Visual acuity and discrimination learning in the dolphin (Lagenorhynchus obliquidens)

Abstract

This experiment investigated the underwater visual acuity of one adult female Pacific white-sided dolphin (Lagenorhynchus obliquidens) by assessing its ability to discriminate between stimuli comprising one and two vertical lines presented simultaneously in a two-choice discrimination learning apparatus. The experiment was performed in two phases: an initial training phase in which a basic two-line vs. one-line discrimination was established, and a testing phase which utilized this discrimination learning in determination of visual acuity. Visual discrimination learning throughout the training phase, was unexpectedly slow and apparently highly problem-specific, despite the use of a sequence of problems designed to facilitate initial learning. During the testing phase of the study, eight two-line vs. one-line test problems, differing only in two-line gap widths were presented in a random sequence 100 times each during 10 daily sessions, at a minimal viewing distance of 45 cm. A standard psychophysical difference threshold criterion was applied to the performance data, yielding an estimate of underwater visual acuity of approximately 6′ of arc, a value roughly comparable to that for the cat in air. The results of the study are interpreted as suggesting that dolphin vision is probably sufficiently well-developed for it to be a behaviorally significant modality, but that vision may well play only a relatively minor or specialised role in the guidance of normal underwater behavior.