The role of object affordances and center of gravity in eye movements toward isolated daily-life objects.

The purpose of the current study was to investigate to what extent low-level versus high-level effects determine where the eyes land on isolated daily-life objects. We operationalized low-level effects as eye movements toward an object's center of gravity (CoG) or the absolute object center (OC) and high-level effects as visuomotor priming by object affordances. In two experiments, we asked participants to make saccades toward peripherally presented photographs of graspable objects (e.g., a hammer) and to either categorize them (Experiment 1) or to discriminate them from visually matched nonobjects (Experiment 2). Objects were rotated such that their graspable part (e.g., the hammer's handle) pointed toward either the left or the right whereas their action-performing part (e.g., the hammer's head) pointed toward the other side. We found that early-triggered saccades were neither biased toward the object's graspable part nor toward its action-performing part. Instead, participants' eyes landed near the CoG/OC. Only longer-latency initial saccades and refixations were subject to high-level influences, being significantly biased toward the object's action-performing part. Our comparison with eye movements toward visually matched nonobjects revealed that the latter was not merely the consequence of a low-level effect of shape, texture, asymmetry, or saliency. Instead, we interpret it as a higher-level, object-based affordance effect that requires time, and to some extent also foveation, in order to build up and to overcome default saccadic-programming mechanisms.

On the optimal viewing position for object processing.

Numerous studies have shown that a visually presented word is processed most easily when participants initially fixate just to the left of the word's center. Fixating on this optimal viewing position (OVP) results in shorter response times and a lower probability of making additional within-word refixations (OVP effects), but also longer initial-fixation durations (an inverted-OVP or I-OVP effect), as compared to initially fixating at the beginning or the end of the word. Thus, typical curves are u-shaped (or inverted-u-shaped), with a leftward bias. Most researchers explain the u-shape in terms of visual constraints, and the leftward bias in terms of language constraints. Previous studies have demonstrated that (I)-OVP effects are not specific to words, but generalize to object viewing. We further investigated this by comparing the strength and (a)symmetry of (I-)OVP effects for words and objects. To this purpose, we gave participants an object- versus word-naming task in which we manipulated the position at which they initially fixated the stimulus (i.e., a line drawing or the written name of an object). Our results showed that object viewing, just as word viewing, resulted in u-shaped (I-)OVP curves. However, the effect was weaker than for words. Furthermore, for words, the curves were biased to the left, whereas they were symmetrical for objects. This might indicate that part of the (I-)OVP effect for words is language specific, and that (I-)OVP effects for objects are a purer measure of the effect of visual constraints.

The Mind-Writing Pupil: A Human-Computer Interface Based on Decoding of Covert Attention through Pupillometry.

We present a new human-computer interface that is based on decoding of attention through pupillometry. Our method builds on the recent finding that covert visual attention affects the pupillary light response: Your pupil constricts when you covertly (without looking at it) attend to a bright, compared to a dark, stimulus. In our method, participants covertly attend to one of several letters with oscillating brightness. Pupil size reflects the brightness of the selected letter, which allows us-with high accuracy and in real time-to determine which letter the participant intends to select. The performance of our method is comparable to the best covert-attention brain-computer interfaces to date, and has several advantages: no movement other than pupil-size change is required; no physical contact is required (i.e. no electrodes); it is easy to use; and it is reliable. Potential applications include: communication with totally locked-in patients, training of sustained attention, and ultra-secure password input.

The pupillary light response reflects eye-movement preparation.

When the eyes are exposed to an increased influx of light, the pupils constrict. The pupillary light response (PLR) is traditionally believed to be purely reflexive and not susceptible to cognitive influences. In contrast to this traditional view, we report that preparation of a PLR occurs in parallel with preparation of a saccadic eye movement toward a bright (or dark) stimulus, even before the eyes set in motion. Participants fixated a central gray area and made a saccade toward a peripheral target. Using gaze-contingent display changes, we manipulated whether or not the brightness of the target background was the same during and after saccade preparation. More specifically, on some trials we changed the brightness of the target background during the saccade, thus dissociating the preparatory PLR (i.e., to the brightness of the target background before the saccade) from the regular PLR (i.e., to the brightness after the saccade). We show that preparation triggers a pupillary response to the brightness of a to-be-fixated target background already before the eyes have landed on it. We link our findings to the presaccadic shift of attention: The pupil prepares to adjust its size to the brightness of a to-be-fixated stimulus as soon as attention covertly shifts toward that stimulus. Our findings illustrate that the PLR is a dynamic movement that is tightly linked to visual attention and eye-movement preparation.

A comparison of two procedures for verbal response time fractionation.

To describe the mental architecture between stimulus and response, cognitive models often divide the stimulus-response (SR) interval into stages or modules. Predictions derived from such models are typically tested by focusing on the moment of response emission, through the analysis of response time (RT) distributions. To go beyond the single response event, we recently proposed a method to fractionate verbal RTs into two physiologically defined intervals that are assumed to reflect different processing stages. The analysis of the durations of these intervals can be used to study the interaction between cognitive and motor processing during speech production. Our method is inspired by studies on decision making that used manual responses, in which RTs were fractionated into a premotor time (PMT), assumed to reflect cognitive processing, and a motor time (MT), assumed to reflect motor processing. In these studies, surface EMG activity was recorded from participants' response fingers. EMG onsets, reflecting the initiation of a motor response, were used as the point of fractionation. We adapted this method to speech-production research by measuring verbal responses in combination with EMG activity from facial muscles involved in articulation. However, in contrast to button-press tasks, the complex task of producing speech often resulted in multiple EMG bursts within the SR interval. This observation forced us to decide how to operationalize the point of fractionation: as the first EMG burst after stimulus onset (the stimulus-locked approach), or as the EMG burst that is coupled to the vocal response (the response-locked approach). The point of fractionation has direct consequences on how much of the overall task effect is captured by either interval. Therefore, the purpose of the current paper was to compare both onset-detection procedures in order to make an informed decision about which of the two is preferable. We concluded in favor or the response-locked approach.

The pupillary light response reveals the focus of covert visual attention.

The pupillary light response is often assumed to be a reflex that is not susceptible to cognitive influences. In line with recent converging evidence, we show that this reflexive view is incomplete, and that the pupillary light response is modulated by covert visual attention: Covertly attending to a bright area causes a pupillary constriction, relative to attending to a dark area under identical visual input. This attention-related modulation of the pupillary light response predicts cuing effects in behavior, and can be used as an index of how strongly participants attend to a particular location. Therefore, we suggest that pupil size may offer a new way to continuously track the focus of covert visual attention, without requiring a manual response from the participant. The theoretical implication of this finding is that the pupillary light response is neither fully reflexive, nor under complete voluntary control, but is instead best characterized as a stereotyped response to a voluntarily selected target. In this sense, the pupillary light response is similar to saccadic and smooth pursuit eye movements. Together, eye movements and the pupillary light response maximize visual acuity, stabilize visual input, and selectively filter visual information as it enters the eye.