Visual search in hunting archerfish shares all hallmarks of human performance

Abstract

Summary Archerfish are renowned for shooting down aerial prey with water jets, but nothing is known about the ways they spot prey items in their richly structured mangrove habitats. We trained archerfish to stably assign the categories 'target' and 'background' to objects solely on the basis of non-motion cues. Unlike many other hunters archerfish are able to discriminate a target from its background in the complete absence of either self-motion or relative motion parallax cues and without using stored information about the structure of the background. This allowed us to perform matched tests to compare the ways fish and humans scan stationary visual scenes. In humans, visual search is seen as a doorway to cortical mechanisms of how attention is allocated. Fish lack a cortex and we therefore wondered if archerfish would differ from humans in their ways they scan a stationary visual scene. Our matched tests failed to disclose any differences in the dependence of response time distributions, a most sensitive indicator of the search mechanism, on number and complexity of background objects. Median and range of response times depended linearly on the number of background objects and the corresponding effective processing time per item increased similarly - about fourfold - in both humans and fish when the task was harder. Archerfish, like humans, also systematically scanned the scenery, starting with the closest object. Taken together, benchmark visual search tasks failed to disclose any difference between archerfish - who lack a cortex - and humans.