Ongoing Projects

Measuring the Speed of Perception

Accurate measurement is a cornerstone of science. In our case, our research relies critically on time-resolved measurements of cognitive processing (sometimes labeled as "mental chronometry") that are more precise than those obtained with alternative techniques. In our urgent paradigms, the amount of time available to view the relevant cue stimulus (rPT) is limited and is determined for each individual trial. This way we can track the evolution of a choice process millisecond by millisecond, essentially. We can then measure, for instance, how perceptual processing speed differs across individual subjects, their motivation (say, when a large versus a small reward is at stake), their experience with the task, or the visual features being discriminated (say, color versus shape).

In general, we have found that the dynamics of the oculomotor system are much faster and richer at such short timescales than previously appreciated. For example, saccades are considered highly stereotyped in that, in looking repeatedly from point A to point B, the observed movements are nearly identical. Nearly — but not exactly. We found (Seideman et al., 2018) that when the subject looks at B but is guessing, the peak saccade velocity is slightly lower than average, whereas when the subject looks at B knowing that B is the correct choice, the peak velocity is slightly higher than average (see figure below). The time-resolved data unlock many previously hidden clues about the underlying neuronal dynamics.

How Distinct Neuronal Types Contribute to Saccadic Choices

Oculomotor areas contain three classically-defined neuronal types: visual (V), visuomotor (VM), and motor (M). Although their properties are well understood for simple oculomotor tasks, such as looking at a single, bright spot of light on a dark background, their roles in saccadic choices are less clear. For instance, we discovered that the responses of visual neurons depend critically on the salience of visual stimuli (see figure below), such that when the choice is between potential targets that are equally salient, visual neurons do not contribute to the target selection process at all. These cells can only help select objects that stand out. One aim of our research is to further characterize the functional properties of different oculomotor cell types, as well as of different oculomotor areas in the brain.

Computational Characterization of the Saccadic Choice Process

It is well established that a saccade is triggered when the activity of motor neurons in oculomotor areas builds up and crosses a critical, or threshold level; once the threshold is exceeded, the system commits to producing an eye movement within a few tens of milliseconds. Elaborating on this triggering mechanism, we have developed a computational model whereby a saccadic choice is conceived as a race between two (or more) populations of neurons, such that the first population to exceed the threshold determines the direction of the next saccade. This saccadic competition model reproduces in quantitative detail not only the psychophysical behavior of the subjects in our urgent tasks (see figure below), but also many key features of the oculomotor activity recorded during task performance. Many of our studies include a variant of this model. An important goal of our research program is to develop this computational framework further, to generate testable hypotheses and design new experiments.

Relationship between Attention and Saccade Planning

Spatial attention refers to the capacity to preferentially process information from a specific region of space, filtering out information from other regions. Attention can be deployed either overtly, by moving the eyes, or covertly, while maintaining the eyes fixed. Although it is well established that covert spatial attention is controlled by the same brain circuits that generate saccades, its implementation is still uncertain. For instance, attention can be directed either voluntarily (at will) or involuntarily (reflexively), but what is the neuronal basis of this distinction? We suspect that so-called visual neurons in oculomotor areas (see above) are responsible for the involuntary, reflexive effects, and are actively investigating this.

Again, timing is critical. We recently designed an urgent version of the classic antisaccade task, in which a single bright stimulus appears and the participant is instructed to look away from it (Salinas et al., 2019; Goldstein et al., 2022). Under time pressure this is not easy, because there is a natural tendency to look at salient things that appear abruptly; they grab our attention automatically. We were able to characterize this reflexive tendency very precisely, and found that it exhibits a distinct dynamic: it is very strong but only during a brief period of time of about 40 ms, and thereafter it can be overcome by the willful intent to look away (see figure below). The results demonstrate an internal struggle that unfolds extremely rapidly.

Individual Differences

Our behavioral techniques allow us to accurately measure the time course of fundamental perceptual processes. In a dark background, detecting that a light was turned on takes about 100 ms, finding a ripe tomato among many lemons takes about 130 ms, and distinguishing a coin from a scrabble tile about 160 ms. This is very fast; no wonder that most eye movements are preceded by a deliberation period of just 200–250 ms. Importantly, these numbers depend on the participant. For example, the time required to look away from a bright light may vary by 30 ms or more from one individual to another (see figure above). Because such idiosyncratic differences are highly reliable, we are exploring whether they can serve as measures of mental capacity more generally. Such ‘cognitive fingerprints’ could work as diagnostic markers of mental dysfunction for conditions such as ADHD, in which attention and perceptual processing are significantly altered.