Abstract
A number of studies have shown human subjects’ impressive ability to detect faces in individual images, with saccade reaction times starting as fast as 100 ms after stimulus onset. Here, we report evidence that humans can rapidly and continuously saccade towards single faces embedded in different scenes at rates approaching 6 faces/scenes each second (including blinks and eye movement times). These observations are impressive, given that humans usually make no more than 2 to 5 saccades per second when searching a single scene with eye movements. Surprisingly, attempts to hide the faces by blending them into a large background scene had little effect on targeting rates, saccade reaction times, or targeting accuracy. Upright faces were found more quickly and more accurately than inverted faces; both with and without a cluttered background scene, and over a large range of eccentricities (4°–16°). The fastest subject in our study made continuous saccades to 500 small 3° upright faces at 4° eccentricities in only 96 seconds. The maximum face targeting rate ever achieved by any subject during any sequence of 7 faces during Experiment 3 for the no scene and upright face condition was 6.5 faces targeted/second. Our data provide evidence that the human visual system includes an ultra-rapid and continuous object localization system for upright faces. Furthermore, these observations indicate that continuous paradigms such as the one we have used can push humans to make remarkably fast reaction times that impose strong constraints and challenges on models of how, where, and when visual processing occurs in the human brain.
Highlights
It is often said that humans can typically make 2 to 5 saccades per second over a single scene[1,2]
In contrast to the previously mentioned paradigms, in which participants had to rapidly detect visual targets within an isolated image on each trial[9], the behavioral paradigms in the current study did not have a 200 ms “gap” before the trial, there were no significant pauses between trials, there was no previous training, the target locations and background scenes changed on every trial, and the subjects had no foreknowledge of the position of the target in each trial
Our experiment sought to determine whether the visual system works as fast continuously as has been observed in paced experiments, and whether the continuous detection rate is faster for particular types of targets
Summary
It is often said that humans can typically make 2 to 5 saccades per second over a single scene[1,2]. Faces provoked faster and more accurate saccades compared to other visual objects These saccadic choice paradigms typically use long intervals of pauses during which the subject is not doing the task of interest: a period of 1000 ms between trials, 800–1600 ms of a fixation cross, and a 200 ms “gap” with a blank screen between the fixation cross and the stimulus onset[9,10]. While many studies have focused on the surprising speed and accuracy of object detection and recognition[9,12], none to our knowledge have investigated the number of correct saccades towards objects embedded in changing scenes that can be made each second continuously and without significant experimental pauses between trials. Our experiment sought to determine whether the visual system works as fast continuously as has been observed in paced experiments, and whether the continuous detection rate is faster for particular types of targets (i.e., upright faces versus inverted faces)
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