David M. Eagleman

A standardized test battery for the study of synesthesia

Synesthesia is an unusual condition in which stimulation of one modality evokes sensation or experience in another modality. Although discussed in the literature well over a century ago, synesthesia slipped out of the scientific spotlight for decades because of the difficulty in verifying and quantifying private perceptual experiences. In recent years, the study of synesthesia has enjoyed a renaissance due to the introduction of tests that demonstrate the reality of the condition, its automatic and involuntary nature, and its measurable perceptual consequences. However, while several research groups now study synesthesia, there is no single protocol for comparing, contrasting and pooling synesthetic subjects across these groups. There is no standard battery of tests, no quantifiable scoring system, and no standard phrasing of questions. Additionally, the tests that exist offer no means for data comparison. To remedy this deficit we have devised the Synesthesia Battery. This unified collection of tests is freely accessible online (http://www.synesthete.org). It consists of a questionnaire and several online software programs, and test results are immediately available for use by synesthetes and invited researchers. Performance on the tests is quantified with a standard scoring system. We introduce several novel tests here, and offer the software for running the tests. By presenting standardized procedures for testing and comparing subjects, this endeavor hopes to speed scientific progress in synesthesia research.

Human time perception and its illusions

Why does a clock sometimes appear stopped? Is it possible to perceive the world in slow motion during a car accident? Can action and effect be reversed? Time perception is surprisingly prone to measurable distortions and illusions. The past few years have introduced remarkable progress in identifying and quantifying temporal illusions of duration, temporal order, and simultaneity. For example, perceived durations can be distorted by saccades, by an oddball in a sequence, or by stimulus complexity or magnitude. Temporal order judgments of actions and sensations can be reversed by the exposure to delayed motor consequences, and simultaneity judgments can be manipulated by repeated exposure to nonsimultaneous stimuli. The confederacy of recently discovered illusions points to the underlying neural mechanisms of time perception.

Vividness of mental imagery: Individual variability can be measured objectively

When asked to imagine a visual scene, such as an ant crawling on a checkered table cloth toward a jar of jelly, individuals subjectively report different vividness in their mental visualization. We show that reported vividness can be correlated with two objective measures: the early visual cortex activity relative to the whole brain activity measured by functional magnetic resonance imaging (fMRI) and the performance on a novel psychophysical task. These results show that individual differences in the vividness of mental imagery are quantifiable even in the absence of subjective report.

Visual illusions and neurobiology

The complex structure of the visual system is sometimes exposed by its illusions. The historical study of systematic misperceptions, combined with a recent explosion of techniques to measure and stimulate neural activity, has provided a rich source for guiding neurobiological frameworks and experiments.

The Effect of Predictability on Subjective Duration

Events can sometimes appear longer or shorter in duration than other events of equal length. For example, in a repeated presentation of auditory or visual stimuli, an unexpected object of equivalent duration appears to last longer. Illusions of duration distortion beg an important question of time representation: when durations dilate or contract, does time in general slow down or speed up during that moment? In other words, what entailments do duration distortions have with respect to other timing judgments? We here show that when a sound or visual flicker is presented in conjunction with an unexpected visual stimulus, neither the pitch of the sound nor the frequency of the flicker is affected by the apparent duration dilation. This demonstrates that subjective time in general is not slowed; instead, duration judgments can be manipulated with no concurrent impact on other temporal judgments. Like spatial vision, time perception appears to be underpinned by a collaboration of separate neural mechanisms that usually work in concert but are separable. We further show that the duration dilation of an unexpected stimulus is not enhanced by increasing its saliency, suggesting that the effect is more closely related to prediction violation than enhanced attention. Finally, duration distortions induced by violations of progressive number sequences implicate the involvement of high-level predictability, suggesting the involvement of areas higher than primary visual cortex. We suggest that duration distortions can be understood in terms of repetition suppression, in which neural responses to repeated stimuli are diminished.

Is subjective duration a signature of coding efficiency

Perceived duration is conventionally assumed to correspond with objective duration, but a growing literature suggests a more complex picture. For example, repeated stimuli appear briefer in duration than a novel stimulus of equal physical duration. We suggest that such duration illusions appear to parallel the neural phenomenon of repetition suppression, and we marshal evidence for a new hypothesis: the experience of duration is a signature of the amount of energy expended in representing a stimulus, i.e. the coding efficiency. This novel hypothesis offers a unified explanation for almost a dozen illusions in the literature in which subjective duration is modulated by properties of the stimulus such as size, brightness, motion and rate of flicker.

Time and the Brain: How Subjective Time Relates to Neural Time

Most of the actions our brains perform on a daily basis, such as perceiving, speaking, and driving a car, require timing on the scale of tens to hundreds of milliseconds. New discoveries in psychophysics, electrophysiology, imaging, and computational modeling are contributing to an emerging picture of how the brain processes, learns, and perceives time.

Causality and the perception of time

Does our perception of when an event occurs depend on whether we caused it? A recent study suggests that when we perceive our actions to cause an event, it seems to occur earlier than if we did not cause it.

The objectification of overlearned sequences: A new view of spatial sequence synesthesia

In the phenomenon of spatial sequence synesthesia (SSS), subjects can articulate explicit spatial locations for sequences such as numbers, letters, weekdays, months, years, and other overlearned series. Similarly, abstract sequences can take on implicit spatial representations in non-synesthetes, as evidenced by the spatial numerical association of response codes (SNARC) effect. An open question is whether the two findings represent different degrees of the same condition, or different conditions. To address this, we developed computer programs to quantify three-dimensional (3D) month-form coordinates in 571 self-reported spatial sequence synesthetes; this approach opens the door for the first time to quantified large-scale analysis. First, despite the common assumption that month-forms tend to be elliptical, we find this to be true in only a minority of cases. Second, we find that 27% of month forms are in the shape of lines, consistent with the assumed shape of implicit spatial forms in the SNARC effect. Next, we find that the majority of month forms are biased in a left-to-right direction, also consistent with the directional bias in the SNARC effect (in Western speakers). Collectively, these findings support the possibility that SSS is directly related to the sequence representations in non-synesthetes. While the search for neural correlates has concentrated on areas in the parietal lobe involved in numeric manipulation and coordinate systems, we propose that the basis of this synesthesia may be the close proximity of temporal lobe regions implicated in sequence coding and visual object representation.

Ready...go: Amplitude of the FMRI signal encodes expectation of cue arrival time.

A neuroimaging study reveals novel insights into how the brain responds to an anticipated event, such as a starting gun or responding to a green light.