate regions of one photoreceptor. When the test flash and the conditioning flash were presented to the same region, enhancement was present; but when the flashes were applied to separate regions, enhancement was nearly absent. This result indicates that enhancement is localized to the region of illumination. We discuss mechanisms that may account for enhancement.     

Significance of presetting an analyzer system in discriminating light intensity in the cat

EEG and behavioural reactions were studied at the action of signal and nonsignal light flashes. The minimal time of stimulus exposition necessary for preserving differences in EEG activation reactions to nonreinforced stimuli of different intensity (in a diapason from 3.5 to 250 lk) was shown to exceed 1 s. After giving the signal meaning to the flashes the minimal time of stimulus exposition when the animals were capable to discriminate intensity of the flashes (in a diapason from 3.5 to 30 lk) just at the first presentation, was equal to 0.1 s. Decrease of the minimal time of stimulus exposition necessary for discrimination of the light signals was supposed to be stipulated by their biological significance and presetting of the analyzing system being organized by preliminary learning a crude analysis of stimuli. Discrimination of the light stimuli of short duration was impossible without such presetting.     

Time course and action spectrum of vibrotactile adaptation

In a series of experiments designed to explore the processes underlying adaptation of the sense of flutter-vibration, vibrotactile threshold was measured on the pad of the index finger, using Békésy tracking. Unadapted thresholds were first measured, for a number of frequencies (4-90 Hz) and contactor sizes (1-8 mm diameter). As expected, these measurements indicated the presence of (1) a Pacinian system possessing spatial summation and increasing in sensitivity, as frequency was raised, at the rate of 12 dB/octave; and (2) a non-Pacinian system showing little spatial summation, and with a frequency characteristic matching that of the NP I mechanism of Bolanowski et al. (1988). These baseline data of Experiment 1 guided the selection of stimulus parameters for subsequent experiments, in which threshold for a test stimulus was measured before, during, and after periods of vibrotactile adaptation. In Experiment 2, test stimuli of 10 Hz and 50 Hz were combined factorially with 30-dB SL adapting stimuli of the same two frequencies. When the test stimulus was 10 Hz, the two adapting frequencies were equally effective in raising threshold; however, when the 50-Hz test stimulus was used, the 50-Hz adapting stimulus raised threshold by a greater amount than did the 10-Hz adapter. These results confirm on the finger the independence of adaptation in Pacinian and non-Pacinian channels, a result previously established on the thenar by other workers. For all four frequency combinations, threshold rose exponentially with a time constant of 1.5-2 min. In Experiment 3, an action spectrum was determined, showing the adapting amplitude needed at each of a series of frequencies to raise the threshold of a 10-Hz stimulus by 10 dB; this spectrum was essentially flat from 30 to 90 Hz. The results, taken in conjunction with what is known about rapidly adapting cutaneous mechanoreceptors, imply that the effectiveness of an adapting stimulus is not determined solely by the amount of activity it generates in first-order afferents.     

Time Course and Action Spectrum of Vibrotactile Adaptation

In a series of experiments designed to explore the processes underlying adaptation of the sense of flutter-vibration, vibrotactile threshold was measured on the pad of the index finger, using Békésy tracking. Unadapted thresholds were first measured, for a number of frequencies (4-90 Hz) and contactor sizes (1-8 mm diameter). As expected, these measurements indicated the presence of (1) a Pacinian system possessing spatial summation and increasing in sensitivity, as frequency was raised, at the rate of 12 dB/octave; and (2) a non-Pacinian system showing little spatial summation, and with a frequency characteristic matching that of the NP I mechanism of Bolanowski et al. (1988). These baseline data of Experiment 1 guided the selection of stimulus parameters for subsequent experiments, in which threshold for a test stimulus was measured before, during, and after periods of vibrotactile adaptation.

In Experiment 2, test stimuli of 10 H_z and 50 H_z were combined factorially with 30-dB SL adapting stimuli of the same two frequencies. When the test stimulus was 10 Hz, the two adapting frequencies were equally effective in raising threshold; however, when the 50-Hz test stimulus was used, the 50-Hz adapting stimulus raised threshold by a greater amount than did the 10-Hz adapter. These results confirm on the finger the independence of adaptation in Pacinian and non-Pacinian channels, a result previously established on the thenar by other workers. For all four frequency combinations, threshold rose exponentially with a time constant of 1.5-2 min.

In Experiment 3, an action spectrum was determined, showing the adapting amplitude needed at each of a series of frequencies to raise the threshold of a 10-Hz stimulus by 10 dB; this spectrum was essentially flat from 30 to 90 Hz. The results, taken in conjunction with what is known about rapidly adapting cutaneous mechanoreceptors, imply that the effectiveness of an adapting stimulus is not determined solely by the amount of activity it generates in first-order afferents.     

