The cerebral cortex is a rich and diverse structure that is the basis of intelligent behavior. One of the deepest mysteries of the function of cortex is that neural processing times are only about one hundred times as fast as the fastest response times for complex behavior. At the very least, this would seem to indicate that the cortex does massive amounts of parallel computation.This paper explores the hypothesis that an important part of the cortex can be modeled as a connectionist computer that is especially suited for parallel problem solving. The connectionist computer uses a special representation, termed value unit encoding, that represents small subsets of parameters in a way that allows parallel access to many different parameter values. This computer can be thought of as computing hierarchies of sensorimotor invariants. The neural substrate can be interpreted as a commitment to data structures and algorithms that compute invariants fast enough to explain the behavioral response times. A detailed consideration of this model has several implications for the underlying anatomy and physiology.

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Cortical connections and parallel processing: structure and function

When negotiating a narrow gap, honeybees tend to fly through the middle of the gap, balancing the distances to the boundary on either side. To investigate the basis of this “centering response,” bees were trained to fly through a tunnel on their way to a feeding site and back, while their flight trajectories were filmed from above. The wall on either side carried a visual pattern. When the patterns were stationary vertical gratings, bees tended to fly through the middle of the tunnel, i.e. along its longitudinal axis. However, when one of the gratings was in motion, bees flying in the same direction as the moving grating tended to fly closer to while bees flying in the opposite direction tended to fly closer to the stationary grating. This demonstrates, directly and unequivocally, that flying bees estimate the distances of surfaces in terms of the apparent motion of their images. A series of further experiments revealed that the distance to the gratings is gauged in terms of their apparent angular speeds, and that the visual system of the bee is capable of measuring angular speed largely independently of the spatial period, intensity profile, or contrast of the grating. Thus, the motion-sensitive mechanisms mediating range perception appear to be qualitatively different from those that mediate the well-known optomotor response in insects, or those involved in motion detection and ocular tracking in man.

/pdf/range-perception-through-apparent-image-speed-in-freely-nv8t2tp94k.pdf

Range perception through apparent image speed in freely flying honeybees.

Flying insects have evolved sophisticated sensory capabilities to achieve rapid course control during aerial maneuvers. Among two-winged insects such as houseflies and their relatives, the hind wings are modified into club-shaped, mechanosensory halteres, which detect Coriolis forces and thereby mediate flight stability during maneuvers. Here, we show that mechanosensory input from the antennae serves a similar role during flight in hawk moths, which are four-winged insects. The antennae of flying moths vibrate and experience Coriolis forces during aerial maneuvers. The antennal vibrations are transduced by individual units of Johnston's organs at the base of their antennae in a frequency range characteristic of the Coriolis input. Reduction of the mechanical input to Johnston's organs by removing the antennal flagellum of these moths severely disrupted their flight stability, but reattachment of the flagellum restored their flight control. The antennae thus play a crucial role in maintaining flight stability of moths.

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Antennal mechanosensors mediate flight control in moths.

Visual orientation and course-stabilization of flying insects rely essentially on the evaluation of the retinal motion patterns perceived by the animals during flight. Apparent motions of the entire surrounding indicate the direction and speed of self-motion in space and are used as visual feedback signals during optomotor course-control manoeuvres. Discontinuities in the motion pattern and relative motions between pattern-segments indicate the existence of stationary or moving objects and represent the basic visual cues for flight-orientation during fixation-and tracking-sequences, and possibly also for the avoidance of obstacles, and the selection of landing sites.

