Response characteristics of the pigeon's pretectal neurons to illusory contours and motion.
ABSTRACT Misinterpretations of visual information received by the retina are called visual illusions, which are known to occur in higher brain areas. However, whether they would be also processed in lower brain structures remains unknown, and how to explain the neuronal mechanisms underlying the motion after-effect is intensely debated. We show by extracellular recording that all motion-sensitive neurons in the pigeon's pretectum respond similarly to real and illusory contours, and their preferred directions are identical for both contours in unidirectional cells, whereas these directions are changed by 90 deg for real versus illusory contours in bidirectional cells. On the other hand, some pretectal neurons produce inhibitory (excitatory) after-responses to cessation of prolonged motion in the preferred (null) directions, whose time course is similar to that of the motion after-effect reported by humans. Because excitatory and inhibitory receptive fields of a pretectal cell overlap in visual space and possess opposite directionalities, after-responses to cessation of prolonged motion in one direction may create illusory motion in the opposite direction. It appears that illusory contours and motion could be detected at the earliest stage of central information processing and processed in bottom-up streams, and that the motion after-effect may result from functional interactions of excitatory and inhibitory receptive fields with opposite directionalities.
Article: A precedence effect resolves phantom sound source illusions in the parasitoid fly Ormia ochracea.[show abstract] [hide abstract]
ABSTRACT: Localizing individual sound sources under reverberant environmental conditions can be a challenge when the original source and its acoustic reflections arrive at the ears simultaneously from different paths that convey ambiguous directional information. The acoustic parasitoid fly Ormia ochracea (Diptera: Tachinidae) relies on a pair of ears exquisitely sensitive to sound direction to localize the 5-kHz tone pulsatile calling song of their host crickets. In nature, flies are expected to encounter a complex sound field with multiple sources and their reflections from acoustic clutter potentially masking temporal information relevant to source recognition and localization. In field experiments, O. ochracea were lured onto a test arena and subjected to small random acoustic asymmetries between 2 simultaneous sources. Most flies successfully localize a single source but some localize a 'phantom' source that is a summed effect of both source locations. Such misdirected phonotaxis can be elicited reliably in laboratory experiments that present symmetric acoustic stimulation. By varying onset delay between 2 sources, we test whether hyperacute directional hearing in O. ochracea can function to exploit small time differences to determine source location. Selective localization depends on both the relative timing and location of competing sources. Flies preferred phonotaxis to a forward source. With small onset disparities within a 10-ms temporal window of attention, flies selectively localize the leading source while the lagging source has minimal influence on orientation. These results demonstrate the precedence effect as a mechanism to overcome phantom source illusions that arise from acoustic reflections or competing sources.Proceedings of the National Academy of Sciences 04/2009; 106(15):6357-62. · 9.68 Impact Factor
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ABSTRACT: A saccadic eye movement causes a variety of transient perceptual sequelae that might be the results of corollary discharge. Here we describe the neural circuits for saccadic corollary discharge that modulates activity throughout the pigeon visual system. Saccades in pigeons caused inhibition that was mediated by corollary discharge followed by enhancement of firing activity in the telencephalic hyperpallium, visual thalamus and pretectal nucleus lentiformis mesencephali (nLM) with opposite responses in the accessory optic nucleus (nBOR). Inactivation of thalamic neurons eliminated saccadic responses in telencephalic neurons, and inactivation of both the nLM and the nBOR abolished saccadic responses in thalamic neurons. Saccade-related omnipause neurons in the brainstem raphe complex inhibited the nBOR and excited the nLM, whereas inactivation of raphe neurons eliminated saccadic responses in both optokinetic and thalamic neurons. It seems that saccadic responses in telencephalic neurons are generated by corollary discharge signals from brainstem neurons that are transmitted through optokinetic and thalamic neurons. These signals might have important roles in visual perception.Nature Neuroscience 06/2008; 11(5):595-602. · 15.53 Impact Factor
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ABSTRACT: The motion aftereffect (MAE) describes an illusory motion in the opposite direction after the sudden halt of a prolonged moving visual stimulus. Behaviorally, this illusion was mostly analyzed in humans and other mammals. Up to now, birds were never tested. Since a new neural mechanism for the MAE was recently discovered in the pigeons' midbrain, the aim of this study was to investigate if pigeons can perceive this illusion. In two successive experiments, we trained animals to discriminate black and white grating patterns with two moving directions: upward or downward, or standing still. During transfer tests, animals were shortly confronted with the static pattern after prolonged exposure to a moving stimulus. The choice behaviors of these animals were highly indicative for the perception of an MAE. The possible neuronal substrate for the movement aftereffect is discussed.Behavioural Brain Research 04/2008; 187(2):327-33. · 3.42 Impact Factor