The oscillatory responses of skate electroreceptors to small voltage stimuli

Tonic nerve activity in skate electroreceptors is thought to result from spontaneous activity of the lumenal membranes of the receptor cells which is modulated by applied stimuli. When physiological conditions are simulated in vitro, the receptor epithelium produces a current which flows inward across the lumenal surface. This epithelial current exhibits small spontaneous sinusoidal fluctuations about the mean that are associated with corresponding but delayed fluctuations in postsynaptic response. Small voltage stimuli produce damped oscillations in the epithelial current similar in time-course to the spontaneous fluctuations. For lumen-negative, excitatory stimuli, these responses are predominantly an increase over the mean inward current. For inhibitory stimuli they are predominantly a decrease. Increased inward current across the lumenal membranes of the receptor cells increases depolarization of the presynaptic membranes in the basal faces leading to increased release of transmitter and an excitatory postsynaptic response. Decreased inward current decreases depolarization of the presynaptic membranes leading to a reduction in transmitter release and an inhibitory postsynaptic response. Clear changes in postsynaptic response are detectable during stimuli as small as 5 microV with saturation occurring at +/- 400 microV. The evoked oscillations in epithelial current are damped and the postsynaptic responses decline during maintained stimuli with large off-responses occurring at stimulus termination. The initial peak of the off-response is similar to the response produced by onset of an oppositely directed stimulus. These observations substantiate the role of receptor cell excitability in the detection of small voltage changes.     

Correlation of subjective assessments with somatosensory evoked potentials to electrical stimulation of the finger in man

The relationship between 5 positive components of somatosensory evoked potentials (EPs) and subjective response to electrical stimuli, which were recorded in the same human subjects, was assessed in the present study. Five levels of tactile stimuli and 6 levels of noxious stimuli were applied to the tip of the right index finger. The relationship between the magnitude of subjective response and stimulus intensity was well expressed by a power function. Of 5 major positive components in an EP, P30 and P50 were localized at contralateral primary somatic projection area, while P90, P190 and P270 were at the vertex area. The amplitude of the 5 components systematically increased as increasing stimulus intensity, and also increased with the magnitude of subjective response. A significant correlation between the amplitude of P30 or P50 and stimulus intensity was found when the effect of subjective response was partially out. By contrast, the amplitudes of P190 and P270 were associated with subjective response when the effect of stimulus intensity was partially out. These results suggest that the earlier EP components reflect sensory signal processing, while the latter ones concern the subjective evaluating system.     

Role of intracellular calcium and sodium in light adaptation in the retina of the honey bee drone (Apis mellifera, L)

In the honey bee drone, the decrease in sensitivity to light of a retinula cell exposed to background illumination was found to be accurately reflected by the difference in amplitude between the initial transient depolarization and the lowest steady depolarization evoked by the background light. It is shown that both the decrease in sensitivity to light and the accompanying drop in potential from the transient to the plateau can be prevented by injecting EGTA intracellularly. A decrease in duration and amplitude of responses to short test flashes such as observed immediately after illumination was found to occur too when Ca or Na, but not K, Li, or Mg injected into dark-adapted retinula cells. Injection of EGTA into a retinula cell maintained a steady state of light adaptation, was found to cause an increase in amplitude and duration of the response to a short test flash, thus producing the effects of dark adaptation. It is suggested that, in the retina of the honey bee drone, an increase in intracellular calcium concentration plays a central role in light adaptation and that an increase in intracellular sodium concentration, resulting from the influx of sodium ions during the responses to light, could lead to this increase in intracellular free calcium.     

Stability of rapidly adapting afferent entrainment vs responsivity

Spike discharge activity was recorded from low-threshold, rapidly adapting, skin mechanoreceptive afferents (RA afferents) dissected from the median (forelimb) or tibial (hindlimb) nerves in anesthetized monkeys and cats. The spike activity was evoked by delivery of controlled sinusoidal vertical skin displacement ("flutter") stimuli to the receptive field (RF). The stimuli (15-30 Hz; 30-400 mum peak-to-peak amplitude; duration 0.8-15 s) were superimposed on a static skin indentation (0.5-1.0 mm) which was either maintained continuously throughout the run or applied trial-by-trial. The neural activity and the analog signal of the position of the stimulator probe were digitized at 10 kHz resolution and stored for off-line analysis. The main goal was to determine whether changes in the RA afferent response to skin flutter stimulation may be responsible for the enhanced capacity to discriminate stimulus frequency that accompanies a relatively brief (approximately equal to 1 min) pre-exposure to such stimulation in humans. To this end, the spike train data were evaluated using methods that enabled independent measurement of entrainment and responsivity. Responsivity (response intensity) was measured as the average number of spikes/stimulus cycle, while entrainment (the degree to which evoked spike train activity is phase-locked to the stimulus) was quantitatively assessed using statistical techniques developed for the analysis of "circular" (directional) data, supplemented by methods based on the calculation of power spectra from point process data. The methods are demonstrated to enable quantification of RA afferent entrainment over a range of stimulus durations and amplitudes substantially greater than reported in previous studies. While RA afferent responsivity was found to decline to a minor extent (10-20%) both across and within stimulus trials, entrainment remained consistently high and stable, and exhibited no temporal trends or dependence on any other measured factor. The average phase angle of the entrained RA afferent response also remained stable both within and across trials, showing only a tendency to increase slightly during the initial 100-500 ms after stimulus onset. The results imply that the improved capacity to discriminate stimulus frequency that develops in response to an exposure to cutaneous flutter stimulation is not attributable to a change in RA afferent entrainment per se.     

