Evaluating the contribution of shape attributes to recognition using the minimal transient discrete cue protocol

Visual encoding of partial unknown shape boundaries

Prior research has found that known shapes and letters can be recognized from a sparse sampling of dots that mark locations on their boundaries. Further, unknown shapes that are displayed only once can be identified by a matching protocol, and here also, above-chance performance requires very few boundary markers. The present work examines whether partial boundaries can be identified under similar low-information conditions. Several experiments were conducted that used a match-recognition task, with initial display of a target shape followed quickly by a comparison shape. The comparison shape was either derived from the target shape or was based on a different shape, and the respondent was asked for a matching judgment, i.e., did it "match" the target shape. Stimulus treatments included establishing how density affected the probability of a correct decision, followed by assessment of how much positioning of boundary dots affected this probability. Results indicate that correct judgments were possible when partial boundaries were displayed with a sparse sampling of dots. We argue for a process that quickly registers the locations of boundary markers and distills that information into a shape summary that can be used to identify the shape even when only a portion of the boundary is represented.

Very small faces are easily discriminated under long and short exposure times

Acuity measures related to overall face size that can be perceived have not been studied quantitatively. Consequently, experimenters use a wide range of sizes (usually large) without always providing a rationale for their choices. I studied thresholds for face discrimination by presenting both long (500 ms)- and short (17, 33, 50 ms)-duration stimuli. Face width threshold for the long presentation was ~0.2°, and thresholds for the flashed stimuli ranged from ~0.3° for the 17-ms flash to ~0.23° for the 33- and 50-ms flashes. Such thresholds indicate that face stimuli used in physiological or psychophysical experiments are often too large to tap human fine spatial capabilities, and thus interpretations of such experiments should take into account face discrimination acuity. The 0.2° threshold found in this study is incompatible with the prevalent view that faces are represented by a population of specialized “face cells” because those cells do not respond to <1° stimuli and are optimally tuned to >4° faces. Also, the ability to discriminate small, high-spatial frequency flashed face stimuli is inconsistent with models suggesting that fixational drift transforms retinal spatial patterns into a temporal code. It seems therefore that the small image motions occurring during fixation do not disrupt our perception, because all relevant processing is over with before those motions can have significant effects.

NEW & NOTEWORTHY Although face perception is central to human behavior, the minimally perceived face size is not known. This study shows that humans can discriminate very small (~0.2°) faces. Furthermore, even when flashed for tens of milliseconds, ~0.25° faces can be discriminated. Such fine acuity should impact modeling of physiological mechanisms of face perception. The ability to discriminate flashed faces where there is almost no eye movement indicates that eye drift is not essential for visibility.

Evaluating persistence of shape information using a matching protocol

Many laboratories have studied persistence of shape information, the goal being to better understand how the visual system mediates recognition of objects. Most have asked for recognition of known shapes, e.g., letters of the alphabet, or recall from an array. Recognition of known shapes requires access to long-term memory, so it is not possible to know whether the experiment is assessing short-term encoding and working memory mechanisms, or has encountered limitations on retrieval from memory stores. Here we have used an inventory of unknown shapes, wherein a string of discrete dots forms the boundary of each shape. Each was displayed as a target only once to a given respondent, with recognition being tested using a matching task. Analysis based on signal detection theory was used to provide an unbiased estimate of the probability of correct decisions about whether comparison shapes matched target shapes. Four experiments were conducted, which found the following: a) Shapes were identified with a high probability of being correct with dot densities ranging from 20% to 4%. Performance dropped only about 10% across this density range. b) Shape identification levels remained very high with up to 500 milliseconds of target and comparison shape separation. c) With one-at-a-time display of target dots, varying the total time for a given display, the proportion of correct decisions dropped only about 10% even with a total display time of 500 milliseconds. d) With display of two complementary target subsets, also varying the total time of each display, there was a dramatic decline of proportion correct that reached chance levels by 500 milliseconds. The greater rate of decline for the two-pulse condition may be due to a mechanism that registers when the number of dots is sufficient to create a shape summary. Once a summary is produced, the temporal window that allows shape information to be added may be more limited.

Consciousness weaves our internal view of the outside world

Low-level consciousness is fundamental to our understanding of the world. Within the conscious field, the constantly changing external visual information is transformed into stable, object-based percepts. Remarkably, holistic objects are perceived while we are cognizant of all of the spatial details comprising the objects and of the relationship between individual elements. This parallel conscious association is unique to the brain. Conscious contributions to motor activity come after our understanding of the world has been established.

The unsteady eye: an information-processing stage, not a bug

How is space represented in the visual system? At first glance, the answer to this fundamental question appears straightforward: spatial information is directly encoded in the locations of neurons within maps. This concept has long dominated visual neuroscience, leading to mainstream theories of how neurons encode information. However, an accumulation of evidence indicates that this purely spatial view is incomplete and that, even for static images, the representation is fundamentally spatiotemporal. The evidence for this new understanding centers on recent experimental findings concerning the functional role of fixational eye movements, the tiny movements humans and other species continually perform, even when attending to a single point. We review some of these findings and discuss their functional implications.

Shape recognition elicited by microsecond flashes is not based on photon quantity

It is generally thought that the perceptual impact of a brief flash of light is determined by the quantity of photons the flash delivers. This means that only the total quantity of photons is important below a critical duration of about 30-100 ms. Recent findings have challenged this concept and the present work provides additional evidence that it is not correct. The first experiment reported here delivered a given quantity of photons in under 200 μs, either as a single threshold-intensity flash or as multiple flashes at the same intensity. The single flash was ineffective at eliciting recognition, but multiple flashes became progressively more effective as the number of flashes was increased. A second experiment varied the number of 10 μs flashes. The effectiveness of multiple flashes was far higher than would be expected on the basis of the total quantity of photons being delivered. The results of both experiments suggest that the brief transitions of intensity provided by the flashes are far more important than the quantity of photons. A final experiment examined the combined impact from two threshold-intensity flashes as the interstimulus interval was increased. The pair members were able to combine their influence for at least 100 ms. These results call for more attention to how very brief light flashes generate signals that convey image content.

Violation of Bloch's Law that specifies reciprocity of intensity and duration with brief light flashes

For more than a century researchers have been reporting that the visual impact of a very brief flash is determined by the quantity of photons that the flash delivers. This has been variously described as the Bunsen-Roscoe Law or Bloch's Law, often specified as reciprocity of intensity × duration. Prior research found no evidence for such reciprocity when microsecond-duration flashes from a light-emitting diode array were used to display the major contours of nameable shapes. The present work tested with flash durations ranging up to 100 ms and also found no reciprocity. This departure from classic principles might be due to the specific range of wavelengths of the light-emitting diodes and to a mesopic level of ambient light, which together would preclude activation of rods. The reciprocity of intensity and duration may only be valid with full dark adaptation and very dim flashes that activate rods.

Shapes displayed with durations in the microsecond range do not obey Bloch's law of temporal summation

Shape patterns were displayed with simultaneous brief flashes from a light-emitting diode array. Flash durations in the microsecond range and luminous intensities were adjusted to vary the degree of successful shape recognition. Four experiments were conducted to test whether Bloch's law would apply in this task. Bloch's law holds that for very brief flashes the perceptual threshold is determined by the total number of photons being delivered, i.e., there is reciprocity of intensity and duration. The present results did not find that effectiveness of flashes was based on the total quantity of photons, as predicted by Bloch's law. Additionally, the evidence points to a visual mechanism that has ultra-high temporal precision that either registers the rate of photon flux or the duration of flashes.