Three types of looming-selective neurons have been found in the nucleus rotundus of pigeons, each computing a different optical variable related to image expansion of objects approaching on a direct collision course with the bird. None of these neurons respond to simulated approach toward stationary objects. A detailed analysis of these neurons' firing pattern to approaching objects of different sizes and velocities shows that one group of neurons signals relative rate of expansion tau (tau), a second group signals absolute rate of expansion rho (rho), and a third group signals yet another optical variable eta (eta). The rho parameter is required for the computation of both tau and eta, whose respective ecological functions probably provide precise 'time-to-collision' information and 'early warning' detection for large approaching objects.

Throughout the animal kingdom, the sight of a rapidly approaching object usually signals danger and elicits an escape response. Gibson suggested that the symmetrical expansion of an object's image (looming) is the critical variable determining that the object is on a collision course with the observer. Similarly, large expanding flow-fields like those produced by locomotion may precipitate manoeuvres such as turning or landing. From such observations it has been shown that the optic flow parameter, tau, which specifies time to contact with the approaching object best fits the behavioural data. We describe a subpopulation of neurons in the nucleus rotundus of the pigeon brain that respond selectively to objects moving on a collision course towards the bird.

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A newly developed flight simulator allows monarch butterflies to fly actively for up to several hours in any horizontal direction while their fall migratory flight direction can be continuously recorded. From these data, long segments of virtual flight paths of tethered, flying, migratory monarch butterflies were reconstructed, and by advancing or retarding the butterflies' circadian clocks, we have shown that they possess a time-compensated sun compass. Control monarchs on local time fly approximately southwest, those 6-h time-advanced fly southeast, and 6-h time-delayed butterflies fly in northwesterly directions. Moreover, butterflies flown in the same apparatus under simulated overcast in natural magnetic fields were randomly oriented and did not change direction when magnetic fields were rotated. Therefore, these experiments do not provide any evidence that monarch butterflies use a magnetic compass during migration.

The responses of single cells to luminance, color and computer-generated spots, bars, kinematograms, and motion-in-depth stimuli were studied in the nucleus rotundus of pigeons. Systematic electrode penetrations revealed that there are several functionally distinct subdivisions within rotundus where six classes of visual-selective cells cluster. Cells in the dorsal-posterior zone of the nucleus respond selectively to motion in depth (i.e. an expanding or contracting figure in the visual field). Most cells recorded from the dorsal-anterior region responded selectively to the color of the stimulus. The firing rate of the cells in the anterior-central zone, however, is dramatically modulated by changing the level of illumination over the whole visual field. Cells in the ventral subdivision strongly respond to moving occlusion edges and very small moving objects, with either excitatory or inhibitory responses. These results indicate that visual information processing of color, ambient illumination, and motion in depth are segregated into different subdivisions at the level of nucleus rotundus in the avian brain.

Cells in intermediate and deeper layers of the pigeon optic tectum respond best when a textured background pattern is moved in the opposite direction to a moving test spot. Complete inhibition occurs when the background moves in the same direction as the test stimulus. Most noteworthy is the invariance of this relationship over a wide range of test spot directions. These cells represent a higher level of abstraction in a motion-detecting system and may play a role in figure-ground segregation or the discrimination of the motion of an object from self-induced optical motion.

Most neurons in cat striate visual cortex show inhibitory effects when moving contours are presented beyond the limits of classic receptive field regions. Facilitatory effects are also present in about 40% of simple cells. Here, we report a highly specific form of this facilitation, mediated only by neurons possessing both an orientation tuning matched to the test unit, and a receptive field position aligned with its long axis. This finding illustrates one of the intracortical interconnection schemes hypothesized by Mitchison and Crick (1982). Periodic clustering in long, intrinsic axons may signify a neuron seeking specific functional interactions like these across columnar systems in both the spatial and orientation domains.

Self-movement of an organism through the environment is guided jointly by information provided by the vestibular system and by visual pathways that are specialized for detecting 'optic flow'. Motion of any object through space, including the self-motion of organisms, can be described with reference to six degrees of freedom: rotation about three orthogonal axes, and translation along these axes. Here we describe neurons in the pigeon brain that respond best to optic flow resulting from translation along one of the three orthogonal axes. We show that these translational optic flow neurons, like rotational optic flow neurons, share a common spatial frame of reference with the semicircular canals of the vestibular system. The three axes to which these neurons respond best are the vertical axis and two horizontal axes orientated at 45 degrees to either side of the body midline.