Anterior Decerebration Blocks Visual Habituation in the Larval Salamander (Ambystoma punctatum )

PAUL PIETSCH and CARL W. SCHNEIDER

Department of Visual Sciences, School of Optometry, Indiana University,
Bloomington, Indiana 47405, USA and
Department of Psychology,
Indiana University of Pennsylvania, Indiana, Pennsylvania 15705 USA

web contact:pietsch@indiana.edu

-- adapted from an article in Brain Research Bulletin 25:613-615, 1990.

ABSTRACT

Amputation of the rostral half of the cerebrum induces a compulsion-like reaction in larval Ambystoma punctatum towards Enchytraeus protected within glass vials. Normal and craniotomized larvae are visually attracted to worm- containing vials, as revealed by time-lapse video taping but, after several unsuccessful attempts to get the prey, habituate and depart. The video tapes revealed that anteriorly decerebrated animals spent as much as 100 of 120 minutes at the worm-containing vial, repeatedly but futilely attacking the glass. The data indicate that the telencephalon plays an active negative role in the salamander larva's visually guided behavior.

MATERIALS AND METHODS

All experiments were conducted with Ambystoma punctatum larvae, siblings from the same egg clutch in a specific volley that had simultaneously passed through given Harrison embryonic stages [7] and had attained a cranio-caudal length of 30 mm at surgery. Operations were conducted with the animal anesthetized in MS 222 (see under Tricaine in Merck's Index), 1:5000 in 5 percent Holtfreters's solution, on a clay-lined Petri dish lid, and under a stereoscopic microscope. Subtotal decerebration involved a) snipping the olfactory nerves; b) freeing the cerebrum of underlying connections; c) amputating the anterior half of the telencephalon on an imaginary transverse line -- marked with an eye-piece reticle -- through the rostral poles of the eyes. The immediate goal was a readily reproducible lesion, which the visible landmarks provided, confined to the telencephalon, which sparing the posterior cerebrum insured. Other operations included 1) craniotomy alone, with the still unossified skull cap removed over the forebrain and midbrain; 2) -- removal of the roof of the cerebral aqueduct [4, 8]; 3) bilateral Nerve II rhizotomy -- intracranial resection of the optic nerves; 4) bilateral ophtalmectomy -- removal of both eyes without entering the cranium; 5) anesthesia alone.

Apparatus and Evaluations.

The testing device consisted of 6 glass liquid scintillation vials equidistantly arranged in a circle within a 20 cm diameter by 7 cm deep finger bowl (fig. 1);the bowl and vials contained 10 percent Holtfreter's solution. The fluid level in the vials and bowl was maintained at 3 cm. A space of 6 mm between the vial and wall of the bowl permitted the salamander to swim freely around the vial's perimeter. The apparatus was equally divided into pie-shaped sectors, a vial in each. Vial sectors were numbered. The prospective worm vial was randomly selected and to it were added 6 Enchytraeus chytra from a laboratory-grown culture. The salamander was decanted into the center of the array, in effect presenting it with a roulette-like array of choices in which the random chances of its being in the target sector became vanishingly small with time. Each animal was video-taped for 2 hours with a Sony black-and-white video camera and a Panasonic nv-8050 time-lapse videorecorder set to record at the 48 hr mode, which permitted playback of 1 hr of recording time in 87 sec. Striking and hovering behavior around the worm vial was scored during fast time playback with a cumulative millisecond timer and converted to real time at a ratio of 87/3600. Test sessions (carried out in volleys with controls represented in each) were run during the first week (4-7 days) postoperatively. Variations in specific day or time of day of run (necessitated by the long duration of the run and evaluated in control subjects) had no effects on results, and for the sake of brevity will not be discussed further. Survivors were retested at approximately a month (28-31 days) postoperatively. Statistical analyses were performed on a VAX 8600 series computer using RS/1 and Speakeasy software.

RESULTS

Their eyes removed (ophthalmectomy group in Table 1), salamander larvae ignored the worm vial and exhibited none of the attack behaviors of the normal animals. Cutting the optic nerves (optic nerve rhizotomy in Table 1) had the same initial effect as ophthalmectomy. However, the optic rhizectomy retest (chronic) values were in minutes: 25, 7, 0 an 18; i. e., 3 of 4 were showing normal or near-normal responses to the visual target, doubtless the result of optic nerve regeneration [4,5].

Tectectomy severely curtailed but did not totally eliminate the time subjects spent at the worm vials. Among tectectomized subjects, individual variations were large, the physical conditions of the subjects poor and the survival rate low.

Craniotomize animals spent somewhat more time at the worm vials than did unoperated subjects (27 and 35 minutes for test and retest, respectively), but the differences were not significant; the attack response and apparent habituation reaction were normal.

The partially decerebrate subjects were attracted to the worm vials where they tended to remain for prolonged intervals, circling the vial, intermittently striking, sometimes breaking away for a brief swim around the bowl but always returning to resume what appeared as futile attempts to get at the worms.

Figure 1. A video freeze-frame from the worm-testing trial of a decerebrate A. punctatum larva. Notice the animal to the left of the vial at the bottom of the picture. (back to narrative)

DISCUSSION

Habituation occurred in craniotomized animals. Thus our main findings are directly related to the cerebral lesion. However, further investigations will be necessary to decide if the observed effect is a function of what was in the removed pieces or what was left behind in the flawed brain.

Tectectomy severely debilitated the animals, and we were unable to decide about the reasons underlying the reduced attraction of these subjects to worm vials. Germane to the present observations, however, is that failure of habituation was not one of this group's characteristics, their extensive brain damage notwithstanding. The tectectomized subject attracted to the worm vial departed within a few minutes.

The salamander's visual pathways are widely dispersed throughout its brain [1,5] with the tectum generally accepted as playing the dominant role in central visual processing [8] and telencephalic structures regarded as marginally involved, at best, in the optic functions of some adult urodeles [6]. Believing that the cerebral hemispheres played virtually no role in the larva's vision, we originally conceived of the operation as a control for tectal lesions (because the mass removed is roughly equivalent in each case). As the current experiments show, however, resection of the anterior cerebrum nullifies an active-negative component of the salamander larva's visually guided behavior, a component whose absence evokes displays reminiscent of obsessive compulsions in higher organisms.

REFERENCES

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• 5 Pietsch, P.; Schneider, C. W. Transplanted eyes of foreign donors can reinstate the optically activated skin camouflage reactions in bilaterally enucleated salamanders (Ambystoma). Brain Behav.Evol. 32:364-370; 1988.
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