Directionally Sensitive Neurons in the Frogs Brain

Reddy (1977) examined single visual units found in the frog's brain exhibiting directional sensitivity (DS) to moving visual objects. Reddy made extracellular recordings from visual units in the tectum, thalamus, and cerebellum using glass-coated Pt/Ir microelectrodes. Only 9% of the motion-sensitive units found in these sites were truly DS.

A DS unit by definition fires at a maximum rate for an object having constant speed in a preferred direction (PD). The neuron fires at slower rates for directions of motion different from the PD. Some DS units have a null direction (usually 180° different from the PD) for which the DS unit fires at its slowest rate, often lower than its spontaneous background rate (if it has one). Many animals, vertebrate and invertebrate, have DS visual units. The vertebrate DS unit list includes the frog, goldfish, pigeon, ground squirrel, rabbit, and cat. Arthropod DS units have been found in every flying insect studied (locusts, flies, dragonflies, moths, butterflies) and in such nonflying species as the lubber grasshopper, Romalea microptera, and in crabs.

Some workers have hypothesized that DS units are used for eye and head movement control (visual tracking), or flight stabilization, wherever applicable. However, it is well known that frogs lack eye tracking movements; they can only retract or elevate their eyes (Walls, 1967). Frogs also lack head movements. (Frogs do not have necks that allow head movements independent of their bodies.) Because they cannot move their eyes or heads to track moving prey, frogs must use information from DS units to predict where and when prey (e.g., a fly) can be struck at successfully. Several workers who have recorded from frog GCs (optic nerve fibers) have been unable to locate DS units per se (Grusser-Cornehls et al., 1963; Gaze and Keating, 1970). Reddy was motivated to look for DS cells in the frog's brain in the belief that the animal's prey capture behavior and its escape from danger behavior required DS unit information.

Reddy moved visual objects for his frogs at constant velocity on a large-bed, XY recorder. Object direction vectors relative to the frog were shifted in 22.5° increments (16 major directions) relative to anterior, left, right, and posterior. Object motion was controlled by a triangle wave generator. Different object sizes, shapes, and contrasts were used. Most tests were done with contrasting spot objects. Monocular stimulation was used; the contralateral eye was masked. A window circuit was used to isolate single units; the total number of spikes a unit fired for a given object direction and speed (vector velocity) was counted electronically for a preset number of object motions in a given direction, and also for the reverse motion. This data were used to make polar plots of DS unit vector sensitivity. Most of Reddy's DS units were found in the deep (100 to 300 |im) tectum layers; a few were recorded in the superficial tectum, and in the thalamus and cerebellum.

Figure 6.2-1 illustrates the vector response of a DS unit located in the superficial, contralateral tectum (see caption for details). Note the directional response is significantly sharper than a cosine(29) law. Figure 6.2-2 shows the vector response of a DS unit recorded in the contralateral deep tectum. Frog tectal DS units have an optimum object speed for maximum response. Figure 6.2-3 illustrates this phenomenon for a DS unit in the contralateral, superficial tectum. Maximum response occurred for object speed of 1.85°/s.

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