Visual responses of crayfish ocular motoneurons: an information theoretical analysis

Polarization vision in crayfish motion detectors

Motion detector interneurons were examined to determine their responsiveness to the motion of polarized light images (i.e. images segmented by spatial variations in e-vector angle). Computer generated images were displayed as intensity contrasts or polarization contrasts on a modified LCD projection panel. The stimuli included the motion of a single stripe (45 degrees -55 degrees /s) and the global motion of a square wave grating (3.3 degrees /s). Neurons were impaled in the medulla interna. Of the neurons which exhibited a directional response to the motion of intensity contrast stimuli, about 2/3 were also directional in the response to polarized light images. Transient (nondirectional) stimuli included looming and jittery motions. The responses to the transient motions of the polarized light images were roughly comparable to those elicited by intensity contrast. The results imply that behavioral responses to polarized light images (i.e. optokinetic and defense reflexes) may have a basis in the polarization sensitivity and synaptic organization of the medulla interna.

Characterization of lobula giant neurons responsive to visual stimuli that elicit escape behaviors in the crab Chasmagnathus

In the grapsid crab Chasmagnathus, a visual danger stimulus elicits a strong escape response that diminishes rapidly on stimulus repetition. This behavioral modification can persist for several days as a result of the formation of an associative memory. We have previously shown that a generic group of large motion-sensitive neurons from the lobula of the crab respond to visual stimuli and accurately reflect the escape performance. Additional evidence indicates that these neurons play a key role in visual memory and in the decision to initiate an escape. Although early studies recognized that the group of lobula giant (LG) neurons consisted of different classes of motion-sensitive cells, a distinction between these classes has been lacking. Here, we recorded in vivo the responses of individual LG neurons to a wide range of visual stimuli presented in different segments of the animal's visual field. Physiological characterizations were followed by intracellular dye injections, which permitted comparison of the functional and morphological features of each cell. All LG neurons consisted of large tangential arborizations in the lobula with axons projecting toward the midbrain. Functionally, these cells proved to be more sensitive to single objects than to flow field motion. Despite these commonalities, clear differences in morphology and physiology allowed us to identify four distinct classes of LG neurons. These results will permit analysis of the role of each neuronal type for visually guided behaviors and will allow us to address specific questions on the neuronal plasticity of LGs that underlie the well-recognized memory model of the crab.

Orientation by polarized light in the crayfish dorsal light reflex: behavioral and neurophysiological studies

In decapod crustaceans, the dorsal light reflex rotates the eyestalk so that the dorsal retina faces the brightest segment of dorsal visual space. Stepwise displacements of white stripes elicit eyestalk rotations in the same direction as that of the stripe. Conversely, stepwise displacements of black stripes on a white background elicit eyestalk rotations in the opposite direction as that of the stripe. The reversal of the response with contrast inversion distinguishes the dorsal light reflex from an optokinetic reflex. When the visual scene is composed of polarized light, segmented by variations in e-vector orientation, displacement of segments containing near vertical e-vectors elicit responses similar to those elicited by a white stripe. Displacement of polarized stripes containing near horizontal e-vectors elicit eyestalk rotations similar to those elicited by a black stripe. The results are consistent with the use of polarized light in orientation. The stimulus conditions described above were also applied to visual interneurons (sustaining fibers) and oculomotor neurons and the results were generally in accord with the behavior. In the neural studies, it was possible to show that responses to polarized stripe displacements are predictable from the receptive field location and the neuron's polarization tuning function.

Polarization contrast and motion detection

Form and motion perception rely upon the visual system's capacity to segment the visual scene based upon local differences in luminance or wavelength. It is not clear if polarization contrast is a sufficient basis for motion detection. Here we show that crayfish optomotor responses elicited by the motion of images derived from spatiotemporal variations in e-vector angles are comparable to contrast-elicited responses. Response magnitude increases with the difference in e-vector angles in adjacent segments of the scene and with the degree of polarization but the response is relatively insensitive to the absolute values of e-vector angles that compose the stimulus. The results indicate that polarization contrast can support visual motion detection.