Auditory motion aftereffects of approaching and withdrawing sound sources

Auditory motion aftereffects of approaching and withdrawing sound sources were investigated in the free field. The approaching and withdrawing of a sound source were simulated by means of differently directed changes in the amplitude of impulses of broadband noise (from 20 Hz to 20 kHz) through two loudspeakers placed 1.1 and 4.5 m away from the listener. Presentation of the adapting approaching and withdrawing stimuli changed the perception of test signals following them: a stationary test signal was perceived by listeners as moving in the direction opposite to one of the movement of the adapting stimulus, whereas a test stimulus slowly moving in same direction as the adapting signal was perceived as stationary. The specific features of the auditory aftereffect of signals moving in a radial direction were similar to those of sound sources moving in a horizontal plane.     

Stability of rapidly adapting afferent entrainment vs responsivity

Spike discharge activity was recorded from low-threshold, rapidly adapting, skin mechanoreceptive afferents (RA afferents) dissected from the median (forelimb) or tibial (hindlimb) nerves in anesthetized monkeys and cats. The spike activity was evoked by delivery of controlled sinusoidal vertical skin displacement ("flutter") stimuli to the receptive field (RF). The stimuli (15-30 Hz; 30-400 microm peak-to-peak amplitude; duration 0.8-15 s) were superimposed on a static skin indentation (0.5-1.0 mm) which was either maintained continuously throughout the run or applied trial-by-trial. The neural activity and the analog signal of the position of the stimulator probe were digitized at 10 kHz resolution and stored for off-line analysis. The main goal was to determine whether changes in the RA afferent response to skin flutter stimulation may be responsible for the enhanced capacity to discriminate stimulus frequency that accompanies a relatively brief (approximately 1 min) pre-exposure to such stimulation in humans. To this end, the spike train data were evaluated using methods that enabled independent measurement of entrainment and responsivity. Responsivity (response intensity) was measured as the average number of spikes/stimulus cycle, while entrainment (the degree to which evoked spike train activity is phase-locked to the stimulus) was quantitatively assessed using statistical techniques developed for the analysis of "circular" (directional) data, supplemented by methods based on the calculation of power spectra from point process data. The methods are demonstrated to enable quantification of RA afferent entrainment over a range of stimulus durations and amplitudes substantially greater than reported in previous studies. While RA afferent responsivity was found to decline to a minor extent (10-20%) both across and within stimulus trials, entrainment remained consistently high and stable, and exhibited no temporal trends or dependence on any other measured factor. The average phase angle of the entrained RA afferent response also remained stable both within and across trials, showing only a tendency to increase slightly during the initial 100-500 ms after stimulus onset. The results imply that the improved capacity to discriminate stimulus frequency that develops in response to an exposure to cutaneous flutter stimulation is not attributable to a change in RA afferent entrainment per se.     

Light-induced reduction in excitation efficiency in the trp mutant of Drosophila

In the transient receptor potential (trp) mutant of Drosophila, the receptor potential appears almost normal in response to a flash but quickly decays to baseline during prolonged illumination. Photometric and early receptor potential measurements of the pigment suggest that the pigment is normal and that the decay of the trp response during illumination does not arise from a reduction in the available photopigment molecules. However, there is reduction in pigment concentration with age. Light adaptation cannot account for the decay of the trp response during illumination: in normal Drosophila a dim background light shortens the latency and rise time of the response and also shifts the intensity response function (V-log I curve) to higher levels of light intensity with relatively little reduction in the maximal amplitude (Vmax) of response. In the trp mutant, a dim background light or short, strong adapting light paradoxically lengthens the latency and rise time of the response and substantially reduces Vmax without a pronounced shift of the V-log I curve along the I axis. The effect of adapting light on the latency and V-log I curve seen in trp are associated with a reduction in effective stimulus intensity (reduction in excitation efficiency) rather than with light adaptation. Removing extracellular Ca+2 reduces light adaptation in normal Drosophila, as evidenced by the appearance of "square" responses to strong illumination. In the trp mutant, removing extracellular Ca+2 does not prevent the decay of the response during illumination.