/pdf/neural-mechanisms-of-visual-course-control-in-insects-3054qvqth5.pdf

Neural Mechanisms of Visual Course Control in Insects

Two aspects of spatial visual orientation in insects constitute the central theme of this chapter: The detection of movement and the evaluation of the position of contrast elements in the visual world. In the first section the visual stimulus situation of an insect moving freely in its natural surround is described. The received “flow field” can be decomposed into three components which result from rotatory and translatory self-movement of the animal and from moving objects (e.g. birds). The next section on basic behavioural phenomena outlines the techniques of open- and closed-loop experiments and describes a few simple experiments on visual movement and position detection in flies. On the basis of these experiments an expression is derived which describes the rotatory component of a fly’s flight path through space. The equation is equivalent to the phenomenological equation of Reichardt and Poggio (1976). The third section investigates the basic principles underlying movement and position detection. Comparison of two schemes for movement detection, the gradient scheme and the correlation scheme, with measured behavioural responses demonstrates that the visual system of flies utilises a correlation-based mechanism for the detection of large- field movement. For position detection again two schemes are discussed which are based on flicker detection and movement detection. A final decision on which of the two schemes might be more relevant for fixation and tracking of moving objects by flies seems not yet possible. In the fourth section interactions between elementary movement and position detectors are deduced from behavioural experiments. In the discussion various more sophisticated aspects of visual spatial orientation behavior of insects are reviewed with emphasis on recent literature (1979 & later).

Behavioural Analysis of Spatial Vision in Insects

1

Descending, movement-sensitive visual interneurons in the ventral nerve cord of the dragonfly,Anax junius, fall into two categories, based upon their responses to a variety of stimulus patterns One group (object-movement detectors) is sensitive only to movement of small patterns; the other (self-movement detectors) responds maximally to movement of very large patterns or to rotation of the animal in the lighted laboratory




2

Object-movement-detector responses to repeated identical stimuli habituate very rapidly The habituation is region specific; pattern movement elsewhere in the receptive field elicits a renewed response (Fig 3) The habituation is very long lasting and is not subject to dishabituation by mechanical or visual stimulation Self-movement detectors, in contrast, show little or no habituation (Fig 3)




3

Increasing the extent of the stimulus pattern in the direction of motion decreases responses of object-movement detectors slightly and greatly increases self-movement-detector responses (Fig 4)




4

Increasing the length of the advancing edges perpendicular to the line of motion dramatically reduces object-movement-detector responses (Fig 5) Such increases enhance self-movement-detector responses only slightly, unless they result in the pattern occupying especially sensitive regions of the receptive field (Fig 7)




5

Over a wide velocity range, self-movement-detector responses are not dependent on pattern wavelength (Fig 8)




6

These results indicate that the parameter upon which the object/world discrimination is based is different for the two groups of interneurons The critical parameter for the self-movement detectors is the extent of the pattern in the direction of motion, whereas for the object-movement detectors, the critical parameter is the extent of the pattern perpendicular to the direction of motion

Object- and self-movement detectors in the ventral nerve cord of the dragonfly

1.

The self-movement detectors whose detailed properties are introduced here are among the largest interneurons in the ventral nerve cord of the dragonflyAnax junius. They combine information from a wide variety of sensory structures including the compound eyes, mechanosensory frontal hairs, antennae, neck proprioceptors, and wind sensitive hairs in the neck region (Fig. 6). They fall into 8 classes, each with a specific pattern of multimodal inputs (Fig. 1).




2.

Their visual receptive fields are large, anteriorly weighted, and selective for animal rotation in a given direction (Figs. 1, 2).




3.

They show directionally selective responses to air puffs delivered to the front of the head and to shifts in direction of a stream of air onto the head (Fig. 8).




4.

They show directionally selective responses to air puffs from the sides, above, and below the animal into the neck region (Figs. 3, 4, 5).




5.

They respond directionally to rotations of the head in still air in the dark (Figs. 3, 4, 11). The receptors mediating these proprioceptive responses are probably different from those mediating the responses to air puffs to the neck.




6.

Four of the eight classes respond when the animal actively flexes its abdomen (Fig. 12).




7.

Within each modality, they are inhibited by stimulation in the non-preferred direction. Pairwise presentations of stimuli to different inputs showed that each sensory channel is capable of inhibiting self-movement-detector responses to stimulation through any other channel (Figs. 13, 14).




8.

For each class, the receptive fields of the various inputs are all aligned to detect rotation of the animal or the head within the same plane in space (Figs. 3, 4, 5). Two classes are aligned in the horizontal plane, and the other six in oblique planes (Fig. 1).




9.

The precise alignment of directional selectivity of several input modalities within the same plane, combined with the finding that individual self-movement detectors drive adjustments in wing position (Olberg 1978) indicates that these interneurons are critical elements of a feedback control system for flight control.

Robert M. Olberg

Papers

Object- and self-movement detectors in the ventral nerve cord of the dragonfly

Parallel encoding of direction of wind, head, abdomen, and visual pattern movement by single interneurons in the dragonfly