Binocular visual integration in the crustacean nervous system

Although the behavioral repertoire of crustaceans is largely guided by visual information their visual nervous system has been little explored. In search for central mechanisms of visual integration, this study was aimed at identifying and characterizing brain neurons in the crab involved in binocular visual processing. The study was performed in the intact animal, by recording intracellularly the response to visual stimuli of neurons from one of the two optic lobes. Identified neurons recorded from the medulla (second optic neuropil), which include sustaining neurons, dimming neurons, depolarizing and hyperpolarizing tonic neurons and on-off neurons, all presented exclusively monocular (ipsilateral) responses. In contrast, all wide field movement detector neurons recorded from the lobula (third optic neuropil) responded to moving stimuli presented to the ipsilateral and to the contralateral eye. In these cells, the responses evoked by ipsilateral or contralateral stimulation were almost identical, as revealed by analysing the number and amplitude of the elicited postsynaptic potentials and spikes, and the ability to habituate upon repeated visual stimulation. The results demonstrate that in crustaceans important binocular processing takes place at the level of the lobula.

Analysis and simulation of gain control and precision in crayfish visual interneurons

Impulse trains in sustaining and dimming fibers of crayfish optic lobe (in situ) were elicited with sinusoidal extrinsic current and sine-wave illumination. Extrinsic currents and currents derived from postsynaptic potentials (PSPs) were used to compute the time course of the spike train with an adaptive integrate-and-fire model. The neurons exhibit variations in gain and spike timing precision related to the frequency of stimulation. These phenomena are influenced by spike-frequency adaptation and nonlinearities in the PSP. Dimming fibers exhibit relatively strong spike-frequency adaptation and an associated increase in gain with the frequency of sinusoidal extrinsic current and sine-wave illumination. The dimming fiber IPSP promotes spike train rectification, and rectification contributes to spike timing precision. Sustaining fibers exhibit weaker spike-frequency adaptation and the gain of the current-elicited response is less sensitive to stimulus frequency. The sustaining fiber excitatory PSP, however, exhibits a strong frequency-dependent nonlinearity that influences the frequency response. Spike timing precision is a function of stimulus frequency in all cells and it is enhanced by rectification of the discharge and/or resonance. In rectified responses the jitter in spike times is closely related to the variance in the times the membrane potential reaches spike threshold. These gain and spike timing results are well approximated by the simulated responses. Because the nonlinearity of the sustaining fiber PSP entails a high rate of depolarization, the PSP can increase the precision of spike timing by 10- to 100-fold compared with the response to pure sine-wave stimuli. This enhanced precision has implications for crayfish oculomotor reflexes that are driven by sustaining fibers and highly sensitive to impulse timing during transient excitation.

Encoder adaptation modulates the visual responses of crayfish interneurons

The responses of sustaining and dimming fibers were characterized by the time varying firing rates elicited by extrinsic current and flashes of light. These data were simulated by an adaptive integrate-and-fire model. A postimpulse shunt conductance simulated spike-frequency adaptation. The correlation between observed and model current-elicited impulse rates was 0.94-0.98. However, except for a difference in input resistance (both measured and simulated), the voltage to impulse encoders of the two cell groups was similar and exhibited comparable degrees of spike-frequency adaptation (40 to 45%). The encoder model derived from current-elicited responses (with fixed parameters) was used to simulate visual responses elicited by light flashes. These simulations included a synaptic current derived from the time course of the postsynaptic potential (PSP). The sustaining fiber visual response consisted of a large excitatory PSP and high-frequency transient burst that adapted (by approximately 80%) to a low-frequency plateau discharge. The simulations indicated that spike-frequency adaptation had no effect on the transient discharge but reduced the plateau firing rate by approximately 60%. Encoder adaptation enhances the sustaining fiber response to the time derivative of the stimulus. In dimming fibers, the light flash elicits an inhibitory PSP that interrupts the "dark discharge" and an off response following the end of the flash. The simulations indicated that spike-frequency adaptation reduces the firing rate of both the dark discharge and the off response. Thus the model suggests that different effects of encoder adaptation on the two cell types arise from the same encoder mechanisms, but different actions are determined by differences in impulse rate and the time course of the discharge.